Control blade degradation test under temperature gradient in steam atmosphere
The core of a boiling water reactor (BWR) consists of an array of fuel assemblies with cross-shaped control blades located between these assemblies. The typical control blade consists of a control blade sheath made of 316L-type S.S., a center pole made of 316L-type S.S., and control rods. In order to understand the degradation process of the control blade channel, control blade degradation tests were performed under various atmospheres with a temperature increase condition and in the presence of a temperature gradient. A test device was built. Four carbon heaters are set on both sides at the upper and the middle parts of the test device. The bottom ceramic part of the test device is cooled by water. Heating in the upper region and cooling at the bottom of the test piece give rise to the temperature gradient in the test piece. Temperatures of the test piece are measured by five W-Re type thermocouples placed close to the test piece. Steam carried by the argon gas is supplied from the bottom of the device and flows to the top inside a square carbon muffle in which the test piece is set. Three windows for visual observations are installed, with one located above the furnace and the others located at the back of the furnace. In the steam atmosphere with a steam flow rate of 0.0125 g/s/rod, the control blade, channel box, and fuel rods were degraded, especially at the upper part of the test piece. This result was similar to that observed in the argon atmosphere test. However, the observed degradation was rather different at a higher steam flow rate of 0.0417 g/s/rod. At the upper part of the test piece, only the control blade was degraded preferentially and did not react with the channel box wall. In contrast, the eutectic reaction of the stainless steel/boron carbide melt and zircaloy occurred at the lower part by coexisting with the relocated melt from the top part. These observations suggested the existence of a threshold condition for the control rod degradation between 0.0125 and 0.0417 g/s/rod which is significantly affected by the thickness of the oxide layer on zircaloy.