REPPIR Approach to Consequence Assessment and Associated Risk - PowerPoint PPT Presentation

REPPIR Approach to Consequence Assessment and Associated Risk Framework Authors: C Boyd Version 1.– 15.2.2020

Contents • Summary of Previous REPPIR Approach • IAEA Approach • REPPIR ACOP and the Risk Framework • Uncertainties in Consequence Assessment • Relevant Good Practice in Consequence Assessment • How licensees have addressed REPPIR Consequence Assessment • Results/Conclusions Slide 2 of 50

Summary of Previous REPPIR Approach • Reasonably foreseeable event • NRPB-M-1311 • Schedule 2 values determined (ground level, 30 minute release, D5 best estimate Gaussian dispersion, 100 meter distance, 1 year dose) • Criteria: 5 mSv effective dose, 50 mSv organ dose. • If exceeded then Emergency Plan required. DEPZ based on the 5 mSv dose contour (typically 1-7 km). Slide 3 of 50

5 mSv contour Pre REPPIR 2019 DEPZ Vulnerable group Boundary marked by geographical landmarks, roads, postcodes, etc. Town and village bisected B A Vulnerable group Political/ county boundary Slide 4 of 50 4

IAEA Approach IAEA-GS-G 2.1 • The Precautionary Action Zone (PAZ) is the area within which arrangements should be made to implement precautionary urgent protective actions before or shortly after a major release with the aim of preventing or reducing the occurrence of severe deterministic effects. • The Urgent Protective Action planning Zone (UPZ) area where preparations are made to promptly shelter in place, to perform environmental monitoring and to implement urgent protective actions on the basis of the results of monitoring within a few hours following a release. Slide 5 of 50

IAEA Approach • Examples • Reactors > 1000 MW th PAZ 3-5 km UPZ 5-30 km. • For radioactive sources where A = Activity and D 2 = “Dangerous quantity”, A/D 2 100 to 1000 (For Pu-239 this would be a dispersible quantity of >6 TBq), No PAZ and UPZ 0.5 km Slide 6 of 50

REPPIR19 ACOP • First assess inventories against schedule 1 (also schedule 2 for fissile quantities). If < then REPPIR19 does not need to be considered further. • If > then assess dose (using PHE data tables for low risk sites) and appropriate methods (at least 2 day and 1 year doses) for other sites to show <1 mSv. • If > 1 mSv, Risk Framework Slide 7 of 50

REPPIR19 Risk Framework REPPIR RISK FRAMEWORK Detailed emergency planning Region where regulatory No emergency planning Outline planning required required by REPPIR required action taken to prohibit or curtail activity - Detailed emergency planning required Catastrophic Significant Impact Moderate Minor Lower ERL - Sheltering Limited Medium Low High Very Low Very High Events not considered in the design Likelihood Slide 8 of 50

REPPIR19 ACOP • DEPZ determined for events with a predicted return frequency from around >1x10 -6 to 1x10 -5 per annum. (REPPIR19 Guidance 5(1)-5(2) 152). • DEPZ determined at lower shelter ERL for averted dose (also thyroid dose). • Outline planning as per schedule 5 or as specified by MOD. For sites regulated by HSE, the risk framework is used to determine whether an outline planning zone is required. Slide 9 of 50

REPPIR19 ACOP • Present results to local authorities for DEPZ for faults with shelter, evacuation and thyroid blocking distances. • Also provide dose to emergency workers, food restriction distances and long term residual dose. Slide 10 of 50

ERL contour REPPIR 2019 DEPZ OPZ Vulnerable group Boundary marked by geographical landmarks, roads, postcodes, etc. Town and village bisected B A Vulnerable group Political/ county boundary Slide 11 of 50 11

Uncertainties in Consequence • Source Term • Dispersion/Deposition • Food transfer • Dosimetric combined (inhalation/ingestion/physiology etc…) Slide 12 of 50

Source Term Slide 13 of 50

Exposure Pathways for Man Release to Air Release to Water Direct Air irradiation Inhalation Deposition Shoreline Irradiation Air Submersion Ingestion Consumption (Water) Ingestion (Foods) Consumption Ground Irradiation (Fish / Ingestion (Milk) Seaweed) Ingestion SRP South West Conference – 16 th & 17 th October 2019 Slide 14 of 50

Some food transfer and dosimetric factors • Transfer rates from soil to food • Removal rates from soil by erosion etc… • Processing times for food • Animal food source (pasture or feedstock) • Fraction of food sourced locally • Consumption rates • Inhalation rates • Indoor/outdoor occupancy, building reduction factors • Physiology • Dose/risk factors Slide 15 of 50

Uncertainties in Consequence Assessment • From ONR TAG-45 (*Dosimetric combined) ; Uncertainty in best Uncertainty in best estimate of risk estimate of risk from I-131 released from Pu-239 from a large break released from a fire loss of coolant at a at a fuel PWR reprocessing facility Source term 5 to 50 10 to 1000 Dispersion 10 20 Food transfer 2 to 20 2 to 20 Dosimetric (ingestion, inhalation, physiological etc…)* 4 12 Slide 16 of 50

Combining Uncertainties • Consequence = source x dispersion x uptake x dose per unit uptake x risk per unit dose • Uncertainties are then combined as; • =√{(uncertainty source) 2 +(uncertainty dispersion) 2 +(uncertainty food transfer) 2 +(uncertainty dosimetric) 2 } • So for iodine LOCA release using geometric means of 17 for source term and 6 for food uptake, with a dispersion uncertainty of factor of 10 and 4 for dosimetric; the uncertainty in the consequence assessment is √ {17 2 +10 2 +6 2 +4 2 } = 20. With a theoretical yet to exist dispersion and deposition model with a factor of 1 to 2 uncertainty, this reduces to 18. Slide 17 of 50

Relevant Good Practice in Consequence Assessment • R-91 is a Gaussian plume methodology developed by NRPB with associated subsequent reports (R-157, R- 322, W-41 etc…) dealing with building wake, wet and dry deposition, habits etc… • ONR TAG-45 states “R-91 is a “medium” range model, considered reliable between 100 metres to several tens of kilometres from the release point in constant conditions. .” • Provided sensitivities are performed, this is fit for purpose for REPPIR19 consequence assessment. Slide 18 of 50

Relevant Good Practice in Consequence Assessment • ONR recognise that for long range assessment (e.g societal risk target in SAPs, which require modelling of the entire UK), or post fault consequence predictions, Gaussian plume modelling is limited in it’s applicability. • ONR are currently assessing whether advanced methods can be used for societal risk safety assessments (awaiting V&V status). • For the very near field, ONR await research work by Imperial College (ADMLC contract). Slide 19 of 50

How licensees have addressed REPPIR19 Consequence Assessment • One licensee has performed screening using Gaussian plume methods and then on higher consequence faults, used advanced methods (NAME/PACE 95 th percentile dose) to determine the geographical extent for Local Authorities to determine DEPZ etc… • Some lower hazard sites have performed hazard analysis to determine release fractions of inventories and then shown that these are less than schedule 1 of REPPIR19. Slide 20 of 50

Results/Conclusions • The minimum recommended DEPZ for sites has changed – Some have slightly decreased due to the removal of hazards e.g. Magnox defueling – Others have increased due to the use of more pessimistic weather categories in the methodology recommended by PHE • In most locations the Local Authority proposes to maintain a similar DEPZ to that currently in place, but modifying to try to use physical features to set boundaries, and aiming to try to prevent bisecting communities Slide 21 of 50

Results/Conclusions • Outline planning has been brought into emergency planning. Slide 22 of 50