Leak Before Break and Leak Detection Systems of CANDU Fuel Channels

Abstract

In the CANDU reactor each of the approximately 400 hot pressure tubes containing the fuel bundles and the pressurized heat transport water is surrounded and insulated from the cold moderator by a calandria tube. The pressure tubes are made from cold worked Zr-2.5%Nb, and the calandria tubes are made from annealed Zircaloy-2. The annulus between these two tubes contains a gas whose dewpoint is measured to ascertain if leaks have occurred into the annulus.Leak-Before-Break provides the operators of CANDU reactors with advanced warning of potential unstable failure of pressure tubes. It relies on matching the characteristics of the leak detection procedure to the characteristics of the cracking phenomena.A procedure for leak detection and reactor response has been built up from the use of the annulus gas. The characteristics of the crack - its shape, its length at wall penetration and its growth rate by delayed hydride cracking, and the expected leak rate at various lengths are used to establish the response time for leak detection. The reactor is required to be shutdown when the crack has a length much less than the critical crack length. This critical crack length, determined using burst tests on slit tubes, is the crack length at which the crack growth becomes unstable.New CANDU reactors will continue to emphasize the importance of the annulus gas system for leak detection. Emphasis has been placed on the design of this system to increase its sensitivity and to shorten the response time. These requirements have led to investigation of new means and concepts of detecting pressure tube cracks or leaks.Current developments of an upgraded detection system have focused on in-situ and remote moisture measurement on individual channels with sensors that can detect all possible ranges of leaks but particularly small ones. The performance of various detectors for in-situ application in high temperature and radiation environments is currently being evaluated. Examples are fibre optic spectrophotometry, and ceramic plate detectors where the capacitance varies with moisture. The characteristics of new detectors such as accuracy, sensitivity and drift in the gas annulus environment must be established.Leak location or identification practice will be improved by a modified system configuration and the use of multiport indexing valves to intermittently sample and record moisture at various positions. The use of a bulk moisture differential monitoring technique using current instrwnentation circuits promises to increase system sensitivity and alleviate the problem of instrwnent drift. Analytical tools have been established to evaluate system response characteristics to different leaks at different positions in the systems.Combining Leak-Before-Break technology and improved techniques of leak detection provide increased margins between the time at which a crack penetrates a pressure tube and the time the operator receives the information and takes corrective action to prevent unstable failure.