Mika Tahara

Heat Removal Capability of Core-Catcher With Natural Circulation

Toshiba has developed a core-catcher system. It is to be installed at the bottom of the lower drywell in order to stabilize a molten core flowing down from a reactor vessel. It consists of a round basin made up of inclined cooling channels arranged axisymmetrically, and the structure including risers, downcomers and a water chamber to get natural circulation of the flooding water. So it can cover entire pedestal floor and can work in passive manner.In order to confirm the heat removal capability of the core catcher with natural circulation, we have conducted full scaled tests in several conditions. Some important dimensionless numbers obtained from fundamental equations of the natural circulation are used for the tests.Using dimensionless number and to compare with several analysis, we can verify that the experiment is adequate to simulate the actual plant.Copyright

Heat Removal Capability of Core-Catcher in Consideration of Transformation of Cooling Channel

It is necessary to stabilize high temperature molten core in a severe accident for long time without electrical power. The core-catcher is to be installed at the bottom of the lower drywell in order to settle the molten core flowing down from a reactor vessel.Toshiba’s core-catcher system consists of a round basin made up of inclined cooling channels to get natural circulation of the flooding water. So it can cover all pedestal floor and can work in passive manner.We have been confirming an applicability of the core-catcher to actual plants. We have conducted full scaled tests with a unique cooling channel which has inclined rectangular flow section and changing the section area along flow direction in several conditions to evaluate the influence of the parameters on the natural circulation and heat removal capability. The test results showed good heat removal performance with nucleate boiling.However, we should consider a transformation of the cooling channel, for example, by the falling corium. So we calculate the assumed transformation of the cooling channel and conduct natural circulation tests with obstruction in the cooling channel. We confirm that natural circulation flow is stably continues and the cooling channel can remove prescribed heat, even if a flow area have got narrow locally.Copyright

Flow Characterization and Heat Transfer of Core-Catcher in Passive Safety System

A mitigation system which can keep core melt stable after a severe accident is necessary to a next generation BWR design. Toshiba has been developing a compact core catcher to be placed at the lower drywell in the containment vessel. The cooling water for the core catcher is supplied from the passive flooder and PCCS drain line. After the core catcher is flooded, the molten core would be cooled by both overflooding water and inclined cooling channels, in which water is boiling and natural circulation is established. So the core catcher can operate in passive manner and has no active component inside the containment.This paper summarizes flow dynamics and heat removal capability in an inclined cooling channel of core catcher when cooling water flows by the natural circulation.Copyright

Numerical Analysis of Catalytic Recombiner Performance Considering a 3-Dimensional Gas Flow

Metal-water reaction and radiolysis of water generate hydrogen during a severe accident in a light water reactor. To prevent hydrogen combustion, a flammability gas control system (FCS) is installed in reactor containment. Most of the current FCS combine hydrogen with oxygen by heating and employ active devices to maintain the gas flow through the FCS. Recently a catalytic recombiner has been developed as passive FCS. The catalytic recombiner performs its function passively and has the advantages of the robustness during an accident, easy maintenance and low cost compared with the current active FCS. The hydrogen depletion rate of the catalytic recombiner is affected by the local thermal hydraulic conditions during an accident. To evaluate hydrogen depletion by the catalytic recombiner considering these phenomena in the containment, a 3-dimensional fluid dynamics analysis is useful. A theoretical catalytic recombiner model has been developed in which the flammable gas depletion rate is estimated accounting for the gas transfer rate in porous catalyst material. The model has been incorporated with a thermal hydraulic model for the fluid dynamics in a containment that has been developed using a 3-dimensional CFD code STRA-CD. This catalytic recombiner model has been confirmed using a catalytic recombiner performance test that was carried out in the Battelle Model Containment (BMC). Further verification of the analysis model has been conducted using the test data described in NUREG/CR-6580 which addressed the wall effect on the catalytic recombiner performance. The predicted performance of the catalytic recombiner shows a good agreement with the test data, and especially the parameters effects on the recombiner performance is well described, which include the effects of the containment wall, gas flow rate to the catalytic recombiner and gas concentration distribution in the containment.Copyright