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Markets are changing as the result of (1) the addition of variable wind and solar that causes highly volatile electricity prices and (2) the goal of a low-carbon economy. These changes require econ...

Lithium-based molten salts have attracted significant attention due to their applications in energy storage, advanced fission reactors, and fusion devices. Lithium fluorides and particularly 66.6%LiF-33.3%BeF2 (Flibe) are of considerable interest in nuclear systems, as they show an excellent combination of favorable heat transfer, neutron moderation, and transmutation characteristics. For nuclear salts, the range of possible local structures, compositions, and thermodynamic conditions presents significant challenges in atomistic modeling. In this work, we demonstrate that atom-centered neural network interatomic potentials (NNIPs) provide a fast method for performing molecular dynamics of molten salts that is as accurate as ab initio molecular dynamics. For LiF, these potentials are able to accurately reproduce ab initio interactions of dimers, crystalline solids under deformation, crystalline LiF near the melting point, and liquid LiF at high temperatures. For Flibe, NNIPs accurately predict the structures and dynamics at normal operating conditions, high-temperature-pressure conditions, and in the crystalline solid phase. Furthermore, we show that NNIP-based molecular dynamics of molten salts are scalable to reach long time scales (e.g., nanosecond) and large system sizes (e.g., 105 atoms) while maintaining ab initio density functional theory accuracy and providing more than 3 orders of magnitude of computational speedup for calculating structure and transport properties.

Interest in molten salts has increased significantly over the last decade due to their potential application in various clean-energy technologies including hydrogen generation, solar heat storage, advanced fission nuclear power plants, and compact fusion energy systems. In nuclear fission and fusion power plants, high heat capacity molten salts allow operation at high temperature and atmospheric pressure, which could dramatically increase efficiency, reduce capital cost, and enable passive safety features. In many of these systems, the hydrogen isotope is of particular importance due to its ability to corrode structural materials as 3H+ in fluoride salts, and its potential to cause significant radioactive release as diffusive 3H0, which are cited as key barriers to technological deployment. Yet, the chemistry and transport behavior of the hydrogen species remain poorly understood due to the difficulties in handling toxic salts and radioactive materials. Here, using ab initio molecular dynamics, we present a coupled examination of hydrogen speciation in the most common prototypical salts 66.6% LiF–33.3% BeF2 (Flibe) and 46.5% LiF–11.5% NaF–42% KF (Flinak). Using extensively validated calculations on the local structure and dynamics, we find significant difference between 3H0 and 3H+ transport behaviors that are usually overlooked. We find that 3H0, which always exists as H2, diffuses 3–5 times faster than 3H+, which can be ascribed to hydrogen bonding and complexation in solution. This work explains contradicting experimental results and provides useful species transport data for designing hydrogen capture and corrosion control systems for molten salts.

Electricity markets are changing because of (1) the addition of wind and solar generating capacity and (2) the goal of a low-carbon electricity grid. The large-scale addition of wind and solar phot...

Energy markets are changing because of (1) the addition of nondispatchable wind and solar electric generating capacity and (2) the goal of a low-carbon energy system. The large-scale addition of wi...

Liquid salts have become more attractive as coolants for low-carbon power generation due to needs for high-temperature heat and affordable energy storage. Of particular interest are halide salts ut...

A low-carbon world needs a replacement for the combined-cycle gas turbine to provide variable heat and electricity. We describe a nuclear combined cycle gas turbine (NACC) with a thermodynamic topping cycle where the high-temperature heat is provided by an electrically-heated thermal energy storage system or hydrogen. During base-load operations the NACC compressed air is heated up to 700°C with heat from a high-temperature nuclear reactor to produce electricity. At times of low electricity prices, electricity from the NACC and the grid is converted into high-temperature stored heat in the form of hot firebrick. At times of high electricity prices compressed air is heated up to 700°C by the reactor and then further heated by sending the compressed air through hot firebrick to heat the air up to 1427°C before going through the turbine and heat recovery steam generator to produce added peak electricity. The round-trip electricity-to-heat-storage-to-electricity efficiency is above 70%; that is, if one unit of electricity is sent to heat storage, more than 0.7 units of peak electricity are returned. The enabling technology is a new doped electrically-conductive firebrick that enables conversion of electricity to very-high-temperature stored heat. Stored heat addresses daily variations in demand. Hydrogen can be used for long-term seasonal energy storage but at lower peak electricity efficiency because the efficiency of the electricity-to-hydrogen conversion step is near 75% versus near 100% for electricity-to-heat.

