The Loss Of Coolant Accident (LOCA) is one of the most considered accidental scenario in nuclear power plant design. It occurs due to a break of the primary cooling circuit, which induces a strong pressure decrease, this leads to an overheating of the fuel cladding. The water steam and high temperature environment undermine the claddings and can lead to their burst with the release of fission products.
The Light-water One Rod Equipment for LOCA Experimental Investigations (LORELEI) is a device that will be dedicated to such accident simulations and studies. It will be implemented in the Jules Horowitz research Reactor (JHR) which is under construction at the CEA Cadarache center. The main objective of the LORELEI device will be the study of fuel claddings behaviors under extreme reactor conditions. In such experiments, the cladding surface temperature monitoring is essential; it allows linking the burst conditions with the temperature. However, to minimize the perturbation and avoiding any change of the burst conditions, this measurement needs to be non-invasive and preferably contactless, which excludes the use of conventional temperature measurement techniques such as thermocouples and thermistors. The IR pyrometry based temperature measurement technologies present a suitable solution for this application. It allows a contactless temperature measurement with a good accuracy, high speed and reliability. However, the conventional pyrometers are not adapted for harsh environments application and could not survive under the extreme core conditions. In order to overcome this issue, we developed a sensor prototype based on the infra-red multispectral pyrometry with an optical collection head (figure 1) that can withstand both reactor core nominal and accidental conditions; high radiations levels (about 1020 fast neutron/cm² fluence and several GGy of gamma dose), high temperatures (up to 1000°C during the accident) and pressure (about 90 bars).
The use of multispectral pyrometry technique allows to estimate both temperature and emissivity, which makes possible to overcome the issue of variation in the emissivity of the cladding and the changes of the system transmission during the operation. The infra-red wavelength range is selected due to the high performances of silica-based materials and their high resistance to ionizing radiations compared to the UV-VIS domain.
Before the sensor integration in the LORELEI experience, validation and qualification procedures are required. This will estimate the reliability and durability of the system under conditions that are representative of the final application field.
In order to provide preliminary results on the reactor core environment effects on our system, we separated the core constraints and the device was tested for each one using different CEA facilities. In addition, we performed several studies to estimate the combined effects of those constraints on the system. We focused in particular on the combined high radiation/temperature effects on the optical element response and the temperature/pressure effects on the sensor body. Finally, we reached a system accuracy with less than ~ 2% temperature measurement error in a range of 700 to 1200 °C.