Patent Number: 039322173
Section: summary

This invention relates to a method and a device for the passive protection of a nuclear reactor and especially a fast reactor. More precisely, the invention is concerned with a method and a device which serve to produce a negative reactivity in the event of excessive increases in reactivity or temperature within the reactor if the active protection systems provided within this latter failed to come into operation. It is known that, in a nuclear reactor, the absorbing rods which have the property of capturing neutrons can perform both the function of fine control of the reactor power and a safety function which consists in inserting said absorbing rods into the reactor core as rapidly as possible in order to achieve a subcritical condition immediately after indication of a fault which is liable to result in an accident (loss of power supply to control devices, loss of pressure or of means for circulating coolant, excessive increase in reactivity, temperature or pressure and so forth). Safety of the installation is accordingly obtained by introducing a certain degree of redundancy into the control means. The neutron-absorbing rods can be divided into separate groups actuated in a safety function by means of independent circuit systems for measurement, relay operation and control which are physically separate in all their paths. Whatever provision may be made for redundancy of the reactor shutdown means, it is always found preferable to have recourse to a passive action rather than an active action in order to introduce into the reactor core the negative reactivity which is required in order to bring the reactor to a subcritical state. Thus in the event of a failure of the power supply which ensures control of the reactor or a loss of coolant flow, passive action under forces developed by gravity can be produced by means of an electromagnet which is operated from the same power supply system and initiates the free fall or "trip" of neutron-absorbing rods previously suspended above the reactor core. Up to the present time, however, protection against excessive increases in reactivity or temperature has always entailed the use of a control loop formed by a detector which performs the measurement, by a device for processing said measurement and by an electromechanical relay system for transmitting motion to a neutron-absorbing rod. This control loop is therefore an active system with sections located outside the reactor core proper and is consequently more sensitive to hazards such as aircraft crashes, fires, sabotage, earthquakes or floods. The present invention is precisely directed to a method and a device which circumvent the disadvantages mentioned in the foregoing insofar as they are strictly passive and located inside the reactor core. The method of passive protection of a nuclear reactor core essentially consists in placing within said core a predetermined number of vertical fuel elements or so-called "safety" elements each provided with an axial duct of sufficiently large diameter to ensure that the fuel which forms the wall of said duct and which will melt first by reason of its initial temperature is capable of flowing freely under the action of gravity between the mid-height of said fuel element and the base thereof whenever a dangerous excursion of neutron-flux density occurs. In a preferred alternative mode of execution of the method, the total number of normal fuel elements is replaced by safety elements. Economic studies have in fact shown that it is more advantageous to design all the fuel elements in the form of safety elements having internal ducts of large diameter. The safety effect which is claimed to arise from differentiation of fuels remains valid insofar as refuelling does not involve the entire reactor core but only a portion of the core. In this case, the reactor accordingly contains at any given moment fuels of different ages in different states of reactivity and thermal equilibrium. Moreover, even in the case of identical fuels which have been loaded at the same moment, the position within the reactor core and the position with respect to the control rods produce different states of thermal equilibrium under normal operating conditions. As a natural consequence of the foregoing, heterogeneity of the fuel elements is always ensured to a substantial degree and internal melt-down of fuel elements which are all assumed to be of the safety type according to this patent will therefore not be simultaneous. Full advantage is thus taken of the intrinsic safety of the novel type of fuel in which the fuel material does not remain trapped in the central portion of the reactor core which has the highest temperature and is the most subject to can failures, with the result that fuel in the liquid state is much less liable to be ejected towards the coolant under the action of the internal pressure of the fission gases. The device for carrying out the method is characterized in that each safety element is constituted in the same manner as normal fuel by a stack of fuel pellets enclosed in a can but differs in that each pellet is provided with a central opening having a cross-section such that the portion of molten fuel is capable of flowing under gravity to the bottom portion of said fuel element. In accordance with a further characteristic feature of the invention, each safety element is provided in the lower portion thereof with a zone corresponding to the lower blanket assembly of the reactor core, said zone being formed of refractory material and provided over part of its height and at the top portion thereof with a central duct having substantially the same diameter as the duct formed in the fissile portion of the fuel element. In a first alternative embodiment, the lower portion of the zone of refractory material is constituted by a solid pellet which is also formed of refractory material. In a second alternative embodiment, the zone of refractory material comprises a central duct having a smaller diameter and opening into a chamber for the fission gases which is formed within the can of the fuel element beneath said zone, the lower portion of said chamber being such as to terminate in a crucible of refractory material.