Re: CANDU nuclear reactor failure and possibility of Chernobyl-scale disaster
From: Karl Johanson (karljohanson_at_shaw.ca)
Date: 11/25/04
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Date: Thu, 25 Nov 2004 20:23:21 GMT
"Corey Snow" <cyclometh@gmail.com> wrote in message
news:co1a7m$kfh$1@nnrp.atgi.net...
>
> "Ian St. John" <istjohn@noemail.usa> wrote in message
> news:EcVod.36175$Le1.864786@news20.bellglobal.com...
>
> Thanks for the reply, Ian!
>
>> Corey Snow wrote:
> <snip>
>>> I have been asked to detail a CANDU failure mode that could result
>>> in
>>> a disaster on par with or greater than the Chernobyl disaster, and
>>> also for a pebble-bed modular reactor.
>>>
>>> My question to the learned individuals here is: Is there a failure
>>> mode in either or both of the above reactors that could result in a
>>> large-scale disaster on the scope of the Chernobyl incident?
>>
>> The CANDU reactor design is not "passive safe". The increased safety
>> of the
>> CANDU comes from the fact that by utilising lower enrichment and
>> 'opitimising' the reaction, there is a lower difference between
>> idling and
>> full throttle. That means that, where a PWR reaction with 5% enriched
>> fuel
>> may 'go critical' in five minutes leaving the operators little time
>> to
>> diagnose or react to the situation ( and thus dependent on the design
>> of
>> safety systems and a 'scram' system as a last resort ) the CANDU will
>> take
>> hours to get to the same state and thus leave plenty of time to
>> discuss,
>> investigate, and take appropriate action. Not completely safe, but
>> with a
>> margin of safety that exceed the PWR or BWR by a LARGE margin. Along
>> with
>> the containment vessel, the design is safe for anyone outside the
>> reactor
>> and cannot release radiation as the Chernobyl desing ( without
>> containment
>> vessels ) did. As well, the 'china syndrome' is rather unlikely since
>> the
>> fuel would lose it's slight positive reactivity if it melted. Dangers
>> are
>> pretty much financial, rather than radiation.
>>
>
> Is it possible (however unlikely) that a situation where the reactor
> went critical that it could potentially breach the containment vessel?
Conceivable, but I can't think of a route that would cause it. Then you
ask, is the fuel still contained in the cladding (or is some of it)?
Then you ask, how much of the fuel might escape the pressure vessel
(noting that it wouldn't have blown apart, ala Chernobyl). Then note
that fuel escaped from the pressure vessel would be inside a steel
reinforced concrete containment building, unlike Chernobyl. Even if
someone then hit the containment building with several jumbo jets, we're
still looking at a smaller scale accident than Chernobyl, as you
wouldn't have a fuel mass which continued to fission for days. (And the
accident would be in an area where people use iodized salt, reducing
radio-iodine exposures to people's thyroid glands.) We could also be
happy that whoever rammed the reactor with half a dozen jumbo jets
didn't instead decide to ram containers of natural gas inside cities, or
huge tankers in harbours next to cities, as that would almost certainly
cause far greater loss of life.
>I'm talking about release of radiation or radioactive debris/dust into
>the environment. Can you further define "safe for anyone outside the
>reactor"? Does this mean there's no possibility of a catastrophic
>series of events that could cause an environmental disaster?
There is no safe energy (although I note that not having access to
useable energy tends to lead to pretty short life expectancies). It's
conceivable that nefarious sorts could cause some deaths by causing
enough damage to a Canadu. At which point we cheer nuclear energy for
drawing the attention of terrorists who could have caused far more death
& destruction by attacking other energy related targets instead.
Karl Johanson
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