16.2 - Severe Accident Analysis (RRC-B) - EDF Hinkley Point

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 149 / 295Document ID.No.UKEPR-0002-162 Issue 04A significant heat-up on the embedded frame during this first phase is avoided by the smallthickness of the upper part of the steel frame and by the fact that the “chutes” on its outerperiphery have a low profile and do not reach into the upper half of the concrete. The lower partof the steel frame, which extends below the level of the gate, is also much thicker and thusrepresents a high thermal inertia, which helps to prevent early failure. This is because theenergy needed to melt a certain volume of steel is higher than the energy to decompose thesame volume of concrete (based on the materials physical/chemical properties).As a consequence, the embedded support frame is expected to support the gate until thesacrificial concrete cover of the melt plug is eroded and the melt reaches the level of the gate.The frame is additionally supported mechanically by a circumferential weld that connects thevertical and horizontal part of the frame at the bottom, at a position directly above the zirconiabricks. This connection is able to carry the weight of the melt and the residual melt plug, evenwithout crediting the surrounding chutes.In the unlikely case that the support frame would thermally fail before the gate is locallypenetrated by the melt, the entire frame, including locked-in gate would completely drop-out intothe transfer channel under the hydrostatic pressure of the accumulated melt. This willinstantaneously open a large cross-section for melt release.In all other cases, after penetrating the sacrificial concrete, the melt will expose and melt throughthe aluminium gate with the support frame intact. The outflow cross-section is determined by thesize of the initial contact area between melt and gate and by the growth of the resulting hole inthe gate due of the heat transfer between flowing melt and surrounding concrete. The aluminiumgate itself does not act as a thermal resistance for the flow because of the low melting point ofthe material and the fact that it will interact exothermically with the melt.2.4.1.3.1. Validation strategyDue to the large diameter of the reactor pit and the fact that the surface area of the gate is lessthan a tenth of the total surface area of the pit bottom, it can be expected that concrete ablation,across the surface of the gate, will be practically homogeneous and therefore the erosion frontwill be fairly even.Therefore, a large region of the gate is predicted to contact the melt within a short period of time.Such quasi-uniform contact is further promoted by the low thermo-mechanical stability ofresidual, dried-out concrete layer that would surround the initial contact area. This would causea large area of the gate to open, thus allowing a very fast release of the accumulated melt intothe transfer channel and core catcher. It is evident that for this most likely case no specificanalysis is necessary.However, because the assumption of a large initial contact area cannot be reliably proven due tothe absence of MCCI experiments at reactor-scale, the possibility of a small contact area alsoneeds to be assessed. The cross-section and location of the initial contact area cannot bepredicted, as they are determined by the course of the preceding MCCI. Therefore, a minimumconceivable size is chosen consistent with observations from available large-scale experiments.The most relevant MCCI experiments are the tests performed at ANL in the framework of theMACE [Ref], and OECD-CCI projects [Ref]. They combine large cross-sections (from50cm*50cm to 1.2m*1.2m) with the use of a prototypic oxidic melt. Among these experiments,the OECD-CCI tests are the best-documented with respect to tracking local erosion. TestsCCI-1 and CCI-3 are most representative for the EPR due to the use of a silicate-type concrete.