Speaker
Description
A high number of new fission and fusion reactor designs are set to operate at high temperatures, on the account of increasing the overall electrical energy production efficiency, rely on passive heat dissipation for cooling after shutdown, or use other types of coolant than water such as liquid metal or molten salt. The behavior of these materials and the associated instrumentation must therefore be rigorously tested in a representative, high temperature radiation environment.
Within the framework of the JSI-CEA bilateral agreement, a project was undertaken to design and implement a highly insulated tube-type furnace with a maximum operating temperature of 900°C. The facility should be designed to integrate seamlessly with the existing irradiation infrastructure of the JSI TRIGA reactor. It must operate at an external temperature below 100°C and exhibit low levels of residual radioactivity following irradiation. Additionally, the facility should be positioned within a representative neutron and gamma radiation field. Due to these stringent limitations, the aim was to identify an appropriate insulating material. We have decided on using nanoporous silica material which has a low thermal conductivity of between 0.020-0.045 Wm-1K-1, depending on the temperature. The thermal analysis using the COMSOL software confirmed that using this material, an irradiation facility with a central hot zone diameter of 5 cm and uniformly heated length of 60 cm would reach the temperature of 900 °C with just 300 W of injected power, while still fitting inside a horizontal irradiation facility with diameter of 15 cm and maintaining the outside temperature some 20 K above ambient. Although these results consider perfectly closed thick furnace cap and a bulk insulation material, not layers of materials with potential gaps, the margins are satisfactory to proceed with the design. Apart from the nanoporous silica material, pieces of rigid, machinable calcium silicate insulation material were also selected to perform the function of structural holders. Samples of both materials were procured and tested for neutron activation potential, which was negligible. We also tested whether sheets of these materials could be cut using a laser cutter, which was remarkably effective. Hence, we ordered sheets of both insulating materials, that would fit the laser cutter bed and have the appropriate thicknesses for the best cutting performance.
The second part was procuring a suitable heating element, that can withstand the desired temperatures, while being made of materials that do not readily become radioactive under neutron irradiation. Commercial silicon carbide heaters were selected, due to their excellent performance, high purity silicon carbide and aluminized contacts. A small sample heater was obtained, to perform a test irradiation and assess neutron activation. The activity of the sample was of the same level as the background level a few hours after the test irradiation.
While silicon carbide is a particularly good material in terms of neutron activation, it is very brittle, and the first silicon carbide heater for the irradiation facility broke during shipping, which delayed our testing for a month.
Nevertheless, we have obtained all the insulating materials, cut them to size using the laser cutter, and inserted them into a custom-built aluminum enclosure. The heating element was also inserted, with the rigid calcium silicate insulation holding it in place. Type k thermocouples were put on the outside of the aluminum enclosure, to monitor the outside temperatures, and another one inserted into the hot zone, using the Zircalloy-4 rod as a guide-tube. The heater in the initial test was powered using a variable AC supply, although a high-power DC power supply is envisioned for the final experiment, to remove unwanted AC interference. Inner zone temperature of above 700 °C was reached in 45-50 min with the injected power at approximately 700-800 W was reached during the initial testing, while the wall remained cold to the touch. The maximum temperature on the outside of the enclosure reached the maximum temperature of about 50 °C an hour after the shutdown.
In terms of device manufacturing, we are currently waiting for the shipment of the power supply and some more testing, with the final design finished by the end of 2024. Efforts on temperature monitoring and heater control are also underway, with the aim of making a programmable application, with a simple user interface, where the user would be able to program the desired temperature profile.
