HTGR Knowledge Base

Conference Article: Preliminary safety assessment of the 450 MWT MHTGR

Dunn, T.D.; Datta, K.; Hoot, C. (General Atomics, San Diego, CA (United States))

Abstract

The Modular High-Temperature Gas-Cooled Reactor (MHTGR) design has been reevaluated to improve its economic potential. As part of a plant cost reduction study, a range of core designs from the reference 350 MWT design to a maximum of 600 MWT have been studied. Based on an evaluation of key accidents, the recommended 450 MWT MHTGR appears to offer a comparably high level of safety as did the reference 350 MWT plant. The design features of the 450 MWT plant that most impact safety include a larger radionuclide inventory, a larger fueled annular core with its subsequent impact on heat transfer, a core power density about the same as the 350 MWT, and the replacement of fertile thorium fuel with natural uranium. The fissile fuel remains low enriched UCO. These combined design changes result in lower decay heat during accidents than the reference plant. The preliminary 450 MWT safety analysis investigated key design basis conduction cooldown events that included simultaneous loss of all forced cooling and helium coolant. These assessments used a revised accident fuel performance model and a decay heat evaluation specific to the 450 MWT fuel cycle. The fuel performance model used is based on recent FRG fuel sphere heating data that augment the current data base. The limiting MHTGR safety requirement is the Protective Action Guide's preventative thyroid inhalation dose at the Exclusion Area Boundary. Iodine radionuclides released to the environment are the dominant contributors to this dose. Iodine release during conduction cooldown indicates that both the 350 MWT and the 450 MWT cores have very low releases from failure of the standard fuel particles. During depressurized conduction cooldown the peak fuel temperature in the 450 MWT plant is about 1500 deg. C (i.e. less than 20 hours above 1495 deg. C), well below conditions where the fuel coating integrity is challenged. The dominant sources of releases are heavy metal contamination and fuel defects that are a result of the fuel manufacturing process. The thyroid dose at the 425m Exclusion Area Boundary for the 450 MWT plant is below the 5 rem preventative PAG. Thereby, providing acceptable plant safety.

view the full text of this article (6 pages, format: PDF, size= 378kB)

key words: Gas Cooled Reactor, Nuclear Technology
Reference:
Behaviour of gas cooled reactor fuel under accident conditions. Proceedings of a specialists meeting held in Oak Ridge, 5-8 November 1990
International Atomic Energy Agency, Vienna (Austria). International Working Group on Gas-Cooled Reactors
IWGGCR--25, pp:111-116