rad monitor setpoints

[JOHN.RICH@sargentlundy.com]

Radsafers,



I'm looking for some guidance on set points for radiation monitors that

initiate safety actions at nuclear power plants.  The question is whether

or not a full instrument string uncertainty calculation should be done for

a setpoint that's based on 2 x background.



We have a couple of monitors that initiate containment isolation, so

they're safety-related.  Usually, safety-related setpoints have a loop

uncertainty calculation attached to them to ensure that a safety limit

isn't exceeded.  This makes sense for pressure and temperature that have

true design limits.  However, it seems like an unecessary refinement for a

setpoint that's based on 2 x background, because the intent here is to have

the monitor perform its function when the situation is not normal, but

there's no safety limit involved.   It also moves the setpoint closer to

the background level which could cause spurious trips.  That's why I'd like

to avoid lowering the setpoint for loop uncertainty.



If anyone has some good guidance (or any guidance) I'd appreciate the help.



Thanx much  - - jmr



John Rich

john.rich@sargentlundy.com





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