Progress in Nuclear Energy 120 (2020) 103239 Contents lists available at ScienceDirect Progress in Nuclear Energy journal homepage: http://www.elsevier.com/locate/pnucene Radiological risk assessment of a proposed site for a generic VVER-1000 using HotSpot and InterRas codes K. Gyamfi a,c,*, S.A. Birikorang b,c, E. Ampomah-Amoako b,c, J.J. Fletcher c a Ghana Atomic Energy Commission, National Nuclear Research Institute, P. O. Box LG 80, Legon, Accra, Ghana b Nuclear Regulatory Authority, Ghana, P. O. Box AE 50 Atomic Energy, Accra, Ghana c Graduate School of Nuclear and Allied Sciences, University of Ghana, Legon, Accra, Ghana A R T I C L E I N F O A B S T R A C T Keywords: An assessment of radiological risk of a proposed site for a generic VVER 1000 MW nuclear power plant has been Nuclear power plant conducted using international radiological assessment system (InterRAS) code and HotSpot Health Physics Code Accident in view of Ghana’s plan to add nuclear energy to her energy mix. The radiological risk assessment was estimated Site selection by considering a hypothetical accident event for a generic VVER 1000 MW at the proposed site. The kind of Risk assessment Total effective dose equivalent radionuclides to be release from the fuel meat to the gap between the meat and the clad defined the intervention measures and countermeasures. The available radionuclide for the hottest fuel rod was determined by depleting the core, a method sometimes termed as “source term estimation”. The direction of trajectory and Total Effective Dose Equivalent (TEDE) received with the corresponding ground deposition of the released radionuclides were estimated. The right protective actions were determined by estimating the appropriate intervention distances. The maximum TEDE calculated were 3.7 � 10 1 Sv at 0.1 km and 3.7 � 10 1 Sv at 0.18 km for InterRAS and HotSpot codes respectively towards the north-east of the release point. Radiological doses of 10 mSv and above was limited to 1.0 km from the point of release. The intervention level for evacuation (50 mSv) ends at 0.5 km for InterRAS code and 0.7 km for HotSpot code. The intervention level for sheltering (10 mSv) also ended at 1.5 km for both InterRAS and HotSpot code. The highest total radionuclide ground deposition was estimated to be approximately 4.0 � 106 kBq m 2 at 0.1 km and 3.8 � 106 kBq m 2 at 0.18 km for InterRAS and HotSpot respectively. Beyond 5.0 km distance, the ground deposition was in the range of 0.1–1 kBq m 2. Generally, the estimated annual effective dose for the public was less than the 1 mSv limit, which is the annual allowable limit for the public. Therefore, with respect to the outcome of the estimated results, there wouldn’t be any radiological risk above the allowable limit, hence the site can be considered as the candidate site for the construction of the proposed nuclear power plant. 1. Introduction sustainability being the key for the development of every country. The total generation of electricity from nuclear energy worldwide keeps For developing nations like Ghana, the need for reliable, cost effec- increasing yearly and more countries are showing the interest to become tive, and environmentally friendly source of energy is more funda- nuclear countries (WNA, 2018: Shamsuddin et al., 2017). mental. Such kind of energy system can boost the economic indexes and With all the benefits of nuclear power and it role in decarbonizing even save lives. The country needs reliable energy to expand its industry, energy mix that helps in limiting global warming there are other chal- mechanise agriculture, increase trade and improve transportation sys- lenges that are very critical. Some of the major challenges facing the tem. These have been the core ideas of the country, as such Ghana has nuclear industry include dealing with public acceptance, ensuring decided to add nuclear energy to its energy mix (Adu-Gyamfi et al., environmental safety against radiation, protecting reactors from natural 2017; Birikorang et al., 2012). A stable energy resource like nuclear disasters and external aggression and finding effective solutions for energy plays a very significant role as a major contributor to energy long-term waste management. Construction of new nuclear power plant * Corresponding author. Ghana Atomic Energy Commission, National Nuclear Research Institute, P. O. Box LG 80, Legon, Accra, Ghana. E-mail addresses: k.gyamfi@gaecgh.org (K. Gyamfi), s.birikorang@gnra.org.gh (S.A. Birikorang), e.ampomah-amoako@gnra.org.gh (E. Ampomah-Amoako), fletcherjjf@yahoo.com (J.J. Fletcher). https://doi.org/10.1016/j.pnucene.2019.103239 Received 21 August 2019; Received in revised form 11 December 2019; Accepted 30 December 2019 Available online 7 January 2020 0149-1970/© 2020 Elsevier Ltd. All rights reserved. K. Gyamfi et al. P r o g r e s s i n N u c l e a r E n e r g y 120 (2020) 103239 demands critical and cautious assessment before siting the plant due to shows the various parameters of the generic VVER reactor under study. its complexity. The environment and the atmosphere could adversely be affected by the emission of fission products as a result of radiological 2. Materials and methods releases from nuclear power plant (NPP) accident. (Van Dorsselaere et al., 2011). Nuclear energy acceptance will depend on how these HotSpot Health Physics Code and International Radiological challenges are met. Assessment System (InterRAS) are the two main codes used in this study. Safety is a critical feature during NPP’s sitting to ensure that the The codes are field-portable set of software tools for evaluating incidents public and the environment will be in a safe condition in case of acci- involving radioactive material. They are simple model codes which are dents. It is a requirement by law in Ghana that a detailed radiological run on personal computers covering the entire path through the envi- and environmental assessment be carried out regarding site selection ronment, including the food and water pathway, and covering essen- preceding the construction. This requirement aims to avoid any unjus- tially a lifetime of exposure to a contaminated environment. The codes tifiable exposure during any accident. (Nuclear Regulatory Authority, are used by health physics personnel, emergency response personnel and 2015). In assessing the radiological risk in NPP’s operations, one critical emergency planners. This makes HotSpot and InterRAS codes appro- issue is to estimate radiation dose and means of demonstrating how priate codes for estimation of radiological consequence assessments for radiation dispersion of released radionuclides during radiological acci- the propose site for nuclear power plant construction in Ghana. dent can be addressed. This is done mostly to fulfil the nuclear safety objective (IAEA, 2003). This assessment which needs to be done to aid 2.1. HotSpot Health Physics Codes site selection and the subsequent construction is the main research objective of this study. The HotSpot Health Physics Code or HotSpot program, provides a As Ghana is making all the needed effort to include nuclear energy in first-order approximation of the radiation effects associated with the its energy generation mix, it is important to evaluate the impact NPP will atmospheric release of radioactive materials. The HotSpot program was have on the environment before the start of construction. It is also a way created to equip emergency response personnel and planners with a fast, of identifying the candidate site for the construction. Assessment of field-portable set of software tools for evaluating incidents involving possible environmental impact using design-basis (DB) accident radioactive material. The software is also used for safety-analysis of approach is therefore essential. The design-basis accident assessment facilities handling radioactive material. This program is designed for scenario adopted in this study is based on hypothesis. The hypothetical short-range (less than 10 km), and short-term (less than a few hours) accident scenarios used was based on the prediction of loss of coolant predictions. (Homann and Aluzzi, 2013). HotSpot includes atmospheric accident as a result of break in one of the primary cold leg pipelines as dispersion models, the module calculates the 95th percentile of the dose shown in Fig. 1. distribution for up to 20 radial centerline distances in each of 16 wind To evaluate the radiological risk associated with the proposed site, direction sectors (direction dependent), and all 16 sectors (direction the direction of trajectory of the released radionuclides is determined, independent). total effective dose equivalent (TEDE) received from the released ra- dionuclides is calculated, the activity of the deposited radionuclides is 2.2. The International Radiological Assessment System (InterRAS) code calculated and the intervention distances