Experimental Investigation into Consequences of Fuel Channel Failure in a CANDU Reactor Under Normal Operating Conditions

Abstract

In CANDU reactor safety and licensing analysis, one of the hypothetical accidents considered is the simultaneous failure of a pressure and calandria tube. This accident is also considered as the design basis event for the evaluation of structural integrity of the calandria vessel and in-core structures due to the high pressure, high temperature primary coolant discharge into the low pressure moderator following a sudden channel rupture1. In order to provide experimental data to validate the theoretical models used in accident analysis, an experimental program has been on going to simulate the sudden failure of a full scale fuel channel in a specially designed multichannel test facility. The multi-channel burst test facility consists of a 3x3 array to represent nine CANDU fuel channels housed inside a containment reservoir which is 6 m long, has a 1.18 m inside diameter and a 19 mm wall thickness. The schematic of the test loop with the main components identified (ie., the 2 m^3 supply tank, the electrical powered boiler, the pressure bump system, control and isolation valve, etc.) is shown in the attached figure. The central fuel channel was chosen as the burst channel for the initial tests and all the channel components were prototypical. The surrounding calandria tube on the burst channel was also defected to ensure that the calandria tube would fail due to pressure tube failure. This arrangement of pressure tube and calandria tube defects was chosen to ensure that the top central channel will be impacted by the burst pressure tube while the bottom central channel will be subjected to the discharging jet forces and the impact of the ejected fuel bundles. The remaining six channels in the array were simulated channels made up of carbon steel pressure tubes each surrounded by a stainless steel calandria tube. The test facility is very extensively instrumented with fast response thermocouples, pressure transducers, strain gauges, accelerometers and displacement transducers placed at various locations on test facility. The instrumentation is connected to three data acquisition systems. This paper presents a detailed description of the test facility and the unique design features that enable reuse of many components of the faculty for subsequent tests. A comprehensive description of the instrumentation and the test procedure is also included. The results from the initial commissioning test and the salient results from the subsequent tests are also included. The implication of these tests, from the perspective of CANDU reactor accident analysis, is briefly discussed.