Patent Number: 040595390
Section: summary

The present invention relates to an improved form of UN. More particularly, it is concerned with a single-phase uranium-zirconium mononitride composition which is characterized by an enhanced thermal stability at temperatures of 1600.degree. C. and above in comparison to uranium mononitride. Uranium mononitride (UN) is an attractive nuclear fuel because of its high uranium density (14.32), high melting point (2850.degree. C. at 2.5 atmospheres), and usefully high thermal conductivity (0.06 at 1000.degree. C.). Because of its high uranium content, UN can, for example, be substituted for UO.sub.2 and occupy approximately 30 percent less volume at an equivalent uranium content. This combination of properties makes UN an excellent candidate fuel for use in high-temperature (fuel temperatures greater than 1500.degree. up to 1700.degree. C.) fast reactors, especially those designed to operate in outer space. A limiting factor which mitigates against its use in such and similar context is its dissociation into liquid uranium and nitrogen gas under reduced pressures at these higher temperatures. It is accordingly a principal object of this invention to provide a modified uranium mononitride composition which does not dissociate at a temperature in the range 1500.degree.-1700.degree. C. In order to derive maximum benefit from UN it is generally held desirable to make it available for use as a homogeneous single-phase structure as opposed to a multiphase structure in which disruptive phase changes may occur during use or where homogeneity is difficult to obtain. It is therefore an additional object of this invention to provide a homogeneous single-phase uranium mononitride composition having improved thermal stability. A further object is to provide a modified sintered UN compact having a lowered dissociation constant or rate of decomposition at 1600.degree. C. to 1700.degree. C. in comparison to pure UN.