Decrease of Cesium Release from Irradiated UO₂ Fuel in Helium Atmosphere under Elevated Pressure of 1.0 MPa at Temperature up to 2,773K

Abstract

In the case of severe accidents, the radionuclides release from fuel could mostly occur at high temperature under elevated pressure. The effect of temperature on the release has been clarified in many previous studies while the pressure influence has been scarcely investigated so far due to difficulty in the experimental operation. To investigate the effect of pressure on the release, two tests under the same conditions except for the system pressure were performed in the VEGA program at JAERI by heating up the irradiated UO₂fuels up to 2,773 K in inert helium. The test results uniquely showed that the release rate of cesium for the temperatures below 2,773 K at 1.0 MPa could be suppressed by about 30% compared with that at 0.1 MPa. This article describes the outlines of the two tests and the observed effects of system pressure on cesium release as well as the results of various post-irradiation examinations. Moreover, the mechanisms and models that explain the pressure effect are discussed.

KEYWORDS:

• VEGA program
• severe accident
• cesium release
• pressure effects
• irradiated fuel
• diffusion
• open pores
• uranium dioxide grains
• temperature dependence