Young-Jin Oh

Effect of Pipe Restraint on the Conservatism of Leak-Before-Break Design of Nuclear Power Plant

In light water reactor designs, the concept of leak-before-break (LBB) can be applied for piping systems to exclude the dynamic effects of pipe rupture. The crack opening displacement (COD) and the J integral are important parameters in the LBB design. From preceding researches, it was revealed that when the restraint of pressure induced bending (PIB) is not considered in LBB evaluation, COD can be overestimated, resulting in a decrease in conservatism of the LBB design. If the pipe restraint is not considered, however, applied moment can also be overestimated. Thus, to take the pipe restraint effect into account when conducting the LBB analysis, the decrease of COD and the decrease of the applied moment must be considered simultaneously. In this regard, the authors have developed the restraint coefficient to account for the pipe restraint effect on both COD and the applied moment at the cracked section in earlier research. In this paper, the restraint coefficient was validated by comparing with an elastic-plastic FEA model including restraint boundary conditions. The effects of the restrained COD and the effective applied moment on the LBB evaluation were then investigated using the piping evaluation diagram. As a result, it was confirmed that the decrease in applied moment had greater influence on LBB evaluation than the decrease in COD. Therefore, the current practice of LBB evaluation which assumes the pipe is free to rotate can provide more conservative results than the case in which the pipe restraint is considered.Copyright

DETERMINISTIC EVALUATION OF DELAYED HYDRIDE CRACKING BEHAVIORS IN PHWR PRESSURE TUBES

Pressure tubes made of Zr-2.5 wt% Nb alloy are important components consisting reactor coolant pressure boundary of a pressurized heavy water reactor, in which unanticipated through-wall cracks and rupture may occur due to a delayed hydride cracking (DHC). The Canadian Standards Association has provided deterministic and probabilistic structural integrity evaluation procedures to protect pressure tubes against DHC. However, intuitive understanding and subsequent assessment of flaw behaviors are still insufficient due to complex degradation mechanisms and diverse influential parameters of DHC compared with those of stress corrosion cracking and fatigue crack growth phenomena. In the present study, a deterministic flaw assessment program was developed and applied for systematic integrity assessment of the pressure tubes. Based on the examination results dealing with effects of flaw shapes, pressure tube dimensional changes, hydrogen concentrations of pressure tubes and plant operation scenarios, a simple and rough method for effective cooldown operation was proposed to minimize DHC risks. The developed deterministic assessment program for pressure tubes can be used to derive further technical bases for probabilistic damage frequency assessment.

The Conservatism of Leak Before Break Analysis in Terms of the Applied Moment at Cracked Section

In leak before break (LBB) analysis, applied moment at the position of a postulated crack is a key parameter. The current procedures of LBB analysis use the value of design basis loads that are calculated with the assumption that the pipe does not contain a crack and follows the linear elastic behavior. Therefore, this can lead to conservative results of LBB evaluation compared with the case that considers the effect of plastic behavior of cracked pipe on the calculation of the applied moment at the cracked section [2].This paper aims to quantitatively investigate the conservatism of the existing LBB analysis in terms of the applied moment at the cracked section. The calculation results using the current procedures with linear elastic pipe model without a crack were compared with the results from the cracked pipe analysis. To consider nonlinearity of crack behavior and pipe material, the time history analysis methods that were verified using the simulated seismic pipe system experiment in the IPIRG-2 program were employed.The comparison results indicated that the applied moment at the cracked section decreases when the effect of crack and nonlinear behavior are considered in the analysis. Therefore, the current procedures of LBB analysis are significantly conservative compared with the elastic-plastic time history analysis. Base on this detailed analysis, the additional safety margin can be secured in the LBB analysis.Copyright

Finite element residual stress analysis of induction heating bended ferritic steel piping

Recently, there is a trend to apply the piping bended by induction heating process to nuclear power plants. Residual stress can be generated due to thermo-mechanical mechanism during the induction heating bending process. It is well-known that the residual stress has important effect on crack initiation and growth. The previous studies have focused on the thickness variation. In part, some studies were performed for residual stress evaluation of the austenitic stainless steel piping bended by induction heating. It is difficult to find the residual stresses of the ferritic steel piping bended by the induction heating. The study assessed the residual stresses of induction heating bended ferriticsteel piping via finite element analysis. As a result, it was identified that high residual stresses are generated on local outersurface region of the induction heating bended ferritic piping.

Butt-fusing Procedures and Qualifications of High Density Polyethylene Pipe for Nuclear Power Plant Application

Abstract In nuclear power plants, lined carbon steel pipes or PCCPs (pre-stressed concrete cylinder pipes) have been widely used for sea water transport systems. However, de-bonding of linings and oxidation of PCCP could make problems in aged NPPs (nuclear power plants). Recently at several NPPs in the United States, the PCCPs or lined carbon steel pipes of the sea water or raw water system have been replaced with HDPE (high density polyethylene) pipes, which have outstanding resistance to oxidation and seismic loading. ASME B&PV Code committee developed Code Case N-755, which describes rules for the constructionof buried Safety Class 3 polyethylene pressure piping systems. Although US NRC permitted HDPE materials for Class 3 buried piping, their permission was limited to only 10-year operation because of several concerns including the quality of fusion zone of HDPE. In this study, various requirements for fusion qualification test of HDPE and some regulatory issues raised during HDPE application review in foreign NPPs are introduced.Key Words : High density polyethylene (HDPE), Code case N-755, Fusion qualification procedure