A Fukushima, what we feared - and that is already taking place - it's a meltdown accident, that is to say that the heart of central or spent fuel located in hot pools because they are no longer cooled and eventually melt. Yes. In Chernobyl, the accident involved a single reactor in Fukushima, there are six reactors potentially hazardous: three of whom were active during the earthquake and have been shutdown by the quake, and three were already shut for maintenance.
So potentially, the total radioactivity could be released from the center of Fukushima is more important than that of Chernobyl. But in reality, we do not know exactly what is called the inventory of nuclear material at the site of Fukushima, and it is very difficult to assess radioactive contamination that could occur in the worst case scenario.
According to the Institute for Radiological Protection and Nuclear Safety French, the total radioactive release that could be released into the environment in Fukushima could represent between 10% and 50% of waste generated at Chernobyl. No containment, at least on the old reactor, was designed to resist a merger of the heart.
It is a risk that was realized with the accident at Three Mile Island. If cooling of reactor cores is no longer possible - and that is what is happening in Fukushima - it is impossible to avoid overheating and melting hearts total, ie ie fuel (on the reactor 1, the fuel would have already melted at 70%).
The question is then whether the tanks of steel reactors can withstand the heat and keep the heart melted. At Three Mile Island, 45% of heart had melted, but the cooling circuits were restored quickly, and the heart was not out of the tank. Fukushima A, it is feared, given the enormous difficulties faced by the operator of cooling, it is impossible to prevent the molten heart - we're talking about Corium - pierces the tank.
In the worst scenario, if the molten heart pierces the tank, there is still a barrier of protection: this is called the dish, that is to say a concrete base which is part of the enclosure and containment measures several meters thick. If your heart melt reaches this layer of concrete, its heat can decrease, but the models show that the outcome depends on the composition of the concrete.
Ultimately, if the layer of concrete is also perforated, then all the radioactive material can reach the subsoil and contaminate the environment, ground water ... The difficulty is that radiation levels are now on the site such that human intervention becomes virtually impossible. This morning there was an attempt to sprinkle of the site using a helicopter, and this operation has been interrupted due to excessive radioactivity from the site.
So, unless deliberately sacrifice of men, we can not now consider this solution. The reactor shutdown does not pose a safety problem in themselves, a priori, since the fission reactions are interrupted. However, in the building of reactors, there are also so-called storage pools in which spent fuel - which have been previously burned in these reactors - are stored.
These spent fuels are also very hot and radiating, and we must continue to cool them permanently in a water bath. Apparently, the site operator has focused on cooling the three reactors in operation. In the reactor pool 4, he started to run out of water, the water evaporates, the water levels drop, and fuel cladding are exposed.
The risk is that these sheaths break or melt. They are made of a metal alloy containing zirconium and degradation, which liberates hydrogen on contact with oxygen from ambient air, can explode. This applies to the reactor pool 4, but also for Units 5 and 6, which also apparently contain pools of spent fuel.
And the risk is even higher than these pools are not protected by a containment building and that any release of radioactivity that can be emitted directly into the atmosphere. Indeed, if no solution is found to cool the reactors and spent fuel pools, one must expect the worst in the few coming days.
However, the site operator is considering new options: firefighters or the military should carry out a spray using vacuum trucks. The United States reportedly sent to Japan pumps that could be used by the operator. But it is true that so far no solutions have been tested successfully. This is apparently a bit the same type of intervention has been tried this morning with one or more military helicopters, but had to abandon because of too strong release of radioactivity.
In theory, there are ways that can determine in real time, inside the speaker, the state of the reactors. But the damage, which is not known precisely gravity, certainly make it very difficult today to have an accurate statement of the situation. However, the operator has given fairly accurate figures on the state of hearts on the first reactor, the level of merger would be 70% and the reactor 3, it would be 30%.
There would also be partial melting of the reactor 2. Very probably. It depends of course on the evolution in the next few days, but even in the case of massive releases of radioactivity, there will probably not as violent explosion at Chernobyl, the plume of radioactive materials will rise less high in the atmosphere, and very highly contaminated area will be smaller.
