It is inaccurate to say “The plant suffered major damage from the magnitude 9.0 earthquake and tsunami that hit Japan on March 11, 2011.” All Light Water Reactors in Japan survived the earthquake either undamaged or with minor damage (covered in Wikipedia articles), and all damage at other sites was easily repaired. All operating reactors in Japan had begun or completed shutdown between the time the earthquake was detected and when it reached the reactors. All Light Water Reactors throughout Japan shut down properly.
No reactors had more than minor damage from the tsunami itself. All that was important that was damaged by the tsunami at Fukushima Daiichi, was the backup diesel generators. (Not all industrial sites fared as well. Except for Fukushima-Daiichi reactors, all other nuclear power plants in Japan fared much better than some other power plants, such as natural gas plant fires.)
If the diesel backup generators had been protected, or if replacement generators had been flown in (from Russia? China? Hawaii? Australia?), there would have been no loss of coolant accident. If the diesel generators worked, there would not have been any of the damage that followed from the LOCA. If one of the reactors had been restarted at minimal power, enough to generate a little electricity, there would have been no loss of coolant accident.
The Fukushima-Daiichi nuclear reactor loss of coolant, hydrogen explosion, core meltdown, reactor vessel breach, and radioactive water flowing into the ocean was clearly one of the worst industrial accidents — yet the entire problem was easily preventable.
For every part of that accident, if simple safety precautions had not been left out, that would have prevented the entire accident!
The Onagawa reactor site was closer to the center of the earthquake, so it got hit harder, and it after the tsunami was fully functional. The management company had such a good relationship with the community, had listened to their concerns so well, and took care of the safety issues the community brought up so thoroughly, that when the tsunami warning sounded, the people in the nearest town went to the reactor site for safety, where the cafeteria was already set up as a shelter with cots, bedding, food, water, first aid.
TEPCO caused the entire Fukushima-Daiichi accident, through mundane cost cutting and not taking basic safety precautions.
What Was the Actual Source of the Disaster?
You will easily understand how the disaster was Not caused by the design of the nuclear reactor. It wasn’t even from the design or construction of these specific reactors on this site. And of course it was not caused by some vague “nuclear power going to destroy all life on earth” nonsense.
The problem was lack of basic safety measures. Safety to prevent the backup generators getting flooded, is one example.
We don’t stop using automobiles because some drunken idiot might spill gasoline on himself and light a cigarette. We don’t stop using automobiles because a turn on a mountain road can only be taken at 20 mph but stupid kids keep taking it at 50 mph followed by “look ma, I’m flying”. Maybe we build a very sturdy wall on that mountain road turn. Maybe we teach everyone in the area how dangerous that curve is and let the idiots kill themselves, and then cry at their funerals.
We should have oversight at all industrial sites for safety, and we should use the safest type of nuclear reactors (LWR is good but not the best). But a good “culture of safety” at nuclear reactors and all other industrial sites is able to overcome good-but-not-great designs; a bad safety culture will thwart the best designs.
Pressing a company to fix problems that aren’t real, doesn’t help. Pressing a company to fix actual problems does help.
Are the fire extinguishers properly charged? Are people coming to work drunk? When a small accident happened, did they get to the source of the problem and make it less likely to recur; or did they put duct tape on it and a fresh coat of paint? Did required maintenance get done, and done properly?
Did the Earthquake or Tsunami Destroy the Reactors?
There are warning signs, carved in stone, throughout the area around the Fukushima-Daiichi site. They say basically “tsunami danger, do not build below this level”. There was a huge tsunami a few hundred years ago, so the signs were put up and the warning signs are still displayed.
TEPCO was explicitly warned of huge tsunamis possible. Other nuclear power operators in the area built adequate sea walls; TEPCO lowered the natural sea wall to make construction easier, and then didn’t build adequate sea wall. That isn’t some complex nuclear reactor design problem, but stupid management ignoring basic safety.
Procedures during earthquakes, which are frequent in Japan, say to shut down all nuclear reactors as soon as the earthquake sensor network detects one large enough to cause damage. The “speed of earthquake” is much slower than the speed of light (signals via radio and via electric wires); there was enough time for the reactors to be shutting down or fully shut down, before the earthquake hit.
