Olin Lathrop wrote:

> Walter Banks wrote:
> > The control rods in an enriched uranium plant is like having a
> > throttle attached to an engine with elastic bands. The control rods
> > when inserted absorb neutrons essential for a nuclear reaction but it
> > takes some
> > time to slow the reaction down to the point where a minimum
> > amount of heat is produced.
>
> According to the article James linked to, inserting the control rods stop=
s
> the chain reaction immediately.  However, that chain reaction before it w=
as
> stopped produced several isotopes that take a few days to decay away.  It=
 is
> the heat released from this decay that takes time to slow down.  There is
> nothing the control rods or anything else can do about it since these ato=
ms
> will decay completely on their own.  This heat is a small fraction of the
> total reactor power when the chain reaction is going on fully, but still
> significant enough to eventually melt everything if it's not cooled.

The timing of the reactor shutdown will depend a lot on the reactor
design. I believe the reactors used in the Japanese plant were GE
designed in the mid 70's. We should be able to find out quite a bit
on the timing specifics

I haven't read James's article yet (but will) On shutdown in an enhanced
uranium reactor the decay energy energy will be about 7-8% of the
operating power on shutdown with a time constant of about an hour,
ie 2-3% at the end of the first hour.

For whatever reason there is still a lot of concern over neutron production
in the reactors in Japan. They are adding borax to the mix they are loading
into the reactor for example. It maybe that they know they have melted
fuel rods that have an un-moderated fuel mass. If this is so they probably
want to slow as much as possible these fuel masses from becoming a
neutron source for the unaffected fuel bundles

w..

--=20
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