[ExI] Two Japanese reactors on red alert
sjatkins at mac.com
Tue Mar 22 02:59:15 UTC 2011
On 03/18/2011 10:58 AM, Jordan Hazen wrote:
> 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.
A BWR is a different in many ways from a PWR. With the control rods in
the power was at most at a 6% level. If the control rods were left out
until after power, plus diesel backup (plus battery?) failed then we
would have had a potentially much worse situation as the reactor would
have been far hotter at the beginning of loosing power to its pumps.
I doubt that power is diverted or easy to divert from the output of the
generators back to the pumping system of the reactor and this would only
have worked until the water in the reactor vessel boiled of anyway.
That is a pretty small window for the pumps to catch, and introduce
enough fresh water for the process to become self-sustaining.
> 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).
As I understand it, in a BWR there is not a lot of waste heat. It heats
water to around 250 C. Water drives turbine, hits the condenser and
circulates. As long as the water flows it runs relatively cool and at
much lower pressure than a PWR.
>> 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.
The reactor was scheduled to hit its 40 year end of life in a few
weeks. It was an old GE design. I am not sure that model had this
particular subsystem. If it did I was no mention of it.
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