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Fission-product release and fuel behaviour in severe-accident conditions : results of the first VERCORS RT experiments concerning UO2 and UO2/ZrO2 debris bed.



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Y. Pontillon, F. Ceccaldi, G. Ducros, P.P. Malgouyres, M. Kissane, D. Boulaud, J.M. Evrard, CSARP meeting, 6-8/05/2002, Albuquerque.


Over the last decades, due to the potentially severe consequences of a nuclear incident and/or accident for surrounding populations as well as the environment, international safety authorities launched R&D programs in support of general policy on exploitation of nuclear energy. In France IPSN (Nuclear Protection and Safety Institute, now IRSN, Nuclear Radioprotection and Safety Institute) and EdF (Electricité de France) initiated several experimental programs devoted to the source term of fission products and actinides released from PWR fuel samples in severe accident conditions. In this context, the Department of Fuel Studies (DEC), part of the Nuclear Energy Directorate (DEN) of the Comissariat à l’Energie Atomique (CEA), has acquired considerable experience in this field of research. In order to attain the required capabilities, specific technical facilities set up in shielded hot cells at the CEA-Grenoble have been developed around the so-called “VERCORS” program. The VERCORS experimental program was launched in order to clarify the mechanisms of radionuclide release and quantify the corresponding source term in PWR severe-accident conditions up to loss of fuel integrity; this is considered complementary to the in-pile integral experimental program “PHEBUS FP”. Up to 1996, several tests were performed dealing with fuel degradation, fission-product (FP) behavior, aerosol characteristics, etc. These tests provided experimental data of high interest and led to a large database regarding release of FPs and actinides from UO2. Since 1996, a new VERCORS series (VERCORS HT and RT) is in progress focusing on improvement of this release database during later phases of an accident, i.e. including liquefaction. Furthermore, other effects have been studied: the influence of the nature of the fuel (high burn-up fuel and MOX versus UO2); fuel morphology (intact or fragmented); chemical experimental conditions (oxidizing or reducing) and addition of neutron-control materials (Ag, In, Cd and boric acid) for their impact on FP transport. The present communication focuses on the VERCORS RT3 and RT4 experiments performed respectively with UO2 and UO2/ZrO2 both in debris-bed configurations. RT4, conducted in 1999, was the first test where release from a debris bed sample was studied in a mixed steam-hydrogen atmosphere. RT3, carried out the same year in less oxidizing conditions, used a mixture of unre-irradiated and a re-irradiated fuel fragments. As observed in tests with an intact UO2 sample, the total release of volatile FPs (such as Cs, I, Te and Sb) was almost complete for the two tests: as expected, there was no particular effect of the initial morphology of the sample in severe-accident conditions. However, the release rate of these elements was much higher in this type of morphology than for an intact configuration. Concerning semi-volatile, low volatility and non-volatile FPs, neither the more severe degradation of the RT3 fuel nor the initial debris-bed morphology increased their release; the liquid corium formation in RT3 seems even to retain a fraction of theses FPs in comparison with solid fuel, excepted for Ba which presents near-total release. The notable difference between these two tests concerns, on the one hand, the release rate of the volatile species, particularly 137Cs, and on the other hand, the fuel degradation temperature (i.e. ~2950K for RT3 (melting) instead of ~2500K for RT4 (start of delocalisation due to liquefaction). This work has been done with the collaboration of the Commissariat à l'Energie Atomique.