Patent Number: 041860500
Section: claims

1. A nuclear reactor having a large prompt negative temperature coefficient of reactivity, which reactor comprises a reactor tank,  a quantity of water in said tank which serves both as coolant and as moderator,  a reactor core assembly within the water in said tank which includes a plurality of fluid-tight fuel elements which are spaced within said tank to provide liquid-flow passageways surrounding each of said elements,  means for permitting water to flow through said passageways in heat-exchange relationship with the surfaces of said elements,  each of said elements containing a solid homogeneous mixture of zirconium hydride, uranium and erbium,  wherein the improvement comprises,  said uranium constituting between about 20 and about 50 weight percent of said mixture, said zirconium hydride constituting between about 79 and about 50 weight percent of said mixture and said erbium constituting between about 0.5 and about 1.5 weight percent of said mixture,  said uranium in said mixture being not greater than about 20 percent U-235 with the remainder substantially U-238, the ratio of hydrogen atoms to zirconium atoms being between about 1.5:1 and about 1.7:1, and said erbium being distributed uniformly throughout said uranium-zirconium hydride mixture. 2. A nuclear reactor in accordance with claim 1 wherein said reactor includes a vertically extending tank and a reflector surrounding said core assembly, and wherein the region of said tank interior of said reflector which is occupied by said core assembly is not greater than about two cubic feet for each one thousand KW of rated power. 3. A nuclear reactor in accordance with claim 1 wherein the prompt negative temperature coefficient of reactivity is at least about 5.times.10.sup.-5 /.degree.C. averaged over the operating temperature range. 4. A nuclear reactor in accordance with claim 1 wherein the core has a burn-up life of at least about 1200 NW days.