Presentation on theme: "Metal Decontamination Techniques used in Decommissioning Activities"— Presentation transcript:
1Metal Decontamination Techniques used in Decommissioning Activities Mathieu PonnetSCK•CEN
2Summary Objectives and selection criteria Full System Decontamination Decontamination of components/partsConclusionsOne detail example (BR3 case) in each part
3DefinitionDecontamination is defined as the removal of contamination from surfaces byWashingHeatingChemical or electrochemical actionMechanical actionOthers (melting…)
43 main reasonsTo remove the contamination from components to reduce dose level in the installation (save dose during dismantling)To minimize the potential for spreading contamination during decommissioningTo reduce the contamination of components to such levels that may beDisposed of at a lower categoryRecycled or reused in the conventional industry (clearance of material)
5Decontamination for decommissioning In maintenance work, we must avoid any damage to the component for adequate reuseIn decommissioning, decontamination techniques can be destructive, the main goal being the removal of as much activity as possible (high DF)
6Decontamination before Dismantling Objectives : Reduction of occupational ExposurePipe Line System DecontaminationClosed systemPool, TankOpen systemHydro jet MethodBlast MethodStrippable coating MethodChemical MethodMechanical method
7Decontamination after Dismantling Objectives : Reduction of radioactive waste or recyclingPipes, ComponentsOpen or closed systemChemical Immersion MethodElectrochemical MethodBlast MethodUltrasonic wave MethodGel Method
8Decontamination Of Building Objectives : Reduction of radioactive concrete waste or Release of buildingConcrete SurfaceConcrete DemolitionMechanical MethodScabblerShaverBlast MethodExplosivesJackhammerDrill &Spalling
9Selecting a specific decontamination technique Need to be consideredSafety: Not increase radiological or classical hazardsEfficiency: Sufficient DF to reach the objectivesCost-effectiveness: should not exceed the cost for waste treatment and disposalWaste minimization: should not rise large quantities of waste resulting in added costs, work power and exposureFeasibility of industrialization: Should not be labour intensive, difficult to handle or difficult to automate.
10Parameters for the selection of a decontamination process Type of plant and plant processOperating history of the plantType of components: pipe, tankType of material: steel, Zr, concreteType of surface: rough, porous, coated…Type of contaminants: oxide, crud, sludge…Composition of the contaminant (activation products, actinides… and radionuclide involved)Ease of access to areas/plant, internal or external contaminated surfaceDecontamination factor requiredDestination of the components after decontaminationTime required for applicationCapability of treatment and conditioning of the secondary waste generated
11Some examples about the type of material Stainless steel: Resistant to corrosion, difficult to treat, needs a strong decontamination process to remove several µmCarbon steel: Quite porous and low resistance to corrosion, needs a soft process but the contamination depth reaches several thousand µm (more secondary waste)Concrete: The contamination will depend of the location and the history of the material, the contamination depth can be few mm to several cm.
12Right decontamination technique Some examples about the type of surfacePorous: Avoid wet techniques which are penetrant.Coated: Do we have to remove paint ? (contamination level, determinant for the use of electrochemical techniques)Presence of crud: what are the objectives ? (reduce the dose or faciliting the waste evacuation)Right decontamination technique
13Soft decontamination process Thorough decontamination process Some examples about decontamination factor requiredPrimary circuit of BWR and PWR reactorsDFSoft decontamination processOuter layer : Fe2O3, Iron rich1-5 µm1-5Intermediate layer (CRUD) :FeCr2O4, Cr2O3, Chromium rich2-10 µm5-50Thorough decontamination process5 – 30 µmBase alloy : Fe, Cr, Ni50-10,000Right decontamination technique
14Pipes, tanks, pools… Right decontamination technique Some examples about the type of componentsPipes, tanks, pools…Decontamination in a closed system? (avoids the spreading of contamination…)Decontamination in an open system after dismantling? (secondary waste…)Connection to the components, dose rate, the total filling up of the component, auxiliary…Right decontamination technique
15Some examples about the treatment of secondary waste Availability of a facility to treat secondary waste from decontamination (chemical solutions, aerosols, debris, …)Final products (packaging, decontaminated effluent,…) have to be conform for final disposal.In decontamination processes, the final wastes are concentrated, representing a significant radiation source.
