Description of Circumstances
An incident occurred in late 2001, while a boiling water reactor (BWR) unit was operating at rated power. The utility was performing a periodic surveillance of the high-pressure coolant-injection (HPCI) system. Immediately after the test began, the HPCI system automatically isolated and the reactor building fire detectors actuated. The unit was then manually shut down. An examination of the residual heat removal (RHR) system revealed that a pipe elbow had ruptured near the high point in the RHR branch steam supply line leading to one of the two RHR heat exchangers (steam condensing mode line) in the reactor building. Fragments from the piping rupture caused some damage to equipment in the general area, but no significant damage to any safety-related equipment. The subject line supplies steam to the RHR heat exchanger when the heat exchanger is operated in the steam condensing mode. The utility had modified the RHR piping by adding a water barrier upstream of the RHR steam supply isolation valve in order to reduce leakage through this valve.
The utility initially reported the pipe rupture as a water hammer event; however, subsequent investigation has led the utility to re-characterize the rupture as a combustion event. This re-characterization was based in part on an analysis of gases in the sister BWR unit at the same RHR piping location. This analysis found the hydrogen levels to be 46 percent by volume (percent vol), 23 percent vol oxygen, and 31 percent vol steam. A metallurgical analysis of the resulting piping fragments indicated ductile fracture from excessive internal over-pressure.
A second event occurred one month later while the unit was operating at 100-percent power. The utility interpreted a containment pressure increase, concurrent with an acoustic monitoring response, as a flange leak in the reactor head spray line. The utility uses the reactor head spray after shutdown to shorten outages. The utility isolated the drain-and-keep-fill line valve for the reactor head spray, which appeared to stop the leak. An inspection found that 2 to 3 meters (6.25 to 10 feet) in length of the 10-centimeter (about 4-inches) diameter head spray line had been destroyed; however, the effects of the ruptured piping were not reported to have caused any significant damage to any nearby safety-related equipment. Although the root cause of the pipe rupture has not yet been established, the utility is investigating the possibility of a hydrogen combustion event inside the pipe as a result of the buildup of hydrogen and oxygen gases created by radiolysis.
Radiolysis occurs normally in the reactor core region when radiation decomposes some reactor coolant system (RCS) water into hydrogen and oxygen. Excess hydrogen and oxygen that does not recombine into water is normally removed by the off-gas system. Hydrogen is added in those BWRs utilizing hydrogen for reactor water chemistry control in order to scavenge excess oxygen from the RCS.
In these two foreign events, it appears that hydrogen and oxygen gases accumulated in system high points instead of being removed by the plants' off-gas system. Further, since the ignition energy for the above combustion events is predicted to be extremely small, the ignition source(s) may not be conclusively identified.
These events show the importance of preventing combustible gas mixtures from accumulating in piping. In both of the above described events, hydrogen and oxygen gases apparently accumulated to a combustible level which then catastrophically failed these piping systems. Proper venting or other considerations to prevent accumulation of combustible gases in piping high points might alleviate conditions leading to hydrogen combustion.