Fuel Pool Cooling High Dose Rates

["Mitchell, C. Brad" <cbmitchell@tva.gov>]

The Tennessee Valley Authority's Browns Ferry Nuclear plant is a  three
unit 1100 MWe  Boiling Water Reactor (BWR) General Electric power
generating plant.  The Unit 3 reactor was restarted in November 1995
after an extended shutdown period (approximately 10 years).  The unit is
being operated with depleted Zinc injection chemistry to mitigate Co-60
accumulation on Reactor Recirculation system piping.  After completion
of the first fuel cycle after restart, spent fuel was removed from the
reactor and placed in the spent fuel storage pool.  Shortly thereafter
fuel storage pool water radioactivity increased to 1.36E-2 uCi/ml.  At
about the same time period, the in service fuel pool cooling pump
contact dose rates increased from approximately 150 mR/hr (normal) to
5000 mR/hr.  The fuel pool cooling pump is an electric motor driven
centrifugal pump that circulates water from the fuel storage pool
through heat exchangers and demineralizers and then back to the storage
pool.  The pump is in continuous service.  An investigation for the
abnormally high pump dose rates is in progress.  Thus far it has been
determined that Ag-110M accounts for approximately 95% of the total
radioactivity in the pump.  Zn-65 and Co-60 isotopes account for the
remaining radioactivity.   Dose rates throughout the fuel pool cooling
system are slightly higher but, are considered normal due to the recent
spent fuel inventory.  Further, this phenomenon of high Ag-110M
concentration has not been a problem elsewhere in Unit 3 nor in Unit 2
systems (Unit 1 is not in service).  To mitigate the high dose rate on
the pump, temporary lead shielding will be applied to the pump.

Does anyone have any theories or practical experiences for the cause of
the high concentration of Ag-110M in the fuel pool cooling pump.  Your
suggestions for corrective actions would also be appreciated.  Please
reply via email.

Mike Scarboro
ALARA Specialist

Tennessee Valley Authority      
Browns Ferry Nuclear Plant
POB 2H-BFN
(205)729-2669
mailto:mgscarboro@tva.gov