Top nuclear scientists advise that the reactors at Fukushima damaged by a tsunami and quake cannot explode. Situation is contained.
Dr Josef Oehmen and a team of faculty and staff have set up an information page at MIT's Department of Nuclear Science and Engineering (NSE). For those concerned about the dangers at the Japanese nuclear disaster should visit the newly launched MIT NSE Nuclear Information Hub.
Earlier an initial public statement via a press release (edited by R Allan) advised the public of the following:
“Up front, the situation is serious, but under control. There was and will *not* be any significant release of radioactivity. By "significant" I mean a level of radiation of more than what you would receive on, say, a long distance flight, or by drinking a glass of beer that comes from an areas with high levels of natural background radiation. “
(Hat tip: Peter Dun).
What happened at Fukushima
The earthquake that hit Japan was 5 times more powerful than the worst earthquake the nuclear power plant was built for. (The Richter scale works logarithmically; the difference between the 8.9 quake and an 8.2 quake the plants were built for is not 8.9-8.2= 0.7. It is 5-fold.) So, the first hooray for Japanese engineering... everything held up. When the earthquake hit the nuclear reactors all went into automatic shutdown. Within seconds after the control rods had been inserted into the core and nuclear chain reaction of the uranium stopped. Now, the cooling system had to carry away the residual heat. The residual heat load is about 3% of the heat load under normal operating conditions.
The earthquake destroyed the external power supply of the nuclear reactor. Then the tsunami came much bigger than people had expected when building the power plant. The tsunami took out all sets of backup diesel generators.
When designing a nuclear power plant, engineers follow a philosophy called "Defense of Depth". That means you build everything to withstand the worst catastrophe you can imagine. Then you design the plant in such a way that it can still handle one system failure (that you thought could never happen) after the other. A tsunami taking out all backup power in one swift strike is such a scenario. The last line of defense is putting everything into the third containment which will keep everything, whatever the mess (control rods in or out, core molten or not) inside the reactor.
When the diesel generators were gone, the reactor operators switched to emergency battery power. The batteries were designed as one of the backups to the backups, to provide power for cooling the core for 8 hours. And they did. Within the 8 hours, another power source had to be found and connected to the power plant. The power grid was down due to the earthquake. The diesel generators were destroyed by the tsunami. Mobile diesel generators were trucked in. This is where things started to go seriously wrong. The external power generators could not be connected to the power plant (the plugs did not fit). After the batteries ran out, the residual heat could not be carried away any more.
At this point the plant operators begin to follow emergency procedures for a "loss-of-cooling event". This is the next step along the "Depth of Defense" path. The power to the cooling system should never have failed completely, but it did, so they "retreat" to the next line of defense. All of this, however shocking it seems to us, is part of the day-to-day training you go through as an operator, right through to managing a core meltdown.
It was at this stage that people started to talk about core meltdown. If cooling cannot be restore the core will eventually melt... after hours or days. The last line of defense, the core catcher and third containment, will come into play.
But the goal at this stage was to give the engineers time to fix the cooling systems by managing the heating in the core and keeping the first containment (the Zircaloy tubes containing the nuclear fuel) and second containment (our pressure cooker) intact and operational for as long as possible. Because cooling the core is such a big deal, the reactor has a number of cooling systems, each in multiple versions (the reactor water cleanup system, the decay heat removal, the reactor core isolating cooling, the standby liquid cooling system, and the emergency core cooling system). Which one failed, and when, is not clear at this time. So imagine our pressure cooker on the stove, heat on low, but on. The operators use whatever cooling system capacity they have to get rid of as much heat as possible, but the pressure keeps building up.
To maintain integrity of the pressure cooker (the second containment) pressure has to be released from time to time. Because the ability to do that in an emergency is so important, the reactor has 11 pressure release valves. The operators vent steam from time to time to control the pressure. The temperature at this stage was about 550°C. This is when the reports about radiation leakage starting coming in. Venting the steam releases radiation but it is not dangerous. The radioactive nitrogen and noble gases are no threat to human health as they decay in seconds.
During venting an explosion took place outside of the third containment (our "last line of defense") and inside the reactor building. (Remember, the reactor building is not intended to keep radioactivity in... it is to keep weather out.) It is not yet clear what happened, but this is the likely scenario.
