In the aftermath of last week’s devastating earthquake and tsunami, Japan is trying to prevent a nuclear crisis from escalating. James Acton, who visited a Japanese nuclear plant after a 2007 earthquake, details what happened and what needs to be done as Japan scrambles to avert further problems at damaged nuclear plants.
The reactors at Japan’s seaside Fukushima Daiichi nuclear power station are “boiling water reactors.” The idea behind them is simple. Energy produced via nuclear reactions in the fuel is used to turn water into steam. The steam then drives generators to produce electricity. When a reactor is turned “off” nuclear reactions essentially stop immediately, but the radioactivity in the reactor’s core still produces heat. This means that the reactor needs to be cooled even after it is shut down.
When the earthquake hit last Friday, the reactors automatically turned themselves off—as they were supposed to—by inserting the control rods into the reactors. The problem was that the reactors still needed to be cooled even after shutdown. The water pumps that keep the core cool rely on electricity. There were two external power lines running to the nuclear complex, but both of them were disrupted by the earthquake. As a backup, there were also diesel generators on site. The diesel generators then started as they were supposed to, but the tsunami flooded the generators and prevented them from producing electricity.
This was essentially the start of the crisis. From then on, the plant operators and Japanese safety authorities have been struggling to cool reactors with, it seems, limited electricity and water supplies. It appears as though there were some backup batteries onsite and the operators may have brought more in, but exactly how effective they were and how long they provided electricity is unclear.
If there is too much heating and no way to cool the reactor, then the core can start to melt. The word “meltdown” is frequently used, but in many ways this is an unhelpful word—there’s actually a spectrum of possible outcomes. Japanese authorities already suspect that there has been some melting of some of the fuel in the core of some of the reactors units, but there is a wide range of possibilities with the absolute worst-case scenario being that there is a massive meltdown of the entire core.
There was a very complicated chain of events leading to where we are now, but this is only partially understood at the moment. We do know, however, that the cooling of the reactor cores was inadequate and unit 1 was the first reactor to show serious signs of problems.
In the end, the operators decided to pump seawater directly into the core of the first reactor in order to try and keep it cool. As pumping water into the core will effectively destroy the reactor, this would only have been done if the reactor is expected to be lost anyway.
As this was happening, the cooling system seemed to be working for unit 3. But for unexplained reasons the cooling was cut off and ultimately the operators also decided to pump seawater into the third reactor. Unit 2 also was being pumped with sea water. The process—for reasons that are unclear—hit problems and fuel reactors became fully exposed on Monday. Water levels have now recovered slightly but not to where they should be.
As the reactors overheat a great deal of water is being quickly turned into steam and if the pressure becomes too high the steam needs to be let out or there is a risk of an explosion. The steam was first vented from the reactor vessel into the containment building that surrounds the reactor and, for reasons safety, the steam eventually needed to be vented into the environment.
Based on the information currently at hand, the amount of radioactivity in the steam is low. And if the situation doesn’t get worse, the venting of steam is unlikely to cause significant damage to people or the environment.
In the extreme conditions inside a malfunctioning reactor, steam can react with the metal that surrounds the fuel and produce hydrogen. Hydrogen is explosive. The good news, in one way, is that the safety systems worked, because the hydrogen was vented away from the containment building and surrounding structure. One safety system, however, did not get rid of the hydrogen as they were designed to do, possibly because they lacked power.
Eventually the hydrogen built up and there was an explosion at the first reactor on Saturday. Japanese authorities have said that the containment building itself was not damaged during the explosion. There was a second explosion on Monday when the containment building’s roof blew off at unit 3, reportedly from the same cause.
The reason that we don’t want the core of a nuclear reactor to start melting is the risk of highly radioactive products can then be transferred into the environment much more easily. A core melt, however, does not necessarily lead to large quantities of radiation reaching the environment. At the Three Mile Island accident in 1979 there was significant core melting, but a very small release of radiation. However, Japan is now in an unknown situation once the core starts to melt significantly and there is a high risk of large amounts of radioactivity leaking into the environment.
Although the Chernobyl accident in 1986 is often mentioned, a similar explosion of the reactor vessel in this case is very unlikely. So, the Chernobyl disaster is probably unhelpful for understanding Japan’s current crisis.
The situation at the moment is unclear with Japanese authorities doing everything they can to cool the reactor cores. There is a range of possible outcomes and it’s impossible to know exactly how long the authorities have to cool the reactors. The speed at which the core is melting—if indeed it is melting at the moment—depends on a number of factors from its temperature to how well the cooling systems are working. And it’s impossible to put numbers on this, but Japan could have anywhere from hours to days to bring the damaged reactors under control.
There is also another nearby nuclear complex, Fukushima Daini, where a state of emergency has been declared for three reactors. There is almost no information about what is going on there, but the hope is that this is just a precautionary state of emergency.