Fukushima update: cracked fuel rods threaten meltdown - defect discovered in No. 2

DaveAlpert David R. Alpert
NHK: TEPCO says fuel rods in No.2 reactor at Fukushima nuclear power plant are "likely" to be exposed again

#Japan's Chief Cabinet Secretary said a partial defect has been found inside the containment vessel of reactor 3 at #Fukushima. -Kyodo News


http://www.newscientist.com/blogs/shortsharpscience/2011/03/japan-megaquake-update.html


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Although all four reactors automatically shut down immediately after Friday's earthquake, engineers have struggled to cool down the reactor cores, because pumps that should have driven cooling water into the reactors failed. This meant that the reactors overheated, turning the water into steam.

The buildup of steam meant the pressure inside the reactor increased, making it impossible to pump more cooling water in. So the engineers vented the steam, carrying some radioactive caesium-137 and iodine-131 (both of which are produced by the uranium in the fuel rods) into the environment.

Michael Bluck of Imperial College London explains that the fuel rods are tubes of zircoloid stuffed with uranium dioxide. When these are not cooled enough, they swell up and can crack. At that point, radioactive caesium and iodine gases can escape. Bluck says:

When caesium was detected, that indicated that some of the rods had ruptured.

As the zircoloid heats up, it reacts with the cooling water to form hydrogen, which is a highly explosive gas. This was to blame for the dramatic explosions that damaged the outer buildings of reactors 1 and 3.

However, it is now reactor 2 that is causing the most concern. Replacement pumps intended to inject cooling water have repeatedly failed, meaning that water levels fell and the fuel rods overheated still further. According to Kyodo News:

Water levels sharply fell and the fuel rods were fully exposed for about 140 minutes in the evening as a fire pump to pour cooling seawater into the reactor ran out of fuel and it took time for workers to release steam from the reactor to lower its pressure, the government's nuclear safety agency said.

Then within the last few hours a further accident occurred. Kyodo News reports:

Tokyo Electric Power Co. said Monday fuel rods were fully exposed again in the No. 2 reactor of the Fukushima No. 1 nuclear power plant as of 11 p.m. TEPCO said a steam vent of the pressure container of the reactor that houses the rods was closed for some reason, leading to a sudden drop in water levels inside the reactor.


Meltdown?

This series of coolant failures has increased the chance that the fuel rods will start to melt. Bluck says:

If you fail to cool it, the uranium can melt and it will all fall to the bottom as a big soup.

But even if the rods do melt and sink to the base of the reactor vessel, this shouldn't be a problem unless the vessel itself breaks open. "The big question is whether the containment holds," says Wakeford. "There was a meltdown at Three-Mile Island in New York, but the vessel remained intact."

#2 container was filled one time with emergency water coolant, then inexplicably was found to be completely lacking in water/coolant. Coolant was added again and disappeared again. I am wondering if it cracked following the violent concussion of the hydrogen explosion from adjacent reactor #3. It would have been a highly asymmetric load on the #2 vessel containment. They have said they are most concerned right now with this #2 unit and are paying more attention to it, which seems odd given how relatively "stable" it has been compared to #1 & #3 and was before the #3 hydrogen burst (or at least how stable we believed it to have been given so little information). They may be concerned that this vessel in particular has been compromised.

If one reactor loses containment, the whole site will be hot as hell and completely inhibit work on adjacent reactors or the extremely important spent rod storage pools... not to be alarmist.

Edano says they found a defect with a piece of equipment that converts steam to water, located between the reactor and the containment wall.

"Three-Mile Island in New York"

OK, so they flunked geography, but I am sure they are geniuses in nuclear science.

Give them a break! :cry:

"For the attention of the residents of Prypiat! The City Council informs you that due to the accident at Chernobyl Power Station in the city of Prypiat the radioactive conditions in the vicinity are deteriorating. The Communist Party, its officials and the armed forces are taking necessary steps to combat this. Nevertheless, with the view to keep people as safe and healthy as possible, the children being top priority, we need to temporarily evacuate the citizens in the nearest towns of Kiev Oblast. For these reasons, starting from April 27, 1986 2 p.m. each apartment block will be able to have a bus at its disposal, supervised by the police and the city officials. It is highly advisable to take your documents, some vital personal belongings and a certain amount of food, just in case, with you. The senior executives of public and industrial facilities of the city has decided on the list of employees needed to stay in Prypiat to maintain these facilities in a good working order. All the houses will be guarded by the police during the evacuation period. Tovarishchs, (Comrades) leaving your residences temporarily please make sure you have turned the lights, electrical equipment and water off and shut the windows. Please keep calm and orderly in the process of this short-term evacuation."

This whole situation is making my stomach churn, as I recall the past.