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 T1/2 > 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, 233U, 235U, 238U, 239Pu, 240Pu, and 241Pu, 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 233U, 235U, and 239Pu 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: 237Np, 238Pu, 240Pu, 241Pu, 242Pu, 241Am, 242mAm, 243Am, 243Cm, 244Cm, 245Cm, 247Cm, 249Cf, and 251Cf. 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--231Pa, 234U, 250Cf, 252Cf, and 254Es--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 231Pa. 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, 232Pa (T1/2 = 1.31 day), has a half-life <40 days