4 DECOMMISSIONING Starts at the end of operational phase Hazards from nuclear and non-nuclear activitiesLower overall hazards than the operational phaseends with de-licensing
5 SAFETY OBJECTIVESGeneral nuclear safety objective – protect individual, society and the environmentRadiation protection objective – exposure below dose limits, ALARATechnical safety objective – practical measures to prevent accidents
6 DEFENCE IN DEPTHMultiple layers of protection – multi-barrier protectionLayers must be independentFailure probabilities of independent layers are multiplicativeLayers may be technical and/or human factors
7 HIERARCHY OF LAYERSTechnical layers of defenceHuman factors
8 TECHNICAL LAYERS OF DEFENCE Quality designSafety systems – protective systems, control systems, interlocks etcSafety 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 riskRisk = Probability of an adverse effectorprobability x consequences
10 TECHNICAL LAYERS OF DEFENCE Redundancy – two or more protective layers in parallelDiversity – alternative types to eliminate common mode failureIndependence – to eliminate common cause failureSystems to comply with single failure criteria.
12 HUMAN FACTORSWorking procedures and practices – safety orders, safety manuals, written system of work etcTraining of workforceQuality assurance and controlManagement 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 individualsIndividual doses to comply with ALARA principleCollective doses to comply with ALARA principleSteps to prevent accidentsSteps to mitigate accident consequences
15 DOSE LIMITSBased on ICRP – 60 in 1990.Both European BSS and the International BSS are based on ICRP – 60.
16 STATUTORY DOSE LIMITS Quantity Dose limits / mSv.y-1 workers Apprentices and PublicstudentsEffective dose for 18 y 16 for yEq. dose in:lens of the eyeskinhands and feet
20 ALARA PRINCIPLE A L A R P Unacceptable level of risk > 10-3 y-1 Basic Safety Objective (BSO)Basic Safety Limit (BSL)Risk level: 10-3 y-1Risk level: 10-5 y-1Broadly acceptable level of risk < 10-5 y-1A L A R PUnacceptable level of risk > 10-3 y-1
21 DOSE PREDICTION TOOLSComputer codes to predict individual and collective dosesCoupling workplace environment with 3D modelling to predict total doseUseful in trainingUseful in ALARA Application
23 PROTECTION OF OCCUPATIONAL WORKERS Classification of workplacesClassification of workersMonitoring of workplaces (area monitoring)Individual dose monitoringControl measures
24 CLASSIFICATION OF WORKPLACES Workplaces with dose levels 1 mSv.y-1Supervised 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 availableArea monitoringUnclassified persons work under a system of work
26 CONTROLLED AREA Dose levels 6 mSv.y-1. Area labelled and physically demarcatedBarriers produced and entry restrictedClassified workers workArea monitoringWork under the written system of work
27 CLASSIFICATION OF WORKERS Category A: dose 6 mSv.y-1.work mainly in controlled areasCategory B: dose 1 mSv.y-1.work mainly in supervised areas
28 AREA MONITORING Required for both supervised and controlled areas. Individual dose estimations fromExternal dosesActivity concentration in airSurface contamination
29 Instrumentation AREA MONITORING Installed -monitors in air monitors
30 INDIVIDUAL DOSE MONITORING For both category A and category B workers:TLDs, film badgesPersonal dosimeters – QFDs, portable dosimeters etc.Alpha-in-air portable dosimeterRecord keeping: at least 30 years or until 75 years of a worker
34 PROTECTIVE CLOTHINGOvershoes, gloves, caps and overallsShoes, gloves, pressurised suits containing own air supply or airline
35 RESPIRATORY PROTECTION Simplest face maskPassive respirators (gas masks) where breathing is through a suitable filter materialPositive pressure respirators where pumped air supply to the face mask
36 NUCLEAR SAFETY IN DECOMMISSIONING Knowledge of the facilityNature of operationsKnowledge of any hot spotsTools to be usedOperator training
37 NUCLEAR SAFETY DURING DECOMMISSIONING Open and frequent communicationSupport from health physics and safety departmentStrict quality controlAdaptation of toolsManagement of the facility with flexibility and adaptability
38 CHEMICAL HAZARDS Chemicals – acids, alkalis Asbestos, lead etc. Solvents used in decontamination processSodium in fast reactors
39 HAZARDOUS MATERIALSChemical solvents, complexing agents such as EDTA, DPTAAlkali metals (NaK)Asbestos in thermal insulationLead dust from Pb shieldingMercury vapour
40 PROTECTION AGAINST CHEMICAL HAZARDS Characterisation of chemical hazardsAppreciation of hazards involvedNational and international regulations on chemical hazardous materialsGuidance, operating procedures, safety instructionsEmergency arrangements
41 INDUSTRIAL HAZARDSDecontamination, dismantling involving cutting, lifting, packaging etcCustom-built tools and machines usedLack of training in new methodsHoist and lifting equipment usedCrane collapse, dropped load, collisionsElectrocution
42 Steel toe-capped shoes Hard hats Heavy duty gloves INDUSTRIAL PPESteel toe-capped shoesHard hatsHeavy duty gloves
43 SAFETY DOCUMENTATIONSafety documentation covering design, construction, commissioning, operation and decommissioningDecommissioning Plan (DP)Decommissioning Safety Case (DSC)Post Decommissioning Report (PDR)
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