[ExI] Two Japanese reactors on red alert

[Jordan Hazen]

On Tue, Mar 15, 2011 at 02:12:46PM -0700, Keith Henson wrote:
> As it has turned out, shutting down reactors due to an earthquake was
> almost certainly the wrong thing to do.
> 
> Had they been left running, even if one had gone down, the rest could
> have provided station power.

Most reactors have a minimum stable power level at which they can
operate, which is far greater than what internal plant loads would
be able to soak up.  I don't know what this level is for the BWR-3
and BWR-4's at Fukushima Dai-ichi, but say it's 30%, as with one
PWR design.  300 MWe for a 1000mW unit, and house loads consume 10
MWe at most... with outside power grid connections destroyed by the
earthquake and/or tsunami, to continue running in island mode you'd
need somewhere to dump the remaining 290 MW, and it would have to
switch in almost instantly to avoid a load-rejection / turbine
overspeed trip.  Resistor banks of this power rating are a little
hard to come by.

Of course, with thermodynamic efficiency of just 33% or less, when
operating at full 1000 MWe condensers on the cold side of the
turbine are having to dissipate about 2000 MW anyway, so they may
be able to accept the entire heat output at a lower operating
point, if provisions exist to allow most steam to bypass the
turbine, and if that bypass circuit can be engaged quickly enough. 
Some big "ifs" there.  I know some fossil plants can handle brief
transients by just venting steam to avoid overspeeding the turbine,
but with steam in a BWR coming straight out of the reactor, hence
slightly radioactive (shielding walls run the length of the turbine
hall to protect plant workers), they probably aren't set up for
this.


That also assumes the main steam loop survives unscathed. 
Considering reports that supplies of emergency cooling water at
the plant were fouled by tsunami-driven mud, even if all piping
remained intact there's an excellent chance seawater inlets to the
condensers would have become clogged as well.  (This plant appears
to have lacked any sort of cooling towers, discharging its waste
heat directly into the ocean).

> Of course, it's a bit iffy if they would have run at all with the
> switch gear under water . . . .

An early report on NHK mentioned a fire or explosion in the turbine
hall of one of the reactors.  I never heard any more detail about
that, though.

Another big question mark is why the RCIC (reactor core isolation
system), a small steam loop within the BWR containment that
extracts power from residual decay heat itself to provide some
emergency cooling, apparently failed to work.  Earthquake damage to
the piping is one possible cause.

> Keith


-- 
Jordan.