TABLE OF CONTENTS The response time of nuclear plant temperature sensors is predominantly affected by environmental conditions such as fluid flow rate and temperature, installation into a thermowell (when one is used), and degradation due to aging. These effects on the response time of temperature sensors are described in this section.
Changes in ambient temperature can affect sensor response time because: 1) the heat transfer between the sensor and surrounding fluid is dependent on temperature; and 2) dimensional changes occur as temperature changes.
The heat-transfer coefficient at the sensor surface changes with temperature because water thermal conductivity, specific heat capacity, and viscosity are dependent on temperature. For example, the film heat-transfer coefficient for a temperature sensor typically decreases by about a factor of two as water temperature increases from 20 C to 300 C.
The effect of temperature on sensor dimensions is a more dominant factor. More specifically, a temperature sensor is composed of several layers of materials. Ideally, these materials are homogeneous and in perfect contact with one another. In actuality, cracks and gaps likely exist within regions and at boundaries. As temperature increases, the gaps and cracks may open or close depending on the temperature coefficient of expansion of the sensor material. Since gas-filled gaps and cracks have a large effect on the heat-transfer resistance, this could be a large (but unpredictable) factor whose net effect might be an increase or decrease in response time with temperature. As such, the effect of temperature on...
