Patent Number: 041860500
Section: summary

The present invention relates generally to nuclear reactors and more particularly to a liquid-cooled nuclear reactor which incorporates an improved nuclear fuel. The TRIGA research reactor, which was developed and is being marketed by the assignee of this application, is an inherently safe reactor utilizing a uranium-zirconium hydride fuel that has a large prompt negative temperature coefficient of reactivity, which is primarily characteristic of the fuel itself. The reactor is described in more detail in U.S. Pat. No. 3,127,325, issued Mar. 31, 1964, the disclosure of which is incorporated herein by reference. As a result of the inherent safety of the reactor core, a single or multiple control rod can be instantaneously removed from the core without having the resulting power pulse damage the core. In fact, one of the normal operating modes of the TRIGA reactor is termed the pulse mode, wherein such a fast ejection is performed in order to produce a high energy pulse of radiation for experimental purposes. The temperature coefficient of the reactor is prompt because of the intimate mixture of the nuclear fuel with a large portion of solid moderator in the form of zirconium hydride. Thus, the fuel and solid moderator temperatures rise together instantaneously, with no heat transfer delays before the occurrence of moderator-related temperature coefficient effects. The prompt negative temperature coefficient for TRIGA reactors is considered to be a result of the following three contributing components: (1) thermal neutron spectrum hardening effects, (2) Doppler broadening of resonances and (3) neutron leakage from the reactor core. In the standard TRIGA reactors operating throughout the world today, the thermal spectrum hardening effects contribute the largest share to the total prompt negative temperature coefficient. The extensive thermal spectrum hardening is caused by the unique neutron-moderating characteristics of zirconium hydride. As neutrons gain energy from the spectrum hardening, the probability of their escape from the fuel element before being captured in the fuel is significantly increased. As a result, the ratio of neutron absorptions in the fuel to total absorptions in the core unit cell decreases as the temperature is increased, and this manifestation is termed the "cell effect". The standard TRIGA fuel elements employed in these reactors contain a homogeneous mixture of about 8.5 weight percent low-enriched uranium and about 91.5 weight percent zirconium hydride, wherein the uranium contains about 20 percent U-235 and about 80 percent U-238. In such reactors utilizing these standard TRIGA fuel elements, more than 50 percent of the prompt negative coefficient results from the phenomenon of thermal spectrum hardening whereas the remaining amount is contributed about equally by the other two factors. For applications where a long burn-up lifetime of fuel is deemed economically desirable, a fuel referred to as TRIGA-FLIP (Fuel Lifetime Improvement Program) was developed. This fuel is designed to be usable in the standard TRIGA reactors, as well as in other similar pool-type research reactors, and it uses 70 percent enriched uranium (i.e., 70 percent of the atoms were U-235). The FLIP fuel is a homogeneous mixture of about 8.5 weight percent uranium, about 1.6 weight percent erbium and the remainder zirconium hydride. The erbium is a strong contributor to the prompt negative temperature coefficient as a result of the interaction of its low energy resonances and the spectrum hardening effects of the zirconium hydride. It also serves as a burnable poison to compensate for the excess reactivity provided by the high-enriched uranium and thus maintains the reactivity balance of the fuel relatively flat throughout the lifetime of the overall reactor core loading. The TRIGA-FLIP fuel elements also demonstrate inherent safety characteristics, similar to the standard TRIGA fuel elements. In the FLIP fuel core, an even greater percentage of the prompt negative temperature coefficient is contributed by thermal spectrum hardening, i.e., an amount of more than 85 percent. The contribution to the coefficient by the Doppler effect decreases slightly, and the contribution resulting from the increase in thermal neutron leakage decreases by about 75 percent. Because these decreases are more than offset by the increase resulting from thermal spectrum hardening, the overall prompt negative temperature coefficient of a FLIP fuel core is equal to or slightly greater than that of a reactor operating with the standard TRIGA fuel core. The United States Government is presently pressing forward with non-proliferation policies which place limits upon the amount of enrichment that may be included within nuclear reactor fuel. In accord with these policies, 70 percent enriched fuel may not continue to be supplied. Accordingly, it is an object of the invention to provide an improved long-life reactor core for a pool-type reactor, such as the TRIGA, which utilizes low-enriched uranium but which also exhibits the desired prompt negative temperature coefficient. It is a further objective to provide fuel elements which can be employed in existing TRIGA reactors to provide a long-life fuel core without the incorporation of highly enriched uranium. It has been found that a pool-type reactor, such as a TRIGA reactor, can be provided with a long-life reactor core loading by fabricating nuclear fuel elements from a homogeneous mixture including between about 20 and about 50 weight percent of low-enriched uranium, which for purposes of this application is defined as having not more than about 20 percent enrichment. By homogeneously mixing this uranium with zirconium hydride and with a small amount of erbium, preferably between about 0.5 and about 1.5 weight percent, it has been found that the resulting core unexpectedly has a prompt negative temperature coefficient which compares very favorably with that of a reactor fueled with standard TRIGA fuel. Past experience had indicated that a reactor core of this proportion would have a sizeably reduced prompt negative temperature coefficient compared with a reactor incorporating the standard TRIGA fuel because: (1) the large increase in the amount of uranium-235 would drastically reduce the cell effect, i.e., the thermal neutron leakage from the fuel-moderator material to the surrounding coolant, (2) the larger amount of uranium would considerably reduce the total amount of zirconium hydride in the fuel elements and thus the amount of hydrogen, the moderating effect of which is one of the major factors in the mechanism for creating a large prompt negative temperature coefficient, and (3) the decrease of the amount of hydrogen in the fuel mixture also decreases the overall reactivity of the core assembly and thus would substantially lessen the amount of erbium that could be added to contribute to the prompt negative temperature coefficient, thereby offsetting some of the losses incurred by items (1) and (2), and to offset excess reactivity.