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Consideration of Off-site Emergency Planning and Response using Probabilistic Accident Consequence Assessment Models M. Kimura, J. Ishikawa and T. Homma.

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Presentation on theme: "Consideration of Off-site Emergency Planning and Response using Probabilistic Accident Consequence Assessment Models M. Kimura, J. Ishikawa and T. Homma."— Presentation transcript:

1 Consideration of Off-site Emergency Planning and Response using Probabilistic Accident Consequence Assessment Models M. Kimura, J. Ishikawa and T. Homma Nuclear Safety Research Center Japan Atomic Energy Agency (JAEA) NREP Conference, April 22, 2009

2 Background IAEA Safety Requirements (GS-R-2, 2002) Safety Guide (GS-G-2.1, 2007) Management approach Practical goals (8 goals) Prevent the occurrence of deterministic health effects Prevent, to the extent practicable, the occurrence of stochastic health effects Intervention principles Efficient and cost-effective system ICRP (Publ.103, 2007) Optimization of overall strategy Japan (NSC’s Guideline on emergency preparedness) No practical guidance for protective measure strategy As you know, it is recognized that good preparedness in advance of an emergency can substantially improve the emergency response to an accident. Recently, international organizations published very important documents in the field of nuclear emergency preparedness and response. IAEA provided safety standards series for preparedness and response for a nuclear or radiological emergency, such as safety requirements GS-R-2 and safety guides GS-G-2.1. In these documents, a management approach was used, in which the practical goals are first established, and based on the intervention principles, a response strategy is formulated, from which detailed planning and response requirements are derived. Then, the most efficient and cost-effective system is developed to meet those requirements. This approach clearly emphasizes planning to respond effectively to achieve the goals, rather than simply responding to a situation. Also, recently the ICRP new recommendation, Publication 103 was published, in that ICRP recommends focusing on optimization with respect to the overall strategy, rather than the individual protective measures. In Japan, Nuclear Safety Commission provides the Guideline on emergency preparedness for nuclear facilities. First, it was established after TMI accident, then several revisions were made, in particular, after Tokai-mura criticality accident in 1999. This guideline provides several numerical criteria, for example, Emergency Planning Zone for each nuclear installation and intervention levels for each protective measure such as sheltering, evacuation, administration of stable iodine, and food restrictions. However, this document does not provide a practical guidance for developing the strategy of protective measures.

3 Objectives Perform a risk informed analysis of protective measures using Level 3PSA code (OSCAAR), in order to formulate the technical basis for the effective strategy of protective measures Introduce a metabolic model of iodine to accurately evaluate the effect of reducing thyroid dose by administration of stable iodine Combination of protective measures, sheltering and evacuation with administration of stable iodine To develop the overall response strategy, it is very important to provide technical guidance for the development of protective measures, based on a comprehensive threat assessment that takes into account the full range of postulated events. Therefore, probabilistic accident consequence assessment models can be very useful for providing a quantitative basis for discussing the effective emergency plans including intervention levels and emergency planning zones for the appropriate protective actions. In this study, in order to formulate the technical basis for the effective protective action strategy, we have performed the consequence calculations with a probabilistic accident consequence assessment code, OSCAAR originally developed by JAEA (the former Japan Atomic Energy Research Institute). First, we introduced a metabolic model of iodine with a probabilistic accident consequence model to accurately evaluate the effect of reducing thyroid dose by administration of stable iodine. And then, for the representative source term derived from the level 2 PSA, the preliminary analysis has been performed using OSCAAR to evaluate the effectiveness of protective action strategy involving a combination of evacuation, sheltering and administration of stable iodine.

4 Accident Consequence Model
OSCAAR (Off-Site Consequence Analysis of Atmospheric Releases of radionuclides) CHRONIC Atmospheric dispersion Protective measure Source term MS Chronic exposure ADD EARLY PM ECONO Population Agricultural data HE Health effect HEINPUT HINAN CURRENT DOSDAC Economic loss Early exposure Meteorological sampling Deposition Meteorological data At first, I’m briefly introducing the Level 3 PSA code OSCAAR. This slide shows the schematic structure of OSCAAR code. OSCAAR consists of a series of interlinked modules and data files, which are used to calculate the atmospheric dispersion and deposition of selected radionuclides for all sampled weather conditions, and the subsequent dose distributions and health effects in the exposed population. ADD module calculates the advection and diffusion of radioactive substance in the atmosphere and the deposition onto the ground. EARLY module calculates short-term dose to the public during or after plume passage. CHRONIC module calculates long-term dose caused by long half-life radionuclides which are deposited in the ground. PM module calculates the effect of dose reduction by emergency protective action. HE module estimates the health effect for exposure. ECONO module estimates the economic impact for the nuclear accident. Several stand-alone preprocessor codes can be used to prepare, in advance, input data for OSCAAR such as meteorological sequences, dose conversion factors, population and agricultural product distributions, and lifetime risks for the exposed population. OSCAAR module Preprocessor code

