In-Containment Aerosol Source Term Behaviour- Analyses in Support of MYRRHA Safety Assessment

The Multi-purpose HYbrid Research Reactor for High-tech Applications, MYRRHA is being designed at SCK CEN since 1998. The MYRRHA programme aims at demonstrating the principle of the Accelerator Driven System (ADS), with the main objective to study the efficient transmutation of high-level nuclear waste, and at providing a flexible and multi-purpose irradiation facility. It features a pool type reactor with subcritical core fed by a linear accelerator to sustain the fission reactions via spallation. Lead-bismuth eutectic (LBE) is both the selected coolant and the spallation target material. The present study is carried out in support to the safety assessment of MYRRHA. For this technology, one of the main safety concerns is related to the presence of radioactive impurities (namely radiotoxic isotope 210Po, activation product of bismuth) in the LBE coolant. The analysis postulates the accidental release of coolant into the primary containment of the reactor, with subsequent generation of an LBE aerosol source term. The aim is to assess the efficiency of removal of the airborne radioactive material by means of natural aerosol deposition processes (i.e. gravitational settling and diffusion by Brownian motion). The study is performed with the aerosol dynamics model implemented in MELCOR code. A preliminary sensitivity analysis is performed with the release of a monodisperse aerosol source, to assess the influence of the initial particle size and mass of LBE released on the evolution of the suspended mass in the Primary Containment. It is followed by an uncertainty analysis aimed at experimenting the statistical methodology and tools to derive a Figure Of Merit (FOM, i.e. the suspended mass of radioactive aerosol) value compliant with the 95/95 criterion and at quantifying the influence of selected input parameters on the output value of the FOM, by means of correlation coefficients. The selected uncertain parameters are: Mass Median Diameter (MMD) and Geometric Standard Deviation (GSD) of the aerosol source, aerosol dynamic shape factor and LBE mass released at accident onset. The analysis shows a dominant influence of the LBE mass released on the FOM during the first hour after accident onset. It is followed by an increasing influence of the remaining parameters, which indicate the higher influence that the release of smaller sized particles (small MMD and high GSD) and the departure from the ideal spherical shape of the particles (dynamic shape factor higher than one) assume towards the end of a one-week transient. Overall, a significant decrease of the aerosol source term can be achieved by accounting for natural deposition phenomena in the primary containment. The value of the aerosol decontamination factor one week into the transient is about 100 for the limit value of the FOM obtained. In the final part of the study, a potential mitigation strategy is investigated: it consists in the injection (into the reactor primary containment atmosphere) of a monodisperse source of non-radioactive aerosols to enhance radioactive LBE aerosol deposition. One week after accident onset, a reduction of the in-containment source term up to a factor 5 can be achieved if such mitigation strategy would be implemented.