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Seismic engineering of a horizontal, compact high temperature gas reactor
Abstract
Boston Atomics have proposed a radical redesign of the high temperature gas reactor by first integrating essential reactor components including the reactor vessel, steam generator, and helium circulator into a unified unit and second by rotating the longitudinal axis of the unit from vertical to the horizontal. The integration and the horizontal reorientation of the reactor components drastically reduces the height of the reactor building, resulting in an increase in the power density, lower overnight capital cost, and an accelerated construction schedule. The innovative horizontal design necessitates analysis and design studies across multiple disciplines including nuclear, mechanical, and structural engineering, which are being pursued now as part of a DOE-sponsored ARC-20 project. This paper discusses work completed to date on the structural and earthquake engineering of the horizontal, compact high temperature gas reactor (HC-HTGR), with a focus on characterizing its seismic response from the plant level (i.e., building + soil domain) to individual structures (e.g., reactor building), systems (e.g., integrated reactor primary system), and components (e.g., core barrel), down to the fuel-block assemblies inside the reactor core. Effective strategies to mitigate the impact of the seismic load case are identified and a clear pathway to standardization is presented.
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Seismic isolation offers significant improvements to building and equipment performance where earthquake shaking is a design consideration. It has been applied to non-nuclear, mission-critical infrastructure in the United States for more than 40 years. A robust regulatory pathway can be useful for applicants seeking US Nuclear Regulatory Commission (NRC) approval to incorporate seismic isolation in nuclear power plants. This report, sponsored by the US Department of Energy under its Industry Opportunities for Advanced Nuclear Technology Development (Industry FOA), has been developed to present a pathway for review and consideration of endorsement by the NRC for an applicant to follow to develop, document, and qualify a seismic isolation system. The report presents a performance-based approach to implement a seismic base isolation system for a reactor building and presents sample calculations for several systems to illustrate the pathway. The report does not propose a risk target for an isolation system but rather describes how a target value can be achieved. For demonstration purposes, an archetype reactor building is sited at Clinch River, East Tennessee, and ground motions are derived using a seismic hazard calculator developed by the US Geological Survey. Guidance is provided on the specification of seismic isolation systems and their commercial grade dedication. The report will be formally submitted to the NRC in late 2024 for review and issuance of a safety evaluation report.
In 2018, nuclear energy generated 55% of United States’ and one third of the world’s carbon free electricity, making nuclear energy a key tool in efforts to mitigate climate change before 2050. However, the current nuclear technology, light water reactors (LWRs), is limited to 300°C, so it cannot be used to decarbonize industrial process heat which accounts for 12% of US greenhouse gas emissions. High temperature gas reactors (HTGRs) can meet the high temperature demand with carbon free nuclear heat. The estimated cost of HTGRs, such as the Next Generation Nuclear Plant (NGNP), are even higher than state-of-the-art LWRs. In this paper, we expanded our nuclear cost estimating tool to include HTGRs and find that the NGNP overnight capital costs were 32% higher than an advanced LWR per unit capacity. The higher cost will naturally result in larger risk to cost overrun as recently experienced by larger LWRs in western nations. With a design-to-build mindset to minimize cost and construction risk, we introduce the horizontal, compact HTGR (HC-HTGR). The reactor core and steam generator are mounted horizontally on rails and in-line with one another, decreasing the size of the reactor building relative to the power capacity four times when compared to traditional HTGRs. The HC-HTGR reduced overnight civil structure costs by 42%, indirect costs by 38%, and total capital costs by 20% from NGNP. We discussed the required engineering of new systems for the HC-HTGR including vessel supports, the reactor cavity cooling system, and steam generator design. Finally, we estimated the fuel and operations costs of the HC-HTGR, and a survey of low-carbon industrial process heat technology showed the HC-HTGR can deliver a highly competitive levelized cost of heat in the range of $6.13–12.48/GJ.
Current federal regulations in the United States require soil-structure-interaction (SSI) analysis for the design of conventionally founded large light water reactors, unless they are founded on very hard rock. Such reactors are typically massive and stiff, which are two key ingredients for significant SSI on soil sites. Advanced small modular and micro-reactors are one or more orders of magnitude lighter and smaller than their GW-scale predecessors. This reduction in physical scale, coupled with seismic isolation, which adds base flexibility, may effectively eliminate SSI and the need for such analysis. This paper explores the importance of SSI for isolated reactors through simulating the seismic response of an archetype advanced reactor building for multiple isolation systems, soil domains, seismic inputs, and intensities of shaking. The results presented here show that SSI effects are insignificant for this seismically isolated advanced reactor and that its design can proceed using surface free-field representations of ground shaking. Companion results for other reactor buildings, seismic isolation systems, soil domains, and seismic inputs, to be presented later, support this recommendation.
Soil-structure-interaction (SSI) analysis is required for the design of conventionally founded large light water reactors in the United States (U.S.), unless the shear-wave velocity of the supporting rock exceeds 2,400 m/sec. This regulatory requirement is driven by the legacy assumption that reactor buildings are both very heavy and stiff: the attributes needed for significant SSI on soil sites. Many of the advanced and micro reactors proposed for possible deployment in the U.S. are one or two orders of magnitude lighter and smaller than gigawatt-scale large light water reactors (LLWRs). Seismic isolation, which can substantially reduce the impact of the seismic load case, adds flexibility at the base of advanced reactors and micro reactors. Taken together, SSI may be negligible for seismically isolated advanced and micro reactors and that is the subject of the study reported here. Analysis is performed for two seismically isolated advanced reactor buildings, a horizontally configured high temperature gas reactor and a fluoride salt-cooled high temperature reactor, for multiple isolation systems, soil domains, seismic inputs, and intensities of shaking. The preliminary analysis results indicate that SSI analysis is not needed for the design of seismically isolated advanced small modular and micro reactors unless the horizontal frequency of the supporting soil domain is very close to that of the isolation system.
The United States Department of Energy is funding the conceptual development of a horizontally configured high temperature gas reactor (HC-HTGR) under its Advanced Research Demonstration Program (ARDP). Unlike traditional HTGRs, the HC-HTGR design integrates key reactor components, including the reactor vessel, steam generator, and helium circulator, into a single horizontal unit. The horizontal configuration offers several advantages, including a significant reduction in the volume of civil materials (e.g., concrete, steel) and a much smaller overall height for the reactor building. The reorientation of the reactor components, from vertical to horizontal, necessitates analysis, design, and optimization studies that are being supported by ARDP. This paper focuses on structural engineering aspects of the HC-HTGR, advancing its design by evaluating the seismic robustness of the reactor building, integrated reactor primary system, and critical internals, and devising effective strategies to mitigate the impact of the seismic load case. Analysis results presented in this paper directly support progression of the HC-HTGR design and facilitate future licensing.
Dynamic response characteristics of fuel-block assemblies in a horizontal, compact HTGR
- S S Parsi
- E Velez-Lopez
- R W Stewart
- K Shirvan
- M V Sivaselvan
- A S Whittaker
S. S. Parsi, E. Velez-Lopez, R. W. Stewart, K. Shirvan, M. V. Sivaselvan, and A. S. Whittaker,
"Dynamic response characteristics of fuel-block assemblies in a horizontal, compact HTGR," in
Transactions: International Congress on Advances in Nuclear Power Plants (ICAPP-24), Las
Vegas, NV, 2024.