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Update on Fukushima Reactor

Update at 11pm EST Saturday 3/12/11:

On Saturday March 12 at 3:36 pm local time (1:36 am EST) an explosion occurred in the Unit 1 reactor building at the Fukushima Dai-ichi nuclear plant. (Original reports suggested that the explosion took place in an adjacent turbine building, but we no longer believe this is the case.)

The explosion was very likely a hydrogen explosion. Hydrogen apparently collected somewhere in the reactor building outside of the primary containment (see diagram below). The primary containment consists of the drywell and the wetwell. The top section of the reactor building, known as the refueling bay, has walls of sheet metal, in contrast to the concrete walls of the lower part of the building. The pressure caused by a large enough explosion anywhere in the reactor building would have caused the sides of the refueling bay to blow out since that is the weakest part of the structure and is not designed to withstand high pressures.

            Figure adapted from BWR Systems

The hydrogen was likely produced by the hot fuel. All signs are consistent with the fact that some fuel damage has occurred at Fukushima Unit 1. Last night (U.S. time) the plant owner, TEPCO, reported that the water level had dropped below the top of the fuel by nearly six feet. This means that roughly half of each fuel rod was exposed. The fuel rods are clad in zirconium, and a few minutes after the water level dropped below the top of the fuel, the zirconium would have become hot enough to react with the surrounding steam and produce hydrogen.

The reactor core is in the reactor vessel, or pressure vessel, which is surrounded by a steel containment vessel. The steel containment vessel is surrounded by a reinforced concrete shell. The explosion took place outside of this shell. It is not clear whether the concrete shell was damaged in the explosion, but the steel containment vessel was reportedly not damaged.

The control room and many of the control and power cables for the emergency equipment used to cool the reactor core are located outside the primary containment, and the extent to which the explosion impaired these vital functions is not known at this time.

Radioactive releases and iodine tablets

Once the water level in the reactor core drops to the point where the fuel is exposed, the zirconium cladding would begin to erode and after about an hour, this would release some radioactive material—primarily noble gases, iodine-131 and cesium-137. (During normal operation, this material accumulates in the gap between the fuel and the cladding.) Some of this material could have been released by the controlled venting, which could explain the cesium detected at the plant boundary.

In addition, the primary containment in this type of reactor typically has a leak rate of about 1% of its volume per day. The secondary containment (the walls of the reactor building) is important since it keeps any leaked radioactive gas from escaping into the environment. The secondary containment is kept at a negative pressure with respect to the outside so that air inside does not leak out. The air in the building is then sent through filters to remove the radiation before it is released through the stack. With the walls blown off the top of the reactor building, this radioactive gas would instead be released directly into the air.

Thus, contrary to some news reports, the detection of cesium outside the reactor does not necessarily indicate that the primary containment has been breached.

Iodine-131 is one of the most radioactive isotopes released in a nuclear accident. It has a half-life of 8 days, meaning half of it will have decayed after 8 days, and half of that in another 8 days, etc. Therefore, it is of greatest concern in the days and weeks following an accident. It is also volatile so will spread easily. In the human body, iodine is taken up by the thyroid, and becomes concentrated there, where it can lead to thyroid cancer in later life. Children who are exposed to iodine-131 are more likely than adults to get cancer later in life. To guard against the absorption of iodione-131, people can proactively take potassium iodine pills so the thyroid becomes saturated with non-radioactive iodine and is not able to absorb any iodine-131

Cesium-137 is another radioactive isotope that has been released. It has a half-life of about 30 years, so will take more than a century to decay by a significant amount. Living organisms treat cesium-137 as if it was potassium, and it becomes part of the fluid electrolytes and is eventually excreted. Cesium-137 is passed up the food chain. It can cause many different types of cancer

What next?

The cooling systems for the Unit 1 reactor have not been operating and, as the core heats up, the water surrounding the fuel has evaporated to the point where the fuel becomes exposed to the air. Unless there is a way to replace the water the fuel will continue to heat up.

To attempt to cool the reactor, TEPCO has been pumping sea water into the reactor vessel. Since this is very corrosive and will seriously damage the reactor, this is an option of last resort and indicates that they do not expect to get the cooling systems back online.

Reports note that boric acid is being added with the sea water. Boric acid is a soluable form of boron, which is very good at absorbing neutrons. By adding this to the water around the fuel rods, it would capture neutrons that could otherwise cause additional atoms to fission. This is being added to the reactor to make sure it does not become critical again, which might happen in two ways: (1) fuel rod damage that results in fuel rod segments dropping to the bottom of the reactor vessel, where they could form a critical mass, or (2) withdrawals of the control rods caused by malfunctions of the hydraulic control units that move the control rods in and out of the core.

Recent reports state TEPCO has succeeded in filling the reactor vessel with water, which would mean the fuel rods are no longer exposed to air. But some form of cooling will still be required.

March 11, 2011

Containment at Fukushima

Update at 6pm EST Friday 3/11/11:

The Japanese Nuclear and Industrial Safety Agency is now saying the containment pressure at Unit 1—not Unit 2, whose core cooling was said to have failed—has risen to about double its normal value.

The Tokyo Electric Power Company (TEPCO) has announced it will “implement measures to reduce the pressure of the reactor containment vessel for those units that cannot confirm certain level of water injection by the Reactor Core Isolation Cooling System, in order to fully secure safety.” It is not clear if this refers just to Unit 1, or to the other two affected units as well.

