Yuanzineng kexue jishu (Jan 2023)
Transient Performance of Nuclear Reactor Pressure Vessel Main Seal
Abstract
As one of the most critical equipment in reactor primary circuit system, nuclear reactor pressure vessel undertakes extremely important system functions and safety functions. Therefore, the analysis and design method for the main seal of reactor pressure vessel has always been one of the core technologies in nuclear power station and related industrial field. And, ensuring the safety and reliability of the main seal is of great significance to guarantee the nuclear safety. With the aim to study the mechanical properties and sealing characteristic of the main seal assembly under transient condition in reactor pressure vessel, the mechanism of stress change in bolt section, the axial separation curve at flange sealing interface, and the cumulative deformation at flange surfacing layer under cycle conditions were explored in this paper. Firstly, the three-dimensional numerical analysis model of the seal assembly including bolt, sphere head and vessel flanges was investigated and established. Using sequential thermal-mechanical coupling analysis technique, the temperature and stress distribution characteristics of the seal assembly under typical transient conditions such as start-up and shut-down were studied. Then, through analyzing the flange and bolt coordination deformation mechanism, the variation rules and interval causes for bolt stress change under transient conditions were systematically explored and concluded. In addition, mechanism of the axial separation between sphere head and vessel flanges at sealing interface versus time was summarized. Last, the cumulative plastic deformation within the sealing region and surfacing layer, and the evolution trend of axial separation at flange sealing interface under cycle transient condition were predicted. It is found that, there is a high resistance to the heat transferring between bolts and flange due to the existence of bolt holes. Consequently, temperature hysteresis effect appears in the bolt relative to the surrounding flange which leads to a great stress amplitude in the bolt section under transient condition. Furthermore, both the transient temperature and pressure fluctuations have great influences on the axial separation at flange sealing interface. In detail, a rapid pressure rising possibly can make a fast increasing of the axial separation curve. Particularly, under the cycle transient condition comprised of start-up and shut-down, the axial separation at flange sealing interface shows characteristic of periodic cycle curve. The cycle curve rises slowly versus cycle number and has almost the same shape in each cycle. However, after 10 cycles, both the cycle curve and the local plastic deformation at flange surfacing layer reach stable stages. In addition, the distribution of overall plastic deformation trends to be uniform.
