Fast High-Energy X-Ray Imaging for Severe Accidents Experiments on the Future PLINIUS-2 Platform

The future PLINIUS-2 platform of the Commissariat à l'Énergie Atomique (CEA) of Cadarache will be dedicated to the study of corium interactions in severe nuclear accidents, and will host innovative large-scale experiments. The nuclear measurement laboratory of CEA Cadarache is in charge of real-time high-energy X-ray imaging setups, for the study of the corium-water and corium-sodium interaction, and of the corium stratification process. Imaging such large and high-density objects requires a 15 MeV linear electron accelerator coupled to a tungsten target creating a high-energy bremsstrahlung X-ray flux, with corresponding dose rate about 100 Gy/min at 1 m. The signal is detected by phosphor screens coupled to high-framerate scientific CMOS cameras. The imaging setup is established using an experimentally validated home-made simulation software High-Energy MODeling for RAdiography and TOmography (MODHERATO). The code computes quantitative radiographic signals from the description of the source, object geometry and composition, detector, and geometrical configuration (magnification factor, etc.). It accounts for several noise sources (photonic and electronic noises, and swank and readout noise), and for image blur due to the source spot-size and to the detector unsharpness. In a view to PLINIUS-2, the simulation has been improved to account for the scattered flux, which is expected to be significant. This paper presents the scattered flux calculation using the Monte-Carlo N-particle transport code, and its integration into the MODHERATO simulation. Then, the validation of the improved simulation is presented, through confrontation to real measurement images taken on a small-scale equivalent setup on the PLINIUS platform. Excellent agreement is achieved. This improved simulation is therefore being used to design the PLINIUS-2 imaging setups (source, detectors, and cameras). Simulation will be presented for different configurations, as well as preliminary tests of alternative detectors considered for PLINIUS-2.

I. Introduction

The study of severe nuclear accidents is a major research topic at Commissariat à l”Énergie Atomique (CEA) (French atomic energy agency). In this scope, the PLINIUS experimental platform situated in Cadarache studies the behavior of corium (melt fuel-structure mixture), through several experimental setups [1]. The tests performed on this facility provide crucial data and validation for the models elaborated at CEA. Among the test facilities of PLINIUS, the KROTOS facility studies the corium–water interaction [2], through the release of corium into a water tank [3], [4] (see Fig. 1). It includes high-frequency X-ray imaging through the whole process, which allows a worldwide unique real-time visualization of the reaction [5] (see Fig. 2). Fig. 1.

Schematic of the KROTOS setup.

Fig. 2.

High-energy X-ray visualization of the corium–water interaction on the KROTOS facility [5].