How long until molten nuclear fuel at Fukashima cools and solidifies?

[FishmanGeertz]

For weeks after the Chernobyl disaster, many tonnes worth of molten "lava-like" nuclear fuel inside Chernobyl's shattered reactor vault eventually cooled down and formed into a hardened substance similar to ceramic.

When nuclear fuel overheats and melts, how long does it remain in it's glowing-hot molten state before it cools down and solidifies? If there is a lot of molten fuel probably at the bottom of the ruined reactor buildings of the Fukashima NPP units 1-4. How long before it cools itself down?

Is it true that this white-hot molten fuel can burn it's way through the floor of the reactor building, through the ground beneath the plant, and into the underground natural water basin causing a massive geothermal explosion that sends giant geysers of extremely radioactive steam shooting up through the general area surrounding the melted-down reactor? Isn't there a reinforced structure beneath the reactor vessel to stop this from happening?

So far we have not seen any geysers of steam shooting up from the ground at the Fukashima NPP. But this disaster is still fairly young. Apparently they are planning to flood and encase the reactor buildings with concrete as a permanent solution to this accident. Much like how the Chernobyl reactor was flooded with neutron-absorbing materials such as boron, and then a massive containment structure or "sarcophagus" was built on top of Chernobyl's reactor.

This photo from the Chernobyl NPP shows a large mass of a fuel/sand/graphite mix inside of a room in the basement located directly beneath the reactor, which solidified into a ceramic-like material. It is still immensely radioactive, and simply approaching it would mean certain death. Many years ago, I believe some Russian scientists sent a remote-controlled robot with a camera attached to it to snap the photograph seen below.

If cores of the Fukashima reactors or the spent fuel assemblies are melting, and are still burning, how long will it be until they cool and solidify? Weeks? Months? I thought the self-sustaining nuclear reaction lasts indefinitely. Which is why nuclear waste has to be constantly cooled 24/7 in giant refrigerated pools of water pretty much forever.

 

[Hologram0110]

Yes and no. The chain reaction in the reactor has been shut down since the earthquake hit. All the problems have been due to decay heat which is much lower than the chain reaction power. This is the energy released when radioactive materials decay into less radioactive materials. You CAN'T stop this, it is a passive process. Since nuclear reactors produce so much radioactive materials in the fuel, the fuel keeps producing heat until it is stable. The amount of heat it produces is dependent on how long the fuel was used for, at what power the fuel was used at and how long since the reaction has stopped.

This spent fuel needs to be kept in water when it comes out the reactor because air won't remove enough heat and it would melt. After a number of years (something like 10 years depending on the above) the fuel produces a small enough decay heat that it can be removed from the water and stored in "dry-casks". Basically concrete blocks, without water in them.

There is something called a "core-catcher" under the reactor that is designed to prevent molten core from melting through the containment. The floor is designed to spread the material out so it can be cooled more easily. It also has a very high melting temperature so that it is EXTREMELY unlikely that it it could ever make it through the floor by melting. They also flooded this area with sea water, so it would have to melt the floor while being cooled by sea water.

Even if it did melt into the ground it wouldn't produce radioactive geysers, the power isn't high enough. The melted mixture of stuff would just keep mixing until it was too big to keep mixing with stuff. It could contaminate the ground water however.

How long it takes to solidify basically depends on how quickly you can remove heat, there is no one answer here. It shouldn't take long. Like on the order of minutes or hours.

Personally, I think the reactors will end up entombed, but I think we should wait to decide if that is best and how to do it. There isn't enough information. We need to stabilize the situation and get robots in there to figure out the details.

 

[heke]

Is there any estimations how the possible the fuel failure (due to overheating) influences on the decay heat time?
Concerning the plant in question, is there any separate core-catcher? Reinforced concrete containment exists, of course. However, the molten fuel has to go first through the reactor pressure vessel, which doesn't happen in the case of TMI in late 70's.
(all the text including comments etc. are just my personal opinions)

 

[Hologram0110]

Is there any estimations how the possible the fuel failure (due to overheating) influences on the decay heat time?
Concerning the plant in question, is there any separate core-catcher? Reinforced concrete containment exists, of course. However, the molten fuel has to go first through the reactor pressure vessel, which doesn't happen in the case of TMI in late 70's.
(all the text including comments etc. are just my personal opinions)

There is no basically no dependence of decay heating on fuel failure. Basically the fuel will continue to produce heat regardless of it's state. That it it doesn't care what chemical form it is in. It doesn't care if it it a solid, liquid or gas. The unstable ATOMS in the fuel will continue to decay.

Just remember heat!=temperature. It will continue to produce heat, but if you cool it you can keep it from getting hot.