The Very High Temperature Reactor (VHTR) is a new reactor that uses helium gas as primary coolant for conventional graphite matrix, coated fuels. It enables the achievements of high thermal efficiency and can supply heat with a high temperature of about 900–1000 °C. During the loss of coolant process, the fuel hot spot temperature should not get over a criteria value due to the temperature limitation of the fuel assembly. Traditionally, the VHTRs are designed to deal with loss of forced circulation conditions by using a passive mode decay heat removal system for the cavity cooling. However, even passive systems may experience some failure due to multiple undesired conditions even though the possibility is extremely low. Therefore, the VHTRs are now expected to be designed as naturally safe reactors with inherent safety features. Which means the decay heat removal is fully dependent on natural convection and radiation. To accomplish the tough task, a clear understanding of the heat transfer process during flow decay transient condition is quite necessary.
This study was conducted to investigate the transient heat transfer process between the solid surface and coolant (helium gas) in VHTR under flow decay conditions. Forced convection transient heat transfer for a horizontal cylinder under flow decay transient condition was experimentally studied. The experiment was conducted by using the helium gas as coolant. A uniform heat generation rate was added to the heater. With a certain flow rate of the helium gas, the heater temperature was maintained at a designed value. Then, the flow rate of the helium gas starts to decrease according to designed linear functions with different decreasing speed. Platinum cylinder with 1 mm in diameter was used as the test heater. The heat transfer coefficient and surface temperature were measured during the flow decay transient process under wide experimental conditions such as initial flow rate, flow decay time. It was found that the temperature of the test heater increases in curve shape with different gradients during this process, with a shorter flow decay time the increasing rate of heater surface temperature would be higher. The heat transfer coefficient versus time during the flow rate decreasing process was also obtained.