MIT Nuclear Information Hub weighs in on worst case scenarios in Japan

[flamingdem]

http://mitnse.com/2011/03/17/on-worst-case-scenarios/

* It would be nice to get an update

>MORE AT LINK: this is just an intro

On prediction of “worst case” scenarios

The blog has received a great number of questions surrounding worst case scenarios. This is not surprising given that such scenarios, with varying degrees of scientific merit, have been advanced in the media. The intent of this blog is to educate, using our best available information, and so we intend to refrain from making predictions of our own. We do, however, want to review some of the terminology used in these predictions, and describe the methods used by government agencies and scientific organizations to determine what actions must be taken to inform the public.

Meltdown

The term meltdown describes melting of the zirconium alloy cladding, and uranium oxide (or mixed oxide, in the case of Unit 3) fuel pellets. These two structures are the first two barriers to fission release, since radioactive fission products normally exist as either solids within the fuel pellet, gases within pores in the fuel pellet, or gases that escape the pellet but remain in the cladding. When a reactor is shut down, these fission products continue to decay, generating heat. This amount of heat is produced at first at 7% of its initial rate, and then decreases as the isotopes responsible for generating it decay away. If this decay heat is not removed by cooling water, the fuel and cladding increase in temperature.

At temperatures above 1200 C, the corrosion reaction which is constantly ongoing in the zirconium cladding accelerates dramatically. The reaction’s products include zirconium oxide, hydrogen (for more on this hydrogen, see our post “Explanation of Hydrogen Explosion at Units 1 and 3), and heat. This heat continues to both fuel the corrosion reaction, and to prevent the fuel rods from being cooled. Because of the self-catalyzing nature of this reaction, safety systems are usually actuated in such a fashion as to provide a large margin of safety to the clad reaching 1200 C. --------------snip

[flamingdem]

1. Quotes to consider from the MIT blog:

"In the event that molten corium does, as has been the case in some experiments, penetrate the lower head of the reactor vessel, it will drop onto the concrete basemat of the containment and spread out as far as possible. The interaction of corium with concrete is known to produce a buildup of non-condensable gases within the containment, a process called molten-core concrete interaction (MCCI). <Many studies available if you google MCII>

..............
The experiments are performed by producing a melt of un-irradiated uranium dioxide (extremely low levels of alpha radioactivity, easily avoided by the experimenters), zirconium alloy, and structural steel, in the proportions that would be present in a reactor core. This melt is sent through a nozzle used to simulate a pressure vessel lower head breach, and dropped onto concrete. Measurements are taken during the hours-long experiment using thermocouples and camera equipment, and the solidified material is examined after completion.

The experiments have shown that without water quenching, corium under conditions similar to those present at Fukushima Dai-ichi will ablate the meters-thick concrete pad at a rate of just millimeters per minute. Gases would build up within the containment at a rate which would require filtered ventilation of the containment in order to prevent rupture.

If, however, water is supplied to quench the corium as it spreads onto the reactor floor, the ablation occurs at 5-7% of the pre-quench rate, and production of gases is suppressed. The rate of ablation continues to undergo fits and starts, as the corium forms a solid crust, and then this crust is broken and re-formed."

[bananas]

2. That blog has a serious credibility problem

The first post on it said there would be no significant radioactivity release.

[CRH]

3. MIT sold their credibility south in the seventies, ...

Remember the Rasmussen Report? Do you really think anything has changed? Would you go to Lawrence Livermore Lab for unbiased information on the dangers of nuclear technology? This site is wantabe nuclear scientist students, pretending expertise.