Ping TNP re gridwatch

[Tim Streater]

Are the Japanese ones graphite-moderated? And reading accounts of Chernobyl, it's clear that poor operator procedures contributed a *lot* to the situation.

[The Natural Philosopher]

I don't think so.

a substantial fraction of the chernobyl core was released - that's as bad as it can get.

No reactor can sustain a chain reaction if its physically compromised - the worst that can happen is full core meltdown and breach of containment. At that point you have a hot radioactive puddle of crap.

Chernobyl was worse because it wasn't totally damaged and I don't think the chain reaction stopped for some time.

to be honest it wouldn't have been a lot worse...in some ways if the radioactive material had ended up in a site flooded full of seawater, the overall radioactivity wood not have been much more and it wouldn't have been in the air..it would have been in the sea.

[The Natural Philosopher]

Not sure about the moderators..Boron I think in the case of the GE BWR..I think its likely they are..on earthquake the rods are dropped /pushed straight into the core to shut the chain reaction down. That happened at Fuku and in fact on about half Japans total fleet..

The problem was that the radioactive products in the reactor were as you probably know, still capable of generating a lot of heat as they decayed into more stable compounds.

each reactor was about 3GW of thermal output and the decay heat was about 3MW.

3MW was enough to boil the coolant and melt the fuel rods in the absence of pumps.

The GEN 1 GE BWR is reckoned to be the most dangerous and badly contained reactor still in use..later ones had more concrete and steel around them to contain them if things went wrong.

The irony is that even at Fuku , if they had had the diesels and tanks up high and not in the basement, very little would have happened.

[dennis]

There would have been a huge steam explosion if the Japanese reactors had been severely damaged by the quake before the tsunami hit them.

Lets get this right, Chernobyl couldn't sustain a reaction because it was physically damage, but it did?

Chernobyl had tons of boron sand put on it to stop the reaction as it was still running even though it was on fire and had partially melted down. The helicopter pilots that did that died IIRC.

[The Natural Philosopher]

"The interaction of very hot fuel with the cooling water led to fuel fragmentation along with rapid steam production and an increase in pressure. The design characteristics of the reactor were such that substantial damage to even three or four fuel assemblies can ? and did ? result in the destruction of the reactor. The overpressure caused the

1000 t cover plate of the reactor to become partially detached, rupturing the fuel channels and jamming all the control rods, which by that time were only halfway down. Intense steam generation then spread throughout the whole core (fed by water dumped into the core due to the rupture of the emergency cooling circuit) causing a steam explosion and releasing fission products to the atmosphere. About two to three seconds later, a second explosion threw out fragments from the fuel channels and hot graphite. There is some dispute among experts about the character of this second explosion, but it is likely to have been caused by the production of hydrogen from zirconium-steam reactions.

Two workers died as a result of these explosions. The graphite (about a quarter of the 1200 tonnes of it was estimated to have been ejected) and fuel became incandescent and started a number of firesf, causing the main release of radioactivity into the environment. A total of about 14 EBq (14 x 1018 Bq) of radioactivity was released, over half of it being from biologically-inert noble gases.*

*The figure of 5.2 EBq is also quoted, this being "iodine-131 equivalent" - 1.8 EBq iodine and 85 PBq Cs-137 multiplied by 40 due its longevity, and ignoring the 6.5 EBq xenon-33 and some minor or short-lived nuclides.

About 200-300 tonnes of water per hour was injected into the intact half of the reactor using the auxiliary feedwater pumps but this was stopped after half a day owing to the danger of it flowing into and flooding units 1 and 2. From the second to tenth day after the accident, some

5000 tonnes of boron, dolomite, sand, clay and lead were dropped on to the burning core by helicopter in an effort to extinguish the blaze and limit the release of radioactive particles."

So it looks like the chain reaction has ceased pretty much as soon as the rids melted. The boron was simply there as part of a general dousing and limitation of neutrons type exercise.

No, it wasn't 'running'

Essentially it was fully melted down with a large carbon fire carrying almost the entire contents of the reactor out in a totally uncontrolled fashion.

[Andy Champ]

Some of the kite surfers use inflatable kites. Does that count?

Andy

[The Other Mike]