Subject: Re: dispersed Plutonium
From: "Karl Dernetz" <firstname.lastname@example.org>
Date: Mar 30 1997
Dennis Nelson wrote:
> It's not the UO2 in the pellets which is the problem, it's the
> fission products.
It's not that fission products may be released, it is the amount of fission
products that are released. BWR's are extremely suceptible to minute fuel
element leakage - especially fission product gases. The plant designs take
this into consideration. See below for further discussion.
> Depending on the burnup of the fuel a cladding failure can release
> considerable amounts of these intensely radioactive uranium progeny
> into the primary coolant system. This could pose a safety hazard to
> the operators and create a large contamination problem in the plant.
That is one reason why light water reactor plants have such an extensive
clean up system attached to the reactor coolant system. In a Westinghouse
PWR, the Chemical Volume and Control System (CVCS) continuously cleans up
the Reactor Coolant System by the letdown and makeup flow paths. In the
letdown portion, the reactor coolant is passed through resin beds to remove
ALL undesirable impurities, especially fission products. The former
coolant that leaves these demineralizers has been purified to the point
that little shielding is required and radiation levels are low enough to
allow workers into most of the areas. Of course, the demineralizer and
filter rooms are off limits to workers most of the time. All PWR's have
similar systems, and BWR's, because of their nature, have other systems
that perform a similar function.
> Other things besides non-nucleate boiling
> can block the coolant flow and create hot spots.
Yes, other things can, and have caused flow blockages. Some of these have
been severe enough to cause fuel damage. This is usually the result of
poor maintenance practices.
> After a period of operation
> a sludge or goop, made up of corrosion products, can build up in the primary
> coolant and either block coolant channels or build up on the surface of the
> cladding and impair heat transfer.
No, corrosion products are removed by the cleanup systems previously
described. With the exception of the chemicals added for corrosion
control, the reactor coolant systems are maintined at very high purity
levels. In addition to fouling heat transfer surfaces, corrosion products
would also increase radiation levels in the plant during and AFTER
shutdown. I emphasized the the word "after" because this affects the
ability of the plant personnel to perform routine and corrective
maintenance. If the radiation levels are not kept down, then the plant
operator has the added expense of hiring extra bodies to perform the
necessary maintenance. 10CFR20 provides limits on the amount of radiation
the workers can be exposed to. The workers dose reports are reviewed by
the NRC which has imposed civil penalties for violations of this code and
can bring criminal charges if warranted.
> I think the PWRs have a harder neutron spectrum because they operate
> at a higher temperature than BWRs and also in general they occupy a
> smaller volume because there is less water coolant/moderator around.
> PWRs have a much smaller water to fuel ratio than do BWRs. A lower
> moderator to fuel ratio has the effect of increasing the average
> neutron energy, hence a harder neutron spectrum. On the other hand if
> the moderator is not water, and is graphite instead, then the above
> conclusions do not hold. I believe the Westinghouse Testing Reactor
> southeast of Pittsburg, which had an excursion in 1960 damaging the
> core and releasing radioactivity into the primary coolant, was a PWR.
You are correct that the neutron energy levels are higher in the upper fuel
regions of a BWR because there is less liquid water to serve as a
moderator. The biggest problem with neutron embrittlement is with the
reactor vessel, not with the cladding on fuel rods. The fuel rods are not
pressure boundaries (well they are because of fission product gases, but
much less so than the vessel because of relative pressures and because they
are designed for the gases - there are void spaces for the gases to expand
a helpful byproduct of using fuel pellets) and are changed out as the fuel
enrichment is depleted. The reactor vessel is there for the life of the
plant and its embrittlement determines the life of the plant. Boston
Edison (?) tried to convince the NRC to perform in-place treatment of one
of their vessels to extend that plant's life. They gave up on it because
of the expense involved with proving to the NRC, and other stakeholders,
that the proposed method would work.
And yes, the Shippingsport WTR was a PWR. Actualy, WTR is somewhat of a
misnomer since it was operated for the government by Westinghouse. Yes it
was owned by Westinghouse, if you could say that any corporation owns a
reactor plant, but most of the tests were performed under contract to the
government, especially the LWBR core loaded near the end of its life.
> Dennis Nelson
Dennis, you would probably get more useful information from people who are
more knowledgable than me if you phrased you questions as questions rather
than as statements. Since you obviously have little real knowledge on this
subject, your statements can be easily misconstrued.
It would also help us in responding to some of your questions if you would
provide supporting documentation and/or links. Not all of us have access
to the vast storehouse of records that you appear to. We could then
investigate these claims of yours and give you a more informed answer or
confirm yor worst fears :-)