How the Industry Is Making Used Fuel Storage Pools Even Safer
Nov. 7, 2013—Nuclear energy facilities are installing new instruments that will alert operators of changes in the level of water in used fuel storage pools even under extreme conditions.
On orders from the U.S. Nuclear Regulatory Commission following the 2011 earthquake and tsunami that damaged the Fukushima Daiichi facility in Japan, companies must add the new equipment by 2016 or after two refueling outages, whichever is sooner.
Several solutions for measuring water level are available and acceptable to the industry and the NRC, based on the requirements the industry guidance has developed to comply with the order. Some vendors have developed specialized techniques.
One vendor is offering a system that uses “through-air” microwave radar pulses directed at the pool surface from above to detect the pool level. The time it takes the pulse to bounce back from the pool surface indicates the level of the water.
Another radar-based technology uses “guided wave radar sensors” in the form of a stainless steel probe that extends into the pool to almost the top of the stored fuel. Radar waves travelling down and back up the probe show the water level.
Suppliers are offering other ways to measure water levels in the pools. One vendor has a “thermal dispersion” system that measures the difference in how heated probes cool in water and in air. Other manufacturers offer a suite of systems using both radar and the buoyancy of magnetic weights. Yet another measures water levels from pressure differences on in-pool strain gauges.
All these methods can provide reliable readings in adverse conditions such as high humidity, high temperature, smoke and radiation.
When the power failed at Fukushima Daiichi immediately after the tsunami struck, concerns arose that the workers on site did not have reliable information on the condition of the water in the pools that store used nuclear fuel at each of the reactors.
Unsure whether the pools may have developed leaks or the water was starting to boil off, which may have ultimately caused some of the fuel in the pool to catch fire depending on the age of the fuel and its arrangement in the pool, workers at the site made several attempts during the following days to ensure the pools were filled.
It turned out that the pools were not in danger of leaking or boiling dry. However, one of the lessons learned by an NRC task force on Fukushima established shortly after the accident was that “confusion and misapplication of resources” can result when adequate information is not available during severe events. One of the task force’s “high-priority” recommendations was for enhanced instruments to be installed for used fuel storage pools at U.S. reactors. The NRC said the instrumentation would help responders “effectively prioritize emergency actions” and provide a substantial increase in protection to public health and safety.
Noting that the pool level instruments currently in place at U.S. reactors are only capable of monitoring normal and slightly off-normal conditions, with indications mostly at the pool, the agency’s March 2012 order for the additional instrumentation called for ensuring reliable readings through a wider range of pool water levels, including under extreme conditions.
The order provides for redundant, physically separated instrument channels with independent electrical power and alternate backup power. The readout display must be in an accessible area during and after an extreme event and the instruments should be mounted with earthquake-resistant brackets. The instruments also must be reliable under high-temperature, high-humidity and high-radiation conditions and must remain functional “until additional off-site resources are obtained, deployed and spent fuel pool conditions are stabilized.”
Reactor operators submitted their “overall integrated plans” to the NRC earlier this year, explaining how they would comply with the agency’s order. The NRC in October began issuing interim safety evaluations of the utility plans. NEI is planning to discuss with the agency its approach to the evaluations in November.