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.
Highlights
THE characterization of radioactive waste packages is a major challenge that has to be addressed to ensure the safety during their management
We propose to exploit gamma-ray ratios of some photofission products emitting multiple gamma rays as attenuation indicators to estimate the depth at which nuclear materials are located inside the radioactive waste package
This paper presents the experimental setup deployed at CEA Cadarache (France) using a high-energy electron linear accelerator (LINAC) in the CINPHONIE irradiation cell [9] to interrogate uranium samples placed in a concrete matrix, and measure the photofission delayed gamma rays with a HighPurity Germanium detector (HPGe)
Summary
THE characterization of radioactive waste packages is a major challenge that has to be addressed to ensure the safety during their management (transport, interim storage, final repository). Among the active nondestructive methods implemented to obtain characterization information in the case of large and dense packages, such as concrete drums [1] -[5] , Active Photon Interrogation with highenergy X-rays (i.e. photofission) is the only one that could bring sufficient information on the nuclear material content inside the package. This interrogating technique relies on the fact that all actinides undergo fission when using photons of energy higher than 6 MeV. 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]
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