Natural nuclear reactor at Oklo and variation of fundamental constants: Computation of neutronics of a fresh core

Using modern methods of reactor physics, we performed full-scale calculations of the Oklo natural reactor. For reliability, we used recent versions of two Monte Carlo codes: the Russian code MCU-REA and the well-known international code MCNP. Both codes produced similar results. We constructed a computer model of the Oklo reactor zone RZ2 which takes into account all details of design and composition. The calculations were performed for three fresh cores with different uranium contents. Multiplication factors, reactivities, and neutron fluxes were calculated. We also estimated the temperature and void effects for the fresh core. As would be expected, we found for the fresh core a significant difference between reactor and Maxwell spectra, which had been used before for averaging cross sections in the Oklo reactor. The averaged cross section of Sm-149(62) and its dependence on the shift of a resonance position E-r (due to variation of fundamental constants) are significantly different from previous results. Contrary to the results of previous papers, we found no evidence of a change of the samarium cross section: a possible shift of the resonance energy is given by the limits -73 <=Delta E-r <= 62 meV. Following tradition, we have used formulas of Damour and Dyson to estimate the rate of change of the fine structure constant alpha. We obtain new, more accurate limits of -4x10(-17)<=alpha center dot/alpha <= 3x10(-17) yr(-1). Further improvement of the accuracy of the limits can be achieved by taking account of the core burn-up. These calculations are in progress.