We propose a low-enriched-uranium, thermal-neutron-spectrum sodium-cooled reactor with a peak sodium temperature of 700°C coupled to an air-Brayton power cycle for electricity and heat. Three low-enriched-uranium, thermal spectrum sodium-cooled reactors were built in the 1960s and 1970s; but there has been no examination of such systems for many decades. We develop a pre-conceptual design based on “new” enabling technologies since the 1970s including yttrium hydride as the high-temperature neutron moderator, commercial gas turbines and secure decay heat removal systems. We define the reactor as a sodium hydride reactor (SHR). The initial application is as a fission battery. The concept of the fission battery (FB) is a “plug and play” nuclear reactor defined by multiple characteristics: economics enabled by factory fabrication of large numbers of identical units, easy installation and removal, unattended operation and highly reliable operations. FBs are designed to be a low-carbon replacement for fossil fuels by industrial and commercial companies that require energy to produce some product (manufactured goods, chemicals, education, data centers, ship transportation, etc.). The reactors may be owned or leased by the company. The SHR is at an early stage of development.

The fluoride-salt-cooled high-temperature reactor and some proposed fusion reactors use clean fluoride salts as reactor coolants that have melting points above 450°C and generate tritium. Tritium d...

Recent developments in high-magnetic-field fusion systems have created large incentives to develop flibe (Li2BeF4) salt fusion blankets that have four functions: (1) convert the high energy of fusi...

The adsorption mechanisms of hydrogen on porous carbon materials were examined using constant volume chemisorption at 700 °C, the operating temperature of various advanced fission devices. It is recognized that hydrogen diffuses and adsorbs into carbon-based materials through various processes, making it difficult to interpret experimental data. Further, previous hydrogen-carbon interaction studies have predominately focused on storage applications at high pressures and cryogenic temperatures or on specific types of structural or neutron-moderating materials at atmospheric pressures and temperatures exceeding 1000 °C. Meanwhile, it has recently been shown that different carbon allotropes may also provide the ability to select and design for desired tritium transport behavior at high-temperature reactor conditions, motivating a need for deeper and more general understanding of these interactions across a range of adsorbents. In this study, it was found that total chemisorption consists of different adsorption modes that vary in binding energy. These interactions were grouped into strong interactions, which require significant energy and temperature to promote desorption, and weak interactions, which have a low desorption activation barrier. The weak interactions were thermodynamically modeled and they were found to follow a dissociative Langmuir isotherm, implying dissociative adsorption and therefore desorption that is driven by recombination of atomic hydrogen. Further, it was shown that strong adsorption can be modeled accurately by difference between total and weak adsorption isotherm models. Both the weak and strong adsorption was found to increase with increasing microporosity (

In the absence of an affordable and deployable energy storage option, the intermittency of renewable energy creates mismatches in supply and demand that limit the viability of a low-carbon electricity grid. High temperature electrically-heated thermal energy storage (E-TES) is a largely unexplored approach to alleviating the problem of low-value renewable energy. Evaluated herein is one E-TES concept, called Firebrick Resistance-Heated Energy Storage (FIRES), that stores electricity as sensible high-temperature heat (1000–1700 °C) in ceramic firebrick, and discharges it as a hot airstream to either (1) heat industrial plants in place of fossil fuels, or (2) regenerate electricity in a power plant. FIRES storage media and heater options are reviewed, and discharge cycling is simulated with Crank-Nicolson finite difference schemes and evaluated over the parameter design space. We report that systems of 100–1000 s MWh may be cycled daily, and discharged at a constant heat rate typically for 70–90% of the storage capacity. Traditional insulation can reasonably limit heat leakage to less than 3% per day. Preliminary cost estimates indicate a system cost near $10/kWh, substantially less expensive than batteries. Northwestern Iowa market analysis shows payback within 2 years and economic profitability.

