What happens to the nuclear power plant when the cooling system fails in it?

Answer 1

The chances of a nuclear disaster is very low. But if one happens in the Scale of Level-7,

We would have an Explosion which will leak highly radioactive materials into the air and ground around it. It will poison the air around it. The land will be uninhabitable for around 100,000 Years.

And to contain the explosion after it would require Billions and more than 10,000 Lives.

Result:

More than 50,000 Dead and more cases due to cancer.

Answer 2

First of all, there are no nuclear power plants which use fusion. All of them presently use fission.

Second, there is a difference between critical and out of control...

The fission reaction needs to be critical in order to work. Criticality is simply that point where the reaction is self sustaining and is neither increasing nor decreasing, i.e. the neutron flux is constant, or the fission rate is constant, or the energy production is constant.

Out of control is a different matter. In this case, we have lost the ability to control the reaction. It might be because we have lost cooling capability and the fuel is overheating. It might be because the control rods are stuck and criticality is increasing.

However, it is important to understand that out of control along with supercriticality is unlikely. The basic design of the reactor core makes this so. It is extraordinarily difficult to hold the core together in a supercritical geometry long enough for it to fully convert, such as in a bomb. In fact, the technological difficulties in doing do makes the design of the implosion components of the bomb more problematic that the actual nuclear core design. What happens is that prompt dispersal makes the core "want" to get larger, which in turn makes it go subcritical, and the pressures behind prompt dispersal are immense.

Back to the power reactor. Only a few reactors have experienced super prompt criticality excursions. Two examples are Chernobyl and SL-1. What usually happens instead is that the core gets damaged, not necessarily from heat from criticality, but from heat from decay radiation. At that point, the core is already subcritical but, due to loss of coolant, it overheats, and sometimes partially melts. This happened at Three Mile Island, and it happened at Fukushima Daiicha.

Answer 3

The cooling system removes heat from the reactor. In normal mode, the extraction of steam, conversion to electricity, and condensation back to water is the primary cooling system. In shutdown mode, this normal steam process is not available, so alternative methods must be used.

The simplest explanation is that, if the cooling system fails, the reactor is going to get hotter and, eventually, fail due to overheating.

The slightly more complex explanation requires understanding that there are two sources of heat in a reactor. First, there is heat generated due to the continual fissioning of uranium. This is the heat that the normal steam process removes. Second, there is heat generated from the decay of mixed fission byproducts. This heat is generated even though the reactor has been made sub-critical and is no longer having continuous fissioning of uranium.

This decay heat must also be removed, otherwise the fuel will overheat and ultimately become damaged.

Initially, decay heat is controlled by virtue of the fact that there is a large volume of water in the reactor and suppression pool, and it takes time for that water's temperature to increase. Secondly, pumps must start circulating that water through the reactor, so that localized boiling within the fuel does not occur. Thirdly, other pumps must start circulating cooling water from another heat sink, usually the sea, through heat exchangers so that the primary cooling water is not allowed to get too hot.

These cooling systems are multiply redundant and diverse, so that the probability of a total cooling system failure is reduced as much as possible. There are multiple sources of power, and multiple ways to circulate water through the reactor. One of the first ways to circulate water is the high pressure coolant injection system. This system does not require external power - it only requires station battery power and a source of steam to drive its turbine. It runs long enough to get the backup diesel generators running. Until they are running, you have cooling water, but you do not have a heat-sink, so you are depending on thermal mass of that volume of water.

In the event of total cooling system failure, the fuel overheats. The water overheats and over pressurizes, and it tends to flash to steam, exposing the fuel and leaving it without any cooling. Ultimately, the fuel assemblies will fail, and you have release of fuel into the cooling system and, eventually, into the environment.

There is a second consideration as well. The spent fuel assemblies that have been removed from the reactor in prior refueling operations also have decay heat. They sit in a pool of water, typically 55 feet deep, so that there is always 40 feet of water above the fuel. There is a fuel pool cooling system, which also requires power and a heat sink to function. If that fails, the pool will overheat and boil off, exposing the spent fuel, with the same possible results as exposing the fuel in the reactor. The difference is pressure control. The spent fuel pool is maintained at atmospheric pressure because it is not located inside the pressure vessel boundary. This means that the water will boil at a lower temperature than it would while within the reactor.

There is a third consideration. Normally, hydrogen gas that is created from the interaction of water and neutrons is collected and recombined back into water, so that hydrogen does not build up in the reactor. There is a second way to create hydrogen, however, and that is through a catalytic reaction between water and the zirconium cladding on the fuel assemblies when the temperature gets too high. If the hydrogen re-combiners can not keep up with the hydrogen production, hydrogen can accumulate. Adding water under certain conditions can create a hydrogen explosion, damaging various things, something that did occur in Japan.

