By David W. Spitzer
Nuclear power was the rage back in the 1960s and 1970s until the incidents at Three Mile Island (1979) and Chernobyl (1986) brought nuclear power plant development to a grinding halt. Despite these setbacks, nuclear energy represents approximately 20% and 80% of the electricity generated in the United States and France, respectively. Recent fossil fuel price increases and concerns regarding global warming refocused attention on the development of more nuclear power plants because nuclear fuel is less expensive and because nuclear plants inherently do not generate greenhouse gases.
Nuclear power plants were typically licensed to operate for 40 years, and their design, construction and operation are strictly regulated. Rigid documentation requirements form significant impediments to upgrading instrumentation in nuclear power plants, especially when the operating license nears expiration.
However, the nuclear industry is constructing new nuclear power plants. In addition, many nuclear power plants are now renewing or in the process of renewing their licenses, so they can operate for another 20 years. Newer technology, plus additional years of operation, combine to increase the viability of upgrading the obsolete and cumbersome existing instrumentation systems that are difficult to maintain.
Larry Witt, director of applications of the Nuclear Division at Weed Instrument classifies nuclear power plant instrumentation, based upon its operating environment. The most demanding environments (Class 1E harsh) are inside the containment structure with exposure to high levels of radiation. These instruments are typically applied to important monitoring, safety and shutdown applications. They are qualified to environmental standards such as IEEE 323 and IEEE 344 that describe testing for seismic effects, thermal aging, radiation aging, vibration and loss-of-cooling accident.
Not Everywhere, But Lots of Places
Some instruments cannot be adequately age-tested for the full 40-year plant life. Witt adds that, temperature elements and other simple devices can usually be age-tested for 40 years. However, pressure and temperature transmitters can often be age-tested for 10 to 20 years, so they are replaced when their age corresponds to the age-test period in the environment to which they are subjected. Microprocessors in particular do not fare well in high-radiation environments, so analog instrumentation is commonly applied in these harsh locations.
Less demanding environments (Class 1E mild) are generally located outside of containment and used for safety functions. Less rigorous thermal and radiation testing is required to qualify instruments in these locations. In locations with low levels of radiation, microprocessor-based instruments can be applied. However, the required software verification and validation of these devices is both extensive and expensive, so many plants opt to install analog instrumentation in these locations, even though microprocessor-based transmitters are superior and might allow the plant to generate more electricity.
Nonetheless, fieldbus instruments could be used in these applications with appropriate qualification and testing. Willard Killough Jr., PE, instrumentation engineer at Duke Energy Carolina Oconee Nuclear Station in Seneca, S.C., is using, fieldbus instrumentation in some mild non-safety environmental areas including the borated water storage tank, bleed holdup tank and concentrated boric acid storage tank.
However, there is a significant amount of instrumentation in nuclear power plants that is located outside of these areas, where the equipment will not be exposed to radiation including the boiler, generator, cooling water, cooling tower and utility systems. These locations are often termed balance of plant (BOP) applications, where microprocessor and fieldbus equipment are acceptable. Witt estimates that the amount of instrumentation in Class 1E (harsh), Class 1E (mild) and BOP is approximately 10%, 20% and 70%, respectively.
Nuclear power plants in North America are typically operated continuously at or near their operating limit because this strategy tends to reduce the purchase of imported fuel while reducing cumbersome load changes. Shutdowns are generally limited the 20-to-30 day window required to refuel approximately every 18 months. Killough says, It is costly and difficult to make changes due to documentation issues that include drawings, calculations and the review process that entails operational and safety reviews. As such, the cost of hardware, software, configuration and installation is often minor compared to the paperwork involved.
Easier, Quicker, More Consistent
Killoughs experience indicates that upgrading to fieldbus was not that complicated.
Replacing 48 old recorders with multi-channel monitors was more difficult than upgrading to fieldbus because the logistics of working on the monitors in a concentrated area were more challenging, he says. In contrast, the fieldbus bricks (junction boxes) were installed prior to the outage. The outage time was used to replace the pneumatic system with a fieldbus system. The fieldbus system underwent both a factory acceptance test and a site acceptance test where approximately 90 transmitters were calibrated and configured in 4 days.
Figure 1. These new cabinets are located in the cable- spreading room in a mild environment that is air conditioned and heated to 66 °F.
He adds that, during the outage, the new instrument cabinets were installed, and the racks were replaced in approximately 45 days. The net result is that the plant now has the infrastructure to add points onlineeven though the plant typically prefers to wait to make changes during an outage.