New measuring channel for continuous monitoring of steam generator leak rates

In order to assess the primary to secondary leakage, radioactive isotopes, formed in the primary coolant as a result of fission or neutron capture, are usually monitored in the pressurized water reactor (PWR) secondary coolant. Some years ago, it appeared necessary to improve both leak rate assessments and the steam generator tube rupture (SGTR) detection. Studies and tests were started with the following goals in mind: (1) to set up a highly sensitive, convenient, and reliable measuring channel to assess the leak rates continuously and accurately; (2) to enable an early detection of an SGTR from the hot shutdown state to full power; and (3) to check the steam volumetric activity. In a first step, French efforts focused on the monitoring of 16N activity within the secondary coolant, allowing the high-energy gamma rays to be detected through the steam pipe wall and predictability of the rate of 16N production. The total gamma dose rate due to 16N at the touch of the steam pipe is rather low. For this reason, EdF has developed and tested a spectrometric method. In a second step, it was decided to analyze the low-energy gamma spectrum with the same apparatus in order to meet the second and third requirements. These gamma rays are emitted by noble gas fission products, which are assumed to be still present in the primary coolant while the reactor is in a hot shutdown state