What is critical mass


Critical mass In nuclear physics, denotes the minimum mass of an object consisting of a fissile nuclide, from which the effective neutron production can maintain a nuclear fission chain reaction. With every nuclear fission, at least one neutron is produced on average, which causes a new nuclear fission, while the other neutrons leave the object or can be absorbed without triggering a fission. The critical mass therefore also depends on

  • the density and shape of the object,
  • the presence of moderating or neutron absorbing substances
  • and the presence of a neutron reflector around the object.

The higher the density, the lower the critical mass. An object has the lowest critical mass when it is spherical. A neutron absorber increases the critical mass, a moderator reduces it, as does a reflector such as e.g. B. beryllium, tungsten carbide or uranium-238.

If one compares critical masses of different nuclides, these usually refer to a homogeneous uncompressed sphere made of the pure material without a surrounding neutron reflector. For shapes other than a sphere, the critical mass changes. These are summarized in the following list with the reflected and unreflected critical mass for fast, unmoderated systems.

isotope Critical mass link
unreflected reflected
Protactinium-231 580-930 kg - kg
Uranium-233 16.5 kg 7.3 kg [1]
Uranium-235 49.0 kg 22.8 kg [2]
Neptunium-237 68.6 kg 64.6 kg [3]
Plutonium-239 10.0 kg 5.42 kg [4]
Plutonium-240 158.7 kg 148.4 kg [5]
Plutonium-241 10 kg - kg [6]
Plutonium-242 100.0 kg - kg [7]
Americium-241 113.5 kg 105.3 kg [8]
Americium-242m 9-18 kg - kg [9]
Americium-243 50-150 kg - kg [10]
Curium-244 23.2 kg 22.0 kg
Curium-245 12 kg - kg [11]
Curium-246 70 kg - kg [12]
Curium-247 7 kg - kg [13]
Californium-251 9 kg - kg [14]

In the case of an atom bomb, subcritical masses are compressed into a supercritical mass with the help of a chemical explosive device. Various techniques are used to reduce the critical mass:

  • compression: Sometimes a full sphere of the fissile material is compressed by the chemical explosion in such a way that the increase in density pushes the critical mass below the mass of the sphere.
  • Neutron reflector: By combining strong compression and a neutron reflector (e.g. water), the critical mass for plutonium-239 can be reduced to 0.53 kg. However, the total weight of such an explosive device is relatively high because of the complex chemical detonator.
  • Moderator: The critical mass can also be greatly reduced by the presence of a neutron moderator, which is particularly important during the preparation of the nuclear fuel, because the fissile nuclides are often in solution here. However, since the time required for the thermalization of the neutrons is relatively long, if the critical mass is exceeded in such a system, no explosion takes place, but only strong heating (accompanied by radioactive radiation), which leads to expansion and partly to Bring the solution to a boil. As a result, i. a. the density, the critical mass increases again and the chain reaction ends. The critical mass here is often only in the range of a few hundred grams. An underestimation of the moderation effect has often led to serious accidents, most recently in 1999 in Tokaimura (Japan).


  • Among other things, a list of critical masses

Categories: Nuclear Technology | radioactivity