Prairie Island: putting CZT on the map

The Prairie Island nuclear plant (PINGP) has implemented a new and highly effective dose reduction initiative using cadmium zinc telluride (CZT) monitoring. Two types of CZT instruments have been used: spectroscopic imaging and process monitoring. These have reduced the dose and risk associated with: forced oxidation monitoring, temporary shielding installation, hot particle identification and shipping survey verifications. The ultimate goal is to use the monitors to support continued dose reduction efforts by providing a real time picture of plant conditions for enhanced decision making.

Prairie Island’s use of CZT monitoring began in 2013 with the purchase of a spectroscopic imaging detector for temporary shielding package verifications. The use of the technology rapidly expanded throughout the commercial nuclear industry and in 2015 a CZT working group was established through the North American Technical Center (NATC). Founding members, who include the DC Cook, Palisades, and Prairie Island plants, share their experiences to improve the utilisation of the technology. This group has monthly conference calls, and provides assistance during refuelling outages. Prairie Island has assisted DC Cook with imaging of its auxiliary building and Palisades with a plant radionuclide analysis. Together, the group has used its collective experience to establish a set of guidelines for CZT programme implementation (see box).

The group uses solid-state detectors with a 3D position-sensitive CZT detector to perform gamma spectroscopy and imaging. These systems have an energy resolution of less than 1.1% at 662keV, rivalling high-purity germanium (HPGe) detectors without the need for cryogenic operating temperatures. The spectroscopic imaging detector takes a visual image and overlays it with a radionuclide specific heat map. This produces a visual means of communicating radiation fields and can provide verification of traditional dose rate surveys. The imager was used to verify the location of the highest dose rate for a radioactive material shipping container and to evaluate the adequacy of a temporary shielding package. The evaluation of the temporary shielding package determined there was no radiation streaming, but that additional shielding needed to be placed in the coloured area of the image.

Process monitoring

The process monitors provide a means of non-destructive real-time radionuclide characterisation. The temporary monitors are installed to observe plant transients and quantitatively determine radionuclide activity in process piping. Monitoring data are provided in real time to the plant’s Remote Monitoring System (RMS), allowing for faster analysis of data and real-time decision-making. Combined with preloaded efficiency curves and operational data, the system quantitatively determines the various radionuclide concentrations in process piping.

The monitors were deployed during a refuelling outage to observe forced oxidation and cleanup of the reactor coolant system (RCS). Two process systems were monitored: the RCS letdown heat exchanger inlet and outlet, and residual heat removal (RHR) letdown and return lines. Real-time monitoring of primary dose-contributor concentrations during cleanup will reduce the need for chemistry sampling and provide a real-time correlation with operating parameters. This will reduce worker dose and costs associated with chemistry sampling during forced oxidation activities.

The RHR system removes the residual heat produced by the core after shutdown, when secondary side heat removal through the steam generators is not available. Two pumps and two heat exchangers remove the heat. After the RCS temperature and pressure have been reduced to 350°F and 425psig, respectively, RHR is initiated by aligning the pumps to take suction from the hot legs of both reactor coolant loops and discharge through the heat exchangers into the cold leg of a reactor coolant loop.

Monitoring this system allows for detection of Ag-110m pollution, circulating hot particles and determination of Co-58 concentrations during forced oxidation. Figure 1 compares the RCS concentration of Co-58, as determined by chemistry sampling, to the monitoring system. This demonstrates that the system is capable of accurately determining Co-58 concentrations during forced oxidation conditions. A similar examination for Co-60 showed that activity plated onto the piping masked the Co-60 being transported through the RCS. No hot particles were observed migrating through the RHR system during the data-collection period. Missing data, indicated by straight lines within the dataset, were due to mistiming of system activation relative to the start of forced oxidation.

The RCS letdown subsystems act together to maintain a continuous ‘feed and bleed’ flow on the RCS. Coolant discharged from the RCS flows into the letdown system, which passes through the shell side of the regenerative heat exchanger. Letdown pressure is reduced before passing through the letdown heat exchanger. Chemistry sampling occurs near the outlet of this heat exchanger before it is directed through a mixed-bed demineraliser. Due to the changing temperature and pressures of the coolant as it passes through the letdown heat exchanger, monitoring units were stationed near its inlet and outlet to monitor for plate out. Data collected from the outlet unit were satisfactory, but due to logistical complications, the inlet monitor was not installed until after cleanup was nearly completed. Analysis compared the RCS chemistry to the monitoring system for Co-58 concentrations. The consistency between the measurements from the RHR system and the letdown heat-exchanger outlet show that the instruments provide accurate real-time information, and that the chemistry letdown samples are representative of the overall RCS system for Co-58.

The letdown heat exchanger outlet monitor detected trace amounts of Ag-110m during the forced oxidation that had not been observed by chemistry. In general, the most likely cause for Ag-110m contamination within the RCS is leakage from the neutron-absorbing alloy (Ag-In-Cd) contained in control rods, or wear from silver-plated pressure vessel seals. Although Prairie Island has both control rods and seals containing silver, the Ag-110m activity is suspected to be from normal wear on pressure vessel seals.

The ability to detect Ag-110m in real time is of special significance, because its removal is extremely difficult and costly. Ag-110m is preferentially deposited in the cold points of auxiliary system heat exchangers, and may plate-out in piping before being sampled by chemistry. By monitoring multiple points in the RCS, early silver leak detection is made more likely.

Further development

Process monitoring work has demonstrated that the monitors are versatile and that the technological fundamentals for real time RCS radiological characterisations are sound. Further development of CZT technologies will be pursued in the form of native incorporation of collimators, deployment procedures, and totalisers. Using monitors as totalisers on resin-bed inlets and outlets will quantify the radionuclide activities of the bed in real time. This would provide significantly more reliable information for ALARA planning and provide an avenue to reduce radiological shipping risk by changing resin beds before they reach Class B levels.

The overall value proposition of developing CZT programmes is in providing a clearer radiological picture of the station, optimising operations to reduce personnel dose and risk.  

About the author: Brad Boyer is radiation protection manager at Prairie Island (Brad.Boyer@xenuclear.com)