In 1980, not a moment too soon, GE issued three important modifications for the Mark I containment structures currently in use. The “ram’s head” terminals on the steam pipes into the torus, which could exert a rocket-like thrust when steam rushed through them, were to be unbolted, thrown away, and replaced with spargers that would distribute and release steam in small doses under the water. Deflectors were to be welded into the insides of each torus, to prevent water waves from developing in the wet well under steaming conditions, and the support irons underneath the torus were to be bolstered. All plant owners executed these modifications, including TEPCO for the five reactors having Mark I containments at Fukushima I.[262]
This was hardly the end of the matter, and over the next few years there were several studies, simulations, and scale-model tests of the mercilessly persecuted Mark I by Lawrence Livermore Laboratories, the University of California Berkeley, and the Nuclear Regulatory Commission. Of particular interest was how the dry well would perform with 787,200 gallons of water sloshing back and forth in it under earthquake conditions.[263] By this time, GE had introduced its radically improved design, the Mark III containment, and the Mark I was supported as a thoroughly vetted, field-modified, and improved legacy system.
All General Electric BWRs were equipped with several reactor safety systems, designed to prevent fuel meltdowns, radiation release to the environment, and general damage to the system in the event of the worst accident that could possibly happen. This hypothetical event, named the “design basis accident,” was planned assuming that the operating staff had lost control of the power plant and that there were multiple equipment failures. Safety systems were designed to begin operating automatically, with no help from the staff. Any such system could be deactivated by an operator, but if there were no operators present and making decisions, the safety systems would sense the condition of the power plant using electronic instrumentation and digital logic, and thus were capable of operating as designed without human intervention.
Unit 1 at Fukushima, the oldest BWR, was equipped with an isolation condenser. In any emergency shutdown (scram), the turbine shuts down and the steam line to the reactor is shut off immediately. This action prevents any radioactive debris from possible fuel leakage from contaminating the turbine, but it also isolates the reactor vessel from its normal cooling loop. Even in full shutdown, a reactor generates significant heat for several days, so something must take the place of the cool water that returns from the steam condenser underneath the turbine.[264] The isolation condenser was designed for this purpose, operating as an emergency alternative to the primary cooling loop without needing any external power.
The condenser is located in the reactor building, on the refueling floor, above the reactor vessel and the dry well. A pipe runs from the top of the reactor to a small condenser in a tank of water. The cool-water return pipe from the condenser connects to the bottom of the reactor. As the water in the tank absorbs heat from the reactor, it boils off and leaves the building through a pipe in the wall, blowing off into the air. There is enough water kept in the tank to cool the reactor for three days, and there is an external connection for a fire truck to refill it. Water circulates through the condenser loop by gravity. The hot water rises from the reactor top into the condenser, and the heavier cooled water is pushed by gravity into the bottom of the reactor. A remote-controlled valve in the cold leg is used to turn the system on and off. The isolation condenser is simple and seemingly foolproof.[265]
In addition, all of the reactors at Fukushima that were in service were equipped with high-pressure coolant injection systems (HPCI) for emergency use. The HPCI (pronounced “hip-see,” with the accent on the first syllable) is designed to inject great quantities of water into the reactor while it is maintaining normal operating pressure in shutdown mode, assuming that there is not a major steam pipe broken. This system does not rely on external electrical power to run the high-pressure water pump. It is turned by its own, dedicated steam turbine, which is connected to the steam pipe on top of the reactor, exhausting into the wet well. It spins up 10 seconds after a scram, and delivers cooling water taken from the pool in the torus at the rate of 3,003 gallons per minute.[266] For it to work, nine valves must be open.
Units 2, 3, 4, and 5 were BWR/4s, and were nearly twice as powerful as Unit 1. An early BWR/4 was rated at 2,381 megawatts thermal, whereas Unit 1 was rated at 1,380 megawatts thermal. These reactors dispensed with the isolation condenser for emergency use, and instead used the more aggressive reactor isolation cooling system (RCIC, pronounced “rick-see”). Its operation is similar to the HPCI, using a steam-driven pump to circulate water through the reactor vessel when the normal cooling loop is shut off, and it can even compensate for coolant leaking from broken pipes. Another nine remote-control valves must be open for it to operate, and its use must be monitored and controlled, else it will overfill the reactor and send water down its own steam pipe. It takes water from a dedicated water tank, which is also used to maintain the water supply in the torus “wet well.”
