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940-3N Main Stream 16N Monitor

Main Stream 16N Monitor


The 940-3N monitor consists of a field-mounted detector shield, a gain-stabilized gamma scintillation detector and an electrical junction box for each steam line. A remote, control-room-located Universal Digital Ratemeter (UDR) provides the detector high voltage (automatically adjusts the high voltage for gain changes), provides DC operating voltages, processes the detector output, performs limit checks, generates analog output signals and displays the 16N value for each detector.


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Key Features

  • 4 Pi lead shield with rear door
  • Temperature compensated NaI (Tl) scintillation detector
  • Integral preamplifier capable driving 1,500 ft of cable
  • Microprocessor controlled
  • Seismically and environmentally qualified
  • Detector anti-jam circuit
  • Analog Input processing
  • Single channel analyzer
  • Automatic high voltage control
  • 16N and/or fission product detection


Current regulatory requirements necessitate operating pressurized water reactors (PWRs) to monitor primary to secondary system leakage. This is normally accomplished by monitoring the main steam lines (MSL), condenser air ejector discharge (CAED) and steam generator blowdown (SGB) for fission and/or activation products. During normal reactor operation, the neutron activation of oxygen in the reactor coolant results in the production of significant amounts of the radioactive isotope Nitrogen-16 (16N). The amount of 16N produced is directly proportional to the reactor power level. In the event of primary to secondary system leakage, the 16N will be present in the steam generator. The 16N that migrates into the main steam lines prior to its decay will be detected by the Model 940-3N 16N monitor. By applying the actual reactor power level and plant unique primary and secondary system factors to the raw monitor output, the primary to secondary leak rate, in terms of gallons per day, may be calculated. The plant unique parameters may be adjusted to agree with the actual leak rate determined by the plant chemistry personnel. Because the origin of the leak is unknown, and considering the short half-life of 16N (7.13 seconds) and the complex hydraulic characteristics of the steam generator, monitor sensitivity is dramatically affected by leak location and monitor location. MSL, CAED and SGB monitors are available as backups to support 16N monitor results.

  • Process radiation monitor
  • Primary to secondary leak detector
  • Gamma scintillation detector
  • Detects 16N in the main steam line
  • Optional main steam fission product detector


The Model 940-3N consists of a field mounted detector shield, a gain stabilized gamma scintillation detector and an electrical junction box for each steam line. A remote, control room located, universal digital ratemeter (UDR) provides the detector high voltage (automatically adjusts the high voltage for gain changes), provides DC operating voltages, processes the detector output, performs limit checks, generates analog output signals and displays the 16N value for each detector. The UDR is provided with a single channel analyzer, electrically adjusted to respond to the 16N 6.13 MeV gamma photon. Where additional system features are required, the Model 960 digital process radiation controller may be supplied in lieu of the UDR.


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 detector provided for this application is a Model 943-37LS Gamma Scintillation Detector. This detector utilizes a 2 inch diameter by 2 inch thick NaI (Tl) scintillation crystal coupled to a 2 inch diameter photomultiplier tube. The detector includes a cable driving preamplifier and provisions for an LED (light emitting diode) for gain stabilization. To maximize the integrity and life of the optical coupling, the crystal, light pipe and photomultiplier tube are provided as a sealed, integral assembly. The detector is provided with a 5 foot cable with an “MS” type connector for termination in a Model 844-211LS Junction Box. In addition to providing a method to interconnect the detector to the control room cable, the junction box includes a stable temperature reference and LED driving circuitry. Secondary standard solid source sets and a standard field calibration fixture are available for on-site re-calibrations.

Universal Digital Ratemeter

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.

Set Points

The firmware in the UDR allows for the entry of the following monitor specific set points:

  • Warn Alarm
  • High Alarm
  • Fail Alarm
  • Overrange Limit
  • Detector Dead Time
  • Background Subtract
  • Detector Conversion Constant
  • Calibrate Timer
  • Analog Full Scale
  • Analog Low Scale
  • 16N Delay Factor


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.



Model Name


940-3N Main Stream 16N Monitor, includes: Field-mounted detector shield Gain-stabilized gamma scintillation detector (943-37LS) Electrical junction box (844-211LS) Universal digital ratemeter (942A)
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