16Overview of decontamination process for metals Chemical processIn closed system (APCE, TURCO, CORD, SODP, EMMA, LOMI, DFD, Foams or various reagents…)In open system on dismantled components (MEDOC, Cerium/nitric acid, CANDEREM, DECOHA, DFDX or various reagents, HNO3, HCl, HF,…)Electrochemical process (open or close system)Phosphoric acid, Nitric acid, Oxalic or citric acid, sulfuric acid or others processPhysical process (open system)Wet or dry abrasives, Ultrasonic cleaning, HPW, CO2 ice blasting, others…
17Decontamination Techniques Used in Decommissioning Activities Objectives and selection criteriaFull system DecontaminationGeneral considerationChemical reagentsSpent decontamination solutionsGuidelines for selecting appropriate FSDThe case of BR3 (CORD)Decontamination of components/partsConclusions
18Full system and closed system Decontamination ObjectivesReduce the dose rate and avoid spreading of contamination during dismantlingTypical decontamination factor 5 to 40ApplicationDecontamination of the primary circuit (RPV,PP, SG and auxiliary circuits) directly after the shutdown of the reactorDecontamination of components in a closed loopPractical objectivesRemove the crud layer of about 5 to 10 µm inside the primary circuit
19Chemical process Chemical process commonly used Siemens England Russia CORD Chemical Oxidizing Reduction Decontamination based on the used of permanganic acid (AP).LOMI Low Oxidation State Metal Ion (AP)APCE Process based on the use of permanganate in alkaline solutionNITROX or CITROX based on the use of nitric or citric acid.EPRI DFD (Decontamination For Decommissioning) based on the use of fluoroborique acid.SiemensEnglandRussiaWestinghouseEPRI
20Multi-step decontamination process Oxidation stepOxidation of the insoluble chromium with permanganate in alkaline or acidic media, Nitric acid or fluoroboric acidDecontamination stepA dissolution step is carried out with oxalic acid to dissolve the crud layerThe reduction / dissolution step is enhanced by complexing agentPurification stepThe excess of oxalic acid is removed using permanganate or hydrogen peroxideThe dissolved cations and the activity are removed using Ion Exchange Resins.
22The Full System Decontamination of the primary system of the BR3-PWR reactor with the Siemens CORD ProcessObjectivesReduce the radiation dose rate by a factor of 10Remove the surface contamination, the so-called CRUD to avoid dispersion of contamination during dismantling of contaminated loopswith a particular attention to:Minimize the amount of secondary wastesMinimize the radiation exposure of the workersMinimize the modifications to be done to the plant for the decontamination operation.
24Full System Decontamination of the primary and auxiliary loops in 1991 CORD®: Chemical Oxidizing-Reducing Decontamination3 Decontamination Cycles at 80 to 100 °C in 9 daysFor each cycle : 3 stepsoxidation step with HMnO4Reduction step with H2C2O4Cleaning step with anionic and cationic IEX resins and removal of excess oxalic acid by oxidation with HMnO4 or with H2O2 on catalysts
25Chemistry of the process Oxidation Step with Permanganic Acid HMnO4 at 0.3 g/lFor the oxidation of the Chromium from Cr3+ to Cr6+Temperature 100°CDecontamination step with Oxalic Acid H2C2O4 at 3 g/lDissolution step for the hematite dissolution and the activity dissolutionTemperature 80°C to 100°CCleaning step: Destruction of the excess oxalic acid by oxidationwith permanganic acid or with hydrogen peroxide on a catalystCombined with fixation of corrosion products on Ion Exchange resinsTemperature: 80 to 60°C (last cycle)
26Process steps for each cycle Ion exchangeResinsChemicals in solutionProcess StepsStep nr 1: OxidationInjection of permanganic acidCirculation during several hoursMnO4-Step nr 2:Reduction + DecontaminationInjection of oxalic acid - CirculationPurification on ion exchangeC2O42-Cr, Fe oxalatesanionic speciesNi2+, Mn2+, Co2+Fixation oncationic IEXStep nr 3: CleaningDestruction of organics +purification on ion exchangeWater, CO2Cr, Fe oxalatesFixation onanionicIEX
28Primary loop Decontamination factors ! In some points, still some hot spots due to redeposition in dead zoneshorizontal line of the pressurizer, dead zones in heat exchangers..
