International Journal of Nuclear Energy Science and Engineering Volume 3 Issue 4, December 2013 doi: 10.14355/ijnese.2013.0304.05
www.ijnese.org
Experimental and Analytical Study of a Simulated Collapsed PHWR Reactor Channel Under Accident Condition Deb Mukhopadhyay1, P. K. Vijayan Reactor Design and Development Group, Bhabha Atomic Research Centre, Mumbai 400085, India dmukho@barc.gov.in
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Abstract Heat transfer study has been carried out to characterize the effect of submergence on the heatup behavior of a collapsed PHWR reactor channel. The situation is simulated with a heat generating body enclosed in a both side open ended tube, undergoing a pool boiling condition. Under this study, the heat generating cylindrical body is rested inside the tube thus making the flow path no-uniform over the heat generating body. Experimentations are carried out at 6 and 8 kW power levels with variation of submergence level of the heater body, which ranges from 100%-12%. It is observed that for fully submerged level (100%) to a partially submerged level of 24%, a counter current stratified flow of steam and water is set up in the annulus fow path. Steam leaves from the upper section of the annulus and the pool water enter from the bottom section, thus a continuous steam generation and circulation is maintained. The convective heat removal by the steam is found to be sufficiently high to avoid dryout in the largely steam voided upper section of the heated body. However, at very low submergence level (12%) steam generation is found to be insufficeint, thus leading to a temperature excursion. The estimated generated steam mass flow rate has been found to vary between 3.55 –0.26 gm/s depending on the submergence level and power. The convective heat transfer coefficients for the exposed and submerged section of the heater body are estimated and found to vary between 0.7 kW/m2 K to 15.5 kW/m2 K. Heat transfer coefficients correlations are proposed for exposed and submerged heater body. Keywords Severe Accident; Collapsed Channel; Heat Generating Body; Pool Boiling; Enclosure Effect; Submergence Level; Dryout
Introduction The reactor core of Pressurised Heavy Water Reactor [Bajaj et al.] consists of several reactor channels which houses nuclear fuel bundle. Under a postulated late phase severe accident scenario, the reactor channels are likely to get heated up and disassembled from its own position and collapsed into a pool of water, thus
forming a debris bed, as described [Lee et al.]. The pool water enters into all the submerged disassembled channels from both ends. A reactor channel typically consists of short nuclear fuel bundles with internal heat generation and is housed in Pressure Tube (PT). The pool boiling behavaiour of these collapsed channels need to be understood well to develop physical models which eventually predicts the coarse debris bed heatup behaviour. The developed model will be useful for predicting severe accident progression and for developing planning Severe Accident Management Guidelines (SAMG). A pool boiling experimentation has been carried out which nearly simulates a single collapsed reactor channel. Under this study, a configuration as illustrated in Figure 1 is considered where a submerged cylindrical body (FR) rests on an open ended tube (PT). This configuration simulates an open ended collapsed reactor channel with nuclear fuel bundles enclosed in the PT. Based on the heatup behavaiour observations from fully submerged to nearly exposed condition of the heat generating body, a heat transfer characterization has been carried out.
PT Non-uniform annulus flow path
FR
FIG. 1 SCHEMATIC OF SS HEATER ROD (FR) IN TUBE ENCLOSURE (PT)
As the scenario demands for experimentation at different power levels, the study has been carried out at two heat flux levels. The heat flux levels of 31.0 kW/m2 (6 kW) and 41.3 kW/m2 (8 kW) simulates different decay powers of 2%-3% for the fuel bundle. As the level varies from fully submerged to fully exposed condition, experimentation has been done at
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