MSR Research Program

SALIENT-01 was the first irradiation of fissile salt in a test reactor in 45 years and is well-known in the MSR community.
The main goal was to gain knowledge and experience in molten salt irradiation: designing an experiment with liquid fuel, handling fresh and irradiated fuel salts in inert environments, and processing the resulting waste.
In summary, mixtures of lithium fluoride and thorium fluoride were placed in graphite holders and exposed to intense neutron and gamma radiation in the HFR Petten. The irradiation caused fission reactions in the salt samples, producing the heat required to melt the salt.

The follow-up to SALIENT-01, SALIENT-03 focuses on metal corrosion by molten fluoride salt in a radiation field and on measuring gas release during irradiation.
Lessons from SALIENT-01 were incorporated into the design. The experiment is conducted in collaboration with the European JRC, which produces and characterizes the salts and contributes to the design process.

Nickel alloys (and potentially certain steels) are considered materials for MSR primary systems due to their good corrosion resistance. However, nickel is sensitive to thermal neutron fields, where neutron capture can produce helium. This helium production can lead to embrittlement of the material.
The ENICKMA irradiation aimed to map this embrittlement effect in candidate alloys for MSRs, including Hastelloy N. Over 100 tensile specimens were irradiated at MSR-relevant temperatures. Results from irradiated samples were compared with unirradiated reference samples. The findings showed significant embrittlement in nickel alloys, while a reference steel's properties were barely affected by neutron exposure.

A successor to ENICKMA, this irradiation also investigates radiation-induced embrittlement of MSR construction materials, but takes it a step further by also examining mechanical creep during irradiation.
The project has two main parts. The lower half replicates ENICKMA but with higher doses and a greater focus on steel alloys based on prior results. The upper half involves a tensile-creep test on three stainless steel samples, measuring their slow elongation during irradiation. The same test is also conducted in an oven to distinguish between temperature and radiation effects.
IFE (Norway) is a partner in this project, contributing expertise in in-pile instrumentation and in the design, construction, and operation of instrumented irradiation facilities. The project is part of the OECD/NEA FIDES II program.

SAGA aims to study gas formation when fluoride (and possibly chloride) salts are exposed to a strong gamma field at temperatures below 100–150°C. This phenomenon could pose problems for temporary storage of spent fuel salts and in certain accident scenarios.
The reusable SAGA facility includes six gas-tight sample holders, each connected to a pressure sensor. These sensors record gas pressure buildup during irradiation of the facility in the gamma field of spent HFR fuel rods.

For design support and safety analyses one should know the molten salt flow and heat transport in the reactor vessel and/or the cooling system. For this, so-called thermal-hydraulics system codes were used. The nuclear reactor is modelled in such codes by ways of 0- or 1-dimensional elements. Originally, such codes were developed for water cooled reactors.

The simulation code needs to be adapted for molten salt reactors. Besides, validation is required to gain trust in the simulation outcomes. The SPECTRA code of NRG has been adapted for the specific and unique requirements put forward by modelling molten salt reactors. The following models are now available in the code

• Flexible user input for molten salt material and transport characteristics
• Fission product transport
• Delayed neutron precursor drift

Besides, the flow in the reactor vessel demands 3-dimensional modelling. For that reason, NRG started to prepare also a 3-dimensional tool for simulation of molten salt reactors.