29Radiological aspects The total dose amounted to only 159 man*mSv Phase I : Preparatory phasemanual closure of the reactor pressure vesselmaintenance of the componentsmodifications to the circuitsPhase II : Decontamination operationhot run3 decontamination CyclesPhase III : Post decontamination operationsevacuation of the liquid wastesevacuation of the solid wastes135.3 man*mSvman*mSvman*mSvThe total dose amounted to only 159 man*mSvThe dose saving up to now is over 500 man*mSv
30Main data and results Contaminated surface treated 1200 m2 Primary system volume 15 m3Corrosion products removed 33 kgMean Crud layer removed 5 µmIEX Waste volume produced m3Final waste volume 8 m3Dose rate in primary system mSv/hDose rate purification system mSv/hMean Decontamination factor ~ 10Collective Dose exposure man.Sv
31Lessons drawn from the operation … Expected ...Smooth process, minor operational problemsCareful and detailed preparation is a mustRequires a reactor in full satisfactory conditionsTo be performed shortly after the operationMan-Sv savings for future dismantling justify the operationUnexpected ...More ion exchange resins needed and higher liquid waste volumePollution of the reactor pool during reactor opening due to the presence of insoluble iron oxalate and loose crud: could be easily removed by the plant filtrationInternals of RPV remarkably clean facilitating inspection and dismantling and allowing to evacuate waste in a lower category LAW vs MAW
32Guidelines for selecting appropriate FSD Objectives in terms of Decontamination FactorType of material: Acidic solution is not appropriate for carbon steelVolume of secondary waste: preferred regenerative process (Lomi, DfD, CORD…)Composition of secondary waste: avoid organic element like EDTA (Complexing agent)Type of oxide layer: Select an oxidizing process for high chromium content in the CRUDCapability of treatment and conditioning of the secondary waste generated (Evaporation, IEX, Precipitation, filtration…)
33Decontamination Techniques Used in Decommissioning Activities Objectives and selection criteriaFull system decontaminationDecontamination of components/partsGeneral considerationsChemical decontaminationElectrochemical decontaminationMechanical decontaminationDecontamination by meltingGuidelines for selecting appropriate decontamination techniquesThe case of BR3 (ZOE - MEDOC)Conclusions
34Decontamination of components/parts To reduce the contamination of components to such levels that they may beDisposed of at a lower category - decategorizationRecycled or reused in the conventional industry (clearance of material)The decontamination can be applied:In a closed system on an isolated component (circuits, steam generator…)In an open system on dismantling material in batch treatment.
35Chemical decontamination Multi-step processesSame processes : Lomi, Cord, CanderemProcesses in one single step (Hard decontamination process)Cerium IV process : SODP, REDOX, MEDOCHNO3/HFHBF4 : Decoha, DfD..
36Cerium IV process The cerium IV process is a one step treatment. The cerium is a strong oxidizing agent (Eo = 1.61 V) in mixture with acid (Nitric acid or Sulfuric acid)The cerium IV dissolves oxide layer and the base metal.Cerium can be regenerated and recycled.The neutralization of cerium IV and the treatment of the solution for final conditioning are simple.
37Cerium IV process T (°C) Acid Regeneration Origin Appli- cation Speed SODPAmbHNO3O3SwedenClosed loopLowREDOX60-80°CElectro-chemicalJapanOpen systemHighMEDOC80°CH2SO4BelgiumOpen and closed loop
38MEDOC process at BR3The MEDOC process has been selected for its high decontamination efficiencyObjectivesClearance of material
39MEDOC : Only one step treatment Cerium solutionO2Ce 4+ContaminatedMaterialOzonegasCe 3+FreereleaseRegenerationof cerium IVDecontamination
40BR3 industrial plant is characterized by three stages O2Rinsingloop21Decon.loop3wastetreatmentO3
41Effluents are partially treated by SCK and transported to Belgoprocess < 5 %AsphaltWaste15 kg/m3 total4 Gbq/m3Ph NeutralizationPrecipitationFiltrationCerium neutralizationNitric acidSCK-CENBelgoprocess
44Safety precautions taken in the MEDOC installation Due to the combined radioactive and chemical hazardsconstruction materials selected to resist to the aggressive processunreacted ozone thermally destroyed before releaseO3 and H2 detectors with automatic actions on the processtwo independent ventilation systems
4625 tons of contaminated material have already been treated Treatment capacity is 0.5 ton per treatment (20 m2)Average corrosion rate 2.5 µm/hThe treatment time is about 4 to 10 hoursVery low residual contamination < 0.1 Bq/gSpecific activity of materialafter decontamination in200 Liters drums
47Steam generator and pressurizer decontamination in May 2002 Main goalMake the demonstration of large components decontamination using MEDOCReach the clearance contamination level after meltingSteam generator characteristics (primary loop - SS)30 tons of mixed stainless and carbon steelNumber of tubes 1400 in stainless steelTotal length of tubes 15 kmTotal surface 620 m2Volume 2.7 m37,94 m
48Handling of the SG before decontamination The SG has been removedand placed horizontallyto allow the total filling upof the primary side
50Workload 30 decontamination cycles are needed : Decontamination (2 hours)Regeneration of cerium IV (4 hours)After 15 cycles, the SG was rotated for homogenous attack on the primary side.60 hours decontamination and 130 hours of regeneration about 3 weeks with 2 working teams.
