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Update on the situation at Japan’s Fukushima nuclear plant March 15, 2011

Posted by apetrov in Near Physics, Physics, Science, Uncategorized.
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The situation at Japan’s Fukushima nuclear plant remains fluid, but it makes sense to do an update. It turns out that the situation is more challenging then I originally thought. To recreate what is happening (based mainly on TEPCo’s press releases and Japan’s Nuclear and Industrial Safety Agency (NISA) press releases), let us take a look at the Mark 1 BWR reactor (for a short description of physics of the nuclear power generation and schematics, please see my earlier post):

This picture was modified (by me) from the materials provided by Department of Energy’s Nuclear Regulatory Comission’s (NRC) website. It is Mark-1 BWR-type nuclear power reactor supplied by General Electric.

The Fukushima Daiichi plant operates six reactors, Units 1, 2 and 6 are supplied by General Electric (Unit 1 is the oldest, built in the 70′s — I’ve heard it was supposed to be decommissioned this Spring), while Units 3-5 are supplied by Toshiba and Hitachi. So, what is happening there?

As you already know, the magnitude 9.0 on Richter’s scale earthquake hit Japan. The reactors at the Fukushima plant were designed to withstand the 8.2 magnitude quake. Nevertheless, the structures held (note that the Richter scale is logarithmic, meaning that 9.0 earthquake releases 10 times energy than 8.0). Since Japan is located in seismically-active zone, there exist provisions on what to do in case of one, especially for the nuclear power stations.Reactors 1-3 were operational at the time of the earthquake, while reactors 4-6 were in a shutdown mode.

So, first and foremost, control rods (containing boron, neutron-absorbing material) were automatically inserted. According to  TEPCo’s press release, this was done successfully at all three units that were in operation. There was an alarm on Unit 1 that one of the rods was not fully inserted. The alarm then went away. It is now believed that all control rods were fully inserted and chain reaction in fuel assemblies was stopped. Even after this, one must keep circulating water in order to continue cooling fuel assemblies due to the heat produced by decays of nuclear reaction products in the fuel rods. It needs to be done for several days.

It appears that over the course of three days reactor cooling systems kept failing, which resulted in increasing steam pressure in the reactor pressure vessel (see the picture above). In this case you really don’t want to keep the pressure rising, as it eventually would simply blow up the containment vessel and you’d get pretty much what happened in Chernobyl. So, the idea is to gradually release pressure by disposing the (slightly radioactive) steam through the vent line (see above picture). The steam is only slightly radioactive because one is using purified water, which does not get activated by the radiation from the fuel assemblies. This was done at all three units. You have to still keep cooling the core, which was done at Units 1 and 3 with injection of seawater into the Primary Containment Vessel and at Unit 2 with seawater injection into the Reactor Pressure Vessel. Injecting seawater is a desperate move, as it contains salt and other staff that can get activated. Which means that the reactor will be decommissioned regardless of whether there is a meltdown or not. Along with seawater, they injected boric acid to capture neutrons.

Now, if the cooling is ineffective (as it appears is at Fukushima) and you keep disposing steam, you lose the amount of water you have in your reactor (think of a boiling teapot). This leads to water levels in the reactor dropping to the point that the fuel assemblies get exposed to steam. This is what happened at Fukushima. This is bad, because this drops cooling efficiency and fuel rods start to heat up (recall the decays of radioactive decay products that are still going on). At some point, zirconium in the ziralloy (the alloy of zirconium and tin that makes up the fuel rod casings) starts react with water vapor. Here is the chemical reaction:

2 H20 + Zr = 2 H2 + Zr O2 + energy

which means that you start producing hydrogen (H2), some of which will escape into the reactor building. Most likely, escaped hydrogen exploded in units 1-3, blowing off the roofs of the reactor building hosting Unit 1, 2, and 3, like this:

This picture is done by the local TV station and posted on Wikipedia. According to the power station owners, the containment vessels are still intact, which is precisely what they are designed to do. Let’s hope that this is an accurate assessment.

