Measurements of the High Dose Rate Profiles Inside a Shutdown CANDU^® Reactor

Abstract

The positions of the components of a reactor can change over time, due to radiation damage, sagging, etc. Thus, it is important to determine their positions. To satisfy this requirement of the staff at the Point Lepreau Generating Station, a method to determine the positions of reactor components has been developed and demonstrated. This method combines the use of dose rate measurements and Monte Carlo simulations. It first involves measuring the high γ-ray dose rates as a function of position within a reactor. Then it entails comparing these measurements with Monte Carlo simulations. In order to perform such measurements, a silicon diode detector and a scan drive system have been developed. In 2009, measurements of the γ-ray dose rate profile of the shut down Point Lepreau Generating Station reactor were conducted. By comparing the locations of the local peaks in the dose rate data, it was possible to determine the distances between the steel reactor components. The measured data were then compared with Monte Carlo simulations to determine how precisely one could locate the positions of the adjuster rods. Using this technique, it was found that the retracted adjuster rods were 440 ± 60 mm below their designed positions.