Critical masses of bare metal spheres using SCALE/XSDRN
More than 200 actinide isotopes are known; most of them have very short half-lives (only 45 with T{sub 1/2} > 40 days). Only 41 have been predicted capable of a self-sustaining nuclear chain reaction, some of them with fast neutrons and others with either thermal or fast neutrons. Of these 41 there are 13 nuclides for which the average production is >1 g/tonne for pressurized water reactors or boiling water reactors (35 GWd/tonne--power 35 W/g; cooling time, 90 days). Six actinides, {sup 233}U, {sup 235}U, {sup 238}U, {sup 239}Pu, {sup 240}Pu, and {sup 241}Pu, have cross sections that are relatively well known. Cross sections for the other actinides are not as well known. In the United States, criticality safety guidelines for nuclides other than {sup 233}U, {sup 235}U, and {sup 239}Pu is provided by the American National Standard for Nuclear Criticality Control of Special Actinide Elements, ANSI/ANS-8.15. The standard appeared in 1981 and was reaffirmed in 1987 and 1995. The standard provides guidance for 14 nuclei: {sup 237}Np, {sup 238}Pu, {sup 240}Pu, {sup 241}Pu, {sup 242}Pu, {sup 241}Am, {sup 242m}Am, {sup 243}Am, {sup 243}Cm, {sup 244}Cm, {sup 245}Cm, {sup 247}Cm, {sup 249}Cf, and {sup 251}Cf. The ANS-8.15 Standard Work Group is in the process of revising the standard. Five nuclides will be added to the list of nuclides included--{sup 231}Pa, {sup 234}U, {sup 250}Cf, {sup 252}Cf, and {sup 254}Es--resulting in a total of 19 nuclides in the revised standard. Subcritical mass limits in the current standard are based on calculations by Clark and Westfall. The calculations were based on ENDF/B-IV and preliminary ENDF/B-V evaluations. For several of the actinides, new or revised evaluations are available in ENDF/B-VI. All of the 19 nuclides in the revised standard are included in the current paper with the exception of {sup 231}Pa. In a previous paper, minimum critical mass estimates for metal-water mixtures (spherical geometry), fully reflected by water, for 20 fissile nuclides with values of Z between 92 and 99 were given. A simple exponential fit was developed that gives quite accurate values for the minimum critical masses. The minimum critical masses of metal spheres using the SCALE/XSDRN program have been calculated and are given in Sec. II of this paper. Results for reflected spheres are also available. Results for 28 actinides are included in Table 1; only 1 nuclide, {sup 232}Pa (T{sub 1/2} = 1.31 day), has a half-life <40 days.
- Research Organization:
- Oak Ridge National Lab., TN (US)
- OSTI ID:
- 20104480
- Journal Information:
- Transactions of the American Nuclear Society, Vol. 82; Conference: 2000 Annual Meeting - American Nuclear Society, San Diego, CA (US), 06/04/2000--06/08/2000; Other Information: PBD: 2000; ISSN 0003-018X
- Country of Publication:
- United States
- Language:
- English
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