If the pressure in the reactor vessel reaches the danger level, 1,056.1 pounds per square inch, a steam-relief valve opens and blows down through a pipe into the torus, where the steam and gas are released underwater and are calmed down.[267] The combination of dry and wet well containment is large enough to reduce the high pressure originating in the reactor vessel, but if the pressure in the torus should go beyond the design pressure of 62.4 pounds per square inch, a valve must open or the torus will explode. Under this condition of extreme emergency, the steam and gas are sent up the ventilation stack and into the environment, possibly containing radioactive fission products. This is a last-ditch measure, meant to keep from causing a major break in the containment structure which would allow uncontrollable leakage of the entire reactor contents outside the reactor building. To make it up the stack, the pressurized gas and steam must break a rupture disc in the pipe, calibrated to fail only at desperately high pressure.
The engineers at GE tried to think of everything that could possibly happen, and a system or a workaround was designed for problems that seemed highly unlikely. Units 2 through 6 at Fukushima were even equipped with residual-heat removal systems using four electrically driven pumps to cool down the reactors using seawater. There was one common weakness, however, for all the safety systems: they all depended on electricity.
Each valve in the complex maze of piping required electricity to open or close it.[268] If the plant scrams in an emergency, then it stops providing electrical power to itself. In this condition, it switches to external power off the grid or to a cross-wired connection to the reactor next door. If no power is available on the power-plant site or from the area outside the plant, then each reactor has two emergency diesel-powered generators that come on automatically. Either generator is capable of handling the electrical needs of the entire plant, in case one breaks down or will not start. If the backup generators will not start, then the last resort is a room filled with lead-acid storage batteries, kept charged up at all times. They will supply direct-current power to the control room for eight hours, which is plenty of time to open the necessary valves and start the emergency cooling systems, which will then run on their own without any electricity.[269] The control-room lighting, the system-monitoring instruments, and the valve actuators are built to run on the DC current from the batteries under this extreme emergency. Condensate or coolant pumps will not run on the batteries.
262
All the reactors at Fukushima I were BWR/4s with Mark I containments, except the 6th and last reactor installed. It was a BWR/5 with a Mark II containment. The Mark II was a complete redesign, having a lot of concrete backing up the steel structure. Reactor No. 6 came online on October 24, 1979. Reactors 3 and 5 were built by Toshiba under license from GE, and Reactor 4 was built by Hitachi with a similar agreement.
263
The report from the initial study, “Sloshing of Water in Torus Suppression-Pool of Boiling Water Reactors Under Earthquake Ground Motions” (LBL-7984), was released in August 1978. It arrived at no negative conclusions regarding the behavior of the wet well in an earthquake, but demonstrated a good agreement between the mathematical finite elements model and a 1/60 scale model of the torus given a shake equivalent to the El Centro Earthquake of 1940.
264
In fact, the water returning from the condenser is too cool to be put back in the reactor vessel. A typical GE BWR uses five stages of pre-heater in the return leg of the primary cooling loop. Heat for these pre-heaters comes from “extraction steam,” or steam directly out of the first (high-pressure) stage of the three-stage steam turbine. The extraction steam is then fed back into the condenser and cooled with the rest of the steam from the turbine. Newer BWRs have a simplified system using electrically driven heaters.
265
The same scheme, called “thermo siphon,” was used to cool the engine of the Model T Ford automobile without the use of a motor-driven water pump. Reactor No. 1 had two isolation condensers, in case one failed. Each water tank held 28,002 gallons of water and was capable of handling 116 tons of steam per hour.
266
The number given, 3,003 gpm, is only for Reactor No. 1. No. 2 and No. 3 pump at the rate of 4,249 gpm.
267
That describes the first steam-relief valve. There are eight in a BWR/3 and 16 in a BWR/4, with progressively larger blow-off pressures. The last one opens at 1,130.1 psig.
268
The larger valves are opened and closed by air pressure supplied by an electrically driven air compressor. For air-operated valves to be manipulated in emergency conditions, compressed-air bottles are provided, but the air valve is still opened or closed remotely by an electric signal. It is possible to open up the RCIC system manually, but it is not easy. If there is already radiation leakage in the reactor building, then the time limit for people working in the area to open a valve is strictly limited by the permissible dose and the dose-rate in the building.
269
It says eight hours in the battery advertisements and in the plant specifications, but performance may vary. Running a reactor on storage batteries can be as disappointing as a laptop computer in an airport waiting area when your flight has been delayed. It will power-down due to low battery just as your iPod dies.