are also estimated for the determination of the right protective actions. The International Radiological Assessment System (InterRAS) was This paper presents an assessment of the radiological risk associated developed by the International Atomic Energy Agency (IAEA). It is with generic VVER-1000 reactor at the proposed site. The VVER-1000 designed to be used in the independent assessment of dose projections design is a 3000 MWth pressure vessel type reactor with four-loop sys- during response to radiological emergencies. The system supplements tem. It has gross electric output of 1000 MW. The Russian word Voda assessments based on plant conditions and quick estimates based on Voda Energo Reactor, abbreviated VVER or WWER stands for ‘water- hand-calculation methods. InterRAS code is mainly used by response water energy reactor’, thus water-cooled water-moderated energy personnel to conduct an independent evaluation of dose and conse- reactor. The VVER plants have proven to be highly reliable over more quence projections. The model was developed to allow consideration of than 1300 reactor-years of operation. The VVER is a pressurized water the dominant aspects of source term, transport, dose, and consequences reactor (PWR), the commonest type of nuclear reactor worldwide, (IAEA, 1997; Sehgal, 2011). employing light water as coolant and moderator similar to PWR. The plant design and materials used has characterized the significant dif- 2.3. The HotSpot and InterRAS algorithms ferences between the VVER and other PWR. (Sangiorgi, 2015). Table 1 The HotSpot and InterRAS codes use the Gaussian dispersion model for all assessments, the model has been widely used and verified in the scientific community and is still the basic workhorse for initial atmo- spheric dispersion calculations. The Gaussian model generally produces results that agree well with experimental data in simple meteorological and terrain conditions, and as a result, has found its way into most governmental guidebooks, and is also used and accepted by the US Table 1 Generic VVER-1000 reactor parameters. Reactor parameter Quantity Reactor power 3000 MW(t) Averaged fuel burn-up 40000 MWD/MTU Containment type VVER 1000 Design pressure 414 kPa Coolant mass 2.60 � 105 kg Assemblies in core 151 Containment volume 7.08 � 104 m3 Fig. 1. Schematic diagram depicting the break in the Cold leg. Design leak rate 0.25%/d 2 K. Gyamfi et al. P r o g r e s s i n N u c l e a r E n e r g y 120 (2020) 103239 Environmental Protection Agency. 2007). The source term calculated by the InterRAS code was used in Plume shape and spread vary in response to meteorological condi- developing a source term input deck for the HotSpot code to allow for tions. The general Gaussian dispersion equation that calculate the steady good comparison between the results of the two codes under the similar state concentration of air contaminant in the ambient air resulting from condition for the source term used. a point source is given by: The activity of each radionuclide present in the source term as esti- " !# mated by the InterRAS code is presented in Table 2. The total activity of χ Q y 2 ðx; y; z; HÞ¼ exp the source term was 1.11 � 10 16 Bq. 2πμσyσz 2σ2y The release fraction adopted was based on PWR core inventory � � 2� � 2�� ðz HÞ ðzþ HÞ fraction released into containment as recommended in Regulatory Guide exp þ exp 2σ2 2σ2 1.183 (2000). This was applied to radionuclides assumed to have z z adverse health consequences. where, H - is the Height of the plume ½m�, 3. Results and discussion σy and σz - are respectively horizontal and vertical deviations of plume concentration distribution ½m�, 3.1. Release trajectory of radionuclides � � Q - is the uniform emission rate of pollutants kgs , The simulation of InterRAS and HotSpot computer codes was carried x - Along-wind coordinate measured in wind direction from the out by using the site specific meteorological conditions, the generic source ½m�, VVER 1000 plant parameters and the source term generated. Most tra- y- Cross-wind coordinates direction ½m�, jectory models produce uncertainty results with regard to starting phase z - Vertical coordinate measured from the ground ½m�, and the height of the tracer. With the help of atmospheric dispersion χðx; y; zÞ - Mean concentration of diffusing substance at a point model most of these uncertainties has been reduced by improving on the � � kg accuracy of detective technology. The