TOWARDS A RATIONAL, MODERN SITING METHODOLOGY FOR NEW NUCLEAR POWER PLANTS IN SOUTH AFRICA

Transcription

1 TOWARDS A RATIONAL, MODERN SITING METHODOLOGY FOR NEW NUCLEAR POWER PLANTS IN SOUTH AFRICA E. LAMPRECHT Eskom (Koeberg Nuclear Power Station) Cape Town South Africa Abstract A proposed methodology is presented for the risk-informed determination of appropriate emergency planning (EP) zone sizes for new nuclear power stations in South Africa. The general approach is to estimate the frequency at which the proposed plant would cause the relevant South African National Nuclear Regulator (NNR) intervention levels to be exceeded outside the corresponding EP zones, and to compare this frequency of exceedance to an acceptable cumulative frequency limit. This is essentially an application of a Level 3 PSA where the results from the PSA must comply to prescribed dose limits to an acceptable level of probability. 1. INTRODUCTION South Africa, specifically the Department of Energy (DOE), aims to create approximately 9.6 GW of nuclear-powered electricity generating capacity by 2030 [1], in addition to the existing 1.8 GW Koeberg Nuclear Power Station 1. The need to assess the suitability of sites and technologies for the new-build programme prompted Eskom, the state-owned electric utility company, to investigate and propose siting methodologies for new nuclear power plants. Initially, two broad categories of nuclear reactor needed to be considered, namely third-generation PWRs and the proposed fourth-generation PBMR [2]. Thus, a framework for nuclear siting had to be developed that would not be linked to a particular technology or site. Subsequently, the PWR build programme was delayed and active development of the PBMR was discontinued. However, the nuclear siting work continues, assuming that any new build programme in South Africa will comprise third-generation or similar PWR technologies. At the outset, it was realized that simply emulating the existing Koeberg EP zones (see Table 1) would not be practicable. These were based initially on US practice [3], in accordance with international guidance [4], [5], and were later justified using a combination of risk-informed and deterministic arguments [6], [7]. Draft legislation [8] then out for public comment included a specific requirement for an exclusion zone around any nuclear installation to be established beyond which no evacuation of members of the public would be required in the event of accidents occurring in any new installation. Eskom s interpretation of this requirement was that the utility would, in effect, have to own the Urgent Protective Action (UPZ) around any future nuclear power plant, thus putting a UPZ with an outer radius of 16 km completely out of the question. Comment [e1]: IRP-2010 Comment [e2]: PBMR Comment [e3]: NUREG-0654 Comment [e4]: TECDOC-953 Comment [e5]: EPR-METOD-2003 Comment [e6]: NNR-EPTB-2003 Comment [e7]: EUR Comment [e8]: [## NNR-SITING- REGS-2009 Therefore it was necessary to develop an answer to the following question: is it possible to site a modern nuclear reactor within EP zones significantly smaller than those surrounding Koeberg, while still complying with the letter and spirit of existing South African nuclear regulatory requirements? This paper describes how this answer is being arrived at. 1 Koeberg Nuclear Power Station comprises two identical units, each being a 900 MWe 3-loop Framatome PWR. The station is located inside a nature reserve on the west coast of South Africa, a short distance away from the small town of Melkbosstrand, about 30 km northwest of Cape Town. Koeberg is owned and operated by Eskom, the South African state-owned electric utility company, and has been operational since

2 Table 1: Summary of Koeberg's existing EP zones identifier name Distances* in km EP actions PAZ Precautionary Action 0 5 Pre-emptive evacuation based on in-plant conditions UPZ Urgent Sheltering Protective Evacuation based on in-plant conditions within Action 5 16 approximately 16 hours Planning Iodine prophylaxis Long Term Relocation Protective Food-banning/replacement Action 0 80 (both long-term actions based on environmental Planning monitoring) * Distances are expressed as radii, but although EP zones are roughly circular in contour, they take into account topological and demographic features. 2. DEVELOPMENT OF SITING METHODOLOGY 2.1 Relevant South African nuclear regulatory requirements The current requirements for EP zones, as applicable to Koeberg, were given in their present form by the South African National Nuclear Regulator (NNR) in 2006 [9]. In this regulatory document, the EP zones listed in Table 1 are defined as in Table 2. Note that no specific zone dimensions are prescribed. Comment [e9]: RD-014 Table 2: NNR EP zone definitions (RD-014) identifier PAZ UPZ Definition Where the risk of deterministic effects is sufficiently high to warrant the establishment of plans for the implementation of pre-emptive protective actions based on plant conditions, before a release or shortly thereafter. Where the risk of stochastic effects is sufficiently high to warrant the establishment of plans to implement protective actions based on environmental monitoring or on plant conditions. Where preparations for the effective implementation of protective actions to reduce the risk of deterministic and stochastic health effects from long term exposure to deposition and ingestion must be developed in advance. Within these zones, the NNR nuclear licensees are required to make provisions for the implementation of protective actions based on the corresponding recommended generic intervention levels presented in Table 3.

