Fission chambers are widely used for on-line measurements of neutron fluxes in irradiation reactors. When the flux is high with a significant thermal component, the measurement of the fast component is a concern and implies a careful choice of the deposit material. Indeed, most fissile materials have a cross section much higher with thermal neutrons than with fast ones. Moreover, a deposit chosen to favour fissions by fast neutrons will make radiative captures in the thermal domain, leading to a gradual modification of the isotopic compositions of the deposit that may dramatically increase the fissions with thermal neutrons. Within the framework ot the FNDS (Fast neutron Detector System) project, we performed simulations of the evolution of the isotopic composition and fission rate for several deposits under high fluxes. We show that Pu242 is the best choice to extract the fast component from a flux with a significant thermal component, provided that the impurities in Pu241 and Pu239 are small enough. However, an independent measurement of the thermal component appears to be necessary to compensate for the evolution of the Pu242 deposit.
|Title of host publication||Proc. NPIC&HMIT 2009|
|Place of Publication||La Grange Park, Illinois, United States|
|State||Published - Apr 2009|
|Event||Sixth American Nuclear Society International Topical Meeting on Nuclear Plant Instrumentation, Control and Human-Machine Interface Technologies (NPIC&HMIT2009) - ANS, Knoxville, Tennessee|
Duration: 5 Apr 2009 → 9 Apr 2009
|Conference||Sixth American Nuclear Society International Topical Meeting on Nuclear Plant Instrumentation, Control and Human-Machine Interface Technologies (NPIC&HMIT2009)|
|Period||2009-04-05 → 2009-04-09|