Scaled Experiments on Air Ingress in the Very High Temperature Reactor (VHTR)

Open Access
Author:
Baird, Jenna J
Graduate Program:
Nuclear Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
None
Committee Members:
  • Seungjin Kim, Thesis Advisor
Keywords:
  • air ingress
  • generation IV reactors
  • very high temperature reactor
  • gravity driven exchange
Abstract:
In the postulated Depressurized Loss of Forced Convection (D-LOFC) for the Very High Temperature Reactor (VHTR), the gravity driven air ingress phenomenon is of particular interest. This accident scenario involves depressurization of helium from the reactor vessel due to a rupture in the primary coolant pipe, followed by the exchange of the helium with the air driven by the density difference. This ingress of air could potentially lead to oxidation of graphite structure in the reactor and ultimately to a release of radioactive fission products. The present study performs scaled adiabatic separate-effects experiments using air and helium as working fluids to highlight the hydrodynamic effects in the process of air ingress. A scaling analysis is performed to design a test apparatus using the Gas Turbine Modular Helium Reactor (GT-MHR) as the reference prototypic reactor. The scaling is performed to preserve the exchange time ratio between the scaled and prototypic case to be close to unity. The test vessel is made of a carbon steel cylinder with 60.96 cm (24 in) in diameter and 182.88 cm (72 in) in length. It is sealed at the top and bottom with flat plates via flanges. It is designed to accommodate three break locations, namely, two horizontal breaks on the side and one vertical break on the top of the vessel. These represent the horizontal primary coolant pipe and the vertical refueling standpipe, respectively. To investigate the geometric effects on the ingress phenomenon, the break locations are designed to accommodate pipe breaks of various length-to-diameter ratios (L/D) and break angles. In the present study, the air ingress via two horizontal side breaks is investigated with a pipe break of L/D = 3. The oxygen concentration is measured with an oxygen analyzer at several axial and radial locations within the test apparatus. It is found that the majority of the local data follows the same general trend. Oscillations are observed in the data for locations within close proximity as a result of the helium-air interface. The transient behaviors of both the local and the averaged oxygen concentration data show characteristics of hydrodynamic effects on the air ingress.