3 Table 3: NNR generic intervention levels (RD-014) Action Intervention level* URGENT PROTECTIVE ACTIONS (UPZ) Sheltering (for max. 2 days) Evacuation (for max. 7 days) Iodine prophylaxis 10 msv** 50 msv** 100 mgy*** LONGER-TERM PROTECTIVE ACTIONS () Temporary relocation 30 msv in first 30 days** 10 msv in next 30 days** Permanent resettlement 1 Sv in lifetime** Recommended activity limits in foods given in RD-014. Food banning/replacement (cf. [4]) * Intervention levels are averages over the affected part of the population, not the most exposed individuals. However, projected doses should be kept below thresholds for deterministic effects. ** Expressed as avertable whole-body effective dose. *** Avertable committed absorbed dose to the thyroid from radioiodine. The South African Department of Energy drafted and subsequently promulgated new regulations on licensing of sites for new nuclear installations [10], which provided simpler emergency planning zone requirements as given in Table 4, again without prescribed dimensions. The explicit requirement that no evacuation of the public is allowed outside the EZ was modified in promulgated version of the DOE siting regulations, but it was still necessary to be able to justify small EP zones with newgeneration nuclear technologies. Comment [e10]: TECDOC-953 Comment [e11]: ## NNR siting regs, promulgated Table 4: EP zones defined by the DOE (SA) siting regulations as promulgated. identifier EZ* EPZ name Exclusion zone Emergency planning zone Long Term Protective Action Planning DOE regulatory definition [A] radius determined for the purposes of evacuating persons in the event of a nuclear accident. Within the boundaries of that zone or within any erven intersecting with that zone there must be no members of the public resident, no uncontrolled recreational activities, no commercial activities, or institutions which are not directly linked to the operation of nuclear installations situated within this zone, or for which an authorization has been not been granted; [to be] of such size that emergency or remedial measures must be considered where the potential exists that any members of the public may receive more than an annual effective dose of 1 msv due to the source term; where preparations for effective implementation of protective actions to reduce the risk of stochastic health effects from long term exposure to deposition and ingestion must be developed in advance consistent with international standards. * Identifier chosen for reference purposes, and not listed in promulgated version of the Government Notice.

4 2.2 Eskom s initial proposed siting methodology To open a dialogue with the regulator on new siting methodologies, part of the European Utilities Requirements (EUR) [11] was selected as a model for the initial Eskom proposal. For beyond design basis accidents and severe accidents, the EUR document specifies a probabilistic safety target for four environmental impact targets referred to as Criteria for Limited Impact (CLI) that a nuclear installation must fulfil, as shown in Table 5. The probabilistic safety target requires that the cumulative frequency of exceeding the CLI must be less than 1E-6 per reactor year. Comment [e12]: ## EUR Rev. C Table 5: EUR design targets relating to beyond design basis accidents EUR design target No emergency protection action beyond an 800 m site boundary No delayed action beyond 3km No long term action beyond 800 m Limited economic impact Elaboration Emergency protection actions are actions involving public evacuation, based on projected doses up to 7 days, which may be implemented during the emergency phase of an accident, e.g., during the period in which significant releases may occur. Delayed actions are actions involving public temporary relocation, based on projected doses up to 30 days caused by groundshine and aerosol resuspension, which may be implemented after the practical end of the release phase of an accident. Long term actions are actions involving public resettlement, based on projected doses up to 50 years caused by groundshine and aerosol resuspension. Doses due to ingestion are not considered in this definition. No specific definition provided in the EUR document, but subsequent literature [7] states the requirement that economic impact is limited to areas of 30 km2 after 1 month and 10 km2 after 1 year following an accident, and allows for free trading of foodstuffs provided a 5 msv dose to individuals eating contaminated food for 1 year is not reached. Comment [e13]: ## EUR Eskom s initial proposal was based, to some extent, on the EUR zone-sizes and design targets, but in order to comply with regulatory requirements in force at the time (and draft legislation then about to be promulgated) it had to consider different actions and calculation methodologies. Moreover, the proposal had to be technology-independent, and criteria had to be expressed per site and not per reactor. The proposal attempted to take into account the then present and future EP zone definitions, and to set probabilistic design targets in the context of these EP zones. The EP zones, actions and intervention levels that were considered are summarized in Table 6.

