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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 18 / 295Document ID.No.UKEPR-0002-0162 Issue 041.3.2 RecriticalityThe sub-criticality of a PWR is maintained under all accidental conditions by the followingfeatures:• the geometry of the fuel assemblies,• the distribution of control assemblies containing neutron absorbers in the reactorcore,• the presence of burnable absorbers distributed within the fuel rods,• boric acid in the reactor coolant.In the progression of a severe accident, resulting from a loss of coolant and the loss of corecooling, the features listed above are significantly altered. For example the geometry and theposition of the absorber rods, which will melt much earlier in the accident sequence than theother fuel assembly structures, will be altered. The loss of moderator is beneficial forpreventing recriticality of the core, however in the event of flooding of the core during thecore degradation the recriticality risk for the resulting configurations must be assessed. Thisassessment is presented in this sub-section.For the melt mitigation and stabilisation proposed for the EPR, the following states areconsidered:• an undamaged or only slightly damaged reactor core without control rods,• a partially damaged fuel lattice with free fuel pellets, fragments of fuel elements,absorber elements and structural materials,• a liquid fuel pool within the lower head of the RPV in combination with potentiallyremaining core fragments and liquid fuel within the core region,• a molten fuel pool within the reactor pit which contains different fractions ofsacrificial concrete,• liquid corium equally distributed within the spreading area,• solidified corium, partially fragmented and distributed on the cooling structurewithin the spreading compartment.The potential for recriticality is assessed following the addition of water to the configurationsdescribed above. The source of the water could arise from RIS [SIS] reflooding before vesselfailure or by flooding of the corium within the spreading compartment. The individual stateslisted above can be grouped into three categories as follows:• in-vessel configurations,• reactor pit configuration,• spreading compartment configurationsThe potential for recriticality in each of these categories is assessed in the sub-sectionsbelow.