None of the Fukushima-Daichi reactors (or any other reactors throughout Japan) were damaged by the earthquake, even though it was “the most powerful earthquake ever recorded to have hit Japan, and the fourth most powerful earthquake in the world since modern record-keeping began in 1900”.
Some of the other nuclear reactors in Japan had minor damage, easily repaired. I have read the reports of one reactor requiring operator actions to keep the reactor safe; it was never in any danger of a loss of coolant accident like at Fukushima-Daiichi.
People in California, not knowing that CA has different types of earthquake (horizontal slipping) in softer soil, not understanding that CA earthquakes don’t cause as much damage to buildings as the Japanese earthquakes (one tectonic plate diving under another), think CA nuclear reactors are going to “destroy the world”. LWR in Japan survived an earthquake 100 times as powerful as what hit the San Francisco or Los Angeles areas recently. The LWR design, including the reactor vessel, the cooling system, and the steam containment building, was adequate for even such a massive earthquake.
What Did the Tsunami Ruin?
The main damage at Fukushima-Daiichi from the tsunami was to the diesel generators that provided electricity to the cooling system if grid power wasn’t available and the reactors were shut down.
Other reactor sites throughout Japan had diesel backup generators inland and high, to protect them from flooding. This site had diesel backup generators in the basement, where they were ruined by the tsunami.
The other reactor operators in the area, including the Onagawa reactor site, had sea walls adequate to stop the tsunami. TEPCO had reduced the natural sea wall to make construction easier, and not even replaced that. TEPCO had received multiple warnings about the tsunami risk.
Similarly, while other reactors in the area had the diesel generators inland and above the possible flood levels, TEPCO had the generators in the basement, despite being specifically warned of the risks.
What Were The Actual Sources of the Loss of Coolant Accident, Hydrogen Explosion, and Meltdown?
The Light Water Reactor is cooled by water. The water has to be circulating to remove heat.
With the reactors shut down and not generating electricity, and the backup diesel generators ruined, there was no electricity to pump the water through the cooling system.
Though the reactors were all properly shut down (no more fission generating intense heat), the fission products were still decaying and generating heat. Also, the temperature in the fuel pellets in a Light Water Reactor is always extremely hot while the reactor is operating and for a while after.
Eventually the zirconium cladding of the fuel rods got hot enough to oxidize (taking oxygen from water, releasing hydrogen and creating still more heat). When steam pressure broke pipes (or if any pipes had cracked in the earthquake), hydrogen was released, gathering in the steam containment building.
The hydrogen could have been vented (there is equipment in the building for filtering and venting gasses), but the “don’t release radioactive gas” rule that makes sense during everyday operation was still being followed, though venting the hydrogen would have prevented the hydrogen explosions that blew apart the steam containment buildings, releasing all radioactive material in the air inside the buildings.
Then the fuel rods melted, allowing the fuel to get more dense at the bottom of the reactor vessel, heating up enough to melt through the reactor vessel. The fuel is now in the concrete floor of the reactor building. The concrete is too thick for the fuel to melt through, especially since the concrete makes the fuel less dense, and the spread-out fuel will cool faster than dense fuel (just like a hot pot of tea stays hot much longer than a pot of tea spilled onto the floor, even if the pot is a heat conductor).
Fission doesn’t happen except in the right geometry, you need enough fuel and neutrons hitting the fuel; in the fuel rods there is little more than enough neutrons to maintain fission. In the bottom of the reactor vessel there would not be enough neutrons for fission. In the concrete there is definitely not enough at the density required.
What Problems Are Affecting Fukushima Now?
Now the biggest problem at Fukushima is the contaminated water. There is water that was used for cooling the reactors, far more than if the circulation systems had been used. There is water leaking from the flooded buildings. There is ground water entering the site; when the site was built they removed part of a hill, but they didn’t divert ground water away from the site; that water also has to be stored and decontaminated.
Contaminated water is leaking into the ocean. There is concern that the spent fuel cooling pond water could leak into the ocean. TEPCO is currently installing ways to seal up the ground so less water will get into the ocean.