simulated results generated by the ðx; y; zÞ m3 and codes show that the radionuclide emission moves towards the north-east h i direction as depicted in Figs. 2 and 3. μ - Mean wind velocity affecting the plume along the x - axis ms The areas that falls within the plume of the radionuclide emission include parts of the Western region of Ghana, which is from Axim to- 2.4. Site meteorological conditions wards Isakro Township as shown in Fig. 4. The proposed site chosen for this study is Axim which is a coastal area located in the western Region of Ghana with an elevation of 38 m, 3.2. Calculation of TEDE doses lying on Latitude 4.86992 north and Longitude 2.24046 west. A 10 year period (2008–2017) meteorological data was used for this study. The HotSpot and InterRAS codes were able to calculate the doses The meteorological data of Axim indicate annual average wind speed of (TEDE) within different stability classes as shown in Figs. 5 and 6. The 1.5 m s 1 with an annual average temperature of 27.3 �C with no pre- doses decreases with increasing wind speed and vice versa due to inverse cipitation. The wind direction is mostly south-western. The principal relationship between the estimated dose/concentration and the mean atmospheric stability class that pertains in Axim is stability class A. The wind velocity in the Gaussian equation. atmospheric stability class defines the terms of tendency of a parcel of More so, at constant wind speed, the maximum dose (TEDE) de- air that moves upward or downward after it has been displaced verti- creases as one transitions from an unstable stability class to a more cally by a small amount. Essentially, unstable atmospheres of stability stable stability class, thus from Stability class A to Stability class F. class A tend to develop vertical updrafts which increase boundary-layer turbulence intensity (Zoras et al., 2006; Arya, 1999; Wart et al., 1998). Table 2 Accident source term for the generic VVER-1000. 2.5. Source term and accidental release scenario Nuclide Activity Nuclide Activity Nuclide Activity (Bq) (Bq) (Bq) The InterRAS code has the capability of calculating the source term Ba- 5.98Eþ13 Mo-99 7.15Eþ12 Te-127 7.56Eþ12 for the type of reactor selected for the analysis. The InterRAS code 137m source term estimations for nuclear reactor accidents analysis are Ba-140 2.25Eþ14 Np-239 1.03Eþ14 Te- 7.72Eþ09 127m dependent largely on the source term estimation methods as discussed Ce-144 5.78Eþ12 Pr-144 5.74Eþ12 Te-129 4.76Eþ13 by McKenna and Glitter (1988) in US Nuclear Regulation Commission Cs-134 1.06Eþ14 Pr-144m 1.03Eþ11 Te- 1.94Eþ13 Guide-1228 (NUREG-1228). The method selected for the simulation of 129m the InterRAS code was based on the assumption that the release was Cs-136 1.89Eþ13 Pu-239 2.77Eþ06 Te-131 8.11Eþ12 Cs-137 6.82Eþ13 Rb-88 2.76Eþ14 Te- 3.60Eþ13 started when there was stop of flow of coolant to the reactor vessel 131m exposing the fuel, thus ‘the time the core was uncovered’. This method Cs-138 5.28Eþ11 Rh- 5.32Eþ12 Te-132 4.03Eþ14 could be the most powerful and important source term type that Inter- 103m RAS uses. The hottest fuel rods was assumed to contain nearly all of the I-131 4.69Eþ14 Rh-106 2.60Eþ12 Xe- 2.67Eþ13 radioactivity at an NPP. Mostly a large release is possible when many 131m I-132 6.18Eþ14 Ru-103 5.38Eþ12 Xe-133 4.58Eþ15 fuel rods are substantially damaged. The surest way this can realistically I-133 7.66Eþ14 Ru-106 2.60Eþ12 Xe- 1.56Eþ14 occur is by loss of water from the primary coolant system, thus rendering 133m the core of the reactor uncovered by water. Most of the parameters used I-134 8.88Eþ12 Sb-127 2.04Eþ13 Xe-135 1.51Eþ15 in the simulation are described in Table 1. I-135 4.03Eþ14 Sb-129 3.46Eþ13 Xe- 1.91Eþ14 135m When one inputs the duration that a reactor core is left uncovered Kr-85 3.42Eþ13 Sr-89 1.33Eþ14 Xe-138 3.39Eþ09 with water, the InterRAS code quantify the amount of core damage that Kr-85m 2.13Eþ14 Sr-90 1.15Eþ13 Y-90 3.01Eþ11 is likely to occur and as a result estimate the activity of each fission Kr-87 3.79Eþ13 Sr-91 9.05Eþ13 Y-91 8.10Eþ12 product nuclide that will be released from the core. The core damage Kr-88 3.36Eþ14 Tc-99m 7.02Eþ12 Y-91m 3.75Eþ13 estimation is based on the damage timings for PWRs (McGuirea et al., La-140 1.85Eþ13 3 K. Gyamfi et al. P r o g r e s s i n N u c l e a r E n e r g y 120 (2020) 103239 stability class A with a corresponding wind speed of 1.5 m s 1. It was observed that the