5 Table 6: NNR (RD-014) and DOE EP zone definitions, with anticipated EP actions and associated intervention levels corresponding to each zone. NNR RD-014 zone PAZ DOE zone Proposed zone radius 0 to UPZ radius Anticipated EP actions and their associated intervention levels Pre-emptive evacuation based on in-plant conditions, to avoid deterministic effects (thresholds in IAEA EPR-Method-2003). UPZ EZ 800 m Sheltering for a maximum of 2 days, with an intervention level of 10 msv (averted), estimated as the sum of the 2-day projected whole-body effective dose via cloudshine, groundshine, resuspension and deposition on skin and clothing, and the long-term (50-year*) projected whole-body effective dose via inhalation. Evacuation for a maximum of 7 days, with an intervention level of 50 msv (averted), estimated as the sum of the 7-day projected whole-body effective dose via cloudshine, groundshine, resuspension and deposition on skin and clothing, and the long-term (50-year*) projected whole-body effective dose via inhalation. Iodine prophylaxis, with an intervention level of 100 mgy (averted) thyroid dose due to radioiodine, estimated as the 50-year* projected (committed) thyroid dose from the iodine component of the source term in question via the inhalation and resuspension pathways. a. 3 km b. 800 m c. 40 km** Temporary relocation, with an intervention level of 30 msv (averted) in the first 30 days, estimated as the whole-body effective dose via the groundshine and resuspension pathways over this period. Permanent resettlement, with an intervention level of 1 Sv (averted) over a lifetime, estimated as the 50-year* whole-body effective dose via the groundshine and resuspension pathways. Food-banning/replacement, if the NNR generic action levels for foodstuffs are exceeded. EPZ 40 km Defines whether an emergency plan is required and if required, its overall extent. Must consider the implementation of emergency or remedial measures in the area where the potential exists that any members of the public may receive more than an effective dose from the release of 1 msv per year, including ingestion. * This guideline is designed for users of PC-COSYMA, which calculates long-term doses to the fixed committed period of 50 years. ** To be performed as indicated by environmental monitoring, with the possibility of extending beyond the preplanned if necessary.

6 The details of the Eskom proposal were necessarily quite complex, but may be summarized by the following guiding principle: Any proposed nuclear installation must not be capable of giving rise to deterministic effects anywhere more frequently than 1E-6 per site. If this is not achievable for certain technologies, it will be necessary to define a PAZ within the EZ boundary; cause the avertable dose intervention levels for sheltering, evacuation and iodine prophylaxis to be exceeded beyond the UPZ boundary (i.e,. further than 800 m from the reactor) more frequently than 1E-6/y for the site; cause the dose-based intervention level for temporary relocation to be exceeded beyond a radius of 3 km from the reactor more frequently than 1E-6/y per site, and must not cause the dose-based intervention level for permanent resettlement to be exceeded beyond a radius of 800 m from the reactor more frequently than 1E-6/y per site; cause the thresholds for food banning/replacement to be exceeded beyond a radius of 40 km more frequently than 1E-6/y per site; or cause any individual to receive an annual whole-body effective dose exceeding 1 msv, including ingestion, beyond a radius of 40 km from the reactor more frequently than 1E-6/y per site. In order to make use of the above criteria, it is necessary to have a complete PSA (Levels 1 and 2 plus dose vs. distance calculations from Level 3), including an assessment of external event frequencies for the site. The Level 1 and 2 data provide frequencies and source terms for postulate releases, and together with the dose vs. distance calculations from the Level 3, it can be demonstrated whether the combination of a given technology on a particular site complies with the above set of criteria. Work done in the process of compiling this proposal showed that modern reactors can be sited within a relatively small PAZ and UPZ (or EZ, in the DOE definition) if a risk-informed approach similar to the one outlined above is employed for nuclear siting. 2.3 Regulatory position on a EP technical basis for new nuclear installations The NNR has produced a draft EP technical basis position paper for future nuclear installations, which has been distributed for comment and is in the process of being finalised [12]. The following is a summary of preliminary information given in the draft. In some areas, further detail is required and this has been requested in the Eskom review comments. Comment [e14]: ## NNR PP-0014 In line with the promulgated DOE siting regulations, the draft NNR document defines the EP zones previously listed in Table 4 above in terms of generic dose criteria for actions within these zones. Although the NNR provides more detailed information on criteria for specific EP actions, Error! Reference source not found. provides generic criteria for emergency actions within the different EP zones to be used for the preliminary determination of the required zone-sizes. The process of determining EP zone sizes is risk-informed. The preliminary radius of each EP zone is selected such that the cumulative frequency of exceeding the dose criteria for the actions outside that zone shall be less than 1E-6/y. It is also stated as a requirement that a group of reference accidents be selected in line with a generic criterion, and further optimized below a reference level of 50 msv (effective dose, residual) as part of the protection strategy, although further clarity is required on this point. Although the NNR position is not yet final, it is clear that a risk-informed siting approach is being developed, and that modern reactor technologies will be sited within EP zones, smaller (this will

