Current concentrated solar power plants (CSP) use molten salt at 565°C as a heat transfer and energy storage fluid. Due to thermal energy storage (TES), these solar plants can deliver dispatachable electricity to the grid; however, the levelized cost of electricity (LCOE) for these plants is 12-15 c/kWh, about 2.5 times as high as fossil fuel electricity generation. Molten salt technology limits peak operating temperatures to 565°C and a heat engine efficiency of 40%. Liquid metal (LM), however, can reach >1350°C, and potentially utilize a more efficient (60%) heat engine and realize cost reductions. A 1350 °C LM-CSP plant would require ceramic containment, inert atmosphere containment, additional solar flux concentration, and redesigned internal receiver. It was initially unclear if these changes and additions for LM-CSP were technically feasible and could lower the LCOE compared to LS-CSP. To answer this question, a LM-CSP plant was designed with the same thermal input as a published LS-CSP plant. A graphite internal cavity receiver with secondary concentration heated liquid Sn to 1400°C and transferred heat to a 2-phase Al-Si fluid for 9 hours of thermal energy storage. Input heat to the combined power cycle was 1350°C and had 60% thermal efficiency for a gross output of 168 MW. The cost of this LM-CSP was estimated by applying material cost factors to the designed geometry and scaling construction costs from published LS-CSP estimates. Furthermore, graphite was experimentally tested for reactivity with liquid Sn, successful reaction bonds, and successful mechanical seals. The result is switching to molten metal can reduce CSP costs by 30% and graphite pipes, valves, and seals are possible at least at 400°C.

The Liquid-Salt-Cooled Very High-Temperature Reactor (LS-VHTR) was modeled using the neutronics analysis code SCALE6.0 and the thermal-hydraulics and kinetics modeling code RELAP5-3D with objective to devise, analyze, and evaluate the feasibility and stability of a start-up procedure for this reactor using natural circulation of the coolant and under the Loss Of Offsite Power (LOOP) conditions. This Generation IV reactor design has been studied by research facilities worldwide for almost a decade. While neutronics and thermal-hydraulics analyses have been previously performed to show the performance of the reactor during normal operation and for shutdown scenarios, no study has heretofore been published to examine the active or passive start-up of the reactor. The fuel temperature (Doppler) and coolant density coefficient of reactivity of the LS-VHTR were examined using the CSAS6 module of the SCALE6.0 code. Negative Doppler and coolant density feedback coefficients were calculated. Two initial RELAP5 simulations were run to obtain the steady-state conditions of the model and to predict the changes of the thermal-hydraulic parameters during the shutdown of the reactor. Next, a series of step reactivity additions to the core were simulated to determine how much reactivity can be inserted without jeopardizing safety and the stability of the core. Finally, a start-up procedure was developed, and the restart of the reactor with natural convection of the coolant was simulated. The results of the simulations demonstrated the potential of a passive start-up of the LS-VHTR.