Molten salt reactors (MSRs) are being considered as one of the potential nuclear options to meet future energy demands. While the MSR designs are drastically different from the more traditional light water reactor, many of the waste streams are similar between the concepts. The purpose of this paper is to outline strategies for the treatment and processing of MSR-type wastes from concepts of reconditioning and recycle of certain components to partitioning and direct immobilization of other waste components. To help bridge science and technology gaps, knowledge gained from similar efforts such as pyroprocessing of Experimental Breeder Reactor II salt wastes can be leveraged to develop concept-to-disposition pathways.

Three reactor types can be designed with pebbles (carbon spheres) as the reactor core: the pebble-bed high-temperature gas-cooled reactor (PB-HTGR), the pebble-bed fluoride-salt-cooled high-tempera...

A low-carbon world needs a replacement for natural gas-fired power to provide variable heat and electricity. The coupling of simple or combined cycle gas turbines (CCGTs) with advanced electrically-heated thermal energy storage (E-TES) systems is an alternative approach to energy storage with cost advantages over batteries or hydrogen production. CCGTs with E-TES may use stored low-value electricity to run the power cycle in place of fossil fuels. This (1) saves money for the power plants by allowing them to switch heat sources based on price, and (2) reduces carbon emissions by making use of otherwise curtailed renewable energy. The development of electrically conductive firebricks enables temperatures approaching 2000°C, hotter than existing E-TES options, sufficient to run CCGTs. Levelized cost of storage (LCOS) calculations show that the use of CCGTs with novel E-TES increases the cost of energy by less than a factor of 2, compared to a factor of 9 increase when using lithium-ion batteries. Unlike batteries, the CCGT with E-TES, provides assured generating capacity by normal operation of the gas turbine. A case study of CCGT coupled with E-TES is included based on 2019 electricity prices in Southern California, which showed an 18% reduction in fuel consumption and $11M savings based purely on the arbitrage case. The arbitrage case is expected to improve dramatically over the decade as deployment of renewable energy in California increases.

The retention of hydrogen in a variety of disordered carbon materials including graphene nanoplatelets and amorphous carbon was measured by volumetric adsorption and modeled using thermodynamic methods. To date, high temperature hydrogen adsorption research in carbon has predominately focused on graphite used as plasma-facing materials in fusion systems, moderators in high temperature gas reactors, and fuel components in molten-salt cooled reactors. Tritium, a hydrogen isotope that is produced in these systems, adsorbs on carbon surfaces that could be used effectively as sinks to remove tritium. Since hydrogen has chemical similarity to tritium, it can be used as a surrogate to simplify experimental studies, greatly expediting materials exploration. Thus, an understanding and prediction of hydrogen behavior on carbon materials is essential for evaluation of performance and safety of present and future reactor designs. Chemisorption experiments were conducted on 9 different materials with a range of surface areas, pore volumes, and pore size distributions. The results showed that carbons with higher surface area and pore volume exhibit higher hydrogen adsorption, which is attributed to larger quantities of active sites. Further, hydrogen adsorption was found to increase in materials with greater micropore (

In a molten salt nuclear reactor system, the redox potential must be controlled for mitigating corrosion of structural materials. The paper presented a critical review on the available knowledge of redox potential control in molten fluoride salt systems. The major phenomena that affect the redox potential and material corrosion are fission, TF production by transmutation, and salt contamination with metal fluorides or other oxidizing impurities. Redox potential control methodologies include gas sparging, contacting the salt with a reducing metal, and adding soluble salt redox buffers to the salt. Redox potential measurement technologies include electrochemical sensors and optical spectroscopy. The paper also analyzed the current technology issues and recommended near future studies.

In a low-carbon world (nuclear, wind, solar, and hydro) there is the need for assured dispatchable electricity to replace the historical role of fossil fuels. Base-load reactors can provide variabl...

Nuclear reactors produce heat and thus can couple to heat storage systems to provide dispacthable electricity while the reactor operates at full power. Six classes of heat storage technologies couple to light-water reactors with steam cycles. Firebrick Resistance-Heated Energy Storage (FIRES) converts low-price electricity into high-temperature stored heat for industry or power. FIRES and brick recuperators coupled to nuclear brayton power cycles may enable high-temperature reactors to buy electricity when prices are low and sell electricity at higher price.

