New measurements of cumulative photofission yields of $^{239}$Pu, $^{235}$U and $^{238}$U with a 17.5 MeV Bremsstrahlung photon beam and progress toward actinide differentiation

International audienceIn the frame of a long-term research program on the characterization of large radioactive waste packages by photofission, the Nuclear Measurement Laboratory of CEA IRESNE has measured cumulative yields of 239Pu, 235U and 238U photofission products by using a Bremsstrahlung photon beam produced by a 17.5 MeV linear electron accelerator. A characterization of the energy of the Bremsstrahlung photon beam has been carried out by photon activation analysis with different samples of gold, nickel, uranium, zinc and zirconium. The contribution of neutron fission in the different samples 2 has also been estimated by MCNP simulations in order to assess as precisely as possible the photofission yields. Finally, 26 cumulative photofission product yields are reported for 239Pu, 28 for 238U and 26 for 235U, with half-lives ranging from 14 min to more than 3 days, some of them being not recorded so far in the literature. Among these reported photofission product yields, 18 have been measured for all 3 actinides, which can thus be used for their discrimination. A differentiation criterion based on delayed gamma-ray ratios has been established to determine the most efficient photofission product couples to estimate the enrichment of a 235U/238U mixture or the fissile fraction (235U+239Pu)/actinide mass in a mixture of uranium and plutonium