Apr 7, 2015

Nuclear Power Plant Instrumentation for Level Control

According to the World Nuclear Association, there are over 435 commercial nuclear power reactors operable in 31 countries, providing over 11% of the world’s electricity as continuous, reliable base-load power, without carbon dioxide emissions. As the world continues to leverage nuclear power as a clean energy source, the demand for improved performance from existing and next-generation reactors increases. Nuclear power plant instrumentation, including level measurement and control devices, must meet the challenge to enhance the efficiency and safety of these facilities.

This blog article addresses level instrumentation applications in reactor containment structures. The pressurized water reactor (PWR) and the boiling water reactor (BWR) represent the world’s two most common forms of nuclear power generation. The level applications discussed below utilize instruments for process control. Typically, a separate level control using a diverse technology serves as an alarm for spill detection.

Emergency Coolant Tanks
Application: The Emergency Core Cooling System (ECCS) supplies cooling water to the reactor during an interruption of the reactor’s normal cooling system. Upwards of 250,000 gallons of emergency make-up water is drawn from Refueling Water Storage Tanks (RWST) during the injection phase and from a containment sump during the second recirculation phase.
coolant tanks

Challenges: Level control of Refueling Water Storage Tanks is essential for emergency cooling operations. Low levels in these tanks can trigger actuation of pumps which bring additional coolant from accumulators, deaerators, de-mineralized water tanks, and treated condensate tanks. The ECCS can be tripped by an indication of coolant pressure loss or by low level of reactor coolant.
Level Instrumentation: 
-Models A10 or B10 Displacer-Actuated Switches for point level
-Eclipse® Model 706 Guided Wave Radar Transmitter or Pulsar® Model R95 Pulse Burst Radar Transmitter for continuous level
-Atlas™ or Aurora® Magnetic Level Indicators for visual indication

SCRAM Discharge Volume Tanks
Application: A SCRAM is a rapid shutdown of a nuclear reactor whereby control rods are inserted between the fuel rods in the reactor core to discontinue the fission reaction. The SCRAM is actuated manually by an operator or automatically when parameters are exceeded. When control rods are inserted, radioactive coolant is displaced by the rods and routed to a storage tank. This “hot” coolant is later processed and routed back to the recirculation system.
scram tank

Challenges: Level instrumentation in the Discharge Volume Tank is an important control in the Reactor Protection System (RPS). The level controls must be approved for radioactive service in a steam environment. Conventional float switches are frequently specified as they meet these requirements with high reliability.
Level Instrumentation: 
-Model B40 Float-Actuated External Cage Switch for point level
-E3 Modulevel® Displacer Transmitter (remote version only) for continuous level
-ATLAS Magnetic Level Indicator for visual indication

Steam Generator
Application: Primary coolant circulating in a PWR is heated under extremely high pressures to prevent boiling. The heated coolant enters two or more boilers called Steam Generators (SG) and boils the secondary loop coolant in a heat transfer process accomplished without mixing the fluids together. The coolant turns to steam which drives the turbine-generator.
steam generators

Challenges: 30% of emergency PWR shutdowns are attributable to SG level control problems. Controls balance feedwater to steam flow under all operating conditions. High-high levels can trip the turbine. Abnormally low levels can actuate emergency feedwater or a reactor shutdown. Measurement accuracy is challenged by thermal reverse effects known as “shrink and swell” and by static pressure effects.
Level Instrumentation:
-Series 3 Float-Actuated External Cage Level Switch; or B40 Float-Actuated Level Switch for point level
-E3 MODULEVEL Displacer Transmitter or ECLIPSE Model 706 Radar Transmitter for continuous level
-ATLAS or AURORA Magnetic Level Indicators for visual indication

Containment and Drainage Sumps
Application: A plant has many low-lying drainage reservoirs known as sumps. Small sumps include pump enclosures and tank rupture basins that contain leakage. The reactor’s large, containment sump is an essential reservoir of the ECCS whose function is to continuously circulate coolant through the reactor once all coolant storage tanks are depleted.
Challenges: Small sumps are monitored for leak detection with simple, float-operated level switches designed for bracket mounting in floor level sumps or troughs. These switches detect leaks or spills from pumps, valves, vessels, and pipelines. Levels of the large containment sump, or ECCS sump, are monitored during the recirculation phase of residual heat removal when the reactor’s primary coolant system is down.

Level Instrumentation:
-Models A10 or B10 Displacer-Actuated Switches for point level
-ECLIPSE Model 706 Guided Wave Radar with Single Rod Probe (remote version only) for continuous level
-ATLAS Magnetic Level Indicator for visual indication

Spent Fuel Pool
Application: One-third of the total fuel load of a reactor is removed from the core every 12 to 18 months and replaced with fresh fuel. Spent fuel rods generate intense heat and high radiation and are stored underwater in pools with depths of 20 to 40 feet. The water cools the fuel and provides radiation shielding. Spent fuel is later sent for reprocessing or dry cask storage.
spent_fuel_pool

Challenges: Without cooling, the spent fuel pool water will heat up and boil. Exposed fuel assemblies will overheat, melt or combust. Pool level is tightly controlled and water is continuously cooled by recirculation to heat exchangers and then back to the pool to resume cooling. High and low level alarms as well as redundant continuous level indication are typically required.
Level Instrumentation:
-Models A10 or B10 Displacer-Actuated Switches for point level
-PULSAR Model RX5 Radar Transmitter for continuous level

Be sure to check out the Magnetrol blog next week for the second part of our discussion on level control applications for nuclear power plants.