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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 238 / 295Document ID.No.UKEPR-0002-162 Issue 04The assessment of effective dose takes account of the 4 exposure routes (exposure todirection radiation from the plume, inhalation, ingestion and exposure to radiation fromdeposits on the ground or other surfaces). The assessment also takes account of foodstuffrestrictions, if appropriate (see above).In general, dose calculation results are presented in a generic way for adults and for a 1-year-old infant at the site boundary (500 m) over 7 days, and for an adult at a distance of 2km from the point of release over 50 years. The equivalent thyroid dose will also be assessedfor these population groups. For a given nuclear site, the doses have to be calculated for themost exposed group (nearest habitation).The presentation of dose results is supplemented with explanation in each case of how theradiological objectives are met (limited sheltering, evacuation only to the site boundary,absence of permanent relocation) with reference to the thresholds mentioned above.The EPR reactor is a French-German design and the original method used to assess theradiological consequences of accidents was German. The method is presented in section 3.2for the reference source term evaluation and in section 3.3 for the off-site consequences withthe associated results.Construction of the EPR in France made it necessary to apply the methodology used forother reactors operating in France to EPR. In terms of its functional principles the EPRreactor is similar to the reactors currently in operation in France – the main differences beingdesign improvements such as design measures against severe accidents. Methods andassumptions adopted for operating French NPPs for evaluating fission product release intothe environment during a severe accident can be applied, except where identified designdifferences require variations from the usual methods. A simplified implementation of thisassessment method has been used for this safety report. The corresponding results aregiven in section 3.4. The approach will be reviewed to address UK specific requirements laterin the licensing process.3.2. REFERENCE SOURCE TERMThe assumed radiological consequences of RRC-B core melt sequences are based on abounding reference source term, calculated with reasonably conservative and simplifiedbounding assumptions, independent of the accident scenario but which relate to the coremelt sequence. These bounding assumptions relate to fission product behaviour and to thebehaviour of the plant (containment leakage, filter efficiency, etc).The reference source term, given in section 3.2.6., has been assessed with the originalFrench-German design method. The corresponding assumptions are summarised in thefollowing paragraphs, as well as the main differences between German and French NPPassumptions. The new features of the EPR are discussed when they might lead to differentresults.3.2.1. Core InventoryThe reactor core activity inventory was determined assuming a reactor thermal output of4900 MW(th), an equilibrium core with 5% U-235 enrichment and an average burn up of43 MWd/kg; the precise boundary conditions for the calculation of the activity inventories areshown in Chapter 14 and the activity inventories of a series of nuclides are also listed inChapter 14.The calculations were performed using the computer program ORIGEN-S [Ref].