Avoiding nuclear meltdown

[gsaldivar]

As brought to mind by recent events in Japan:

As I understand it, nuclear fuel must be brought together in close proximity so as to approach critical mass. A combination of control rods, coolant, fuel placement and reactor geometry keeps the fuel in a safe temperature range and allows a controlled reaction to occur which generates heat, steam, electricity, etc...

So when there is a critical failure, such as a mechanical problem where control rods cannot be inserted, or where coolant cannot be pumped into the reactor chamber... the nuclear fuel, due to its close proximity, will continue generating heat on its own leading to a possible explosion or meltdown.

Are there any modern reactor designs where, in the event of an emergency like this, the fuel itself is separated so that it is no longer in danger of becoming critical and no longer capable of generating enough heat as to cause an explosion or meltdown...?

I'm thinking maybe a chamber underneath the reactor, where half of the fuel can be dropped into, in the an emergency. There may still be some damage caused by latent heat from a "hot" reactor scrammed in this way, there shouldn't be a critical danger of a "runaway" reaction taking place because the fuel has been separated and can no longer generate enough heat on its own to cause a meltdown.

 

[pitz]

The reactor vessel itself is a pressure vessel, and pressure vessel integrity is directly proportional to the surface area of the pressure vessel itself, ie: a larger vessel has twice the probability of failure than a smaller vessel.

The huge issue you face, right off the bat, is that installing a lot of mechanical equipment inside such a pressor vessel, the reactor itself, is highly problematic from a maintenance point of view. Reactor vessels and the equipment that contains the fuel rods, becomes highly radioactive during operation of the reactor, due to neutron irradiation. What happens then, if the machinery at the bottom of this vessel requires maintenance? The problem is essentially one of coming up with with a workable design that is maintainable over the service life and conditions typically expected to be encountered in such a reactor over a 30-40 year service life.

Building a reactor is obviously the very calculated decision of engineers balancing off certain risks against others. This incident is related heavily to failure of diesel generators. Three Mile Island was an instrumentation and human factors failure. You never really know where the risks are going to show up, and concentrating on one factor to the exclusion of all others can have its own set of unintended consequences.

 

[gsaldivar]

Any reactor design, is going to be a complex system. I don't think it adds a significant amount of complexity to have some system of scramming half the reactor fuel into a pit under the reactor. The transport method can simply be gravity itself. The size of the reactor vessel would stay the same, the only changes would be the addition of fuel doors at the bottom of the vessel set to open by remote or automatically when a certain temperature/pressure level is reached.

I understand that the failure of the diesel generators is a direct cause of what's happening today. I'm just trying to visualize a system in which the reactor can be made inert - instantly. So that in the event of an emergency, and complete failure of all other "powered" backup systems, it might still be possible to stop nuclear fuel from generating heat and ultimately protect against a meltdown.

 

[PottedMeat]

Are there any modern reactor designs where, in the event of an emergency like this, the fuel itself is separated so that it is no longer in danger of becoming critical and no longer capable of generating enough heat as to cause an explosion or meltdown...?

It's been a while since I've read about it but there was this: Pebble Bed Reactor

http://en.wikipedia.org/wiki/Pebble_bed_reactor

The nice thing was that in addition to control rods was that it could use strong negative feedback.

From article:

When the nuclear fuel increases in temperature, the rapid motion of the atoms in the fuel causes an effect known as Doppler broadening. The fuel then sees a wider range of relative neutron speeds. U238, which forms the bulk of the uranium in the reactor, is much more likely to absorb fast or epithermal neutrons at higher temperatures. [2] This reduces the number of neutrons available to cause fission, and reduces the power of the reactor. Doppler broadening therefore creates a negative feedback because as fuel temperature increases, reactor power decreases. All reactors have reactivity feedback mechanisms, but the pebble bed reactor is designed so that this effect is very strong and does not depend on any kind of machinery or moving parts. Because of this, its passive cooling, and because the pebble bed reactor is designed for higher temperatures, the pebble bed reactor can passively reduce to a safe power level in an accident scenario. This is the main passive safety feature of the pebble bed reactor, and it makes the pebble bed design (as well as other very high temperature reactors) unique from conventional light water reactors which require active safety controls.

It also looks like they've tried to kill one of the test reactors but the link is gone.

 

[Mark R]

Most shutdown systems have a huge safety margin. As a result, if a few (up to 10%) of control rods fail to be inserted, this is not a disaster, and the reactor should be adequately shutdown.

Some reactor designs (e.g. the Canadian Candu) shouldn't suffer from this, as the shutdown rods don't enter a pressurised chamber, but enter an open bath of water with reactor pressure pipes running through it. To shutdown, they are dropped by switching off an electromagnet and fall into the pool into spaces between the pipes.

