BORK, Circuit Judge:
This case presents two questions. The first is whether the Nuclear Regulatory Commission (“NRC” or “Commission”), before issuing a license for the operation of the Diablo Canyon Nuclear Power Plant, is required to hold a hearing concerning the potential complicating effects of an earthquake on responses to a simultaneous but independently caused radiological accident at the plant. The risk of that happening is calculated as being one in several tens of millions. The second question is whether this court should examine transcripts, not a part of the record, of a closed meeting of the Commission.
In San Luis Obispo Mothers for Peace v. NRC, 751 F.2d 1287 (D.C.Cir.1984), a panel of this court affirmed a decision by the NRC to allow issuance of low power and full power licenses for the Diablo Canyon plant. In so doing, the panel majority considered and rejected petitioners’ claim that the Commission improperly excluded from licensing hearings specific consideration of the potential complicating effects of an earthquake on planned emergency responses at the Diablo Canyon facility. The same majority refused to examine the proffered transcripts. See id. at 1323-29. Subsequently, the full court vacated a portion of the original opinion and judgment and granted rehearing en banc to consider the questions more fully. See 760 F.2d 1320. We now affirm the Commission’s decision.
I.
Licensing proceedings for nuclear power plants are typically long and complex and the Diablo Canyon proceedings were no exception. In this section, we set forth only a skeletal history of those proceedings, taken largely from the panel opinion. See 751 F.2d at 1296-97. Additional facts relevant to the specific issues we consider are set forth throughout the opinion.
The petitioners in this case consist of a number of individuals who, and groups whose members, live and work near the Diablo Canyon plant. They have been active in the Commission's proceedings related to the licensing of the plant.
The Atomic Energy Commission (“AEC”), the predecessor to the NRC, issued construction permits to the Pacific Gas and Electric Company (“PG & E”) for Units 1 and 2 of the pressurized water reactor plant at Diablo Canyon in 1968 and 1970. See Docket No. 50-323, 4 A.E.C. 447, 460 (1970), affd, ALAB-27, 4 A.E.C. 652, 664 (1971) (Unit 2); Docket No. 50-275, 4 A.E.C. 89, 98-99 (1968) (Unit 1). Construction began shortly thereafter, based on the assumption that the nearest significant earthquake fault was eighteen to twenty miles away. See ALAB-519, 9 N.R.C. 42, 45 (1979). Four years later, offshore exploration for petroleum revealed the presence of the Hosgri Fault within three miles of the Diablo Canyon site. See id. Petitioners, who had intervened in the administrative proceedings, requested that construction at the facility be stopped until the implications of the discovery could be assessed, but the AEC permitted construction at the plant to continue. See 4 A.E.C. 914 (1972). Following an extensive reexamination, the Commission’s Appeal Board approved the plant’s seismic design on June 16, 1981. ALAB-644, 13 N.R.C. 903 (1981).
On September 21, 1981, the Commission rejected claims that the emergency planning program at Diablo Canyon was deficient and issued a license to PG & E to load fuel and conduct low power testing at Unit 1. See CLI-81-22, 14 N.R.C. 598 (1981). Investigation by PG & E and the Commission’s staff, however, soon uncovered various design errors, see CLI-81-30, 14 N.R.C. 950, 951 (1981), and on November 19, 1981, the Commission suspended PG & E fuel loading and low power test license. Id. at 950. To ensure that the plant would be adequately protected against seismic disturbances, the Commission ordered PG & E, as a condition of reinstatement of the license, to institute an independent design verification program. See id. at 951, 955-58. In addition, several requirements concerning seismic and other design verification issues were imposed on PG & E as conditions of its eligibility for a full power license. See CLI-84-13, 20 N.R.C. 267 (1984).
Professionals expended more than 2,000,-000 hours on the reanalysis and modification of the plant’s design, which were completed in October 1983. CLI-84-5, 19 N.R.C. 953, 971 (1984) (views of Commissioner Bernthal). The NRC staff undertook an independent review of the results of the Independent Design Verification Program after which the Commission progressively reinstated elements of the suspended low power license in late 1983 and early 1984. See CLI-84-5, 19 N.R.C. 953 (1984); CLI-84-2, 19 N.R.C. 3 (1984); CLI-83-27, 18 N.R.C. 1146 (1983). Reinstatement of the license was consistent with the Appeal Board’s findings that “[t]he applicant’s verification efforts provide adequate confidence that the Unit 1 safety-related structures, systems and components are designed to perform satisfactorily in service and that any significant design deficiencies in that facility resulting from the defects in the applicant’s design quality assurance program have been remedied.” ALAB-763, 19 N.R.C. 571, 619 (1984).
On August 10, 1984, the NRC approved issuance of a full power license for the Diablo Canyon plant. CLI-84-13, 20 N.R.C. 267 (1984). Petitioners appealed both the low power and full power orders to this court and, before the license had issued, the court granted petitioners’ motion for a stay. On October 31, 1984, after oral argument, the court lifted the stay, thereby permitting issuance of the full power license and the commencement of operations at Diablo Canyon. On December 31, 1984, the court affirmed the Commission’s decision to permit issuance of the low power and full power licenses. See San Luis Obispo Mothers for Peace v. NRC, 751 F.2d 1287 (D.C.Cir.1984). The court found that the Commission made two legal errors (not related to the issues considered in this en banc proceeding), but that neither warranted judicial relief since one was harmless and the other had already been remedied by the Commission. See id. at 1311-12.
Specifically with regard to emergency planning, the panel majority held that the Commission did not err by excluding consideration of the effects of earthquakes on emergency responses at Diablo Canyon. In addition, the majority denied petitioners’ motion to supplement the administrative record with the transcripts of a closed meeting of the NRC. See 751 F.2d at 1323-29. Judge Wald, dissenting in part, thought that the Commission’s exclusion of consideration of earthquakes was arbitrary and capricious and that the court should make an in camera inspection of the transcripts in deciding whether to grant petitioners’ motion to supplement the record. 751 F.2d at 1329-35.
II.
Petitioners argue that the Commission’s decision to exclude from the Diablo Canyon licensing proceedings consideration of the potential complicating effects of an earthquake on emergency responses “has deprived Petitioners of their right to an on-the-record hearing on a material safety issue ... in violation of § 189(a) of the Atomic Energy Act as applied by this Court in Union of Concerned Scientists v. [NRC, 735 F.2d 1437 (1984), cert. denied, — U.S. —, 105 S.Ct. 815, 83 L.Ed.2d 808 (1985) ].” Supplemental Brief for Petitioners on Rehearing En Banc (“Pet.Supp.Br.”) at 11 (citations omitted).
Section 189(a)(1) of the Atomic Energy Act provides that “[i]n any proceeding under this chapter, for the granting ... of any license ..., the Commission shall grant a hearing upon the request of any person whose interest may be affected by the proceeding.” 42 U.S.C. § 2239(a)(1) (1982). It follows from Union of Concerned Scientists, however, that the “interest” which entitles a person to a hearing is defined by the Commission’s rules and regulations. In that case, we invalidated an NRC amendment to its rule on emergency preparedness. The amendment eliminated the requirement of a hearing on the results of emergency preparedness exercises as a prerequisite to authorization of a license. But those results remained a factor that the Commission was required to consider in its licensing decision. “Since the NRC, by its own regulations, has made correction of deficiencies identified in emergency exercises a requirement of its ultimate licensing decision, it would seem to follow that results of these exercises must be subject to the § 189(a) hearing requirement.” 735 F.2d at 1442; accord id. at 1445.
