[ RadSafe ] Melting a reactor??

[George Sallit]

Doug,

As you are inferring the original article is at best an exaggeration of the 
issue and at worst at bit of a nonsense. However, in dispelling it we need 
to correctly identify what is happening.

"Shut off a reactor" is not really too much of an issue as it takes 
considerable effort to actually make the reactor become critical, so 
shutting it down (make it sub critical) is straightforward. The heat that a 
shut down reactor produces is from decay heat from the fission products and 
typically is about 7% of the thermal power of the operational reactor. 
Certainly there will be sufficent heat to cause problems but with Gen III 
and Gen IV reactors there are sufficent safety systems built into the 
reactors to minimise the impacts of decay heat.

An errant operator would be better off causing huge damage at a chemical or 
gas plant than spend time and effort at nuclear plants. The security at 
nuclear plants tends to be better.

I am not sure what you mean by a critical mass is never safe. It  depends. A 
critical mass of natural uranium in the earth can be totally safe, although 
early in the Earth's history there was sufficent enrichment for the then 
natural uranium and the right considitions to sustain a critical reactions 
for many years at Oklo. A full reactor's core of PWR fuel can be, and is, 
maintained safely at all nuclear power stations. If you use burnable poisons 
then it can be very safe.

Not sure what you mean by 'worst experience' causing 'arguable marginal 
increase'; TMI, Chernobyl or a LOCA? All three have been used as examples by 
those opposed to nuclear power and unfortunately out of context. You are 
right that the risks are worth the benefits from nuclear power, if they were 
not then nuclear power would not be Justified as required by the first ICRP 
principle on Justification.

Not everyone accepts hormesis and I must confess that I do not and believe 
the case is 'not proved'. ICRP also believes the case is not proven.

----- Original Message ----- 
From: "Doug Huffman" <doug.huffman at wildblue.net>
To: <radsafe at health.phys.iit.edu>
Sent: Monday, January 03, 2011 12:40 PM
Subject: Re: [ RadSafe ] Melting a reactor??


>
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> The statement, "shut off a reactor" begs the question, modus ponens,
> what does shutting off a heat source like a reactor mean?  It continues
> to produce heat from sub-critical fission while 'shut-off'.
>
> It has been a while, but I may recall that various alloys and chemical
> reactions all occur at elevated temperatures of a magnitude achieved in
> a loss of coolant accident.  Even in a LOCA there is plenty of hydrogen
> available for hydriding chemical reactions.  ZrH2 autoignition occurs by
> 300C.
>
> A critical mass is never safe.  Carefully managed, it's benefit is worth
> the risk that is comparable to that enjoyed in everyday life.
>
> "The cooling system" is integral to the whole design of a modern
> facility and has been shown empirically robust.  The worst experience
> caused only an arguable marginal increase in morbidity/mortality.
> Remember that we argue radiation hormesis.
>
> On 1/2/2011 20:15, Steven Dapra wrote:
>> Jan. 2
>>
>>     This quote is from an editorial page article written by Charles
>> Faddis and published in the NY Times.
>>
>> "But there's no way to quickly shut off a reactor: the heat that builds
>> up inside it is so intense that even if something goes wrong, cooling
>> water must continue to circulate through its systems for days before it
>> is safe.
>>
>> "If the cooling system malfunctions, even if the rest of the plant is
>> operating safely, the heat will literally melt the reactor and its
>> concrete containment shell, releasing radioactive gas into the
>> atmosphere  in other words, a partial nuclear meltdown like that at
>> Three Mile Island in Pennsylvania in 1979."
>>
>>     Is it true that reactor heat will melt the containment vessel
>> ("shell")?
>>
>>     The link to the Faddis' NYT article is:
>>
>> http://www.nytimes.com/2010/05/06/opinion/06Faddis.html?_r=1
>>
>> Steven Dapra
>>
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