Raluca O. Scarlat

Tritium Control and Capture in Salt-Cooled Fission and Fusion Reactors: Status, Challenges, and Path Forward

Three advanced nuclear power systems use liquid salt coolants that generate tritium and thus face the common challenges of containing and capturing tritium to prevent its release to the environment. The fluoride salt–cooled high-temperature reactor (FHR) uses clean fluoride salt coolants and the same graphite-matrix coated-particle fuel as high-temperature gas-cooled reactors. Molten salt reactors (MSRs) dissolve the fuel in a fluoride or chloride salt with release of fission product tritium into the salt. In most FHR and MSR systems, the baseline salts contain lithium where isotopically separated 7Li is proposed to minimize tritium production from neutron interactions with the salt. The Chinese Academy of Sciences plans to start operation of a 2-MW(thermal) molten salt test reactor by 2020. For high-magnetic-field fusion machines, the use of lithium enriched in 6Li is proposed to maximize tritium generation—the fuel for a fusion machine. Advances in superconductors that enable higher power densities may require the use of molten lithium salts for fusion blankets and as coolants. Recent technical advances in these three reactor classes have resulted in increased government and private interest and the beginning of a coordinated effort to address the tritium control challenges in 700°C liquid salt systems. We describe characteristics of salt-cooled fission and fusion machines, the basis for growing interest in these technologies, tritium generation in molten salts, the environment for tritium capture, models for high-temperature tritium transport in salt systems, alternative strategies for tritium control, and ongoing experimental work. Several methods to control tritium appear viable. Limited experimental data are the primary constraint for designing efficient cost-effective methods of tritium control.

Design Summary of the Mark-I Pebble-Bed, Fluoride Salt–Cooled, High-Temperature Reactor Commercial Power Plant

Abstract The University of California, Berkeley (UCB), has developed a preconceptual design for a commercial pebble-bed (PB), fluoride salt–cooled, high-temperature reactor (FHR) (PB-FHR). The baseline design for this Mark-I PB-FHR (Mk1) plant is a 236-MW(thermal) reactor. The Mk1 uses a fluoride salt coolant with solid, coated-particle pebble fuel. The Mk1 design differs from earlier FHR designs because it uses a nuclear air-Brayton combined cycle designed to produce 100 MW(electric) of base-load electricity using a modified General Electric 7FB gas turbine. For peak electricity generation, the Mk1 has the ability to boost power output up to 242 MW(electric) using natural gas co-firing. The Mk1 uses direct heating of the power conversion fluid (air) with the primary coolant salt rather than using an intermediate coolant loop. By combining results from computational neutronics, thermal hydraulics, and pebble dynamics, UCB has developed a detailed design of the annular core and other key functional features. Both an active normal shutdown cooling system and a passive, natural-circulation-driven emergency decay heat removal system are included. Computational models of the FHR—validated using experimental data from the literature and from scaled thermal-hydraulic facilities—have led to a set of design criteria and system requirements for the Mk1 to operate safely and reliably. Three-dimensional, computer-aided-design models derived from the Mk1 design criteria are presented.

Datasets for elemental composition of 2LiF-BeF2 (FLiBe) salt purified by hydro-fluorination, analyzed by inductively coupled plasma mass spectrometry (ICP-MS) using two digestion methods

This article shows the elemental analysis of a batch of FLiBe prepared from LiF and BeF2 and purified by hydro-fluorination, see “Batch-Scale Hydrofluorination of Li2BeF4 to Support Molten Salt Reactor Development” (Kelleher et al., 2015), which was performed by the method of inductively-coupled plasma mass spectrometry (ICP-MS), with analysis samples prepared by multi-acid microwave digestion with and without HF acid. Data shows quantification of a total of sixty-five elements and is reported for a total of eight digested samples. Quantification of 6Li/7Li isotopic ratio is reported for a total of eight digested samples.

Spectroscopy (Raman, XPS, and GDMS) and XRD analysis for studying the interaction between nuclear grade graphite and molten 2LiF-BeF2 (FLiBe) at 700 °C

FLiBe-exposed IG-110 graphite and a control IG-110 sample were analyzed by Raman, XPS, GDMS, and XRD, and the complete raw data sets are provided in the Supplementary Information. These data sets enable full reproducibility and transparency of the data analysis we reported in the accompanying research paper titled “Fluorination of Nuclear Graphite IG-110 in Molten FLiBe salt at 700 °C”, published in the Journal of Fluorine Chemistry, and facilitates quantitative comparison with future similar studies by other research groups. In this data article, we provide plots of the peak fitting for all Raman spectra from each sampling point on the graphite surface. We also provide the measured impurity concentrations of the IG-110 samples, as measured by GDMS; this data was not reported nor discussed in the accompanying research paper. The method and software used for peak fitting for the spectra from Raman, XPS, and XRD are listed separately.

Solid fuel, salt-cooled reactors

Abstract FHRs are a Generation IV fluoride salt-cooled, solid-fueled, high-temperature reactor class currently under development in the United States and China, by both research institutions and universities. The concept of an FHR was generated by trying to integrate advantages of other reactor classes (low pressure, high temperature, high efficiency) while eliminating some of their drawbacks (huge containment domes, low efficiencies). Fixed fuel and pebble fuel, SMRs, and gigawatt reactors are being studied. FHRs are a promising reactor technology for the needs of a future electrical grid with highly fluctuating solar and wind power. FHRs might also serve as a stepping stone to liquid fuel molten salt reactors, helping to understand phenomenology and engineering issues prior to having fuel and associated fission products present in the primary coolant.

Tritium Control and Capture in Salt-Cooled Fission and Fusion Reactors

Abstract Three advanced power systems use liquid salt coolants that generate tritium and thus face common challenges to prevent release of the tritium to the environment. The Fluoride-salt-cooled High-temperature Reactor (FHR) uses the same graphite-matrix coated-particle fuel as High-Temperature Gas-cooled Reactors (HTGRs) and clean fluoride salt coolants. Molten salt reactors (MSRs) dissolve the fuel in a fluoride or chloride salt and release the fission product tritium to the salt. High-magnetic-field fusion machines may use liquid salt cooling and blankets because of the very high power densities of this new class of fusion machine. The three technologies can be coupled to a Nuclear Air-Brayton Combined Cycle (NACC) enabling variable electricity with base-load reactor operation. Converging requirements for tritium control in 700°C liquid salts are leading to cooperative programs across technologies; tritium models that combined generation, chemistry, metal corrosion and transport; and new tritium control technologies using advanced carbon forms, metals produced by additive manufacturing and other technologies.