Union of Concerned Scientists holds only that the Commission cannot exclude from a section 189(a) hearing issues that its rules of regulations require it to consider in its licensing decisions. As the opinion stated: “Today, we in no way restrict the Commission’s authority to [limit the purposes for which it considers emergency exercises relevant] as a substantive licensing standard.” 735 F.2d at 1448 (footnote omitted). Thus, to establish, on the rationale of Union of Concerned Scientists, that the Commission in this case impermissibly refused a hearing, petitioners must show that NRC rules or regulations required the Commission to consider the potential complicating effects of earthquakes on emergency responses in deciding whether to license Dia-blo Canyon. Petitioners have made no such showing.
Petitioners assert that “the Commission’s interpretation and application of its own regulations are entitled to no weight,” Pet.Supp.Br. at 20, because “the Commission’s conclusion is undermined both by the language and prior application of the NRC’s regulations,” Pet.Supp.Br. at 12, and also because “ ‘the Commission’s outright refusal to make explicit provision in emergency response plans for an earthquake in a nuclear plant within three miles of a major, active fault in California is by definition an arbitrary and capricious act,’ ” Pet. Reply Br. at 3 (quoting 751 F.2d at 1335 (Wald, J., dissenting)); see also Pet. Supp.Br. at 15-21. We think these contentions do not survive analysis.
A.
We consider first the question whether the Commission’s regulation requires consideration of earthquakes and thereby triggers section 189(a)’s hearing requirement.
1. The Commission’s interpretation of the regulation. We note at the outset that courts are not at liberty to set aside an agency’s interpretation of its own regulations unless that interpretation is plainly inconsistent with the language of the regulations. See United States v. Larionoff, 431 U.S. 864, 872-73, 97 S.Ct. 2150, 2155-56, 53 L.Ed.2d 48 (1977); National Association of Regulatory Utility Commissioners v. FCC, 746 F.2d 1492, 1502 (D.C.Cir.1984). The degree of deference due is great. We “need not find that the agency’s construction is the only possible one, or even the one that the court would have adopted in the first instance.” Belco Petroleum Corp. v. FERC, 589 F.2d 680, 685 (D.C.Cir.1978). As stated by the Supreme Court:
Since this involves an interpretation of an administrative regulation a court must necessarily look to the administrative construction of the regulation if the meaning of the words used is in doubt____ [T]he ultimate criterion is the administrative interpretation, which becomes of controlling weight unless it is plainly erroneous or inconsistent with the regulation.
Bowles v. Seminole Rock Co., 325 U.S. 410, 413-14, 65 S.Ct. 1215, 1217, 89 L.Ed. 1700 (1945).
The only NRC regulation relevant to this case is the regulation dealing with emergency planning. Promulgated in 1980 following the accident at Three Mile Island, that regulation provides in pertinent part that “no operating license for a nuclear power reactor will be issued unless a finding is made by NRC that there is reasonable assurance that adequate protective measures can and will be taken in the event of a radiological emergency.” 10 C.F.R. § 50.47(a)(1) (1984). Though that regulation represents a departure from the Commission’s previous policy of requiring little or no emergency planning, the Commission has consistently interpreted that regulation not to require specific consideration of the potential complicating effects of earthquakes. See Pacific Gas & Electric Co. (Diablo Canyon Nuclear Power Plant, Units 1 & 2), CLI-84-12, 20 N.R.C. 249 (1984); Southern California Edison Co. (San Onofre Nuclear Generating Station, Units 2 & 3), CLI-81-33, 14 N.R.C. 1091 (1981).
Petitioners’ claim that the Commission’s interpretation contradicts the language of the emergency planning regulation is supported only by a quotation of the regulatory language. That language, however, does not contradict, but amply supports, the Commission.
The regulation does not address any particular emergency or natural hazard; rather, it sets forth a general standard that envisions judgment and implies discretion: the Commission is to satisfy itself that there is “reasonable assurance” of “adequate” protective measures. In this case, we think that the Commission’s view — that it need not consider the potential effects of earthquakes to determine “that there is reasonable assurance that adequate protective measures can and will be taken in the event of a radiological emergency” — is not, by any stretch of the imagination, “plainly inconsistent” with the regulatory language.
Petitioners assert, however, that the Commission’s interpretation is “undermined” by an NRC staff report referred to in the emergency planning regulation. Subsection (b) of the regulation sets forth sixteen specific standards which the onsite and offsite emergency response plans for nuclear power reactors must meet. A footnote to subsection (b) states: “These standards are addressed by specific criteria in NUREG-0654; FEMA-REP-1 entitled ‘Criteria for Preparation and Evaluation of Radiological Emergency Response Plans and Preparedness in support of Nuclear Power Plants — for Interim Use and Comment’, January 1980.” 10 C.F.R. § 50.47(b) n. 1 (1984). NUREG-0654 was a joint project of the NRC and FEMA staffs “to provide a common guidance and reference source for ... State and local governments and nuclear facility operators in the development of radiological emergency response plans and preparedness in support of nuclear power plants.” NUREG-0654 at 1.
We do not think NUREG-0654 undermines the Commission’s interpretation of its emergency planning regulation. Petitioners state that NUREG-0654 contains “general references to ‘natural hazards.’ ” Pet.Supp.Br. at 13. But we can find no reference, general or specific, to “natural hazards” in the body of the document. Petitioners quote two statements from the report in support of their assertion. The first is that “[e]ach State and local organization should have procedures in place that provide for emergency actions to be taken which are consistent with the emergency actions recommended by the nuclear facility licensee, taking into account local offsite conditions that exist at the time of the emergency.” NUREG-0654 at 42. The second statement is that “[t]he organization’s plans to implement protective measures for the plume exposure pathway shall include ... [identification of and means for dealing with potential impediments (e.g., seasonal impassability of roads) to use of evacuation routes, and contingency measures.” NUREG-0654 at 61-63. Petitioners’ argument is that these sentences constitute “references to ‘natural hazards’ ” and that the Commission is therefore required to consider the effects of earthquakes on emergency planning. This argument is unsound.
It is not at all clear that the phrases, “local offsite conditions” and “potential impediments ... to evacuation routes,” were intended to suggest specific consideration of all conceivable “natural hazards.” Taken in context, these phrases constitute broad references. They might suggest some consideration of natural phenomena reasonably anticipated at the plant such as seasonal rains, fog, or “seasonal impassibility of roads.” But petitioners’ reading of these references to require specific consideration of such highly unlikely and infrequent events as an earthquake at the plant sweeps much too broadly. If we accept petitioners’ argument, we can think of no potential natural or unnatural hazards, regardless of their improbability, that the Commission would not be required to consider. That is a prescription for licensing proceedings that never end and plants that never generate electricity. Petitioners themselves attempt to disavow that logical conclusion of their argument. For example, at oral argument petitioners conceded that the emergency planning regulations (and presumably NUREG-0654) do not require the Commission to consider the potential complicating effects of a meteorite striking the plant. Yet we do not see why NUREG-0654 would not require just such consideration given a holding that it requires consideration of potential simultaneous earthquakes and independently caused radiological accidents at the plant. As we will show, the latter is not significantly more likely than the former.
Moreover, our conclusion that NUREG-0654 does not counsel specific consideration of earthquakes is more in keeping with NUREG-0654’s stated policy that “[n]o single specific accident sequence should be isolated as the one for which to plan because each accident should have different consequences, both in nature and degree,” NUREG-0654 at 6, than is petitioners’ contrary assertion.
