940-3N Main Stream ¹⁶N Monitor

An “adjacent to the line” or “on-line” shield assembly, with 4 inches of 4 Pi virgin lead shielding is provided. The purpose of the shielding is to reduce the induced background due to the potential for lower energy fission products in the Steam Line, increasing the monitor sensitivity and range. The shield is provided with a hinged rear door to simplify detector installation and replacement. The shield also provides a convenient location to mount the detector junction box. The shield is designed to mount on a platform or pedestal, adjacent to the main steam line. Due to the penetrating power of the 6.13 MeV photon, the shield should be located to reduce interference resulting from other steam lines in the area. Thermal insulation may be installed between the steam line and the shield (where required) to prevent thermal shock from damaging the detector crystal. The shield is designed to match the diameter of the main steam line, which will affect the actual size and weight of the shield.

The Model 942A UDR is provided to process the detector output and display the 16N reading in units of counts per minute (CPM) or gallons per day (GPD). In operation, the detector output is monitored by the UDR located in the control room. To provide the required auxiliary functions, various optional modules are factory-installed. The Model 942-200-80 Serial Communication Port Module provides monitor status and historical data via a serial port for use by the plant computer or a laptop PC. The Model 942-200-75 Analog Input Module accepts a 4-20 mA input signal representing reactor power. The Model 942-200-90 Single Channel Analyzer/Americium Regulator Module is used to count the pulses that fall into a 4.5 to 7.5 MeV window, capturing the primary 6.13 MeV 16N photo peak. To prevent changes in temperature or high voltage supply drift from causing photomultiplier tube gain changes that could shift the detector output pulse out of the 6.13 MeV window, the Model 942-200-90 also contains a gain stabilization circuit to automatically adjust the detector high voltage. Approximately once every 10 minutes, the Model 942 UDR actuates the gain stabilization LED through a pulsar circuit located in the Model 844-211LS Junction Box. The LED output feeds back through the detector signal cable to the Model 942-200-90 Single Channel Analyzer/Americium Regulator Module in the UDR. The detector output is compared to an internal reference in the Model 942-200-90 and the detector high voltage is adjusted, as required, to agree with the internal reference. If the reference signal cannot be matched within one minute, the detector FAIL alarm is turned on. The gain stabilization LED may also be actuated manually by pressing the UDR Front Panel Check Source pushbutton. The advantage of this method of gain stabilization, over the traditional Americium-241 gain stabilization method, is that the background count rate that would result from the Americium-241 is eliminated, providing a greater operating range.

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.