Air Ingress and Corrosion Potential for PBMR Direct Cycle

The possibility of fuel and graphite degradation due to chemical attack is a perennial issue for HTR’s. For the direct cycle used in the PBMR design, only air ingress is a problem that merits serious attention. Initially, and as reported at a previous conference, investigation of the problem was tackled by assuming worst case conditions for a break at the core outlet pipe to determine what the grace time would be, before counter measures need be taken. The current work identified worst case break positions, quantifying air ingress rates, assuming a Guillotine break. These calculations include first order corrosion reactions in the bottom reflector and the core. Taking the worst possible large break location and the maximum initial air ingress as a determinant, a period of 24 hours was determined to be sufficient to prevent both serious fuel and core structure degradation. The acceptability of the extent of corrosion will be determined by the Safety Analysis Report (SAR), which is under preparation. However, it was realized that a more realistic specification and analysis of the problem was required to enable design decisions to be made, and a more detailed model of the break and the Main Power System (MPS) cavities was developed. This includes the maximum movement of large piping postulating a Double Ended Guillotine Break (DEGB) at worst possible locations. Further calculations on the improved model are described that investigate the influence of various pipe separations i.e. 50 mm and 500 mm at the turbine inlet. A strong correlation between the opening size and total core corrosion rate was confirmed. The simulation also established an approximate duration for air to be expelled to stop further ingress and the volume flow requirements for the inert gas system using helium or nitrogen.