(358c) Experimental Investigation of Air Ingress Scenario during Natural Circulation Cooling of a Very Hot Channel

Apoorva V. Rudra ^*a, Narbeh Artoun ^b, Dinesh Kalaga ^b, Sanjoy Banerjee  ^{a, c}, Masahiro Kawaji ^{b, c}

^a Department of Chemical Engineering, City College of the City University of New York, New York

^b Department of Mechanical Engineering, City College of the City University of New York, New York

^c The Energy Institute, City College of the City University of New York, New York

After the severe accidents at Fukushima Daiichi Nuclear Power Plants, the need for understanding the response of nuclear facilities to externally or environmentally initiated events, such as strong earthquakes, tsunamis, and flooding has become significantly important [1, 2]. VHTRs are one of the important Gen-IV reactor designs which have passive safety features. Although these designs ensure passive cooling in case of loss of forced circulation or pressure [3], it is still unknown how they will behave under all conceivable accident scenarios. One such accidental scenario is air ingress scenario in which a crack/breakage might occur in the reactor due to an externally or environmentally initiated event leading to a pressure front. Due to pressure difference, air from the outside can enter the reactor loop and cause graphite oxidation. It could also induce secondary effects such as increased temperatures due to an exothermic reaction which lowers the graphite’s thermal conductivity, thus causing a degradation in conduction heat transfer.

In this study, we present the experimental measurements of the coolant flow and heat transfer behavior in a flow channel in a graphite block simulating a prismatic core of a Very High Temperature Gas-Cooled Reactor (VHTR). Natural circulation experiments are conducted using a helium-nitrogen mixture representing a helium-air mixture initially present in the lower plenum. A helium analyzer is used to investigate the nitrogen transport from the lower to upper plena as a function of time.

The natural circulation flow rate is measured and the flow rate data is related to the graphite temperatures, operating pressure, and overall concentration of nitrogen in helium. The amount of nitrogen gas injected into the lower plenum and thus the overall nitrogen concentration in the gas mixture is systematically changed to determine its effect at different graphite temperature settings in the riser and downcomer. To what extent the local helium concentrations will differ between the lower and upper plena during both steady and transient natural circulation is determined in order to elucidate the air transport process following a postulated accident involving air ingress into the lower plenum. In order to measure the natural circulation flow rate, a mass flow measurement system was designed. The natural circulation flow and heat transfer data along with the viscosity effects are analyzed to identify possible changes in heat transfer rates from the graphite in the riser.

 [1] ScienceDaily. “Fukushima at Increased Earthquake Risk, Scientists Report.” ScienceDaily. 13 Feb. 2012. Web. 6 Nov. 2012. <http://www.sciencedaily.com/releases/2012/02/120214100819.htm>

[2] Alvarez, Robert. “Spent Nuclear Fuel Pools in the U.S.: Reducing the Deadly Risks of Storage.” Institute for Policy Studies. May 2011. <http://www.ips-dc.org/files/3200/spent_nuclear_fuel_pools_in_the_US.pdf>

[3] Vilim, R.B., Pointer, W.D., and Wei, T.Y.C., 2006, “Prioritization of VHTR System Modeling Needs Based on Phenomena Identification, Ranking and Sensitivity Studies,” ANL-GenIV-071.