Nuclear power

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. Challenge 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. Consult factory for Nuclear Qualified instruments and specific model codes.

Open-system cooling towers reject waste heat from the steam cycle by exposing the cooling water directly to the atmosphere. The majority of heat removed is due to evaporation and the remaining cooled water drops into a collection basin. Level control applications include a high level switch to avoid overflow conditions in the cooling tower basin. In a once-through cooling system, the water intake structure is often a vertical wet pit pump which requires high and low level sensing and possible pump control. Challenge

The hyperbolic cooling tower releasing clouds of water vapor is the iconic image of nuclear power. Warm water from the condenser is pumped to the natural draft cooling tower, distributed to remove waste heat to the ambient atmosphere through evaporation, and collected in a basin prior to being recycled back to the condenser. Challenge The cooling tower’s intake structure, typically a vertical wet pit, requires level sensing and pump control. Water basin level controls maintain level through the addition of make-up water and are frequently configured with high and low level alarms.

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. Challenge 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. Consult factory for Nuclear Qualified instruments and specific model codes.

Waste liquids from sumps, radioactive leakage collectors, the Reactor Cooling System (RCS), and allied systems are collected, stored and processed. Inactive wastes are discharged or reused; active wastes are collected for processing. Radioactive liquids can provide make-up to the RCS, the ECCS, and the spent fuel storage pool. Challenge Waste liquids are collected and stored in large single- and double-walled tanks designed to suit radioactivity levels. Tanks are monitored for activity levels and their contents are processed, released or reused. Tank level instruments, frequently of redundant design, indicate inventory levels and protect against overfilling or underfilling that cavitates pumps. Prevention of tank overfilling.

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. Challenge 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. Consult factory for Nuclear Qualified instruments and specific model codes.

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. Challenge 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. Consult factory for Nuclear Qualified instruments and specific model codes.

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. Challenge Thirty percent 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.