TEDE increases till it peaks, after which it decreases as one moves farther away from the point of release for both codes as can be seen from Fig. 7. Notwithstanding, the maximum dose was estimated to be within 0.2 km from the release point for both codes. The estimated maximum TEDE were 3.70 � 10 1 Sv at 0.1 km and 3.69 � 10 1 Sv at 0.18 km for InterRAS and HotSpot respectively. The doses decreases sharply along the downwind distance after the maximum dose point. The TEDE levels beyond 4.0 km from the release point are less than 1 mSv. Based on the levels of doses shown by the results of this study and the ICRP 103 guidelines (ICRP, 2008), the following analyses were made: a. The dose level within the region of release as a result of the accident were less than 1 mSv, especially areas that are 4.0 km away from the release point within the path of the emission; b. Moreover some areas will be exposed to dose level of less than 0.1 mSv and within the ranges of 0.1 mSv–1 mSv, 1 mSv–10 mSv and 10 mSv–100 mSv as presented in Fig. 8; Fig. 2. Coordinate system showing Gaussian distributions in the vertical and c. Since dose less than 1 mSv can hardly have a detrimental effect on horizontal direction. human health and the environment, such levels are not considered in most assessment and do not need any countermeasures; The maximum TEDEs and the downwind distances where they were d. In such cases, the operators determines the best countermeasures recorded by both codes were very comparable though varied slightly basing the measures on the site regulation documents and seeking due to difference in sigma values coded in both codes. Unstable stability approval from the competent State authority class recorded higher doses than more stable stability class due to the fact that there are better atmospheric mixing within unstable stability Doses of 10 mSv and above will be limited to 1.0 km from the point of class than more stable stability class. release which is expected to be within the vicinity of the power plant or The fundamental factor which yielded the greatest radiological the buffer zone where public occupation is restricted. More so, for dose impact (worst case scenario) with respect to the prevailing meteoro- levels above 10 mSv, there exist a link between radiation and cancer risk logical condition was identified to be stability class A at lower wind according to ICRP 96 (Valentin, 2005). Considering the level of exposure speed. The site meteorological condition coincidentally follows the likely to be received by staff and emergency workers within the 1.0 km worst case scenario, therefore narrowing the research work findings on distance and their risk to cancer, much effort will be needed in reducing Fig. 3. Dose footprint of TEDE from the release of radionuclides showing the direction of the propagation (InterRAS code). 4 K. Gyamfi et al. P r o g r e s s i n N u c l e a r E n e r g y 120 (2020) 103239 Fig. 4. Plume contour TEDE from the release of radionuclides showing the direction of the propagation (HotSpot). Fig. 5. Plume contour showing areas that falls within the path of the radio- nuclide emission. Fig. 7. Maximum TEDE for Stability Classes at varying wind speed (InterRAS). Fig. 6. Maximum TEDE for Stability Classes at varying wind speed (HotSpot). Fig. 8. TEDE (Sv) as a function of downwind distance from the release point in stability class A. 5 K. Gyamfi et al. P r o g r e s s i n N u c l e a r E n e r g y 120 (2020) 103239 the exposure. In such cases all forms of principles of radiation protection methods should be employed. In this study, there is a conservation assumption of 1% probability of risk to cancer for dose levels above 100 mSv. Therefore real numbers could possibly be higher than as presented. Even though, the levels of dose to the public is expected to be insignificant, the total affected area is likely to cover a sizeable area which would predominantly be occupied by facility workers. Measures should be put in place to reduce the level of exposure as low as reasonably achievable. Monitoring and assessment of individual expo- sure should periodically be performed and made available for radio- logical emergency planning coupled with regular training in radiation measures. 