Nuclear criticality

American National Standardfor Nuclear Criticality in Operations with Fissionable Materials Outside Reactors, ANS Standard N 16.1-1975, American Nuclear Society, LaGrange Park, lU., 1975. 

The principal ha2ards of plutonium ate those posed by its radioactivity, nuclear critical potential, and chemical reactivity ia the metallic state. Pu is primarily an a-emitter. Thus, protection of a worker from its radiation is simple and usually no shielding is requited unless very large (kilogram) quantities are handled or unless other isotopes are present. 

Handbook of Nuclear Criticality, Rev. 3, RFP-4657, EG G, Rock Flats, Golden, Colo., 1991. 

Nuclear criticality, preventing, 17 547 Nuclear cycle, water treatment issues in, 23 235-236... 

To avoid nuclear criticality, each batch of plutonium must be homogeneously dissolved before new batches are added. Total plutonium in the system should be kept below 5 kg so that criticality (nuclear chain reactions) cannot occur. The melter geometry may also limit plutonium content in the system. 

Another unique consideration is the prevention of nuclear criticality within the cells. In the dissolver and first cycle, criticality is prevented by the presence of the uranium-238, which absorbs neutrons. Later in the process, the plutonium is separated from the uranium. Criticality is prevented by proper design of the vessels and piping. This includes the cell floor and sumps, where materials would collect in case of leakage from the equipment. To prevent criticality, the vessels are limited in either diameter or thickness. Vessels and piping are placed in arrangements designed to avoid a critical array. 

Crucibles made of materials resistant to liquid fissile metals and designed to avoid nuclear criticality. 

Table 4.11 Nuclear criticality limits for uniform aqueous solution reflected by an effectively infinite thickness of water ...

Data from American National Standard for Nuclear Criticality Safety in Operations with Fissionable Materials Outside Reactors [A2] and J. T. Thomas [Tl]. 

Mixer-settler contactors of much larger scale are used in the solvent extraction operations associated with production of natural uranium (cf. Chap. 5), wherein nuclear criticality is not... 

As in the case of uranium metal, production of pure plutonium metal presents many difficulties. It forms very stable compounds with oxygen and carbon, it oxidizes rapidly in air when in the form of powder, it cannot be deposited electrol3 ically from aqueous solution, and it boils at too high a temperature to be purified by distillation. Additionally, the extreme radiotoxicity of plutonium and neutron production from (a, n) reactions require that production operations be carried out in airtight and shielded enclosures. Nuclear criticality limits the amoimt of plutonium produced in any production operation to batch sizes of no more than a few kilograms. Methods that have been used to produce plutonium metal are... 

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

M. C. Brady and T. L. Sanders, A Validated Methodology for Evaluating Bumup Credit in Spent Fuel Casks, Proceedings of International Conference on Nuclear Criticality Safety, Christ Church, Oxford, United Kingdom, September 9-13, 1991. 

RESEARCH ON NUCLEAR CRITICALITY SAFETY AND ACCIDENT RISK EVALUATION FOR NUCLEAR FUEL CYCLE FACILITIES... 

Nuclear Criticality Safety and Accident Risk Evaluation for Nuclear Fuel Facilities 45... 

Reviews of designs from the point of view of their nuclear criticality safety, reviews of technical documentation of nuclear danger areas, plants, installation, and equipment if necessary, reviews of technical documentation for methods, tools, and the system of control over nuclear safety parameters final reports and conclusions based on the review results ... 

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