Another Explosion at Fukushima Nuclear Plant | Hacker News Reader

Another Explosion at Fukushima Nuclear Plant

bbc.co.uk

50 pointsolivercameron45 comments

Insightful reporting on the scale of the problem from NYT: http://www.nytimes.com/2011/03/14/world/asia/japan-fukushima...

The major issue seems to be that the power equipment is in a room that is flooded, so regular cooling is impossible. They're pumping in seawater, but they have to release pressure as it builds up and this released steam is irradiated. Until some kind of regular cooling system is rebuilt, this process to prevent meltdown will continue for the forseeable future.

How long does this post-shutdown cooling process take?

I thought I'd read something like 48 hours in the immediate aftermath of the quake, but that's obviously not correct.

I saw a figure earlier today (can't recall where sorry) that once the control rods are in and the reactor is shutdown it still produces 6% of the usual amount of heat, which is enough to still require some cooling.

The decay heat is about 5% of full reactor power at shutdown, but decreases exponentially thereafter, to about 1% at 4 hours, etc. Daiichi units 1 & 3 are about 1.4 and 2.4 MW(th) respectively.

I'd read that the problem was residual heat, as the control rods should have dropped as soon as coolant flow halted. This means that no new heat is being added to the system, but that the reactor will experience a gradual decline from its 3000 MW operating capacity. All of that energy is stored as irradiated steam, which can be forced into pools of seawater beneath the plant to cool it, but only so much.

I don't think it's true to say no new heat is being added. The uranium fission process has stopped, but there are secondary radioactive elements created by the decay of uranium. These elements will continue to decay, as such the core is still producing the heat. Though eventually this heat will decline to a safe level, provided there isn't a meltdown beforehand.

The design oversight seems to be that they depend on external AC power or diesel generators for backup. Surely there should be an emergency generator that can run off the heat that the core still produces in order to at least partially operate the cooling system. Much like aeroplanes have a small dynamo that runs off a tiny wind emergency wind turbine so they can still operate the hydraulics with no engines running. A saving grace in the case of Gimli Glider (http://en.wikipedia.org/wiki/Gimli_Glider).

Ah, thanks for the explanation. As I said, that was just what I had read. I appreciate the further info. What you said makes sense, and explains why steam pressure is still building.

No problem. Further to my comments about the design, I read in another story linked to on HN that generation III reactors now use convection based cooling. I gather this means the water is taken elsewhere as steam to cool then returns via gravity to the core. Therefore you wouldn't have this same kind of issue with a modern reactor.

Interesting. That seems like a smart way of doing it.

My understanding is that new heat continually gets added because the nuclear byproducts decay. It's exponential decay with a long half-life.

I hadn't thought of this, but it makes sense, considering that normally the cooling rods are kept in ponds for years after being removed from the reactor.

this released steam is irradiated

Just as a pedantic point, irradiated steam would not be hazardous. Steam with radioactive particles mixed in, on the other hand, would be.

Thanks, my mistake! Irradiation is the process of exposing to radiation while in this case the steam actually contains radioactive particles.

Yah, the steam itself is perfectly safe. It's only what is mixed in that is a problem.

I can't imagine there is very much mixed in though, water boils at much lower temperatures than the other things there. And I assume they are venting from the top, so it should be pretty pure. (Worlds most expensive steam distillation :)

> and this released steam is irradiated

I wonder where this steam is going to end up. All those lambasting us about how this is "not another Chernobyl" seem to forget mentioning this, as if all this mess isn't serious enough if it's not on par with Chernobyl.

Timeline from BBC Live Page: (Reverse Chronological Order)

0328: Seven people are missing and three people have been injured by the explosion at the Fukushima 1 nuclear plant, the AFP news agency reports, quoting an official from Tepco, the company which operates the plant.

0247: Mr Edano said major radiation leaks were unlikely from Reactor 3.

