Removing Heat from a Reactor in Shutdown
Boiling water reactors of the Fukushima vintage were constructed with multiple, overlapping alternatives for removing decay heat during shutdown situations.
By Bruce Mrowca
Shutting down a nuclear power plant is not as simple as flipping a switch. Following a plant shutdown, decay heat, a byproduct of nuclear fission, initially produces about 6 percent of the steady state power. That heat decreases over time at a rate dependent on the fuel type, reactor history, and power levels experienced during operation. Decay heat removal has been long recognized as an important plant safety function and is typically mitigated with many layers of defense and redundant systems.
Those systems are usually of no major concern to anyone outside the power plant. But in the weeks since March 11, when a combination of an earthquake and a tsunami shut down the boiling water reactors at Fukushima Daiichi Nuclear Power Plant in Japan, the means by which engineers at a shutdown reactor remove decay heat has taken on worldwide importance.
The Fukushima plant features boiling water reactors, known as BWRs, which operate at lower pressures than the other main light-water reactor technology, the pressurized water reactor. Because of the lower pressure, when the demineralized water flows through the reactor core, the absorbed heat makes the water boil. In normal operation, after water droplets are removed, the resulting steam is fed to a turbine that powers a generator.
Plants like those at Fukushima Daiichi are designed with the recognition that decay heat will need to be removed for an extended period following a plant shutdown. Their designs contain both normal and backup systems to prevent the overheating of the nuclear fuel. For decay heat removal to be successful, enough cooling water must be maintained to cover the core and the energy must be removed, through the use of heat exchangers or condensers, or by bleeding steam.
For BWRs, heat is normally removed through steam lines to the turbine’s main condenser and condensate is returned to the reactor through the main feedwater system. However, this process is dependent on off-site power sources and is lost when off-site power is lost.
In some BWRs, such as Fukushima’s Unit 1, an isolation condenser (IC) provides heat removal if the main condenser is unavailable or isolated from the reactor. ICs condense steam from the core and return condensate directly to the core, rejecting the heat to an external pool of water through natural circulation. During isolation condenser operation, the water on the shell side of the condenser boils and needs to be replenished. Makeup water is typically provided from on-site water storage tanks through pumps powered by either off-site power or on-site emergency diesel generators if off-site power is unavailable. Other sources of water—from, say, the fire protection system—could also be used. However, their use will likely require manual actions.
Newer designs, such as those at Fukushima Units 2, 3, and 4, employ a steam turbine-driven reactor core isolation cooling (RCIC) system to control primary system inventory during abnormal transient events, including the loss of off-site power and conditions known as “station blackout” where both on-site and off-site power is lost. Energy from the reactor may be removed through the safety relief valves to the suppression chamber, a toroidal-shaped chamber located within the reactor building that is an integral part of the Mark 1 containments, the kind used by Fukushima Units 1 through 4.
The suppression chamber, often referred to as the “torus,” contains a large volume of water and a vent system connecting the upper portion of the primary containment which contains the reactor, referred to as the drywell, to the space within the suppression chamber containing the water, the wetwell. Steam from the reactor vessel drives the RCIC turbine, and the exhaust is directed to the suppression chamber.
Water for the RCIC system comes initially from a condensate storage tank, with the suppression pool providing an alternative source.
This mechanism for decay heat removal does not require on-site ac power to initiate. But the RCIC typically requires dc control power from batteries, and unless they are recharged from an ac power source, the batteries typically will fail in eight hours or less. The RCIC may also degrade or fail because of high suppression pool temperature or pressure due to the lack of suppression pool cooling.
All current generation BWRs have an emergency core cooling system (ECCS) in order to maintain coolant inventory and decay heat removal under design basis accident conditions. (A design basis accident is one against which a nuclear power plant has been designed; the challenge to the fuel and the release of radioactive material do not exceed authorized limits.) The ECCS employed at Fukushima includes the following subsystems: high-pressure core injection, core spray, low-pressure coolant injection/containment cooling, and the automatic depressurization subsystem.
