Speaker
Description
The prevention of nuclear accidents is partly sized by a precise assessment of the quantity of radionuclides potentially emitted from nuclear fuel and transported into the environment for a given accidental sequence. LAMIR (a French acronym for Laboratory for the Analysis of the Migration of Radioelements) has specialized for many years in the development of experimental devices deployed in high activity cells. Generally speaking, these types of set up provide access to the kinetics and release rates of fission products from irradiated nuclear ceramics submitted to given temperature transients representative of accident conditions (typically Reactivity Initiated Accident (RIA), Loss Of Coolant Accident (LOCA) or Severe Accident (SA)).
MERARG in operation in the LECA-STAR Nuclear Installation of CEA-Cadarache’s centre is a perfect illustration of this point. The corresponding experimental assembly consists of three main parts: the annealing treatment furnace itself located in a high activity cell, the gamma spectrometry detector and finally the gloves box including the gas micro-chromatography and the gas recovery apparatus. The fuel pellet to be characterized (generally with its cladding) is placed in a metal crucible. It is positioned at the centre of the induction coil of the furnace supplied by a high-frequency power generator. It therefore also acts as a susceptor with respect to the HF heating. The sealed enclosure of the furnace consists of a quartz tube inside which the crucible is located. During a typical experiment, this whole assembly is swept by circulation of a so-called "sweeping" gas (argon or dry air) at a low flow rate (approximately 60 cm3/min).
In addition to the fission product release measurement capabilities, MERARG has recently been equipped with a system for imaging and recording the evolution of the sample morphology correlated with a measurement of the surface temperature by thermal radiation spectrometry as a function of the thermal sequence applied. It is thus possible to monitor online and in real time the fragmentation of the fuel, which occurs during a thermal transient, and to link these observations to the release of fission gases and/or fission products (also measured online) and the temperature of the sample.
After recalling the main characteristics and performances, in terms of possible temperature transients, of the MERARG device, the development, the so-called “cold qualification (i.e. outside the active laboratory) phase” and, finally, the active commissioning of the new imaging and surface temperature measurement system will be described in detail. The first results obtained on high burnup UO2 fuel will then be presented in order to highlight the unique capabilities to date of the experimental set up, thus making it possible to better understand the mechanisms involve in the release of fission products. Future experimental developments planned to allow the application of thermal sequences representative of a severe accident (i.e. up to fuel fusion) in an oxidizing atmosphere will be the subject of the last part of this presentation.
