As I understand it, the problem at the Japanese nuclear plant started when they decided to shut down the 3 running reactors, and it takes several days to cool down the reactors, and then their power and all of their backup power failed, so the cooling pumps stopped, leading to the overheating, explosions, etc.
Would they have been better off to have not ever have shut down these reactors in the first place?
I don’t see that the plant suffered any great damage from the tsunami waves. Nor does the distribution grid they are connected to seem damaged.
What would have happened if they had kept the reactors running? Are there other problems that would have been caused by that?
Ironically you may be right. But its a statistical sort of thing. While this current scenario could be bad, it would be way worse if the reactor hand not been properly shut down, then they lost coolant, then things broke and they couldnt shut it down.
When things are shaking like hell, and you don’t know when it will stop, and you don’t know if/when things will break or already have, generally STFD is probably the best move.
One problem is that you can’t leave a plant running if there is no load. The power the reactor makes has to go somewhere, and that somewhere is pretty much restricted to being the customers on the grid. When the disaster hit, a huge amount of infrastructure was lost, including in all likelihood the connectivity to get the power from the generators to the grid.
Right now, the problem they face is still one of sending the residual power being generated in the reactor somewhere. As the various isotopes decay this problem rapidly reduces in magnitude, and by now is probably reaching manageable levels.
In both cases the way they get the power out of the reactor is by boiling water. When the generators have load they can use he steam to generate power. No load, the generators would simply tear themselves apart and then all hell would break loose. It may be that it is possible to keep the generator’s steam generation system running by venting the steam, however the enormous amounts of energy involved, and the amount of water needed to sustain the process may make such an idea infeasible. The plant would need to be designed fro the outset to allow such an idea, and clearly they thought they would not need to,
Possibly. The shutdowns however were completely automatic and probably happened in the first few seconds of the earthquake. Nobody made a decision.
Whether it would have been possible to say, restart ONE reactor and use it to power all the pumps for the whole plant is a good question. Having somewhere for the power to go to is an issue. Whether it would have been prudent to do that straight after an earthquake, when the diesel power had kicked in and was running fine, is an easy one - no it wouldn’t have been! Not without thoroughly checking the reactors over first.
Whether it would have been possible once the emergency power had failed after the tsunami to use the decay heat from the reactors to generate power and run the pumps is another good question. Theoretically yes, but practically? Also bear in mind that they had battery power and an expection that mobile generators would solve the problem.
I’m not an expert at commercial-sized power plants, but this doesn’t sound right to me in the least.
An electrical generator will only produce as much electrical power as there is electrical load. In addition, nuclear reactors are designed to match reactor power to the steam demand. You would only run into a problem if the steam demand were less than the minimum possible heat being produced by the operating reactor, which would likely be equivalent to the decay heat that the plant is now having to deal with. The difference would be that power would be available to run pumps, etc., because the plant would never have been shut down.
Whatever system the reactor normally uses to exhaust its waste heat would presumably still be available, along with all the power necessary to run it.
The reactor doesn’t care if it’s shut down and producing only decay heat, or if it’s still critical and running at minimal power (equal to or slightly greater than 7% or so reactor power). If, during normal shutdowns, the reactor has cooling systems sufficient to remove decay heat, then the reactor can certainly operate indefinitely at the same power level, even with no load (or minimal load)–so long as electrical power is available.
But isnt running at 7 percent and staying at 7 percent (until things melt) worse than starting at 7 percent and residual heat production exponentially dropping the whole time?
My WAG is that a shutdown reactor has a day or two to get cooling back up and running before things get bad. OTOH, a reactor that hasnt been shut down has more like a fraction of a day to get the cooling back up and running.
That makes sense, but is there the facility to bypass the turbines and route steam direct to the (presumed) seawater heat exchanger/condenser?
My understanding is that these reactors aren’t designed to be throttled by control rods. Instead they run hotter at low power so the density of the water is lower and its moderating effect is less efficient, reducing the chain reaction, and vice-versa. The minimal power level might well be a lot more than 7%.
I suspect we mostly agree, I was stating the boundaries of the situation. What isn’t clear is that there is an available situation where the reactor can run at 7% and keep the generators running, nor is it clear that even if you could, that the system could stay stable just powering the cooling pumps. One imagine it would be possible to design the reactor from scratch to do this, at some unknown additional cost and complexity. Clearly the designers thought such a thing to be unnecessary. History may well be proving them wrong.
