lundi 2 juillet 2012 à 12:20
Radioprotection measures needed in the nuclear fuel cycle require accurate knowledge of the radioactive sources involved. For innovative nuclear reactors such as Gen IV designs radiation sources (alpha, beta, gamma and neutron) in the spent fuel need to be calculated in order to understand the radioprotection needed in all aspects of the fuel cycle (transport, reprocessing, fuel fabrication, waste storage, etc.). For this purpose we have developed CHARS a set of source characterisation tools coupled to our code MURE (MCNP Utilities for Reactors Evolution) which is a precision research code for fuel evolution. MURE determines inventories of around 800 nuclei during irradiation and cooling via a series of MCNP5 calculations and numerical integration of Bateman's equations.
With the CHARS package using the ENSDF libraries, it's possible to generate alpha, beta and gamma spectra of the nuclear fuel at the end of cycle from any given reactor design. Beta spectra are calculated using a simplified Fermi theory where all the transitions are allowed. Gamma spectra take into account gammas from alpha,beta, EC decay, isomeric transitions and also bremsstrahlung from beta particles (for the latter a further MCNP5 calculation is required). The neutron spectrum calculation takes into account neutrons from (alpha,n) reactions, using stopping powers and (alpha,n) cross sections, and neutrons from spontaneous fission using a watt distribrution.
These complex source definitons are then used to generate automatic MCNP5 inputs for radioprotection calculations, allowing us to undertake a wide range of radioprotection studies. They also allow assessment of radiation damage in materials (i.e, displacement per atom) and gas production rates.
The first use of these tools was to estimate additional shielding in the French fuel cycle in case of switching from the current uranium (U/Pu) cycle to the thorium (Th/U) cycle. Irradiated thorium-based fuels produce small quantities of 232U, which has a realtively short half life (69y) and emits a hard gamma of 2,6 MeV at the end of its decay chain. 232U is synthesized in mainly two ways : 233U(n,2n) and 232Th(n,2n) followed by 231Pa(n,gamma). From the results of radioprotection calculations we estimate additional thickness of shielding required for the back end of the fuel cycle and show that the greatest constriants occur for the fuel manufacturing. On the other hand the neutron activity for some thorium-based fuels will be lower in Th/U than in U/Pu cycle and as a result necessary neutronic shielding will be reduced.