18th International Conference on Structural Mechanics in Reactor Technology (SMiRT 18) Beijing, China, August 7-12, 2005 SMiRT18-F05-6 REVIEW OF STRESS ANALYSIS RESULTS ACCORDING TO DECOUPLING CRITERIA CHANGE AND SUGGESTION OF ALTERNATIVE SOLUTION Joong-Kyo Shin * Kyoung-Mo Yang Korea Power Engineering Company, inc. Korea Power Engineering Company, inc. 360-9 Mabuk-ri, Guseong-eup, Yongin-si, 360-9 Mabuk-ri, Guseong-eup, Yongin-si, Gyeonggi-do, 449-713, Korea Gyeonggi-do, 449-713, Korea Phone: 82-31-289-3737, Phone: 82-31-289-3636, Fax: 82-31-289-4109 Fax: 82-31-289-4105 E-mail: [email protected] E-mail: [email protected] ABSTRACT For OPR 1000(Optimized Power Reactor 1000) which was called as KSNP(Korean Standard Nuclear Plant), if moment-of-inertia ratio of run to branch pipe (Ir/Ib) is larger than 7 to 1, or diameter ratio of the run to the branch pipe (Dr/Db) is larger than 3 to 1, the branch pipe could be decoupled from the run pipe in piping stress analyses. But, EPRI URD and WRC Bulletin 300 criteria, Ir/Ib ≥ 25, are more difficult to accommodate the design sequence and designers’ convenience, than Ir/Ib ≥ 7. If the branch pipe, branching off the run pipe, cannot be decoupled in a piping stress analysis according to the criterion, Ir/Ib ≥ 25, the design sequence of the branch pipe should be parallel with that of the run pipe. However, in general, the design process for run pipe always precedes the process for the branch pipe, because the works for run pipe closely interfaces with other works such as calculation of penetration loads and size of embedded plate for the pipe support design. Although, OPR 1000 adopts its own decoupling criterion, the difference between the two existing criteria and thereafter the effects on the piping design process are not yet investigated in detail. Therefore, the difference between the two criteria was intensively reviewed and the impact on the piping design process was investigated. To do these, the three representative piping systems in the OPR 1000 having a branch pipe and 7 < Ir/Ib <25 were selected. In order to compare the effect of including the branch pipe in the stress analysis, two type of stress analyses were also performed for each piping system; that is, one is to carry out an analysis by including the branch pipe in the run pipe analysis whereas the other is by excluding it. Analysis results show that although pipe stresses, equipment nozzle loads and piping support loads increase when using the criterion of Ir/Ib ≥ 25, almost all stresses and loads satisfy the design requirements. However, it is thought that some parts of current piping design procedure have to be modified. Based on these results, recommendations are made to develop more appropriate guideline for piping stress analysis of OPR 1000. Keywords: Decoupling Criteria, Run Pipe, Branch Pipe, Moment of Inertia, Pipe Diameter 1. Introduction The purpose of decoupling the branch pipe from the run pipe is to overcome the functional limitation of stress analysis program and to increase the efficiency in design and construction processes for nuclear power plant (NPP). Especially, there is a big difference in design schedule between large-bore piping (2inch over in diameter) and small-bore piping (2inch and less in diameter). The large-bore piping design always precedes the small-bore piping design in design schedule. For the design sequence between large- and small-bore piping, the layout and stress analysis of large-bore piping are performed first in connection with large-bore piping spool RTM (Release to Manufacture). And it is 1212 Copyright © 2005 by SMiRT18 difficult to conduct the design of large and small-bore piping simultaneously because the analysis results of a run pipe are reflected in design of small-bore piping. Although the small-bore pipe design has been partially completed, when it interferes with works for large-bore pipes, electrical cables, ducts or any other systems, the possibility of design change is high for the reason that the design change of the small-bore pipe is much easier than that of large-bore pipe. For the above-mentioned reasons, the stress analysis of large and small-bore piping is separately performed. It is an essential condition in analyzing the run and the branch pipe separately that the effect of the branch pipe on the run pipe can be negligible. Whether the branch pipe gives a negligible impact on the run pipe is determined by the stiffness ratio of the run to the branch pipe. If the stiffness ratio of the run to the branch pipe meets the design requirement, then the stress analysis of the run pipe can be performed separately from the branch pipe. However, if the design requirement