Petitioners also claim that a reference to earthquakes in NUREG-0654’s appendix undercuts the Commission’s interpretation of the applicable regulations. See Pet. Supp.Br. at 13. The appendix contains a list of “example initiating conditions” that could lead to a “site area emergency” that includes: “Severe natural phenomena being experienced or projected with plant not in cold shutdown.” NUREG-0654 app. 1 at 1-13. It is to be noted that this example refers to an earthquake that causes a radiological emergency, not an earthquake that complicates emergency responses. The former risk, to which the example pertains, was the subject of extensive hearings and is not under review here. Under this example is listed: “Earthquake greater than SSE levels.” Id. “[T]he SSE is the most powerful earthquake ever expected to occur at the plant site.” ALAB-644, 13 N.R.C. 903, 911 (1981). For Diablo Canyon, the SSE was calculated to be an earthquake of 7.5 magnitude. Id. at 910. Far from being “projected” for the Diablo Canyon, an earthquake greater than SSE levels, by definition, is never expected to occur at the plant site.. Indeed, the Commission has noted that the probability that an earthquake at the SSE level will occur has “typically been estimated to be on the order of one in a thousand or one in ten thousand per year.” CLI-84-4, 19 N.R.C. 937, 948 (1984). Evidence before the Licensing Board indicated that “there have not been recurrent earthquakes above 6.5 magnitude on the Hosgri in the past 17,000 years.” LBP-79-26, 10 N.R.C. 453, 482 (1979). The fact is particularly significant because the Hosgri is 90 miles long, see id. at 472, and only a small portion of it is near the Diablo Canyon plant. As the panel majority stated in a portion of its opinion not vacated by our May 1, 1985 Order: “We must assume, therefore, that the likelihood that an earthquake will trigger a nuclear accident at the facility is so small as to be rated zero.” 751 F.2d at 1304 (footnote omitted).
Moreover, even if we agreed with petitioners’ claim that NUREG-0654, in its body or appendix, suggests consideration of earthquakes, the emergency planning regulations’ reference to NUREG-0654 makes plain that it is a staff document intended simply to provide guidance to parties in complying with the standards set forth in the emergency planning regulations: “NRC staff has developed ... a joint NRC/FEMA report, NUREG-0654 ... to provide guidance in developing plans for coping with emergencies.” 10 C.F.R. Part 50 app. E n. 1 (1984). Under the regulations, the Commission is required to make its own finding that emergency plans “provide reasonable assurance” of “adequate protective measures” and meet the specified regulatory standards. See 10 C.F.R. § 50.47(a)(1) & (b) (1984). These regulatory standards contain no references to “natural hazards,” to say nothing of earthquakes. To accept petitioners’ argument, therefore, we would have to hold that NU-REG-0654, a staff document intended as guidance, supersedes the regulation itself. The only virtue of that approach is novelty.
2. The Commission’s applications of the regulation. Petitioners’ next argument is that the Commission’s interpretation conflicts with “prior application of the NRC’s regulations.” If petitioners suggest an inconsistency with prior Commission applications, their assertion is false. The Commission has never applied its regulation in any way except the way it did here. Indeed petitioners’ only support for their claim is apparently that the Commission’s staff has called for emergency plans to consider the potential complicating effects of earthquakes. The position of an agency’s staff, taken before the agency itself decided the point, does not invalidate the agency’s subsequent application and interpretation of its own regulation.
The facts are as follows. In December, 1980, a member of the NRC’s staff sent PG & E a letter requesting that it evaluate “the potential complicating factors which might be caused by earthquakes which either initiate or follow the initiation of accidents,” Record, vol. 69, exh. 117 (Letter from Tedesco (NRC) to Furbush (PG & E) (Dec. 16, 1980)). See Pet.Supp.Br. at 5, 13. A staff member wrote a memorandum on November 3, 1980 requesting that the Federal Emergency Management Agency review the adequacy of state and local capabilities for emergency response to a radiological accident occurring during an earthquake. See Pet.Supp.Br. at 5-6 (citing Record, vol. 69, attachment to exh. 117 (Memorandum from Grimes (NRC) to McConnell (FEMA) (Nov. 3, 1980))). Petitioners ignore the fact that both of these documents were written before the Corn-mission itself had interpreted its emergency planning regulation.
The regulation was promulgated in 1980 and the question whether it required consideration of earthquakes first came before the Commission in 1981, after it was raised by the Atomic Safety and Licensing Board in the context of licensing the San Onofre Nuclear Generating Station. The Commission decided “that its current regulations do not require consideration of the impacts on emergency planning of earthquakes which cause or occur during an accidental radiological release.” San Onofre, CLI-81-33, 14 N.R.C. at 1091. In so interpreting its regulation, the Commission stated:
A review of the rulemaking file associated with the Commission’s emergency planning regulations reveals that ... [t]hree commenters suggested that the NRC specifically require the occurrence of earthquakes or severe natural phenomena to be part of the basis for emergency response planning, but the comments were not accepted in the final rule. The current regulations are designed with the flexibility to accommodate a range of onsite accidents, including accidents that may be caused by severe earthquakes. This does not, however, mean that emergency plans should be tailored to accommodate specific accident sequences____
San Onofre, CLI-81-33, 14 N.R.C. at 1092 (citations omitted). Thus, the 1980 staff documents on which petitioner rely in no way affect the legitimacy of the Commission’s subsequent decision not to require consideration of earthquakes on emergency planning at Diablo Canyon. The positions of an agency’s staff do not preclude the agency from subsequently reaching its own conclusion.
The San Onofre rule has been followed since. The Appeal Board relied explicitly on San Onofre to reject a challenge to the Licensing Board’s authorization of low power testing at Diablo Canyon on the ground that it should have required consideration of earthquakes in emergency planning. See ALAB-728, 17 N.R.C. 777, 792-93 (1983), aff'g LBP-81-21, 14 N.R.C. 107 (1981). The Commission itself then summarily declined review. See CLI-83-32, 18 N.R.C. 1309 (1983).
Prompted in part by two staff memoran-da, the Commission in 1984 decided to consider whether “the circumstances of [the Diablo Canyon] case ... provide a basis for departure from its decision in” San Onofre. See Diablo Canyon, CLI-84-12, 20 N.R.C. at 249. Specifically, the Commission requested that petitioners, PG & E and the NRC staff submit comments addressing “whether NRC emergency planning regulations can and should be read to require some review of the complicating effects of earthquakes on emergency planning for Diablo Canyon.” CLI-84-4, 19 N.R.C. 937, 938 (1984). After receiving and considering these comments, the Commission reaffirmed its original interpretation “that the NRC’s regulations ‘do not require consideration of the impacts on emergency planning of earthquakes which cause or occur during an accidental radiological release.’ ” Diablo Canyon, CLI-84-12, 20 N.R.C. at 250 (quoting San Onofre, CLI-81-33, 14 N.R.C. at 1091). Thus, there can be no doubt that the NRC’s position has not only been consistently applied by has been thoughtfully reconsidered in this very proceeding.