Now, if there is a meltdown (fuel rods are damaged), some of the reaction products might get into the atmosphere (the troubling news is that the monitoring stations did detect small amounts of iodine nearby the reactor). The most immediate concern are radioactive Iodine (half-life of 8 days) and Cesium (half-life is 30 years). Iodine can accumulate in human’s thyroid gland – so the first line of defense is to saturate the gland with non-radioactive Iodine. This is why the population around the station is given iodine tablets as a precaution. The detected amounts of iodine are not of a concern for the US West Coast (too far).

In the case of a serious meltdown, the melted fuel will likely remain in the reactor containment below the rector pressure vessel. This would be bad, but still nowhere near Chernobyl’s explosion. BTW, I was on a school trip in Kiev when the Chernobyl power station blew up. I had to bury my shoes because the radioactivity levels on them were too high (dust)…

To add to the problem, rector unit 4 (which was not operational at the time of an earthquake) developed problems of its own. In particular, it appears that the personnel missed that the water level in the spent fuel pool came down. This exposed spent fuel rods that contain more long-lived radioactive isotopes. You want to keep spent fuel rods in the water to cool them, as the decays still produce heat. In this case, usual convection cooling (warm water is rising and is replaced by cooler water) is sufficient to keep them cool. That is, if there is water! There was report of a fire at the spent fuel pond. This might indicated that the water level in the pool went down and spent fuel caught on fire. This might be bad, as this would release radioactive material in the air. Japanese scientists monitor the situation.

I’ll try to keep you posted as well.

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Comments»

1. Update on the situation at Japan's Fukushima nuclear plant … | H2O Report - March 15, 2011

[...] the original post:  Update on the situation at Japan's Fukushima nuclear plant … a-nuclear-plant, about, cosmic-variance, feeds, first, fuel, japan, monitoring, nuclear, reactor, [...]

2. Mark - March 15, 2011

Information I heard is that it was a lube oil fire… not the spent fuel in the fuel pool….. fire is out.

apetrov - March 15, 2011

That would be great! Thanks for the info!

3. Nick Dawson - March 16, 2011

Thank you for these detailed posts, most informative

4. Hugo - March 16, 2011

Assuming all its correct, and i believe so (dont take me wrong, but without full information i can only assume) since it came from schematics and other reliable sources, including apetrov that i’m not putting in cause in anyway, i raise up only one question (well not really a question, but more like an asking for comment), that it not seems so clear to me:

(Note that i dont know the specifics of the various types of reactors that are talked along the text)

when apetrov talks about the reaction between water and zirconinum, as i can interpret from the 1st schematic present in the previous post (http://apetrov.wordpress.com/2011/03/12/what-is-happening-at-japans-fukushima-daiichi-power-plant/)this reaction always happens, since water is always in contact with the fuel bars (casing), or, this reaction is not supposed to happen (and by this i mean the casing of the bars is somehow protected/inhibited to have this reaction). So we have at least two possible scenarios.
1º an hydrogen explosion, how its appointed (that i really dont understand so much by looking to the schematics). If so (and we must not forget that gas/liquid interaction are not so easy to explain)the explosion would be inside the reactor since its there the reaction is happening and where the hydrogen has the potential to be ignited (exotermic reaction + pressure + some O2 that may exist in this form).

2º a pressure explosion (that i understand better). In this case the pressure was released from the reactor to the reactor building (secondary chamber)that even if it was some way to release pressure to the outside this release way has is limmits (and from the schematics i dont see any emergency release valve). So what we have is steam contacting with the “hot” fuel bars and producing super heated steam (up to 150 ºC if i’m not mistaken) that will do more steam by evaporating even more water in the reaction chamber and that would cause pressure to raise up even more. As i safety measure the pressure was release to the secondary chamber that couldn’t hold it.

Thank you for the atention.

Hugo.

5. Rick - March 18, 2011

One of good things is -11 C overnight temperature, who may help the cooling process in
the Spent fuel storage.

6. SELVA NATHAN - March 29, 2011

A country like Japan until a few mths ago the worlds 2nd largest economy this can happen what not if this were to happen in another developing country.
My suggestion is ban nuclear fuel if you are really intrested in saving the world as this is the Final warning from the one above But if you still want to use it should be monitored by a group of experts fron the 4 corners of the world and if they say shut doun the plant all must agree to it and not put money 1st,its a tall order choise is yours. Tks


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