In the MYRRHA gen IV Accelerator Driven System (ADS) under development at SCK·CEN, liquid lead-bismuth (LBE) is foreseen as the reactor coolant and as spallation target for providing the required neutrons to the sub-critical reactor core. MYRRHA is a pool type reactor, with a large cover gas space and equipped with external LBE loops for chemistry control of the coolant.
Aerosols will be formed during operation of a heavy liquid metal cooled reactor such as MYRRHA. Although these aerosols can present an important hazard for safe operations, their formation and properties under MYRRHA operating conditions are not well understood.
Aerosols can be formed wherever a gas flow comes into contact with a liquid or solid, or due to nucleation and growth of particles from an oversaturated LBE vapor. In MYRRHA, various routes are available such as e.g. rupture of a gas pipe inside the LBE, formation of dust by gas flow induced release of condensed particles from e.g. the vessel walls. Furthermore, temperature gradients or presence of airborne nucleation sites can lead to formation of particles from vapor condensation in the cover gas.
Under accident conditions, hot-spots, the presence of dispersed fuel particles and fission products in the LBE or the presence of water in the cover gas may lead to enhanced aerosol formation and/or or changes in their deposition properties.
Due to the various mechanisms involved, aerosol particle sizes ranging from about 50 nm (freshly formed nuclei) to well over 100 µm (due to entrainment or coagulation) are expected. Their size, shape and composition will determine whether and where these aerosols are deposited. Besides possible clogging of e.g. gas exits, aerosols can also act as carriers for hazardous radionuclides such as 210Po. The most critical aerosol size range, from a radiological viewpoint, is the one that minimizes depletion by natural deposition. From a preliminary assessment by means of the MELCOR code, this size range has been approximately identified as 0.3 - 50 μm (particle diameter values).