Petitioners cite the two staff memoranda just referred to for the proposition that “[s]ince 1980, the Commission’s staff has frequently advocated the view that consideration of the effects of earthquakes on emergency planning ‘may be warranted’ for reactor sites in California because of their ‘relatively high’ seismic risk.” Pet. Supp.Br. at 6 & n. 15 (citing Memoranda of Jan. 13, 1984 and June 22, 1982, attachments 1 & 2 to CLI-84-4, 19 N.R.C. 937 (1984)). Petitioners support their assertion that the staff “frequently advocated” a view contrary to the Commission’s with the following parenthetical: “(‘planning for earthquakes which might have emergency preparedness implications may be warranted in areas where the seismic risk to offsite structures is relatively high (e.g., California sites...)’).” Pet.Supp.Br. at 6 n. 15. This is a single occasion, not a frequent event. Worse, the claim that it constitutes “advocacy” is completely misleading. Petitioners have taken the quoted language out of context.
The language in question comes from the January 13, 1984 memorandum. The memorandum first recounts the substance of the Commission’s San Onofre decision and the Commission’s statement that it would consider whether its regulations should be changed. The memorandum then states that the Commission’s Secretary directed the staff to undertake such consideration and that the staff responded in a memorandum dated June 22, 1982. In a footnote, the memorandum then states:
To very briefly summarize the Staff’s position as expressed in its June 22nd response, the Staff concluded that the Commission’s regulations do not require amendment since (1) for most sites there is only a very low likelihood that an earthquake severe enough to disturb on-site or offsite planned responses will occur concurrently with or cause a reactor accident, and (2) while planning for earthquakes which might have emergency preparedness implications may be warranted in areas where the seismic risk to offsite structures is relatively high (e.g., California Sites and other areas of the Western U.S.), current review criteria set forth in NUREG-0654 (which are derived from the Commission’s regulations in 10 C.F.R. § 50.47) are considered adequate.
Attachment 2 to CLI-84-4, 19 N.R.C. at 947 n. 2.
Petitioners substantially mischaracterize the staff’s views. The staff was summarizing its reasons for rejecting an amendment to the Commission’s emergency planning regulations that would have required specific consideration of the effects of earthquakes on emergency planning. Similarly, since the staff was expressing its views about an amendment to the regulation and not the regulation itself, there can be no suggestion that the staff was expressing an opinion about the correctness of the Commission’s interpretation of the existing regulation. Moreover, since both memoranda were written after the Commission’s San Onofre decision, the staff was well aware of the Commission’s interpretation.
We have now reviewed the sum of petitioners’ arguments and find disingenuous petitioners’ assertion that the Commission’s refusal to allow consideration of the effects of earthquakes on emergency responses for Diablo Canyon was “in disregard of its own technical staff’s longstanding practice of considering earthquakes in their emergency planning reviews for California nuclear power plants.” Pet.Supp.Br. at SA 3. By petitioners’ own admission, “the earthquake risk affects only two nuclear plants, Diablo Canyon and San Onofre.” See 751 F.2d at 1308; Opening Brief for Petitioner at 44-45. With respect to the licensing of the San Onofre plant, petitioners’ claim that the NRC staff “considered earthquakes” is unsupported. Further, it appears inaccurate. In San Onofre, the Commission stated that the issue “whether emergency planning should be concerned with earthquakes” was “raised sua sponte by the Atomic Safety and Licensing Board.” CLI-81-33, 14 N.R.C. at 1091. After concluding that its current regulations do not require consideration of earthquakes, the Commission “directed [the Licensing Board] not to pursue this issue.” Id.
In the case of Diablo Canyon, it is true that the NRC staff requested PG & E to consider the effects of earthquakes in its emergency plans. But as already noted, this request came before the Commission’s San Onofre decision. There is no indication that the staff persisted in requiring consideration of earthquakes in Diablo Canyon emergency plans after San Onofre was decided. Indeed, it seems unlikely that they would have done so given that the Commission’s Appeal Board and then the Commission itself specifically rejected challenges to Diablo Canyon licenses on the ground that emergency plans failed to consider earthquakes. We do not believe that the one instance cited by petitioners constitutes a “longstanding practice.”
3. Consistency with Atomic Energy Act. Though petitioners make only a cursory assertion that the Commission’s interpretation of its emergency planning regulation contradicts the Atomic Energy Act, we consider this contention briefly since the Supreme Court has stated that “regulations, in order to be valid, must be consistent with the statute under which they are promulgated.” See United States v. Lario-noff 431 U.S. 864, 873, 97 S.Ct. 2150, 2156, 53 L.Ed.2d 56 (1977).
Enacted in 1946, the Atomic Energy Act provides that “[i]n the performance of its functions the Commission is authorized to ... make, promulgate, issue, rescind, and amend such rules and regulations as may be necessary to carry out the purposes of this chapter.” 42 U.S.C. § 2201(p) (1982). This is a broad grant of authority. One of the stated purposes of the Act is to provide for “a program to encourage widespread participation in the development and utilization of atomic energy for peaceful purposes to the maximum extent consistent with the common defense and security and with the health and safety of the public.” Id. § 2013(d).
Petitioners’ argument must be that emergency planning regulations that do not specifically require consideration of earthquakes are inconsistent with “the health and safety of the public” as those terms are used in the Act. But this argument disappears when it is recalled that prior to 1980, there were no emergency planning regulations at all. Apparently the Commission did not think the Act required such regulations and, so far as we can tell, no litigant claimed that the Act did so. It would be a strange reading of the statute to say that it permits no emergency planning at all (the situation for over thirty years), but that, once an emergency planning regulation is promulgated, it must mandate consideration of earthquakes. The current regulation does not contradict, but furthers, the Act’s stated purposes. Under these circumstances, we cannot say that the current emergency planning regulation, as interpreted by the Commission, is in any way inconsistent with the Atomic Energy Act.
The Commission has consistently interpreted its emergency planning regulation not to require consideration of earthquakes. This interpretation contradicts neither the regulatory language nor the Atomic Energy Act and is therefore controlling. Thus, we must uphold the Commission’s decision to exclude from the Dia-blo Canyon licensing proceedings consideration of the potential complicating effects of earthquakes on emergency planning unless we find that the action was arbitrary, capricious, or an abuse of discretion.
B.
Petitioners' final argument that the Commission’s exclusion of consideration of earthquakes is arbitrary and capricious is somewhat difficult to follow. The argument appears to take two forms. The first is that the danger of simultaneous but independent events (an earthquake and a radiological emergency) is so great that it must be considered in licensing, and hence be the subject of a hearing. In that form, the argument goes beyond anything said in Union of Concerned Scientists and has already been answered. If the Atomic Energy Act and the emergency planning regulation do not require such consideration, then petitioners may not ask this court to rewrite the statute and regulation to deal with their concerns.
The second form of the argument is that the Commission already interprets its regulation to require consideration of such complicating phenomena as fog and heavy rain. It follows, petitioners contend, that it is arbitrary and capricious to refuse to consider earthquakes. This contention rests upon the assumption that the probabilities of fog, heavy rain, and an earthquake are similar. If the probabilities are not similar, it is rational to consider some but not others. At some point the probability of an occurrence becomes so infinitesimal that it would be absurd to say that a hearing about it is required. Thus, no one would argue, or so we assume, that the Commission had to consider the possibility that a space satellite might fall on the Diablo Canyon plant. And, as we have already pointed out, petitioners agree that no hearing is required on the possibility that a meteorite might strike the plant. It can be shown that the danger posited by petitioners here falls into the same range of improbability. We will first establish that this case concerns only the likelihood of the simultaneous occurrence of an earthquake and a radiological emergency arising from an independent cause. We then turn to the probability of such an event and show why the Commission’s decision to exclude its consideration was by no means arbitrary.
