After a supposed accident lead the reactor to shutdown, the residual heat generated from the decay will be removed out by the cooling system. A cooling water flow will be pumped into the steam generator for cooling the reactor after the pressure of the secondary side of the steam generator is decreased to a lower design value with opening of the pressure relief valve which is installed on the live-steam pipeline. But the temperature of the heat-exchange tubes and the tube plate at the outlet of the steam generator whose temperature in the steady-state operation is 570° high, need to be cooled down to about 200° for the material limitation before the cooling water is injected, to avoid the cold-shock damage on the components. In this paper, exampled as the 200MWe high temperature gas cooled reactor (HTR-PM), a general thermal-hydraulic system analysis code is used for modeling and simulating numerically the pressure relief transient after the accident. The results show the structure components of the steam generator will be cooled down effectively to mitigate the cold-shock damage from the cooling water during the transient with the designed pressure value of 1MPa where the pressure relief valve in 30mm diameter will be closed. It provides supports for the evaluation on the feasibility of cooling water injection and also for the material stress analysis on some relevant components in the steam generator design.

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