Patent Number: 048045160
Section: summary

BACKGROUND OF THE INVENTION The invention relates to fuel assemblies for a nuclear reactor in which a coolant is circulated upwardly through the nuclear core including the assemblies and disposed in the reactor vessel. Each assembly is of the type comprising a bundle of fuel elements each formed by a metal tubular sheath containing a stack of nuclear fuel pellets and closed by gas tight caps. The elements are held in a rigid structure including end pieces connected together by tie rods and spacer grids spaced a part along the tie rods. The elements are distributed at the nodal points of a square network by means of the spacer grids arranged at predetermined intervals. The invention is particularly suitable, although not exclusively, for assemblies of pressurized water nuclear reactors, called PWRs, having fuel elements with metal sheath made with a zirconium base alloy. A certain number of separate functions are performed by grids of fuel assemblies of the type which have just been described. On the one hand the grids support laterally and brace the fuel elements containing the fissile material. The grids also ensure structural protection of the assemblies against transverse shocks during handling, during incidents of earthquake origin or operational accidents. On the other hand these grids allow the mixing of the coolant streams circulating upwardly through the core. Such mixing of the coolant is important to allow optimal and continuous regular extraction of the heat released by the fuel elements containing the fissile material, and to avoid local temperature increases within the coolant which could result in a local boiling of the coolant. With an adequate mixing it is thus possible to obtain a general enthalpy increase for the coolant, which is more homogeneously transversely distributed in the core than without mixing, in order to allow a maximum output thermal power dissipation from the core while avoiding local boiling. Various types of assemblies are already known comprising spacer grids spaced at predetermined intervals and providing the above-mentioned functions. Nuclear assemblies in the prior art in PWR reactors typically comprise grids arranged at regular intervals from top to bottom of the assembly. French document No. 84 18 645 describes assemblies of this type comprising a skeleton or rigid structure having two end pieces connected together by elongated elements such as guide tubes and a plurality of grids spaced apart along the guide tubes and forming support cells for the elements of a bundle of fuel elements, said elements being distributed at the nodal points of a regular network. The grids are of different types and consist in median grids, designed so as to resist lateral shocks and provided with fins to create turbulance in the flow of the coolant along and inside the assembly, lower grids and upper grids ensuring the bracing of the elements and creating a lesser pressure drop for the coolant than the median grids. This solution solves numerous problems. However, in the particular case of zirconium-based alloy sheath of the fuel elements, applicants have noticed that corrosion appear to a greater extent at the top part of the elements than at their bottom part. It is known that the local corrosion phenomena depend from the temperature of the coolant and the local thermal power, locally dissipated by each fuel element, and that the temperature of the coolant existing at the top of the core is inevitably greater than at the bottom part of the core. These two factors: the temperature of the coolant and the thermal power locally released by the core, could explain to the technician skilled in the art the differential corrosion between the top and the bottom parts of the fuel elements, but by the same token will convince him that this phenomenon is unavoidable. SUMMARY OF THE INVENTION It is an object of the invention to provide an improved fuel assembly for a nuclear reactor core. It is a more particular object to provide an assembly which reduces corrosion at the top part of the elements. For that purpose, it has firstly been necessary to provide a new analysis of the phenomena while noticing that it should exist other causes of acceleration of corrosion at the top part of the sheath than the causes known by the prior art. This analysis has led to the hypothesis that the oxide layer, formed on the sheath or cladding of the elements and due to corrosion, renders worth the heat exchange between the fuel element and the pressurized water surrounding the elements and so aggravated the corrosion. And, due to the fact that the thickness increase of the oxide layer is more rapid at the top part of the fuel elements where the temperature is higher than at the bottom part, the phenomenon of corrosion is further greatly amplified. This analysis has been verified by measuring the oxide layer thickness which is formed at the surface of the sheath of an irradiated fuel element at a plurality of points along the sheath. The measurements were performed by means of an eddy current probe. They have shown that the oxide thickness increases considerably from the bottom to the top. For example, this thickness, for one of the fuel sheaths analyzed and which was irradiated at more than 40,000 MWj/tU, was found to be around 18 .mu.m at the bottom end portion of the sheath and reaches 50 .mu.m at the top end portion of the sheath. Applicants have also noticed that the oxide thickness is increasing as the increase of the interface temperature between the metal and the oxide which occurs while the fuel element remains in the reactor. The increased thickness of oxide at the top end portion therefore induces an increased temperature at the interface between the metal and the oxide and hence accelerates the formation of oxide. In order to reduce this phenomenon of increased corrosion at the top part of the sheath, Applicants have sought to increase the heat exchanges which occur between each fuel element and the coolant, i.e. the pressurized water in a PWR at the top part of the element. For this purpose it is an object of the invention to provide particularly a fuel assembly of the above-defined type, wherein the upper grids situated in the upper part of the assembly are disposed at smaller intervals than the lower grids situated at the lower part of the assembly, said upper grids providing an improved flow mixing of the coolant streams (pressurized water) circulating through said upper grids with a more intense mixing of said coolant than the mixing that provide the upper grids of the prior art. It is thus possible to obtain: a delay in the appearance of corrosion on the surface of the sheath of the fuel elements at the top part of the assemblies, this being due to: better mixing of the coolant at the top part of the assembly, while preserving an acceptable pressure drop or head-loss, compatible with an operation of the reactor in good conditions. a delay in the formation of oxide at the top part of the sheath, without however penalizing the bottom part of the assembly. The invention also provides an assembly having upper grids provided with fins and lower grids devoid of them. Preferably, the last grid located close to the upper end of the assembly is also devoid of fins. It is another object of the invention to provide upper grids having a rather light constitution, such as, for example, grids of the type described in French document FR No. 86 02 501. Such grids include a peripheral belt and at least two series of parallel plates, the plates of one series being disposed at an angle to the plates of the other series so that the two series define cells for the passage of the fuel elements; the parallel plates are distributed in at least two beds spaced along the longitudinal direction of the assembly, said plates being provided with half-fins or with fins having respective different orientations respectively in each of the two beds. The invention will be better understood from the description which follows of a particular embodiment, given by way of non-limiting example.