The GPD calculation is a measurement of the radioactivity seen by the detector.
The equation for instantaneous GPD is: GPD = GC - (Ba x Ka) x (PA/PN) x Kn where:
GPD: gallons per day
GC: gross concentration, CPM x Ka
CPM: detector output, counts per minute
Ka: GPD to CPM conversion constant setpoint at normal power. Ka is the isotopic conversion constant for the detector/sampler geometry, and is defined as the inverse of the isotopic efficiency (i.e., 1/efficiency) stated on the sensitivity data sheet
Ba: background subtract setpoint, current in CPM
PN: normal power, 100%
PA: actual reactor power (obtained as an analog input)
Kn: other operator or automatically entered constants or variables relating to the GPD calculation (i.e. steam flow, steam generator pressure, steam temperature, steam density, transit time, etc.)
The display is updated once per minute, and is the result of the sum of the last 60, one second values. Longer counting times, up to 20 minutes, are available through use of the statistical accuracy jumper options provided on the UDR.
As an option, a second UDR may be provided for monitoring fission products present in the main steam line. The detector output signal would be electrically connected to a second UDR, set to monitor fission products in the 80 keV to 2 MeV range. In this configuration, both fission products and 16N in the main steam may be independently monitored, using the same detector and shield.