Noble Gases Emission Monitoring and Control At Bruce Nuclear Generating Station A (BNGSA)

Abstract

Noble gases are produced in the fuel, as fission products, typically Xe¹³³ and Xe¹³⁵, and in the liquid zone and moderator cover gas systems, as the activation product, Ar-41.

The station target for noble gas emission is one percent of the AECB approved Derived Emission Limit (DEL). This target has been consistently exceeded at Bruce N.G.S. 'A', with emissions (apparent and actual) y averaging between 2 to 4% DEL until early 1980 when emissions were successfully reduced to below target level.

The achievement of this goal, was a result of improvement in two major areas: the control of the production and subsequent pathway of noble gases to the environment and also the capability of monitoring and measurement of noble gas releases.

The decrease in the production of noble gases was partially due to improved manufacturing techniques of the fuel, bundles with fewer associated fuel defects. In addition, our methods of defective fuel identification have progressed to allow speedy removal of identified failed fuel from the reactor core.

In the area of control of the movement of gases prior to release, the commissioning of the off gas management system has had a significant effect. This system draws noble gases from major escape points from the heat transport system and retains them in charcoal delay beds resulting in a reduction by a factor of 100 of the noble gas activity.

Also instrumental in the reduction of emissions was the increase in the length of the pathway, and thus the time spent by the noble gases within the station. When a significant release of noble gas occurs at a certain contaminated exhaust, the ventilation pathway is changed, forcing the gas to travel. to adjacent units for discharge. Thia provides an effective delay time, permitting decay of many of the noble gas radionuclides.

The second area of concern involved improvements to the noble gas monitoring system. Highly variable background fields were inadequately compensated for by the continuous background subtract function of the monitor. Efforts made to reduce this inherent design weakness succeeded in reducing the measured emissions by at least a factor of two.

Further attention is currently being directed to the sensitivity of the detectors employed in the monitoring system.

The combination of the above mentioned improvements have enabled Bruce N.G.S. 'A' to success--fully meet the station operating target of 1% DEL noble gas emission.