Cherenkov Radiation vs Power

Motivation

Cherenkov radiation is emitted when a charged particle (such as an electron) passes through a dielectric medium at a speed greater than the phase velocity of light in that medium. In nuclear reactors, this phenomenon is observed as a characteristic blue glow in the water pool, primarily due to high-energy electrons and positrons produced during the decay of fission products. Since the intensity of Cherenkov radiation is directly proportional to the number of charged particles, and thus to the fission rate, it can be effectively used to estimate reactor power.

Utilizing Cherenkov radiation for power estimation offers several advantages:

• Unlike traditional detectors such as fission chambers, which can suffer from depletion and require frequent calibration, a camera-based system is less susceptible to these issues as long as the camera itself does not corrode.
• To understand the overall flux distribution within a reactor, multiple expensive detectors would typically be required. However, with just a few cameras, we can estimate both radial and axial power distributions.
• At medium and high power levels, Cherenkov radiation measurements can exhibit lower noise compared to conventional detectors, providing more stable and reliable data.

Method

To investigate this, we installed a camera just below the water pool surface during normal reactor operation and recorded video for approximately 30 minutes. From the recorded video's RGB channels, we isolated the blue channel, which corresponds to Cherenkov radiation. The blue channel values across the entire screen were then averaged to obtain a single representative value. This data was subsequently calibrated and aligned using corresponding detector data. Two experiments were conducted: one with the reactor bay lights on and another with the lights off. The aligned graphs from these experiments are displayed below. The top two graphs are presented in a linear scale, while the bottom two graphs are generated in a logarithmic scale.