Study on pressure relief system design for high temperature gas cooled reactor

• Pressure relief transient is studied numerically.
• Effects of different pressure relief system design on the transient are compared.
• Analysis results provide support for steam generator pressure relief system design.

Under some postulated accidental conditions, the secondary loop of the reactor needs to release its pressure for consequent system action. During the pressure relief transient, the residual fluid mixture of water and the steam will pass through the heat-exchange tubes of the steam generator and the live-steam pipeline, which decreases the steam generator pressure and the steam generator temperature due to taking the heat stored in the components away. The pressure and the temperature during the transient needs to be controlled to protect the steam generator from the potential damage. A 250 MWth high temperature gas cooled reactor is selected to evaluate this countermeasure effect during the pressure relief transient in this paper. A thermal-hydraulic system analysis code is used for modeling and simulating the pressure relief transient after the accident occurs. The temperature change of the fluid and the components in the steam generator are relative to the pressure relief transient, which is dominated by the design of pressure relief valve. Larger diameter of pressure relief valve will result in more intensive blown-down transient which causes faster pressure decrease and stronger heat transfer between the inside walls of the components in the secondary loop and the fluid. The designed pressure value where the pressure relief valve closes will affect the transient also. The designed pressure value dominates the time of the pressure relief transient which affects the amount of the fluid from the pressure relief valve. Different amount of the fluid will take away different quantity of heat from the components in the secondary loop of reactor. The pressure relief transients with a series of different diameter of pressure relief valve under some typical designed pressure value were simulated and compared. The results indicate that the pressure and the temperature of the steam generator during the pressure relief transient could be controlled effectively with reasonable design on the pressure relief system, which will provide protection and benefit for the steam generator after the postulated accident.