Primary-to-Secondary Coolant Leak Monitoring

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General Atomics radiation monitoring technology offers effective monitoring of primary-to-secondary leakage for the nuclear power industry. this technology is available through four different detection methods to meet various plant-specific performance and design criteria. These methods are:

Method 1 Detection of fission products (noble gases and iodines or activation products (Nitrogen-16) in a main steam line (MSL).
Method 2 Detection of fission products (noble gases) in condenser air ejector (CAE) lines.
Method 3 Detection of Tritium in secondary coolant, continuously or before and after steam generator blowdown (SGB).
Method 4 Detection of soluble fission products (Cesium-137) in SGB lines or SGB sample lines.

The following discussions present monitor configuration for each detection method, and include detection results from actual steam generator leak conditions.

Method 1 MSL Fission Products / Nitrogen-16

GA  offers two detector configurations to monitor fission and activation products in the MSL. These configurations are shown combined on one microprocessor in figure 1.

The MSL monitor detector assembly is designed to respond to gamma dose rates that accompany noble gas and iodine concentrations present in the MSL for a major primary-to-secondary coolant ingress, i.e., 10-4 to 101 R/hr. This detector assembly is used in GA monitor systems to meet USNRC Regulatory Guide

1.97 MSL monitoring requirements. For this configuration, the microprocessor provides direct digital display of MSL effluent activity rate for fission product concentrations between 10-3 to 103 _Ci/cc.

The Adjacent-to-Line (ATL) detector assembly, which employs temperature and gamma equivalent energy stabilization, uses the RM-2000 Multichannel Analyzer (MCA) or the RM-80 Single Channel Analyzer (SCA) capability to determine the presence and concentration of Nitrogen-16 in the MSL. An insulated, lightly shielded 2“ x 2“ Sodium Iodide scintillator subassembly provides the gamma sensitivity required to respond to Nitrogen-16 at 6.13 M3V. In this case, the microprocessor provides direct, digital display of the leak rate in gallons per day (GPD).

Figure 1 - Main Steam Line N-16 and Fission product Detector Configurations

Figure 1 - Main Steam Line N-16 and Fission product Detector Configurations

Figure 2 - Detector Response Main Steam Line 1

Figure 2 - Detector Response Main Steam Line 1

Figure 3 - Detector Response Main Steam Line 2

Figure 3 - Detector Response Main Steam Line 2

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 confidence level.


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.

Figure 5 - CAE WRGM Low range Nobe Gas Channel Response to Primary-to-Secondary Coolant Leakage in a PWR

Figure 5 - CAE WRGM Low range Nobe Gas Channel Response to Primary-to-Secondary Coolant Leakage in a PWR

Figure 6 - Comparison of Leak Rates from Chemistry Sample and WRGM

Figure 6 - Comparison of Leak Rates from Chemistry Sample and WRGM

Figure 7 - In-Line Detector Assembly

Figure 7 - In-Line Detector Assembly

Figure 8 CAE Rate vs. Leak Rate - PWR #1

Figure 8 CAE Rate vs. Leak Rate - PWR #1

Figure 9 CAE Count Rate vs. Leak Rate - PWR #2

Figure 9 CAE Count Rate vs. Leak Rate - PWR #2

Method 3 Tritium Detection

There are several factors that make Tritium detection and measurement a desirable method for determining primary-to-secondary leaks. Among these characteristics are its high activity level in the primary coolant and relative ease of detection combined with the inherent accuracy of the measurement process. The efficiency of Tritium concentration measurement as a means of determining primary-tosecondary leaks has been fully documented in the article —Modeling PWR Systems for Monitoring Primary-to-Secondary Leakage Using Tritium Tracer“ 1 by David G. Peiffer of Northeast Nuclear Energy Company.

To determine the Tritium concentration in a continuous stream of secondary coolant sample line flow, Sorrento Electronics utilizes a small light tight detector assembly (Figure 10) which is interfaced with the sample. A matched pair of photomultiplier tubes view the detector scintillator. Use of the tube outputs in pulse height and coincidence analysis circuits allow determination of Tritium concentration in the order of 1E-4 _Ci/cc.

A second Tritium detection system uses the flow-through gas monitor shown in Figure 11 and operates in conjunction with the output of a gas chromatograph. It utilizes a counting gas to determine Tritium concentration in a laboratory sample of secondary coolant.

Figure 10 - Tritium Detection Assembly

Figure 10 - Tritium Detection Assembly

Figure 11 - Flow Through Gas Monitor

Figure 11 - Flow Through Gas Monitor

Figure 12 - RD-53 Offline Detector Assembly

Figure 12 - RD-53 Offline Detector Assembly

Method 4 Fission Products

Detection of soluble fission products in steam generator blowdown or blowdown samples is accomplished with the RD-53 Offline Detector assembly shown in Figure 12. Concentrations of Cesium-137 as low as 2.6 x 10-8 _Ci/cc in a 0.1 mR/hr Cobalt-60 background are attainable using a monitor with this detector. This detector is used primarily for leaking loop identification.