TitleDesign 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

Fluoride-salt-cooled, high-temperature reactor (FHR) technology combines the robust coated-particle fuel of high-temperature, gas-cooled reactors with the single phase, high volumetric heat capacity coolant of molten salt reactors and the low-pressure pool-type reactor configuration of sodium fast reactors. FHRs have the capacity to deliver heat at high average temperature, and thus to achieve higher thermal efficiency than light water reactors. Licensing of the passive safety systems used in FHRs can use the same framework applied successfully to passive advanced light water reactors, and earlier work by the NGNP and PBMR projects provide an appropriate framework to guide the design of safety-relevant FHR systems. This paper provides a historical review of the development of FHR technology, describes ongoing development efforts, and presents design and licensing strategies for FHRs. A companion review article describes the phenomenology, methods and experimental program in support of FHR.

Modern large air Brayton gas turbines have compression ratios ranging from 15 to 40 resulting in compressor outlet temperatures ranging from 350 °C to 580 °C. Fluoride-salt-cooled, high-temperature reactors, molten salt reactors, and concentrating solar power can deliver heat at temperatures above these outlet temperatures. This article presents an approach to use these low-carbon energy sources with a reheat-air Brayton combined cycle (RACC) power conversion system that would use existing gas turbine technology modified to introduce external air heating and one or more stages of reheat, coupled to a heat recovery steam generator to produce bottoming power or process heat. Injection of fuel downstream of the last reheat stage is shown to enable the flexible production of additional peaking power. This article presents basic configuration options for RACC power conversion, two reference designs based upon existing Alstom and GE gas turbine compressors and performance of the reference designs under nominal ambient conditions. A companion article studies RACC start up, transients, and operation under off-nominal ambient conditions.

The near and mid-term future of the existing Polish coal-fired power fleet is uncertain. The longer-term operation of unabated coal power is incompatible with climate policy and is economically challenging because of the increasing price of CO2 emission allowances in the EU. The results of the techno-economic analysis presented in this paper indicate that the retrofit of existing coal-fired units, by means of replacing coal-fired boilers with small modular reactors, may be an interesting option for the Polish energy sector. It has been shown that the retrofit can reduce the costs in relation to greenfield investments by as much as 35%. This analysis focuses on the repowering of a 460 MW supercritical coal-fired unit based on the Łagisza power plant design with high temperature small modular nuclear reactors based on the 320 MWth unit design by Kairos Power. The technical analyses did not show any major difficulties in integrating. The economic analyses show that the proposed retrofits can be economically justified, and, in this respect, they are more advantageous than greenfield investments. For the base economic scenario, the difference in NPV (Net Present Value) is more favorable for the retrofit by 556.9 M€ and the discounted payback period for this pathway is 10 years.

Because molten fluoride salts can deliver heat at temperatures above 600 °C, they can be used to couple nuclear and concentrating solar power heat sources to reheat air combined cycles (RACC). With the open-air configuration used in RACC power conversion, the ability to also inject natural gas or other fuel to boost power at times of high demand provides the electric grid with contingency and flexible capacity while also increasing revenues for the operator. This combination provides several distinct benefits over conventional stand-alone nuclear power plants and natural gas combined cycle and peaking plants. A companion paper discusses the necessary modifications and issues for coupling an external heat source to a conventional gas turbine and provides two baseline designs (derived from the GE 7EB and Alstom GT24). This paper discusses off-nominal operation, transient response, and start-up and shutdown using the GE 7EB gas turbine as the reference design.

This study suggests a new approach to diffusion bonding (DB) 316L stainless steel: a low-pressure procedure that includes a nickel interlayer. In this approach, relatively lower pressure is applied to the sample before the DB process, in contrast to the usual approach in which higher pressure is applied during the DB process. This new procedure was tested on mock-up 316L stainless steel tube-to-tubesheet joints, which simulated similar joints in coiled-tube heat-exchanger applications. This study confirms that the new procedure meets the overall success criteria, namely, a pull-out force exceeding the force required for tube rupture. It also shows that the DB joint is improved by the use of a Ni interlayer; the joint strength increased by approximately 33% for a 0.25 μm Ni interlayer and by approximately 18% for a 5 μm Ni interlayer. The joint cross sections were qualitatively examined using optical microscopy (OM) and scanning electron microscopy (SEM); the observations suggest that only portions of the interface were diffusion bonded, as a result of the low-pressure procedure and the surface roughness (due to the sample fabrication). The portions that were diffusion bonded, though, were sound, as characterized by the fact that the steel grains grew through the interface line to create a continuous metallographic structure.

Coiled tube air heaters (CTAH) are heat exchangers currently under investigation at the University of California, Berkeley for application to Reheat Air-Brayton Combined Cycles (RACC). In a CTAH, molten fluoride salt on the tube side heats air on the shell side in a cross-counterflow arrangement, leading to high heat exchange effectiveness, while the coiled geometry provides good transient mechanical performance during thermal transients. This paper describes the important phenomenology involved in CTAH design and operation, such as salt freezing, flow induced vibration, start-up and shut-down control, and air circulating power requirements. The THEEM simulation tool used in the design of CTAHs and its validation against simple water to air test sub-bundles is presented. Finally, an initial design for a demonstration 370 kWth CTAH closed air test loop is presented. This test loop design, when constructed, would enable demonstration of salt-to-air heating under conditions relevant to RACC power conversion (air pressures from 4 to 20 atm and air temperatures from 400 to 650°C).

No abstract