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Critical mass
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Smallest fissile material mass which under fixed conditions (type of fissile material, geometry, moderated/immoderate system, etc.) initiates a self-perpetuating chain reaction. The table contains the minimum critical mass for some nuclides under certain conditions.
| Isotope |
Smallest critical mass in spherical
shape for aqueous solution at optimum moderation |
Smallest critical mass in spherical shape for metal (fast systems) |
||
| Unreflected |
Water-reflected |
Unreflected |
Steel- |
|
| U-233 |
1.080 |
0.568 |
15.8 |
6.1 |
| U-235 |
1.420 |
0.784 |
46.7 |
16.8 |
| Np-237 |
- |
- |
63.6 |
38.6 |
| Pu-238 |
- |
- |
9.5 |
4.7 |
Pu-239 |
0.877 |
0.494 |
10.0 |
4.5 |
| Pu-240 |
- |
- |
35.7 |
19.8 |
| Pu-241 |
0.511 |
0.246 |
12.3 |
5.1 |
Am-241 |
- |
- |
57.6 |
33.8 |
Am-242 |
0.042 |
0.020 |
8.8 |
3.0 |
Cm-243 |
0.280 |
0.127 |
8.4 |
3.1 |
Cm-244 |
- |
- |
26.6 |
13.2 |
Cm-245 |
0.116 |
0.054 |
9.1 |
3.5 |
Cm-247 |
4.060 |
2.180 |
6.9 |
2.8 |
Cf-249 |
0.129 |
0.060 |
5.9 |
2.4 |
|
Cf-251 |
0.048 |
0.025 |
5.5 |
2.3 |
Smallest critical masses for some fissile material under certain boundary conditions