940-4FP

When used with an accident range noble gas detector, the Model 940-4FP consists of an in-line field mounted detector shield, two beta scintillation detectors and an electrical junction box. A remote, control room located, Model 942A Universal Digital Ratemeter (UDR) provides the detector high voltage and DC operating voltages, processes the detector output, performs limit checks, generates analog output signals and displays the net detector count rate or fission product activity for each detector. Where additional system features are required, the Model 960 Digital Process Radiation Controller may be supplied in lieu of the UDR.

Shield Spool Piece

An In-line spool piece and shield assembly, with up to 4 inches of 4 pi virgin lead shielding is provided. The spool piece is designed to mount directly into the CAED discharge line. The spool piece is manufactured from 300 series stainless steel, 4 inch in diameter, with standard raised face pipe flanges. A 300 series stainless steel removable well is bolted into the spool piece to position the detector in the effluent to maximize sensitivity and to permit detector removal without violating the integrity of the process pressure boundary. The spool piece and shield may be designed for either vertical or horizontal installation. To support the weight of the shield, a pedestal or other support structure is required. The purpose of the shielding is to reduce the induced background, enhancing the response to the low energy fission products present in the CAED and increasing monitor sensitivity. Detector installation and replacement is accomplished via removable lead caps, located on the top of the shield. Process-drain and vent valves, which may also be used to inject a calibration gas, and a pressure transmitter are also provided. The shield provides a convenient location to mount the detector junction boxes. The spool piece is designed to match the diameter of the CAED line.

Detector

Two detectors are provided with this Regulatory Guide 1.97 Monitor: Model 943-25T and 943-27 Beta Scintillation Detectors. The Model 943-25T detector utilizes a 0.002 inch thick titanium end window, and a 2 inch diameter BC434 plastic scintillation crystal coupled to a 2 inch diameter photomultiplier tube. The detector includes a pulse shaping and cable driving preamplifier. It is provided with an 8 foot cable with a “MS” type connector for termination in a Model 844-211 Junction Box. The junction box also provides a method to interconnect the detector to the control room cable. Secondary standard solid source sets and a standard field calibration fixture are available for on-site re-calibrations. The Model 943-27 detector utilizes a 0.0002 inch thick titanium end window, a 1.125 inch diameter by 0.005 inch thick calcium fluoride plastic scintillation crystal coupled to a 1.125 inch diameter by 0.125 inch thick quartz light pipe and a 1.0 inch diameter photomultiplier tube. To operate over the extended range required, the detector is operated in the current, vs. pulse, output mode. The detector is supplied with a Model 943-227-15 “smart” Digital Preamplifier. The preamplifier contains a microprocessor controlled electrometer, which converts the detector current output (approximately 1.0 x 10^-12 A/µCi/cc) into a digital value, and transmits the value to the readout device via a serial data port. The detector is provided with an integral high voltage and signal cable. Each cable is 6 feet long, and terminates on mating connectors in the preamplifier enclosure. The preamplifier enclosure contains the electrometer circuit board and the communications circuit board and also provides a method to interconnect the detector to the control room cable. Secondary standard solid source sets, which slide directly onto the detector body, are available for on-site re-calibrations.

Leak-Rate Monitor

When used with a steam generator leak rate detector, the Model 940-4FP consists of an in-line field mounted detector shield, a gamma scintillation detector and an electrical junction box. A remote, control room located, Model 942A Universal Digital Ratemeter (UDR) provides the detector high voltage and DC operating voltages, processes the detector output, performs limit checks, generates analog output signals and displays the net detector count rate (i.e. the steam generator fission product leakage) for the detector. Where additional system features are required, the Model 960 Digital Process Radiation Controller may be supplied in lieu of the UDR.

Shield Spool Piece

An In-line spool piece and shield assembly, with up to 4 inches of 4 pi virgin lead shielding is provided. The spool piece is designed to mount directly into the CAED discharge line. The spool piece is manufactured from 300 series stainless steel (available in 4, 6 or 8 inch diameter) with standard raised face pipe flanges. A 300 series stainless steel well is welded into the spool piece to position the detector in the effluent to maximize sensitivity and to permit detector removal without violating the integrity of the process pressure boundary. The spool piece and shield may be designed for either vertical or horizontal installation. To support the weight of the shield, a pedestal or other support structure is required. To simplify installation, the shield may be provided in sections, for installation on the spool piece in the field. The purpose of the shielding is to reduce the induced background, enhancing the response to the low energy fission products present in the CAED and increasing monitor sensitivity. Detector installation and replacement is accomplished via an access port in the shielding, located on the top of the shield. The shield provides a convenient location to mount the detector junction box. The spool piece is designed to match the diameter of the CAED line.

Detector

The detector supplied is a Model 943-36 or 943-36L Gamma Scintillation Detector. This detector utilizes a 1.5 inch diameter by 1.0 inch thick NaI (Tl) scintillation crystal coupled to a 2 inch diameter, 10 stage photomultiplier tube. The detector includes a pulse shaping and cable driving preamplifier. The Model 943-36L detector includes provisions for an LED (light emitting diode) to function as an integral check source. To maximize the integrity and life of the optical couplings, the crystal, light pipe and photomultiplier tube are provided as a sealed, integral assembly. The detector is provided with an 8 foot cable with a “MS” type connector for termination in a Model 844-211 or 844-211LS Junction Box. In addition to providing a method to interconnect the detector to the control room cable, the 844-211LS Junction Box includes the LED driving circuitry for the 943-36L detector. Secondary standard solid source sets and a standard field calibration fixture are available for on-site re-calibrations. Based on the guidance provided by EPRI TR-104788-1, the counts per minute (CPM) received from the detector may be converted directly into leakage, in gallons per day.

Universal Digital Ratemeter

The Model 942A Universal Digital Ratemeter (UDR) is used to process the detector output and display the reading in units of CPM. In operation, the detector output will be monitored by the UDR located in the control room. A Model 942-200-80 Serial Communication Module is available to provide monitor status and historical data via a serial port for use by the plant computer or a laptop PC.

Set Points

The firmware in the UDR includes the ability to enter 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

Radioactivity

The radiation value calculation is a measurement of the average radioactivity seen by the detector. The equation for the radiation value is: CPM_net = (CPM – CPM_BKRD) x Ka) where: CPM_net: net detector output, counts per minute CPM: detector output, counts per minute; this is the average of the 60 previous counts per second values, updated once per second CPMBKRD: set point, detector output due to ambient background radiation, counts per minute Ka: User enterable radiation value compensation factor. May be used for sample volume pressure compensation, flow compensation, or engineering units conversion 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.