Also, running a reactor in such a mode will require continual supervision and control. The designers would likely have decided that if something bad had happened the best plan is to shut it down. You can’t predict exactly which form of bad happens. Indeed most bad things they used to worry about involved failures of plant, many of which would likely have disabled power generation. (We don’t actually know that state of the power generation systems anyway, they could also be seriously damaged.)
As I understand it, the earthquake knocked out both the primary and secondary cooling systems, leaving the tertiary which ran on batteries. After about eight hours the batteries were exhausted. No more pumps, no more cooling; disaster ensued.
Why gennys and fuel could not be airlifted in was not explained.
I read that generators were trucked in but the switchgear and connectors were flooded. There may also have been compatibility problems between a modern genny and the 40-year-old system. Batteries were apparently airlifted in.
Oh, man. Parts of Japan are at 50Hz, parts at 60. It’s a legacy from Gen MacArthur who ruled the place for a while. I’m picturing a big-ass crate being eagerly unpacked and – it’s the wrong kind.
You are correct the reactors automatically shut down (rods dropped in) as soon as the earthquake hit. The plant was designed to do that and, while not a great thing, is fine.
My understanding is once a reactor is shut down like this (along with all the ancillary bits such as the generators) getting it started again is not as simple as flipping a light switch to “On”.
These things are complex creatures.
Further, initially, all seemed well. The reactors SCRAMed, things shut down, backup systems were working. I imagine they figured to let things ride out and settle down then restart everything. No biggie.
Then the tsunami hit (more than one). That whomped their generators and whomped their connection to the grid.
At this point it is a done deal. Restarting the reactors to provide electricity to themselves is out of the question. They have shut down reactors generating too much heat as is and cannot cool them. The pumps were dead. There was no way to start generating electricity without melting the whole thing instantly.
Rock and a hard place. Bad news for them.
ETA: Some new designs do not rely on pumps. They rely on physics of cold water moving towards hot water to provide circulation. This is a very old reactor though so does not have such nifty features.
The short of it is, SO MUCH has to go wrong that I’d love to know exactly what happened on all three reactors. The normal cooling route is to send the steam that goes to the turbines through a bypass direct to the condensing heat exchanger and return the cold water to the reactor. Then there’s two high pressure emergency cooling systems, one fully automatic and powered by steam pressure (of which there should be plenty, for days) and not needing any external power at all. The other is powered directly by diesel backups - NOT the emergency power backups generators that failed but independent diesels. Four of them.
In the event that the two high pressure emergency coolant systems fail, there’s two low-pressure systems AS WELL. One of which probably needs external power but the other of which could get by on battery power.
I’d seriously like to get the technical story on this. Maybe when the movie comes out.
I have not read all your links but here is my understanding of it:
Reactors are running producing power (normal)
Earthquake hits. Reactors SCRAM automatically (control rods drop in the core to stop the reaction). Other systems shut down as well (e.g. turbines that run generators).
Backup diesel generators kick in to provide power to the coolant pumps.
Tsunami hits. It swamps the diesel generators and disables them.
Plant shifts to grid power. Grid gets knocked out by tsunami.
Plant shifts to batteries. This works but batteries only have several hours of charge.
Batteries fail and there is nothing left to run the coolant pumps.
Reactor, despite being shut down, still produces a lot of heat. Water in the core is now “idle” (not being circulated). Water boils away.
Heat from core causes fuel rods to melt. Fuel rods are clad with a zirconium alloy which, upon overheating, produces hydrogen gas (among other things).
Hydrogen gas builds up and explodes.
So yeah, lots of things had to go wrong. Frankly most disasters are an exercise in numerous things going wrong.
It is rare (if ever) that major disasters occur due to one screw up/oversight.
In the case of this reactor they had numerous backups. They messed up on the diesel backup placement not thinking a tsunami would breach the wall.
It did.
It also blasted other power they’d have gotten from the grid.
Well there’s even more than that. There’s a steam-turbine powered, fully-automatic cooling system called RCIC that should run WITHOUT backup power. What happened to that? Then there’s a seperate high pressure emergency cooling system called HPCI which is indicated on the status updates but apparently was never available for use. What’s the story there?
As stated above, restarting a reactor after it has shut down isn’t as easy as flipping a switch. Also, there are probably very strict re-inspection procedures that have to be performed before restarting a reator following a 9.0 earthquake. Pipes, wires, computers, etc may have damaged. I can’t see how you could have trained inspectors check and test all that equipment to restart that plant in the span of 8 hours. Even if the tsunami didn’t happen, I’m sure those reactors would have been shut down for days.