In 2015 NRG started together with the European Joint Research Centre (JRC) in Karlsruhe a series of nuclear fuel irradiation experiments to support the development of  Molten Salt Reactor technology. Such experiments were not performed since the 60’s, when a large scale investigation was carried out by the Oak Ridge National Lab. The first experiment in the series named SALIENT-01 (SALt Irradiation ExperimeNT) was irradiated in Petten’s High Flux Reactor between August 2017 and August 2019. The experiment was cooled down for one month before transport to the Hot Cell Lab for examination. SALIENT-01 focuses on the interaction between nuclear fuel and graphite, as well as on the spread of fission products within the reactor. Overall, the experiment was meant to  develop ‘hands-on’ experience with nuclear fuel salt.

SALIENT-01 consists of a hermetically closed sample compartment of 60 cm length and 3 cm diameter, containing 4 small nuclear fuel salt samples (each one smaller than 1.5 cm³) in open graphite crucibles. These  fuel salt samples consist of mixtures of LiF and ThF₄ having a melting temperature of 568 ^oC.

The samples were irradiated for 18 cycles of 30 days in the HFR under a strong neutron field at temperatures of around 600 ^oC. During irradiation, ²³²Th captures a neutron and transmutes into ²³³U (via ²³³Pa) , which is fissionable. The fission of ²³³U produces energy in the salt samples in the form of heat. At the same time the fission process contaminates the salt due to the formation of radioactive fission products.

In the Hot Cell Labs, measurements and inspections  are performed to understand the transformations of the samples occurring inside the reactor core. Some important questions to be answered are:

• Are fission product compounds released from the salt, and if so, where have they relocated to?
• Some fission product compounds form small metal particles. What is their typical size? Do these metal particles attach preferentially to metal or to graphite surfaces?
• Is there any noticeable chemical reaction between salt and graphite?

The main techniques that will be used are analysis of gamma radiation generated by the samples, chemical composition of plenum gas and (electron) microscopy of salt and graphite particles.

Samples of irradiated salt will also be sent back to JRC Karlsruhe. In Karlsruhe, the temperature will be increased slowly until the samples are completely vaporized. The evaporation temperature of each salt component is then characterized. The obtained data can be used to understand the source term for radioactivity in safety assessment studies.