0243: Japanese government spokesman Yukio Edano has just spoken on TV. Says that water injection at Reactor 3 seems to be continuing, and the containment vessel is still safe.

0242: Reactor 3 withstood the explosion(s), its operator says - Japanese news agency Jiji.

0240: The governor of Tokyo orders radioactivity levels in the city to be measured - Kyodo.

0239: The 600 people still living within 20km of the plant where the explosion(s) occurred are ordered to get inside buildings - Kyodo.

0236: The wall of a building collapsed as a result of the blast(s) at Reactor 3 - Japanese TV.

0232: Update on the explosion(s) at Reactor 3: "We believe it was a hydrogen explosion. It is not immediately known if it affected the reactor" - nuclear safety agency spokesman Ryo Miyake.

Another update: Tepco has confirmed that 11 engineers were injured in the blast. No word on their condition.

To be clear this is Unit 3, not the same reactor that blew up previously (Unit 1). Though dramatic, these are NOT nuclear explosions but hydrogen explosions that result from venting steam from the reactor. The primary containment and the reactor pressure vessel remain intact. Tepco (the Japanese utility) previously warned that this might occur to Unit 3.

Video Here: http://my.firedoglake.com/scarecrow/2011/03/13/japan-nuclear...

Wow, I know nothing about nuclear plants and the fail safe's they have built in, but that looks like a scarily powerful explosion.

NHK is saying it's another blast like Fukushima Daiichi #1 on Saturday/Sunday - Hydrogen collecting in the reactor building because of the fuel cladding burning and then being vented with the steam to reduce pressure.. according to the authorities so far, the reactor containment was undamaged and workers in the area are to remain indoors.

I don't think it's that big. It has no visible effect on the nearby buildings (the towers).

Wow, I know nothing about computers or hacker ethos, but the name "Hacker News" sounds scarily powerful.

Do we really want to keep these computer criminals up to date?

A scaled up aerial photo of the blown out building #3 now the smoke has cleared.

http://gigazine.jp/img/2011/03/14/fukushima_daiichi_3rd/Capt...

Why can't the hydrogen and oxygen be safely vented out and burned as soon as they are produced?

Hydrogen and oxygen only react at significant rate at high temperatures. In a hydrogen explosion, the rate initially starts off low, but as the reaction is highly exothermic, heat from the reaction increases the temperature and causes the reaction to accelerate until reactant concentrations are sufficiently depleted.

This means that hydrogen and oxygen can only react quickly if the concentration of hydrogen in a volume is high enough that the increase in temperature from the exothermic reaction is faster than the dissipation of heat, so the temperature will rapidly expand, causing an explosion. For this reason, hydrogen tends to build up and then explode, rather than burning continuously.

It might be possible to trap hydrogen in a small volume and burn it in a series of small explosions; however, I don't think the reactors are designed that way.

Couldn't you stick a piece of platinum at the mouth of a vent and just vent it steadily?

IANANS, however from what I understand the problem is this: -> The reactor is a BWR type, hence even though it is scrammed it is still generating some amount of power from fission products. -> The ECCS failed, meaning that the core can't maintain sufficient cooling. -> So what water was left in the core is then exposed to very high temperatures and pressures. -> That apparently then creates conditions sufficient to cause the water to react with the zirconium alloy which is used in the fuel rods, forming zirconium dioxide and hydrogen. (2H2O + Zn -> ZnO2 + 2H2). -> As reactors generally don't create much gaseous waste (The only waste products that I can think of would be, for example, radon, and that would be in small quantities), and the core isn't expected to be above the autoignition temperature of whatever gas that is being vented. What could be happening is that the gas is being vented but is being concentrated at some point where it comes into contact with something hot enough to ignite it thus causing an explosion. For example, if the gas is being vented out of the core, but into a secondary containment vessel, if the vessel is filled with air and if it contains say a piece of metal hot enough to ignite the hydrogen, it would simply fill with gas until it reaches a point where the mixture is explosive, before then it then explodes.

