THERMAL-HYDRAULIC AND LBB EVALUATIONS TO JUSTIFY SHORT-TERM PLANT OPERATION WITH A CRD RETURN LINE SUSCEPTIBLE TO POTENTIAL THERMAL STRATIFICATION

摘要

Most of the BWR reactor pressure vessels have control rod drive hydraulic return (CRDHR) line nozzles. Each vessel has one such nozzle, typically 3-4 inches in diameter, and generally located 68-100 inches above the top of the active fuel. The CRDHR line was designed to provide a reactor pressure reference to the CRD system and to return to the reactor vessel exhaust water from CRD movement and water in excess of system requirements. The horizontal section of the piping near the CRDHR line nozzle is susceptible to thermal striping. Many of the BWR plants have capped this line. Recently at an overseas plant that had not capped this line, an axial through-wall fatigue crack of approximately 1-inch length was observed at the safe end connected to this nozzle. Based on this overseas operating experience (OE), a domestic plant that also did not cap the line developed a comprehensive analysis, inspection and repair plan to address the OE. Thermal-hydraulic and leak-before-break (LBB) evaluations were conducted to justify continued plant operation at this plant until the upcoming planned mid-cycle maintenance outage when the inspection of the line could be conducted and if necessary any repairs/modifications could be implemented. A thermal-hydraulic model was developed considering the geometry, the density difference between the hot and cold streams, the frictional and local losses, and the external flow effects, to predict thermal stratification. The model was validated against the test data from a foreign and a domestic BWR plant. This model was conservatively applied without taking any credit for the external flow and predicted that at the typical flow rates at the plant, thermal stratification in an approximately 45-inch long horizontal segment of the piping cannot be ruled out. However, later plant testing showed that thermal stratification does not appear at 20 to 26 gpm of cold injection flow, and the model predicts the plant testing when a moderate external flow effect is considered. The model determined a flow rate that would eliminate the phenomenon. However, the hardware limitations precluded the increase in the flow rate. The question that needed to be addressed was whether any fatigue cracking initiated from the previous operation could lead to failure of the affected piping segment during operation until the next refueling outage. The piping material is Type 304 stainless steel with a nominal diameter of 3-inches. Several LBB evaluations were conducted assuming different levels of part through-wall and through-wall cracking. Limit load equations of Appendix C of ASME Section XI were used to calculate the limiting critical crack lengths and depths. The leak rates were calculated using a modified two-phase flow model. The LBB evaluations concluded that short-term plant operation to next refueling outage is justified. The inspection findings, the temperature monitoring hardware installation, and the monitoring results obtained during the mid-cycle outage are also discussed.