A speech I gave at Toastmasters recently. This was Speech 7, ‘Research your speech’, and my goal was to explain the basics of how nuclear power plants work to a lay audience, using the Fukushima Dai-Ichi nuclear power plant as the prime example. Also the word of the day was’perturbed’. :-)
Note: My aim here was not to accurately describe nuclear power or detail the complex events that occurred at multiple reactors over two weeks – more to give a simplified overview of how the plant works and the kind of challenges the engineers faced.
Chair, fellow toastmasters and guests.
Two weeks ago we were perturbed to hear of the terrible earthquake and tsunami which devastated Japan. In the aftermath, one of the main problems was the breakdown of cooling systems at the Fukushima Dai-ichi Nuclear Power Plant, which threatened to release radioactive material, making the disaster even worse.
What happened at Fukushima?
To explain this question we first need to understand how a nuclear power plant works. Then we will look at what happened during the earthquake, and why the problems at the plant occurred.
Fukushima Dai-Ichi, which means Fukushima Number One, is one of the biggest nuclear power plants in the world with 6 reactors producing 4.7 GW of electricity. It was built in the early 1970s using a design called the Boiling Water Reactor.
How does the power plant work?
Basically, Fukushima is pretty similar to other kinds of power plants, like our Huntly coal power station. There is a heat source which is used to boil water into steam. The expansion of the steam powers turbines to produce the electricity.
After the turbine, you’ve got all this steam which needs to be turned back into water. This happens in the condenser, which is basically like a giant radiator. It is usually cooled by another water source; at the Fukushima power plant, it was cooled by seawater.
After being cooled by the condenser, the water can be run through the boiler again. You can see that this is a closed loop, so it doesn’t waste huge quantities of water.
The main difference between the Huntly coal power station and a nuclear power plant is that instead of coal, the nuclear power plant uses nuclear fission as the heat source. Each of the six reactors contains around 50 tons of enriched uranium.
Uranium nuclear fission starts with a stray neutron, which is a tiny particle. If the neutron gets absorbed by a uranium atom, that atom becomes unstable and splits to produce two smaller atoms. This splitting releases a lot of energy. It also releases more neutrons, which can cause the same thing to happen to neighbouring uranium atoms.
In a nuclear explosion, there is a cascading chain reaction which splits all of the atoms in a few milliseconds, causing a nuclear explosion.
In a nuclear reactor you want these reactions to occur at a steady rate, producing an even amount of heat over time. The way you do this is by having the uranium spread out inside the reactor, and by controlling the neutrons.
So you can see this is the inside of the nuclear reactor. The uranium is in fuel rods, which are several metres long. They are separated into a grid, and between the fuel rods there are control rods, made of graphite, which absorbs neutrons. You can slide the control rods in or out of the reactor to control the fission of the U-235.
If you want to shut it down, you slide in the control rods, the neutrons will be absorbed, and the fission will slow down, and ultimately come to a halt.
So that’s an overview of how a nuclear reactor works. But what happens when disaster strikes?
Japan, like New Zealand, is a geologically active country, and the plant was designed to resist both earthquakes and tsunamis.
Unfortunately, the 9.0 earthquake which occurred on March 11 was an extremely powerful event. It exceeded the earthquake design strength by 100%. Despite this, the plant wasn’t particularly damaged by the earthquake itself.
Within minutes, the reactors were shut down, with the control rods being fully inserted.
About ten minutes after the earthquake a 12 metre tsunami struck the plant. The plant is surrounded by sea walls designed to resist a tsunami – but the walls were designed with a 6 metre tsunami in mind.
When the tsunami struck, it did not damage the reactor buildings directly.
What it did damage was almost all of the associated infrastructure surrounding the reactor building.
Whole buildings were washed away or inundated with saltwater and debris. The electricity lines which bring power to the plant from outside were destroyed. The diesel generators which operate the pumps were washed away. And the pumps themselves were affected too.
As I said, the reactor cores have been shut down. Unfortunately, shutting down a nuclear reactor isn’t like turning off a light switch. The reactor still produces around 6% of peak heat. This called decay heat and it will gradually stop over time.
Now 6% doesn’t sound like much. But these are massive, industrial power plants. The decay heat was 140 MW, and it takes a lot of cooling water to remove this heat from the reactor. The reactor is designed to operate with water running through it all the time, taking away the heat. Usually, pumps take care of this with no problem, even in an emergency when plant power is lost.
Without any water running through it, this heat is going to build up inside the reactor.
The engineers at the plant were able to get some pumps running to push water through the reactor, but that water wasn’t being cooled by the ocean water. So the water in the primary loop became very hot, 100 degrees. At that point, its not doing actually any useful cooling. So the reactor core continues to build up heat.
Like anything else, the fuel rods have a melting point. The reactor got so hot that the fuel rods started to melt. This can change the distribution of the uranium within the reactor, making the situation worse.
As the heat continued to rise, the pressure inside the reactor was increasing. The reactor normally operates like a pressure cooker, at about 75 times normal atmospheric pressure. Its built pretty strong to contain that pressure, but at some point it can’t take any more. To avoid a blowout which would damage the reactor vessel, the workers started to vent the steam, to relieve the pressure. Along with the steam and some mildly radioactive material, hydrogen was released – one of the byproducts of fission. Hydrogen gas is explosive, and at some point the hydrogen gas ignited and blew the roof off the reactor building. The reactor itself wasn’t damaged, but the loss of the roof means that the vented steam was carrying the radioactive material directly into the atmosphere.
Meanwhile, the engineers were working to restore a power connection to the plant and get more water flowing through the reactor. They were eventually able to get power connected to the plant by rebuilding 1 km of power lines. They then started pumping seawater directly through the reactor to cool it down.
They were able to prevent a meltdown, which would cause damage to the reactor and possibly release a greater amount of radioactive material into the atmosphere.
Now keep in mind that these reactors were built in the 1970s, before modern computers. Compared to modern reactor designs, the reactors are ancient, creaking hulks.
With almost all of their cooling infrastructure destroyed, plant workers were able to ensure that the reactors did not melt down and prevented a major radioactive disaster. In the face of this remarkably powerful and destructive event, they did a remarkable job.