1 safety aspects in decommissioning a rahman rwe nukem ltd (uk) name, company and chapter
TRANSCRIPT
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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)