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The in-reactor tests referred to as the IRDMR (In-Reactor Diameter Measuring RIG) experiments or the 'In-Reactor Fuel Element Diameter Measurements', consisted of two experiments, Exp-FIO-118 and Exp-FIO-119. Exp-FIO-118 consisted of two single-element irradiations on elements ABS and ABH; Exp-FIO-119 consisted of five single element irradiations on elements ACH, ACA, ACC, ACK and ACG.
Irradiation tests on elements ABS, ABH and ACH were performed to investigate the effect of fuel density on fuel element dimensional response to power changes. The remaining four elements, ACA, ACC, ACK and ACG, were involved in a series of power ramp irradiations. These experiments were conducted at AECL's Chalk River Laboratories in the NRX pressurized heavy water reactor using the In-Reactor Diameter Measuring Rig (IRDMR) with seven fuel elements between 1978 and 1983. The IRDMR was used to measure diametral changes of single fuel elements while at power.
The objectives of the tests on the seven elements were:
- to determine the effect of various design and operating parameters on the dimensional response of current CANDU power reactor fuel elements, and
- to provide quantitative data for code validation.
Coolant for the test was pressurized light water at a nominal pressure of 9 MPa and a flowrate of 1.0 kg/s, and nominal temperature of 200°C.
The seven fuel elements used in the Exp-FIO-118 and Exp-FIO-119 series of irradiation tests were assembled using enriched (3.5 wt% U-235 in U) uranium dioxide fuel pellets and clad in Zircaloy-4 sheath. The inner sheath surfaces of the elements were coated with a graphite layer.
Standard loop instrumentation included inlet and outlet temperature and pressure measurements, and flow measurement. Neutron flux was monitored with 10 vanadium, two cobalt, and two platinum, self-powered, neutron detectors mounted on the X-6 test section within the region of the He-3 coil, used for changing the neutron flux in the test region of the X-6 loop. He-3 pressure was monitored and controlled by an out-reactor pressure control system. The loop and neutron flux data were logged on magnetic tape by the loop data acquisition system.
Diameter measurements were taken by recording the strains induced in two pairs of cantilever beams by moving the fuel element back and forth. Calibration steps on the element end caps were used to calibrate the strains and to help eliminate the long-term problem of irradiation-induced drift. The diameter measurements were done at power, at shutdown, and during power changes caused by reactor start-up or shutdown, or He-3 power cycling. During irradiation, the fuel diameter was measured and flux detector signals were recorded.
Post-irradiation examination (PIE) included dimensional changes, fission-gas release, fuel burnup analysis and ceramography that included grain size measurement.
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Keywords: CANDU, fuel behaviour, loss-of-coolant accident.