A question about Fukushima

[Willis888]

Is it true that GE's BWR-3 with Mark 1 containment does not have a core catcher?

If it is, does this mean there is a possibility of molten fuel falling in such a way that a blob of critical mass forms and explodes like a dirty bomb?

[Zamfir]

No. A nuclear bomb requires a pretty careful arrangement to work, it needs in particular highly enriched uranium. The chain reactions in a bomb grow exponentially, and at some point the reactions start producing enough energy to destroy the bomb itself. So if you want a nuclear bomb that can do more than destroy itself, it has to generate lots and lots of energy in the short period between the moment that it start to rip itself apart, and the moment that the pieces are spread out too far to keep the chain reaction going. So the exponent of the exponential increase has to be very high, or the bomb will fizzle.

That is impossible with the very impure uranium in a nuclear reactor, where most of the material is U238 which isn't fissile. In fact, it takes quite a bit of effort to get a chain reaction going at all in that stuff. If you use lowly enriched uranium you need water (technically hydrogen, or another moderator) to slow down neutrons, plus a very large volume of material (so escaping neutrons have to escape a long way), and a specific arrangement of water and fuel, so that most neutrons pass though enough water to be slowed down, but not so much water that they are captured by the water. And after all that, you still cannot get anywhere close to the high exponent of reaction growth that you would need for a bomb.

It is not completely impossible that molten fuel together with trapped water would form the right arrangement for a working reaction again (perhaps just for a few seconds at a time). It wouldn't explode, but it would keep making a lot more heat than it would otherwise. It's unlikely, but to be extra sure they only pump in water with a solution of boric acid in it in Fukushima. Boron is an effective capturer of neutrons. A reaction needs water with not too much boron, and this water has lots of boron.

So in sort, no, the molten fuel cannot become bomb, not by a long stretch. The worst possibility is that limited reactions flare up once in while, but that can only happen if it melts into a very specific shape together with pure water, so the water there is all borated to make that impossible.

[Jplus]

It is not completely impossible that molten fuel together with trapped water would form the right arrangement for a working reaction again (perhaps just for a few seconds at a time). It wouldn't explode, but it would keep making a lot more heat than it would otherwise.

FYI, I once heard about a "natural nuclear reactor" that worked pretty much like this. IIRC there was a lake with a significant amount of (slightly enriched) uranium in the soil. It would periodically start a chain reaction, heating the lake so much that it completely evaporated, which stopped the reaction again until the lake re-filled.

If anyone knows what I'm talking about and can find back the source, please do.

[Qaanol]

A dirty bomb uses conventional explosives to propel radioactive material into the atmosphere. So unless someone’s been putting conventional explosives into the nuclear plant, there is no possible way it can explode “like a dirty bomb”.

[Zamfir]

It is not completely impossible that molten fuel together with trapped water would form the right arrangement for a working reaction again (perhaps just for a few seconds at a time). It wouldn't explode, but it would keep making a lot more heat than it would otherwise.

FYI, I once heard about a "natural nuclear reactor" that worked pretty much like this. IIRC there was a lake with a significant amount of (slightly enriched) uranium in the soil. It would periodically start a chain reaction, heating the lake so much that it completely evaporated, which stopped the reaction again until the lake re-filled.

If anyone knows what I'm talking about and can find back the source, please do.

I think you mean the Oklo mine, where there is evidence of a natural reactor 2 billion years ago that operated on and off for thousands of years. That age is very relevant, in those days natural uranium had the U235 content of modern reactor-grade uranium.

[Jplus]

I think you mean the Oklo mine, where there is evidence of a natural reactor 2 billion years ago that operated on and off for thousands of years. That age is very relevant, in those days natural uranium had the U235 content of modern reactor-grade uranium.

Yes! That's almost certainly what I was talking about.

[Willis888]

that can only happen if it melts into a very specific shape together with pure water, so the water there is all borated to make that impossible.

Thanks for the clarification.

The way I imagined it happening is that once a sufficiently large ball of molten uranium was brought together, it would begin to fission. The force of it would blast the ball apart, stopping the reaction but spraying molten fuel in all directions.

[firechicago]

Is it true that GE's BWR-3 with Mark 1 containment does not have a core catcher?

If it is, does this mean there is a possibility of molten fuel falling in such a way that a blob of critical mass forms and explodes like a dirty bomb?

