Japan was dealt a double blow with the enormous earthquake causing secondary problems with their nuclear power plants. To clarify and explain some of the problems that the Japanese are occurring with the plants, here’s a brief explanation of the design and what could be going wrong.
The design of failing nuclear power plants in Japan are known as a Boiling Water Reactor or BWR. As the picture below shows, the reactor vessel serves as the boiler for the nuclear steam supply system. The steam is generated in the reactor vessel by the controlled fission of enriched uranium fuel which passes directly to the turbo-generator to generate electricity. The water then goes into a another vessel where it is cooled further before returning to the reactor vessel in a closed loop.
As you can see, the pump must be kept active for the cooling to continue, a critical component in the design. To assure that the pumps keep going, several redundant systems are built in. If the electricity fails, backup diesel generators are standing by to supply electricity. If the diesel generators fail, batteries are available to supply a short amount of pump run time. But the batteries are not meant for any extended period of pump operation, just for short power outages.
If anything can be learned about relying on technology for safety, Mother Nature has a way of dealing out situations that haven’t been planned for. The record magnitude earthquake, measuring 9.0 on the scale, has knocked out the power grids for an extended amount of time.
In addition, diesel generators were knocked out because of tsunami waters flooding the switching circuitry that was installed to switch over from main power to the diesel generators. The backup to the backup generators, the batteries, only give about 8 hours of operation, but the AC grid never was able to come back online because of it’s damage. So now the cooling pumps cannot operate.
The reactor core fission process can be shut down in a matter of seconds but it will remain superheated for a very long time. Without that pump cooling loop that is used to carry away the heat, the reactor core is so hot that it will actually begin to melt it’s surrounding structure. Another side effect is that the metal that clads the fuel rods, zirconium, reacts with the extreme heat producing hydrogen, a very explosive gas.
But the reactor designers have even planned for that scenario because a containment structure is built around the entire reactor structure itself. A protective shield that encapsulates the process. That’s the good news.
The bad news is that the Fukushima Daiichi reactors are a GE Mark I reactor containment vessel design.
Our own Nuclear Regulatory Commission did an analysis of the potential failure of the Mark I under accident conditions. They concluded in a 1985 report that Mark I failure within the first few hours following core melt would appear rather likely.
In 1986, Harold Denton, then the NRC’s top safety official, told an industry trade group that the “Mark I containment, especially being smaller with lower design pressure, in spite of the suppression pool, if you look at the WASH 1400 safety study, you’ll find something like a 90% probability of that containment failing.
To address this design concern, it was deemed necessary to have the capability to vent any high pressure buildup. As a result, a vent system was designed as another safety precaution. Called the “direct torus vent system” it was designed into all Mark I reactors.
Designed to be used as a last ditch operation to save the containment from rupturing, it’s operated from the control room of the plant. Although it’s a venting of radioactive high pressure steam, it’s considered a better option than allowing the containment vessel to rupture by explosion. But the venting releases high levels of radiation, particularly Cesium-137 and Iodine-121.
To complicate things even more, Unit 3 of the group of nuclear reactors was using something called MOX for fuel. This is the normal reactor fuel, Uranium-235, mixed with plutonium for the purpose of disposing of bomb grade weapons material. This gives the additional hazard of venting not only radioactive Cesium-137 and Iodine-121 but plutonium and other actinides, which are far more hazardous.
Japan is facing an unimaginable crisis from a multitude of fronts. The world needs to give any and all support that is asked for, not just for the earthquake victims but to avoid a catastrophe on a world wide scale.
Of the 104 nuclear plant operating in the United States, 23 of them are of the GE Mark I design.