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

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SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 245 / 295Document ID.No.UKEPR-0002-162 Issue 04• At 50 years: The 50 year dose represents the effects integrated over the life ofan individual. In addition to the doses received during the passage of aradioactive cloud, the doses received during this phase include those due to thepersistency of the contamination deposited on the ground. Individuals livingclose to the power plant are subjected to external exposure to deposits on theground and to internal exposure by ingesting contaminated foodstuffs, for aduration of 50 years. These doses are evaluated 2km from the point of release.3.4.1. Dose calculation methodThe atmospheric concentration integrated over the timescale of the passage of the plume isobtained using a differential equation representing atmospheric diffusion. The model used isa Gaussian plume model with a standard deviation evaluated using the 2 class Doury model[Ref].The calculations were performed taking into account standard meteorological conditionswhich ensure a broad coverage of atmospheric dispersion effects. The conditions of weakdiffusion with a wind speed of 2 m/sec were used. It can be seen that these cover around90% of the conditions encountered whichever French NPP site is considered (even if the siteis located on the sea shore such as Flamanville). Changes in meteorological conditions suchas wind speed, wind direction and diffusion are taken into account depending on the releaseduration by suing a correction factor, ranging from 1 to 5, applied to the horizontal standarddeviation.The conversion factors for dose and respiratory flow rates are chosen in accordance with theInternational Commission on Radiological Protection (ICRP) and Euratom guidance andrecommendations [Ref].Data concerning life habits, exposure conditions, integration time and transfer ofradionuclides into the environment used in the EDF methodology is realistic in terms of allFrench sites [Ref].The preliminary dose calculations obtained by applying the EDF methodology, and presentedhere, are considered to be generic in nature.3.4.2. Reference source termThe design options available for the EPR are such that the risks of core melt are very low. Asa result of the reinforced defence in depth concept, design provisions are adopted whichmake it possible to exclude severe accident phenomena which could lead to large earlyreleases. Low pressure core melt sequences are dealt with in the design by specificprovisions aimed at ensuring the integrity of the containment under such situations.An examination of RRC-B conditions shows that, taking account of the design provisionsadopted, the radiological objectives associated with these situations are met (see section3.1.2. of this sub-chapter).The main assumptions of the reference source term are as follows:• 100% core melt,• releases of the most significant radionuclides, from the point of view ofradiological consequences (noble gases, iodine and caesium) have beenmaximised (100% release in the containment vessel),
SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 246 / 295Document ID.No.UKEPR-0002-162 Issue 04• quantity of aerosols in suspension in the containment falls because of naturaldeposition only. The effectiveness of the containment spray system has not beentaken into consideration.During a severe accident in an EPR, the integrity of the containment vessel is ensured byspecific provisions which justify the use of the PCC assumptions [Ref] to assess releases tothe environment:• an internal containment leak rate of 0.3% by volume/day (maximum internalcontainment leak rate at its absolute design pressure of 5.5 bar),• filtration which allows 99.9% of the aerosols and molecular iodine and 99% ofthe organic iodine to be retained. Noble gases cannot be filtered.Given the conservative assumptions taken, the reference source term bounds that from lowpressure core melt accidents.3.4.3. Radiological consequences calculationsUsing the assumptions stated above into account, the preliminary reference source term isshown in the table below. It is expressed as percentages of the core activity released to theenvironment for selected radionuclides.Radionuclides133 Xe131 I137 Cs132 Te90 Sr106 Ru141 Ce241 PuSource Term(% C.I.) 3 1.5x10+0 6.1x10 -5 7.0x10 -6 5.1x10 -6 1.3x10 -6 2.6x10 -7 2.6x10 -7 4.6x10 -8From the fractions released into the environment and the methods and assumptionsdescribed in section 3.2 of this sub-chapter, the short term and long term doses as a functionof the distance around the reactor have been evaluated [Ref] and are shown below (see alsoSection 16.2.3 - Figures 16 to 19).Severe AccidentRRC-BDose 500 m(7 days)Dose 2 km(50 years)Dose (Sv) Adult (Sv) Infant 1y (Sv) Adult (Sv)Effective whole bodyThyroid1.5 10 -2 1.6 10 -23.0 10 -31.3 10 -2 2.0 10 -2 2.4 10 -3These results confirm that the main radiological objectives are met:• evacuation or relocation of the population is not necessary: only limitedsheltering very close to the site boundary is envisaged;3 % C.I. : percentage of Core Inventory
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16.2 - Severe Accident Analysis (RRC-B) - EDF Hinkley Point

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