The LBE Coolant Chemistry R&D Programme for the MYRRHA ADS: Chemistry and Control of Oxygen, Corrosion and Spallation Products

Compatibility with structural materials and activation are major challenges of the use of heavy liquid metal (HLM) spallation targets and coolants such as lead-bismuth eutectic (LBE).
Steel exposed to HLM is prone to corrosion which results in the release of steel elements in the HLM coolant. A commonly accepted strategy to reduce corrosion rates is to maintain a sufficiently elevated dissolved oxygen concentration in order to stabilize a protective oxide layer on the steel surface in contact with HLM. We present a brief overview of the oxygen sensing and control technology for LBE, developed in the frame of the MYRRHA accelerator driven system (ADS) demonstrate its performance on a large scale in the MEXICO chemistry loop (Mass Exchanger In Continuous Operation). MYRRHA stands for Multi-purpose hYbrid Research Reactor for High-tech Applications accelerator driven system and is currently under development at the Belgian Nuclear Research Centre, and will operate between 200 and 400 °C with a target oxygen concentration level of 10−7 wt-%. Numerical simulations allowed mapping of the local oxygen concentration in MYRRHA, enabling identification of the regions in the core which could be prone to corrosion.

On the other hand, oxygen concentrations must be sufficiently low to avoid formation of solid lead oxide (PbO) in the primary circuit. When designing a nuclear system such as an ADS, one must take into account accidents that may lead to increase of the oxygen concentration in the LBE and assess the probability for PbO formation and its consequences. In this context, we have studied the formation of lead oxide from oxygen-oversaturated LBE and determined the metastable limit for PbO nucleation.

Corrosion products that are released in the LBE will interact with dissolved oxygen in LBE to form corrosion product oxides. These interactions influence the concentration of both dissolved oxygen and dissolved corrosion products. This in turn can cause changes in the corrosion process itself by altering the driving force for steel element dissolution or protective oxide layer decomposition. Experimental results will be presented which provide evidence for the precipitation and dissolution of oxides of iron impurities in LBE. Numerical simulations based on computational fluid dynamics (CFD) and chemical equilibrium complement experiments in understanding the interactions between coolant chemistry and reactor thermal-hydraulics.

A similar approach was adopted for simulating the chemistry of spallation and other radioactive impurities in the primary LBE of MYRRHA. Simulations allow the prediction of released fraction, vapor composition and precipitation/dissolution phenomena in the LBE, as function of the oxygen concentration in the LBE and of the oxygen and humidity content of the cover gas in contact with the LBE. Simulations of the chemical behavior of several critical radioactive impurities in the coolant such as iodine and polonium are discussed and compared with experimental results.