Wednesday, March 16, 2011

One Woe Doth Tread Upon Another's Heels ...

... so fast they follow.

Fire has again broken out at the quake-stricken Fukushima Daiichi nuclear plant in northern Japan. The new blaze began at reactor four... on Tuesday morning, a third blast hit the building of reactor two, while a fourth damaged the building of reactor four, where a fire also broke out in the unit's spent fuel storage pond. Reactor four had been shut down before the quake for maintenance, but its spent nuclear fuel rods were still stored on the site.

Officials said the explosions at the first three reactors, and possibly the fourth as well, were caused by a buildup of hydrogen. Chief Cabinet Secretary Yukio Edano said officials were closely watching the remaining two reactors, as they had begun overheating slightly.

He said cooling seawater was being pumped into reactors one and three - which were returning to normal - and into reactor two, which remained unstable.

What ? Number 4's meant to have been closed down before ever the quake and tsunami hit ! And when was the first explosion in #4 (apparently just before the explosion in #2 yesterday)- we were told about a fire in the fuel storage pool, not an explosion ?

This morning it's not looking too good :

A rise in radiation levels at Japan's stricken Fukushima nuclear plant has forced workers to suspend operations, a government spokesman says. He was speaking after smoke was seen billowing from reactor three. Earlier, a blaze struck reactor four for the second time in two days.

Reactor 3 has uranium/plutonium fuel - you really don't want bits of plutonium in the air.

There are a lot of things that don't make a huge amount of sense to ignorant me :

1) how's the hydrogen being created for these explosions ? Is it created by the fuel's cladding reacting with steam or boiling water, which will create hydrogen only, or is it as some are saying thermal decomposition of water, which requires a temperature of 2000C and which will create hydrogen and oxygen (which are likely IMHO to recombine explosively the moment temperature or pressure drops - like when the core is vented) ?

2) where's the hydrogen coming from ? In #1 and #3 we know they were venting the pressure, so that's a source of H2 - although you'd think they'd put a hole in the roof of #3 to let it escape, following the bang at #1. But I'm not sure about #2 - did it ever get vented ? Yet it went pop all the same.

3) what's with 4, 5 and 6 needing more cooling? Presumably the residual heat is the problem.

4) so why are the pumps proving such an issue (other than that they've been subject to frequent explosions) - what's been the problem with getting more pumps and more power supplies on site? I presume they're pretty specialist beasts and you can't buy them at Machine Mart - but there are aircraft to collect them and helicopters to deliver, along with the fuel. Is there not enough of that specialist deionised, ultra-pure cooling water available ? Obviously not.

It would be nice if the reactor company could keep the Japanese people (and the rest of the world) updated, but I guess it's not exactly a priority. In a situation like this, when there's big trouble and it needs to be fixed fast if at all possible, the role of management is to keep everyone off the backs of the people doing the work, so that they don't have to waste time and mental energy on anything but what's in front of them.

UPDATE - if this is true, it's not looking good. Let's hope it isn't.

At the plant, desperate and improvisational measures have become the rule. Japanese Self-Defense Forces helicopters took off from a nearby base Wednesday afternoon carrying giant red buckets on a line used to scoop up seawater to douse the plant's Unit 3 reactor building. Tepco told nuclear safety officials they had no other way of cooling the reactor's fuel rods. Kyodo later reported that the helicopters were unable to drop water due to high levels of radiation.


CJ Nerd said...

I'll answer briefly, from what I've picked up in various places.

But first I'd suggest you read here:

This is a blog by a geologist whose father was a nuclear engineer for the US Navy, then worked at various American power stations, including one using similar designs of reactor.

CJ Nerd said...

"1) how's the hydrogen being created for these explosions ?"

"2) where's the hydrogen coming from ?"

The fuel rods are made of zirconium. When steam hits zirconium at over 2200 deg F, the zirconium breaks the H2O down by nicking the oxygen, and the hydrogen floats away.

CJ Nerd said...

"3) what's with 4, 5 and 6 needing more cooling? Presumably the residual heat is the problem."

When fuel is in a working reactor, uranium nuclei are getting hit by neutrons and broken down into smaller nuclei (plus energy). These smaller nuclei are what we refer to as the "fission products".

When the chain reaction is stopped, these fission products remain. These are unstable things. Typically, they have one or two more neutrons more than they 'ought' to have. Such unstable things have no business existing (which is why they don't exist in nature).

Because they're unstable,they fall apart- and when they do so, they break into some further products, which may themselves be similarly unstable, and fall apart too. Eventually- sometimes after up to half a dozen steps- they end up as something stable.

Now, many of these decay events produce energy. So, even though the chain reaction has been shut off, the fuel rods sit there with their fission products decaying away, and producing heat.

So, you can't just switch the reactor off and see it go to zero energy production. It's not an on-off switch, it's more like a variable control that goes from 100% down to about 7%.

So, when the reaction stops, you have to keep cooling the reactor for a few days while that 7% drops to the point where it won't cause problems.

All the reactors were shut down as of a couple of minutes after the quake. Some were already shut down anyway for various reason, and those that were running were shut down PDQ.

And then the backup cooling systems started working. All fine and dandy... till the tsunami took out the cooling systems.

CJ Nerd said...

"4) so why are the pumps proving such an issue ?"

The reactors are inside containment structures, where the pressure is elevated well above atmospheric pressure, because of all that boiling water and hydrogen.

If you want to pump water into there, the pump isn't just moving water from A to B. It has to push harder, because the pressure inside the vessel will try to push the water straight back out again.

A normal common-or-garden water pump from Machine Mart would have difficulty doing this. I presume even fire brigade pumps would struggle.

I did hear an hour or two ago that the USAF has just flown in some suitable pumps from America.

Of course, there are also now problems with lack of electric power, bits of blown-up building being in the way, and radiation leaks from the various reactors.