TUBE PLUGGING INDUCED TEMPERATURE NON-UNIFORMITY IN ONCE THROUGH STEAM GENERATOR

Multi-layered helically coiled tube bundles are used as heat-transfer area of High Temperature Gas-cooled Reactor-Pebble Module (HTR-PM) Once Through Steam Generators (OTSGs). Tube plugging is very common in Pressurized Water Reactor (PWR) steam generators. Though the corrosion in HTGR OTSG is not so severe as that in PWR SG, it has large possibility to plug tube. However, plugged tubes will lead to temperature non-uniformity in the shell side helium in the steam generator. Large temperature non-uniformity will raise the tube working temperature. Excessive temperature non-uniformity will also lead to high thermal stress of the tube sheet. Thus, a precise prediction of temperature distribution after tube plugging is essential for the safety and efficiency of HTGR OTSG. An in-house code for tube and shell side coupled thermal analysis of HTGR SG was developed based on porous media assumption. This code is used to simulate the tube and shell side temperature distributions after plugging 1 or 2 tubes. The effect of molecular diffusion, turbulent diffusion and the dispersion are considered using the effective thermal diffusivity, which is normalized as the Peclet number (Pe). However, Pe of cross flow over tube bundles varies a lot in previous investigations. Thus, in the current investigation, a large range of Peclet number values are used for simulation. When Pe increases, outlet steam temperature non-uniformity become more severe. Numerical simulation shows that plugging one tube of inner layer will lead to more severe temperature non-uniformity than that of outer layers. The temperature non-uniformity will become obvious when the number of plugged tube increases. The code is validated using experiments carried out on the Engineering Test Facility for Steam Generator (ETF-SG) of HTR-PM. Experiments were performed for cases with 1 or 2 tubes plugged. Comparison shows that the present code can predict the experimental measurements well.