The increase in containment pressure resulted from the loss of alternating-current (AC) power to the reactors, which stopped the containment cooling system. There are large water-cooled air conditioning units inside containment. Motor-driven pumps send cool water to the units. Motor-driven fans blow air inside the containment across the metal tubes containing the cool water. But without AC power, the pumps and fans don’t work and can’t provide cooling. The heat radiating off the hot reactor vessel (over 500F) and the hot piping heats up the air in the containment building very rapidly, which causes an increase in pressure.

The rising pressure reduces the ability of the containment to absorb the energy released from a pipe rupture, should one occur. The volume of air in the containment building and its wall thickness are designed to contain a specified level of energy being dumped into containment. If the pressure gets too high, then an energy release like a broken pipe, should it occur, could over-pressurize the containment and cause it to fail. So emergency procedures call for venting air from the containment to reduce the pressure if it gets too high.

If the containment structure was weakened by the earthquake, then what pressure it could withstand is not known.

The reactors have a containment ventilation system that can be used to vent air from the containment building. In this situation, the vented air would be routed through a high-efficiency particulate air (HEPA) filter, charcoal beds, and another HEPA filter to remove as much radioactivity as possible before being released from a very tall stack to dilute the flow as much as possible.

If there has been no appreciable reactor core damage, the air vented from containment will contain minute but detectable amounts of radiation. The filtration systems are designed to lower that radioactivity release by nearly a factor of 100.

The latest news is that evacuation around the plants is being expanded from a 3 km to a 10 km radius, which suggests the crisis isn’t over yet.

For the next update, click here.

Nuclear Crisis at Fukushima

As of 2:30 pm EST Friday 3/11/11:

The massive earthquake off the northeast coast of Japan has caused a potentially catastrophic situation at one of Japan’s nuclear power plants. The situation is still evolving, but here is a preliminary assessment based on the facts as we currently understand them.

The plant’s owner, Tokyo Electric Power Company (TEPCO), reported that at 2:46 p.m. local time (12:46 a.m. EST) “turbines and reactors of Tokyo Electric Power Company’s Fukushima Daiichi Nuclear Power Station Unit 1 … and Units 2 and 3 … automatically shut down due to the Miyagiken-oki Earthquake.”

These reactors are 3 of the 6 operating reactors at the Fukushima I nuclear facility. All are boiling water reactors. Unit 1 has a rated output of 460 megawatts, and Units 2 and 3 each have a rated output of 784 megawatts.

TEPCO went on to state the shutdowns were caused by the loss of off-site power “due to malfunction of one out of two off-site power systems.” This loss of power triggered emergency diesel generators, which automatically started to provide backup power to the reactors.

However, at 3:41 p.m. local time (1:46 a.m. EST), the emergency diesel generators shut down “due to malfunction, resulting in the complete loss of alternating current for all three units,” according to TEPCO. The failure of the diesel generators was most likely due to the arrival of the tsunami, which caused flooding in the area. The earthquake was centered 240 kilometers from Japan, and it would have taken the tsunami approximately an hour to reach the Japanese islands.

This power failure resulted in one of the most serious conditions that can affect a nuclear plant—a “station blackout”—during which off-site power and on-site emergency alternating current (AC) power is lost. Nuclear plants generally need AC power to operate the motors, valves and instruments that control the systems that provide cooling water to the radioactive core. If all AC power is lost, the options to cool the core are limited.

The boiling water reactors at Fukushima are protected by a Reactor Core Isolation Cooling (RCIC) system, which can operate without AC power because it is steam-driven and therefore does not require electric pumps. However, it does require DC power from batteries for its valves and controls to function.

If battery power is depleted before AC power is restored, however, the RCIC will stop supplying water to the core and the water level in the reactor core could drop. If it drops far enough, the core would overheat and the fuel would become damaged. Ultimately, a “meltdown” could occur: The core could become so hot that it forms a molten mass that melts through the steel reactor vessel. This would release a large amount of radioactivity from the vessel into the containment building that surrounds the vessel.

The containment building’s purpose is to keep radioactivity from being released into the environment. A meltdown would build up pressure in the containment building. At this point we do not know if the earthquake damaged the containment building enough to undermine its ability to contain the pressure and allow radioactivity to leak out.

According to technical documents translated by Aileen Mioko Smith of Green Action in Japan, if the coolant level dropped to the top of the active fuel rods in the core, damage to the core would begin about 40 minutes later, and damage to the reactor vessel would occur 90 minutes after that.

Concern about a serious accident is high enough that while TEPCO is trying to restore cooling the government has evacuated a 3-km (2-mile) radius area around the reactor.

Bloomberg News reported that the battery life for the RCIC system is eight hours. This means that the batteries would have been depleted before 10 a.m. EST today. It is unclear if this report is accurate, since it suggests that several hours have elapsed without any core cooling. Bloomberg also reported that Japan had secured six backup batteries and planned to transport them to the site, possibly by military helicopter. It is unclear how long this operation would take.

There also have been news reports that Fukushima Unit 2 has lost its core cooling, suggesting its RCIC stopped working, but that the situation “has been stabilized,” although it is not publicly known what the situation is. TEPCO reportedly plans to release steam from the reactor to reduce the pressure, which had risen 50% higher than normal. This venting will release some radioactivity.

More information about the cooling issue is available in this New York Times story.

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