Saturday, March 12, 2011

Don't These Reactors Have Control Rods ?

Apparently the Japanese reactors are BWR reactors, where the control rods have to be inserted into the reactor from beneath rather than dropped in from the top. Wiki :

In most reactor designs, as a safety measure, control rods are attached to the lifting machinery by electromagnets, rather than direct mechanical linkage. This means that automatically in the event of power failure, or if manually invoked due to failure of the lifting machinery, the control rods will fall, under gravity, fully into the pile to stop the reaction. A notable exception to this fail-safe mode of operation is the BWR which requires the hydraulical insertion of control rods in the event of an emergency shut-down, using water from a special tank that is under high nitrogen pressure.
Did the earthquake damage the insertion mechanism or rupture the nitrogen pressure tank ? Or is it that even after full insertion of the control rods the beast needs continuous cooling ?

Anyone know ? Call me naive, but I assumed in the event of trouble you dropped the rods in (or in this case shoved them up) and that was that.

UPDATE - "is it that even after full insertion of the control rods the beast needs continuous cooling ?" is indeed the answer. As laid out in the post above, there's still residual heat - because nuclear reaction products are in the fuel. These typically have short half-lives, which means they give off a lot of heat as they (quickly) decay - but not quickly enough in the absence of cooling water flow.

The estimated death-toll seems to be rising rapidly. The images of the water rolling towards people and vehicles looked very nasty, and gave the impression of potential casualties in the thousands or tens of thousands. I've not been able to gather from the news how much time the people of Sendai and the north-east coast had to flee in between the earthquake and the tsunami.

Social solidarity seems to have been magnificent. I heard a TV report that supermarkets were cutting food prices, and couldn't but wonder if in similar case they'd be raised over here. I bet there's either none or very little looting. In the July 2007 UK floods, not only were many abandoned cars broken into, but when free packs of bottled water were left on the pavement for those who needed it there were reports of people loading hundreds of bottles into vans - presumably to sell.


dearieme said...

Even if you turn off the chain reaction you've still got the decay heat to worry about. I'm assuming that that's what their current concern is. But then I've never paid much attention to BWRs, I admit.

JuliaM said...

There's something like 10000 listed as 'missing'...

One of the big vending machine companies immediately switched all it's machines to 'free vend' yesterday morning. Can you imagine a UK company doing that? We don't even have the technology, for a start.

Laban said...

Surely the decay heat won't be that much, will it ? Aha - it's not the heat of the uranium decay, but the heat from the fission products. Wiki :

"On a SCRAM for a reactor that held a constant power for a long period of time (greater than 100 hrs), about 7% of the steady-state power will initially remain after shutdown due to the decay of these fission products."

Laban said...

Wiki again.

"The removal of the decay heat is a significant reactor safety concern, especially shortly after normal shutdown or following a loss of coolant accident. Failure to remove decay heat may cause the reactor core temperature to rise to dangerous levels and has caused nuclear accidents, including the nuclear accident at Three Mile Island."

CJ Nerd said...

Yes, it's the heat from the decay of the very unstable fission products produced while the reactor was operating.

An engineer interviewed on BBC News 24 said that would account for about 10% of the full power.

Putting control rods in would therefore only shut off about 90% of the power produced.

Foxy Brown said...

Social solidarity seems to have been magnificent.

The cultural glue is strong because Japan is almost homogeneous, save for a few foreign workers who work on temporary visas. I've just checked Wiki, and citizenship is rarely awarded to outsiders.

The lack of looting could be put down to Japan being a shame culture. A shame culture guides personal conduct and compels individuals to behave with honour and dignity. In other words, what others think of you and how you are regarded by them is very important.

staybryte said...

I've been to Japan. Social solidarity, I predict, will remain very strong.

I wish them well.

Anonymous said...

Yes its all well and nice that they are giving away free stuff, but as you know its not really free.

Someone is paying the cost for that, and the large corporations can only afford to do it a small amount, unless they get reimbursed from the government.

Wouldn't surprise me if they had a special deal with the government to cover these kind of situations, as Japan prepares for just this kind of emergency.

Anonymous said...

Still surprising to me that they can't cool the reactor if as you say its only running at 10% of normal..
There cooling system can usually cool 10 times as much and probably has a backup tolerance of at least 2x for safety.
So really if the cooling system is working at all if should be able to do enough?

If they are still having a problem, the cooling system must have packed up completely. Which i guess is why, last I heard, they are trying to flood it with sea water instead.

Robert Carruthers said...

There is a good explanation here:

Furor Teutonicus said...

XX or rupture the nitrogen pressure tank ? XX

Could explain the explosions?

Anonymous said...

The temperature of the steam splits the molicules into Oxygen & Hydrogen and the Hydrogen is what caused the explosions.

Anonymous said...

They are using (pumping) sea water to cool the reactor - where does the heated (polluted) water then go - back in the sea! For a country of fish eaters that might not be great news!

safe meds said...

I think that The lack of looting could be put down to Japan being a shame culture. A shame culture guides personal conduct and compels individuals to behave with honor and dignity.

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