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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 2 / 295Document ID.No.UKEPR-0002-162 Issue 041. APPROACH TO SEVERE ACCIDENT CONTROL1.1. BASIC STRATEGYSevere accidents involve phenomena which pose an immediate threat to the integrity of thecontainment. An early failure of the containment in a severe accident situation would havemajor consequences (in terms of radiological dose) for the public. The main objective istherefore to avoid as far as possible any risk of early containment failure. A further, equallyimportant objective for the EPR is the preservation of the containment integrity in the longterm.In response, a two-staged approach is pursued. The first stage focuses on the practicalelimination of highly energetic phenomena which have the potential to breach thecontainment early into the accident and thus result in large early releases. The second stageis then concerned with maintaining the containment integrity in the long-term. These tasksare achieved by influencing and controlling the accident progression with design measuressuch that well-determined states can be achieved and thus the safety objectives are fulfilledwith traditional technologies. This approach has, therefore, a significant economic benefit andadditionally allows the efficient conduct of supportive research and development.Practical elimination is achieved by specific engineered safety features that concern thefollowing phenomena:• Core melt under high pressure and direct containment heatingHigh pressure failure of the reactor pressure vessel (RPV) following melting of the core canlead to direct containment heating (DCH) and to the formation of RPV missiles. Briefly, DCHdenotes heating of the free containment volume by finely fragmented and dispersed melt,which results in a fast pressure build-up. The RPV missile denotes an RPV which detachesfrom its anchoring due to the thrust provided by failure of the RPV at high internal pressure.High pressure core melt is practically eliminated by deliberately depressurising the primarysystem, which transforms high pressure into low-pressure core melt sequences. In addition,the design of the reactor pit avoids any direct path for the core melt to escape into thecontainment.• Large steam explosions which can threaten the containmentSteam explosions develop from the contact between molten reactor materials and water.Regarding in-vessel steam explosion, analyses [Ref] have verified that the RPV wouldwithstand the corresponding loads and thus no consequent effects on the containment wouldoccur. Ex-vessel steam explosions are prevented by avoiding the presence of water in thereactor pit at the time of pressure-vessel failure and during the corium flow out of the RPV, aswell as in the spreading compartment before the spreading phase. Specifically, wateringression into the reactor pit as a result of a loss-of coolant accident (LOCA) leading to coremelt is prevented by the design of the pit, which isolates it from the rest of the containment.Therefore, there exist no direct pathways for water to enter the pit. The only possibility wouldbe a break of one of the main coolant lines (MCL) directly at the connecting welds betweenthe MCL and the RPV nozzles. Such a break is, however, highly unlikely due to the breakpreclusion concept applied in the UK EPR.