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SAFETY ASPECTS IN DECOMMISSIONING

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SAFETY ASPECTS IN DECOMMISSIONING

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  1. SAFETY ASPECTS IN DECOMMISSIONING A RAHMAN RWE NUKEM Ltd (UK) Name, company and chapter

  2. SAFETY ASPECTS IN DECOMMISSIONING Safety is a culture extending from • Design • Construction • Operation • Final shutdown • Decommissioning and • Waste management

  3. SAFETY ASPECTS IN DECOMMISSIONING • Nuclear safety – radiological • Non-nuclear safety – industrial, chemical.

  4. DECOMMISSIONING Starts at the end of operational phase • Hazards from nuclear and non-nuclear activities • Lower overall hazards than the operational phase ends with de-licensing

  5. 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

  6. 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

  7. HIERARCHY OF LAYERS • Technical layers of defence • Human factors

  8. TECHNICAL LAYERS OF DEFENCE • Quality design • Safety systems – protective systems, control systems, interlocks etc • Safety monitoring systems – installed monitors, portable monitors, alarms etc

  9. 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 risk Risk = Probability of an adverse effect or probability x consequences

  10. 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.

  11. TECHNICAL LAYER Redundancy

  12. 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

  13. Radiological Protection To protect individuals, society and the environment from the harmful effects of radiation

  14. 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

  15. DOSE LIMITS Based on ICRP – 60 in 1990. Both European BSS and the International BSS are based on ICRP – 60.

  16. 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 - STATUTORY DOSE LIMITS

  17. DOSIMETRIC QUANTITIES • Absorbed dose, • Equivalent dose, • Effective dose,

  18. POSSIBLE DOSES • External dose • Internal dose • Body wounds

  19. PROTECTION FROM EXTERNAL IRRADIATION

  20. Unacceptable level of risk > 10-3 y-1 Risk level: 10-3 y-1 Basic Safety Limit(BSL) A L A R P Risk level: 10-5 y-1 Basic Safety Objective(BSO) Broadly acceptable level of risk < 10-5 y-1 ALARA PRINCIPLE

  21. 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

  22. VISIPLAN ALARA PLANNING TOOL

  23. PROTECTION OF OCCUPATIONAL WORKERS • Classification of workplaces • Classification of workers • Monitoring of workplaces (area monitoring) • Individual dose monitoring • Control measures

  24. 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.

  25. SUPERVISED AREA • Area labelled and physically demarcated • Working instructions available • Area monitoring • Unclassified persons work under a system of work

  26. 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

  27. 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

  28. AREA MONITORING Required for both supervised and controlled areas. Individual dose estimations from • External doses • Activity concentration in air • Surface contamination

  29. AREA MONITORING Instrumentation • Installed -monitors •  in air monitors • -in-air monitors

  30. 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

  31. ELECTRONIC DOSIMETER

  32. PERSONAL PROTECTIVE EQUIPMENT (PPE) • Protective clothing • Respiratory protection

  33. PROTECTIVE CLOTHING

  34. PROTECTIVE CLOTHING • Overshoes, gloves, caps and overalls • Shoes, gloves, pressurised suits containing own air supply or airline

  35. 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

  36. NUCLEAR SAFETY IN DECOMMISSIONING • Knowledge of the facility • Nature of operations • Knowledge of any hot spots • Tools to be used • Operator training

  37. 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

  38. CHEMICAL HAZARDS • Chemicals – acids, alkalis • Asbestos, lead etc. • Solvents used in decontamination process • Sodium in fast reactors

  39. HAZARDOUS MATERIALS • Chemical solvents, complexing agents such as EDTA, DPTA • Alkali metals (NaK) • Asbestos in thermal insulation • Lead dust from Pb shielding • Mercury vapour

  40. 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

  41. 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

  42. INDUSTRIAL PPE • Steel toe-capped shoes • Hard hats • Heavy duty gloves

  43. SAFETY DOCUMENTATION Safety documentation covering design, construction, commissioning, operation and decommissioning • Decommissioning Plan (DP) • Decommissioning Safety Case (DSC) • Post Decommissioning Report (PDR)