The Color Coded Grading System for
Nuclear Plants: "Decoding the Nuclear Crayon"
Supposedly, the crayon selection is determined by the risk
associated with the violation -- the more steps taken towards
core damage, the fewer steps remaining to complete that journey
the worser the crayon color -- Green to White to Yellow
to Red.
The NRC picked Green for the High Pressure Service Water pump
discharge check valve problem.
They picked White (preliminarily) for the Reactor Protection System
power supply glitch that triggered an automatic scram and containment
isolation.
The HPSW violation seems to involve many more steps than
the RPS violation. If anything, the crayon selections should
be reversed.
The HPSW problem dates back to 2016 and remained uncorrected for years.
The RPS problem dates back hours and remained uncorrected for minutes.
The HPSW problem was "analyzed" to be Green based on no concurrent failures.
Duh! No problem, except perhaps gross rupture of the reactor
pressure vessel, is anything but Green if no concurrent failures are
assumed.
Probabilistic Risk Assessment ("PRA") does not work that way, at least
not when done properly.PRA looks at the likelihood of success when a safety
component is needed. By assuming zero concurrent failures, NRC
didn't PRA they prayed for good luck.
With the HPSW check valve stuck open, the flow in the running pump dropped
to nearly have the needed flow (5,000 gpm to 3,300 gpm). Which means the
cooling flow provided to emergency systems was nearly halved.
For the RPA glitch, the NRC ditched its zero concurrent failures ploy and
assumed that concurrent failures could be present and lead to core damage.
To be fair, the NRC's inconsistent treatment might have benefit.
Consistency runs the risk of being wrong all the time.
Inconsistency increases the chances of being right some of the time, even
if you don't know when.
I suspect Constellation the owner will negotiate the final finding back to
a Green from the preliminary White.
