Recent news reports have focused on how the emergency venting system at the Fukushima Daiichi nuclear plant in Japan responded to the massive earthquake and tsunami in March. The reports also pose questions about the performance of U.S. plants with a similar venting system. Here is an update from GE on these two issues:
Mark I venting system
The Mark I containment incorporates a system of emergency vents for safely relieving a buildup of pressure inside the primary containment, which is the protective steel and concrete housing that surrounds the reactor pressure vessel and its associated systems. In 1989, the U.S. Nuclear Regulatory Commission (NRC) recommended improvements to the emergency venting system in the Mark I containment in Generic Letter 89-16. Specifically, the NRC recommended the addition of what is known as a “hardened vent,” which is a separate vent pipe designed to withstand higher loads during an accident such as a station blackout (a complete loss of power) and routed to an elevated point outside the reactor building.
Recognizing that plant designs do differ in some respects, the NRC recommended that each plant operator conduct a plant-specific evaluation and design and install an appropriate hardened vent capability. In response to this letter, operators of U.S. nuclear power plants with Mark I containment systems followed this recommendation and designed and implemented hardened vent systems. Operators in Japan, including TEPCO, did the same. While general requirements for a vent system were established by a BWR (boiling water reactor) owners group which GE helps facilitate, each individual plant owner designed and installed the hardened vent to meet its specific design criteria.
The purpose of the emergency venting system in the Mark I containment design is to prevent damage to the primary containment caused by a rise in internal pressure, which may occur in the event of an accident. Although we continue to learn more about the events at Fukushima, it appears that when the loss of backup electrical power disabled the reactors’ core cooling systems, the buildup of steam and gases inside the primary containment caused an increase in pressure. In such a scenario, the hardened vent is designed to provide a path for releasing the steam and/or gases directly into the atmosphere, thereby relieving the pressure, and discharging the steam and gases outside the reactor building. In an accident scenario involving an increase in primary containment pressure, the steam and gases inside the primary containment may contain fission products. Thus a hardened vent system usually incorporates a set of isolation valves designed to prevent an unintended release of fission products into the environment. In order to relieve the pressure, these valves must be activated by the plant operator either electrically or manually.
We do not have sufficient information to know how the venting system installed at Fukushima performed in the days and weeks following the earthquake and tsunami, or why TEPCO may have experienced difficulties with the venting system. This is one aspect of the ongoing fact-finding and analysis being conducted by public and private experts. Engineers from GE Hitachi Nuclear Energy continue to work with TEPCO, the plant operator, to understand the chain of events that occurred at these reactors. Although the reactors at Fukushima Daiichi originally were designed by GE or built under a license of GE technology, GE did not design or build the hardened vent system at Fukushima Daiichi or, for that matter, at any other nuclear power plant—whether in Japan, the United States, or elsewhere.
Hardened Vents in U.S. Plants
Reactors in the United States with containments similar to the Fukushima Daiichi plants in Japan include features that allow operators to vent gases from the primary containment in an emergency. These boiling water reactors use specially designed vents with emergency valves used to open and close these vents, and feature multiple backup operational capabilities. These include:
- remote electrical operation from the plant control room;
- backup methods to control the valves in the vent; and
- in some cases, valves or other methods of operating the vent that are located outside the reactor building, enabling plant operators to control them manually, if necessary.
Under NRC regulation in the U.S., reactor operators in the control room have full authority to vent gases from the primary containment immediately, if needed. Licensed control room managers do not need additional authorization from either their company’s management or government regulators to open vents when they determine this is needed to prevent damage to the primary containment system. Reactor operators also spend considerable time and resources training for possible contingencies, including emergency events. This training takes place in a replica of the control room at each nuclear energy facility and incorporates containment venting scenarios. Plant operators must continually undergo requalification training in venting and other emergency procedures, and all of these emergency procedures are reviewed periodically as part of emergency response drills.
GE Hitachi Nuclear Energy agrees with the statements already made by TEPCO and others that there will no doubt be lessons learned as we all come to better understand what happened at Fukushima Daiichi. Like our industry peers and the customers we serve, GE Hitachi Nuclear Energy is committed to participating in the learning process and supporting the continued safe operation of nuclear power plants.
