Fissile materials criticality

Field desorption mass spectrometry radiopharmacological agents, 6,976 Filter dyes photography, 6, 104 Fissile material criticality... 

H. C. Paxton, Criticality Control in Operations with Fissile Material, LA-3366 (rev.), Los Alamos Scientific Laboratory, Los Alamos, N.M., 1972. 

Nuclear energy can be extracted by arranging for a nuclear chain reaction to take place in a critical mass of fissionable material. with neutrons as the chain carriers. A moderator is used to reduce the speeds of the neutrons in a reactor that uses fissile material. 

The dissolver solution is treated with chemicals to adjust the acidity, valence, and concentrations of the species involved. The HNO3 concentrations are 2-3 M, the U02(N03)2 concentrations are 1 -2 M, and the Pu is stabilized as Pu(IV) using N2O4 or hydroxylamine. In these and subsequent manipulations of these solutions, attention must be given to criticality control. This is done by regulating the solution geometry, the concentrations of fissile materials, and by the addition of neutron absorbers such as Gd. 

Most radioactive particles and vapours, once deposited, are held rather firmly on surfaces, but resuspension does occur. A radioactive particle may be blown off the surface, or, more probably, the fragment of soil or vegetation to which it is attached may become airborne. This occurs most readily where soils and vegetation are dry and friable. Most nuclear bomb tests and experimental dispersions of fissile material have taken place in arid regions, but there is also the possibility of resuspension from agricultural and urban land, as an aftermath of accidental dispersion. This is particularly relevant to plutonium and other actinide elements, which are very toxic, and are absorbed slowly from the lung, but are poorly absorbed from the digestive tract. Inhalation of resuspended activity may be the most important route of human uptake for actinide elements, whereas entry into food chains is critical for fission products such as strontium and caesium. 

To achieve a critical mass (more than 1 thermalized neutron per atom of fissile material) you need a spherical mass of 900 g of 92U235 / or 283 g of 94P u239. 

Handling of fissile material (plutonium and enriched uranium) requires strict observance of criticality conditions. As elucidated in section 11.1, criticality depends on the properties and the mass of fissile material in the system, its concentration (including local concentrations) and the presence of a moderator such as water. In all... 

The shorter half life of compared with that of meant that at this time, ca. 1.8 x 10 y ago, natural uranium contained some 3% rather than its current level of 0.7%. Thus, in regions of the ore body where uranium concentrations greater than 10% were present in seams over 0.5 m thick, there was sufficient fissile material for the moderating effect of percolating groundwater to lead to a fission chain reaction. When the heat released was sufficient to boil the water and expel it, the moderating effect would be lost, so that the reactor zone would cycle between critical and subcritical conditions depending upon its water content. It has been estimated that the principal reactor zone at Oklo must have operated in this way for at least 1.5 x lO years at a power level between 10 and 100 kW. 

The fission of one nucleus of produce about 200 MeV of usable energy, to be compared with 4 eV produced by the oxidation of one C atom. During the overall process, a huge amount of heat is produced through a controlled nuclear chain reaction in a critical mass of fissile material. Potential future developments (fission in fast neutron reactors (breeders), also known as fourth generation nuclear, which... 

The critical mass of fissile material required to maintain the fission process is roughly inversely proportional to the neutron-absorption cross section. Thus the critical mass is lowest for plutonium in thermal reactors, larger for the uranium isotopes in thermal reactors, and much greater in fast reactors. For this reason, as well as others, thermal reactors are the preferred type except when breeding with plutonium is an objective then a fast reactor must be used. 

For given fuel composition (factors 1 and 2 speciHed), the simplest but most restrictive condition to ensure subcriticality is one of items 3, 4, S, or 6 (limitation of mass, dimensions, volume, or concentration of fissile material). These soolled single-parameter limits for fissile nuclides are spelled out in American National Standard ANSI N16.1-1975 [A4], They were abstracted in Table 4.11 of Chap. 4 and are amplified somewhat in Sec. 8.2, following. These single-parameter limits give the largest mass, size, volume, or concentration that will be safely subcritical no matter what other criticality-limiting conditions may be present. 

American Nuclear Society American National Standard, Nuclear Criticality Safety in the Storage of Fissile Materials, Report ANSI N16.5-1975, La Grange Park, 111. 

---