Patent Number: 039322173
Section: description

There will now be described the execution of the method as applied to the case of fast reactors in which the fuel elements are placed vertically and formed of pellets of sintered uranium oxide or any other solid chemical compound containing uranium and plutonium encased in vertical cylindrical cans which extend in a single piece to the full height of the reactor core. Said sintered pellets are either solid or pierced vertically by a small central capillary orifice and are superposed so as to form a vertical column within the can. A few hundred of these vertical fuel elements are grouped together in the form of "fuel assemblies" and a few hundred assemblies in juxtaposed relation form the reactor core. The core which is thus formed imposes on each constituent fuel element a neutron flux density which is always higher at the center than at the periphery by reason of neutron leakage towards the exterior. Along a vertical axis, the evolution of heat per unit of volume of fuel (which is assumed to be homogeneous in its enrichment in fissile isotopes within one and the same fuel element) is distinctly greater (at least twice as much) at the mid-height of the reactor core than at the bottom and top limits of this latter. In normal operation, the hottest point (approximately 2300.degree.C) is therefore the center of the pellet which is located at the mid-height of the reactor core. In the event of an excessive temperature build-up resulting from an insufficient coolant flow rate or from an excessive increase in neutron flux density, the hottest point aforesaid will begin to melt at about 2800.degree.C whereas along the can, the fuel which is cooler by 1500.degree. to 2000.degree.C will remain in the solid state and protect said can, the integrity of which is an essential and characteristic element of our method. There is shown in FIG. 1 a fuel element 2 constituted by a stack of pellets 4 of uranium oxide or mixed oxide of uranium and plutonium surrounded by a metallic can 6 (for example of austenitic steel). There has been formed in the stack of oxide fuel pellets 4 an axial capillary duct 8. It is apparent that said axial duct has a small diameter in order to ensure that the volumetric void fraction within the reactor core remains of very small value: for example, if the diameter of the fuel element is 7 mm, the diameter of the duct has a value of 2 mm. Under the action of an excessive value of neutron-flux density and temperature prevailing within the reactor, it is observed that fusion of the oxide fuel takes place in the zones 10 over the entire periphery of the axial duct 8 within the central zone of the fuel element 2 as designated in the figure by the reference P. The molten fuel 12 collects in a drop which forms a plug on account of the small diameter of the duct 8. It can therefore be stated that the entire quantity of molten fuel thus remains within the zone P. There is shown in FIG. 2 a fuel element 2' or so-called safety element in accordance with the present invention as constituted by oxide fuel 4' and its protective can 6' with an axial duct 8' of large diameter. For example, this diameter has a value of 10 mm in respect of a diameter of the fuel element of 15 mm, which gives the same thickness of fuel in both cases. It is found that, in the zone P corresponding to the mid-height of the fuel element, fusion of the oxide takes place in the event of excessive elevation of the neutron flux density within the reactor. The fuel melts in the annular zone 10'. Since the diameter is of substantial value, the molten fuel 12' does not form a drop but moves down in the form of a ring along the internal walls of the duct 8' until it reaches the lower end of the fuel element. As long as the molten fuel 12' remains subjected to the neutron flux of the reactor core, the fuel is superheated in a liquid form and continues to flow downwards not only without solidifying against the wall but while even having a tendency to carry down part of this latter in liquid form by reason of the ovethickness of fuel which exists at the time of downward motion of said liquid ring 12' . It is therefore apparent that, by virtue of this method, part of the fissile material is automatically removed in the zone of maximum neutron flux and transferred into a zone of lower flux, thereby reducing the core reactivity and consequently even stopping the reactivity excursion. It should be pointed out that the device comes into operation only after and then in parallel with the negative temperature coefficients in order to control reactivity and temperature excursions. In consequence, the function of the device is solely to enhance a counter-reaction when this latter has not been of sufficient value to control the excursion before core meltout takes place. The time constant which is inherent to this safety device and is related to the time constant of an absorber rod which falls under the action of gravity is of undoubted significance when it is necessary to stop a prompt reactivity excursion at a low level but, when utilized in combination with the very short time constant of the Doppler effect, it can result in a general counter-reaction which is both effective and reliable. As will be readily understood, the diameter to be given to the axial duct 8 or 8' is clearly dependent on the nature of the fissile fuel and more precisely on the viscosity of the liquid formed. If it should prove as a result of initial experimentation with an internal bore having vertical walls that sliding of the liquid ring 12' is neither sufficiently rapid nor of sufficient duration, the internal walls of the axial duct 8' can be given any shape whose intended function is to promote the formation of drops which move down at least partially in free fall and break-up by successive impacts. By way of example and as shown in FIG. 3, the duct can be given a sawtooth profile 14 which can readily be obtained by stacking of sintered pellets having a frusto-conical internal hole. There is shown in FIG. 4 an alternative form of construction of the safety element in which an assembly for receiving the molten fuel is added to the lower portion of the fuel element (beneath the fissile zone). The safety element 2' (only the lower portion of which is illustrated) is provided in the central zone corresponding to the reactor core with a stack 4' of fissile pellets which may be formed of UO.sub.2 or of UO.sub.2 and PUO.sub.2, for example. Said