Patent Number: 044938098
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS A fuel for a nuclear reactor includes a fissile material dispersed through a thorium hydride (ThH.sub.X) matrix. The atomic ratio of hydrogen to thorium may be between about 1:1 and 3:1. Thorium dihydride, ThH.sub.2, is preferred due to its relative stability at elevated temperatures. In certain lower temperature applications, ThH.sub.3 or intermediate forms may be selected to provide additional hydrogen. The fertile material may be fissile uranium, preferably .sup.235 U, included in particles primarily composed of .sup.238 U. The fuel may be formed by reacting a thorium-uranium alloy with hydrogen at elevated temperatures. The uranium content of the proposed fuel may be between about 5% and 10% and preferably about 6% so that the hydride formed by reaction of the alloy with hydrogen is primarily thorium hydride, with uranium present as a uniform dispersion of fine particles. An important advantage of ThH.sub.2 and other thorium hydrides is that the .sup.232 Th atom can absorb a neutron to form fissile .sup.233 U. The thorium hydride thus serves as fertile material for breeding additional fuel in addition to serving the moderating and safety functions of other metal hydride fuels. As is well known, .sup.232 Th is far more plentiful than .sup.235 U, so the provision for breeding allows for more economical reactor operation and extends the practical lifetime time of natural uranium reserves. Some of the bred .sup.233 U fissions so as to contribute directly to the output of the incorporating reactor. As breeding progresses, the neutronic and power contribution of the bred .sup.233 U may equal that of the .sup.235 U. The use of ThH.sub.2 permits a vastly improved power per unit volume ratio. Unburned .sup.233 U may be recovered for subsequent use by reprocessing. Another advantage of thorium is that it is capable of bonding more hydrogen than zirconium is. Thorium forms hydrides with up to a 3:1 atomic ratio of hydrogen to thorium, whereas zirconium hydride is limited to a 2:1 ratio. Thus, the use of thorium hydride permits a greater density of hydrogen atoms. The hydrogen serves as the primary moderator in a metal hydride fuel, so the greater hydrogen density provides more effective moderation of the fission neutrons, which in turn increases the efficiency of the reactor. The additional hydrogen also may enhance the inherent safety of an incorporating reactor by enlarging the prompt negative temperature coefficient of reactivity. Furthermore, the preferred ThH.sub.2 is more stable than ZrH.sub.1.7 at elevated temperatures. For example, tests conducted at General Atomic Company determined that the equilibrium hydrogen pressure of the ZrH.sub.1.7 is 1 atm. at 760.degree. C. ThH.sub.2 achieves a 1 atm. equilibrium hydrogen pressure at 883.degree. C. Due to the facts that the initial concentration of hydrogen is higher and that hydrogen is better retained, ThH.sub.2 has better moderating characteristics and a larger prompt negative temperature coefficient of reactivity than ZrH.sub.1.7. ThH.sub.2 does not react strongly with the materials and chemicals normally employed as structural members of fuel elements or as coolants, e.g., stainless steel, zirconium, CO.sub.2, Na, water, or organic coolants. Consequently, even upon the breach of the cladding of a fuel element, adverse chemical interactions are avoided. This chemical inertness contributes to the safety of an incorporating reactor and permits flexibility in reactor design. Also, ThH.sub.2 should exhibit excellent irradiation stability and fission product retention. ThH.sub.2 has appropriate physical properties, such as high thermal conductivity and good heat capacity so as to facilitate heat transfer through and from the core. The stability of the preferred ThH.sub.2 may be enhanced by adding hydrogen to the coolant. The hydrogen can diffuse through fuel element cladding materials, such as stainless steel. The resulting partial pressure of hydrogen within the cladding increases the equilibrium level of hydrogen within the fuel matrix. A net loss of hydrogen from the matrix may be effectively eliminated by externally providing a partial hydrogen pressure equal to that of the fuel. Conveniently, the rate of hydrogen diffusion through the cladding increases with temperature for most cladding materials so that the hydrogen pressure correlates with the hydrogen pressure of the fuel. During reactor operation, irradiation, fission transmutations and the build up of gaseous fission by-products can result in distortion of the fuel matrix and damage and/or breach of the fuel element cladding. The resistance of ThH.sub.2 to the change due to fission burnup has not been determined precisely, but is expected to be quite good. UThH.sub.2 fuel may be favorably compared with UZrH.sub.x fuels. The improved moderating characteristics and neutronics performance of the UThH.sub.2 fuel provide greater power per unit volume and per unit mass, given equivalent quantities of fissile uranium upon insertion of the fuel into a reactor. In other words, for a given power output specification, the UThH.sub.2 permits the design of a more compact and lightweight fuel element. The reduction in fuel element size further permits reduction in the size of a reactor core, and, in turn, the size of an entire reactor. The reduction in reactor size corresponds to a reduction in reactor mass. The inclusion of breeding material in the fuel, with little, if any, sacrifice of fuel compactness or performance provides for an extended fuel element recycling time and for more efficient utilization of natural uranium reserves. The improved compactness and fuel recycling times made possible by the fuel of the present invention allow the design of reactors especially well suited for remote and mobile applications. Many variations upon the preferred embodiments are possible. The inventive fuel may be used alone or in combination with other fuels, including other metal hydride fuels. The ratio of hydrogen to thorium can be varied, as can the ratio of uranium to thorium hydride. These and other embodiments are within the spirit and scope of the present invention.