The Administrative Procedure Act, 5 U.S.C. § 706 (1982), made applicable by 42 U.S.C. § 2231 (1982), establishes the scope of our review: “The reviewing court shall ... hold unlawful and set aside agency action, findings, and conclusions found to be ... arbitrary [and] capricious____” Id. § 706(2)(A). This “standard of review is a narrow one. The court is not empowered to substitute its judgment for that of the agency.” Citizens to Preserve Overton Park v. Volpe, 401 U.S. 402, 416, 91 S.Ct. 814, 824, 28 L.Ed.2d 136 (1971). Moreover, the party challenging an agency’s action as arbitrary and capricious bears the burden of proof. See, e.g., National Association of Regulatory Utility Commissioners v. FCC, 746 F.2d 1492, 1502 (D.C.Cir.1984). We note that in determining whether agency action is arbitrary and capricious, the Administrative Procedure Act directs that “[t]he court shall review the whole record or those parts of it cited by a party.” 5 U.S.C. § 706 (1982). Thus, that the Commission did not include citations to specific pages of the record in its Diablo Canyon decision provides no basis for overturning the Commission’s decision.
Under its emergency planning regulations, the NRC cannot issue an operating license for a nuclear power reactor unless it makes “a finding ... that there is reasonable assurance that adequate protective measures can and will be taken in the event of a radiological emergency.” 10 C.F.R. § 50.47(a)(1). Thus, the Commission’s decision to exclude consideration of earthquakes from the Diablo Canyon licensing hearings was in effect a decision that it could find that emergency plans for Diablo Canyon that do not plan specifically for the effects of potential earthquakes provide the requisite “reasonable assurance” of “adequate protective measures.”
The Commission cited three considerations in support of its decision. We turn to these now.
1. Earthquake-initiated radiological emergency. The Commission considered the possibility that an earthquake might cause a radiological emergency at Diablo Canyon and observed: “For earthquakes up to and including the Safe Shutdown Earthquake (SSE), the seismic design of the plant was reviewed to render extremely small the probability that such an earthquake would result in a radiological release. While a radiological release might result from an earthquake greater than the SSE, the probability of occurrence of such an earthquake is extremely low.” Diablo Canyon, 20 N.R.C. at 251 (footnotes omitted).
The Commission’s reasoning is rational and supported by the record. “[T]he SSE is the most powerful earthquake ever expected to occur at the plant site.” ALAB-644, 13 N.R.C. 903, 911 (1981). For Diablo Canyon, the SSE was calculated to be an earthquake of 7.5 magnitude. Id. at 910. The Licensing Board found that value to be “very conservative.” LBP-79-26, 10 N.R.C. 453, 485 (1979). Thus, the Commission could properly conclude that the possibility of an initiating earthquake of a magnitude greater than 7.5 is so low that specific consideration is not justified.
Similarly, the Commission could rationally exclude from consideration earthquakes of magnitudes 7.5 or smaller. The Commission determined that Diablo Canyon’s seismic design is more than adequate to withstand the forces of an SSE without releasing dangerous quantities of radioactivity. See CLI-84-12, 20 N.R.C. at 251-52. This means that such earthquakes pose no material threat to the plant. Since petitioners have not challenged this conclusion on appeal, we have no grounds to conclude that the Commission’s exclusion of such consideration was arbitrary and capricious. As the panel majority stated in a portion of its opinion not vacated by our May 1, 1985 Order: “We must assume, therefore, that the likelihood that-an earthquake will trigger a nuclear accident at the facility is so small as to be rated zero.” 751 F.2d at 1304 (footnote omitted). The original panel was unanimous on this point. As Judge Wald stated in her partial dissent, the Commission’s first conclusion “is adequately supported by findings in the record.” Id. at 1332. Thus, the only risk to be considered here is that of the simultaneous occurrence of an earthquake and a radiologic release for reasons unrelated to the earthquake.
2. Simultaneous occurrence of an earthquake and an independently caused radiological emergency. The Commission determined that “earthquake[s] that would complicate emergency response” as well as occur contemporaneously with “a radiologic release from the plant caused by an event other than an earthquake” are “so infrequent that their specific consideration is not warranted.” Diablo Canyon, CLI-84-12, 20 N.R.C. at 252. This determination is supported by the record, not merely adequately but, we think, conclusively.
The NRC estimates the representative probability of a serious core melt accident with offsite radiation release requiring protective action (sheltering or evacuation) to be one in a hundred thousand per year. See Technical Guidance for Siting Criteria Development, NUREG/CR-2239, SAND 81-1549 at iii, 2-11, 2-12 & table 2.3.1-1 (1982). In 1981, the Commission’s Appeal Board considered evidence relating to the Diablo Canyon operating basis earthquake (“OBE”) and rejected the claim that the Diablo Canyon plant is located in an area of high seismicity. The OBE is defined as “that earthquake which, considering the regional and local geology and seismology and specific characteristics of local subsurface material, could reasonably be expected to affect the plant site during the operating life of the plant.” 10 C.F.R. Part 100 app. A § III.(d)(1984). By definition, less severe earthquakes are not expected to affect the Diablo, Canyon plant site.
Petitioners argue, however, that earthquakes of a magnitude smaller than the OBE might complicate emergency response and therefore should have been considered. Petitioners have cited no support for this assertion. Moreover, the record supports the Commission’s decision to exclude consideration of earthquakes of any size. Record evidence indicates that seismic activity of any magnitude occurs infrequently along the Hosgri Fault and more particularly, in the San Luis Obispo area. For example, in reviewing the geologic setting of the Diablo Canyon plant site, the Atomic Safety and Licensing Board observed that “[i]n the main southern part of the Coast Ranges province, no [faults other than the San Andreas] show evidence of more than minor seismic activity during Holocene time (the last 10,000 years).” LBP-79-26, 10 N.R.C. 453, 469 (1979). Evaluation of the site prior to the discovery of the Hosgri Fault “established that it is an area of relatively low seismicity.” Id. at 470. Indeed, a major reason the Hosgri was not discovered sooner was “the absence of seismic activity that would indicate a nearby significant fault.” Id.
Petitioners have cited nothing to contradict the Commission’s conclusion that earthquakes of sufficient magnitude to disrupt emergency responses occur very infrequently. On this point, petitioners cite only a portion of Commissioner Asselstine’s dissent, see Pet.Supp.Br. at 18 n. 39, which states:
Publicly available information compiled by the U.S. Geological Survey (USGS) would seem to indicate that earthquakes of sufficient magnitude to cause possible damage, obstruction or disruption to roads, buildings, bridges and communication networks occur throughout many parts of California, including the San Luis Obispo area, with some regulari-ty____ According to this information, four earthquakes have occurred in the immediate San Luis Obispo area since 1830....
CLI-84-12, 20 N.R.C. 249, 263 n. 2 (1984) (dissenting views of Commissioner Assel-stine).
Petitioners’ citation of Commissioner As-selstine’s information does not contradict, but amply supports the Commission’s conclusion. The source on which Commissioner Asselstine was relying indicates that only four earthquakes of any magnitude have occurred at or near San Luis Obispo during the last 200 years. See National Oceanic & Atmospheric Administration, U.S. Dep’t of Commerce, Pub. No. 41-1, Earthquake History of the United States 155-86 (Rev.ed.1973). Moreover, none of these earthquakes is reported to have caused any damage that would interfere with emergency responses. The earthquake of 1830 damaged a church. Id. at 156. The earthquake of December 17,1852 knocked down part of an adobe dwelling and fractured the walls of two others. Id. Although the earthquake of June 11, 1903 was felt at San Luis Obispo, the only damage (fallen chimneys) occurred near San Jose. Id. at 162. Similarly, the earthquake of December 6, 1906 was felt at San Luis Obispo, but the damage was limited to a cracked lighthouse at Piedras Blancas. Id. In short, the information on which petitioners rely in no way undermines the Commission’s observation that “earthquakes of sufficient size to disrupt emergency responses at Diablo Canyon would be so infrequent that their specific consideration is not warranted.” CLI-84-12, 20 N.R.C. at 252.
