A Strategy to Prevent the Next Fukushima

Date: 2015-10-07; view: 722.

Background

There was a series of equipment failures, nuclear meltdowns and releases of radioactive materials at the Fukushima I Nuclear Power Plant, following the Tōhoku earthquake and tsunami on 11 March 2011. It is the largest nuclear disaster since the Chernobyl disaster of 1986 and only the second disaster (along with Chernobyl) to measure Level 7 on theInternational Nuclear Event Scale.

The plant comprised six separate boiling water reactors originally designed by General Electric (GE) and maintained by the Tokyo Electric Power Company (TEPCO). At the time of the earthquake, reactor 4 had been de-fueled and reactors 5 and 6 were in coldshutdown for planned maintenance.^[8] Immediately after the earthquake, the remaining reactors 1–3 shut down automatically and emergency generators came online to power electronics and coolant systems. However, the tsunami following the earthquake quickly flooded the low-lying rooms in which the emergency generators were housed. The flooded generators failed, cutting power to the critical pumps that must continuously circulate coolant water through a Generation II reactor for several days to keep it from melting down after shut down. After the pumps stopped, thereactors overheated due to the normal high radioactive decay heat produced in the first few days after nuclear reactor shutdown (smaller amounts of this heat normally continue to be released for years, but are not enough to cause fuel melting).

As workers struggled to cool and shut down the reactors, several hydrogen-airchemical explosions occurred. It is estimated that the hot zirconium fuel cladding-water reaction in each reactor produced 800 to 1000 kilograms of hydrogen gas, which was vented out of the reactor pressure vessel, and mixed with the ambient air, eventually reaching explosive concentration limits in units 1 and 3, and due to piping connections between units 3 and 4, unit 4 also filled with hydrogen, with the hydrogen-air explosions occurring at the top of each unit, that is in their upper secondary containment building.

There were no deaths caused by radiation exposure, while approximately 18,500 people died due to the earthquake and tsunami. Future cancer deaths from accumulated radiation exposures in the population living near Fukushima are predicted to be extremely low to none.

On 5 July 2012, the Japanese National Diet appointed The Fukushima Nuclear Accident Independent Investigation Commission (NAIIC) submitted its inquiry report to the Japanese Diet, while the government appointed Investigation Committee on the Accident at the Fukushima Nuclear Power Stations of Tokyo Electric Power Company submitted its final report to the Japanese government on 23 July 2012. Tepco admitted for the first time on October 12, 2012 that it had failed to take stronger measures to prevent disasters for fear of inviting lawsuits or protests against its nuclear plants.

Among the most striking elements of the catastrophe at Fukushima Daiichi nuclear reactors in Japan were the hydrogen explosions that destroyed the upper parts of some of the reactor buildings. The hydrogen was released by a metal called zirconium in the overheated core.

Since that accident researchers have been looking at a variety of ways to prevent a repetition. At the Electric Power Research Institute, a nonprofit utility consortium, scientists think they have zeroed in on one strategy: replacing some of the zirconium with a ceramic.

Zirconium is used not for its strength or for its resistance to heat or its price but because it is nearly transparent to neutrons, the subatomic particles that are released from the nucleus when an atom is split and go on to split other nuclei in a chain reaction.

Zirconium has always been known to release hydrogen when overheated, and that gas will burn or explode at a variety of concentrations, making it particularly troublesome. And under some circumstances, the fire cannot be extinguished with water.

The most prominent use of zirconium at nuclear reactors is in making the long metal tubes that hold the pellets of uranium fuel. In a boiling water reactor of the type that melted down at Fukushima Daiichi, each group of zirconium tubes, called a fuel bundle or assembly, sits inside a so-called channel, a tall metal box that is open at the top and bottom so that fuel can flow through.

Forty percent of the zirconium in the core is in those channels, said Christine King, the director of nuclear fuels and chemistry at the research institute. During an accident, she said, operators hope they can buy “coping time” to stave off the release of hydrogen from the channels.

That expression is widely used by nuclear scientists to refer to how long the reactor can cope with problems like a loss of electrical power, which led to the meltdown at Fukushima. “A few hours can make a big difference,'' she said.

The institute, with $800,000 in research funds from the Energy Department, is looking at the feasibility of using silicon carbide in place of the zirconium. That is the ceramic that nuclear engineers use for the “pebble bed” type of reactor: the ceramic wraps around the fuel so that it cannot get hot enough to melt, and the ceramic will not burn.

It would be easier to persuade the Nuclear Regulatory Commission to approve its use in the reactor's channels than in the fuel itself, they say.

Another potential use that could win early approval is using silicon carbide in the four-bladed control rod that is inserted between the channels to choke off the flow of neutrons when it is time to shut a reactor down.

Among the challenges is to manufacture the channels, which are about 10 feet long, in such a way so that they will not easily shatter or deform as they are heated, which could block the functioning of the control rods.

Beyond the Nuclear Regulatory Commission, scientists working toward a solution on replacing zirconium will need to convince reactor owners, who will be reluctant to introduce anything new that could go wrong.

In normal operation, the existing fuel, with the zirconium fuel and the zirconium channels, performs nearly perfectly, said Kurt Edsinger, director of nuclear fuels at the institute.

1. What triggered the Fukushima Daiichi meltdown?

2. Why did the workers of the plant fail to prevent the explosion?

3. Did the meltdown cause deaths in the adjacent territories? Are any fatal predictions?

4. What was the fatal role of zirconium in the meltdown?

5. What is “coping time” for the reactor? Why is this term widely used by scientists?

6. What substitution of zirconium is offered as zeroing in on the prevention of the meltdown?

7. How can silicon carbide be used in control rods?

8. What is the basic challenge in introducing the new material with reactors?

5. Watch the slide presentation “Man-made Disasters”. Work in two groups, A and B. Without looking up the presentation again, Group A chooses a man-made disaster and its representative puts on the blackboard its features and the representative of Group B puts down on the blackboard preventative measures to avoid it. The winner is the group which gives the most exhaustive list.

6. Look through the pictures of the dam accident in Sayanoshushenkaya Power Plant. Make a subsequent report about the event using the comments to the photos.

7. Watch the fragments “Toxic sludge in Hungary” and “Explosion of the derailed train carrying crude oil in Quebec”

(a) Fill the gaps in the following video scripts.