Development of a characterization method for concrete radioactive waste packages using photofission

The non-destructive characterization of large, concrete radioactive waste packages is a major challenge for criticality safety aspects and long-term management. On the one hand, the fissile mass present inside must be estimated precisely for their transport and storage in the safest possible way. On the other hand, a precise knowledge of their alpha activity makes it possible to send them towards the most appropriate final repository, which potentially implies significant cost savings. The 870 L concrete packages produced by the CEA and stored at the Cadarache center cannot be characterized by gamma spectrometry or passive and active neutron measurements because of insufficient sensitivity and very penalizing uncertainties associated with the high attenuation of neutrons and gamma rays in the waste or in the surrounding concrete layer. Therefore, Active Photon Interrogation (API) is being investigated to locate and quantify fissile material in these bulky concrete waste drums. This technique is based on the photofission phenomenon when actinides are interrogated with photons of energy higher than 6 MeV. Since all actinides have a similar photofission cross section, the method does not allow a direct estimate of the mass of fissile material (e.g. ²³⁵U, ²³⁹Pu) or the alpha activity (mainly due to plutonium isotopes). It is therefore necessary to use a differentiation method to evaluate the proportions of the actinides of interest. To do this, differences in photofission product yields between actinides are exploited via the measurement of delayed gamma-ray ratios with a high purity germanium detector. New experimental data concerning the production rates of photofission products of ²³⁵U, ²³⁸U and ²³⁹Pu have been measured by irradiating uranium and plutonium samples with a Bremsstrahlung photon beam produced with a linear electron accelerator located in the CINPHONIE irradiation cell of the IRESNE institute, at CEA Cadarache. A preliminary characterization of the Bremsstrahlung photon beam and of the photoneutron flux produced by the accelerator was carried out via photon and neutron activation measurements of reference materials. From the cumulative photofission yields measurements, the most effective photofission product pairs for actinide differentiation were then identified for different mixtures of ²³⁵U, ²³⁸U and ²³⁹Pu isotopes. A method for localizing actinides inside a mock-up of a concrete waste package was also developed and tested, based on both experimental data (differential attenuation of delayed gamma rays emitted by ¹³⁸Cs) and Monte-Carlo simulations with the MCNP6 code. Localization is an essential step to correct for matrix effects, since the differential attenuation of delayed gamma rays affects actinide discrimination, but also to estimate the uncertainty on the fissile mass evaluated by API. Finally, detection limits ranging from a few grams to tens of grams depending on the waste characteristics and the position of actinides in an 870 L waste package were estimated by simulation with the experimentally validated MCNP model. Ways of improving these detection limits are being studied, in particular by reducing the active background or by measuring high-energy delayed gamma rays with NaI(Tl) scintillation detectors. (author)