5 Models of OSCAAR Atmospheric Dispersion and Deposition
Multi-puff trajectory model with two scale wind fields Take account of temporal changes in weather conditions and variable long duration releases Meteorological Sampling Stratified sampling scheme appropriate for use with the trajectory dispersion model was designed and developed Select a representative sample of weather sequences for analysis Exposure Pathways Realistic estimates for all possible exposure pathways with protective measures with simple models such as sheltering, evacuation, relocation and food ban Shielding and filtering factors for sheltering Radial evacuation Additionally, I explain characteristics of models in OSCAAR code. The atmospheric dispersion and deposition calculations are based on a multi-puff trajectory model that can take into account the temporal changes in weather characteristics and variable long duration releases. In probabilistic consequence assessments, the atmospheric dispersion and dose calculations must be repeated for a large number of sequences of meteorological conditions to predict the full distribution of consequences that may occur. For this purpose, the stratified sampling scheme appropriate for use with the trajectory dispersion model was designed and developed. In order to estimate received dose realistically, OSCAAR calculates it for all possible exposure pathways such as cloud shine, ground deposition and inhalation and considers the effect of dose reduction by protective measures with simple models such as sheltering, evacuation, relocation and food ban. For example, shielding and filtering factors for sheltering and radial evacuation.

6 Metabolic Model of Iodine
Johnson model (1981) Reduction in the committed thyroid dose to man (for adult, 100mg) GUT or LUNG, Y 1 THYROID, Y 3 ORGANIC IODINE, Y 4 BLADDER EXCRETA INORGANIC 2 s r GUT or LUNG, Y1 THYROID, Y3 IODINE, Y4 IODINE, Y2 0.0 0.2 0.4 0.6 0.8 1.0 -80 -60 -40 -20 20 40 Time (h) before or after an intake of radioiodine (Intake of stable iodine/ No measurement) Reduction factor In this study, we introduced a metabolic model of iodine with OSCAAR to evaluate the effect of reducing thyroid dose by administration of stable iodine. In this case, the metabolic model developed by Johnson was used to calculate the distribution and retention of the radioactive iodine and then calculate the committed dose to thyroid. The advantage of using this model is to take account of radioactive iodine intake as a function of time, and also age dependence of the thyroid dose reduction. This model can also include the effect of stable iodine intakes on radioactive iodine uptakes. The left figure shows the metabolic model of iodine in man used in this study. It basically consists of three compartment models. All of inhaled or ingested iodine is assumed to be absorbed by the blood (inorganic compartment), from which it is either taken up by the thyroid or excreted in urine. Iodine that leaves the thyroid bound to thyroid-produced organic molecules is assumed to be evenly distributed throughout soft tissue (organic compartment). This organic iodine is slowly catabolized to inorganic iodine, or excreted. The model forms a set of coupled differential equations to be solved, with a following weak coupling parameter, which is defined as the rate of radioactive and stable iodine uptake by the thyroid. The right figure shows an example of the thyroid dose reduction as a function of time before and after the intake of iodine-131, in case of the administration of 100 mg of stable iodine for adults. The horizontal axis is the time before or after an intake of radioiodine. And vertical axis is the reduction effect of thyroid dose. For example, 0 hour shows that the man intakes stable iodine as soon as inhales or ingests radioactive iodine. This figure shows the most effective time for administration of stable iodine is before or immediately an intake or ingestion of radioactive iodine. Then, the effect of stable iodine decreases as the delay of intake time. : the rate of uptake of radioactive and stable iodine by the thyroid : the rate constant for uptake or excretion of iodine from each compartment

