Level 3 PSA - EDF Hinkley Point

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HPC-NNBOSL-U0-000-RES-000028 Version 1.0 Level 3 PSA NOT PROTECTIVELY MARKED The annualised probability of death of the most exposed individual due to accidents is then given by: where F Min[1.0, 510 i 5 D ] i F i D i = the frequency of an individual receiving an effective dose in dose band i = magnitude of the dose in mSv and the summation is taken over the five dose bands. To give a conservative result D i is taken as the effective dose at the upper limit of the dose band except for dose band 5 where a unit probability of individual death is assumed 1 . Applying the above equation, with the data in Table 17, results in a risk of individual death of 2.8x10 -7 /ry, which is considered pessimistic. If the risk from a single unit is doubled [AS-001] to take into account HPC is intended to be a twin unit site then this risk becomes 5.6x10 -7 per year. Both the risk from an individual unit site and the risk from a twin reactor site meet the target set by Safety Design Objective SDO-6. 4 ASSESSMENT OF SOCIETAL RISK One of the goals of the EPR design philosophy is to reduce the frequency of releasing substantial amounts of radioactivity into the off-site environment, compared to the current generation of PWRs. Consideration of the frequency of accidents leading to such large releases is an appropriate way to assess societal risk. SDO-8, set out in the NSDAPs [Ref. 1], sets a target of 1.0E-07 per year for the frequency of exceeding 100 immediate or eventual fatalities in members of the public. SDO-8: The total predicted frequency of on-site accidents resulting in more than 100 fatalities (either immediate or delayed) of members of the public will be below 1x10-7 per year and/or demonstrated as ALARP The Level 3 PSA identifies those accident sequences with the potential to result in environmental source terms sufficient to cause 100 or more fatalities in the wider population, both immediate and eventual, taking appropriate responses into account. The likelihood of early radiation induced fatalities is very low. The majority of the off-site fatalities are ‘statistical deaths’ arising from integrating low doses over very large populations over a significant period of time. These calculations are performed with a probabilistic accident consequence assessment system called PC COSYMA. This model has been updated for use in the UK by the Health Protection Agency (HPA). The software calculates the dispersion and deposition of radioactive material for a given source term using a segmented Gaussian Plume dispersion model. It produces a probability distribution of results based upon a sample of historical meteorological conditions. The detailed methodology is set out in [Ref. 17] and the PC COSYMA calculations and analysis are set out in [Ref. 18]. 1 It should be noted that this is slightly different from the approach taken in the Worker Risk Assessment presented in section 5 where the mid-point of the dose band is used in the calculation to obtain a less conservative result. UNCONTROLLED WHEN PRINTED NOT PROTECTIVELY MARKED Page 16 of 60
HPC-NNBOSL-U0-000-RES-000028 Version 1.0 Level 3 PSA NOT PROTECTIVELY MARKED 4.1 Input Data 4.1.1 Source Terms There are two stages of source term input data used to assess Societal Risk. The first stage is the extraction of data from the Level 2 PSA, and then the second stage is to convert that data into meaningful and representative input data for PC COSYMA. The Level 2 PSA [Ref. 19] identified a number of Release Categories, 25 of which have been analysed in this work – Dose Band 3 releases are assumed to result in zero probability of 100 deaths. Two additional NCD (Non Core Damage) accidents identified in the level 1 PSA have also been analysed, details given in [Ref. 17 and Ref. 18]. The NCD accidents are LOCAs (Loss of Coolant Accidents) with clad failure and containment bypass and release duration of 100 hours. Each NCD accident is assigned to a Release Category for analysis within the Level 3 PSA. The Release Category containing the NCD accident with 10% clad failure is assigned to Dose Band 5 and the Release Category containing the NCD accident with 1% failure is assigned to Dose Band 4. The data used for the PC COSYMA calculation is split into 11 groups of isotopes, these 11 groups are derived using the Level 2 PSA Modular Accident Analysis Program (MAAP) Release Fractions, which are split into 13 groups of isotopes. The conversion is described in Appendix B6 of the Societal Risk Assessment [Ref. 18]. The Iodine Release Fractions required for PC COSYMA are also derived from the Level 2 PSA MAAP Release Fractions, with the conversion also described in Appendix B6 of the Societal Risk Assessment. A constant energy and uniform rate of release are assumed over the duration of the release. A maximum of six phases are used to model the total duration, with each phase lasting one hour. The phases are not necessarily consecutive and are evenly spaced over the duration of the release. Details about the releases for each RC considered can be found in the Societal Risk Assessment [Ref. 18] and Methodology [Ref. 17] At this time some site specific buildings are not yet fully considered within the PSA. Most of these buildings are intended to contain limited amounts of potentially radioactive material. The only buildings not currently considered within the PSA that are likely to cause an increase in risk to the probability of 100 fatalities are the Interim Spent Fuel Store (ISFS) and the Intermediate Level Waste Store (ILWS). Future actions to close this gap are identified on the Forward Work Plan [Ref. 20]. 4.1.2 Meteorological Data Two years worth of hourly historical meteorological data supplied by the Met Office has been used [AS-002]. The model uses wind speed, wind direction, rainfall, atmospheric stability category and the depth of the mixing layer. A cyclic sampling scheme is used to sample the meteorological data to generate 141 sequences for each source term calculation. The wind direction can change with each phase of release. Each phase of material continues to travel in a constant direction once released, but the different phases can be dispersed in different directions depending on the wind direction at the start of each phase. UNCONTROLLED WHEN PRINTED NOT PROTECTIVELY MARKED Page 17 of 60
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