Spontaneous Fission In Fukushima Daiichi Unit 2

Kyusaku Natsume writes "Tokyo Electric Power Co. said Wednesday that some of the melted fuel in reactor 2 at the Fukushima No. 1 nuclear plant may have triggered a brief criticality event. Tsuyoshi Misawa, a reactor physics and engineering professor at Kyoto University's Research Reactor Institute, said that if Tepco's data are correct, 'it's clear that the detection (of xenon-133 and -135) comes from nuclear fission.' Tepco spokesman Junichi Matsumoto said the test results suggest that either small-scale fission occurred in the melted fuel, or conditions to trigger criticality were temporarily met for some other reason. He said the same thing could also happen at reactors 1 and 3. But because the reactor's temperature and pressure level have not changed, the fission would not have been large-scale, Matsumoto said, adding that it would not thwart Tepco's schedule for achieving a cold shutdown at the reactors. In response, boric acid water was injected again on November 2. On the plus side, the concentration of radioactive materials in the air is low enough that workers inside some areas of Fukushima Daiichi workers soon will not have to use full face masks."

Not due to criticality

[gstrickler]

From Mainichi Daily News [mainichi.jp]

Tokyo Electric Power Co. said Thursday the detection of radioactive xenon at its stricken Fukushima Daiichi power plant, indicating recent nuclear fission, was not the result of a sustained nuclear chain reaction known as a criticality, as feared, but a case of "spontaneous" fission.

Cooling itself a danger

[Anonymous Coward]

Colder water increases the chance of a criticality. Colder water is denser, therefore a better neutron moderator. As the temperature in the core drops, it probably crossed the threshold for a (briefly) sustained reaction, which probably then melted or reformed into a shape no longer capable of sustaining the reaction. As the shape and condition of the fuel is currently a complete unknown, this could happen again at any time until all the way down to room temperature. /former US Navy reactor operator

Not too surprising

[Animats]

Down at the bottom of Reactor 1, they have a melted core. It's not surprising that they have a criticality event once in a while. Nobody seems to have a clue how to get in there, remove the core bit by bit, and transport the mess in small pieces to some disposal location. TEPCO is saying that in 10 years, they might be able to start on that. Meanwhile, they have to continuously remove about 2MW of heat or things get worse.

One bright spot in this is that the plant is built on bedrock, and the containment vessel seems to have held. It needs to hold for another decade or two.

A lesson to be learned from train braking

[goffster]

One of the best inventions for a train was its braking system.
You have to apply energy to *prevent* a train car from braking.
This prevents run-away cars.

A successful nuclear reactor would have something similar
where you have to apply energy to keep the coolant at bay.

i.e. The core is at the bottom of the ocean and energy
is spent by the reactor to keep ocean water from rushing in.

Re:Subject

[DaveAtWorkAnnoyingly]

I'm trying to work out if you're being sarcastic or not. Of course you can have uncontrolled criticalities in a shutdown reactor. All you need to do it put enough fissionable material together and you'll get a criticallity event. They're usually just flashes and last fractions of a second, but it does happen. History is littered with these events. A shutdown reactor with the right levels of boron, still with core geometry intact will not have un-controlled criticalities, in that you are correct. However, this reactor does not have core geometry anymore and you can therefore not prove that the boron is getting everywhere and that the fuel hasn't managed to arrange itself into fissionable quantities.

stop calling it 'critical'

[e3m4n]

As a former navy nuclear enlisted personnel; I can tell you that reactors operate at criticality all the time. The mere definition of critical is when all the thermal neutrons born from fission go on to cause more fission reactions. Critical = steady state. 'Prompt critical' or 'supercritical' is when its critical without the contribution of thermal (delayed) neutrons.

Every single reactor startup, we calculate exactly what rod height we expect to reach when the reactor goes critical. Once we are critical we then allow steam demand and thermal coolant temperature to drive reactor power output. higher temps are less dense thereby thermalising fewer neutrons lowering reactor power. If steam demand or load increases coolant temperature subsequently lowers making the coolant more dense in turn thermalising more neutrons increasing reactor output. Its all driven back to steady state. This is commonly referred to as a negative reactivity coefficient. Critical = steady state and Steady state is a good thing.

Re:After so much disinformation...

[afidel]

Actually, the field mice around Chernobyl have a much higher rate of mutation including a 300% increase in fertility rates compared to those outside the exclusion zone which more than makes up for the slightly increased morbidity rate. It was one of the more interesting tidbits in a recent PBS show on research in the exclusion zone.

Re:No (fission) Nukes

[Solandri]

Judging nuclear power's safety by a first generation reactor design that was built nearly 40 years ago, and that despite a M9 earthquake and 15m tsunami has not killed anyone, and is predicted to eventually cause up to 100 deaths from cancer is foolish. It's like judging hydro power by the dams that have burst and flooded and killed thousands

The worst power generation-related accident in history was the cascade failure of a series of hydroelectric dams [wikipedia.org]. It killed nearly a quarter million people, damaged or destroyed 6 million buildings, and forced the evacuation of 11 million residents. Basically, it was as bad as or worse than the tsunami in Japan.

In stark contrast to nuclear accidents, it is almost never brought up as an argument against hydroelectric power.