Title: Experimental Assessment of Accident Scenarios for the High Temperature Reactor Fuel System
Citation: ATW-INTERNATIONAL JOURNAL FOR NUCLEAR POWER vol. 58 no. 11 p. 618 - 625
Publication Year: 2013
JRC N°: JRC84794
ISSN: 1431-5254
URI: http://publications.jrc.ec.europa.eu/repository/handle/JRC84794
Type: Articles in periodicals and books
Abstract: The High Temperature Reactor (HTR) is characterized by an advanced design with passive safety features. Fuel elements are constituted by a graphite matrix containing sub-mm-sized fuel particles with TRi-ISOtropic (TRISO) coating, designed to provide high fission product retention. During a loss of coolant accident scenario in a HTR the maximum temperature is foreseen to be in the range of 1600- 1650 °C, remaining well below the melting point of the fuel. Two key aspects associated with the safety of HTR fuel are assessed in this paper: fission product retention at temperatures up to 1800°C is analyzed with the Cold Finger Apparatus (KüFA) while the behaviour of HTR-relevant fuel materials in an oxidizing environment is studied with the Corrosion Apparatus KORA. The KüFA is used to observe the combined effects of Depressurization and LOss of Forced Circulation (DLOFC) accident scenarios on HTR fuel. Originally designed at the ForschungsZentrum Jülich (FZJ), an adapted KüFA operates on irradiated fuel in hot cell at JRC-ITU. A fuel pebble is heated in helium atmosphere for several hundred hours, mimicking accident temperatures up to 1800°C and realistic temperature transients. Nongaseous volatile fission products released from the fuel condense on a water cooled stainless steel plate dubbed "Cold Finger". Exchanging plates frequently during the experiment and analyzing plate deposits by means of High Purity Germanium (HPGe) gamma spectroscopy allows a reconstruction of the fission product release as a function of time and temperature. To achieve a good quantification of the release, a careful calibration of the setup is necessary and a collimator needs to be used in some cases. The analysis of condensation plates from recent KüFA tests shows that fission product release quantification is possible at high and low activity levels. Another relevant HTR accident scenario is air ingress into the reactor vessel as a consequence of a DLOFC incident. In case of breaches on the vessel or in other components, the pressure drops and air enters the reactor cavity. This scenario can affect the stability of graphite, which is used as a structural material for parts of the reactor core and the fuel. The presence of an oxidizing atmosphere leads to graphite corrosion and increases the risk for mechanical failure of TRISO coated particles, impeding the fission product retention barriers of the fuel and particularly leading to a sudden release of fission gases. In order to quantify such releases KORA was designed and operated in FZJ between 1992 and 1996: a high temperature furnace was installed in hot cell and able to simulate accident conditions in an oxidizing atmosphere. A successive version is planned to be installed at JRC-ITU in order to perform more tests. Currently, a non-radioactive “cold” prototype is operated to investigate the oxidation behaviour of materials relevant for the HTR fuel system. Recent tests have been conducted on nuclear graphite.
JRC Directorate:Nuclear Safety and Security

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