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http://hdl.handle.net/11375/18045
Title: | MECHANISTIC MODELLING OF CRITICAL HEAT FLUX ON LARGE DIAMETER TUBES |
Authors: | BEHDADI, AZIN |
Advisor: | LUXAT, JOHN C. |
Department: | Engineering Physics and Nuclear Engineering |
Keywords: | CRITICAL HEAT FLUX;TWO PHASE FLOW |
Publication Date: | Nov-2015 |
Abstract: | Heavy water moderator surrounding each fuel channel is one of the important safety features in CANDU reactors since it provides an in-situ passive heat sink for the fuel in situations where other engineered means of heat removal from fuel channels have failed. In a critical break LOCA scenario, fuel cooling becomes severely degraded due to rapid flow reduction in the affected flow pass of the heat transport system. This can result in pressure tubes experiencing significant heat-up during early stages of the accident when coolant pressure is still high, thereby causing uniform thermal creep strain (ballooning) of the pressure tube (PT) into contact with its calandria tube (CT). The contact of the hot PT with the CT causes rapid redistribution of stored heat from the PT to CT and a large heat flux spike from the CT to the moderator fluid. For conditions where subcooling of the moderator fluid is low, this heat flux spike can cause dryout of the CT. This can detrimentally affect channel integrity if the CT post-dryout temperature becomes sufficiently high to result in continued thermal creep strain deformation of both the PT and the CT. A comprehensive mechanistic model is developed to predict the critical heat flux (CHF) variations along the downward facing outer surface of calandria tube. The model is based on the hydrodynamic model of \cite{Cheung/Haddad1997} which considers a liquid macrolayer beneath an elongated vapor slug on the heated surface. Local dryout is postulated to occur whenever the fresh liquid supply to the macrolayer is not sufficient to compensate for the liquid depletion within the macrolayer due to boiling on the heating surface. A boundary layer analysis is performed, treating the two phase motion as an external buoyancy driven flow, to determine the liquid supply rate and the local CHF. The model takes into account different types of flow regime or slip ratio. It is applicable for a calandria vessel as well, under a sever accident condition where a thermal creep failure is postulated to occur if sustained CHF is instigated in the surrounding shield tank water. Model shows good agreement with the available experimental CHF data. The model has been modified to take into account the effect of subcooling and has been validated against the empirical correction factors. |
URI: | http://hdl.handle.net/11375/18045 |
Appears in Collections: | Open Access Dissertations and Theses |
Files in This Item:
File | Description | Size | Format | |
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Azin_thesis_final.pdf | 3.08 MB | Adobe PDF | View/Open |
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