It's worth noting that this is actually more or less what happened. As Zamfir points out, it's not possible to get an explosive chain reaction out of the low-enriched uranium used in most reactors. But the water in the reactor reacted with the cladding of the rods to produce hydrogen gas, and the resulting explosion cracked the containment and released an awful lot of radioactive material. In this way, the Fukushima Daiichi accident looks very much like a (conventional) dirty bomb.

[aoeu]

Is it true that GE's BWR-3 with Mark 1 containment does not have a core catcher?

If it is, does this mean there is a possibility of molten fuel falling in such a way that a blob of critical mass forms and explodes like a dirty bomb?

It's worth noting that this is actually more or less what happened. As Zamfir points out, it's not possible to get an explosive chain reaction out of the low-enriched uranium used in most reactors. But the water in the reactor reacted with the cladding of the rods to produce hydrogen gas, and the resulting explosion cracked the containment and released an awful lot of radioactive material. In this way, the Fukushima Daiichi accident looks very much like a (conventional) dirty bomb.

Not really. They weren't prepared for a tsunami so large so it caused a loss of power, which lead to them not being able to cool the reactor. As the reactor temperature rose hydrogen gas began to form and the fuel rods melted, releasing the radioactive contents into the coolant water (a large part of which was steam at that point). Pressure rose and to keep it within safe levels they had to let out some of the steam. They did it multiple times. Because the system which was supposed to safely burn off the hydrogen failed too (because it didn't have power? idk), the steam exploded after there was an adequate concentration of oxygen. The explosions dispersed the radioactive steam far and wide and also caused the leaks of water, which later ended up in the ocean. Because most of the radioactive release was from the leaking I would not call it a dirty bomb.

[Minerva]

Criticality is not some sort of horrible, terrible, destructive catastrophe. In fact, criticality is the normal, intentional operational state of a nuclear fission reactor. Ideally, criticality is the state that your nuclear power reactor is actually supposed to be kept in all of (or most of) the time.

Criticality does not mean explosion.

Criticality just means that it emits a large amount of ionising radiation (even larger than what you'd get from the fission products in the used fuel) and heat.

Anyway, let's suppose you've got a power reactor; a GE Mk. 3 BWR, or whatever.

Let's suppose that you completely melt the hell out of its core.

So, what do you have then? You have a semi-molten mass of used nuclear fuel (with its ²³⁵U content already partially burned up, because it's not fresh fuel), mixed with molten Zircaloy cladding and various bits of steel and/or Zircaloy from the core fuel assembly mounting and supporting hardware in the core around the fuel assemblies, which is mixed with the control rods which were fully inserted prior to core damage, so you've got a full set of control rods now molten and mixed through the molten mixture, plus you've got the stable fission products which accumulated in the used fuel prior to the damage, such as ¹⁴⁹Sm, which are also contributing to negative reactivity in the mixture. You've also got whatever boron or boric acid or gadolinium has been injected by the operators in the emergency situation and/or by the Standby Liquid Control System, so that also contributes substantial negative reactivity to the system (enough to keep a full water-covered core subcritical without any control rod insertion if control rod insertion fails).

If you've got a core damage accident, that also means that you've got little or no water covering the core, because you can't have a meltdown unless the water has first been lost or has boiled away. Less water covering the core means less reactivity, and removing most of the water from the core means it will be subcritical even without the control rods and/or soluble poison injection present. You can't have criticality in LEU without some moderator, no matter how much LEU you've got, and even if there were no other negative reactivity contributions from these other factors as well.

So to get criticality following a core damage accident, you would have to wait until the damaged melted fuel has cooled down a bit and then re-cover it with water, water that doesn't have soluble poisons such as B or Gd added, and then maybe if you've got enough LEU in a compact enough geometry and you've got no other control rods melted into the mixture then you might, maybe, get recriticality.

Basically, given the negative reactivity contributed by all these different factors, I don't think it's credible to consider any realistic possibility of any re-criticality in molten damaged fuel sitting in the bottom of the reactor pressure vessel following a Fukushima like situation.

[Willis888]

Thank you Minerva for the in-depth explanation. I hadn't considered the melted control rods being mixed in.

I'm having trouble picturing why a theoretical arbitrary quantity of pure low enriched uranium could not self-moderate. The way I've heard it described before, is that fast neutrons will ricochet off nuclei and thermal neutrons are absorbed. Do they lose so much energy with each collision that it becomes unlikely for them to ever have the exact energy of a "thermal" neutron?

[exporito]

Do they lose so much energy with each collision that it becomes unlikely for them to ever have the exact energy of a "thermal" neutron?

As far as I know they lose almost all their energy