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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
(kg)

Water-reflected
(kg)

Unreflected
(kg)

Steel-
reflected
(kg)

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

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