Radioactive particles are then dispersed course by winds and precipitated to the ground by the rains. But it is likely that the contaminated area will be much smaller, although the level of contamination is very high. There will certainly an area of \u200b\u200bno man's land around the site of Fukushima.
What extent is impossible to say today. As it stands, the Japanese authorities, but also the Institute for Radiological Protection and Nuclear Safety French, consider that the evacuation zone in a radius of 30 km is sufficient. If the situation deteriorates sharply, as is feared, there will, in an area that is now impossible to determine, a prohibited zone.
As for cancer, an evaluation is impossible today to give. One can simply recall that Chernobyl, besides the thirty people died by acute radiation just after the accident - I do not speak here of "liquidators" - the estimated number of expected deaths from cancer from a few thousand and tens of thousands.
But beyond the immediate vicinity of the site, despite a rise in the level of radioactivity recorded to Tokyo, the levels did not give rise to particular concern. But that situation could change dramatically if the operator does not find new ways of cooling and the process of merging cores and spent fuel, which would be inevitable happens.
The Chernobyl accident occurred in 1986, twenty-five years later, the site is still a forbidden zone. In terms of radiological consequences, the fact whether a MOX fuel, that is to say a mixture of uranium and plutonium, does not fundamentally change things. Anyway, during the fission reaction, it produces plutonium, spent fuel that are in storage pools also contain plutonium, the fuel has been whether to start MOX fuel, and they also contain, of course, extremely radiotoxic fission products.
In fact, systems that have been damaged by the earthquake and tsunami are the power systems of the plant. There is no certainty, but one can imagine that the earthquake destroyed the influx of power (high voltage pylons, etc.). Then the backup systems on site-specific, that is to say diesel engines seem to have temporarily taken over.
But they could not be used permanently for reasons that are poorly understood. Since it is a loss of power, there is no way to ensure a normal cooling facilities for the water system. Of course we can all imagine, provided that the remedy (... hydrogen bomb) is not worse than the disease.
In any event, the problem is not cool the containment, but to get to the heart to cool the reactors. That is why, from the beginning, the operator injects or trying to inject seawater (which, for a nuclear operator, is a heresy in normal times), or directly into the tank which surrounds the heart or in the containment building is located in the heart.
Until now, no. Tomorrow? Their experience may of course be useful, and Japan agreed earlier this week international aid. The nature of the accident is different, but fundamentally, the problems are the same. The point is that intervention on the site seems to become increasingly difficult.
I have already partially answered this question. In fact, the worst possible scenario on the site of Fukushima, that is to say, the total fusion of hearts and their migration into the environment, never happened. There have been partial melts hearts on reactors, but to my knowledge, they are never out of the tanks.
No one has reference and comparison to assess the consequences. The "China Syndrome", a term used to describe a scenario in which the heart of an American reactor, after being melted, drilled the tank and the concrete base, traverse the entire Earth to arrive in China, is science- fiction.
The Soviet Union sent over 600,000 Chernobyl "liquidators". May decide that the Japanese government? It seems unthinkable that he does not try at all costs to mitigate the catastrophe. But at what human cost? Unless an absolute deluge, it is not the rain that cools the plant. However, the effect of rain is to precipitate the ground radioactive particles escaping from the site and are dispersed by winds.
Much the same thing. In the case of spent fuel, the radioactivity will be released - and perhaps began to be released - directly into the air. If the heart melts and reaches the ground, the radioactivity spread in the basement, water, that is to say throughout the environment. Today, no.
For the rest, everything depends on the amount of radioactivity released, the wind direction, etc.. Remember just after Chernobyl, where, again, radioactive materials had been projected up to 3 km high, and where the engine had burned for twelve days, there has been, never to speak of the territories overseas but in mainland France and Western Europe more generally, that the benefits certainly significant, but reduced.
The issue of dose is always very difficult. As a benchmark, a dose of 100 mSv is the one from which, in France, one advocates taking iodine tablets (to prevent thyroid cancer). In Japan, from 50 mSv. What is certain is that if Tepco (the operator), the Japanese authorities, the army, or whatever, does not arrive within the next few hours to cool the facilities, thus bringing them to One way or another of water, then yes, there will be little hope.
