PASSIVE HEAT REMOVAL EXPERIMENTS FOR AN ADVANCED HTR-MODULE REACTOR PRESSURE-VESSEL AND CAVITY DESIGN

In the search for further reducing the residual risks of possible majo r reactor pressure vessel failure during and in the aftermath of sever e accidents of modular HTRs, an alternative RPV has been designed and a sample vessel already fabricated by the firm Siempelkamp, Krefeld, F RG. This alternative RPV design is made of high quality, ductile spher o cast iron with axial and circumferential wire or more recently circu mferentially flat band prestressing. This specific Siempelkamp design has been tested and qualified already on behalf of a series of experim ents in the sample test vessel. Also, this design was used for the con trol gas vessel in the THTR under operational service conditions. In o rder to demonstrate reliable decay heat removal under most severe cond itions, a 1 : 1 scale, 20-degrees sector of the vessel/cavity, termed INWA-facility (inactive decay heat removal) was fabricated and tested at Siempelkamp. The cavity was cooled by natural circulation of water flowing in tubes embedded into the cast iron structure of the cavity. A total of 5 experiments were performed with this setup examining a va riety of changes in constructive details, surface and cooling conditio ns. Each experiment was performed both for operational conditions and depressurization transient, typical for a 200 MW(th) HTR-module. Exper imental test times ranged up to 1000 hours. Pre- and post-test predict ions with the TOPAZ-code accompanied the INWA test series. The paper d escribes the INWA-facility and reports some of the experimental result s as well as the predictive capability of TOPAZ by comparing the data with computational results. The INWA-results qualify the pre-stressed cast iron vessel together with the natural circulation cooled cavity e ven for the worst of severe accident conditions. Even in case of total failure of all cooling capabilities in or at the cavity structure the vessel surface temperature remains below critical values.