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RE: [cdn-nucl-l] CNA letter to Ottawa Citizen regarding anti-nuclearopinion ...
I agree with Julian Ginniver that other reactors have backup shutdown systems.
Even the N-reactor, a plutonium production reactor on the Hanford Site, now decommissioned, had a borated ball drop system similar to the one Julian describes.
The xenon situation is also mis-described in the original letter. It is not the equilibrium xenon poison that causes the problem after a shutdown, but rather the buildup of xenon poison as its precursor decays after shutdown. With equilibrium levels of the precursor and the xenon poison, there will be a few minutes during which the reactor can be restarted, followed by a number of hours during which the xenon poison level becomes too great to restart, continues to buildup due to precursor decay, and finally starts decaying away itself.
I have been told by former submariner friends that submarine reactors had (have?) a scram "battle" override switch for situations in which losing power for several hours was/is not acceptable. I have no first-hand knowledge whether that is true.
Pacific Northwest National Laboratory
These comments are mine and have not been reviewed and/or approved by my management or by the U.S. Department of Energy.
From: JGinniver@aol.com [mailto:JGinniver@aol.com]
Sent: Sat 8/30/2003 3:16 PM
Subject: Re: [cdn-nucl-l] CNA letter to Ottawa Citizen regarding anti-nuclear opinion ...
I not sure that the following two paragraphs extracted from the letter to the
Ottowa Citizen are correct. Since we are criticising Mr McKay for his
inaccuracies I think we have to make sure that we are 100% correct in the
statements that we make in response. The first paragraph states that CANDU reactors
are the only nuclear reactors in the world to have two dedicated emergency
shutdown systems. It is my understanding that both types of Gas Cooled Reactor in
the UK three shutdown systems. For the Magnox reactors these are Boronated
steel Shutdown Control Rods which drop under gravity, a Boron Ball Shutdown
Device (BBSD), This relases small (about 1" dia. I think) borated steel balls into
the reactor, this is in case a very severe accident has taken place which has
shifted the graphite core to a position where the control rods can't insert,
and finally the teriary shutdown system is based on the injection of Boron
Dust. A system that has never been used at it is irreversible and the reactor
can never be restarted. the secondary shutdown device, BBSD, is regularly
tested to confirm that it operates satifactorily, usually during the shutdown to a
planned outage. The balls can be recovered and reloaded during the outage.
The AGRs use Boron shutdown rods as the primary means of shutdown, these are
stainless stell alloyed with about 5% by weight boron, the second system is
Nitrogen Injection, the third (tertiary) is either a BBSD or for Dungeness B &
Hinkley B is water injection.
Sizwell B also has (nominally) three seperate shutdown systems. The primary
method is shutdown control rods, the second is boration of the reactor
coolant, the third is automatic "injection" significant amounts oof boron from an
Emergency Boration System. Sizewell B is unique in having this last system, as
it was required by the UK regulator and has been fitted to any other PWR to my
I'm also not convinced about the second statement (below) which says that
Xenon prevents the restart of a reactor for several days, and also states that
any nuclear plants which shutdown completely were going to be out of service for
the xenon effect. It is my understanding that this is not necessarily true
that a PWR will need to remian shutdown due to Xeonon poisoning. It depends on
which point in it's operating cycle that this shutdown occurs and how quickly
the reactor can be returned to service. If I have undertstood correctly,
early in the operating cycle of a PWR there is sufficient additional reactivity
in the core that it can be restarted by removing some of the Boron present in
the coolant. By removing extra Boron, over and above that which would be
required for current burn-up in the fuel, from the coolant the reactor can return
to service, although as the power is raised and levels of Xenon return to the
levels present during normal operation the level of Boron has to be
increased. This does result in a lot of liquid radwaste being produced, but does allow
the reactor to be restarted even from a full shutdown. There is however a
point during the operating cycle where there is insufficient excess reactivity
left in the reactor and the level of Xenon poisoning cannot be compensated for,
even by removing all the Boron in the reactor coolant.
If any of the above is inaccurate, I apologise, and would be grateful for any
corrections anyone might wish to give as I'm neither a reactor operator, or a
reactor physicist/nuclear engineer.
> In his interest in building an anti-CANDU case, Mr. McKay comments on the
> shutdown of Ontario's nuclear reactors by gadolinium injection. Gadolinium
> is used to ensure the shutdown of a nuclear reactor under any circumstances.
> It seems that Mr. McKay is now criticising nuclear reactors for having a
> second shutdown system to ensure public safety under any emergency
> conditions. I am confused by his criticism: are nuclear power plants too
> safe, or not safe enough? CANDU reactors are the only nuclear reactors in
> the world to have two dedicated emergency shutdown systems rather than just
> one. This feature alone helps make CANDU reactors the safest in the world.
> Would Mr. McKay prefer to trade off safety for personal convenience?
> Mr McKay also ignored the fact that any nuclear plants which shut down
> completely were going to be out of service from the xenon effect. Xenon
> exists in nuclear fuel after prolonged periods of operation. After a
> shutdown, it prevents restart of a reactor for several days until the xenon
> has decayed away. This is a condition which affects all reactors, not just