3.3. Ground deposition of released radionuclide The ground deposition of the released radionuclide impacted greatly Fig. 10. Activity footprint of Surface concentration from the release of radio- within 1.0 km from the release point. The highest total radionuclide 6 2 nuclides (InterRAS). ground deposition was estimated to be 4.0 � 10 kBq m at 0.1 km and 3.8 � 106 kBq m 2 at 0.18 km for InterRAS and HotSpot code respec- tively on the north-east direction along the plume trajectory. The ac- tivity decreases along the downwind distance. For both codes, the activity values rises and peaks then starts a sharp decrease till it reaches very low levels as shown in Figs. 9 and 10. From 2.0 km onwards, the activity values for both codes are almost the same as illustrated in Fig. 10. The top ten radionuclide according to activity contribution for the highest ground deposition of 4.0 � 106 kBq m 2 at 0.1 km from the release point is shown in Fig. 11. Iodine-131 (I-131) has the highest contribution of 2.8 � 105 kBq m 2 in terms of activity. Beyond 5.0 km, the activities decrease significantly. Based on the outcome of results, surrounding water bodies, planta- tion and other terrestrial environment within the 5.0 km distance will experience minimal effects from the accident since the highest contributor, thus, I-131 has a half-life of 8 days. Thus, in 8 days, half the initial activity of I-131 will decay. It will decay from the ground after few half-lives within a short period. Cs-137 isotope with half-life of Fig. 11. Surface Deposition concentration as a function of downwind distance 30.17 years which is also potentially harmful when it enters the human from the release of radionuclides. body has less activity that will only have negligible effect on the envi- ronment. Beyond the 5.0 km distance, the ground deposition is in the 3.4. Estimation of intervention distances range of 0.1–1 kBq m 2. Though these levels of concentration do not pose much harmful effect, putting in place measures to monitor radia- In accordance with IAEA Safety Standards GS-R-2 (IAEA, 2011), sizes tion of the water bodies, plantation and soil in the areas that are within of emergency planning zones which are dependent on the estimated the ground deposition plume contour line is very important. intervention distances are to be defined in the course of site assessment of NPP. The intervention level for evacuation (50 mSv) and sheltering (10 mSv) were achieved at slightly varied distances for both code as seen in Fig. 8. The intervention level for evacuation (50 mSv) ended at 0.5 km for InterRAS and 0.7 km for HotSpot code. Again, the intervention level for sheltering (10 mSv) ended at 1.5 km for both InterRAS and HotSpot code. The outcome can be a way of helping in the determination of the emergency planning zones without any trans-boundary issues. It can also lessen the cost that may be incurred during possible evacuation process. 4. Conclusion The radiological risk assessment for a proposed site for a generic VVER 1000 MW is conducted using HotSpot and InterRAS codes. The direction of trajectory was observed to be towards the north-east from the point of release. Doses were estimated at various distances and it was realised that the doses decreases significantly with increasing distance from the point of release. The maximum TEDE calculated were 3.7 � 10 1 at 0.1 km and 3.7 � 10 1 at 0.18 km for InterRAS and HotSpot codes respectively. Radiological doses of 10 mSv and above was limited Fig. 9. Dose footprint of TEDE from the release of radionuclides showing the to 1.0 km within the vicinity of the plant or the buffer zone where no levels of doses at different points (InterRAS code). public occupation is allowed. The radionuclide concentration beyond 6 K. Gyamfi et al. P r o g r e s s i n N u c l e a r E n e r g y 120 (2020) 103239 4.0 km within the path of the emission was less than 1 mSv. Therefore Arya, S.P., 1999. Air Pollution Meteorology and Dispersion, vol. 6. New York: Oxford the public will not receive dose level more than 1 mSv which is the University Press, USA. Birikorang, S.A., Gbadago, J.K., Akaho, E.H.K., Nyarko, B.J.B., Ampomah-Amoako, E., annual allowable dose limit for the public. It was realised that the Odoi, H.C., Abrefah, R.G., Debrah, S.K., Sogbgaji, R.B.M., Boafo, E., Boffie, J., 2012. intervention level for evacuation (50 mSv) and sheltering (10 mSv) were Prospect of nuclear power today as part of Ghana energy mix and socio-economic achieved at slightly varied distances for the two codes. The intervention development. Environ. Res. Eng. Manag. 60 (2), 67–76. 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