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.
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.