7 depend on the site specific analysis) than those which currently surround the Koeberg Nuclear Power Station, commensurate with the risk posed by the facility. Table 7: NNR Draft EP technical basis document criteria for preliminary determination of EP zone sizes EP zone ID EZ Generic dose criteria (projected effective) for actions, for use in determining zone sizes Acute external exposure (<10 h) and internal exposure from acute intake (30 days) criteria for the avoidance of deterministic effects. 100 msv in first 7 days Example actions Sheltering, evacuation, decontamination, food restrictions, contamination control, public reassurance overall EPZ 100 msv in 1 year 1 msv per year Temporary relocation, decontamination, food-replacement, public reassurance Preparations for iodine prophylaxis, with an action level of 50 msv equivalent thyroid dose in the first 7 days. 3. CONCLUSION Risk-informed siting methodologies are currently in development in South Africa that allow for nuclear facilities to be sited on smaller EP zones than those that currently surround the Koeberg Nuclear Power Station, commensurate with the reduced risk posed by more advanced technologies. These developments assist in making nuclear energy a viable future contributor to South Africa s electricity generation capacity. REFERENCES: [1] Electricity Regulations on the Integrated Resource Plan (South Africa, Department of Energy: Government Notice R.400, Regulation Gazette No. 9531, Government Gazette Vol. 551, No , 2011) [2] Details on the PBMR project can be found on the website of PBMR (Pty) Ltd. Comment [e15]: IRP-2010 Comment [e16]: PBMR [3] NUREG 0654 FEMA-REP-1 (Rev 1), Criteria for Preparation & Evaluation of Emergency Response Plans, including Supplement 3 and Addenda. [4] IAEA-TECDOC-953, Method for the Development of Emergency Response Preparedness for Nuclear or Radiological Accidents, IAEA,1997 [5] EPR-METHOD-2003, Method for Developing Arrangements for Response to a Nuclear or Radiological Emergency, IAEA, 2003 [6] The NNR Report on the Technical Basis for Emergency Planning at the Koeberg Nuclear Power Station, National Nuclear Regulator (SA), July 2000 [7] EUR-23280, Risk Informed Support of Decision Making in Nuclear Power Plant Emergency Zoning, European Commission Joint Research Centre, Institute for Energy, 2008.

8 [8] Invitation for the Public to Comment on Proposed Draft Regulations on the Siting of New Nuclear Installations (South Africa, Department of Minerals and Energy National Nuclear Regulator Act, 1999, Government Notice 914, Government Gazette No , 3 July 2009) [9] RD-014, Emergency Preparedness and Response Requirements for Nuclear Installations, Rev. 0, National Nuclear Regulator (SA), 2005 [10] The Regulations on Licensing of Sites for New Nuclear Installations (South Africa, Department of Energy, Government Notice R927, Government Gazette No , 11 November 2011) [11] British Energy, et al, European Utility Requirements for LWR Nuclear Power Plants, [12] PP-0015, Emergency Planning Technical Basis for New Nuclear Installations Rev. 0 (DRAFT), National Nuclear Regulator (SA), 2012