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

Abstract

Nuclear heating inside an MTR reactor needs to be known in order to design and to run irradiation experiments which have to fulfill target temperature constraints. To improve the nuclear heating knowledge, an innovative calorimetric system CALMOS has been studied, manufactured and tested for the 70MWth OSIRIS reactor operated by CEA. This device is based on a mobile calorimetric probe which can be inserted in any in-core experimental location and can be moved axially from the bottom of the core to 1000 mm above the core mid-plane. Analysis of obtained results and advantages brought by the first CALMOS-1 mobile equipment have been already presented. However, some difficulties appeared with this first version. A thermal limitation in the cell did not allow to monitor nuclear heating up to the 70 MW nominal power, and some significant discrepancies were observed at high heating rates 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 13W.g -1 and improving the “zero method” measurement thanks to a 4-wires technique. In addition, the CALMOS-2 calorimeter has been designed as a real operational measurement system, well suited to characterize and to follow the radiation field evolution throughout the reactor cycle. To meet this requirement, a programmable system associated with a specific software allows automatic complete cell mobility in the core, the data acquisition and the measurements processing. This paper presents the analysis of results collected during the 2015 comprehensive measurement campaign carried out with the new CALMOS-2 prototype. The 4-wires ;technique was tested up to around 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 neutron flux distributions, obtained with the Self-Power Neutron Detector suited to the CALMOS-2 calorimetric cell, are compared with those obtained with current devices. Finally, this analysis tends to emphasize advantages brought by the human machine interface automation, which deeply refined the profiles definition.