Design of an X-ray Imaging System to Analyse Corium-water Interaction

The Krotos experience requires fast imaging (10 to 100 images/sec) of a dynamic process involving highly attenuating corium (density 8) and detection of low density variations (distinction between water and steam), with millimetric spatial resolution. In response to this difficult problem, various solutions were devised and simulated. We will show that the only feasible option is to use a very high energy (8 MeV) and very powerful source (0.3 Gy/s), associated with a scintillator screen optically linked to a high sensitivity CCD camera. This solution has been validated in the experimental phase to ensure its performance: results confirm the possibility of monitoring with 10 images/sec with a single X flash per image. Introduction: The study of "serious accidents" involves imagining the case of a nuclear reactor core fusion: resulting from a cooling fault, all structures (mainly zircaloy) and nuclear fuel (in uranium oxide form, to which fission products are added) melt to form a liquid called "corium". Depending on the scenario envisaged, this may break through the main tank and flow into the building hall. Starting up the spray system (normally used in the event of incidents to cool the tank), the hall floor can be flooded with several tens of cm of water. In this context, the corium-water interaction requires careful examination. The heat energy provided by the corium is such that a risk of steam explosion is potentially possible. Studies therefore attempt to find out more about the corium–water interaction, to determine the conditions favourable to steam explosion and assess the energy dissipated by such an explosion. In particular, the development of a simulation tool must compare its results with experimental data. To study the corium–water interaction, various experimental systems have been produced throughout the world. Among them, the experimental system Krotos [1, 2] enables liquid corium, heated to 2800°C to be poured in a test section (internal diameter 200 mm) filled with water. The aim is to determine the parameters of the pre-mix obtained: number and size of corium fragments, location of the so-called “coherent” (unfragmented) jet, location and size of the vapour films formed around the corium. These phenomena occur very quickly: the jet of corium propagates in the water at a speed of 3 m/sec, and the experiment only lasts a few seconds. The objective is to monitor these experiments with millimetric spatial resolution, at a rate of 10 to 100 images per second. Visible observations are rapidly unusable because of the water vaporisation caused by the melted metal. Use of an X-ray imaging system was therefore envisaged. After weighing up the problem, we will present the simulation results for a number of configurations, then the experiment results obtained on a high energy system. We will then explain the differences observed between the simulations and experiments and the apparent contradiction between our results and those obtained by other systems. Method: An imaging system is a compromise between three parameters: integration duration, spatial resolution and measurement accuracy (or density resolution). Improving one of these parameters always leads to deterioration of one or both of the others. In the Krotos experiment, the challenge is a tricky one, as the system must be very fast, with quite accurate precision to distinguish a vapour film in the water, while penetrating the corium, and all this with millimetric spatial resolution that remains restrictive in the light of the other requirements. In other words, the compromise between the three above-mentioned parameters will be difficult to determine. First, we devised not one but two imaging chains, operating simultaneously. Each chain is turned 90° from the other. One, called "low energy" (LE; X generator of 100 to 400 kV) aims to view the vapour film; the other, called "high energy" (HE; linear electron accelerator, maximum energy greater than 1 MeV) is used to observe fragments and the coherent jet. A dual system of this kind has been used by Wisconsin-Madison university, to monitor an analog experiment [3]. We have these results that actually show the ability of the LE chain to view a vapour film around the corium jet. The results of the HE chain are less convincing however, with only slight contrast, which seriously limits location of the coherent jet. In view of the speed restrictions and current technologies, a system using a scintillator screen, optically linked to a camera seemed to be the only viable solution. This solution has already been tested in HE dynamic inspection systems [4, 5]. Another technology, using CdZnTe strip detectors [6, 7] was also envisaged. However, the dynamic aspect of the Krotos experiment prevents radiography by translation scanning and this detector can only give a series of profiles.