The high-pressure core injection (HPCI) subsystem is similar to RCIC, in that it can provide makeup to the reactor vessel from either the condensate storage tank or the suppression chamber. It is also a single turbine-driven pump system and, as with RCIC, may degrade and/or fail due to the loss of dc power or as a result of high suppression pool temperature or pressure.
The core spray subsystem sprays cooling water over the top of the core, providing a sink for the heat radiated from the fuel rods during loss-of-coolant events; the energy is transferred to the suppression chamber. The core spray subsystem requires ac to power its pumps.
The low-pressure coolant injection/containment cooling subsystem provides the ultimate heat sink by transferring the core’s energy through heat exchangers to secondary cooling water systems and ultimately to the sea.
The automatic depressurization system provides a means to depressurize the reactor. It is used as a backup to the HPCI subsystem to depressurize the reactor through automatic opening of the relief valves in order to vent steam to the suppression chamber and to allow core spray or low-pressure coolant injection to provide adequate core cooling.
With the exception of the HPCI, these systems contain multiple redundant trains. For the HPCI, redundancy is provided by the diverse functions of the automatic depressurization and core spray. The events at Fukushima did not begin as a loss-of-coolant accident, but the coolant inventory was eventually lost as a result of lifting of the reactor safety release valves. Had ac power been available, these systems could have been employed.
The low-pressure coolant injection subsystem discussed above is an integral part of an important heat removal system that is also used in response to a normal reactor shutdown. This system is known as either shutdown cooling or residual heat removal and can be configured in several operating modes to remove decay heat. For example, suppression pool cooling could be used as an indirect decay heat removal system, removing heat from the reactor vessel via the SRVs and the suppression pool.
Ultimately, the operation of the shutdown cooling or residual heat removal systems is required to cool the reactor or suppression pool and therefore to maintain long-term decay heat removal. This system, by either name, contains multiple redundant trains and requires on-site ac or emergency backup to power its pumps.
A final option that is typically available at BWRs of the Fukushima vintage is controlling containment pressure by venting. Venting keeps the containment pressure below its pressure limit, maintains the function of equipment inside the containment, and helps in the removal of decay heat, but it does not provide inventory to replenish the water that is being lost through the venting process. (Maintaining this inventory requires one of the systems discussed above.)
For plants in the United States with a Mark 1 containment, a “hardened vent” has been installed to enable the venting of the suppression chamber to a location outside the reactor building, therefore reducing the potential buildup of hydrogen within the reactor building. The requirement to install these vents stems from actions by the U.S. Nuclear Regulatory Commission and was implemented on a plant-specific basis in order to account for possible unique design differences.
Boiling water reactors of the Fukushima vintage were constructed with multiple, overlapping alternatives for removing decay heat during shutdown situations. There is, however, a degree of dependency on ac or emergency backup power for all the alternatives. This dependency is not surprising: Although some design and operation requirements have been established to address multiple failures, such as the U.S. requirements for station blackout, the failure of multiple emergency diesels is considered to be beyond the design basis of the plant and exceeds the design practice of accounting for the worst-case single failure.
The March 11 earthquake and subsequent tsunami knocked out off-site power to the Fukushima plant and disabled many, if not all, of the 13 on-site emergency diesel generators. The installed decay heat removal systems then failed, either directly due to equipment failure or indirectly due to the loss of ac and dc power. That left only the recovery of these systems or the use of portable equipment and non-standard inventory sources, such as seawater, as a means to remove the decay heat.
Up to now, assessments of the importance of decay heat removal have found the risk of its loss to be low due to the combination of the low likelihood of a postulated event that challenges decay heat removal and the large degree of redundancy of the available systems to mitigate these challenges. In addition, experience has shown that there is typically a high likelihood of an early recovery of off-site power.
It’s likely that the recent events at the Fukushima plant will be the focus of much discussion for years to come.
Bruce Mrowca is a senior vice president at Information Systems Laboratories, Inc., in Rockville, Md. He has over 25 years of experience in commercial nuclear power design, maintenance and operation, and specializes in the application of probabilistic risk assessment techniques to nuclear safety and security.
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