The probability of any size earthquake occurring in San Luis Obispo in any given year is about one in fifty. If the operating life of the plant is forty years, the probability that any size earthquake and an independent radiologic emergency both will occur at Diablo Canyon during a single year during the life of the plant is one in 125,-000. The probability that the two events will occur contemporaneously in a single week during the life of the plant is approximately one in 6,500,000. Thus, it is no objection that the Commission did not hold hearings to determine the size earthquakes required to interfere with emergency responses since earthquakes of any size are very infrequent events in the San Luis Obispo area.
The probabilities are even smaller when we consider the OBE, a somewhat larger earthquake that might more conceivably interfere with emergency responses. For Diablo Canyon, the OBE was calculated to be an earthquake with maximum vibratory ground acceleration of 0.2g. See ALAB-644, 13 N.R.C. 903 (1981). The Appeal Board observed that for the Diablo Canyon OBE, “the lowest average return period computed by any of the methods used in the analyses is 275 years.” Id. at 992 (emphasis added). Based on its review, the Appeal Board concluded:
The record ... does not bear out the claim that the Diablo Canyon site is one of “high seismicity.” The term refers to the frequency of seismic events. Drs. Anderson and Trifunac plotted for the years 1950 through 1974 the known epicenters in the region, centered around Diablo Canyon, between 33° and 37° north latitude and 119° to 123° west longitude. That plot, and the calculated low recurrence rate of an earthquake of the magnitude assigned the OBE, indicate that the region is at most one of low to moderate seismicity.
ALAB-644, 13 N.R.C. at 993-94 (emphasis added) (footnotes omitted).
Based on these figures, the Commission could properly conclude that the probability of the two events occurring contemporaneously is extraordinarily low. The record establishes that the probability of an OBE at Diablo Canyon in any given year is, at most, one in 275. The probability that an independent radiological emergency will occur in a given year is one in a hundred thousand. Since the operating life of the plant is forty years, this means that the probability that an OBE and a radiological emergency will both occur at Diablo Canyon within the space of a single year during the life of the plant is one in 687,500. The probability that the two events will occur contemporaneously (say, within the space of a single week during the life of the plant) is approximately one in 35,750,-000. The possibility that an earthquake would disrupt a response to a radiological emergency is so extremely low as to be, for any practical purpose, non-existent. If the NRC is required to hold hearings on the emergency plans to deal with contingencies of that level of improbability, we can think of no speculative danger that would not require a hearing. Such a conclusion would serve no purpose other than to enable petitioners to hold up licensing for many more years, and probably for a period long enough to make the construction of nuclear power plants entirely economically unfeasible.
Perhaps petitioners’ real objection is not that the Commission erroneously concluded that the probability is exceedingly low that an earthquake and an independent accident will occur contemporaneously, but that the Commission acted arbitrarily in refusing to consider earthquakes while permitting consideration of other natural phenomena. See Pet.Supp.Br. at 17-18. “Hence,” petitioners argue, “[the Commission’s] exclusion of earthquakes in the context of emergency planning is not only arbitrary, but irrational.” Id. at 18. Despite this assertion, it is clear that the Commission’s differential treatment of these phenomena is entirely rational.
Petitioners assert that “the Commission does consider ... volcano[es], hurricanefs], [and] tornado[es],” and then state: “Notably, with respect to Diablo Canyon, the Commission allowed Petitioners the opportunity to litigate the potential impacts of tornadoes and hurricanes on emergency response, but provided no basis upon which to conclude that their occurrence is any more probable than an earthquake — the potential impacts of which were excluded from the hearing process.” Pet.Supp.Br. at 17-18 (emphasis in original). These are strong statements, and so it is remarkable that petitioners offer no support in their initial brief for either of those assertions. In their reply brief, however, petitioners cite as support, and their only support, Commissioner Asselstine’s Diablo Canyon dissent. The only possibly relevant portion of that dissent states:
The probability that a tornado will travel through a particular 10-mile area and thereby initiate or disrupt response to an emergency at a nuclear plant must be quite low; yet, the Commission requires consideration of that issue for certain plants. Similarly, the probability of a hurricane striking the San Luis Obispo coastal area and initiating or disrupting an emergency response must also be quite low; yet the Commission considered that very issue in the Diablo Canyon case.
Diablo Canyon, CLI-84-12, 20 N.R.C. at 263 (dissenting views of Commissioner As-selstine).
With respect to tornadoes, petitioners simply misstate Commissioner Asselstine’s position. His dissent clearly states that “the Commission requires consideration of [tornadoes] for certain plants. ” Id. (emphasis added). It does not, as petitioners assert, state that the Commission considered tornadoes in the case of Diablo Canyon or that the Commission considers tornadoes for most plants.
Commissioner Asselstine does assert in his dissent that the Commission considered the possibility of a hurricane striking the Diablo Canyon plant. Id. But Commissioner Asselstine, like petitioners, provides no record citation to establish that such consideration in fact took place. Indeed, PG & E asserts that Commissioner Assel-stine was simply mistaken. PG & E Brief at 17. Thus, petitioners’ reliance on Commissioner Asselstine’s dissent does nothing to advance their claim. At oral argument, petitioners repeatedly emphasized Commissioner Asselstine’s dissent. Petitioners were then specifically asked whether they could produce any citations of instances where the Commission had required consideration of infrequent natural phenomena other than earthquakes. Petitioners’ reply was that they could not. As a matter of law, petitioners’ reliance on Commissioner Asselstine’s unsubstantiated assertion is insufficient to establish that the Commission considered any highly infrequent natural phenomena in its review of the Diablo Canyon emergency plans, and therefore acted arbitrarily in excluding consideration of earthquakes.
In its Diablo Canyon decision, the Commission observed that “[w]ith one exception, the focus has always been on frequently occurring natural phenomena.” CLI-84-12, 20 N.R.C. at 252. “The one exception is Trojan, for which consideration has been given to the effects of volcanic eruption due to the expectation that another explosion is imminent at Mt. St. Helens.” Id. at 252 n.4. The Commission’s consideration of a volcanic eruption at Mt. St. Hel-ens on emergency planning at the nearby Trojan plant does not render arbitrary the Commission’s decision not to consider earthquakes at Diablo Canyon. A major eruption occurred at Mt. St. Helens in May, 1980, and there was scientific evidence that there was a probability of further volcanic activity in the near future. This is in significant contrast to the situation at Diablo Canyon. Petitioners have pointed to nothing in the record to suggest that there has been an earthquake near Diablo Canyon in the recent past that would have posed any threat to the plant or to emergency responses. As we have already discussed, the Commission reasonably concluded that the possibility that an earthquake would occur at the plant contemporaneously with an independently caused radiological release is too small to require specific consideration. Under these circumstances, the Commission’s decision to consider volcanic eruptions at Trojan, but exclude consideration of earthquakes at Diablo Canyon, was entirely rational.
