Few events have resonated within the core of society recently like the earthquake and tsunami that hit the nation of Japan on March 11, 2011. While the destruction wrought by the earthquake itself and the tsunami has been more than substantial, the nuclear emergency at Fukushima Daiichi has been a major event that will likely remain in the consciousness of the world for quite some time. This past week, the Mines campus hosted one of its own, Dr. Jeffrey King of the Nuclear Science and Engineering Program, to speak about the history and effects of this cataclysm.
King emphasized the continuing nature of the event, “It is going to be years before we know many of these things for certain.”
The Fukushima site was the oldest reactor in Japan, older than those at Three Mile Island. The reactors themselves are diverse in design but are all boiling water reactors, or as King said, “Just a fancy way to heat water.” They all consist of a central reactor filled with water. Submerged in the water are bundles of rods filled with fuel pellets. The reactions within the fuel pellets heat the water, which boils, and the steam is used to turn turbines to actually generate the electricity.
At the time of the earthquake, only three of the six reactors at the Fukushima Daiichi site were online. Although the earthquake was around ten times stronger than what the reactors were designed to handle, most of the reactors immediately shut down, as they were programmed to do in such an event.
When a reactor shuts down, it does not entirely stop the reactions within the fuel pellets. The pellets are made up of radioactive materials, which continue to decay as they normally would. Therefore, heat is still being produced in the reactor. There are emergency diesel generators to help keep the reactor cool, which all properly started.
When the tsunami hit the plant, it caused the most damage. The electric disconnects, circuit breakers and fuses were flooded, breaking the connection between the plant and the electrical grid; the spent fuel was either contaminated or taken out to sea; and the diesel generators were damaged or swept away. Losing the generators and connection to the grid was important because, as King noted, “You need power to keep things cool.”
All three affected reactors had emergency passive cooldown methods. However, those emergency methods were not intended to work without other, electrically-powered backup systems. Reactor one had a simple condenser chamber, while reactors two and three each had a toroidal loop which served the same purpose. Both systems were designed to achieve the same result, that is, decrease the pressure in the reactor chamber by condensing the steam back into water and putting it back into the reactor.
If the water is not able to remove enough of the heat produced in the reactor, then the heat stays in the pellets, which can result in the pellets melting, called a core meltdown.
All of the rods are clad in a zirconium-based solid solution which is invisible to neutrons and designed to keep the radioactive pellets from interacting with the water. “We know that we exposed at least two-thirds of the core because the cladding failed,” stated King.
At this time, the parties involved were faced with a tough decision, with the water boiling and many of the materials within the reactor boiling as well, the pressure inside was mounting. Were the reactor to explode, all of the radioactive particles would escape quickly and create a true national emergency. On the other hand, if they vented the reactor, some radioactive material would escape to the environment. “Better to release some than wait and release everything,” said King.
The process of venting the reactors did not go as smoothly as hoped, due to the high level of exposure of the core. The zirconium-based cladding reacted with the steam to produce zirconium dioxide and hydrogen. When this was vented, it created an explosion that some media sources took to be the explosion of the reactor itself. The main effect of this explosion was to blow off the service roof of the building – the containment system was not ruptured.
This process happened for reactors one and three. However, at reactor two it is speculated that, instead of the top blowing off, the torus may have had a slight break, allowing steam, carrying radioactive contaminants, to escape into the environment.
Due to the aid of many organizations, the site was able to be treated after a few days. The first goal of all involved was to cool down the reactors. Where media organizations cited the use of sea water as a rushed reaction of the cleanup crew, according to King, the idea of using sea water was in the plans if pure water could not be readily used. Finally, off-site power was restored and the reactors could resume normal cooling. “Unfortunately,” revealed King, “this is not where the story ends.”
Because of maintenance, the fuel for reactor four was in a spent fuel pool. Normally the fuel rods are stored in neutron-absorbing borated water and in such an orientation that they cannot maintain chain reactions. However, the earthquake may have changed the orientation of the fuel rods, increasing the rate of reactions. This caused the water in the pool to boil off, melting the cladding on the rods. Because the pool did not have as much of a containment vessel as the reactor, it caused many of the same problems as the first three facilities.
The fuel rods were damaged at sites one through four while the main containment remained intact at all sites except for two. Some fuel from reactor two leaked into the ocean, but the contamination was treated with colloidal silicon.
As for the state of the other reactors around the country, most are not affected and are in the process of going back online. The plants closest to the epicenter – Fukushima Daini, Onagawa, and Takai – are still in cold shutdown and there will be a delay until they are producing power again. At Fukushima Daiichi, it is possible that reactors five and six will go back online as they have a different reactor design and are much newer; reactors one through four, though, will be decommissioned.
With the reactors properly cooled and the immediate danger averted, officials have been able to react to the environmental effects. The radioactive material is being extensively monitored to ensure that it does not contaminate the food or water supply. Compared to Chernobyl, significantly more precautions are being taken, “We are not going to see [effects on the scale of Chernobyl] at Fukushima,” asserted King. In the long run, there are plans to repair the containment at reactor two and cover the reactors with filtered fabric covers, instead of cement. “When you cover everything in cement, it is hard to clean it up later,” said King.
Unlike Chernobyl, Fukushima has been carefully dealt with. For instance, where more than half of a million people were subjected to dangerous levels of radiation for Chernobyl, less than 30 workers have received that amount in Japan. From King’s perspective, this shows that the workers were not in panic mode and that the entire operation was not rushed. Fukushima has released only 1/10th of the amount of radioactive material that was released by the Chernobyl incident. “A lot of nearby towns are back to or are close to background radiation,” said King. In addition, few effects have been seen here in the United Sates. Some radioactive iodine has been found in California milk. It is worth emphasizing that someone would have to drink several thousand gallons of milk to get the same radiation as a normal plane flight.
Unfortunately, due to the event, the South Texas Project, an expansion of a nuclear plant in the US, has been delayed. The Tokyo Electric Power Company (TEPC), owner of the Fukushima Daiichi plant, had been looking into buying a 10% ownership stake in the project. Since the TEPC has been dealing with the Fukushima incident, and the chance of getting federal loans has decreased, the deal has been called ‘uncertain’ by analysts.