Re: Chornobyl Radiation Increases

[gj_vargo@ccmail.pnl.gov]

     Monitoring of the reactor remains is accomplished through the use of 
     approximately 30 instrument strings that monitor temperature, neutron 
     and gamma flux levels.  These strings have been placed in boreholes 
     and various openings in the wreckage.  They all feed into a dedicated 
     "control room" (really a monitoring room since there are no control 
     functions as such) inside Unit 3 where they are recorded by a data 
     logger.
     
     The Shelter is not an engineered structure by almost any standard.  It 
     was erected around the damaged reactor in less than 6 months under 
     working conditions that anyone here would find unimagineable.    There 
     are numerous openings to the environment - mainly places where 
     prefabricated panels don't quite meet together - along with windows in 
     stairwells, etc.  Rainwater intrusion is an ongoing problem.
     
     In the past we have had anecdotal reports of neutron count rate 
     increases following significant rainfall.  A likely explanation is 
     that rainwater seeps into parts of the wreckage (including boreholes, 
     fissures, etc.) and causes thermalization of neutrons in the vicinity 
     of the detectors, increasing the observed count rate.
     
     This phenomena should not be unexpected.  Some of the accident 
     management strategies for LWRs in the US use in-core neutron detector 
     readings as a gross indication of water level given a loss of other 
     reactor vessel level indications (very low or zero count rate = no 
     water).
     
     George J. Vargo
     Pacific Northwest National Laboratory