Now we get into the area of theoretical. The term "meltdown" is not a scientific term; it is an anecdotal term, that the public has blown out of proportion; but here is the theory...

Up to this point, the fuel has been maintained in a sub-critical state by virtue of its geometry. There is no continuous fission stream - only decay heat. If you allow the fuel to overheat to the point of fuel assembly failure, not only will you have fuel in the coolant and possibly in the environment, but it is possible that the fuel could melt, leaving the confines of the fuel assembly, and collect in the bottom of the reactor. That represents a change in geometry, and it is theoretically possible that such change in geometry could cause the fuel puddle to go critical again, generating much more heat than just decay heat. This process would feed on itself, and (again, I have to emphasize theoretically) it could get hot enough to burn through the reactor pressure vessel, the primary containment, the secondary containment, and escape into the ground. This is the famous "China Syndrome".

The design of the facility, however, makes such an incredible event unlikely. The containment is designed to not allow such a fuel puddle to accumulate into a critical geometry, it is designed to spread any puddle out into a sub-critical geometry. Additionally, by the time you reach this point, you have probably made the decision to flood the reactor with sea water, what we call the ultimate core cooling connection. When this is done, we would add boron to the mix so as to further reduce the geometry in an attempt to prevent a second criticality.

Do not misunderstand this, however; by the time fuel is sitting in a puddle outside the reactor vessel, you have a disaster, but the question is whether or not you can contain the fuel within the containment structure.

This is why the spent fuel pool is so worrisome. The spent fuel pool is located outside of the primary containment, but inside the secondary containment, so while it does have some protection, it is more difficult to control if it were to meltdown. Mitigating that statement, however, is the fact that by the time spent fuel is removed from the reactor and transferred to the spent fuel pool, it has had time to lose a lot of its mixed fission byproducts that generate decay heat. Yes, there still is decay heat, but it is less.

Answer 4

Nuclear reactors produce heat. Heat is simply the transfer of energy The kind of heat that is meant here is called "decay heat", which means the heat produced as a result of radioactive decay. Fuel rods are radioactive, as they contain radioactive isotopes of uranium (typically) and/or plutonium. When that radiation, typically neutron radiation, interacts with other things (such as air, water, concrete, or whatever else is around), there is a physical impact with other atoms. This impact transfers energy.

The fission in an active reactor is a primary source of decay heat. However, a reactor in shutdown mode that is not engaged in fission will still produce "beta decay", which is produced by the fission products. Although beta decay heat drops off quickly, it is vital that this heat be removed from the reactor or it can raise the temperature of the reactor fuel (which, briefly, is bad). Heat removal from reactors is usually accomplished through water pipes. Water heated inside the reactor travels out of the reactor core, gets cooled, and returns to be heated again and remove more heat. In the case of the Fukushima reactors, the cooling systems in the plant were not operating, and sea water was pumped directly into the reactor to try to remove beta decay heat. This is a highly simplified explanation.

Answer 5

The term "melt down" is very broad and riddled with misunderstanding. It is a theoretical consequence of the ultimate sustained loss of cooling accident at a nuclear power plant. The anecdotal term is "china syndrome", because the core could theoretically melt through the containment, into the ground, and go "all the way to china", so to speak, contaminating everything in its way.

Let's talk reality, shall we... A bit of background, please, first...

Nuclear fuel is generally comprised of hollow tubes of zircalloy with pellets of slightly enriched uranium. The ends are welded shut and they are called fuel pins or rods. They are arranged into bundles, and the bundles are arranged into assemblies along with control rods, and the assemblies are arranged into the reactor core matrix.

There is a source of coolant, usually water, which also serves as the moderator. The moderator is necessary to sustain the fission reaction, because the product of fission generates high energy (fast) neutrons that do not interact well with other uranium atoms. The moderator slows down those neutrons into the thermal range - that's why we call these "thermal reactors" - and the reaction can be sustained and controlled.

The control rods serve as a gross control for startup and shutdown. Once the reactor is critical, power is adjusted by changing water flow and turbine load, with control rods fine tuning the effects of iodine and xenon buildup. This works because the moderator (water) has a negative temperature coefficient, making the reaction self stabilizing.

Now, that is basic reactor design, without all the heavy physics behind it to cloud the explanation. The thing to understand is that the reaction is highly dependent on geometry, geometry being everything from physical fuel position, to temperature, to pressure, to moderator status, etc.

Now to the problem of cooling...

While the reactor is operating, it generates an enormous amount of heat, which is carried away by the coolant. That is used to flash water to steam, spin turbines, and generate electricity. All well and good.

