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Experimental Verification of the Initial Stages of an HTGR Double-ended Guillotine Break

Arcilesi, David J., Jr.
http://rave.ohiolink.edu/etdc/view?acc_num=osu1533356728728114

Abstract Details

2018, Doctor of Philosophy, Ohio State University, Nuclear Engineering.
A critical event in the safety analysis of a High Temperature Gas-cooled Reactor (HTGR) is a depressurized loss-of-forced circulation (D-LOFC) accident followed by air ingress. This accident is initiated, in its worst case scenario, by a double-ended guillotine break of its cross vessel. In an HTGR, the reactor vessel is located within a reactor cavity that is fi lled with air during normal operating conditions. During a D-LOFC event followed by air ingress, an air-helium mixture may enter the reactor vessel following a reactor vessel depressurization. Since air chemically reacts with high-temperature graphite, this could lead to damage of core-bottom and in-core graphite structures as well as core heat-up, toxic gas release, and failure of the structural integrity of the system unless mitigating action is taken. Early studies postulated that the dominant mechanism of air ingress is molecular diffusion, which is a slow process. However, recent studies show that the air-ingress process could be initially controlled by density-driven stratifi ed flow of hot helium and a relatively cooler air-helium mixture in the hot duct. If density-driven stratifi ed flow initially dominates, earlier onset of natural circulation within the core would occur. This would lead to an earlier onset of oxidation of internal graphite structures and, most likely, at a more rapid rate. Thus, it is important to understand both of these air-ingress mechanisms in a HTGR. These mechanisms may be important at different times for different scenarios, specifi cally breaks of varying size, orientation, shape, and location. Also, understanding which ingress mechanism dominates informs the type of mitigating measures that need to be considered for HTGR designs. The principal question of this dissertation is to determine whether density-driven stratifi ed flow is the dominant ingress mechanism during the initial stages of an air-ingress accident in the unlikely event of a double-ended guillotine break (DEGB). As a corollary, this dissertation also examines which ingress mechanism dominates for a smaller break, or more speci cally, an axial break (1/64th area of DEGB) in the hot duct that is accompanied by failure of the cold duct enabling air ingress from the containment. The results of this dissertation demonstrate that during the initial time frame of the double-ended guillotine and axial break density-driven stratifi ed flow is the dominant ingress mechanism. The rate at which ingress flow occurs is different for each case. There is data to substantiate that the rate of ingress is proportional to the cross-sectional flow area of the break. Specifi cally, the ingress rate for the double-ended guillotine break is much larger than the axial break oriented in the top position (ABT). In the DEGB 400 C case, experimental results show that the time required to reach an average oxygen concentration of 10% in the plenum is approximately 45 s. Whereas in the ABT 300 C case, experimental results show that the time required to reach an average oxygen concentration of 10% in the plenum is approximately 9,200 s. The cross-sectional area of the DEGB is 64 times larger than the axial break, but the ingress time for the DEGB is approximately 200 times faster than the ingress time of the axial break oriented in the top position. Therefore, as hypothesized, the ingress time is a strong inverse function of the cross-sectional area of the break. Experimental oxygen concentration data confi rms previous literature fi ndings that oxygen consumption due to graphite oxidation is proportional to the reaction temperature and that in low temperature cases (ABT 300 C and DEGB 400 C) a negligible amount of oxidation occurs. The data also indicates that at higher temperatures a signi ficant amount of graphite oxidation occurs.
Richard N. Christensen, Ph.D. (Advisor)
Xiaodong Sun, Ph.D. (Advisor)
Tunc Aldemir, Ph.D. (Committee Member)
Richard S. Denning, Ph.D. (Committee Member)
Sandip Mazumder , Ph.D. (Committee Member)
568 p.
Nuclear Engineering
HTGR, VHTR, air-ingress accident

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Citations

  • Arcilesi, Jr., D. J. (2018). Experimental Verification of the Initial Stages of an HTGR Double-ended Guillotine Break [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1533356728728114

    APA Style (7th edition)

  • Arcilesi, Jr., David. Experimental Verification of the Initial Stages of an HTGR Double-ended Guillotine Break. 2018. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1533356728728114.

    MLA Style (8th edition)

  • Arcilesi, Jr., David. "Experimental Verification of the Initial Stages of an HTGR Double-ended Guillotine Break." Doctoral dissertation, Ohio State University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=osu1533356728728114

    Chicago Manual of Style (17th edition)

osu1533356728728114
147
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