Assessment of Choking Flow Models in RELAP5 for Subcooled Choking Flow Through a Small Axial Crack of a Steam Generator Tube

Choking flow plays an integral part not only in the engineered safeguards of a nuclear power plant (NPP), but also to everyday operation. Current NPP steam generators operate on the leak-before-break approach. The ability to predict and estimate a leak rate through a steam generator tube crack is an important safety parameter. Knowledge of the maximum flow rate through a crack in the steam generator tube allows the coolant inventory to be designed accordingly while limiting losses during loss of coolant accidents. Here an assessment of the choking flow models in thermal-hydraulics code RELAP5/MOD3.3 is performed and its suitability to predict choking flow rates through small axial cracks of the steam generator tubes is evaluated based on previously collected experimental data. Three sets of the data were studied in this work which corresponds to steam generator tube crack sample 1, 2, and 3. Each sample has a wall thickness, channel length (L), of 1.285 mm to 1.3 mm. Exit areas of these samples are 5.22 mm2, 9.05 mm2, and 1.72 mm2 respectively. Samples 1 and 2 have the same flow channel length to hydraulics diameter ratio (L/D) of 2.9 whereas sample 3 has a L/D of 6.5. A pressure differential of 6.8 MPa was applied across the samples with a range of subcooling from 5 °C to 60 °C. Flow rates through these samples were modeled using the thermal-hydraulic system code RELAP5/MOD3.3. Simulation’s results are compared to experimental values and modeling techniques are discussed. It is found that both the Henry-Fauske (H-F) and Ransom-Trapp (R-T) models better predict choking mass flux for longer channels. As the channel length decreases both models’ predictions diverge from each other. While RELAP5/MOD3.3 has been shown to predict choking flow in large scale geometries, further investigation of data sets need to be done to determine if it is suited well for small channel lengths.

Decommissioning and Waste Treatment Hazard Evaluation and Modeling

Hazards encountered during decommissioning and waste treatment involve different issues, emphasis, and scenarios than encountered during normal facility operations. This paper provides examples from experience in analysis and modeling of diverse facilities, framed in terms of custom phenomena models and their incorporation into integral facility analysis modeling. Models for entrainment of contamination and aerosol behavior are described and applied. The FATE™ (Facility Flow, Aerosol, Thermal, and Explosion) computer program is described, and example calculations are given for contamination release due to a dust explosion in order to demonstrate the sensitivity to boundary conditions and the use of engineered safeguards.

Progress With the International Pipe Failure Data Exchange Project

Certain member countries of the Organisation for Economic Cooperation and Development (OECD) in 2002 established the OECD Pipe Failure Data Exchange Project (OPDE) to produce an international database on the piping service experience applicable to commercial nuclear power plants. OPDE is operated under the umbrella of the OECD Nuclear Energy Agency (NEA). The Project collects pipe failure data including service-induced wall thinning, part through-wall crack, pinhole leak, leak, and rupture/severance (i.e., events involving large leak rates up to and beyond the make-up capacity of engineered safeguards systems). The part through-wall events include degradation in excess of code allowable for pipe wall thinning or crack depth. OPDE also addresses such degradation that could have generic implications regarding the reliability of in-service inspection. At the end of 2006 the OPDE database included approximately 3,700 records on pipe failure affecting ASME Code Class 1 through 3 and non-Code piping. This paper summarizes the unique data quality considerations that are associated with the reporting and recording of piping component degradation and failure. The paper also summarizes the database content and puts it in perspective relative to past efforts to systematically collect and evaluate service experience data on piping performance.