stack is placed within the can 6' of the fuel element and pierced by a central duct 8' having a sufficient diameter to permit the flow of part of the material which is liable to melt. In the zone corresponding to the lower blanket assembly of the reactor core and designated by the letter C, provision is made for a member 14 of fertile material which can be either natural uranium oxide, depleted uranium oxide (containing a very low percentage of U.sub.235) or thorium oxide. The member 14 is provided at the upper end (namely the end nearest the reactor core) with a central duct 16 having a diameter which is equal to or slightly larger than the diameter of the duct 8', the two ducts being intended to communicate with each other. At the lower end thereof, the member 14 is pierced by a central duct 18 which opens into the duct 16 and has a diameter which is distinctly smaller than this latter. By way of example, provision is made for a diaphragm 20 which is rigidly fixed to the can 6' and serves to support said member 14. Beneath the zone C which corresponds to the lower blanket assembly, there is formed within the can a chamber 22 for the fission gases and said chamber communicates through the narrow duct 18 with the ducts 16 and 8'. The base 24 of the can is lined with a crucible 26 of refractory material. In addition to the substances mentioned earlier for the fabrication of the member 14, boron carbide can be employed for the fabrication of the crucible 26. The operation of the lower portion of the safety element is as follows: The molten fissile material moves as far downwards as possible under its own impetus before finally adhering to the refractory wall of the duct 16 of the member 14. This portion of the duct 16 constitutes a first crucible for the molten fissile material. Each safety element thus has its own internal catchpot. If all the fuel elements are safety elements, there is thus provided a molten-core catchpot which is integrated withh the interchangeable fuel assemblies. This arrangement has accordingly solved the problem of fitting a core catcher which is effective and inexpensive and comes into operation before irreparable damage is caused to the reactor block. The presence of the chamber 22 makes it possible to increase the rate of downward motion of the molten fuel within the central duct (8' and 16) and thus to counteract a power excursion more rapidly on condition that a full top blanket is adopted. In fact, at the moment of a power excursion, the fission gas is in pressure equilibrium and its temperature rises sharply within the duct 8' formed in the fissile portion. If the only escape route provided for the gases is the bottom chamber 22 which remains at the same temperature as the inlet sodium, the gas under pressure contained in the central duct 8' is partly expelled towards the bottom and can thus play in accelerating the downward motion of the molten fuel ring and even in entraining finely dispersed droplets at an even higher speed. This phenomenon is accentuated if part of the molten fuel begins to vaporize. The fissile material will then leave the maximum flux zone both in the liquid phase and in a gas phase which accelerates the liquid. The duct 18 permits this gas-phase flow towards the fission chamber 22. Although the greater part of the molten fissile material is intended to undergo resolidification within the first crucible formed by the member 14 and to become attached to this latter, provision is made for a second crucible 26 which serves as an additional safety feature. The refractory crucible 26 forms both a core catcher which is placed as a second line of defense and a second lower blanket which is capable of producing a further attenuation of the neutron flux. Said crucible 26 is placed after a zone in which the thickness of coolant (liquid sodium) will have softened the neutron spectrum and has higher capture cross-sections. In a simplified form of construction, the duct 18 is no longer provided and the crucible 26 is similarly dispensed with. A solid pellet of refractory material is therefore placed at the lower end of the member 14. Should it be found preferable not to replace all the fuel elements by safety elements, the number of fuel elements of the safety type is determined by calculating the mass of fissile product which is intended to be displaced for example from the mid-height of the reactor core to the lower portion of this latter in which the flux is reduced by one-half and the square of the flux is reduced to one-quarter of its initial value. This accordingly determines the required number of safety fuel elements having axial ducts of large diameter for ensuring that the desired negative reactivity is obtained. It is possible either to place all the safety fuel elements in a certain number of fuel assemblies which consist only of elements of this type or to distribute the safety fuel elements within all the fuel assemblies. It can be noted in the first place that it is advantageous in a fast reactor to place all the safety elements at the center of the core as seen in plan since it is at this point that the neutron flux density has the highest value. It will finally be noted that the safety elements in accordance with the invention have the design function both of fuel elements and of neutron-absorbing rods as well as a safety function, with the result that the use of such elements in a reactor core would lead to an advantage in the event that the number of neutron-absorbing rods could be reduced. In some cases this advantage would compensate for the penalty attached to the introduction of a not-negligible void fraction at the center of the reactor core. As stated earlier, the system is intended to reduce to a very considerable extent the consequences of highly improbable accidents of the type described by Bethe and Tait which begins with complete meltout of the central third of the reactor core (including cans) which moves into the coolant location within the lower third. The first stage of the Bethe and Tait accident is reproduced by the safety device but limited in extent and guided within the fuel itself in order to leave the can of the central portion intact. There will thus be no possibility either of subsequent compaction or of reaction with the coolant. Since this safety device is intended to come into operation only in very infrequent instances, it is important to note that its main properties are reliability and absence of spurious reactor trips.