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- We can rewrite the above equation as
- The last term is generally negligible, and hence B can be found explicitly.
- Once B is known, we can use the Table for B and obtain the size.
- In case of spherical reactor, R can be obtained directly. However, if it is a cylindrical reactor, we can obtain the Height or Radius, if the ratio of H/R is assumed.
- Another class of problem is to obtain the material composition for a given geometry.
- For such cases, the procedure followed is outlined.
- The minimum mass of fuel required to make a reactor critical is called Critical Mass.
- For such computations we assume that the reactor is made of fuel and moderator only.
- Let
 .
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- As the fuel is specified, η, can be computed as described earlier using the fuel property data.
- Similarly Lmod, τMod can be obtained from the Moderator property data.
- As described earlier, the value of ε and p will be assumed to be known.
- f can be written as
- As the geometry is known, the value of B can be estimated using the expressions summarized.
- Thus the critticality equation can be written as
- The only unknown in the above equation is R and it can be estimated.
- As
 , we can now compute
the value of
 .
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