Chat moderated by François Béguin 
So potentially, the total radioactivity could be released from the center of Fukushima is more important than that of Chernobyl. But in reality, we do not know exactly what is called the inventory of nuclear material at the site of Fukushima, and it is very difficult to assess radioactive contamination that could occur in the worst case scenario.
According to the Institute for Radiological Protection and Nuclear Safety French, the total radioactive release that could be released into the environment in Fukushima could represent between 10% and 50% of waste generated at Chernobyl. No containment, at least on the old reactor, was designed to resist a merger of the heart.
It is a risk that was realized with the accident at Three Mile Island. If cooling of reactor cores is no longer possible - and that is what is happening in Fukushima - it is impossible to avoid overheating and melting hearts total, ie ie fuel (on the reactor 1, the fuel would have already melted at 70%).
The question is then whether the tanks of steel reactors can withstand the heat and keep the heart melted. At Three Mile Island, 45% of heart had melted, but the cooling circuits were restored quickly, and the heart was not out of the tank. Fukushima A, it is feared, given the enormous difficulties faced by the operator of cooling, it is impossible to prevent the molten heart - we're talking about Corium - pierces the tank.
In the worst scenario, if the molten heart pierces the tank, there is still a barrier of protection: this is called the dish, that is to say a concrete base which is part of the enclosure and containment measures several meters thick. If your heart melt reaches this layer of concrete, its heat can decrease, but the models show that the outcome depends on the composition of the concrete.
Ultimately, if the layer of concrete is also perforated, then all the radioactive material can reach the subsoil and contaminate the environment, ground water ... The difficulty is that radiation levels are now on the site such that human intervention becomes virtually impossible. This morning there was an attempt to sprinkle of the site using a helicopter, and this operation has been interrupted due to excessive radioactivity from the site.
So, unless deliberately sacrifice of men, we can not now consider this solution. The reactor shutdown does not pose a safety problem in themselves, a priori, since the fission reactions are interrupted. However, in the building of reactors, there are also so-called storage pools in which spent fuel - which have been previously burned in these reactors - are stored.
These spent fuels are also very hot and radiating, and we must continue to cool them permanently in a water bath. Apparently, the site operator has focused on cooling the three reactors in operation. In the reactor pool 4, he started to run out of water, the water evaporates, the water levels drop, and fuel cladding are exposed.
The risk is that these sheaths break or melt. They are made of a metal alloy containing zirconium and degradation, which liberates hydrogen on contact with oxygen from ambient air, can explode. This applies to the reactor pool 4, but also for Units 5 and 6, which also apparently contain pools of spent fuel.
And the risk is even higher than these pools are not protected by a containment building and that any release of radioactivity that can be emitted directly into the atmosphere. Indeed, if no solution is found to cool the reactors and spent fuel pools, one must expect the worst in the few coming days.
However, the site operator is considering new options: firefighters or the military should carry out a spray using vacuum trucks. The United States reportedly sent to Japan pumps that could be used by the operator. But it is true that so far no solutions have been tested successfully. This is apparently a bit the same type of intervention has been tried this morning with one or more military helicopters, but had to abandon because of too strong release of radioactivity.
In theory, there are ways that can determine in real time, inside the speaker, the state of the reactors. But the damage, which is not known precisely gravity, certainly make it very difficult today to have an accurate statement of the situation. However, the operator has given fairly accurate figures on the state of hearts on the first reactor, the level of merger would be 70% and the reactor 3, it would be 30%.
There would also be partial melting of the reactor 2. Very probably. It depends of course on the evolution in the next few days, but even in the case of massive releases of radioactivity, there will probably not as violent explosion at Chernobyl, the plume of radioactive materials will rise less high in the atmosphere, and very highly contaminated area will be smaller.
Radioactive particles are then dispersed course by winds and precipitated to the ground by the rains. But it is likely that the contaminated area will be much smaller, although the level of contamination is very high. There will certainly an area of \u200b\u200bno man's land around the site of Fukushima.
