THERMALIZATION OF NEUTRONS IN GRAPHITE - STATUS AND ERROR ANALYSIS

Calculations related to the temperature coefficient of reactivity of e nriched gas-cooled reactors show the high sensitivity of this paramete r to the proper description of thermalization effects in the moderator . Additionally, the calculation of the temperature dependence of the i nelastic-scattering cross section with current ENDF/B formalisms corre lates the errors of the cross sections as functions of the temperature . Neglecting this temperature correlation introduces unnecessary conse rvatism in the estimation of the error of the reactivity coefficient. These two facts drove our efforts to characterize the present status o f the inelastic cross section of graphite and to calculate its covaria nce file. The ENDF/B evaluation of the scattering matrix S(alpha, beta , T) is still based on the approximations (incoherent component only) and phonon spectra of the early 1960s. Subsequent measurements showed that the structure observed in S(alpha, beta, T) cannot be described u sing the incoherent approximation, and soon after the availability of highly intense neutron beams and large specimens of pyrolitic graphite have allowed the direct measurement of elastic constants of relevance for a better calculation of the phonon spectra. Calculations of the p robability distributions of the moment and energy transfer, alpha and beta, in a Maxwellian spectrum allow us to define a range of alpha and beta for which comparison of experimental and theoretical data are of most interest for reactor analysis, and to point out regions of defic ient resolution or excessive details in the present alpha, beta mesh u sed in the ENDF/B files. Because the phonon spectrum defines S(alpha, beta, T), mathematical formulas have been found that allow the calcula tion of the covariance matrix of S by propagating the errors of the ph onon spectra.