THERMAL HYDRAULIC ANALYSIS TOWARDS A ROBUST DESIGN OF LEAK COLLECTION TRAY FOR POOL TYPE SODIUM COOLED FAST REACTORS

摘要

To protect the sodium cooled FBR plant against the hazardous effects of sodium leak into the ambient, one of the passive protection devices used is the Leak Collection Trays (LCT) below the secondary sodium carrying pipelines in the Steam Generator Building (SGB). The design of LCT is based on immediate channeling of burning liquid sodium on the funnel shaped ‘sloping cover tray’ to the bottom ‘sodium holdup vessel’ in which self-extinction of the fire occurs due to oxygen starvation. In the secondary heat transfer circuits of FBRs, leakage of liquid sodium from the pipelines is postulated as one of the design basis accidents with probability of occurrence at 10-2 per reactor year. LCT collect the leaked sodium in a hold up vessel, suppress the sodium fire due to oxygen starvation and guide the sodium to an inerted ‘sodium transfer tank’ located at the bottom most elevation of the SGB. The procedure of draining the leaked sodium into the transfer tank has been envisaged as a defense in depth measure against the handling of un-burnt sodium and to guard against larger leak rates than that can be handled by the LCT effectively. Towards this, a network of carbon steel pipelines are laid out connecting all the LCT and the transfer tank through headers in strategic locations, each having a fusible plug. The fusible plug separates the air environment in LCT and argon environment in sodium transfer tank. Woods metal is the preliminary choice for the fusible plug. It is an alloy of 50 % Bi, 25 % Pb, 12.5 % Sn and 12.5 % Cd with a melting point of 72℃. The transfer tank is filled with argon at～0.03 bars-g pressure. Both the header and the tank are at room temperature during normal conditions. Leaked sodium by virtue of its high temperature has to heat up the fusible plug to melt the same and drain into the transfer tank. Transient thermal hydraulic investigations have been carried out to predict the fusing characteristics of woods metal plug. The numerical results have been validated against analytical solutions for idealized conditions. Detailed parametric studies have been carried out with plug thickness as a parameter. It is established that effective melting of the plug and trouble free draining of the leaked sodium is possible for a 3 mm thick fusible plug.