940-4P Reactor Coolant Letdown Monitor

The sampling assembly consists of a 140 cc, in-line sample volume welded to two (2) 150 # or 300 # raised face flanges that are installed in the reactor coolant let down system. The sample volume, sample tubing and flanges are manufactured from 300 series stainless steel. The system is designed for operation up to 300 psig. ASME Section IX qualified welders perform all welding. To avoid the expense of ASME Section III certification, suitable isolation valves should be installed upstream of the sample volume. The raised face flanges permit removing the sample volume for decontamination. The sample volume is installed into a lead shield. The shield houses a scintillation detector that views the sample volume in an in-line configuration. The shield consists of a 0.24 inch thick steel weldment, filled with five (5) inches of 4 Pilead shielding. The lead shielding surrounds the detector and reduces the response to ambient background radiation. Detector installation and replacement is accomplished via a removable lead cap, located on the top of the shield. The shield is also provided with a lead attenuator, designed to increase the range of the monitor by 2.5 decades. The shield provides a convenient mounting base for a solenoid operated ¹³⁷Cs check source and the detector junction box. Lifting eyes are provided for transporting and installing the shield. Alternate sampling configurations, based on existing in-plant piping configurations, are also available.

Two (2) Model 942A Universal Digital Rate meters are used to process the detector output and display the reading in units of counts per minute (CPM).The first UDR operates in the “gross” mode, displaying the total output of the detector. The second UDR is provided with a single channel analyzer (SCA) option circuit board, set to display the count rate for a specific, control isotope, normally 135I. In the SCA mode, an 135I sensitivity of 1E-4 µCi/cc may be realized to detect fission product activity at the 1% failed fuel element level. Using the attenuator plug, the range may be extended up to 1E+3 µCi/cc, approximately 30 times higher than the expected activity at the 1% failed fuel level. As an option, a single UDR maybe provided, with a front panel mounted Gross/SCA display select switch. In operation, the detector output will be monitored by the UDR located in the control room. An optional 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. 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, gross or SCA, counts per minute. This is the average of the 60 previous counts per second values, updated once per second CPM_BKRD 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 USR.