Induced nuclear reactions: processes – systems – and elements – Handling of fission reactor component structure within... – Fuel component
Patent
1998-03-30
1999-11-02
Wasil, Daniel D.
Induced nuclear reactions: processes, systems, and elements
Handling of fission reactor component structure within...
Fuel component
376419, 264 05, 252638, G21C 362
Patent
active
059784318
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to the production of nuclear fuel pellets. In particular, it relates to the production of mixed oxide (MOX) nuclear fuel pellets.
The use of MOX nuclear fuel pellets in pins or rods in fuel assemblies for light water reactors (LWR), eg pressurised water reactors (PWR) and boiling water reactors (BWR) is known, eg as described in Applicants' EP 627,743A. Incorporation of burnable neutron poisons in MOX fuel is unknown. Such poisons have been included as separate fuel rods or incorporated in the fuel pellets of non MOX fuels.
Such poisons allow thermal hot spots to be controlled, especially in mixed reactor cores containing both uranium oxide and mixed oxide fuels. They also allow higher initial fissile isotope enrichments to be used (providing longer fuel cycles and a higher quantity of electricity from the fuel element before discharge from reactor) due to suppression of the initial reactivity. However, such poisons when added directly to the fuel have a deleterious effect on the size of the grains making up the fuel pellets, adversely affecting fission gas retention in the pellets.
Various methods have been described in the prior art for producing fuel pellets in which steps are provided to counteract the effect on grain size of neutron poisons. However, the fuel pellets produced by these known methods do not show ideal properties. The mechanical and physical properties of such pellets may be inadequate, causing for example the pellets to be damaged easily by chips or cracks. The density of the pellet material may vary across the pellet giving a variation in burn up. Such pellets may contain large inhomogeneously dispersed plutonium agglomerates which may lead to several disadvantages including decreased solubility in nitric acid. Consequently treating spent fuel by conventional reprocessing processes is made more difficult.
Addition of the neutron fuel poison directly to the fuel, as opposed to the incorporation of separate rods within the reactor, mean for example that there would no longer be the need for extra production lines for the production of neutron poison rods, it would increase the even fuel burn up and would not require any special assembly design, thus providing a more economic system.
It is an object of the present invention to provide a method for the production of thermal MOX fuel pellets incorporating burnable poison(s) in which the aforementioned problems are reduced or eliminated.
According to the present invention there is provided a method of producing mixed oxide fuel pellets for use in a nuclear reactor, the mixed oxide comprising oxides of at least two fissile elements, the method including the steps of: (iii) to produce a fuel pellet.
Suitable milling, pressing and sintering steps are known per se and are described for example in Applicants' EP 277,708B.
Preferably, the method includes one or more steps to counteract the grain size limiting effect of the neutron poison. Such steps may be known per se.
The method may include addition of one or more additives, prior to the pressing step or these additives may be added at the milling step. The additives are introduced to reduce grain size limitation. Such additives which are known per se include one or more of Ti, Al, Nb, Cr and Mg, eg in a maximum total concentration of typically in the range 0.01 to 1% by weight. Alternatively, or in addition, the method may include control of heating and/or cooling rate at the beginning or end of sintering or a part of the sintering step, and/or the introduction of a small quantity (eg up to 8 percent by volume) of an oxidising gas (eg moisture or carbon dioxide). This oxidising gas is introduced into the gaseous atmosphere employed during the sintering step (normally a reducing atmosphere), eg at specific selected parts of the sintering step when the sintering temperature has been reduced and additionally, for the adoption of sintering temperatures (eg 2000.degree. C.) which are higher than those conventionally used (eg 1600 to 1700.degree. C.) in at least part of
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British Nuclear Fuels
Wasil Daniel D.
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