Effect of Initial Conditions and Depressurization on Lock Exchange Flow After a Depressurized Conduction Cool-Down Event in the High Temperature Test Facility

Graduation date: 2017The High Temperature Test Facility (HTTF) is an electrically heated, scaled model of a Modular High Temperature Gas Reactor (MHTGR). Five experiments were conducted to study Depressurized Conduction Cool-Down phenomena stemming from a Double-Ended Guillotine Break (DEGB). After a DEGB, the reactor pressure vessel would depressurize until the pressures are equalized between the cavity that surrounds the reactor vessel and the vessel itself. Immediately following the depressurization there would be a large density gradient between the reactor vessel, which is full of hot helium and the cavity mixture of cold air and discharged helium. This density gradient would drive lock exchange flow, where the less dense helium would travel along the top of the pipe and the denser cavity gas would intrude into the bottom of the pipe and propagate towards the lower plenum. The cavity gas would then fill the lower plenum up to the top elevation of the hot leg pipe, which would then diffuse air into the core, potentially compromising core support and fuel integrity by graphite oxidation. Lock exchange flow is a density driven mechanism that depends on the density gradient between the cavity and the pressure vessel. Gas concentrations and temperatures greatly affect density and thus affect key time characteristics of lock exchange flow. The key time characteristics to be studied in the five experiments are: the time for the gas front to arrive at the thermocouple instrumentation in the cross-duct outside the lower plenum, the time to fill the lower plenum, and the times to reach instrumented posts in the lower plenum at various heights. The time to reach the plenum will indicate the speed at which the cold dense gas front is travelling. The time to fill the lower plenum is also the time of onset of molecular diffusion. The time the gas front reaches each instrumented post will display how the cold dense gas front propagates through the plenum. The first three of five tests were conducted at ambient temperatures with initially equalized pressures between the Reactor Cavity Simulation Tank (RCST) and the Primary Pressure Vessel (PPV). The next test was a heated depressurization, and the last test was a heated test with pressures initially equalized. These three categories of tests vary initial conditions that will change lock exchange flow time characteristics