REPPIR Approach to Consequence Assessment and Associated Risk Framework

Authors: C Boyd

Version 1.– 15.2.2020

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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

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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).

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5 mSv contour

DEPZ

Town and village bisected

Vulnerable group

Political/ county boundary

Boundary marked by geographical landmarks, roads, postcodes, etc.

A

B

Vulnerable group

Pre REPPIR 2019

4

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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.

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IAEA Approach

• Examples

• Reactors > 1000 MWth PAZ 3-5 km UPZ 5-30 km.

• For radioactive sources where A = Activity and D2 = “Dangerous quantity”, A/D2 100 to 1000 (For Pu-239 this would be a dispersible quantity of >6 TBq), No PAZ and UPZ 0.5 km

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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

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REPPIR19 Risk Framework

Minor

Moderate

Significant

Lower ERL - Sheltering

Very Low Low Medium High Very HighEvents not considered in the design

Imp

act

REPPIR RISK FRAMEWORK

Limited

No emergency planning required by REPPIR

Outline planning required Detailed emergency planning required

Region where regulatory action taken to prohibit or curtail activity - Detailed

emergency planning required

Catastrophic

Likelihood

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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.

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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.

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ERL contour

DEPZ

Town and village bisected

Vulnerable group

Political/ county boundary

Boundary marked by geographical landmarks, roads, postcodes, etc.

A

B

Vulnerable group

OPZ

11

REPPIR 2019

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Uncertainties in Consequence

• Source Term

• Dispersion/Deposition

• Food transfer

• Dosimetric combined (inhalation/ingestion/physiology etc…)

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Source Term

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Release to Water Release to Air

Shoreline Irradiation

Consumption (Water)

Ingestion

Consumption

(Fish / Seaweed)

Ingestion

Ingestion (Milk)

Ground Irradiation

Ingestion (Foods)

Air Submersion

Air Inhalation

Deposition

Exposure Pathways for Man

Direct irradiation

SRP South West Conference – 16th & 17th October 2019

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Some food transfer and dosimetricfactors

• 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

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Uncertainties in Consequence Assessment

• From ONR TAG-45 (*Dosimetric combined);Uncertainty in best estimate of risk from I-131 released from a large break loss of coolant at a PWR

Uncertainty in best estimate of risk from Pu-239 released from a fire at a fuel reprocessing facility

Source term 5 to 50 10 to 1000Dispersion 10 20Food transfer 2 to 20 2 to 20Dosimetric (ingestion, inhalation, physiological etc…)* 4 12

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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 √{172+102 +62+42} = 20. With a theoretical yet to exist dispersion and deposition model with a factor of 1 to 2 uncertainty, this reduces to 18.

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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.

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Relevant Good Practice in Consequence Assessment

• ONR recognise that for long range assessment (e.gsocietal 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).

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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 95th 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.

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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

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Results/Conclusions

• Outline planning has been brought into emergency planning.