Radioactive wave: Will tsunami lead to meltdown?
Officials in protective gear check for signs of radiation on children who are from the evacuation area near the Fukushima Daini nuclear plant in Koriyama, Japan. (Source: Reuters)
The biggest nuclear crisis since Chernobyl has reignited doubts and fears about atomic energy. So what went wrong in Japan? Professor Andrew Sherry, Director of the Dalton Nuclear Institute at the University of Manchester, explains how a tsunami wreaked havoc.
What happened at the Fukushima No.1 nuclear plant when the earthquake and tsunami struck?
When the earthquake struck all of the reactors did exactly what they were designed to do. Control rods went into the reactor cores and they would have slowed the nuclear reaction process right down.
But the cores still have residual heat within them, which decays over time. The heat is drawn off by cooling water, circulated around the reactor by pumps.
They lost electrical power to those pumps. Back-up diesel generators kicked in as designed but then the tsunami took out those generators. Back-up battery power wasn’t sufficient.
So the prime cause was as simple as a couple of diesel generators being flooded?
I think that’s right. They had to connect a different power source. While they were doing that, the heat in the core was boiling the cooling water. As the water boiled, the pressure increased so they had to vent some of the resulting steam into the outer building.
Just like when we leave a kettle boiling too long, the water boiled away leaving the top of the core of Reactor 1 exposed. That core is made of zirconium-clad uranium oxide pellets. When exposed to steam zirconium reacts and produces hydrogen gas.
So when they vented the steam they also vented hydrogen; the source of the two dramatic explosions we saw affecting Reactors 1 and 3 [on March 12 and March 14].
Those explosions looked catastrophic. How much radiation did they release?
Imagine the reactor as a series of Russian dolls that fit into one another. Each is a barrier to the release of radioactivity. With these reactors there are five barriers.
First is the fuel pellet itself, which retains much of the radioactive products. Second is the zirconium alloy cladding that the pellets sit within. Third is the reactor pressure vessel. Fourth is the reinforced concrete and steel-lined containment structure, and fifth is the airtight building.
What we saw with those explosions was the fifth barrier blowing outwards so there was no breach of either the containment vessel or the reactor pressure vessel.
The only release of radiation was due to venting of steam, about 8 to 10 times more than average background radiation levels—normal in some parts of the world. I think we have seen some radioactive iodine and some caesium and strontium in very small doses.
"Chernobyl was an explosion from the core outwards. The worst-case scenario here is a molten core going downwards and contamination of the ground beneath the reactor structure." (Source: University of Manchester)
If that’s the case why did the government tell people in the area to stay indoors?
That followed the fire at Reactor 4 [March 15] which caused some radioactive release.
The second issue was another explosion, this time at Reactor 2 [March 15], which seems to be different to the explosions at Reactors 1 and 3. The timing of the explosion matched a pressure drop within a component that helps control pressure within the system.
This component takes out radiation products from within the steam as the steam condenses to water and so a leak from this structure would have released more radioactive steam than in the deliberate venting process.
How can people limit their exposure to radiation?
There are two things people should do: stop radioactive particles coming into contact with their skin and avoid ingesting radioactive particles. And so staying indoors is a major thing. The authorities may be handing out iodine tablets: radioactive iodine is major cause of thyroid cancer and non-radioactive iodine can block that.
How do these explosions affect the reactor core and the cooling system?
The priority is to maintain the core cooling and keep the water levels up. The real difficulty they have is injecting cooling seawater into the reactor while the pressure is so high due to boiling water. They can’t force the water in without relieving the pressure by venting [and risking another hydrogen explosion].
If they don’t cool the core it will heat up and the zirconium cladding could fail and lead to a melting within the core and then it will flow down into the lower basin of the reactor pressure vessel.
The ultimate concern is that the core could melt through the reactor pressure vessel into the containment structure and, with a release of pressure, then that gets very, very serious.
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Could there be another explosion, throwing nuclear material into the air like at Chernobyl?
This is a very different reactor system. My understanding is that is highly, highly unlikely to happen.
Chernobyl was an explosion from the core outwards. The worst-case scenario here is a molten core going downwards and contamination of the ground beneath the reactor structure.
The best-case scenario is they manage to complete the cooling of the core to a manageable level. Every day this goes on it should be getting better because this is decaying heat.
Are there better, more reliable cooling system designs?
There are other ways. This reactor was designed in the 1960s but modern Boiling Water Reactors have passive safety systems using gravity and natural convection to cool the core without requiring any electrical power at all.
These systems use natural convection—hot water rises cold water falls—that can create a flow of water through the core. There are back up systems of water reservoirs that are gravity-loaded so if there is a power cut valves automatically open and water flows down into the reactor.
Japan is very earthquake-prone so aren’t these reactors too vulnerable to be safe?
Here we have a 40-year-old reactor plant going through the largest earthquake on record in Japan. Initially it did what it was meant to do. The challenge has been managing the decaying heat from the core.