7 Source Term Development
Release fraction of iodine 環境へのヨウ素の放出割合 BWR5/Mk-II Relative failure frequency (from INS/M03-22) 発生頻度/全格納容器破損頻度 10 10 10 -1 10 -1 10 -2 10 -2 10 -3 10 -3 10 -4 10 -4 (each / all containment failure scenarios) :Relative failure frequency ●: Release fraction of Iodine 10 -5 10 -5 10 -6 10 -6 10 -7 10 -7 10 -8 10 -8 Next, we performed the preliminary analysis using OSCAAR to evaluate the effectiveness of protective action strategy involving a combination of evacuation, sheltering and administration of stable iodine. For selecting the representative source terms for use in the consequence analysis, we have reviewed the Level 2 PSA results by Japan Nuclear Energy Safety Organization (JNES) for a BWR-5 with Mark-II containment. This slide shows the summary of the release fraction of iodine to the environment for various accident scenarios which consist of a specific core damage sequence and containment failure mode with relative failure frequencies. The horizontal axis is the type of core damage sequence and containment failure mode. And vertical axis is release fraction of iodine indicated by black circle and is relative failure frequencies for all containment failure scenarios indicated by white square. It shows that in terms of the fraction of iodine releases to the environment, accident scenarios are classified into three groups, Release fraction is about order of 10-1 corresponds to energetic events, overpressure, ISLOCA. And relative failure frequency is about order of 10-8. Release fraction is about order of 10-5 to 10-4 corresponds to containment vent. And relative failure frequency is about order of 10-3. Release fraction is in order of 10-8 to 10-7 corresponds to termination. And relative failure frequency is about order of 10-3. 10 -9 JAEA evaluation 10 -9 10 -10 10 -10 TB-α TBU-α AE-α TB-μ TBU-μ TB-σ TBU-σ TB-δ TBU-δ AE-δ TW-θ TC-θ V-ν TQUV-α TQUX-α TQUX-μ TQUX-σ TQUV-δ TQUX-δ TB-υ-l AE-υ-l AE-ψ-l TQUV-υ-l TQUX-υ-l TBU-υ-l TQUV-ψ-l TBU-ψ-l  ・10-1 Iodine release fraction :Energetic events, Overpressure, ISLOCA  ・10-5~10-4                  :Containment vent  ・10-7~10-8                  :Termination

8 Source Term Development (Cont’d)
●Release time (hr) □Relative failure frequency (from INS/M03-22) ▲JAEA evaluation ●: Release time (hr) (each / all containment failure scenarios) :Relative failure frequency This slide also shows the summary of the release time for various accident scenarios. The horizontal axis is the type of core damage sequence and containment failure mode. And vertical axis is release time of radioactive substance after scram indicated by black circle and is relative failure frequency for all containment failure scenarios indicated by white square. Judging by the results of this review, we considered that which accident scenarios cover all release pattern of the source term for BWR-5 plants and selected following three representative scenarios for use in the consequence analysis. First, large early release scenario is the accident of hydrovolcanic explosion shown by TQUV-α. Next, large late release scenario is the accident of overpressure shown by TQUV-δ. Finally, control release scenario is the accident of containment vent shown by TQUV- ν-l. For these scenarios, we calculated source term data using Level 2PSA code, THALES-2 originally developed by JAEA. The result of the calculation is indicated by red triangle.

9 Source terms and PM strategy
Three source term scenarios Release scenario Release time (hr after scram) Duration of release (hr) Release fraction of iodine (%) Large early release 3 1 7.9 Large late release 27 7 3.3 Control release 12 22 0.09 Strategies of protective measures Large early release: precautionary evacuation with stable iodine intake Large late release: evacuation and sheltering with stable iodine intake Control release: sheltering with stable iodine intake Site data A reference site is assumed to be located at a coastal area facing the Pacific Ocean (JAEA site at Tokai) Next, for these scenarios, we consider different combinations of protective measures. This table shows source terms data for each release scenario. In case of large early release scenario, the amount of iodine releases to the environment was large and release time was very short. We thought that evacuation before the start of accident will be main protective measurement in the situation. Additionally, we considered whether sheltering is the alternative protective measurements, because there may be difficult situation to implement evacuation. In case of large late release scenario, the release time was longer than 1 day. So it will be enough time to decide which protective measurement to be implemented. We considered evacuation and sheltering with stable iodine intake in the situation. In case of control release scenario, the amount of iodine releases to the environment was smaller and release time was shorter than other scenarios. We considered that sheltering with stable iodine intake is the main protective measurement and evacuation is alternative protective measurement. The site data such as meteorological data and population distribution data was used for the Tokai site at JAEA.