As it is, in most plausible accident scenarios, the difficulty is not inserting the control rods. It's not too difficult to provide reliable hydraulic or pneumatic power (e.g. via batteries or tanks of compressed air) sufficient to drive the rods in, if there is a total power failure. Additionally, each control rod typically has its own independent control and drive system.

However, modern designs have made some differences. First generation fuel bundles were mounted from both ends. The problem was that under fault conditions, if the fuel overheated it couldn't expand and the rods buckled (which did lead to some problems with control rod insertion failure). The more modern designs hold the rod bundles at one end, and allow thermal expansion to slightly splay the rods, moving the fuel configuration away from optimal.

As it is, mass control rod failure is not considered in any current reactor design, even the latest PWR/BWR designs.

Similarly, loss of coolant will only mean a leak - e.g. due to burst of a 3" pipe. Loss of coolant is countered by pouring more coolant in from another tank. Typically, the coolant top-up pumps will be able to cope with a 6" pipe splitting. As it is, if a coolant pipe does rupture, it will rupture into the containment building and flood the space around the reactor, so the coolant is not actually lost.

The problem is most likely to be the aftermath - after the reactor has been shutdown. At both Fukushima and Three mile island. The problems have occurred hours after shutdown, where the reactors were not-critical, due to persistent heat production from radioactive decay. For the first few hours after shutdown the fuel continues to produce heat at 3-5% of operating power. These are decay reactions and cannot be stopped or controlled.

So, simply dropping metal fuel out of the reactor, into separate chambers, isn't sufficient to prevent meltdown. You have to preserve coolant flow around the fuel for at least 72 hours, and the fuel has to remain immersed in water for up to 1 year after use to ensure that the fuel doesn't melt or become otherwise heat damaged.

What probably happened at Fukushima, is that lack of coolant flow led to build up of decay heat, causing boiling of coolant which was then vented into the containment building. Without a working top-up system (which was dependent on electrical power) water level in the reactor dropped exposing the fuel, which melted from decay heat.

Similar things happened at TMI - in that case, a drain valve got stuck open, allowing coolant levels to drop, and the fuel melted from decay heat when uncovered.

The solutions proposed for water cooled designs include:
gravity cooling - using the fact that hot water rises and cold water sinks to circulate the coolant.
Passive heatsinking - using the containment building wall as a heatsink, to condense steam back into cooling water, which can be circulated back into the reactor core.
Depressurisation - the reactor core is depressurised which means it can be topped up from tanks in the containment building roof using gravity pressure alone, or by simply flooding the containment chamber above the reactor level. (Compare this with Fukushima, which didn't have a safe way of emergency depressurisation of the reactor - necessitating 1200 psi high-pressure AC powered pumps to top up the coolant as it boiled off).

Other more radical designs have been proposed which mitigate factors even further:
E.g. the pebble bed design mentioned above. By making the reactors relatively small, the 'skin' of the reactor should be sufficient heatsink to prevent the reactor overheating from decay heat, or even operational fission heat (due to negative feedback effects). Failing that, there can be provision to open trap-doors at the bottom, and drop the pebbles into sb-critical, passively cooled containers.

Molten-salt reactors also have this advantage. In these reactors, the coolant is the fuel. So, overheating or boiling leads to expansion of the fuel and loss of criticality. Further protection is available from the use of freeze valves, which melt if overheated, allowing the fuel to pour out into sub-critical, passively cooled tanks.

 

[BladeVenom]

It's been a while since I've read about it but there was this: Pebble Bed Reactor

http://en.wikipedia.org/wiki/Pebble_bed_reactor

The nice thing was that in addition to control rods was that it could use strong negative feedback.

Another option is the liquid fluoride thorium reactor http://en.wikipedia.org/wiki/Molten_salt_reactor You don't have to worry about a meltdown.

 

[C1]

???????

I am a mechanical engineer and I have worked in the nuclear industry for some time, including the TMI cleanup project. At TMI, none of the melted fuel even had a chance of eating through the bottom of the reactor vessel, much less the containment structure. Statements like the ones that this so-called expert made are unnecessary and reckless. Once the core gets into an un-organized condition, such as occurred at TMI, the chances of its regaining a geometry that supports fission is extremely remote. Other than that, it isn't likely that the decay heat alone would result in that happening, particularly now. Once the core has been shutdown, the decay heat drops off quickly. Their main problem was trying to prevent over pressurizing the primary containment vessel due to steam generation in the core. That was why they vented and released the hydrogen.
My sources in Japan near the plant tell me that they have successfully added seawater to the primary containment to cool the reactor and they believe the worst is over for unit 1. Now they are likely starting to work on the other critical problems at the other units. I hope that all of their hard work and training pays off and stops this from becoming more serious than it already is.
- Bob 21 hours ago


http://news.yahoo.com/s/ap/as_japan_quake_power_plant


No doubt nuclear safety will be a renewed issue across the globe:

http://www.nytimes.com/2011/03/14/world/europe/14atom.html

Japan's Nuclear Crisis Stokes Fear in Europe
By JUDY DEMPSEY
Published: March 13, 2011

BERLIN — The nuclear power emergency that began unfolding at a Japanese atomic power plant during the weekend could lead to a major reassessment in European countries that are already building such plants or are considering a shift from fossil fuels to nuclear energy to combat climate change.

