Piping system safety concern

When a BWR is not in a scrammed state, the scram valves are held closed by control air pressure and reactor coolant is retained on the upstream side of the closed valves. In this state, the scram valves perform reactor coolant boundary and primary isolation functions. During and immediately following a scram the SDV system assumes a reactor boundary function and a primary containment isolation function. It is during this fully pressurized state of the SDV system that there exist a potential safety concern associated with a break in the SDV system piping. 

Safety concerns associated with pipe breaks in the BWR scram system, Office for Analysis and Evaluation of Operational Data, USNRC. 

Thermal expansion is one major concern for the safety of the piping system. For a liquid-filled situation, thermal expansion should be considered for the protection of the piping system. 

The water distribution system in the city of Dayton, OH, uses Southdown concrete water mains to deliver water to its citizens. Routine sampling and testing of Dayton s water supply by the city s Department of Water consistently shows that the levels of metals are well below the Ohio EOA Community Drinking Water Standards, and that these levels have remained constant throughout a nine-year testing period from 1982 to 1990. Because metal leaching has not occurred, there is no reason for concern over the safety of Southdown concrete pipes to transport drinking water. 

Concerning the cooling of the reactor core, loss-of-coolant accident due to piping breech, which is treated as the major accident in high-pressure systems of LWRs, is no more a significant issue in SFRs, since SFR is a low-pressure system. The ECCS, which is significant also for the safety of LWRs, is not required. Instead, a guard vessel is set outside of the reactor vessel to prevent loss of sodium coolant from the core in SFR. 

Another potential problem to watch for concerns safety shower systems that use a Christmas tree arrangement of piping. If someone trips the flow to one shower, the entire system will fill up with water, hence the lines to those showers that were not tripped will have a dead-leg of water that can quickly freeze— leading to line rupture when the water is thawed. 

---