The energy system of the United States requires several million gigawatt hours of energy storage to meet variable demand for energy driven by (1) weather (heating and cooling), (2) social patterns (daily and weekday/weekend) of work, play and sleep, (3) weather-dependent energy production (wind and solar) and (4) industrial requirements. In a low-carbon world, four storage options can meet this massive requirement at affordable costs: nuclear fuels, heat storage, hydrocarbon liquids made from biomass, and hydrogen. Because of the different energy sector characteristics (electrical supply, transportation, commercial, and industrial), each of these options must be developed. Capital costs associated with electricity storage at this scale using, for example, batteries and hydroelectric technologies are measured in hundreds of trillions of dollars for the United States alone and thus are not viable.

Liquid fossil fuels (1) enable transportation and (2) provide energy for mobile work platforms and (3) supply dispatchable energy to highly variable demand (seasonal heating and peak electricity). We describe a system to replace liquid fossil fuels with drop-in biofuels including gasoline, diesel and jet fuel. Because growing biomass removes carbon dioxide from the air, there is no net addition of carbon dioxide to the atmosphere from burning biofuels. In addition, with proper management, biofuel systems can sequester large quantities of carbon as soil organic matter, improving soil fertility and providing other environmental services. In the United States liquid biofuels can potentially replace all liquid fossil fuels. The required system has two key features. First, the heat and hydrogen for conversion of biomass into high-quality liquid fuels is provided by external low-carbon energy sources--nuclear energy or fossil fuels with carbon capture and sequestration. Using external energy inputs can almost double the energy content of the liquid fuel per unit of biomass feedstock by fully converting the carbon in biomass into a hydrocarbon fuel. Second, competing effectively with fossil fuels requires very large biorefineries—the equivalent of a 250,000 barrel per day oil refinery. This requires commercializing methods for converting local biomass into high-density storable feedstocks that can be economically shipped to large-scale biorefineries.

The large-scale deployment of wind or solar energy results in electricity prices below the price of fossil fuels at times of high wind or solar output. Price collapse can be limited by using low-price electricity to heat firebrick to high temperatures, store the heat in firebrick, and provide hot air as needed to industrial furnaces, kilns, power plants and gas turbines. This sets a minimum price on electricity near that of fossil fuels.

Summary The current study explores an innovative option for demonstrating the Fluoride-salt–cooled High- temperature Reactor (FHR) technologies with a reactor-driven subcritical facility. The FHR uses clean salt coolants, carbon-matrix coated-particle fuel similar to that used in High-temperature Gas-cooled Reactors and can be coupled to a nuclear air-Brayton combined cycle. Recent assessments indicate favorable economics and safety characteristics, but no FHR has been built. The question is what experimental facilities should be constructed to reduce technical uncertainties before a decision to build a test or demonstration reactor? The MIT Reactor design and license would allow the construction and operation of a subcritical facility with 700°C salt circulating through multiple full-width partial-height fuel assemblies operating with a power density up to 60% of a commercial FHR. This option would allow hot systems testing as a major step toward building the test or demonstration reactor. Preliminary system design, power control options, testing capabilities, and key nuclear characteristics of such a reactor-driven subcritical facility are described. A method of deriving subcritical multiplicity using surface source has been proposed and verified in this study. Finally, the neutronic impacts on the driver facility, ie, the MIT Reactor, have been evaluated.

A crushed rock heat storage facility with storage capacities of gigawatt-hours is proposed to couple to light water reactors (LWRs) to enable base-load LWR operation with (1) variable electricity to the grid and heat to industry and (2) substantially higher revenue in deregulated electricity markets with significant solar or wind generation capacity. At times of low electricity prices, crushed rock is heated by hot air in a two-stage process. Air is initially heated by a steam-air heat exchanger using LWR steam and then with electric resistance heaters before circulating from the top to bottom of the crushed rock pile. Depending upon the design, peak rock temperatures can be from 250°C to 800°C. Heat is recovered by circulating air from the bottom to the top of the crushed rock pile with the hot air sent to industrial furnaces or thermal electric power plants. For industrial applications the hot air is a partial replacement for the burning of fossil fuels in industrial furnaces. Many of the challe...