Determination of wigner doses in graphite. Neutron spectrum effect

Various models representing the formation of Wigner defects are studied and compared. The Thompson-Wright model is chosen and this choice is justified. It is shown also that in the majority of cases of thermal neutron reactors, it is of no special use to know in detail the neutron spectrum to which the graphite is subjected, and that the Wigner dose W is given simply by a knowledge of the fast flux Φr (E > 1 MeV) and of the flux per unit of lethargy at 1 keV, φ, measured using techniques which can today be applied in research reactors. W/Φr is a logarithmic function, given here, of φ/Φr for the eclectic collection of 40 spectra considered which obey the condition 0.07 ≤φ/Φr≤1.4 (thermal neutron reactors). Finally, the constants required for calculating W directly from a known φ(E) spectrum are given. In particular, a very good approximation can be obtained by calculating the integral Φ(Eo)=∫Eo∞φ(E)dE. The threshold Eo is given here for different types of reactor (∼ 75 keV). (author)