  The Displacement per Atom (DPA) rate is conventionally computed with DPA
cross sections in reactor applications. The method of direct calculation with
energy-angular distributions given in the Center of Mass (CM) frame is proposed
and recommended in the present work. The methods for refining and verifying the
calculations of DPA cross sections are proposed: (i)
Gauss-Legendre-Quadrature-based Piecewise Integration (GLQPI) for ensuring the
numeric convergence of integral over emission angle due to the discontinuity of
integrand; (ii) verification of the convergence for trapezoidal integration
over the secondary energy; (iii) interpolation of double-differential cross
sections. For 56Fe of JEFF-3.1.1, the current numeric integration over emission
angle is shown not convergent, whereas the direct trapezoidal over the
secondary energy and the direct interpolation of energy-angle-integrated damage
are shown accurate. On the other hand, it is shown that the DPA cross sections
are overestimated if isotropic angular distributions are assumed. However, the
DPA cross section is not sensitive to the high-order Legendre polynomials
because the former is an angle-integrated quantity. Numerical results of
neutron elastic scattering show that 2 orders of Legendre polynomials can give
the DPA rates of 56Fe within 0.5% overestimation for fission reactors, while 4
orders are required for fusion reactors. For neutron inelastic
scatterings-induced DPA, the first order Legendre polynomial is sufficient for
both fission and fusion reactors.
