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Developmental Analysis and Design of a Scaled-down Test Facility for a VHTR Air-ingress Accident

Arcilesi, David J., Jr.
http://rave.ohiolink.edu/etdc/view?acc_num=osu1338387523

Abstract Details

2012, Master of Science, Ohio State University, Nuclear Engineering.
A critical event in the safety analysis of the Very High-temperature Gas-cooled Reactor (VHTR) is a loss-of-coolant accident (LOCA). This accident is initiated, in its worst case scenario, by a double-ended guillotine break of the hot duct, which leads to a rapid reactor depressurization. In a VHTR, the reactor vessel is located within a reactor cavity that is filled with air during normal operating conditions. During a LOCA, 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 actions are taken. Therefore, it is imperative to understand the dominant mechanism(s) in the air-ingress process so that mitigating measures can be considered for VHTR designs. Early studies postulated that the dominant mechanism of air ingress is molecular diffusion. In general, however, molecular diffusion is a slow process, and recent studies show that the air-ingress process could be initially controlled by density-driven stratified flow of hot helium and a relatively cool air-helium mixture in the hot duct. If density-driven stratified 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 VHTR. These mechanisms may be important at different times for different scenarios, specifically breaks of varying size, orientation, shape, and location. Since no experimental data are readily available to understand the phenomena and determine which mechanism will dominate for various break conditions, there’s a need to design and construct a scaled-down experimental test facility to generate data. In this thesis, the scaling analysis, the developmental analysis and design of a scaled-down air-ingress accident test facility will be given. As part of the developmental analysis, the non-dimensional Froude number was preserved in establishing hydraulic similarity. On average, the non-dimensional resistance number of the scaled-down facility deviates 2.81% in terms of relative accuracy from the non-dimensional resistance number of the prototypic design. A 1/8th geometric scale is utilized for the entire geometry except for the hot duct length, support column pitch and support column diameter. The exceptions to the 1/8th geometric scale are to avoid large distortion of the loop pressure loss distribution (modified hot duct length) and to preserve the non-dimensional Froude number (modified support column diameter and pitch). A heat transfer characterization of the lower plenum of the prototypic and scaled-down system was performed. The characterization focused on the support columns which are the principal heat source in the lower plenum during an air-ingress accident scenario. This analysis shows that a lumped capacitance approximation for the support columns is valid. Also, the analysis determines an operational heater power ( = 125 W) for shell/heater rods in the scaled-down system so that the rod surface temperature and the rod average radial heat flux ( = 0 W) can be preserved from the prototypic case. In addition, a containment free volume (V = 1 m3) was determined to house the scaled-down facility. With the containment free volume known, initial vessel pressures to preserve the air-to-helium mole ratio (P = 40 psig) and mixed mean temperature (P = 34.2 psig) of the prototype case were calculated. Finally, vessel design drawings and instrumentation are given
Richard Christensen, Ph.D (Advisor)
Xiaodong Sun, Ph.D. (Advisor)
171 p.
Nuclear Engineering
VHTR; Air-ingress Accident; Nuclear Engineering; high temperature gas-cooled reactor

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Citations

  • Arcilesi, Jr., D. J. (2012). Developmental Analysis and Design of a Scaled-down Test Facility for a VHTR Air-ingress Accident [Master's thesis, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1338387523

    APA Style (7th edition)

  • Arcilesi, Jr., David. Developmental Analysis and Design of a Scaled-down Test Facility for a VHTR Air-ingress Accident. 2012. Ohio State University, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1338387523.

    MLA Style (8th edition)

  • Arcilesi, Jr., David. "Developmental Analysis and Design of a Scaled-down Test Facility for a VHTR Air-ingress Accident." Master's thesis, Ohio State University, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=osu1338387523

    Chicago Manual of Style (17th edition)

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© 2012, all rights reserved.
This open access ETD is published by The Ohio State University and OhioLINK.