Implementation of Leak Before Break (LBB) criteria in fast spectrum reactor

Abstract

Sodium is employed as a coolant in the primary and secondary loops of fast reactors to transfer heat from the reactor and deliver it to the expander. However, handling sodium poses challenges and can be hazardous, as it reacts exothermically when exposed to air. Again, though there are radioactive barriers, there will be some radiation in the liquid coolant which makes it even more hazardous. Generally, P91 (9 % Cr and 1 % Mo) material is used in sodium-cooled nuclear reactors for its high-temperature strength, excellent creep resistance, and good corrosion and oxidation resistance, making it ideal for the demanding environment within the reactor and coolant line (Kudiyarasan and Biswas, 2024). Although, fast reactors operate at a lower pressure (1̴2 MPa) compared to the pressurized water reactors (5̴10 MPa), the operating temperature in the coolant line for fast reactors being very high (500 ֯ C̴800 ֯ C) unlike PWRs (̴300 ֯ C), there is a risk of brittle failure, due to factors such as material embrittlement from radiation exposure or stress concentrations, due to environmental interactions or a combination of different hydraulic transients (i.e. water hammering, thermal hammering or creep etc.) that can lead to sudden catastrophic failure under certain conditions. After thermal hammering, the main reason of hydraulic transients in fast spectrum reactor is found to be “Water Hammering”, which is nothing but sudden rise in pressure and propagation of pressure waves (shockwaves) resulting in high stress generation in the pipes. There are two primary reasons for “Water Hammering” i.e. i) Sudden closure or opening valves and ii) Due to sudden starting and stopping of pumps. In our paper, we have done numerical simulation on these hydraulic transient cases and found out the necessity to implement Griffith's criteria on designing the coolant line. Based on Griffith's LBB criteria and considering hydraulic transient scenario, we designed pipes for two different cases to find out the applicability of LBB criteria in transient case, experimentally and found out the criteria to be quite suitable to tackle this kind of problems in practical scenarios. Therefore, it seems obvious to always design the coolant line based on Griffith's criteria, so that leakage would occur before the complete failure takes place and this would give us ample time to successfully repair the coolant line before any catastrophic failure happens.