Spent fuel pool sensors

29 May 2013

The Fukushima fuel pool scare has turned a spotlight on measuring the water level and temperature of spent fuel pools. There are many ways to do so. Will Dalrymple takes a look

In March 2012, the USNRC mandated that all US nuclear power plants must install spent fuel pool water level gauges within two fuel cycles or by the end of 2016. The stations need to install gauges at three water levels: at a height suitable for normal operations, a level that is adequate to provide substantial radiation shielding for someone standing on the operating deck, and a level where the fuel is just covered. It also specified that they install a main and a backup level instrument, provide a display in an accessible area, and provide independent electrical power to each instrument channel, and an alternate remote power connection (see figure).

In July 2012, the US Nuclear Energy Institute developed industry guidance on how to meet these requirements (12-02).
Fluid level measurement is an essential function of any process that deals with stored tanks or material flows, so there are many different types of level measurement detectors available from many vendors. Nuclear vendors have also developed specialised equipment. A range of different technologies are introduced below.

Westinghouse offers a Spent Fuel Pool Level Instrumentation System that consists of a permanently-installed cable probe suspended from the pool deck by a heavy-duty bracket that extends down to the spent fuel racks. In this guided-wave radar system, part of a signal transmitted all the way down the probe reflects back up the probe when it hits the water's surface. The return wave is detected, and based on the time of flight a computer calculates the water level. The instrumentation electronics are mounted away from the pool, and are backed up by a battery. A transmitter sends the level data to a display, which could be mounted elsewhere. Another option is for the transmitter to send the level signal wirelessly.

AREVA announced in January 2013 that an unnamed US utility customer had bought the contactless through-air radar-based system that it promotes for spent fuel pool level measurement. This system, manufactured by VEGA and sold exclusively through AREVA in North America, is installed facing down. It transmits a packet of microwave pulses at the water below from a conical horn antenna; the water surface reflects the pulses back, even if agitated (says AREVA). The time lag is proportional to distance, which can be converted into a height measurement by software. Electronics can again be mounted remotely.

Fluid Components International (FCI) recently launched a multi-function monitor that complies with the NEI's recommendations, according to the manufacturer. The CL86 Plus's level system uses thermal dispersion, which involves measuring the temperature difference between multiple heated probes; those immersed in water will cool at a different rate than those in air, and so register a temperature differential that generates an electrical signal. The CL86 integrates three measurements: continuous level (up to 9m range), point level (up to four wet or dry points) and water temperature (0-100°C). Each sensor output is separate and independent, although all three are mounted in a unified probe assembly immersed in the pool that is manufactured to a customised length. Electronics can be mounted up to 300m away in a steel enclosure. An option enables in-situ verification of level operation.

German manufacturer Krohne manufactures three different sensing systems suitable for spent fuel pools, according to US distributor AZZ/NLI. There is the Optiflex 220 guided-wave radar device, which uses time-domain reflectometry; its protective housing only needs to be open at top and bottom to protect it from debris. Also offered is a non-contact radar sensor, the Optiwave 5200, that beams down a frequency-modulated wave on the spent fuel pool from above. The signal is not susceptible to pool debris. The device's electronics can be mounted locally or remotely. Finally, the BM26 system uses a floating magnet and reed switch to indicate water height; it is also equipped with a non-contact radar system.

For spent fuel pools, US supplier Magnetrol recommends two different systems: a radar-based system (its Pulsar series through-air pulse-burst RADAR level transmitter), and a system based on the buoyancy of a weight. The latter A10 single-stage and B10 dual-stage displacer switches rely on weights (displacers) heavier than water. Immersion of the displacers results in a change in buoyancy force, which reduces the tension on the spring. Spring movement causes a magnetic sleeve to attract a pivoting magnet, actuating a switch mechanism. The single-stage displacer system has one weight; the dual-stage unit has two that can actuate at different field-adjustable heights.

In many industrial environments, pressure transducers are used to measure water level. The device is lowered to the bottom of the pool, where the weight of water presses on a sensor that converts pressure into an electrical signal. Less water translates into a change in signal. For example, the Campbell Scientific CS460-L sensor uses a strain gauge bonded to a pressure-sensitive diaphragm; a connected datalogger records the measurement and converts it into a water level. It also measures water temperature with an operating range of -10°C-80°C, and is temperature-compensated.

USNRC requirements for spent fuel pool sensors

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