 

[pm]

A great technical article about the Japanese nuclear situation.
http://bravenewclimate.com/2011/03/13/fukushima-simple-explanation/#more-3970

 

[Perryg114]

It would seem like a gravity fed emergency cooling system would be a good idea and a way to vent resulting steam/hydrogen would also be a good idea. I would think it would be possible for fuel/damper rods to fail in the shutdown position due to gravity. This being the case, all you need to do is get the residual heat out of the core with the stored water. Maybe I am oversimplifying things?

Perry

 

[BladeVenom]

A great technical article about the Japanese nuclear situation.
http://bravenewclimate.com/2011/03/13/fukushima-simple-explanation/#more-3970

WTF! "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). So after the batteries ran out, the residual heat could not be carried away any more."

They had a nuclear meltdown because they had the wrong plugs. Haven't they ever heard of splicing wires.

 

[Perryg114]

Sounds like something from Apollo 13.

Perry

 

[Matt1970]

According to the news they have flooded a 2nd and 3rd reactor with sea water. That is basicly the method of last resort. The explosions you see on TV are aparently not the actual reactors themselves, that would be the equivilent of Chernobyl.

"Operators knew the sea water flooding would cause a pressure buildup in the reactor containment vessels -- and potentially lead to an explosion -- but felt they had no choice if they wanted to avoid complete meltdowns. Eventually, hydrogen in the released steam mixed with oxygen in the atmosphere and set off the two blasts."

 

[QuantumPion]

WTF! "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). So after the batteries ran out, the residual heat could not be carried away any more."

They had a nuclear meltdown because they had the wrong plugs. Haven't they ever heard of splicing wires.

That sounds like baloney to me. You don't simply plug a diesel generator in to a power plant like plugging in a toaster. Anyways the complications in getting power was due to flooding by the tsunami, damaging the electrical equipment used to distribute power within the plant.

 

[Chiefcrowe]

Thank you, that was a great read and a very good explanation!

A great technical article about the Japanese nuclear situation.
http://bravenewclimate.com/2011/03/13/fukushima-simple-explanation/#more-3970

 

[spikespiegal]

A nuclear reactor design that requires constant active cooling to prevent criticality is a *bad* design, period. Even if active cooling is required temporarily to 'coast down' residual heat it's still a bad design.

Not nearly as bad as having a Positive Void Coefficient, as per a rather infamous Soviet designed reactor, but it's still bad. Knock - knock - you live in an Earthquake zone, morons, and you can't rely on generators or any other power source to supply active cooling.

If the plug is pulled and the reactor can't 'scram' into a self contained mode that can't handle it's own neutron flux passively without turning into the core of a sun then maybe it's time to revisit the designs used for these plants and.....ghad....fire some engineers. Smaller reactors in a loop that use a cascade of heat exchangers to gradually convert a closed loop volume of water to steam would be safer and would be far less likely to have these issues.

 

[QuantumPion]

A nuclear reactor design that requires constant active cooling to prevent criticality is a *bad* design, period. Even if active cooling is required temporarily to 'coast down' residual heat it's still a bad design.

All reactors need to have residual heat removed. You can't redesign nature.

Not nearly as bad as having a Positive Void Coefficient, as per a rather infamous Soviet designed reactor, but it's still bad. Knock - knock - you live in an Earthquake zone, morons, and you can't rely on generators or any other power source to supply active cooling.

You can rely on generators to supply active cooling as long as everything is designed to withstand credible accident and disaster scenarios. The main problem here is that this earthquake and tsunami were beyond the design basis. No matter how safe you design something, there is always a chance that nature will throw at you more than it can handle.

If the plug is pulled and the reactor can't 'scram' into a self contained mode that can't handle it's own neutron flux
passively without turning into the core of a sun then maybe it's time to revisit the designs used for these plants

I think what you mean to say is that reactors should be able to handle their own decay heat load without intervention, even under station blackout conditions, and I agree that is a desirable feature. There are some new designs that have these features (mainly the GE ESBWR). For plants that do require active measures, I'm sure in the coming months and years they will be re-evaluating their safety measures so the same catastrophe is not repeated.

 

[C1]

Yup. Not sure how many of these puppies there are all over the world, but if there ever should become a meteor strike such that installations become unattended or debilitated so that you have a couple hundred simultaneous meltdowns, then most higher life forms on earth might be zilched for thousands of years (if not more).