Petitioners correctly point out that “on-the-record consideration was given to complications resulting from other natural phenomena, such as fog and heavy rain.” Pet. Supp.Br. at 18-19. In Diablo Canyon, the Commission itself stated that “[i]n prior cases, such frequently occurring natural phenomena as snow, heavy rain, and fog have been considered.” CLI-84-12, 20 N.R.C. at 252. The Commission went on to stress, however, that “the focus has always been on frequently occurring natural phenomena.” Id. (emphasis added). Thus, the Commission may require consideration of snow for a plant in Pennsylvania where snow occurs frequently. This does not mean, however, that the Commission acts arbitrarily if it excludes consideration of snow for plants in southern Florida.
We cannot say that the Commission decision to consider such frequently occurring natural phenomena as rain and fog, but not to consider the infrequent phenomenon of a major earthquake, was arbitrary and capricious. There is record evidence that dense fog (visibility of less than a quarter mile) occurs, on average, approximately eighty-eight times a year, see Evacuation Time Assessment for the Diablo Canyon Nuclear Power Plant at 7, Sept. 1980, Record, vol. 102, applicant’s exh. 78 at 7, at Operating License Hearing, Jan. 19-26, 1982, and that heavy rainfall (greater than .31 inches per day) has occurred up to twenty-five times in a given year. See Diablo Canyon Units 1 and 2 Final Safety Analysis Report at 2.3A-44, 2.3A-45 table 7 (applicant’s exh. 5 at Operating License Hearing, Oct. 18, 1977). This establishes that rain and fog are far more likely to occur at the plant than a major, disrupting earthquake.
It is of no significance that the Commission did not announce a general standard for determining what constitutes frequently occurring and infrequently occurring natural phenomena. We are reviewing the Commission’s action in this case to determine if it is arbitrary and capricious. To conclude that the Commission did not act arbitrarily and capriciously in this case, it is sufficient that the record establishes that fog is 24,200 times more likely to occur, and rain is 6,875 times more likely to occur, at Diablo Canyon than is a major earthquake. (Contrary to the dissent’s charge, this comparison relates the frequencies of rain and fog to that of earthquakes and does not involve multiplying either by the chances of an independent nuclear accident.) Under these circumstances, the Commission certainly drew a rational distinction between rain and fog, on the one hand, and earthquakes, on the other. Given the relative probabilities, this court cannot conclude that the Commission’s decision was arbitrary and capricious.
3. Flexibility of emergency plans. The Commission gave a third reason for excluding consideration of earthquakes:
The Commission’s view that it need not give specific consideration to the complicating effects of earthquakes on emergency planning in this case is bolstered by the following consideration. Specific consideration has been given in this case to the effects of other relatively frequent natural phenomena. The evidence includes the capability of the emergency plan to respond to disruptions in communication networks and evacuation routes as a result of fog, severe storms and heavy rain. In the extreme, these phenomena are capable of resulting in area-wide disruptions similar to some of the disruptions which may result from an earthquake. Testimony in the Diablo Canyon record indicates that adverse weather conditions such as the effect of heavy fog could increase evacuation time to approximately 10 hours. Thus, while no explicit consideration has been given to disruptions caused by earthquakes, the emergency plans do have considerable flexibility to handle the disruptions caused by various natural phenomena which occur with far greater frequency than do damaging earthquakes, and this implicitly includes some flexibility to handle disruptions by earthquakes as well.
Diablo Canyon, CLI-84-12, 20 N.R.C. at 252-53.
Petitioners argue that “the Commission majority provides no support whatsoever for its third assertion” and that therefore the Commission’s conclusion about the flexibility of the emergency plans “is complete speculation and nothing more.” See Pet. Supp.Br. at 18-19. Petitioners’ argument is both irrelevant and wrong. The Commission expressly stated that it was citing the inherent flexibility of the emergency plans only to “bolster” its conclusion that specific consideration of earthquakes is not warranted. See CLI-84-12, 20 N.R.C. at 252. At the outset of its decision, the Commission set forth the positions of the parties, attributing the flexibility argument to PG & E and the argument based on probability to its staff. See id. at 251. The Commission then began its analysis by stating: “The Commission agrees with the NRC staff’s analysis in this case.” Id. Thus, even if petitioners’ attack on the flexibility rationale were successful, that would not damage the Commission’s basic argument, which was that the coincidence of two highly improbable events was so radically improbable as not to require a hearing.
In any case, the Commission’s observations about the inherent flexibility of emergency plans does, in fact, support its decision not to consider earthquakes. Those remarks are also entirely consistent with the emergency planning regulation. Both of these conclusions are easily demonstrated. The regulation sets forth sixteen general standards with which emergency plans must comply. For example, these standards require emergency plans to provide and maintain “[ajdequate emergency facilities and equipment to support the emergency response,” as well as to use “[ajdequate methods, systems, and equipment for assessing and monitoring actual or potential offsite consequences of a radiological condition.” 10 C.F.R. § 50.47(b)(8) & (9). The regulations make no reference to specific conditions or accident sequences. As early as 1981, just one year after the regulations were promulgated, the Commission observed in another context: “The current regulations are designed with the flexibility to accommodate a range of onsite accidents, including accidents that may be caused by severe earthquakes. This does not, however, mean that emergency plans should be tailored to accommodate specific accident sequences____” San Onofre, CLI-81-33, 14 N.R.C. at 1092. Thus, the Commission’s observation in this case that the Diablo Canyon emergency plans contain a measure of inherent flexibility is supported by the fact that the plans were designed and approved in accordance with the standards of flexibility set forth in the emergency planning regulation.
As the NRC points out, the emergency response plan already in place to deal with frequent natural phenomena has the capacity to be of assistance in coping with problems that may be expected to occur as the result of an earthquake. For example, in the event that commercial telephone lines go down, the plans provide for back-up communications, including radio transmission and telephone lines dedicated specifically to all critical facilities and organizations. See LBP-82-70, 16 N.R.C. 756, 775, 817-18 (1982). Similarly, if roads become unusable, the plan specifically contemplates the use of helicopters, overland vehicles, and boats. Id. at 773, 814-16, 834-35. Petitioners’ argument is that the Commission should have held a hearing to determine whether these alternate facilities will be useful in the event of an earthquake. But the Commission is not required to hold a hearing to prove what common sense shows, that such backup communication and transportation plans and facilities are likely to prove helpful in the event of an earthquake as well as in the event of a heavy rain. It was, therefore, entirely rational for the Commission to bolster its conclusion with the observation that the emergency response plan already in place has flexibility that would aid in dealing with disruptions caused by earthquakes.
We conclude that petitioners have failed to establish that the Commission’s refusal to require emergency response plans to consider earthquakes was arbitrary and capricious or irrational.
C.
In short, petitioners have been unable to advance any reason why the deference normally accorded to an agency’s interpretation of its own regulations should not be given to the Commission’s interpretation in this case. The Commission has consistently and repeatedly interpreted its emergency planning regulation not to require consideration of the effects of earthquakes in emergency planning and this interpretation is neither plainly inconsistent with the regulatory language nor arbitrary and capricious. Under these circumstances, the Commission’s interpretation is controlling. See United States v. Larionoff, 431 U.S. at 872-73, 97 S.Ct. at 2155-56.
Because the NRC was not required by its regulations to consider the potential complication effects of earthquakes on emergency planning in its decision to license Diablo Canyon, and in fact affirmatively excluded such consideration, there is no merit to petitioners’ claim that they were denied a hearing on this issue in violation of section 189(a) of the Atomic Energy Act. See Union of Concerned Scientists, 735 F.2d 1437.
III.
