High Temperature Gas Reactors: Assessment of Applicable Codes and Standards

Abstract

SummaryCurrent interest expressed by industry in HTGR plants, particularly modular plants with power up to about 600 MW(e) per unit, has prompted NRC to task PNNL with assessing the currently available literature related to codes and standards applicable to HTGR plants, the operating history of past and present HTGR plants, and with evaluating the proposed designs of RPV and associated piping for future plants.Considering these topics in the order they are arranged in the text, first the operational histories of five shut-down and two currently operating HTGR plants are reviewed, leading the authors to conclude that while small, simple prototype HTGR plants operated reliably, some of the larger plants, particularly Fort St. Vrain, had poor availability. Safety and radiological performance of these plants has been considerably better than LWR plants. Petroleum processing plants provide some applicable experience with materials similar to those proposed for HTGR piping and vessels.At least one currently operating plant -HTR-10 -has performed and documented a leak before break analysis that appears to be applicable to proposed future US HTGR designs.Current codes and standards cover some HTGR materials, but not all materials are covered to the high temperatures envisioned for HTGR use. Codes and standards, particularly ASME Codes, are under development for proposed future US HTGR designs. A -roadmap‖ document has been prepared for ASME Code development; a new subsection to section III of the ASME Code, ASME BPVC III-5, is scheduled to be published in October 2011.The question of terminology for the cross-duct structure between the RPV and power conversion vessel is discussed, considering the differences in regulatory requirements that apply depending on whether this structure is designated as a -vessel‖ or as a -pipe‖. We conclude that designing this component as a -pipe‖ is the more appropriate choice, but that the ASME BPVC allows the owner of the facility to select the preferred designation, and that either designation can be acceptable.v

Regulatory Considerations Regarding Potential High Temperature Fluid Releases in Advanced Reactor Designs

“…First and foremost is the consideration of pipe breaks and the associated dynamic and environmental effects. The use of the LBB approach for HTGRs has been considered to potentially alleviate certain requirements regarding dynamic effects [151,152]. However, additional fracture mechanics analysis is likely necessary to demonstrate applicability for the specific HTGR design under consideration.…”

“…Fundamental to this area is the adherence to codes and standards for the design and fabrication of HPB system components, such as the reactor vessel and piping. Several recent studies have explored the applicability of current standards, or those in development, to these areas [151,154,155]. Similarly, different approaches are possible for the estimation of leak frequencies and sizes.…”

"Advanced" isn't Always Better: Assessing the Safety, Security, and Environmental Impacts of Non-Light-Water Nuclear Reactors

“…In 1970, the Atomic Energy Commission even predicted that almost half of US nuclear capacity by the year 2000 would be made up of HTGRs (Shropshire and Herring 2004). General Atomics, a prominent HTGR developer, sold 10 large reactors to US utilities between 1971and 1974(Mcdowell et al 2011. However, the HTGR revolution did not come to pass, in large part because the operating experience with demonstration reactors did not inspire confidence.…”

Technical Needs for Enhancing Risk Monitors with Equipment Condition Assessment for Advanced Small Modular Reactors

“…Intrusion of oil, water, and other contaminants into the coolant system was a problem at HTGR plants. The use of water-lubricated bearings in helium circulators at Fort St. Vrain plant led to moisture intrusion, which contributed to corrosion in an otherwise non-corrosive environment (McDowell et al 2011). Oil leakage through the compressor at Peach Bottom 1 plant resulted in approximately 100 kg of oil in the reactor (Beck et al 2010).…”