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 : 241 / 295Document ID.No.UKEPR-0002-162 Issue 04Ventilation of the annulus: Pressure in the annulus is assumed to be held constantly belowatmospheric pressure by the emergency ventilation system with an extraction rate of200 m³/h (See Chapter 6).Filters: Retention of all substances except noble gases (aerosols, elemental iodine andorganic iodine) by filters reduces the release to the environment to 1% of its value upstreamof the filter.The assumption used for operating French NPPs for filter efficiency is that the release ofmolecular iodine and aerosols is ten times lower than this (i.e. 0.1% of the value upstream ofthe filter [Ref]). The sensitivity study presented in section 3.4 uses this latter value.3.2.6. Release of Radioactive Substances to the Environment [Ref]Radioactive releases to the environment have been calculated for a large number of nuclidesusing an established French-German assessment method for estimating environmentalconsequences.The results for the nuclides Xe-133, I-131 and Cs-137 for a release period of 30 days are asfollows:Xe-133:I-131:Cs-137:1.4⋅10 5 TBq7.5 TBq0.45 TBqThe chemical composition of the iodine 131 is as follows:I-131 aerosol 3.0 TBqI-131 elemental 2.3 TBqI-131 organic 2.2 TBqOnly the dose factor corresponding to the elemental iodine is used for dose calculations butusing conservative assumptions.The cumulative release to the environment of the above three nuclides is given in Section16.2.3 - Figure 3. The calculations were performed using the computer code ACARE 1 .A complete list of all relevant radionuclides, which are released to the environment, is givenin Section 16.2.3 - Table 7 and Section 16.2.3 - Table 8.1 The computer code ACARE (Activity in interrelated compartments and release to the environment)was formerly used as a pre-program for PRODOS (Probabilistic Dose Calculation, section 3.3) tocalculate the activity flow of nuclides in coupled compartments. Both programs were kind ofcomplicated to handle and therefore taken out of use a couple of years ago and are not going to bereactivated.Radioactive decay and production of daughter nuclides were taken into account. Deposition of aerosolsand iodine in the single compartments could be considered in four different ways. The last compartmentrepresented the atmosphere. The results were presented in a time dependent form for the activityinventory in every compartment, the release rates to the atmosphere and the total release.The essential application was to sum over all nuclides the nuclide specific release multiplied with dosefactors. These values were used in PRODOS to calculate probabilistically distributions of radiationdoses in the vicinity of nuclear power plants after accidents.For the calculation of radiation doses the program chain ACARE-PRODOS was replaced by MACCS[Ref]
SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 242 / 295Document ID.No.UKEPR-0002-162 Issue 043.3. CALCULATION OF RADIOLOGICAL CONSEQUENCESIn the first instance, the radiological consequences of the reference source term (see section3.2.6.) have been assessed using the German methodology [Ref].With respect to environmental conditions, the following assumptions are made:1) Time of the accident: The accident is postulated to occur in the growing season(spring to autumn).2) Weather conditions during the accident: Although it is generally agreed that bestestimate methods can be applied in demonstrating compliance with requirements forsuch extreme cases, a more conservative approach is chosen. As required for designbasis accidents, it is assumed that the safety goals are met if they are not exceededfor 95% of the atmospheric dispersion conditions.3) Aerosol removal: The dispersion, fallout, washout and ecological conditions (e.g.washout and deposition rates from the atmosphere, transfer rates of radionuclidesfrom soil to plants, etc.) are selected in compliance with German rules for theevaluation of design basis accidents.4) Exposure of individuals: It is assumed that an individual may be exposed during thewhole accident at any location outside the plant boundary including the mostunfavourable location.5) Biological data for exposed individuals: The same biological data is used (e.g.breathing rates) as prescribed for the evaluation of design basis accidents; thesevalues are based on basic data from recent ICRP Publications (see [Ref])6) Selection of Appropriate Weather Conditions: The dispersion and deposition ofthe released radionuclides depend on the prevailing weather conditions, which arevaried over a range determined by the conditions that have occurred in reality over aperiod of a year or longer. The variation of dispersion and deposition according toprevailing weather conditions means that the results of the evaluated radiationexposure are also influenced to a high degree by the postulated meteorologicalconditions.Weather data (hourly measurements of wind speed, wind direction, diffusion classand precipitation rate) from a recent 4 year period from a seaside site in France wasused. Radiological consequences were evaluated using the computer codePRODOS 2 using this data. The radiological consequences covering 95% of thepossible maximum values have been chosen.2 The computer code PRODOS (Probabilistic Dose Calculation) was formerly used to make predictionof radiological impact in the environment resulting from accidental releases into air and for theassessment of emergency countermeasures after postulated severe accidents inside nuclear powerplants, using input data from the computer code ACARE (Activity in interrelated compartments andrelease to the environment; cf. answer ch. 3.2.6). Both programs were kind of complicated to handleand therefore taken out of use a couple of years ago and are not going to be reactivated.PRODOS calculated probability distributions of consequences to the environment of a plant due to atime depending release of radioactive substances out of this plant using a weather course of a largertime span. The release period was related to each possible weather sequences in the time span.For the calculation of radiation doses the program chain ACARE-PRODOS was replaced by MACCS[Ref].
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16.2 - Severe Accident Analysis (RRC-B) - EDF Hinkley Point

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