Disclaimer: I'm not a nuclear scientist, any corrections/additional information would be very welcome.

As I understand it, anything that gets vented from the outer structure first goes through an activated charcoal scrubber to remove radioactive contaminants. This may limit the rate at which gas can get vented.

Oxygen and hydrogen form explosive mixtures over a very wide range of relative concentrations, which doesn't help.

Any amount of hydrogen vented into the outer building could be burned with environmental oxygen, regardless of how much of it available in the exhaust. Can't be that hard.

Can't be that hard, really? I can't say one way or another but I'd kinda expect the experts to have thought of this at some point if it was a trivial possibility.

I read the article referenced here (http://news.ycombinator.com/item?id=2318980), and since the author seems so certain that the third containment shell would hold the meltdown safely: Can someone answer me why won't they just allow it to happen, so they can clean up the rest and get the whole nuclear plant back online? Is it just an economical reason? Or is there any other risk?

Even if they're 99.9% confident that the containment would hold against a molten reactor NO ONE would want to be the official to actually just "let it happen."

If you allow meltdown, you're looking at contaminating the entire reactor building. Clean up costs would be astronomical. If the pressure vessel remains intact, the whole thing can be lifted out with little effort, leaving an uncontaminated building behind.

Why? Isn't the "last line of defense" prepared to avoid any contamination at all?

Yes, of the surrounding environment. This is a single building more or less single-purpose-build to be contaminated as a last measure.

All these exercises are inherently economical -- they could just have walked away on the first day and have the whole thing melt down, and the surrounding area wouldn't have been any better or worse off, radiation-wise.

As I understand it, spreading out the waste leads to more material being contaminated. Surrounding concrete, pipes, cranes beams etc... Most equipment used ends up contaminated. I know an engineer who cleans these places up, and its mad, bad and very very expensive. Massive amounts of planning for simple jobs. Plotting safe routs, zones, dose rates etc. Maximum containment makes the job way way easier.

I'm going to guess that building a new reactor is a lot cheaper than fixing up a 30 year old reactor that has gone through even partial meltdown, and that no no-one involved in managing the reactor is worried about getting it back online.

40 years in the case of unit 1, and scheduled to be decommissioned in 2011 anyway.

Things get worse as the temperature goes up. Instead of the rods starting to melt, eventually the fuel could melt.

(Also, there's no chance of it going back online after getting flooded with seawater.)

The fact that these explosions aren't damaging the containment actually makes me pretty confident that nothing else will either.

Something has been bothering me: if Japan is so well prepared for earthquakes, how did they have a nuclear reactor right on the coast with no protection from tsunami? Wouldn't a barrier of some sort have prevented this whole mess? Or elevated backup generators?

They actually have lots of--if not all--reactors along the coastline. It's about having access to water and Japan isn't exactly a land of rivers and lakes.

http://i.dailymail.co.uk/i/pix/2011/03/13/article-1365569-0B...

They did have protection from tsunamis. This one was too big.

A couple of stories have covered this, their wave estimation was low. That being said, its clear they can improve and had the imported generators been able to connect to the plant's cooling power plugs this would have been moot.

My guess is that positioning backup generators are safe from tsunamis and that can be moved into position quickly, and are pre-tested to be compatible with the backup infrastructure will get added to the licensing regulations.

Anyone know where all the engineers would be located in a plant like this - an onsite shielded bunker? What if they were to get hit by an explosion like this - remote redundant control far offsite?

I think the main control room and nearby offices are not bunker like but certainly reinforced to withstand damage from accidents. I have been onsite to a couple of reactors and most of the engineers are housed in regular administrative buildings with no special reinforcements but these are outside of the reactor building where the control room is located. There is no remote control room but there are (at least in the USA) dedicated communication channels to industry experts (probably GE engineers that designed these plants) available to assist and advise 24/7 during an accident. Unfortunately, one report said that 11 workers were injured in the explosion at Unit 3.

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