energy to require an accurate determination of the relative energy appearing in the resonance cross section as given in equation (2.12). The relative kinetic energy of the neutron-nucleus collision is


                    E    1       -                 (2.13)
          r-                   n   )

where vn is the velocity of the incident neutron, VN the velocity of the nucleus, and I the reduced mass as defined by (2.8). Expanding (2.13), the relative energy can be written as
                             Pn " P

          E      E -_ E +rn - E                    (2.14)
          r    m  n   M  N    m + M

where the following relations have been used:


        En   - mv2 = kinetic energy of the neutron
        n    2   n

           E 1
        En   2 MVN2 = kinetic energy of the nucleus


        Pn   mvn    = momentum of the neutron PN = MVN    = momentum of the nucleus

Note that, if the nuclei were stationary, the second and third terms of (2.14) would be zero and the expression reduces to (2.6). The kinetic energy of nuclei at the maximum solid core fuel temperature of 30000K is 0.26 eV,


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