Aerosol formation, transport and deposition in lead based fast reactors

Corazza Christophe

Promoter

Vetrano Maria Rosaria, rosaria.vetrano@kuleuven.be

SCK•CEN Mentor

Rosseel Kris
kris.rosseel@sckcen.be
+32 14 33 80 05

SCK•CEN Co-mentor

Aerts Alexander
alexander.aerts@sckcen.be
+32 14 33 80 18

Expert group

Conditioning and Chemistry Programme

PhD started

2018-10-01

Short project description

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).

 

Objective

In this PhD topic we want to advance the knowledge regarding aerosols in heavy liquid metal cooled fast reactors.  Specifically, we want to assess the impact of aerosols in MYRRHA by combining numerical tools and experiments for characterization of the aerosol formation mechanism, their particle size distribution and composition, the amount of aerosols formed and their deposition properties under normal and accident conditions in MYRRHA. 

For the formation mechanism of aerosols, two main mechanism are thought to be at play: entrainment (i.e. due to mechanical gas-liquid interaction) and evaporation.

Formation of aerosols due to entrainment can be expected wherever disturbances occur in a gas-LBE interface, e.g. at the inlet of sump pumps or due to gas bubbling.  Bubbling at low gas flow rates is foreseen in the LBE chemistry control system normal operating conditions.  High gas flow rates may be encountered during accidental release of gas inside the LBE during e.g. a gas line break.  Models exist for water – air for entrainment under various gas flow rates.  These models will be used as a starting point and will be adapted for assessing the entrained amounts under various gas flow rates and atmospheric conditions (dry gas, wet gas, bubbling, mixing) in LBE.

Evaporation of LBE or impurities dissolved therein is strongly dependent on temperature and will only be relevant at high temperatures or in the presence of hot spots.  This is expected to be the dominant aerosol formation mechanism during accident conditions, such as partial or complete core blockage evens. Aerosols formation by evaporation-condensation under accident conditions will be studied in high temperature setups. The capture mechanisms and methods of aerosol particles will be addressed as well. Dedicated LBE setups will be constructed for these fundamental studies, using a cascade impactor and laser aerosol spectrometry for quantification and characterization of the aerosols formed.  Tools such as SEM and XRD are available for characterization of particle shapes and compositions. The experimental results obtained will be used as input for the development and validation of numerical models. 

The developed numerical models will be used to predict and assess the aerosol transport under various conditions prevailing in the MYRRHA cover gas.  This will then be used to determine the source term for radiological hazard calculations and for the development of a cover gas filtration system aimed at minimizing the presence of aerosols over the envisaged particle size range