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tokens: a radionuclide abundance in the comet based on ordinary chondritic values, for all our calculations. The possibility of inhomogeneities of short-lived radionuclides produced in the protosolar disk has been raised (Lee et al. 1998 ), which could lead to a comet enriched or impoverished in radionuclides. However, the alternate scenario of supernova injection, an efficient mechanism to enrich the PSN in radionuclides, involves extensive mixing of radionuclides into the disk (Ouellette et al. 2009 ) and would not lead to significant inhomogeneities. The radiolytic production per unit of volume can be influenced by the size of the body in which it occurs, only if its dimensions are shorter than the typical radiation range. This could have been the case for the building blocks of the comet shortly after CAI, when short-lived radionuclides were most active. Most of their radiation energy would become available for radiolytic production once the boulders become meter-sized. The accretion scenario of Davidsson et al. ( 2016 ), even though its timescale is considerably longer compared to the original scenario of Weidenschilling ( 1997 ), yields meter-sized boulders within 0.56 Myr after CAI, at most. This delay is insufficient to fully negate the production by short-lived radionuclides (see Figure 4 ). Overall, endogenic radiolysis is unlikely to have produced more than a fraction of the O 2 observed in 67P/C-G. Better experimental constraints on the radiolytic production of O 2 , as well as its subsequent emission, are required for a more definitive assessment of this fraction. Additionally, we have investigated the potential production of H 2 O 2 , where our model yields a much higher final abundance (about 0.1%) than that observed by Rosetta (upper limit of 0.0023%). A better understanding of H 2 O 2 production yields and of activation barriers in its dismutation is required to understand how the thermal and/or chemical history parameters led to the comet’s present-day composition. Appendix Appendix Information A.1. Attenuation of <inline-formula> <tex-math> </tex-math> <inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjaad22fieqn32.gif"></inline-graphic> </inline-formula> Particles Exiting a Grain A radionuclide type s inside a dust grain emits, in the form of emission j , a power dP s,j / dV per unit of volume. “Emission j ” designates a given emission in the decay chain (e.g., the 5.013 MeV α particle emission from the 231 Pa to 227 Ac transition in the 235 U decay chain). dP s,j / dV is given by:where n s is the density of radionuclides in the grain (atoms per unit of volume, deduced from the mass fractions in chondrites shown in Table 1 , the grain density and the molar masses of the radionuclides), τ s is the average decay time of radionuclide s (deduced from the half-life values in Table 1 ), E 0, j is the energy of emission j , and I j is the absolute intensity of emission j (i.e., the probability of emission j