Fuel Pellets Phase Composition Evolution During Steam or Air Oxidation

In this paper the results of laboratory tests on oxidation behavior of non-irradiated UO Studies of the physico-chemical phenomena concerning severe accidents in nuclear power stations are very challanging since in this area is not possible to conduct experiments on a real-world scale and only laboratory tests can be used. This type of tests permits each phenomenon to be considered separately The oxidation reactions that may occur on the zircaloy claddings and on fuel pellets in case of a severe nuclear accident lead to a decrease in the fuel mechanical stability and to an encrease in fission products volatility Also, in order to define proper conditions for secure storage of spent fuel it is important to understand the oxidation behaviour of UO 2 fuel pellets at low temperatures The aim of this paper is to obtain experimental data neccessary to the modelling of fuel pellets behaviour in the case of a severe nuclear accident in CANDU reactors. The paper describes the oxidation tests of UO 2 fuel pellets in a SETSYS Evolution SETARAM Thermoanalyzer in air and steam atmosphere in the temperature range 400-1200 0 C. Experimental part Two sets of oxidation experiments of UO 2 pellets in 0% and 95% steam in air atmosphere were performed in a SETARAM SETSYS Evolution Thermoanalyser. The temperature range was 400÷1200 0 C. The time duration of experiments was 4 h , except the D6 experiment where the duration was 2.5 h. The oxidation tests conditions are presented in table 1. The gas flow rate used in tests was 50 mLmin -1 . The samples were non-irradiated UO 2 sintered fuel pellets, manufactured by pressing and then sintering the green pellets for 4h at 1700 0 C in a Degussa VSL 10/18 furnace using natural UO 2 powder. The diameter of pellets was approximately 3mm with 2÷2.5mm thickness, to fit to the100 μL thermo analyzer crucible. The mass of each fuel pellets was about 0.2g UO 2 . The samples were weighed and placed in an Al 2 O 3 crucible. Then the crucible was introduced in the Setsys Evolution Thermoanalyzer furnace. The samples were * email: iulia.dumitrescu@nuclear.ro heated up to working temperature in an inert atmosphere (argon). During the tests various data (temperature, gas flow rate and atmosphere composition) were recorded continnously. The weight change of samples was measured and the TG curves were obtained. The humid atmosphere was created with the WETSYS controlled humidity generator. The samples were dimensionally, morphologically and cristallografically investigated after oxidation using the Horiba Particle Size Analyser, the TESCAN VEGA LMU scanning electron microscope and the X'Pert Pro MPD_PanAlytical diffractometer, respectively. Results and discussions To study the temperature effect on oxidation the samples were exposed in air-steam mixture with a steam content of 0% and 95% at temperature in the range 400÷1200 0 C