Localization of nuclear materials in large concrete radioactive waste packages using photofission delayed gamma rays

Abstract

The characterization of radioactive waste packages is mandatory for their transport, interim storage and final disposal. In this framework, the Nuclear Measurement Laboratory of CEA DES IRESNE Institute, at Cadarache, France, uses a high-energy electron linear accelerator (LINAC) to produce an interrogating bremsstrahlung beam with endpoint energies ranging from 9 to 21 MeV to perform X-ray imaging and high-energy photon interrogation on large concrete packages. In particular, highenergy photon beam induces photofission reactions in both fissile (235U, 239Pu, 241Pu) and fertile (238U, 240Pu, 232Th, etc.) actinides possibly present in the radioactive waste. In order to assess their mass, we use delayed gamma rays emitted by their photofission products, which are measured with a 50 % relative efficiency High-Purity Germanium (HPGe) detector. Actinide differentiation, which is important for the fissile mass estimation, is based on the ratios of gamma rays emitted by different photofission products and requires appropriate corrections for the gamma attenuation in concrete. To this aim, we report here a localization method of point-like nuclear materials in the concrete matrix, based on the differential attenuation of several gamma rays emitted by a same photofission product. We use here the 1435.9 and 2639.6 keV lines of 138Cs, with both experimental data and MCNP numerical simulations to determine the (r,θ) coordinates of nuclear materials. Then, the depth inside the concrete matrix, which is determined with a precision of a few percent, mainly depending on counting statistics on 1435.9 and 2639.6 keV net peak areas, is used to correct for the different gamma ratios used in the actinide identification method. Experimental tests with uranium samples have been performed to validate the localization method.

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

“…Further work will be dedicated to the selection of the most efficient photofission product couples in presence of a waste matrix, causing gamma attenuation effects that depend on actinide localization. To this aim, a new experimental campaign will be carried out to test the differentiation of actinide isotopes inside a concrete matrix, as reported in [18] for actinide localization. One of the main objectives of this long-term R&D program is to assess nuclear materials in heterogeneous technological waste blocked in 870 L cemented drums.…”

“…Besides, the uranium isotopes differentiation in an 870 L waste drum by using delayed gamma-ray ratios has already been investigated experimentally with a mockup package [1] and by using Monte-Carlo simulations in the work of Simon et al [5]. Furthermore, photofission products emitting several gamma rays can be used as attenuation indicators to estimate the depth at which nuclear materials are localized inside the package [18].…”

“…It is important to mention that we are not limited to ratios of close-in-energy gamma rays. This implies that detection efficiency at both energies has to be estimated and requires a prior localization of actinides, for example with the method described in [18];…”