Thermal condition of open KBS-3H tunnel

    Research output: Book/ReportReport

    Abstract

    This report contains the temperature calculations of open KBS-3H type spent nuclear fuel repository, where the fuel canisters are disposed at horizontal position in horizontal tunnels according to the preliminary SKB (Swedish Nuclear Fuel and Waste Management Co) and Posiva plan. The objective of the study is to simulate the operation phase atmospheric conditions in open horizontal tunnels, where the KBS-3H type canister containers and distance blocks are installed. The analyses concern BWR type canisters.

    The analyses were made as heat conduction problem by taking into account radiation over gaps. A perforated steel plate surrounds a canister and bentonite. Heat transfer through a perforated plate and surrounding air gaps is a complicated three-dimensional heat transfer problem. To simplify the analysis, the gaps around a container and a distance block were taken into account by describing them by a homogenous layer having effective thermal properties.

    Convection due to natural circulation of humid air in horizontal gaps between the container and rock was not considered. Convection could reduce the temperature variation in the gap. On the other hand, the perforated steel plate has good conductivity and transfers quite well heat in horizontal gaps.

    Since the actual temperatures of disposal canisters depend in a complicated way on considered time and position, two extreme cases were studied to make the analyses easier. In the first extreme case an infinite queue of canisters are disposed simultaneously. This case overestimates temperatures, since the actual number of canisters is finite and they are not disposed simultaneously. In other extreme case only the first single canister and the first distance block are disposed. This case underestimates temperatures, since the actual number of canisters is greater than one and the canisters heat each other in later phase. The analysis showed that temperatures differ only a little from each other in the two extreme cases. Thus a complicated analysis of several canisters could be avoided. The actual situation is between the two extreme cases and closer to the first case.

    According to the analysis the air temperature in the tunnel between the container and rock wall rises to range of 40-50ºC within 2-3 weeks after the start of disposal operation. Temperature varies rather much axially in the gap between a container and the rock. In the middle of the canister the average temperature in the gap is about 50ºC, whereas in the middle of the distance block the temperature is 24ºC. Thus constant humidity conditions are difficult to achieve in the gap between a container and the rock in horizontal direction.
    Original languageEnglish
    Place of PublicationOlkiluoto
    PublisherPosiva
    Number of pages34
    ISBN (Print)951-652-136-3
    Publication statusPublished - 2005
    MoE publication typeD4 Published development or research report or study

    Publication series

    SeriesPosiva Report
    Number2005-04
    ISSN1239-3096

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