Structural mechanical and thermal hydraulic aspects on the behaviour of crack like leaks in piping

Abstract

The evaluation of fluid flow rates through crack-like leaks in pressurized components of NPPs plays an important role for leak-before-break considerations. When using available simplified methods for the estimation of critical discharge flow rates, a suitable friction factor has to be chosen. Guidelines are given in the German code KTA 3206 based on investigations of leak rate experiments summarized in this paper. The frictional pressure loss in different leak flow approaches has to be chosen consistently in order to obtain comparable results with different models.Thermal hydraulic and structural mechanical analyses were performed for a postulated leak in the pressurizer surge line (SL) of a PWR Konvoi type under design basis accident conditions. The leak was postulated in form of a through-wall crack in circumferential direction. The size of the leak was calculated in the framework of Finite Element calculations with the code ADINA using an analysis model of a cooling loop of Konvoi type. With the calculated leak size, an ATHLET calculation was conducted, also examining the influence of the consideration of a variable leak size depending on the system pressure and mechanical loads. In the investigated case, the implication of the decreasing leak size, especially on the course of the remaining system pressure, cannot be neglected. The reduction of the leak area amounts in the transient examined to ca. 25% after about 1 h due to both the system pressure and temperature decrease and leads therefore also to an approximately 23% smaller leak rate. These results are dependent on the assumed position of the leak.Concerning the determination of the leak rates as critical flow-through rates with simplified methods good agreement has been achieved although the approaches differ considerably in parts. Relevant influence factors are the inflow losses in the crack channel, the consideration of the hydraulic diameter in connection with the leak area and the assumptions on the flow resistance coefficient due to the roughness of the crack surfaces.