A new spent fuel source characterization code CHARS and its application to the shielding of the thorium cycle
Progress in Nuclear Science and Technology
Radioprotection measures needed in the nuclear fuel cycle require accurate knowledge of the radioactive sources involved. For innovative nuclear reactors such as Generation 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 characterization
... ols 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. These complex source definitions are then used to generate automatic MCNP5 inputs for radioprotection calculations, allowing us to undertake a wide range of radioprotection studies. 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 232 U, which has a relatively short half-life (69 years) and emits a hard gamma of 2.6 MeV at the end of its decay chain. 232 U is synthesized in mainly two ways: 233 U(n,2n) and 232 Th(n,2n) followed by 231 Pa(n,γ). 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 constraints occur for the fuel manufacturing. On the other hand the neutron yield 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.