Simultaneous Measurements of Nuclear Heating and Thermal Neutron Flux Obtained with the CALMOS-2 Measurement Device inside the OSIRIS Reactor

Abstract

Nuclear heating inside a materials testing reactor needs to be known in order to design irradiation experiments that have to fulfill temperature constraints. To improve nuclear heating knowledge, an innovative calorimeter, CALMOS, has been studied, manufactured, and tested for the 70-MWth OSIRIS reactor operated by the French Alternative Energies and Atomic Energy Commission. This device is based on a mobile calorimetric probe that can be inserted in any in-core location and moved axially along the core height and above. Obtained results and advantages brought by the first CALMOS-1 equipment already have been presented. However, a thermal limitation in the cell did not allow the monitoring of nuclear heating up to the nominal power, and some significant discrepancies were observed at high heating levels between results deduced from the calibration and those obtained by the “zero method.” Feedback-based, the new CALMOS-2 calorimeter has been designed both for extending the heating range up to 13 W.g-1 and for improving the “zero method” procedure. In addition, the CALMOS-2 calorimeter is an operational measurement system, suited to characterize the radiation field evolution throughout the reactor cycle. To meet this requirement, a programmable system associated with specific software allows automatic cell mobility in the core, the acquisition of data and their processing. This paper presents an analysis of results collected during the 2015 measurement campaign carried out with the new prototype. The four-wire technique was tested up to about a 4 W.g-1 heating level and allowed to quantify discrepancies between “zero” and calibration methods. Thermal neutron flux and nuclear heating measurements from CALMOS-1 and CALMOS-2 are compared. Thermal flux distributions, obtained with a self-powered neutron detector suited to the calorimeter, are compared with those obtained with current devices. Finally, the analysis emphasizes advantages brought by the human machine interface, which deeply refined the profiles definition.