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SAFETY ASPECTS IN DECOMMISSIONING

A RAHMAN

RWE NUKEM Ltd (UK)

Name, company and chapter

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Safety is a culture extending from• Design• Construction• Operation• Final shutdown• Decommissioning and• Waste management

SAFETY ASPECTS IN DECOMMISSIONING

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SAFETY ASPECTS IN DECOMMISSIONING

• Nuclear safety – radiological

• Non-nuclear safety – industrial, chemical.

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DECOMMISSIONING

Starts at the end of operational phase

ends with de-licensing

Hazards from nuclear and non-nuclear activities

Lower overall hazards than the operational phase

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SAFETY OBJECTIVES

• General nuclear safety objective – protect individual, society and the environment

• Radiation protection objective – exposure below dose limits, ALARA

• Technical safety objective – practical measures to prevent accidents

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DEFENCE IN DEPTH

• Multiple layers of protection – multi-barrier protection

• Layers must be independent• Failure probabilities of independent

layers are multiplicative• Layers may be technical and/or

human factors

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HIERARCHY OF LAYERS

• Technical layers of defence

• Human factors

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TECHNICAL LAYERS OF DEFENCE

• Quality design• Safety systems – protective

systems, control systems, interlocks etc

• Safety monitoring systems – installed monitors, portable monitors, alarms etc

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SAFETY ASSESSMENT

• Various methods exist: Failure Mode and Effects Analysis (FMEA) Event Tree Analysis (ETA) Fault Tree Analysis (FTA) Hazard and Operability (HAZOP) Hazard Assessment (HAZAN)

• Evaluation of riskRisk = Probability of an adverse effect

or probability x consequences

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TECHNICAL LAYERS OF DEFENCE

• Redundancy – two or more protective layers in parallel

• Diversity – alternative types to eliminate common mode failure

• Independence – to eliminate common cause failure

Systems to comply with single failure criteria.

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TECHNICAL LAYER

Redundancy

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HUMAN FACTORS

• Working procedures and practices – safety orders, safety manuals, written system of work etc

• Training of workforce

• Quality assurance and control

• Management review of safety standards

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Radiological Protection

To protect individuals, society and the environment from the harmful effects of radiation

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Implementation of Radiological Protection

• Statutory dose limits to individuals• Individual doses to comply with

ALARA principle• Collective doses to comply with

ALARA principle• Steps to prevent accidents• Steps to mitigate accident

consequences

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DOSE LIMITS

Based on ICRP – 60 in 1990.

Both European BSS and the International BSS are based on ICRP – 60.

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STATUTORY DOSE LIMITS

Quantity Dose limits / mSv.y-1

workers Apprentices and Public

students

Effective dose 20 20 for 18 y 1

6 for 16-18 y

Eq. dose in:

lens of the eye 150 50 15

skin 500 150 50

hands and feet 500 150 -

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DOSIMETRIC QUANTITIES

• Absorbed dose,

• Equivalent dose,

• Effective dose,

Gymd

EdkgJ

dm

EdDT 1.

SvwDHR

RTT

SvwHE TT

T

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POSSIBLE DOSES

• External dose• Internal dose• Body wounds

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PROTECTION FROM EXTERNAL IRRADIATION

Shielding

Distance

Time

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ALARA PRINCIPLE

Basic Safety Objective (BSO)

Basic Safety Limit (BSL) Risk level: 10-3 y-1

Risk level: 10-5 y-1

Broadly acceptable level of risk < 10-5 y-1

A L A R P

Unacceptable level of risk > 10-3 y-1

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DOSE PREDICTION TOOLS

• Computer codes to predict individual and collective doses

• Coupling workplace environment with 3D modelling to predict total dose

• Useful in training• Useful in ALARA Application

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VISIPLAN ALARA PLANNING TOOL

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PROTECTION OF OCCUPATIONAL WORKERS

• Classification of workplaces

• Classification of workers

• Monitoring of workplaces (area monitoring)

• Individual dose monitoring

• Control measures

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CLASSIFICATION OF WORKPLACES

Workplaces with dose levels 1 mSv.y-1

• Supervised area: dose levels 1mSv.y-1 but < 6 mSv.y-1.

• Controlled area: dose levels 6 mSv.y-1.

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SUPERVISED AREA

• Area labelled and physically demarcated

• Working instructions available

• Area monitoring

• Unclassified persons work under a system of work

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CONTROLLED AREA

• Dose levels 6 mSv.y-1.• Area labelled and physically

demarcated• Barriers produced and entry

restricted• Classified workers work• Area monitoring• Work under the written system of

work

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CLASSIFICATION OF WORKERS

• Category A: dose 6 mSv.y-1.work mainly in controlled areas

• Category B: dose 1 mSv.y-1.work mainly in supervised areas

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AREA MONITORING

Required for both supervised and controlled areas.

Individual dose estimations from• External doses• Activity concentration in air• Surface contamination

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AREA MONITORING

Instrumentation

• Installed -monitors in air monitors -in-air monitors

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INDIVIDUAL DOSE MONITORING

For both category A and category B workers:

• TLDs, film badges• Personal dosimeters – QFDs, portable

dosimeters etc.• Alpha-in-air portable dosimeter• Record keeping: at least 30 years or

until 75 years of a worker

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ELECTRONIC DOSIMETER

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PERSONAL PROTECTIVE EQUIPMENT (PPE)

• Protective clothing

• Respiratory protection

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PROTECTIVE CLOTHING

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PROTECTIVE CLOTHING

• Overshoes, gloves, caps and overalls

• Shoes, gloves, pressurised suits containing own air supply or airline

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RESPIRATORY PROTECTION

• Simplest face mask

• Passive respirators (gas masks) where breathing is through a suitable filter material

• Positive pressure respirators where pumped air supply to the face mask

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NUCLEAR SAFETY IN DECOMMISSIONING

• Knowledge of the facility• Nature of operations• Knowledge of any hot spots• Tools to be used• Operator training

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NUCLEAR SAFETY DURING DECOMMISSIONING

• Open and frequent communication• Support from health physics and

safety department• Strict quality control• Adaptation of tools• Management of the facility with

flexibility and adaptability

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CHEMICAL HAZARDS

• Chemicals – acids, alkalis

• Asbestos, lead etc.

• Solvents used in decontamination process

• Sodium in fast reactors

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HAZARDOUS MATERIALS

• Chemical solvents, complexing agents such as EDTA, DPTA

• Alkali metals (NaK)

• Asbestos in thermal insulation

• Lead dust from Pb shielding

• Mercury vapour

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PROTECTION AGAINST CHEMICAL HAZARDS

• Characterisation of chemical hazards

• Appreciation of hazards involved

• National and international regulations on chemical hazardous materials

• Guidance, operating procedures, safety instructions

• Emergency arrangements

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INDUSTRIAL HAZARDS

• Decontamination, dismantling involving cutting, lifting, packaging etc

• Custom-built tools and machines used

• Lack of training in new methods• Hoist and lifting equipment used• Crane collapse, dropped load,

collisions• Electrocution

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INDUSTRIAL PPE

• Steel toe-capped shoes

• Hard hats

• Heavy duty gloves

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SAFETY DOCUMENTATION

Safety documentation covering design, construction, commissioning, operation and decommissioning

• Decommissioning Plan (DP)• Decommissioning Safety Case (DSC)• Post Decommissioning Report (PDR)