Pressurized water nuclear reactors generally comprise a generally cylindrical vessel arranged with its axis vertical in service and closed at the top by a removable convex closure head. The reactor core, consisting of adjoining fuel assemblies of prismatic shape, is arranged inside the vessel, in which it is immersed in pressurized water for cooling the reactor when the latter is in service.
When the reactor is operating, measurements of neutron flux must be carried out inside the reactor core, in various places distributed along its cross-section and along its height.
The fuel assemblies comprise a skeleton structure including guide tubes arranged in the lengthwise direction of the assembly and placed in the core, in a vertical arrangement. Measurements of neutron flux are performed by means of measurement conduits which are introduced into the guide tubes of certain assemblies. The measurement conduits consist either of thimbles closed at one of their ends, which is introduced into the reactor core and in which thimbles movable probes can be moved, or of measurement rods in which neutron flux detectors are fixed in predetermined positions along the length of the rod.
In all cases, the measurement conduit comprises an end part which may be introduced into the core, and a second, opposed end part which enters a measurement room arranged in the vicinity of the reactor core. The measurement conduits are mounted slideably in the instrumentation tubes and may be withdrawn or installed merely by pulling or pushing on their ends, from within the measurement room.
The measurement conduits must, in fact, be withdrawn from the fuel assemblies of the core, for example when the reactor core is being recharged.
Moreover, the measurement conduits must enter the reactor vessel through leakproof passages through which guide tubes for the measurement conduits, of great length, are connected linking the vessel with the measurement room.
In an embodiment which is known and widely employed in pressurized water nuclear reactors, the passages for the measurement conduits consist of fittings provided on the convex bottom of the vessel. The instrumentation guide tubes connected to these fittings have a path in the shape of an arc of a circle of long radius of curvature, joining the bottom of the vessel to the measurement room.
The provision of fittings in the convex bottom of the vessel complicates the manufacture of this component and leads to difficulties insofar as the observance of safety standards is concerned.
Moreover, the structure of the reactor building must be designed to permit the passage of instrumentation guide tubes of great length along a trajectory which is a circular arc. As a result of this, the design and the construction of the reactor building are made more difficult.
Furthermore, access to the fittings passing through the bottom of the vessel is very difficult, and this complicates the operations of monitoring these fittings in order to ensure that the reactor operates in complete safety.
Lastly, the instrumentation guide tubes connected to the bottom of the vessel are always filled with the core coolant water, with the result that this water can enter the measurement room in the event of a defect in the sealing of the guide tube.
An instrumentation device has also been proposed and used, in which the measurement conduits pass through the vessel closure head in a leakproof manner. This avoids the disadvantages linked with the need to provide fittings in the bottom of the vessel and a complex reactor building structure to permit the passage of the instrumentation guide tubes towards the measurement room. However, in a layout of this kind, a part of the instrumentation device is carried directly by the closure head, and this complicates the operations of dismantling of the closure head, as well as the instrumentation handling and storage operations associated with the closure head during reactor stoppages.
An instrumentation device for the core of a nuclear reactor, in which the measurement conduits pass through the closure head is described in French Patent 2,065,512.
The instrumentation device comprises guide tubes capable of receiving neutron detectors in a fixed position or detection units which can move inside corresponding tubes. The guide tubes pass through the vessel closure head inside columns and are then distributed along the cross-section of the core by beam-shaped support arms arranged under the vessel closure head, which enable each of the detector tubes to be led towards a guide tube of a core fuel assembly intended to receive it.
In order to perform the recharging of the core, after the vessel has been depressurized, the devices permitting the leakproof passage of the instrumentation columns are disassembled, and then the vessel closure head is disassembled to permit access to the support arms and to the detector tubes.
The withdrawal of the support arms, each carrying a plurality of guide tubes independently of one another, is performed with the aid of a special tool, so as to preserve the shape and the distribution of these conduits. This requires numerous handling operations requiring special tools; the times involved are therefore very long and this increases the length of the period of reactor stoppage.
Furthermore, in order to avoid having a large number of support arms of complex shape, the number of assemblies into which a device for measuring neutron flux can be introduced is limited, at the expense of the accuracy of determination of the flux pattern in the core.
Nuclear reactors cooled by pressurized water comprise, arranged above the core, a structure which is called upper internal equipment, which consists chiefly of two horizontal plates connected by vertical braces, one of which, resting on the upper part of the fuel assemblies, forms the upper plate of the core. The other plate, called a support plate, arranged above the upper plate of the core at a certain distance ensured by the braces, is housed inside the vessel, in a recess in which it is fixed and locked in position when the closure head is closed.
The upper internal equipment also supports vertical columns forming guide tubes for the reactor control rods.
During a reactor stoppage for recharging and maintenance, the upper internal equipment of the reactor may be withdrawn and arranged on a storage stand in the reactor pool.
In the case of a core instrumentation device comprising guide tubes or measurement conduits passing through the closure head, the known arrangements of the prior art do not permit simple and rapid handling of the upper internal equipment during a reactor stoppage for recharging.
The advantages obtained in respect of the design and construction of the reactor, when the core instrumentation passes through the vessel closure head, are therefore accompanied by very considerable disadvantages insofar as the use and maintenance of the reactor are concerned.
For this reason, most nuclear reactors now in service incorporate instrumentation tubes which pass through the bottom of the vessel.
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