



Japanese nuclear disaster is cause for pause, reflection




I. Fukushima Daiichi - a Veteran Installation




II. Disaster at Fukushima Daiichi 1













III. Can a Meltdown be Avoided?





The question now becomes what to do.  





IV. Grim Lessons From the Disaster











Comments     Threshold


Control rods
By drycrust3 on 3/12/2011 5:05:26 PM , Rating: 2
The next step was to halt the reaction by cooling the power rods, composed of uranium-235.

I thought that a nuclear reactor used carbon control rods to "turn on" and "turn off" the reaction in the pile, and that in an emergency these rods would be lowered (automatically or manually) to kill the reaction; but the way everyone has been talking (and the above quote is just one example) is as though either the control rods were damaged and couldn't kill the reaction, or there weren't any control rods.

RE: Control rods
By JonB on 3/12/2011 5:39:11 PM , Rating: 4
Control rods are not carbon (never have been). For BWR and PWR designs, and this is a BWR, the control rods are made of either Halfnium or Silver/Gadolinium. They are neutron absorbers. Carbon is not a neutron absorber.

The Chernobyl design plant (RBMK) used large blocks of carbon as a Moderator to slow neutrons, not absorb them.

This reactor is NOT still running. The control rods did fully insert. But - since it was running at full power and had been for weeks or months, there is a lot of residual heat from nuclear decay that remains for hours and days. That is the heat that must be removed to avoid melting the fuel pellets.

BWR - Boiling Water Reactors - are relatively simple in their core design and emergency cooling methods. But they need electrical power to work and the earthquake and tsunami disabled their diesel generators used for emergency power.

RE: Control rods
By mianmian on 3/12/2011 6:02:10 PM , Rating: 2
I wounder why there is no secondary emergency power generator. Or they do have them but all get wipped out by tsunami?

RE: Control rods
By Solandri on 3/12/2011 8:01:53 PM , Rating: 2
The primary backup power source was diesel generators which were knocked out by the tsunami (unclear if they're damaged or contaminated with water or both).

The secondary backup was batteries. They ran off those, but they only had enough electrical capacity for a few hours.

The little water they are getting into the reactor is probably being pumped in using power from one of the emergency vehicles at the scene.

The irony in all this is that a nuclear reactor used to generate electricity is suffering a serious accident because it lacks electricity...

RE: Control rods
By Tuor on 3/12/2011 11:21:49 PM , Rating: 2
From what I read, they were submerged in salt water due to the tsunami. That will end their usefulness.

As for your last line, here is a grim story:

There was a US Navy submarine (the USS Thresher, IIRC) that was lost because they scrammed the reactor and did an emergency blow. As they ascended towards the surface, the drop in water pressure caused a drop in temperature, freezing certain valves. When the sub surfaced violently, the air used for ballast was lost and the sub began to sink again. The frozen valves prevented them from putting more air into the ballast tanks. The scrammed reactor could not be restarted anywhere near quickly enough, so they had no propeller. They just sank down until they imploded. All hands were lost.

RE: Control rods
By johnsonx on 3/13/2011 1:28:08 AM , Rating: 2
well, sort of. you got the sub's name right, but most of the details wrong.

the wikipedia account seems accurate:

not sure what any of it has to do with an article on nuclear power plant disasters though. all else being equal, the sub's fate would have been the same if the (presumed) broken water pipe had shorted out electrical systems controlling diesel engines.

RE: Control rods
By Tuor on 3/13/2011 10:39:02 PM , Rating: 2
I was commenting on the last line of the previous post: the irony of not being able to fix a problem due to lack of power, when a nuclear plant produces tons of power. The same was true for the Thresher.

RE: Control rods
By Iaiken on 3/12/2011 6:44:09 PM , Rating: 2

JonB forgot to mention that cooling a BWR is a very delicate procedure as cooling it even a fraction too quickly can cause the fuel rods to shatter into thousands of tiny fragments due to thermostatic shock. If this happens, the fuel rods will not cool evenly and you can end up with a situation where the external fragments cool while the center-most fragments melt together and slowly consume the rest of the rod from the inside out.

BWR's are basically like walking a tight rope, a little too far to one side or the other and you've got no choice but to rely on the net, if the net fails... Well, then that's all she wrote.

RE: Control rods
By drycrust3 on 3/12/2011 8:27:04 PM , Rating: 2
My thanks to both you and JonB for your clear and considerate explanations, none of the broadcasts I had heard mentioned any of this, and especially the way the fuel rods are like glass in that they need to be cooled carefully otherwise they will shatter.

RE: Control rods
By JonB on 3/13/2011 3:16:13 PM , Rating: 2
The fuel rods are really long metal tubes. Inside the tubes are ceramic cylinders of uranium. The melting temperature of uranium is very high, so a true "meltdown" is hard to achieve. The more likely scenario is that the hot metal tubes could become cooled too quickly and cracks could form. The salt water won't help here because chlorides will make metals crack even faster when there is heat and stress.

IF the tubes crack, then they could possibly spill their pellets into a pile at the bottom of the reactor vessel. That is potentially good and bad. It gets them directly cooled by water rather than being inside a tube, but any fission product gases that were trapped in the ceramic matrix will escape into the water, bubble to the surface and then possibly released to the atmosphere. Radioactive Iodine is the predominate gas; that is why Iodine tablets are given to people to saturate your thyroid gland with non-radioactive iodine.

RE: Control rods
By rika13 on 3/12/2011 8:34:32 PM , Rating: 2
The RMBK design had carbon-tipped control rods, which screwed with the speed of the reaction as they SCRAMed the reactor. Most SCRAM systems now are based on dead-man switches, that is they are forced to be off and will snap on given half a chance.