 

[beginner99]

Yup. Not sure how many of these puppies there are all over the world, but if there ever should become a meteor strike such that installations become unattended or debilitated so that you have a couple hundred simultaneous meltdowns, then most higher life forms on earth might be zilched for thousands of years (if not more).

If such a meteor strikes, meltdowns would be the least of your problems because humanity would probably be killed off just like the dinosaurs were.

The math is simple build additional nuclear plants and prevent (or better said limit global warming at a very low risk of contaminating small areas) or don't, burn up all coal and just hope a 10°C warmer earth is still habitable by humans (high probability that's not the case).

Or simplified:

nuclear = very small risk of contaminating very small areas (you know what you get)

non-nucelar (=fossil) = 100 % certainty of more extreme global warming with very unpredictable results for the whole earth.

 

[C1]

As far as I can see, the die is already cast. The chemtrails needed to reduce sunlight will eventually create an eco-catastrophe. What dies, dies. Earth life is pruned back until it fits. I suspect that it has happened at least a few times already. (Probably the only way to rid the earth of politicians and lawyers.)

http://www.venusproject.net/ethics_in_action/Global_Warming_Dimming4.html

http://www.willthomasonline.net/Are_Chemtrails_Masking_True_Glo.html

 

[Throckmorton]

If such a meteor strikes, meltdowns would be the least of your problems because humanity would probably be killed off just like the dinosaurs were.

The math is simple build additional nuclear plants and prevent (or better said limit global warming at a very low risk of contaminating small areas) or don't, burn up all coal and just hope a 10°C warmer earth is still habitable by humans (high probability that's not the case).

Or simplified:

nuclear = very small risk of contaminating very small areas (you know what you get)

non-nucelar (=fossil) = 100 % certainty of more extreme global warming with very unpredictable results for the whole earth.

Except under no circumstances would we be able to replace most, or even a large chunk of fossil fuel usage with nuclear.

 

[Sukhoi]

Except under no circumstances would we be able to replace most, or even a large chunk of fossil fuel usage with nuclear.

Why not?

 

[wirednuts]

If such a meteor strikes, meltdowns would be the least of your problems because humanity would probably be killed off just like the dinosaurs were.

The math is simple build additional nuclear plants and prevent (or better said limit global warming at a very low risk of contaminating small areas) or don't, burn up all coal and just hope a 10°C warmer earth is still habitable by humans (high probability that's not the case).

Or simplified:

nuclear = very small risk of contaminating very small areas (you know what you get)

non-nucelar (=fossil) = 100 % certainty of more extreme global warming with very unpredictable results for the whole earth.

i completely disagree with this. i actually doubt humans would go extinct if we got hit by a rock the size that killed the dinos. most of us would die, but a small percentage would live the initial blast (as dinos probably did) but we would know how to adapt to the new climate. dinosaurs had to wait to evolve for their adaptations, and that was far too long of a wait obviously.

but if you had hundreds of nuclear plants completely melt down as well, then yes i would have to imagine most everything would die besides small insects and certain plants because there really is no way for humans to immediately adapt to high levels of radiation.

 

[Eyeless Blond]

i completely disagree with this. i actually doubt humans would go extinct if we got hit by a rock the size that killed the dinos. most of us would die, but a small percentage would live the initial blast (as dinos probably did) but we would know how to adapt to the new climate. dinosaurs had to wait to evolve for their adaptations, and that was far too long of a wait obviously.

but if you had hundreds of nuclear plants completely melt down as well, then yes i would have to imagine most everything would die besides small insects and certain plants because there really is no way for humans to immediately adapt to high levels of radiation.

Why not? It doesn't seem to hurt many other animals. Really, people love to get all heated up over radiation (a leftover from some nasty Cold War coverups), but honestly after the crisis itself the long term effects of radioactive contamination probably won't be nearly as bad as most people think.

 

[wuliheron]

There were so many errors made with this nuclear plant we probably won't ever know all of them. The original design was intended for nuclear subs and never meant to be scaled up to a commercial power plant. Three of the scientists who helped design the plant resigned in protest over the lack of adequate safety. G.E. later addressed some of these problems, but needless to say it was not the safest possible design. It was a relatively cheap design.

Then they built it in Japan which is an island nation the size of Indiana with 108 active volcanoes sitting on top of what is called the "Ring of Fire". This is the most earthquake prone area on earth where some 90% of all earthquakes occur and some 80% of the worst. They provided adequate protection for most bad quakes, but not enough for the very worst.

Nor is the company that ran the reactor exactly a shining example either. Six years ago the president, vice-president, and others resigned in disgrace when a scandal emerged about a conspiracy to hide safety violations including cracks. Hopefully they got everything straightened out before the quake and hopefully the can get the reactors under control, but it certainly doesn't look good at this point.

 

[Ninjahedge]

Where's Homer when you need him?