It is possible for a fuel assembly to be critical at near zero power. These examples only outline the simplest ideal cases: ![]() These attributes have complex interactions and interdependencies. The mass where criticality occurs may be changed by modifying certain attributes such as fuel, shape, temperature, density and the installation of a neutron-reflective substance. Fission can also be initiated by neutrons produced by cosmic rays.Ĭhanging the point of criticality If there is uranium-238 ( 238U) present, the rate of spontaneous fission will be much higher. The probability that one such event will cause a chain reaction depends on how much the mass exceeds the critical mass. For example, a spherical critical mass of pure uranium-235 ( 235U) with a mass of about 52 kilograms (115 lb) would experience around 15 spontaneous fission events per second. In this case, k 1.ĭue to spontaneous fission a supercritical mass will undergo a chain reaction. ![]() A population of neutrons introduced to a subcritical assembly will exponentially decrease. When k = 1, the mass is critical, and the chain reaction is self-sustaining.Ī subcritical mass is a mass of fissile material that does not have the ability to sustain a fission chain reaction. The objective of this report is to summarize the available nitride irradiation performance data base and to recommend optimum nitride characteristics for use in advanced liquid metal reactors.Īuthors: Matthews, R B Publication Date: Fri Jan 01 00:00: Research Org.: Los Alamos National Lab., NM (United States) Sponsoring Org.When a nuclear chain reaction in a mass of fissile material is self-sustaining, the mass is said to be in a critical state in which there is no increase or decrease in power, temperature, or neutron population.Ī numerical measure of a critical mass is dependent on the effective neutron multiplication factor k, the average number of neutrons released per fission event that go on to cause another fission event rather than being absorbed or leaving the material. Irradiation performance data suggest that nitrides have low fission gas release and swelling rates thereby permitting favorable pin designs and long lifetime. Uranium nitride fuel pellet fabrication processes were demonstrated during the SP-100 program, and irradiated nitride fuels can be reprocessed by the PUREX process. Recent analyses done under similar operating conditions show that - compared to metal - fuels mixed nitrides operate at lower temperatures, produce less cladding strain, have greater margins to failure, result in lower transient temperatures, and have lower sodium void reactivity. Nitrides are compatible with cladding alloys and liquid metal coolants, thereby reducing fuel/cladding chemical interactions and permitting the use of sodium-bonded pins and the operation of breached pins. Basically the high thermal conductivity and uranium density of nitride fuels permit high power density, good breeding ratios, low reactivity swings, and large diameter pins compared to oxides. ![]() ![]() The properties and advantages of nitride fuels are well documented in the literature.
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