Advanced monitoring for LWR sustainability9 May 2017
Over the last ten years advanced monitoring techniques have been used in nuclear facilities to optimise maintenance, improve efficiency, reduce costs and manage ageing. H M Hashemian gives an overview of four key tools that are in use at over 50 nuclear plants.
The nuclear industry has benefited from vibration monitoring of rotating machinery for nearly five decades and it still depends on this technique for predictive maintenance of motors, pumps, valves, fans and more. In recent years, vibration-monitoring technology has advanced to the point that a user can not only identify the onset of a failure, but also estimate the residual life of rotating machines and establish optimum maintenance and replacement schedules for them.
The savings that have resulted from efficiency improvements and reduced downtime and maintenance cost amounts to billions of dollars per year in the US industry alone. Combined with other measurements, such as thermography and acoustics, ultrasonics and oil analysis, vibration measurements can provide a very detailed picture of equipment health and remaining useful life. In the last two decades, building on the success of vibration measurement technology, the nuclear industry has developed other condition monitoring tools to go beyond rotating equipment applications. Four examples are:
- Rod control system performance monitoring
- Cable condition monitoring
- Online monitoring to optimise I&C maintenance
- Wireless technology for equipment condition monitoring
These techniques are used in over 50 nuclear power plants and four research reactors, helping them save over 100 days of outage per year. That corresponds to nearly $100 million of maintenance cost reduction and increased revenue over a typical operating cycle.
Rod control system performance monitoring
In pressurised water reactors (PWRs) the drop times of each of the 53 control and shutdown rods (Figure 1) must be measured at each refuelling outage, to verify that there is no obstruction to impede the drop. The requirement is for each rod to drop from the top to the bottom of the reactor in less than about two seconds.
To shut the reactor down, the rods fall under gravity. But normal manipulation of the rods for reactor control is accomplished by control-rod drive mechanisms (CRDMs). CRDMs are typically made of three coils that must work together in timely sequences to move the rods (Figure 2). To verify the timing and sequencing of CRDMs, the activation
and deactivation of the coils must be tracked as the rods are moved in and out of the reactor. This is accomplished by recording the electrical current signals that energise the coils (see Figure 3). The rod-drop time measurements and CRDM tests must be performed after each refuelling cycle, or whenever the reactor vessel head is removed.
Taking advantage of fast multifunctional data acquisition systems, rod-drop time measurements and CRDM testing for PWR plants have been automated so that a set of tests can be completed in less than an hour, compared to the 12-24 hours that it would take using conventional procedures. All 53 rods are dropped at one time and their drop times are measured simultaneously, whereas previously the conventional practice was to testing one rod at a time. CRDM testing is performed on one bank of CRDMs at a time.
In addition, new techniques are available to track the performance of rod control-system electronics. For example, by measuring the current and voltage of CRDM coils, the coil resistance can be measured, and monitored for overheating, degradation and other changes that would interfere with proper movement of the rods. The same principle has been used to verify the health of digital rod-position indication (DRPI) coils, perform online condition monitoring on DRPI power supplies, improve the resolution of DRPI systems and more.
Cable condition monitoring
A nuclear power plant cable is a metallic conductor covered by polymer insulation. During routine plant operation, conductor and connector problems arise and are identified, located and resolved using an existing array of electrical testing technologies. They include insulation resistance (IR) checks, time domain reflectometry (TDR) tests, and loop resistance, capacitance, and inductance (LCR) measurements.
In cable ageing management, in-situ methods have been developed to allow plants to remotely identify and locate insulation degradation, damage and cracks, by sending a signal through the cable and measuring its reflection. Other electrical measurements can also aid diagnosis of cable insulation problems. For example, TDR can identify problems on cable insulation material as well as along conductors. When TDR is combined with frequency domain reflectometry (FDR) and other electrical measurements such as IR and LCR, it is much easier to diagnose insulation problems.
Online monitoring (OLM) has a wide spectrum of applications in nuclear power plants. For example, the condition of pressure, level and flow transmitters can be verified during plant operation using OLM. This has been done at the Sizewell B plant in the UK and at the Advanced Test Reactor (ATR) in the USA. It involves retrieving the normal output of the sensors from existing plant computers and analysing them for evidence of drift. Figure 4 shows an example of online calibration monitoring data for four redundant transmitters at Sizewell B, together with their allowable calibration bands. These data reveal that one of the three transmitters has drifted out of tolerance. This transmitter was calibrated and the remaining three were left untouched, or one of the three is calibrated to provide a reference. With this approach, a plant can save up to 75% of pressure transmitter calibration load. For a plant like Sizewell B, which has hundreds of pressure transmitters, this saving is significant.
A common complaint about online calibration monitoring is that it is a one-point calibration check rather than a full-span calibration verification tool. To overcome this concern, OLM data must be retrieved not only from periods of normal plant operation, but also during startup and shutdown. Figure 5 shows OLM data for three Sizewell B pressure transmitters sampled during startup, normal operation and shutdown to verify the calibration of the three transmitters over their entire operating range.
Sizewell B plant reported that using OLM it can save up to five days of outage time, which would normally amount to nearly $5 million of generation revenues, plus indirect benefits such as reduced radiation exposure to technicians, less damage to plant equipment from potential mistakes during calibration and reduced potential for calibration-induced alarms.
Wireless technology for condition monitoring
As plants age, they require increased maintenance of rotating machinery to identify anomalies as they occur and prevent failures ahead of time. With the advent of wireless sensors, numerous wireless sensors can be deployed in a plant at a low cost. As well as vibration, sensors can measure temperature, humidity and other variables. They can paint a holistic picture of equipment health and reveal other problems such as overheating and leaks. Referred to as “lick and stick” sensors, these wireless measurement devices are becoming available from a number of manufacturers and can be attached to rotating equipment for condition monitoring.
Wireless sensors have been used for condition monitoring of containment cooling fans at Arkansas Nuclear One and the High Flux Isotope Reactor (HFIR) at the USA’s Oak Ridge National Laboratory. Figures 6 and 7 show installations where data is collected, using wireless equipment at a frequency of at least 100 samples per second and analysed to identify any deviation for normal operation. The significance of these developments is in using wireless sensors in nuclear facilities, where interference and cybersecurity is of great concern. The installation at ANO is inside the reactor containment, and is used to monitor vibration of the containment cooling fans located at a high elevation above the reactor.
This work and that at HFIR proved that electromagnetic compatibility and cybersecurity questions are not insurmountable and wireless sensors can be used in nuclear facilities with little fear of interference and intrusion, unless they are used for control of equipment or the plant.
Using wireless technology inside the containment also has benefits in better employee communication during outages. The Arkansas installation of router, access points, antennas and repeaters for the project above allowed the plant voice over internet protocol (VOIP) phones to be used for the first time inside the containment. Previously, plant personnel had to install temporary cabled communication systems with less range and higher risk of contamination and radioactive waste.
Author notes: H.M. Hashemian (firstname.lastname@example.org) is president and CEO, Analysis and Measurement Services Corporation. AMS Corporation, 9119 Cross Park Drive, Knoxville, TN 37923, USA.