From: hawk@iastate.edu (John D Hawkinson)
Subject: AP Wire Story on Japan Reactor 
Summary: Speculation on what might have actually happened 
Keywords: Nuclear Power, ECCS Systems, Media
Organization: Iowa State University, Ames, IA
Distribution: usa
Date: Wed, 7 Oct 1992 04:38:02 GMT

Sorry for the delay in posting...took awhile to unscramble the AP news article
sufficiently to write a reply.  

Assuming the AP article got the age of the reactor right (and screwed up the
power rating and the type), the affected reactor is:

Owner:  Tokyo Electric Power Company
Unit:   Fukushima Daiichi 2  (there's also a Fukushima Daini 2 owned by the 
        same company, but the power listed is way too high....)
Power:  760 MW(e), net to grid
Type:   Boiling Water Reactor
Initial Criticality:  May, 1973
Commercial Start:     July, 1974

Reactor Supplier:     General Electric
Generator Supplier:   General Electric
Architect/Engineer:   Ebasco
Constructor       :   Kajima

O.k.  General Electric BWRs have TWO reactor circulation pumps located outside
of the reactor pressure vessel.  The circulation pumps force water through
piping into the vessel and thence into "jet pump nozzles" located midway down
the vessel.  Jet pumps assist the flow of water through the core--a lot of 
core cooling can occur even without the assistance of the reactor pumps just
from natural circulation (convective flow), but the pumps allow the reactor to
operate at a higher power level. 

As best I can tell from the AP article, an operator inadvertently tripped one
of the two operating reactor pumps, while the reactor was at power.

O.k.  What probably happened next is the reactor protection system (RPS)
detected an interruption in the flow of one of the two reactor coolant
recirculation loops (makes sense, one of the pumps just turned off!!).  This
event triggers a reactor scram, and simultaneously the "trip logic" signals
for the start of the emergency core coolant system (ECCS actuation).

When the ECCS is activated, the High Pressure Core Spray System (HPCS) is
activated and prepares to provide makeup water to the reactor vessel IF NEEDED.
Valves are aligned to feed water into the reactor vessel, pump motors are 
started and run up to speed, spray nozzles are directed to remove decay heat 
from the core, one of the emergency diesel generators starts and is run up to
speed (just in case electrical power should be lost), and a variety of other 
core coolant systems are held in readiness while the control room operators and
the plant computer sort out the situation.  If the situation is actually
getting worse, the plant computers can take further actions to safeguard the
reactor core (mostly by flooding lots and lots of water on it), or the control
room operators can over-ride the plant computer and manage the incident
manually (or as what probably happened in Japan, tell the plant computer the 
trip was accidental and call off the ECCS activation).

The design philosophy behind all this is fairly straightforward--it's better to
scram the plant and start ECCS immediately than delay flooding the core with 
water if it's ever needed.  

Since the AP article didn't mention anything else happening, like the pressure
vessel automatically depressurizing, water actually being shot into the reactor
vessel, or reactor steam being dumped to the supression pool (huge water vats
inside the containment building that cool and condense steam), I'd guess the 
plant operators realized the error and called off ECCS activation within the 27
seconds between the trip signal and time the plant computers would have
autonomously started injecting water into the reactor vessel. 

In the U.S., the plant operators would probably have been required to notify 
the U.S. Nuclear Regulatory Commission of the incident within 4 hours as a 
"non-emergency event", per 10 CFR 50.72b(2)(ii):   "Any event or condition 
that results in manual or automatic actuation of any Engineered Safety Feature
(ESF), including the Reactor Protection System (RPS)."  If it got to the point
where the ECCS actually injected water into the vessel (i.e., the event went
beyond 27 seconds), then the report would be made to the NRC within one hour.
It definatly would NOT have reached even the lowest threshold of the Emergency
Classification Level scheme--Notification of Unusual Event (NUE), and thus
off-site notification would not be required.  (Personal opinion:  It might,
however, be darn smart public relations to make a press release, especially in 
a nuclear-phobic community).

NOTE:  Usual disclaimers--my opinions and interpretations are my own, and
       shouldn't be construed as reflecting ISU or anyone else.