Inverse Problem Approach for the underwater localization of Fukushima Daiichi fuel debris with fission chambers

Jonathan Dumazert (1) , Romain Coulon (1) , Frédérick Carrel (1) , Adrien Sari (1) , Cheick Thiam (2) , Mathieu Trocmé (1) , Q. Lecomte (1) , Hassen Hamrita (1) , Romuald Woo (1) , Frédéric Lainé (1) , Camille Frangville (1) , Mounir Bakkali (1) , Karim Boudergui (1) , B. Krausz (3) , R. Pissarello (3) , R. Delalez (3)

1 LCAE (CEA, LIST) - Laboratoire Capteurs et Architectures Electroniques (CEA, LIST)
2 LNHB (CEA, LIST) - Laboratoire National Henri Becquerel (CEA, LIST)
3 ONET Technologies

Fuel debris have a distinct neutron signature that can be detected to locate the said debris in a damaged nuclear power plant. Neutron measurement in a damaged PCV environment is however submitted to severe deployments constraints, including a high-dose-rate gamma background and limited available space. The study was therefore oriented towards small fission chambers (FC), with U-235-enriched active substrates. To investigate the expected performance of the FC in various irradiation conditions, a numerical model of the detector head was built. We describe the elaboration and experimental calibration of the numerical model and the Monte Carlo study of the fission rate inside U-235 coatings per generated neutron. The evaluation of a representative calibration coefficient then allowed us to carry out a multi-parameter performance study of a FC underwater, aiming at computing an explicit response function linking, on the one hand, the activity and spatial distribution of neutron emitters in a water container, with, one the other hand, the expected count rates measured by a fission chamber as a function of its radial and axial position inside the water volume. The FC underwater behavior was subsequently corroborated by a measurement campaign on a FC response, set at different positions inside a water drum, as a function of its axial and radial distance to a Cf-252 neutron source attached near the center of the container. We finally present an approach in which fuel debris localization is defined as an Inverse Problem, solvable with a Maximum-Likelihood Expectation Maximization (ML-EM) iterative algorithm. The projector matrix is built by capitalization on the results of the previously consolidated numerical studies. The ML-EM was tested on simulated data sets with a varying number of active voxels. Our first results indicate that, for a thermal neutron flux in the order of 10 n.cm−2.s−1 at the detector, originating voxels are identified with a spatial resolution in the radial plane in the order of 10 to 100 cm2.

Mots clés: sensor; calibration; metrology; Maximum-Likelihood Expectation Maximization (ML-EM); signal processing; Neutron localization; Fuel debris; nuclear power plant; Fission chamber; Inverse problem; instrumentation; nuclear instrumentation; ionizing radiation; radioactivity; irradiation; U235; modelling; simulation; Monte Carlo; neutron; gamma-rays; detector

Domaines: Instrumentations et Détecteurs [physics.ins-det]; Physique Nucléaire Expérimentale [nucl-ex]

Jonathan Dumazert, Romain Coulon, Frédérick Carrel, Adrien Sari, Cheick Thiam, et al.. Inverse Problem Approach for the underwater localization of Fukushima Daiichi fuel debris with fission chambers. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2020, 954, pp.161347. ⟨10.1016/j.nima.2018.10.025⟩. ⟨hal-03489692⟩