What extent is impossible to say today. As it stands, the Japanese authorities, but also the Institute for Radiological Protection and Nuclear Safety French, consider that the evacuation zone in a radius of 30 km is sufficient. If the situation deteriorates sharply, as is feared, there will, in an area that is now impossible to determine, a prohibited zone.
As for cancer, an evaluation is impossible today to give. One can simply recall that Chernobyl, besides the thirty people died by acute radiation just after the accident - I do not speak here of "liquidators" - the estimated number of expected deaths from cancer from a few thousand and tens of thousands.
But beyond the immediate vicinity of the site, despite a rise in the level of radioactivity recorded to Tokyo, the levels did not give rise to particular concern. But that situation could change dramatically if the operator does not find new ways of cooling and the process of merging cores and spent fuel, which would be inevitable happens.
The Chernobyl accident occurred in 1986, twenty-five years later, the site is still a forbidden zone. In terms of radiological consequences, the fact whether a MOX fuel, that is to say a mixture of uranium and plutonium, does not fundamentally change things. Anyway, during the fission reaction, it produces plutonium, spent fuel that are in storage pools also contain plutonium, the fuel has been whether to start MOX fuel, and they also contain, of course, extremely radiotoxic fission products.
In fact, systems that have been damaged by the earthquake and tsunami are the power systems of the plant. There is no certainty, but one can imagine that the earthquake destroyed the influx of power (high voltage pylons, etc.). Then the backup systems on site-specific, that is to say diesel engines seem to have temporarily taken over.
But they could not be used permanently for reasons that are poorly understood. Since it is a loss of power, there is no way to ensure a normal cooling facilities for the water system. Of course we can all imagine, provided that the remedy (... hydrogen bomb) is not worse than the disease.
In any event, the problem is not cool the containment, but to get to the heart to cool the reactors. That is why, from the beginning, the operator injects or trying to inject seawater (which, for a nuclear operator, is a heresy in normal times), or directly into the tank which surrounds the heart or in the containment building is located in the heart.
Until now, no. Tomorrow? Their experience may of course be useful, and Japan agreed earlier this week international aid. The nature of the accident is different, but fundamentally, the problems are the same. The point is that intervention on the site seems to become increasingly difficult.
I have already partially answered this question. In fact, the worst possible scenario on the site of Fukushima, that is to say, the total fusion of hearts and their migration into the environment, never happened. There have been partial melts hearts on reactors, but to my knowledge, they are never out of the tanks.
No one has reference and comparison to assess the consequences. The "China Syndrome", a term used to describe a scenario in which the heart of an American reactor, after being melted, drilled the tank and the concrete base, traverse the entire Earth to arrive in China, is science- fiction.
The Soviet Union sent over 600,000 Chernobyl "liquidators". May decide that the Japanese government? It seems unthinkable that he does not try at all costs to mitigate the catastrophe. But at what human cost? Unless an absolute deluge, it is not the rain that cools the plant. However, the effect of rain is to precipitate the ground radioactive particles escaping from the site and are dispersed by winds.
Much the same thing. In the case of spent fuel, the radioactivity will be released - and perhaps began to be released - directly into the air. If the heart melts and reaches the ground, the radioactivity spread in the basement, water, that is to say throughout the environment. Today, no.
For the rest, everything depends on the amount of radioactivity released, the wind direction, etc.. Remember just after Chernobyl, where, again, radioactive materials had been projected up to 3 km high, and where the engine had burned for twelve days, there has been, never to speak of the territories overseas but in mainland France and Western Europe more generally, that the benefits certainly significant, but reduced.
The issue of dose is always very difficult. As a benchmark, a dose of 100 mSv is the one from which, in France, one advocates taking iodine tablets (to prevent thyroid cancer). In Japan, from 50 mSv. What is certain is that if Tepco (the operator), the Japanese authorities, the army, or whatever, does not arrive within the next few hours to cool the facilities, thus bringing them to One way or another of water, then yes, there will be little hope.
Chat moderated by François Béguin
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