TEPCO could have diverted ground water away from the site, during construction. TEPCO could have installed materials to chemically trap each of the fission products in an emergency (or ways to rapidly have them deployed when needed).
The fuel rods melted and breached the reactor vessel. That also needs to be cleaned up. However, it isn’t as urgent as stopping the flow of radioactive water into the ocean. The fuel and fission products that aren’t dissolved in water, aren’t going anywhere.
There are spent fuel rods in cooling ponds. There is concern that another major earthquake may make the cooling ponds leak, both putting radioactive water into the ocean, and removing the cooling from the still-hot fuel rods. These rods should be moved to dry cask storage as soon as possible; dry cask storage is very safe for many decades. Most of the radiation from the fuel rods is fission products; the fission products that are the most radioactive and the most biologically hazardous have already had a few half-lives decay. As long as there is cooling, there isn’t a huge health risk, and this must be cleaned up.
Design Flaw or Basic Safety Precautions?
TEPCO didn’t take basic safety precautions. If TEPCO had taken these basic precautions, the Fukushima-Daiichi reactors would likely be undamaged, like all the other reactors in Japan. TEPCO didn’t keep the reactors in nearly the necessary conditions.
But the LWR design used at Fukushima-Daiichi was adequate for all the other sites hit by the earthquake, even those closer to the center of the earthquake and therefore hit harder; and was adequate for all other sites hit by the tsunami but with the diesel generators not flooded.
How Well Would Molten Salt Reactors Fare at the Fukushima Site in That Earthquake?
Molten Salt Reactors have better inherent safety than LWR designed safety (e.g. coolant that can’t boil away). They have a much simpler design, that would have fared even better than LWR.
The cooling system for MSR requires no emergency power. Power is required to keep the reactor operating, not to shut it down. In emergency, or normal maintenance, cutting power to a freezer, (specifically keeping salt frozen solid in a section of pipe), leads quickly to the frozen salt melting and the fuel salt draining into storage tanks. Fission is not possible in these tanks, the tanks can handle the hottest the fuel salt can possibly get, the tanks are designed for cooling to air.
The salt is chemically bound to the fuel and can’t evaporate or otherwise get away from the fuel; it will rapidly cool without need for electricity or water. Most of the fission products (the elements with high radioactivity) are also chemically bound to the salt; when the salt solidifies the fuel and fission products are trapped.
(In LWR, the coolant can escape and the fuel can get hotter and melt the fuel rods, becoming more dense and hotter still, and the reactor vessel is not designed to handle the higher temperatures. The reactor vessel is designed to withstand the pressure of super-heated water, but not the temperature of un-cooled fuel pellets.)
If an earthquake crimps the MSR fuel drain pipe, the fuel salt would remain in the reactor, and the fuel would continue to fission, but still temperatures wouldn’t get hot enough to damage the reactor vessel, since Molten Salt Reactors are very temperature-stable, and since the reactor materials can handle much higher temperatures than the fuel can possibly get.
Similarly, if the reactor vessel gets broken, the fuel salt would spill onto the building floor, designed to both withstand the temperature and salt, and to drain to storage tanks.
Since all Molten Salt Reactors operate at atmospheric pressure, there is no way for the fuel salt to travel into the air or ground water.
Most MSR designs would include chemical separation of fission products from the fuel. The fission products can be stored until no longer a radioactive risk (83% by mass need under 10 years storage). The risk of each individual storage container being damaged is low, and would release little radiation (remember, even gamma radiation is stopped completely by a few meters of concrete or packed earth). The fission products or the separation chemicals (e.g. fluorine) can have safety containment like used in other industries.
The entire reactor building can be made waterproof, and 10 meters underground, to minimize the effects of earthquake, tsunami, hurricane, tornado, forest fire, bombs, or Godzilla. Most of the problems at Fukushima today are from the leaking water. MSR, even if flooded, would not have radioactive material leaking into the water (any type of salt used in MSR wouldn’t dissolve in water).
No complex equipment can be made with zero risk of accident. But the risks of environmental damage or pollution or radiation release from Molten Salt Reactors are very low.
Some designs of Molten Salt Reactors can be deployed in standard ship/rail/truck shipping containers, to disaster sites, to provide electricity and desalinate water.