Rokkasho Reprocessing Plant: Calibration check in accountancy tank

Full text: Accurate determinations of volume are a fundamental component of the accountancy of nuclear material (NM) in a facility that processes such material in liquid form. In the Rokkasho-mura Reprocessing Plant (RRP), the inlet accountancy is performed in a tank (IAT) that has been initially calibrated during the cold tests in 2002. To ensure that the calibration curve V = f(H) is remaining valid during the whole life of the plant, the operator performs a specific test once a year during a long time shutdown. This test consists in pouring known amounts of water in the tank and measuring the resulting height of liquid with the dip tubes (fast bubbling rate). The IAEA is measuring independently the levels and densities in the tank using its own manometers, part of the SMMS-1 system. The temperature and mass values are operators data. During the test, inspectors randomly survey the data collected by the operator. Samples are taken in the tank before and after calibration, and also in the feeding tank. These samples are independently measured in the OSL. In order to limit the errors, the procedure and conditions of the calibration check are very close to the ones of the initial calibration. Yet some differences cannot be avoided, such as the presence of the heel in the bottom in the tank, the presence of a hydraulic seal on the feeding line, the operation of the ventilation off gas system, the use of humidified bubbling air, and so on. The evaluation of the calibration check results by the IAEA must take into account all the possible errors or biases in order to detect any significant change in the way volumes are measured in this tank. Many technical issues are discussed internally (SGOA2, PSA) and with the operator during technical meetings. (author)