Mike Patterson

Design Option of Heat Exchanger for the Next Generation Nuclear Plant

The next generation nuclear plant (NGNP), a very high temperature gas-cooled reactor (VHTR) concept, will provide the first demonstration of a closed-loop Brayton cycle at a commercial scale, producing a few hundred megawatts of power in the form of electricity and hydrogen. The power conversion unit for the NGNP will take advantage of the significantly higher reactor outlet temperatures of the VHTRs to provide higher efficiencies than can be achieved with the current generation of light water reactors. Besides demonstrating a system design that can be used directly for subsequent commercial deployment, the NGNP will demonstrate key technology elements that can be used in subsequent advanced power conversion systems for other Generation IV reactors. In anticipation of the design, development, and procurement of an advanced power conversion system for the NGNP, the system integration of the NGNP and hydrogen plant was initiated to identify the important design and technology options that must be considered in evaluating the performance of the proposed NGNP. As part of the system integration of the VHTRs and the hydrogen production plant, the intermediate heat exchanger is used to transfer the process heat from VHTRs to the hydrogen plant. Therefore, the design and configuration of the intermediate heat exchanger are very important. This paper describes analyses of one stage versus two-stage heat exchanger design configurations and simple stress analyses of a printed circuit heat exchanger (PCHE), helical-coil heat exchanger, and shell-and-tube heat exchanger.

THEORETICAL DESIGN OF THERMOSYPHON FOR PROCESS HEAT TRANSFER FROM NGNP TO HYDROGEN PLANT

The Next Generation Nuclear Plant (NGNP) will most likely produce electricity and process heat, with both being considered for hydrogen production. To capture nuclear process heat, and transport it to a distant industrial facility requires a high temperature system of heat exchangers, pumps and/or compressors. The heat transfer system is particularly challenging not only due to the elevated temperatures (up to ∼ 1300K) and industrial scale power transport (≥50 MW), but also due to a potentially large separation distance between the nuclear and industrial plants (100+m) dictated by safety and licensing mandates. The work reported here is the preliminary analysis of two-phase thermosyphon heat transfer performance with alkali metals. A thermosyphon is a device for transporting heat from one point to another with quite extraordinary properties. In contrast to single-phased forced convective heat transfer via ‘pumping a fluid’, a thermosyphon (also called a wickless heat pipe) transfers heat through the vaporization / condensing process. The condensate is further returned to the hot source by gravity, i.e. without any requirement of pumps or compressors. With this mode of heat transfer, the thermosyphon has the capability to transport heat at high rates over appreciable distances, virtually isothermally and without any requirement for external pumping devices. Two-phase heat transfer by a thermosyphon has the advantage of high enthalpy transport that includes the sensible heat of the liquid, the latent heat of vaporization, and vapor superheat. In contrast, single-phase forced convection transports only the sensible heat of the fluid. Additionally, vapor-phase velocities within a thermosyphon are much greater than single-phase liquid velocities within a forced convective loop. Thermosyphon performance can be limited by the sonic limit (choking) of vapor flow and/or by condensate entrainment. Proper thermosyphon requires analysis of both.Copyright

Preliminary Design for Conventional and Compact Secondary Heat Exchanger in a Molten Salt Reactor

In this study, the heat transfer coolant utilized in the heat exchanger is a molten salt, which transfers thermal energy to water (steam) for power production by a supercritical Rankine (25MPa) or subcritical Rankine (17MPa) cycle. Molten salts are excellent coolants, with 25% higher volumetric heat capacity than pressurized water, and nearly five times that of liquid sodium. The greater heat capacity of molten salts results in more compact components like pumps and heat exchangers. However, the use of a molten salt provides potential materials compatibility issues. After studying a variety of individual molten salt mixtures, chlorides and fluorides have been given the most serious consideration because of their heat transport and transfer characteristics.In this study thermal designs of conventional (shell and tube), and compact (printed circuit) heat exchangers are carried out and compared for a given thermal duty. There are a couple of main issues that need to be addressed before this technology could be commercialized. The main issue is with the material compatibility of molten salts (especially fluoride salts) and secondarily, with the pressure difference across the heat exchanger. The heat exchanger’s primary side pressure is nearly atmospheric and the secondary side (power production) is pressurized to about 25MPa for supercritical cycle and 17MPa for subcritical cycle. Further in the analysis, the comparison of both the cycles will be carried out with recommendations.© 2012 ASME

Small Modular Molten Salt Reactor (SM-MSR)

The strategic goal of the Small Modular Molten Salt Reactor (SM-MSR) is to broaden the environmental and economic benefits of nuclear energy in the United States by producing power to meet growing energy demands and demonstrating its applicability to market sectors not being served by light water reactors. Of primary importance is demonstrating that the SM-MSR can be operated safely and compete economically with larger reactors. Reactor outlet temperatures (ROTs) of SM-MSRs will likely be much higher (around 700°C) than light water reactors, thereby increasing the efficiency of electricity production and potentially providing process heat for industrial applications, which will help offset the higher per kilowatt costs generally associated with smaller reactors, making the SM-MSR more economically competitive. This paper compares thermal power cycles for given ROT, compares thermal performance using figure of merits and sensitivity study and discusses the comparative advantages of SM-MSRs.Copyright

NGNP Process Heat Utilization: Liquid Metal Phase Change Heat Exchanger

One key long-standing issue that must be overcome to fully realize the successful growth of nuclear power is to determine other benefits of nuclear energy apart from meeting the electricity demands. The Next Generation Nuclear Plant (NGNP) will most likely be producing electricity and heat for the production of hydrogen and/or oil retrieval from oil sands and oil shale to help in our national pursuit of energy independence. For nuclear process heat to be utilized, intermediate heat exchange is required to transfer heat from the NGNP to the hydrogen plant or oil recovery field in the most efficient way possible. Development of nuclear reactor-process heat technology has intensified the interest in liquid metals as heat transfer media because of their ideal transport properties. Liquid metal heat exchangers are not new in practical applications. An important rationale for considering liquid metals as the working fluid is because of the higher convective heat transfer coefficient. This explains the interest in liquid metals as coolant for intermediate heat exchange from NGNP. The production of electric power at higher efficiency via the Brayton Cycle, and hydrogen production, requires both heat at higher temperatures and high effectiveness compact heat exchangers to transfer heat to either the power or process cycle. Compact heat exchangers maximize the heat transfer surface area per volume of heat exchanger; this has the benefit of reducing heat exchanger size and heat losses. High temperature IHX design requirements are governed in part by the allowable temperature drop between the outlet of NGNP and inlet of the process heat facility. In order to improve the characteristics of heat transfer, liquid metal phase change heat exchangers may be more effective and efficient. This paper explores the overall heat transfer characteristics and pressure drop of the phase change heat exchanger with Na as the heat exchanger coolant. In order to design a very efficient and effective heat exchanger one must optimize the design such that we have a high heat transfer and a lower pressure drop, but there is always a tradeoff between them. Based on NGNP operational parameters, a heat exchanger analysis with the sodium phase change is presented to show that the heat exchanger has the potential for highly effective heat transfer, within a small volume at reasonable cost.Copyright