The operators decided to vent the steam from the pressure vessel-- not directly into the environment, but into the space between the third containment and the reactor building (to give the radioactivity in the steam more time to subside). At the high temperature the core had reached, water molecules "disassociate" into oxygen and hydrogen - an explosive mixture. And it did explode, outside the third containment, and damaging the reactor building around it. It was that sort of explosion that caused the Chernobyl disaster, because it happened inside the pressure vessel which was badly designed and not managed properly by the operators. This was never a risk at Fukushima. The problem of hydrogen-oxygen formation is one of the biggies when you design a power plant (if you are not Soviet, that is), so the reactor is built and operated in a way it cannot happen inside the containment. It happened outside. It was not intended, but was OK because it did not pose a risk to the containment.
Steam was vented and the pressure was now under control. But if you keep boiling your pot, the water level will keep falling.
At the start, he core is covered by several metres of water to allow time to pass (hours, days) before the core gets exposed. Once the rods start to be exposed at the top, the exposed parts will reach the critical temperature of 2200 °C after about 45 minutes. This is when the first containment, the Zircaloy tube, fails. And this started to happen. The cooling could not be restored before there was some damage to the casing of some of the fuel rods. The nuclear material itself was still intact, but the surrounding Zircaloy shell had started melting. What happened next is that some of the byproducts of Uranium decay - radioactive Cesium and Iodine - started to mix with the steam. The big problem, Uranium, was still under control, because theUuranium Oxide rods were good until 3000 °C.
It is confirmed that a very small amount of Cesium and Iodine was measured in the steam released into the atmosphere. The operators knew that the first containment on one or more of the rods was about to give. This was the "go signal" for plan B.
Plan A had been to restore one of the regular cooling systems to cool the core. Why Plan A failed is unclear. One plausible explanation is that the tsunami also took away, or polluted, all the clean water needed for the regular cooling systems. The cooling water is very clean and demineralized (like distilled water). Pure water does not get activated much, so stays practically radioactive-free. Dirt or salt in the water will absorb the neutrons quicker, becoming more radioactive. This has no effect whatsoever on the core - it does not care what it is cooled by. But it makes life more difficult for the operators when they have to deal with activated (i.e. slightly radioactive) water. But Plan A failed. Cooling systems were down or clean water was unavailable. Plan B came into effect. This is what it looks like happened.
In order to prevent a core meltdown, the operators started to use sea water to cool the core. The plant is safe now and will stay safe. Japan is looking at an INES Level 4 Accident: Nuclear accident with local consequences. That is bad for the company that owns the plant, but not for anyone else. Some radiation was released when the pressure vessel was vented. All radioactive isotopes from the activated steam have gone (decayed). A very small amount of Cesium was released, as well as Iodine. If you were sitting on top of the plant’s chimney when they were venting, you should probably give up smoking in order to return to your former life expectancy. The Cesium and Iodine isotopes were carried out to the sea and will never be seen again.
There was some limited damage to the first containment. That means that some amounts of radioactive Cesium and Iodine were released into the cooling water, but no Uranium or other nasty stuff. (Uranium Oxide does not "dissolve" in the water.) There are facilities for treating the cooling water inside the third containment. The radioactive Cesium and Iodine will be removed and stored as radioactive waste in terminal storage. The seawater used as cooling water will be activated to some degree. Because the control rods are fully inserted, the Uranium chain reaction is not happening. That means the "main" nuclear reaction is not happening, thus not contributing to the activation.
The intermediate radioactive materials (Cesium and Iodine) are also almost gone at this stage, because the Uranium decay stopped a long time ago. This further reduces the activation. The bottom line is that there will be some low level of activation of the seawater, which will also be removed by the treatment facilities. The seawater will then be replaced over time with "normal" cooling water. The reactor core will be dismantled and transported to a processing facility, just like during a regular fuel change. Fuel rods and the entire plant will be checked for potential damage. This will take about 4-5 years. The safety systems on all Japanese plants will be upgraded to withstand a 9.0 earthquake and tsunami.
I believe the most significant problem will be a prolonged power shortage. About half of Japan's nuclear reactors will probably have to be inspected, reducing the nation's power generating capacity by 15%.