10 Steps for Consequence Evaluation
Calculation of dose from each pathway and time-dependent iodine concentrations in air at receptor points using OSCAAR 248 weather sequences selected by a stratified sampling method Calculation of dose reduction effects by various combinations of protective measures Intervention levels for implementing each protective measure (NSC’s Guideline) Sheltering:10 mSv, Evacuation:50 mSv(effective dose) Administration of stable iodine :100 mSv (thyroid equivalent dose for infants) Inhalation dose due to isotope iodine based on I-131~135 contents in thyroid using Johnson model Then, I explain steps for consequence evaluation for each release scenario. OSCAAR calculates the concentration of radionuclides released in air and on ground and the subsequent dose distributions at a grid point divided by distance bands and angular sectors around the site for each meteorological sequence. For each source term, we calculated dose from external irradiation such as cloud-shine and ground-shine and time-dependent iodine concentrations in air at receptor points using OSCAAR. We used 248 weather sequences selected from all of weather sequences in 1 year by a stratified sampling method. Then, we calculated dose reduction effects by various combinations of protective measures. We used intervention levels for implementing the protective measure shown by NSC’s Guideline. Sheltering is 10 mSv of effective dose and evacuation is 50 mSv of effective dose. Administration of stable iodine is 100 mSv of thyroid equivalent dose for infants. Inhalation dose due to isotope iodine in thyroid was calculated by Johnson model. In this study, the maximum value of dose at each distance was selected from 32 angular sector grids used by OSCAAR for each meteorological sequence. The results of probabilistic distribution for effective dose and thyroid dose from exposure pathways were discussed as a function of distance from the site. Next, I explain the result of the calculation and consideration for three release scenarios. Calculation of maximum dose at each distance from the site and its probability of weather sequence Probability of exceeding a specific dose level Dose at each distance from the site at a specific cumulative probability of weather sequences Find a maximum dose at each distance and each weather sequence

11 Large Early Release Effect of the reduction for the thyroid dose due to the delay of evacuation and SI intake Effect of the reduction for the effective dose due to protective measures Distance from the release point (km) Thyroid dose (Sv) 95% Met 50% Met Stable iodine Distance from the release point (km) Effective dose (Sv) 95% Met 50% Met Sheltering Evacuation This slide shows the results for the large early release scenario. The left figure shows the 50th and 95th percentile values of the effective dose as a function of distance from the release point with no protective measures, sheltering and evacuation. 10 mSv is the generic intervention level for sheltering. So, you can see, for this scenario, sheltering is not enough to reduce the effective dose, even though 50th percentile values. In case of this scenario, the amount of iodine releases to the environment was large and release time was very short. So, evacuation before the start of accident will be main protective measurements in order to avoid the deterministic effect. Therefore, it is important to develop the preparedness action before occurring accident. For example, emergency action level, precautionary action zone. The right figure shows the effect of the reduction for the thyroid dose due to the delay of evacuation and intake of stable iodine. This graph plots the 50th and 95th percentile values of the committed thyroid dose as a function of the distance from the release point. In this case, even when evacuation is initiated with the delay of three hours, administration of stable iodine taken prior to the release can substantially reduce the committed thyroid dose to the people within about 20 km from the site. Pre-distribution of stable iodine might be considered for the people in emergency planning. For large early release, precautionary evacuation is needed before the start of release. It is important to develop the preparedness action before occurring accident (e.g. EAL: emergency action level, PAZ: precautionary action zone) Early stable iodine intake can be very effective to reduce the thyroid dose for the people within about 20 km from the site even the delay of evacuation (consideration of distribution method).

