TED-AJ03-301 DEBRIS BED COOLABILITY IN THE BWR PRESSURE VESSEL

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The objective of the experimental program, performed at RIT (Royal Institute of Technology) was to determine the additional capability for cooling of debris, deposited in the lower head of a BWR (Boiling Water Reactor) by the internal structures in the lower head. The debris bed in the BWR's could be formed as a consequence of a severe accident in this reactor type. In the LWR (Light Water Reactor) severe accident scenario, particulate debris beds are formed when corium melt comes in contact with water. Of particular importance is the coolability potential, what is offered by the control rod guide tubes (CRGTs), which are located in the lower head of the reactor vessel, since (a) there is a large number of these tubes in the lower head (b) each of them offers a substantial additional heat transfer area (c) water may be supplied via the guide tubes. The POMECO (POrous MEdia COolability) facility was constructed. The test section was a stainless steel vessel. Cross section area of the vessel was 350×350mm. Total height of the section was 1400mm. Up to 370mm of the sand bed could be formed to simulate the debris. The test section contained annular pipe of the same dimensions as CRGT in typical BWR. The test section represented a unit cell with one CRGT for the BWR reactor lower head. Test section power was up to 1MW/m3. The dryout heat flux as the limiting parameter for the steady state removal of the generated heat by boiling of the coolant was the subject of these investigations. Focus was placed on low porosity, small particle size and relatively large-scale debris beds. A database on the enhancement of dryout heat flux by the CRGT was obtained, for low porosity uniform beds with heat addition of up to 1MW/m3. The scenario of interest is that of the discharge of a large amount of core melt into the lower head of the BWR, which either leads to : (a) The melt break-up and formation of a large debris bed in water, which may have porosity of 30% to 40%;(b) The vaporization of all the water in the lower head resulting in a dry debris bed;(c) The formation of a melt pool from the dry debris bed. Further, the three variations of interest for the debris bed and for a melt pool are : (i) There is no control rod guide tube;(ii) There is water entry from the bottom into CRGT at the appropriate flow rate and the openings are above the water overlayer (iii) The CRGT openings at the top are in the water overlayer, with possibility of water entering from the top. Several studies of quenching of particle debris beds either by flooding from top or bottom have been reported in the literature [1-4]. Cho and Bova [5] found that during top flooding, the penetration of liquid was faster in the middle of the particulate layer. Ginsberg at al.[6], however, concluded from their experiments that the quenching process was characterized by a two step bi-frontal process with a partial quench front propagating downward and another front traveling upward after the downward front had reached the bottom of the bed. They also proposed a model based on counter-current flooding limitation (CCFL). Tung et al.[7] studied experimentally and analytically the quenching by top flooding when a certain amount of gas was injected at the bottom and heat was generated in the particles. Their model also utilized CCFL. The purpose of the present work is to study experimentally the effect of the presence of a CRGT in an internally heated homogeneous particle bed on dryout heat flux and quenching processes. The CRGT was modeled with and without water flow.

TED-AJ03-301 DEBRIS BED COOLABILITY IN THE BWR PRESSURE VESSEL

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