Japan's Accident

[MikeVV]

Does anyone know specifically what went wrong recently in Japan while mixing radioactive "fuels"? I'm sure that the Japanese are very concerned and will make changes to prevent a future occurence like that last week but wonder if anyone here can explain the process in more detail than the press...

 

[helmley]

There was a minimum limit of fuel to be processed at a time. They overloaded this minimum requirement many times over. Nuclear plant safety has more to do with human committing unsafe acts than machines.<br>
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This is not the only accident that happend in Japan. About 9-10 years ago, the Meihama Nuclear Plant caused some radiation leak. Whether it was contained or not, I can not remember. The caused of the accident was fluid induced vibration on the tubes of the nuclear steam generator. It has since been repaired.<br>
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Japan has many nuclear plants. They may be far ahead than any Western world in the development of nuclear power. I understand there is a research going on in Europe with the US joining. The last news I knew of, is that the US is leaving the research efforts behind.<br>
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Nuclear plants has three redundant controls and safety measures. This means that when one system malfunction, the other two systems override the controls. The chance of accident is greatly reduce, but the risk (cost and consequences) is greater than other plants.<br>
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It is a sad fact that nuclear power generation is tainted with the idea of being like an untamed dragon on a leashed. Please consider also, that any nuclear power project is a political issue.

 

I would like to point out that the accident resulted mainly in high radiation levels rather than a release of contamination. From the radiation levels reported by the press (at 1 km away), the general public received little, if any, radiation exposure, and no environmental contamination occurred (other than cleanup waste).<br>
I disagree with helmley's statement that &quot;Nuclear plants has three redundant controls and safety measures. This means that when one system malfunction, the other two systems override the controls. The chance of accident is greatly reduce, but the risk (cost and consequences) is greater than other plants.&quot; <br>
Most nuclear systems I worked on used what is known as 2-of-3 coincidence logic in their protection schemes. This requires two of three alarm signals to cause a protective action. This way, a single failed alarm channel will neither cause nor prevent a protective action. This scheme is used to ensure both protection and plant operations are maximized without compromising one another.<br>
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[TSSD]

The process of reprocessing nuclear fuels (at least one method of doing so) is to dissolve the spent fuel in acid once in solution the separate elements may be separated chemically.<br>
Natural Uranium (common nuclear fuel) contains 2 isotopes U-238 99.3% and U-238 0.3%.<br>
U235 is fissile i.e. it readily undergoes fission.<br>
U238 is fertile i.e. it produces file material but undergoes fission itself very little.<br>
This is used in the older graphite moderated low temperature reactors (Magnox in the UK).<br>
In more modern Gas cooled (AGR) or pressurised water reactors(PWR) the proportion of U235 to U238 is insufficient to cause a sustainable reaction.<br>
So the ratio of U235 muste be increased (Enrichment).<br>
This is carried out during the acid solution stage, The amount of U235 is increased to an amount that will allow a sustained 'Chain Reaction' in the relevant reactor type.<br>
Uranium can be enriched to a point that will allow a self sustaining chain reaction, without moderation. (used in nuclear bombs).<br>
If a sufficient quantity of U235 or the plutonium isotopes is brought together the mass will go critical (hence Critical mass).<br>
It is this that occurred at Tokaimura, too much of the fissile isotope was added to the vessel, causing a criticality incident.<br>
The heat produced boiled the solution in the vessel with conventional explosive results.<br>
The radiation given off by an unshielded criticality is considerable.<br>
Once the enrichment has fallen below critical mass, (by boiling of the solution or by fission) then the reaction will cease.<br>
Hope this is not too in depth.

 

[drstrangelove]

What really happened at Tokai Mura? Here it is in a nutshell. It was a nuclear criticality accident not unlike those that happened as described in Stratton, "A Review of Nuclear Criticality Accidents". Basically, JCO, the company that handles the conversion of enriched uranium hexafloride to uranium  doxide in the process of making reactor fuel. The reactor fuel they were making was 18.8 wt% U235 for one of their research reactors. Normal processing of fuel (5% or less enrichement ) for power stations is carried out on a different production line.<br><br>Basically, the correct procedure employing a solution tank and dedicated pump only allows uranium solution to be introduced into a precipitation tank in amounts determined to prevent a criticality. In this case however, a stainless steel bucket was used to manually pour uranium solution directly into the precipitation tank. Since this solution was about 7 times the allowed uranium limit, a criticality was obtained.<br><br><br>The cause of the JCO accident was that JCO failed to adhere to procedures because management said schedule was more important than safety.