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 : 243 / 295Document ID.No.UKEPR-0002-162 Issue 04Two types of countermeasures were distinguished outside the area enclosed by the plantfence where specific protective measures are coordinated by the operator:• Countermeasures to be performed before the accident is fully under control(evacuation):oIn this initial stage, intervention will generally be performed over a circulararea with radius r centred on the plant because it is difficult to predictchanges in wind direction with sufficient confidence. In order to evaluatethis kind of countermeasure, maximum dose values were calculated whichwill not be exceeded with a probability of 95% at any location beyond thedistance r from the plant. The evaluation is performed for different values ofr. Dose values are then plotted as a function of r.• Countermeasures to be performed after termination of release of radioactivematerials into the environment:oThis second type of intervention (e.g. relocation and banning the sale offoodstuffs) will only be performed in those areas in which measurementsshow that an intervention level D is exceeded. D may be a dose, a doserate or a contamination level for particular foodstuffs (measured in Bq/kg).For this type of intervention, the areas over which they may have to beapplied are considered more important than the distance from the plant.The evaluation is therefore done for different types of area.The potential radiological consequences depend on:• Release height.• Nuclide - specific activity of releases as a function of time.• Meteorological conditions during a defined, representative time period.The results of these calculations are given in the Section 16.2.3 - Figures 5 to 15.Information about the potential dose originating from the passing radioactive cloud is given inSection 16.2.3 - Figure 5, which gives the effective dose to adults and to infants due toinhalation and direct radiation from the cloud as a function of distance, and Section 16.2.3 -Figure 6, which gives the thyroid dose for adults and for infants due to inhalation only as afunction of distance. The portions of the dose values relating to inhalation account forradiation exposure due to inhaled radionuclides over a time period of 50 years for adults and70 years for infants, respectively (committed dose).For the area-related countermeasures, a corresponding calculation was performed. In thiscase, the main interest is related to the size of areas in which certain dose or contaminationlevels are exceeded. Twenty dose and contamination levels were evaluated simultaneouslyon the basis of the activity release pattern of the EPR Reference Source Term and aparticular weather condition.This calculation was performed for all weather conditions for the given representative timeperiod. For each of the 20 dose and contamination levels, cumulative distribution functions ofarea sizes were calculated. It was then possible to select those area sizes for which thegiven dose and contamination levels are not exceeded in 95% of all evaluated cases.Section 16.2.3 - Figures 7 to 15 show the size of these areas:
SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 244 / 295Document ID.No.UKEPR-0002-162 Issue 04Section 16.2.3 - Figures 7 to 9 show the whole body dose a person would receive which arenot exceeded in the cases 95% due to direct gamma radiation from ground contamination.In addition to inhalation and direct radiation doses, it is necessary to evaluate ingestiondoses. Therefore the direct contamination of leafy vegetables (salad - being representative ofall plants used directly as foodstuffs) and the contamination of milk from animals grazing oncontaminated pastures (representing plants used indirectly) are evaluated. Section 16.2.3 -Figures 10 to 15 show the calculated foodstuff contamination values immediately after theaccident for leafy vegetables and milk, as a function of the size of the area, which is notexceeded in 95% of the cases.For comparison with EU contamination limits for foodstuffs [Ref] only contamination with Cs-137, Cs-134, I-131 and Sr-90 is of concern. The curves labelled “all nuclides except Sr and I”are of minor importance since they include nuclides with relatively short half-lives.All these results confirm that the radiological objectives stated in Sub-section 3.1 are met.The next section presents the radiological consequence evaluation using EDF methodology.It must be noted that as yet no comparison with EU contamination limits for foodstuffs hasbeen made.3.4. EVALUATION OF THE POTENTIAL RADIOLOGICALCONSEQUENCES USING FRENCH NPP ASSESSMENT METHODThe principles involved in assessing the radiological consequences associated with severeaccidents are listed below:• the evaluation of the activity released is based on a reference source term,calculated with reasonably conservative bounding assumptions, independent ofthe accident scenario,• the assumptions used in calculating radiological consequences (dose evaluation)are fixed realistically to give a reasonably conservative evaluation of theradiological consequences.The calculation of the effective dose includes all potential exposure routes: external exposureto radiation plumes and deposits and internal exposure by inhalation and ingestion ofcontaminated foodstuffs. The dose is assessed over a period of up to 50 years.The results are presented:• At 7 days: The doses at 7 days correspond to the exposure of an individuallocated in the immediate vicinity of the site at the time of the release. Theeffective doses received via inhalation and external exposure to the plume andto deposits on the ground are calculated at 500 m from the reactor. In addition,the thyroid dose due to radio-iodine inhalation is evaluated for an adult and a 1-year-old infant at the same distance;
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16.2 - Severe Accident Analysis (RRC-B) - EDF Hinkley Point

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