Petitioners ask this court to supplement the record to consider transcripts of a closed meeting of the Nuclear Regulatory Commission. Petitioners claim that “[t]he illegitimacy of the Commission majority’s decision to exclude earthquakes from emergency planning at Diablo Canyon is confirmed by an examination of the closed meeting transcripts.” Pet.Supp.Br. at 21.
Judicial examination of these transcripts would represent an extraordinary intrusion into the realm of the agency. These transcripts record the frank deliberations of Commission members engaged in the collective mental processes of the agency. In a case reviewing action by the Secretary of Agriculture, the Supreme Court had this to say about the district court’s authorization of deposition of the Secretary:
[T]he Secretary should never have been subjected to this examination. The proceeding before the Secretary “has a quality resembling that of a judicial proceeding.” Such an examination of a judge would be destructive of judicial responsibility. We have explicitly held in this very litigation that “it was not the function of the court to probe the mental processes of the Secretary.” Just as a Judge cannot be subjected to such scrutiny, so the integrity of the administrative process must be equally respected.
United States v. Morgan, 313 U.S. 409, 422, 61 S.Ct. 999, 1004, 85 L.Ed. 1429 (1941) (citations omitted).
As the Supreme Court has stated, “there must be a strong showing of bad faith or improper behavior before [inquiry into the mental processes of the administrative deci-sionmaker] may be made.” Citizens to Preserve Overton Park v. Volpe, 401 U.S. 402, 420, 91 S.Ct. 814, 825, 28 L.Ed.2d 136 (1971). Petitioners have made no such showing in this case.
Petitioners offer nothing but the transcripts to support their motion to supplement the record. Apparently unable to point to any independent evidence of improper conduct by the Commission, petitioners simply assert that the transcripts alone are sufficient to establish the requisite bad faith and improper conduct on the part of the Commission. We reject this approach. Petitioners must make the requisite showing before we will look at the transcripts. We will not examine the transcripts to determine if we may examine the transcripts.
There may be cases where a court is warranted in examining the deliberative proceedings of the agency. But such cases must be the rare exception if agencies are to engage in uninhibited and frank discussions during their deliberations. Were courts regularly to review the transcripts of agency deliberative proceedings, the discussions would be conducted with judicial scrutiny in mind. Such agency proceedings would then be useless both to the agency and to the courts. We think the analogy to the deliberative processes of a court is an apt one. Without the assurance of secrecy, the court could not fully perform its functions.
We deny petitioners’ request to supplement the record in this case since petitioners have failed to make an independent showing that the Commission acted improperly or in bad faith.
The Commission’s decision is, therefore,
Affirmed.
. Courts show deference to an agency’s interpretation of its governing statute. “When the construction of an administrative regulation rather than a statute is in issue, deference is even more clearly in order.” Udall v. Tallman, 380 U.S. 1, 16, 85 S.Ct. 792, 801, 13 L.Ed.2d 616 (1965).
. Nevertheless, the dissent insists that the Commission’s interpretation of its emergency planning regulation is entitled to no weight because it "is inconsistent with the fundamental purposes of those regulations." Dissent at 5. This requirement is found nowhere in the controlling Supreme Court precedent and we decline to adopt it here. Given the dissent’s view of the regulation’s purpose — to plan for the unexpected — any Commission interpretation that declines to require consideration of any particular occurence, no matter how improbable, would be invalid. The dissent repeatedly stresses that the Diablo Canyon plant is "located only three miles from an active geological fault." Dissent at 32; see id. at 31. But under the dissent’s own approach, a plant’s proximity to a fault is irrelevant. Every plant must plan for the disrupting effects of an earthquake regardless of the likelihood that one will actually occur.
Even the dissent, however, stops short of following its analysis to its logical conclusion. For example, at one point it states:
[T]he Commission can ... exclude from consideration (initiating or complicating events] with such low probabilities that they would not warrant prudent risk reduction meth-ods____ While drawing the line between probabilities will sometimes prove difficult, the Commission clearly does not have to consider an event as unlikely as an earthquake greater than the 7.5 magnitude SSE for Diablo Canyon.
Dissent at 51 n. 7 (citations omitted). In this the dissent is obviously correct. But once the dissent concedes that there exist some contingencies which the Commission is not required to consider, it cannot at the same time maintain that the Commission’s refusal to consider an occurrence because of its improbability conflicts with the purpose of the regulation. It follows that the Commission is left with a comparison of relative probabilities — a matter of line drawing. In this circumstance, we think the only inquiry open to us is whether the Commission’s decision that the regulation does not require consideration of earthquakes was rational. The dissent might wish to draw the line elsewhere, but, as will be shown, petitioners have not met their burden of demonstrating that the Commission’s decision was irrational.
. Our conclusion is buttressed by the fact that the emergency planning regulation cites a preliminary version of NUREG-0654 "for Interim Use and Comment," issued in January 1980 before the regulation itself was adopted in August 1980. See 10 C.F.R. § 50.47(b) n. 1 (1984).
. The Commission noted, however, that it "will consider on a generic basis whether regulations should be changed to address the potential impacts of a severe earthquake on emergency planning." San Onofre, 14 N.R.C. at 1092. On the basis of this consideration, the Commission decided that the regulations should not be amended to require consideration of earthquakes and has proposed instead a rule providing explicitly that earthquakes need not be considered in emergency planning. See 49 Fed. Reg. 49,640 (1984).
. The Commission in addition determined that petitioners made no showing of special circumstances within the meaning of 10 C.F.R. § 2.758 (1984) warranting a waiver of the regulations to permit consideration of the effects of earthquakes on emergency planning at Diablo Canyon. See CLI-84-12, 20 N.R.C. at 253-54.
. We note that even if petitioners were accurate in their assertion that the Commission’s staff "frequently advocated" the view that emergency plans should consider the effects of earthquakes, the Commission would be under no obligation to accept the staffs view and either interpret or amend its regulations to require such consideration.
. The probability that the two events will occur in a particular year is 1 in 5,000,000. The probability that the two events will occur during any year during the life of the plant is obtained by multiplying the above probability times 40 years, the life of the plant.
. This probability is derived by multiplying one over 52 times the probability that the two events will occur in any single year during the life of the plant.
. The probability that the two events will occur in a particular year is one in 27,500,000. The probability that the two events will occur during any year during the life of the plant is obtained by multiplying the above probability times 40 years, the life of the plant.
. The probability is derived by multiplying one over 52 times the probability that the two events will occur in any single year during the life of the plant.
. For an earthquake to complicate an emergency response the two events would probably have to occur closer in time than one week. If a period of 48 hours were chosen, for example, the odds against a simultaneous occurrence would be far higher even than those mentioned in the text.
. The dissent states that we unfairly refuse to look at Commission decisions not cited by petitioners, yet "unquestionably accept[ ]" the Commission’s assertion that it had previously considered only frequently occurring natural phenomena. Dissent at 55-56. "Fairness would seem to dictate that both parties cite cases to support their opposing claims before the court accepts either. Here, unfortunately, neither party did so.” Id. In addition to requiring that the Commission prove a negative, this objection ignores the more basic point that the burden of proof is on the petitioners and that, consequently, they bear the risk of nonpersuasion. Moreover, the cases cited by the dissent (involving, for example, the possibility of severe winter storms), while they may consider occurrences that are to some degree “infrequent,” do not compare in degree of rarity with the event whose occurrence is considered here. These are judgments of degree and wherever the spectrum is cut it will always be possible to point out that events on opposite sides of the line are not vastly different.