(712d) Natural Circulation of Single and Binary Gas Mixture in Very High Temperature Reactor

Natural Circulation of single and binary gas mixture in Very High Temperature Reactor
Apoorva Rudra^*a, Dinesh V. Kalaga^b, Masahiro Kawaji^b,c
aDepartment of Chemical Engineering, City College of New York, CUNY, New York, USA.
^bDepartment of Mechanical Engineering, City College of New York, CUNY, New York, USA.
^cThe Energy Institute, City College of New York, CUNY, New York, USA.

Air ingress scenario is one of the scenarios that could lead to hot spots in a very high temperature reactor (VHTR) core [1] and hence needs to be studied in detail. During air ingress, a crack/breakage in the reactor caused due to an environmental or externally initiated event leads to a pressure difference situation [2][3]. The two main gases involved are helium, which is the proposed coolant for VHTRs, and air which enters the reactor loop from outside due to the pressure difference. The air that enters into the reactor can cause oxidation of the graphite core and also induce secondary effects like increased temperatures due to an exothermic oxidation reaction. Thus, it is imperative to quantify the natural circulation behavior of these gases to get an in-depth knowledge of the air ingress scenario.

Natural circulation experiments are performed in a high-temperature and high-pressure test facility incorporating a flow channel in a graphite block simulating a prismatic core of a VHTR. Because air cannot be used owing to the presence of graphite block (possible oxidation), nitrogen is used instead in order to mimic the air flowing into the system. Mass flow experiments are conducted with the help of mass flow measurement (MFM) device, which consists of heating tape to which power is applied and two pairs of thermocouples, which are used to measure the gas temperatures and insulation temperature. The inlet and outlet gas phase bulk temperatures and surface and insulation temperatures are measured at steady state. The energy balance approach has been used to obtain the mass flow rates for different power levels.

The experimental results show interesting trends and relevant conclusions can be drawn from them. First and foremost, there is a clear dependency of mass flow rate on the working pressure because of the relation between pressure and gas density. Secondly, with respect to single gases, mass flow values of helium are found to be faster in comparison to nitrogen owing to the fact that the density of nitrogen is almost 7 times larger than helium density. A less dense gas, Helium in the present study, would be expected to move at a faster velocity. There is a drop in the mass flow if the graphite midpoint temperature (GMT) goes on increasing to a certain extent, which is not expected. Fundamentally, higher temperatures should lead to lower gas density and higher mass flow rates. However, this observation reveals the effect of two opposing properties: density and viscosity. As the helium temperature increases, even though the density decreases the dynamic viscosity increases. This leads to an increment in the flow resistance triggering the decrease in the mass flow rate. Mass flow measurements are also performed for helium-nitrogen gaseous mixture which is the actual case during air ingress scenario. There is also focus on analysis of the mean heat transfer parameters representing heat transport in the VHTR loop for different gaseous mixture concentrations.

References:

[1] Ball, S.J., et al., 2007, “Next-Generation Nuclear Plant (NGNP) Phenomena Identification and Ranking Table (PIRT) for Accident and Thermal Fluids Analysis,” NUREG/CR-6944.
[2] Oh, C., Kim, E.S., Schultz, R.R., Petti, D., Liou, C.P., 2008, “Implications of Air Ingress Induced by Density-Difference Driven Stratified Flow,” Proc. ICAPP ’08, Anaheim, CA, June 8-12, 2008.
[3] Oh, C.H., Kim, E.S., 2011, “Air-ingress analysis: Part 1. Theoretical approach,” Nuclear Engineering and Design, 241 (1) 203–212. doi:10. 1016/j.nucengdes.2010.05.064.