is not met, the branch pipe has to be analyzed separately with intentionally setting up anchors on pipe so that the branch pipe may not influence a severe effect on the run pipe. Seismic category buildings such as the containment building and auxiliary building have available structures which consist of concrete and beams for easy installation of piping restraints and supports; therefore, this kind of intentional design is possible. For non-seismic category buildings, such as the Turbine Generator building, it is difficult to perform the decoupling analysis due to the difficulty in installing anchors. In case of being not feasible for anchor installation as stated above, piping stress analyses of the run and branch pipe have been performed, respectively, in accordance with the unconservative criteria. Namely, OPR 1000 Standard [1], EPRI URD [2] and WRC Bulletin 300 [3] have been used as decoupling criteria in piping stress analysis. The major difference between these criteria is the moment of inertia ratio of the run to the branch pipe (Ir/Ib): OPR 1000 Standard is at least 7, EPRI URD and WRC Bulletin 300 is at least 25. Although, OPR 1000 adopts its own decoupling criterion, the difference between the two criteria and thereafter the effects on the piping design process are not yet investigated in detail. In this paper, therefore, the difference between the two criteria was intensively reviewed and the impact on the piping design process was investigated. To do these, the three representative piping systems in the OPR 1000 having a branch pipe and 7 < Ir/Ib <25 such as main steam piping system, component cooling water piping system and chemical and volume control piping system were selected. In order to compare the effect of including the branch pipe in the stress analysis, two type of stress analyses were also performed for each piping system, that is, one is to include the branch pipe in the run pipe analysis and the other is not to include it. Analysis results show that although pipe stresses, equipment nozzle loads and piping support loads increase when using the criterion of Ir/Ib ≥ 25, almost all stresses and loads satisfy the design requirements. However, it is thought that some part of current piping design procedure have to be modified. Based on these results, recommendations are made to develop more appropriate guideline for piping stress analysis of OPR 1000. 2. Decoupling Criteria The standards or guides, which are generally used for decoupling criteria in piping stress analysis, are OPR 1000 Standard, EPRI URD and WRC Bulletin 300. According to these standards or guides, the piping stress analysis of the run pipe has to be performed by integrating or overlapping the branch pipe, when the decoupling criterion is not satisfied. 2.1 OPR 1000 Standard In case of Dr/Db ≥ 3 or Ir/Ib ≥ 7, the branch pipe can be decoupled from the run pipe in the piping stress analysis. If these criteria are not satisfied, the overlapping analysis should be performed. For overlapping analysis, branch (or run) pipe at least up to two restraints in each orthogonal direction from branch connection should be included in the modeling of run (or branch) pipe for seismic category I and II piping. For seismic category III cold piping, two vertical supports should be included in the modeling of run (or branch) pipe. Then, the higher values of stress, restraint load, etc. are taken from the two overlapping analyses. 2.2 EPRI URD Criteria In case of Dr/Db ≥ 3 or Ir/Ib ≥ 25, the branch pipe can be decoupled from the run pipe in the piping stress analysis. 2.3 WRC Bulletin 300 Criteria Although the branch pipe can be decoupled from the run pipe in case of Ir/Ib ≥ 25, it should not be applied under the following conditions: - If anchors or restraints on the branch pipe are located near the run pipe and significantly restrict the 1213 Copyright © 2005 by SMiRT18 movement of the run pipe, the branch pipe up to the anchor points should be included in the run pipe analysis. - If accurate load evaluations are required at equipment nozzles, penetration, etc., the branch pipe should be included in the run pipe analysis to consider the effect of the branch pipe. 3. Impact on the Piping Design due to the Change of Decoupling Criteria Table 1 shows the diameter and the moment of inertia ratio of the run to the branch. If EPRI URD and WRC Bulletin 300 criteria (Ir/Ib ≥ 25) are applied to the decoupling criteria, the branch pipes of 1~2 inch in diameter can not be decoupled from the run pipes of 2.5~4 inch in diameter. That is, the branch pipe should be included in the run pipe analysis because the stiffness of the branch pipe influences on that of the run pipe.