Analysis of Precooling Injection Transient of Steam Generator for High Temperature Gas Cooled Reactor

Yan Wang,Yanhua Zheng,Lei Shi

doi:10.1155/2017/8521410

Abstract

After a postulated design basis accident leads high temperature gas cooled reactor to emergency shutdown, steam generator still remains with high temperature level and needs to be cooled down by a precooling before reactor restarts with clearing of fault. For the large difference of coolant temperature between inlet and outlet of steam generator in normal operation, the temperature distribution on the components of steam generator is very complicated. Therefore, the temperature descending rate of the components in steam generator needs to be limited to avoid the potential damage during the precooling stage. In this paper, a pebble-bed high temperature gas cooled reactor is modeled by thermal-hydraulic system analysis code and several postulated precooling injection transients are simulated and compared to evaluate their effects, which will provide support for the precooling design. The analysis results show that enough precooling injection is necessary to satisfy the precooling requirements, and larger mass flow rate of precooling water injection will accelerate the precooling process. The temperature decrease of steam generator is related to the precooling injection scenarios, and the maximal mass flow rate of the precooling injection should be limited to avoid the excessively quick temperature change of the structures in steam generator.

Highlights

Emergency shutdown system will be tripped to start up after a postulated accident is detected, and the decay heat will be removed by the residual heat removal system
The results show that, under the case with 1 t/h precooling water injection, the fluid temperature at the steam generator heatexchange tubes (SGHT) outlet is approx. 345∘C at the 8th hour after the precooling injection, while the fluid temperature at the pressure relief valve (PRV) in the live-steam pipeline (LP) is about 400∘C
In Case 2 with 5 t/h precooling water injection, both of the fluid temperatures at the SGHT outlet and at the PRV decrease to 200∘C within 5 hours, which indicates that the SGHT and the LP had been well cooled down by the precooling water injection

Summary

Introduction

Emergency shutdown system will be tripped to start up after a postulated accident is detected, and the decay heat will be removed by the residual heat removal system. Steam generator (SG) still remains with high temperature level after emergency shutdown, and it needs to be cooled down to a certain temperature level before the reactor restarts with faults clearing. Because the mixture of water and steam with high pressure remains in SG after emergency shutdown, it needs to be drained out of SG to decrease the SG pressure for the following cooling water injection into SG. Does the SG temperature change obviously during the precooling water injection stage, and the temperature distribution of the SG structures will be affected due to heat convection during the pressure relief stage to certain extent. The high temperature gas cooled reactor pebble-bed module (HTR-PM) is selected for the study on the precooling transient of SG. Brief descriptions on the HTR-PM are given

The HTR-PM Reactor

Description on the Precooling Injection

Analysis of the Precooling Injection Transient

Conclusions