These reactors were designed to withstand earthquakes and tsunamis but this earthquake and tsunami was greater than the design standards. It withstood the earthquake and the earthquake wasn’t the issue: it was the tsunami taking out the diesel generators.
The biggest nuclear crisis since Chernobyl has reignited doubts and fears about atomic energy. So what went wrong in Japan? Professor Andrew Sherry, Director of the Dalton Nuclear Institute at the University of Manchester, explains how a tsunami wreaked havoc.
What happened at the Fukushima No.1 nuclear plant when the earthquake and tsunami struck?
When the earthquake struck all of the reactors did exactly what they were designed to do. Control rods went into the reactor cores and they would have slowed the nuclear reaction process right down.
But the cores still have residual heat within them, which decays over time. The heat is drawn off by cooling water, circulated around the reactor by pumps.
They lost electrical power to those pumps. Back-up diesel generators kicked in as designed but then the tsunami took out those generators. Back-up battery power wasn’t sufficient.
So the prime cause was as simple as a couple of diesel generators being flooded?
I think that’s right. They had to connect a different power source. While they were doing that, the heat in the core was boiling the cooling water. As the water boiled, the pressure increased so they had to vent some of the resulting steam into the outer building.
Just like when we leave a kettle boiling too long, the water boiled away leaving the top of the core of Reactor 1 exposed. That core is made of zirconium-clad uranium oxide pellets. When exposed to steam zirconium reacts and produces hydrogen gas.
So when they vented the steam they also vented hydrogen; the source of the two dramatic explosions we saw affecting Reactors 1 and 3 [on March 12 and March 14].
Those explosions looked catastrophic. How much radiation did they release?
Imagine the reactor as a series of Russian dolls that fit into one another. Each is a barrier to the release of radioactivity. With these reactors there are five barriers.
First is the fuel pellet itself, which retains much of the radioactive products. Second is the zirconium alloy cladding that the pellets sit within. Third is the reactor pressure vessel. Fourth is the reinforced concrete and steel-lined containment structure, and fifth is the airtight building.
What we saw with those explosions was the fifth barrier blowing outwards so there was no breach of either the containment vessel or the reactor pressure vessel.
The only release of radiation was due to venting of steam, about 8 to 10 times more than average background radiation levels—normal in some parts of the world. I think we have seen some radioactive iodine and some caesium and strontium in very small doses.
If that’s the case why did the government tell people in the area to stay indoors?
That followed the fire at Reactor 4 [March 15] which caused some radioactive release.
The second issue was another explosion, this time at Reactor 2 [March 15], which seems to be different to the explosions at Reactors 1 and 3. The timing of the explosion matched a pressure drop within a component that helps control pressure within the system.
This component takes out radiation products from within the steam as the steam condenses to water and so a leak from this structure would have released more radioactive steam than in the deliberate venting process.
How can people limit their exposure to radiation?
There are two things people should do: stop radioactive particles coming into contact with their skin and avoid ingesting radioactive particles. And so staying indoors is a major thing. The authorities may be handing out iodine tablets: radioactive iodine is major cause of thyroid cancer and non-radioactive iodine can block that.
How do these explosions affect the reactor core and the cooling system?
The priority is to maintain the core cooling and keep the water levels up. The real difficulty they have is injecting cooling seawater into the reactor while the pressure is so high due to boiling water. They can’t force the water in without relieving the pressure by venting [and risking another hydrogen explosion].
If they don’t cool the core it will heat up and the zirconium cladding could fail and lead to a melting within the core and then it will flow down into the lower basin of the reactor pressure vessel.
The ultimate concern is that the core could melt through the reactor pressure vessel into the containment structure and, with a release of pressure, then that gets very, very serious.
Pollution & EcocidesClick on the image to see gallery
more
Could there be another explosion, throwing nuclear material into the air like at Chernobyl?
This is a very different reactor system. My understanding is that is highly, highly unlikely to happen.
Chernobyl was an explosion from the core outwards. The worst-case scenario here is a molten core going downwards and contamination of the ground beneath the reactor structure.
The best-case scenario is they manage to complete the cooling of the core to a manageable level. Every day this goes on it should be getting better because this is decaying heat.
Are there better, more reliable cooling system designs?
There are other ways. This reactor was designed in the 1960s but modern Boiling Water Reactors have passive safety systems using gravity and natural convection to cool the core without requiring any electrical power at all.
These systems use natural convection—hot water rises cold water falls—that can create a flow of water through the core. There are back up systems of water reservoirs that are gravity-loaded so if there is a power cut valves automatically open and water flows down into the reactor.
Japan is very earthquake-prone so aren’t these reactors too vulnerable to be safe?
Here we have a 40-year-old reactor plant going through the largest earthquake on record in Japan. Initially it did what it was meant to do. The challenge has been managing the decaying heat from the core.
These reactors were designed to withstand earthquakes and tsunamis but this earthquake and tsunami was greater than the design standards. It withstood the earthquake and the earthquake wasn’t the issue: it was the tsunami taking out the diesel generators.
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