Containment High Range Area Monitors -Reply

[Charles Willis <CAW@nrc.gov>]

Mr. Maeng,

In the US system, the containment high range monitor (CHRM) is not intended to
cause containment isolation and the set point is not of particular importance.  The
CHRM is on of two devices intended to let people know the status of the
radioactivity in containment so appropriate accident management decision can be
made.  The CHRM should give an immediate measure of the radiation level in
containment, up to 1 E 8 rad/hr or 1 E 7 R/hr.  It does not tell you whether the
activity is airborne or on the floor nor whether it is principally noble gas, halogen,
ruthenium or other, but it is better than the total ignorance we had at TMI.  The
CHRM is to be supplemented by the "Post Accident Sampling System" (PASS). 
The PASS obtains a sample of the atmosphere in containment and analyses it to
determine how much of what nuclides are airborne.  With this information,
reasonable adjustments can be made to the off-site protective measure
recommendations that were (we hope) made and implemented earlier.  The PASS is
permitted to be slow (results after 3 hours) because the requirements were based
on the technology of the 1970s.  

The limitations on containment vent and purge are based on normal operation
considerations.  There are two criteria to be met: (1) the integrated dose from all
airborne releases, including releases from containment, are to be below the
"Appendix I" criteria (e.g. 5 mrem in a year); and (2) the instantaneous dose rate
off-site from airborne releases are not to exceed 500 mrem/yr (the "old" Part 20
concentration limits).  The set points are to be selected accordingly.  Commonly,
these set points have been important after shutdown, when containment is being
purged to permit people to enter containment.  

Others can tell you what they actually are doing, but this is the way it is supposed to
work.

Charlie Willis
caw@nrc.gov
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