Figures 2 and 3 show the RM-2000 MCA displays of the ATL 2“ x 2“ Sodium Iodide scintillator detector subassembly response to fission and activation products present in two MSLs of a pressurized water reactor (PWR). Each MSL is fed by a separate steam generator module which has experienced a 10 gallon per day primary to secondary coolant leak. These leak rates were verified by the concentration of a fission product, Xe-133, present in secondary coolant chemistry samples and by primary coolant chemistry. Figure 4 shows the initiation and history of the leak in steam generator module 1 and the post-initiation history of the leak in steam generator module 2. The responses in Figures 2 and 3 were obtained about one month after the last secondary coolant chemistry reading shown in Figure 4, when the leak rate in each loop had stabilized at about 10 gallons per day. The detection point on each line is at the same location relative to the loop‘s steam generator module outlet. The detector assembly in loop clearly responds to 6.13 MeV gammas expected from the activation product N-16. Quantification of the leak rate required determination of the detector sensitivity at the detection point. To establish sensitivity, an energy and an efficiency calibration are necessary. A reference Cm-244/C-13 gamma source provides 6.13 MeV output for this purpose. Table 1 shows expected source yield for the 2“ x 2“ Sodium Iodide scintillator. Table 2 gives expected sensitivity of an ATL detector that uses the 2“ x 2“ Sodium iodide scintillator in a representation of main steam line geometry. Steam generator geometry, primary coolant N-16 source term, N-16 decay constant, primary coolant flow rate, secondary coolant flow rate, and secondary coolant state have been used to calculate secondary coolant N-16 concentration. For a 10 gallon per day leak rate, concentrations at the detection point use to obtain figures 2 and 3 have calculated range from 2.4E5 to 4E-14 _Ci/cc. The concentrations depend on whether the leaking steam generator tube is active or plugged an also on leak location along the tube. Figure 2 shows response for an active tube leak. Lack of response in figure 3 and knowledge of an existing leak indicate a plugged tube leak. The ATL detector geometry of table 2 can resolve a substantial part of the calculated range with statistical significance in a reasonable length of time.
Table 2 shows calculated detector sensitivity extremes. When the channel lower level of detection (LLD)
is set to provide response to all four N-16 energy groups with buildup, an estimate of the maximum or
gross sensitivity is obtained. Response of the detector for a window centered on and including only the N-
16 photoelectric peak at 6.13 MeV yields an estimate of the minimum or SCA sensitivity. Use of either
sensitivity impacts the counting time required to obtain statistical significance. For example, in the gross
counting mode 1E-6 _Ci/cc can be resolved within 10% at a confidence level of 95% in about ten
minutes. Using single channel analysis at 6.13 MeV this accuracy occurs in about 3.5 days with the same
Table 1 - Yield for 6.13 MeV Calibration Source
Table 2 - ATL Detector N-16 Sensitivity using Microshield 4.0
Method 2 Noble Gases
GA offers two systems for condenser air ejector (CAE) line monitoring. One is the proven USNRC Regulatory Guide 1.97 compliant wide range gas monitor (WRGM) for PWRs with CAE vents that require the full regulatory guide effluent noble gas range. Isokinetic samples from the CAE line are routed through the WRGM offline detector assemblies to provide a noble gas detection range from 1E-7 to 1E+5 _Ci/cc (Xe-133). A WRGM with this capability has been in service for tens years at the PWR with the leakage shown in Figure 4. In fact, figure 5 shows the response of the WRGM low range noble gas channel to the initiation of the leak in steam generator module 1. As the leak commences, noble gas concentration rises from 2E-6 to 2E-5 _Ci/cc (Xe-133) in about 5 minutes. Figure 6 shows the CAE WRGM low range noble gas channel response superimposed on figure 4. When the correction of leak rate to noble gas concentration in Figure 6 is considered, the WRGM low range noble gas channel has resolved a leak rate change of approximately 0.2 GPD/min. figure 6 also shows that the CAE WRGM, unlike the main steam line ATL detectors, senses the leaks in both loops. This is because CAE noble gas concentration is not sensitive to the hold up that occurs in leaking plugged steam generator tubes, i.e., the attenuation of N-16 main steam line concentration due to N-16‘s 7.2 second half-life. The second CAE detector arrangement is used for in-0line monitoring of noble gases. It is employed where CAE discharge is isolated, recirculated, or combined with other fluid streams in a separately monitored effluent path. Figure 7 shows the in-line detector which includes a shielded, temperature stabilized, gamma scintillator capable of resolving 1E-06 to 1E+0 _Ci/cc Xe-133 concentrations. Figures 8 and 9 show the detector responses for two CAE in-line monitors at two different PWRs. Detectors of this type are recommended for the high temperature, high moisture fluid flow encountered in CAE lines. In addition, the in-line CAE detector assembly can be used to monitor primary to secondary coolant leaks not resolvable by N-16 main steam line or steam generator blowdown soluble fission product monitoring.