Analysis of radioactivity releases in the MEGAPIE reference accident

A computer model for the MEGAwatt Pilot Target Experiment (MEGAPIE) was developed, verified and applied to the analysis of the reference accident, resulting from the hypothetical loss of a Pb–Bi inventory and subsequent cooling down of the target with free convection of the atmospheric air. The radioactivity releases caused by evaporation of activation products from the lead–bismuth films on the surfaces of the target inner structures are estimated. The maximum radioactivity releases were found to be from evaporation of the mercury and polonium estimated as 7.0e11 and 6.2e8 Bq, respectively. Thermal conductivity and radiation heat transfer through the gas gaps were found to be more important mechanism of the target cooling down than the atmospheric air convection. The final conclusion made on the basis of the work is that MEGAPIE in the reference accident meets the 1 mSv criterion.