Nuclear power plant is a large-scale complicated system, which includes reactor core, steam generator, turbine and other important components. These components are tightly coupled with each other. Among these components, steam generator is the key link of primary circuit system and secondary circuit system. Heat transfers from primary side to secondary side. Once-through steam generator applied in high temperature gas cooled reactor (HTGR) has the properties of small heat capacity and rapid response speed. In HTGR system, the steam generator should match with the properties of reactor core such as large heat capacity and the slow response. Therefore, accurately simulating the steam generator is a complex task and has a great impact on the coupling property of a reactor system. To address this issue, effects of boundary conditions on the output water quality are analyzed and time integration schemes of backward differentiation formula (BDF) are implemented to HTGR steam generator simulation code BLAST in this work. The introduced BDF is a higher-order approximation to a transient term. It can reduce the numerical error from an explicit time integration scheme. The modified code is numerical tested in a noteworthy HTGR accident operation condition: Pressurized Loss Of Forced Cooling (PLOFC) accident. The performance of HTGR steam generator in the accident is analyzed. The accuracy of the improved algorithm is compared with the original BLAST code. Result shows the safety characteristics of steam generator in PLOFC accident and indicates that the numerical accuracy is significantly improved for both helium and water sides by BDF. For the consideration of accuracy and stability, BDF2 is chosen in the modified BLAST code.

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