Japanese Nuclear Problems

Here’s an overview of the nuclear power crisis in Japan. History will probably overtake this post within a day, but I thought people might like a general picture of the problems.

The worst hit plant is Fukushima Dai-ichi, located about 180 miles north of Tokyo on the east coast of Japan’s main island of Honshu. It is a set of six boiling water reactors, meaning that the water heated by the nuclear reaction is not pressurized, so it boils. This is an important point, which I will get to later.

Three of the six reactors were operating at the time the earthquake hit. They all immediately shut down. That means that a number of rods were dropped into each reactor core, these rods absorbing radiation and stopping the reaction. The problem is that the cores of these reactors don’t immediately cool down, so the cooling water needs to circulate through them for another week. During a normal shutdown the cooling water pumps run on electricity from the utility grid. This being absent, the pumps are powered by diesel generators on site. In this case, the generators were swamped by the tsunami, so the pumps fell back on batteries, the problem being that the batteries can only power the pumps for 8 hours.

Adding to the problem, each reactor has a pool of water holding used fuel rods. These pools need to have water circulated through them to keep the spent rods cool, although to a lesser extent than the reactor cores. Some of these pools lack circulating water due to the multiple failures and are starting to boil off their water. The danger is that if the spent rods are exposed they could catch fire and spew radioactive particles into the air.

Already there have been releases of radiation. Units 1, 2, and 3 are all at risk, with the cores of all three having been exposed several times and partially melted. The pool on Unit #4 is at risk of exposure. Local residents have been evacuated, and even outside the evacuation zone people have been ordered to stay indoors.

Those explosions you may have seen on television were from hydrogen produced when steam reacted with the superheated casings of the nuclear fuel rods. The hydrogen vented out of the steel and concrete containment vessel and into the building around it. Usually any gas that gets vented into this building gets run through filters to remove radioactive particles and is then blown off, but the venting system was overwhelmed and the hydrogen built up and detonated. Now any radioactive gas that gets vented from those primary containment vessels goes directly into the atmosphere.

Japanese utility workers have been flooding the cores of reactors 1-3 with seawater in an attempt to keep the cores cool. Apparently they have also been mixing some boron containing compound into the water as well. Boron absorbs the neutrons that maintain a nuclear reaction, so this gives double duty, cooling and slowing the reaction at the same time.

There is presently a problem in the #2 reactor, in that the operators can't open the pressure relief valves. The pressure has built up to the point where they are unable to pump water into the core. Unless the valves open on their own, this can't end well.

Key concept: Flooding a boiling water reactor with seawater and boron is a permanent shut down. The Japanese authorities decided that the situation was bad enough to write off these reactors forever.

Second key concept: A small percentage of the fuel rods in reactor #3 are mixed oxide, or MOX, meaning that they contain plutonium as well as the usual uranium. This raises the probability of containment failure, because the plutonium helps to keep the reaction going and because it tends to create more gas when the core gets uncovered. It also poses a higher cancer risk than pure uranium.

Third key concept: These reactors are 40 years old, of a type called the General Electric Mark 1. There have always been questions about the probability of catastrophic failure with this design. This is, by the way, the same age and design as our own Entergy/Vermont Yankee plant in Vernon, as well as 22 other plants in the U.S.

A good site for ongoing information about the Fukushima plant, with excellent explanations of the technology, is at the Union of Concerned Scientists website.