Monitoring parameters for fuel channel safety in AHWR - an overview

Full text: Advanced heavy water reactor (AHWR) is a vertical pressure tube type reactor having boiling light water coolant circulating by thermosyphon in reactor coolant channels. The fuel channel monitoring system instrumentation becomes very important for maintaining channel safety and integrity. The various monitoring parameters for fuel channel monitoring system can be classified as (a) continuous monitoring and (b) sample monitoring. The continuous monitoring parameters for fuel channel safety and operation are i) channel inlet pressure ii) channel temperature iii) channel flow and iv) channel power. Coolant activity and Annulus gas monitoring are sample-monitoring parameters for ensuring channel integrity. The inlet pressure to the core being same for all the channels and AHWR being a pressurized boiling water system, pressure and temperature measurement cannot be considered as monitoring parameter for channel safety. Hence the continuously monitoring parameter for fuel channel safety becomes flow and power monitoring only. Since channel DT is negligible, the void fraction measurement is required for getting the channel power. Hence in AHWR the measurement of flow at the single-phase inlet section and measurement of two-phase flow/void fraction at the tail pipe section becomes important for estimating channel flow and channel power for reactor safety and operation. Measurement of two-phase flow in natural circulation system is complex and various attempts were made to understand and develop instrumentation so that an effective channel monitoring can be carried out. The channel integrity is monitored by sampling methods namely annulus gas monitoring and activity monitoring. Since the annulus gap is partially open, the moisture/activity in the annulus air need to be measured accurately to identify and quantify the leak rate and in turn the pressure tube/calandria tube integrity. This paper describes in detail the various aspects of fuel channel monitoring, engineering design features of various systems and design and development of two-phase flow / void fraction measurement sensors with different principles for a reliable channel monitoring