When something happens, the delicate balance is upset, and the reactor shuts down automatically. Control rods are inserted, and the reaction goes subcritical, stopping the creation of this enormous flow of energy. In fact, depending on the nature of the event, the destabilization of the moderator will cause the reactor to go subcritical even before the control rods have a chance to move.

The problem is that there is a second source of energy. We call it decay heat. It comes from mixed fission byproducts - the isotopic result of splitting all of those uranium atoms into other, highly radioactive atoms - and the radioactive decay of those mixed fission byproducts generates heat - lots of heat - not nearly as much as the heat of fission - but still, lots of heat.

Those mixed fission byproducts are trapped inside the fuel rods, which are designed extraordinarily well. But, they still have limits on temperature. So we need to cool them, even after the reactor is shutdown - usually for a significant period of time, in order to allow the mixed fission byproducts to decay to safe levels.

The immediate source of cooling is the thermal mass of the reactor. It takes time to raise its temperature. Another, nearly immediate source of cooling, is the large volume of water in the reactor and in the suppression pool. Even though the reactor has been shutdown, it is still generating steam, and that steam goes to turn emergency turbines that circulate suppression pool water back into the reactor. This requires no outside source of power - only batteries.

If the facility is isolated from the power grid, emergency diesel generators then start up to keep the batteries charged, and to provide additional cooling water for the suppression pool and reactor to keep them cool. There are multiple layers and multiple redundancies in these systems, so that the total loss of cooling for a long enough time to obliterate the core is considered an incredible, i.e. non-probable, accident.

Nevertheless, there have been partial meltdown accidents. Fukushima Daiichi is the most recent. Three Mile Island is the other notable accident. Other accidents that involved major core damage were Chernobyl and SL-1. There were probably others, but these stand out. Chernobyl and SL-1 were not cooling accidents, per se, although Chernobyl was complicated - they were prompt critical accidents that blew apart the core, but that is outside the scope of this answer. I add them only for the sake of completeness.

What happens is that, with no source of cooling, the decay heat in the core causes temperature to rise to the point where the zircalloy pins start to fail. This, by the way, is also an issue with spent fuel that has been removed from the reactor core and placed in the spent fuel pool, as was the case at Fukushima Daiichi.

Once the pins start to fail, the mixed fission byproducts are released into the coolant or the atmosphere of the facility. Eventually, they get released into the public areas of the atmosphere or water. Yes, we do have containment systems that do a very good job, but if pressures build up too much, it is better to bleed off some of the pressure than to allow a catastrophic loss of containment. Complicating this is that hot zircalloy in contact with water generates hydrogen gas that can be explosive, potentially, and as seen at Fukushima Daiichi, damaging parts of buildings, even possibly parts of containment.

Now, we enter into the theoretical zone, one that we have never experienced to date...

The discussion so far has laid the groundwork for this next part. Even though many aspects were covered, aspects there were not necessarily asked in the question, I felt that it was necessary to understand the reality of what we mean by "melt down", and by what we don't mean, or should not imply.

A full blown, theoretical melt down would be the sustained loss of cooling in the reactor (or the spent fuel pool) such that the fuel rods would melt, not just partially, as has happened, but fully. They would continue to heat, and the fuel itself would melt, and start to drip to the floor of the reactor, where it would collect. On the floor of the reactor, there are no control rods, so there are no gross reactivity controls. There is no moderator, either, so fast neutrons cannot be converted to thermal neutrons to restart the fission reaction. Well, sort of... It is possible to start a reaction with fast neutrons, but it is much harder. Theoretically, and I emphasize, theoretically, we could go critical again. If this were to happen, heat output would go up exponentially, and the theory is that the corium (core now assembled into a pool of slag) could melt through the reactor, then through the primary and secondary containment, and into the ground and water tables. This would create a devastating plume and compromise the water supply.

Several things are in place to prevent this incredible outcome...

Long before we get to this point, we flood the reactor and containment with sea water. This has already happened at Fukushima Daiichi, an indication of how seriously the utility and the government are treating the accident. This is known as the ultimate core cooling connection. It is devastating because the sea water will destroy the reactor, but it will prevent (or reduce) further risk to the public.

The reactor and containment structures are enormously strong and thick. It would take a tremendous amount of energy to penetrate them. One reactor that I worked at was designed, amongst other things, to be able to withstand the impact of a fully loaded 747 flying down at an angle of 45 degrees into the base of the reactor building. It was not able to continue operating after that, but it was able to safely shutdown and maintain cooling after that.

Last, remember that all important geometry - well, underneath most reactors, there is an ultra high temperature dike called a corium ring whose sole purpose in life is to prevent the accumulation of corium into a critical mass. While breach of the reactor would be devastating, preventing criticality is even more important at that point.

Answer 6

your recton gets ripped in two.