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Description
Between the 1970s and the late 1980s, the CEA developed several fission chambers operating at high temperature to instrument French sodium-cooled fast reactors (SFR). These developments, carried out in collaboration with the Exosens Company, led to the marketing of ex-core detectors, referenced CFUC, as well as miniature detectors such as referenced CFUE. These developments were put on hold in the late 1990s before being resumed in the 2010s through the ASTRID project. For reactor core monitoring, ex-core fission chambers with 10 decades of neutron flux operational range have been proposed. However, miniature detectors were not targeted. With the renewed interest in SFRs and molten salt reactors thanks to the spread of advanced modular small reactor concepts, development of high-temperature miniature neutron detectors are required.
In this context, two types of miniature high-temperature fission chambers: CFUE43 detectors (dedicated to high temperatures with a diameter of 7 mm), and CF3 detectors (reference detectors for neutron flux monitoring in research reactors at the CEA LDCI lab) were identified for testing. Filling gases are respectively argon and a mixture of argon with 4% nitrogen at 5 bars.
As part of a collaboration between CEA and INL through the US Department of Energy, these two types of detectors have been tested under irradiation at high temperatures in the Ohio State University Research Reactor (OSURR) to check their behaviour at 900 K.
Two CFUE43 and one CF3 were installed in a furnace made of an SiC heating element and silica fibre insulating material. This furnace was settled at the bottom of a dry irradiation channel located in the vicinity of the reactor core. At maximum reactor power, 200 kW, the maximum neutron flux at the irradiation location was 1012 n/s/cm².
Thanks to the tests carried out in the OSURR, it is demonstrated that the CF3 fission chamber operating in pulse mode is a good option for the instrumentation of a high-temperature reactor. Optimisation is underway to be fully operational in such conditions. Mineral cables with magnesia are also being considered for improvement. Future experiments will test the Campbelling mode to determine the measurement range of this high temperature CF3. CFUE43 works well in current mode, but unfortunately the interpretation of the signal is more complicated than in pulse or Campbell mode because there is no gamma compensation.
