From: jgd@dixie.com (John De Armond)
Newsgroups: sci.energy
Subject: Re: AP Wire Story on Japan Reactor
Keywords: Nuclear Power, ECCS Systems
Date: 9 Oct 92 04:11:15 GMT
Organization: Dixie Communications Public Access.  The Mouth of the South.

sxb@fermi.cray.com (Stephen Behling) writes:

>  Another point about ECCS actuation in BWRs...they do not run with borated
>water (or the stuff would plate out on the fuel rods).  The ECCS system is
>borated, so if it does get accuated, the boron would have to be flushed out
>before normal operation could be resumed.  This could cause operators to
>try other recovery techniques before ECCS is activated. [Note, I have no
>knowledge about the actual plant technical specification and procedures
>in this area.]

Here's the rundown on the reactor protection system.  GE came about as
close to a standardized design as any so most plants adhere to this 
scheme.

The borated water system you speak of is known as the Standby Liquid 
Control system and is the last ditch system that would be used only
in extreme emergencies.  As the inside joke used to go, when the 
SLC button is pushed it also drops a ticket to the local concrete
vendor to start hauling in the concrete to fill in the reactor.

The first system is called the HPSI (prounounced Hipsy) system.
This consists of a pair of steam driven pumps located inside the
secondary containment.  The steam comes from the reactor.  The pump 
takes suction from the torus (MkIII) or supression pool (MkIV) and 
injects the water on top of the core.  Anything that actuates the 
ECCS also causes a containment isolation so the excess steam flows
from the reactor to the torus/supression pool and forms a closed loop.
The system is actuated when power is dropped to a fail-to-open valve on the 
steam line.  The pump runs until the steam runs out.  At that point
the LPSI (Lipsy) system takes over.  One pump is typically steam driven
and the others are electric.  They operate the same as the HPSI except
they are higher volume, lower pressure pumps.  

If the LOCA is so severe that all reactor pressure is gone, the Core 
Spray system takes over cooling.  This consists of several multi-thousand
horsepower electric pumps that take suction from the torus/supression
pool and pumps into a sparger header inside the reactor and over
the core.  All these systems can be reconfigured to take suction from
the condensate system or the river.

I've heard nothing more about the particulars of the incident in question.
I still can't imagine what an operator could do that fits even some
reasonable interpretation of the "facts".  Since almost any SCRAM in
a GE reactor causes containment isolation and thus activates the ECCS,
the incident could have been as minor as a technician bouncing a reactor
level transmitter.  This is a common occurance because the trip setpoints
are close to the operating points.  A physical jar or even the act of
valving one of the 4 transmitters out for calibration can jog the other
three into generating a SCRAM.  

The way the reactor protection system works in a BWR is the "one
out of two taken twice" concept.  That is, there are 4 channels
of reactor protection organized into two trains of redundant
pairs.  A single trip from each train causes a half-scram.  Any
trip  on the other channel causes a full scram.  So for example,
a reactor water level transmitter can half-scram Train A and
then something as innocuous as putting a channel of Train B in
calibration mode (generates an automatic half-scram) would trip
the reactor.  Operators work REAL hard to minimize the state of
half-scram :-) 

John
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