12 Distance from the release point (km)
Large Late Release Effect of the reduction for the thyroid dose due to protective measures Effect of the reduction for the effective dose due to protective measures Distance from the release point (km) Effective dose (Sv) 95% Met 50% Met Sheltering Evacuation Distance from the release point (km) Thyroid dose (Sv) 95% Met 50% Met Stable iodine Next, this slide shows the results for the large late release scenario. In this scenario, it will be enough time before the starting of the release. So evacuation will be considered as the main protective measure, but you should need to implement evacuation and sheltering in the range of the appropriate area, based on conditions of the weather. The left figure shows both 50th and 95th percentile values of the effective dose as a function of distance from the release point with no protective measures, sheltering and evacuation. 50 mSv is the generic intervention level for evacuation. So, you can see, for this scenario, evacuation area is very unlikely to be exceeded beyond 20 km. 10 mSv is the generic intervention level for sheltering. So, for this scenario, sheltering area is very unlikely to be exceeded beyond 50 km. The right figure shows the 50th and 95th percentile values of the committed thyroid dose as a function of the distance from the release point for each protective action strategy. This figure shows that it is not enough to implement just evacuation or sheltering. Because the intervention level of 100 mSv for administration of stable iodine is exceeded. So, administration of stable iodine should be considered as a supplement to evacuation or sheltering in planning. For large late release, evacuation and sheltering areas can be decided to consider weather conditions at the time of accident. In this scenario, evacuation and sheltering area are unlikely to occur beyond 20 km and 50 km. Sheltering or evacuation with administration of stable iodine is very important to reduce the thyroid dose.

13 Control Release Effective dose for distance from the release point
Effect of the reduction for the thyroid dose due to sheltering and intake of stable iodine Effective dose (Sv) Distance from the release point (km) Sheltering Evacuation Distance from the release point (km) Thyroid dose (Sv) 95% Met 50% Met Stable iodine Next, this slide shows the results for the control release scenario. The left figure also shows the effective dose as a function of distance from the release point. The calculation of effective dose indicated that the 50th percentile values of effective dose for no protective action did not exceed a criterion of 50 mSv for evacuation at all locations. Even though very severe weather conditions such as 95th percentile values, evacuation are is unlikely to occur beyond a few kilometers and sheltering area is unlikely to occur beyond 10 km. The right figure shows both 50th and 95th percentile values of the committed thyroid dose as a function of the distance from the site for each protective action strategy. In very severe weather conditions, it is not enough to implement just sheltering. Because the intervention level of 100 mSv for administration of stable iodine is exceeded. So, administration of stable iodine should be considered as a supplement to sheltering in planning. For control release, evacuation area is unlikely to occur beyond a few kilometers and sheltering area is unlikely to occur beyond about 10 km. For very severe weather conditions, sheltering with stable iodine intake is needed.

14 Conclusions For the representative source terms, the preliminary analysis has been performed using the probabilistic accident consequence model (OSCAAR) to evaluate the effectiveness of protective action strategy involving a combination of evacuation, sheltering and administration of stable iodine. The study indicated following findings for three release scenarios. Large early release Precautionary evacuation is needed before the start of release. Early stable iodine intake can be very effective to reduce the thyroid dose even the delay of evacuation. It is important to develop the preparedness action before occurring accident (e.g. EAL, PAZ, method for distribution of stable iodine). Large late release Evacuation and sheltering areas can be decided to consider weather conditions at the time of accident. Sheltering or evacuation with administration of stable iodine is very important to reduce the thyroid dose. Control release Evacuation area is unlikely to occur beyond a few kilometers and sheltering area is unlikely to occur beyond about 10 km. For very severe weather conditions, sheltering with stable iodine intake is needed. Finally, I would like to show some conclusions in this study. First, we introduced a metabolic model of iodine with the probabilistic accident consequence model (OSCAAR) to evaluate the effect of reducing thyroid dose by administration of stable iodine. Next, for the representative source terms defined by the level 2 PSA, the preliminary analysis has been performed using OSCAAR to evaluate the effectiveness of protective action strategy involving a combination of evacuation, sheltering and administration of stable iodine. The study indicated following findings for three release scenarios. In case of large early release, Precautionary evacuation is needed before the start of release. Early stable iodine intake can be very effective to reduce the thyroid dose even the delay of evacuation. It is important to develop the preparedness action before occurring accident (e.g. EAL, PAZ, method for distribution of stable iodine). In case of large late release, Evacuation and sheltering areas can be decided to consider weather conditions at the time of accident. Sheltering or evacuation with administration of stable iodine is very important to reduce the thyroid dose. In case of control release, Evacuation area is unlikely to occur beyond a few kilometers and sheltering area is unlikely to occur beyond about 10 km. For very severe weather conditions, sheltering with stable iodine intake is needed. The result of this study will be expected to form the basis for future technical guidance for protective action strategy.


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