940-3MS

A Model 841-7, “adjacent to the line” or “on-line” shield assembly, with 7 inches of 4 Pi virgin lead shielding is provided. The face of the shield is designed to match the diameter of the main steam line, which affects its overall size and weight. The shield is designed to mount on a platform or pedestal, adjacent to the main steam line. The purpose of the shielding is to reduce the induced background due to the low energy fission products present in the steam line, thereby increasing the monitor sensitivity. Because of the small size of the GM detector, it is inserted into the shield through a stainless steel tube welded to the shield frame. A “seal grip” type adaptor is used to seal the detector entry port and prevent moisture from damaging the detector. The detector tube intersects a viewing port that positions the GM detector to view the main steam line directly. The distance between the detector and the main steam line is based on the size of the line, the fission product activity expected and the response of the GM tube detector. The shield also provides a convenient location to mount the detector junction box. The shield should be located to reduce potential interference that may result from other steam lines in the area. Thermal insulation may be installed between the steam line and the shield (where required) to reduce the temperature at the detector. A radioactive check source is installed in the shield weldment to verify detector operation.

The detector is a Model 843-251 Series GM tube gamma detector. The detector is provided with a 6 ft cable with a triax connector for termination in a Model 843-252 Junction Box. The GM detector is housed in a phenolic tube to reduce the potential for damage during handling. The Model 843-252 Junction Box includes a pulse shaping and cable driving preamplifier. In addition, the junction box provides a method to interconnect the detector to the control room cable. Because the GM detector operates on a limited plateau, manufacturing differences between detectors can have a significant effect on detector response. To accommodate these differences, each GM detector is subjected to a primary calibration to determine a unique correction factor. By applying the detector unique correction factor to the counts per minute (CPM) to µCi/cc conversion constant, the variations between detectors are eliminated. Secondary standard solid source sets and a standard field calibration fixture are available for on-site re-calibrations.

The Model 942A Universal Digital Ratemeter is provided to process the detector output and to display the fission product activity reading in units of CPM or microcuries per cubic centimeter (µCi/cc). In operation, the detector output is monitored by the UDR located in the control room. A Model 942-200-80 Serial Communication Module provides 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 µCi/cc calculation is a measurement of the radioactivity seen by the detector. The equation for the radioactive concentration is: µCi/cc = (CPM - CPM_BKRD) x Ka where: µCi/cc: Concentration, µCi per cubic centimeter CPM: detector output , counts per minute. This is the average of the 60 previous counts per second values, updated once per second CPM_BKRD: background subtract setpoint, current in CPM Ka: CPM to µCi/cc constant setpoint. Ka is the isotope conversion constant for the detector/sampler geometry, and is defined as the inverse of the isotopic efficiency (i.e., 1/effeciency) stated on the sensitivity data sheet 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 and gamma scintillation detector may be provided for monitoring ¹⁶N present in the main steam line. The detector output signal is electrically connected to the second UDR and adjusted to monitor the 6.1 MeV ¹⁶N photon. In this configuration, both fission products and ¹⁶N in the main steam are independently monitored, using the same detector shield.