51Reach the clearance contamination level after melting 10 µm or 42 kg of material were removed on the overall surface.2.06 Gbq of Co60The tube bundle was manually rinsed via the primary head with pressurized water.Low radiation level in the primary head (few µSv/h) - no free contamination
52Conclusions on MEDOCContaminated materials are successfully decontaminated using a batchwise technique in MEDOC plant.Up to now, 80% of treated materials have been cleared and sold to a scrap dealer (including primary pipes)Remaining 20% can be cleared after melting (< 1 Bq/g)The loop treatment of the BR3-SG was also a successIt will easy the post-operation dismantling (HPWJC),It will avoid the evacuation of huge components in a waste category.
53HNO3/HF processesThe sulfonitric mixture is commonly used for the etching of stainless steelin batch processin pulverization solutionThe liquid penetrates the oxide layer to attack the base metal (thorough decontamination process).The oxides come off the surface and stay in the solution.The oxides are eliminated by filtration.
54The efficiency increases with the concentration and the temperature. HNO3/HF processesThe efficiency increases with the concentration and the temperature.However, it decreases with the increasing of dissolved material.This is not a regenerative process, new HF has to be added to the solution and produces more effluents.
55Application : Safety HNO3/HF processes Not very attractive in batch treatment due to the consumption of reagentGood result in pulverisation process at low temperature followed by rinsing with pressurised water jet.SafetyNeed of special attention to the worker safety due to the presence of HF and fluoride.
56Decoha or DfD processes HBF4 processesDecoha or DfD processesThe fluororic acid is able to dissolve both the oxide layer and the base alloy on stainless or carbon steelThis process is used in batch treatment or in pulverization processThe fluoroboric acid can be regenerated by electrodeposition of the metal.
58ApplicationCompared to the cerium or sulphonitric process, it is less aggressive (lower rate)Due to the formation of hydrogen in the decontamination and regeneration steps, the process required special safety attention (monitoring, ventilation, dilution with air…)The 137Cs which is not deposited has to be eliminated in IEX or by added chemical treatment.
59Advantages for chemical decontamination Chemical decontamination allows the treatment of complex geometry material (hidden parts, inside parts of tubes,…)With strong mineral acids, DF over 104 can be reached allowing the clearance of materialWith proper selection of chemicals, almost all radionuclides may be removedChemical decontamination is a known practice in many nuclear plants and facilities (experience…)
60Disadvantages for chemical decontamination The main disadvantage is the generation of secondary liquid waste which requires appropriate processes for final treatment and conditioningThe safety due to the chemical hazard with high corrosive products (Acid, gas,…) and by-products (H2, HF, …)Chemical decontamination is mostly not effective on porous surfaces
61Electrochemical decontamination Electrolytic polishing is an anodic dissolution techniqueMaterial to be decontaminated is the anode, the cathode being an electrode or the tank itselfObjectives :removed hot spotlowered dose ratedecategorisation of material
62Electrochemical decontamination +High current density at low voltagebath with acid or salt-ElectrolytePhosphoric acidNitric acidSulfuric acidSodium sulfatechemical orelectrochemical
63ApplicationElectropolishing can be used for the treatment of Carbon steel, Stainless steel, AluminumElectropolishing requires conducting surfaces (the paint must be removed)Not really adapted for small or complex geometry material with hidden parts (current density inside pipes, …)
64Special technique at KRB A plant in Gundremmingen Decontamination ofstainless steel parts withphosphoric acidElectropolishingquick processing timereliabilityless secondary wastemaximal recycling effect
65Principle of electropolishing +6000 Aat low voltagebath with phosphoric acid-beforeafterH2PO4Oxide skinchemical orelectrochemicalBase material
68Regeneration of Phosphoric Acid Recycling of Phosphoric acid byReuse acid for decontaminationThermolysis of iron oxalat- adding oxalic acid- precipitate the dissolved iron as- iron oxalate- extracting the iron oxalate- vaporization- heating the iron oxalate- transformation into iron oxide for final storage
69Schematic principle of Regeneration Dilute acid to concentrated
73Advantages of Electropolishing Commercially available and relatively inexpensiveLarge panel of material and geometry (water box of SG, tanks, large pieces,…) can be treated with this techniqueHigh corrosion rate and quick treatmentLow volume of secondary waste.
74Disadvantages of Electropolishing Electropolishing does not remove fuels, sludge or any insulating materialInside parts of tubes or hidden parts are treated poorlyLike chemical processes, secondary liquid waste are generated. This method is less applicable for industrial decontamination of complex geometries:limited by the size of the batch in immersion processThe access to contaminated parts and free space are required when an electrode (pad) is usedHandling of components may lead to additional exposure to workers
75Mechanical decontamination Mechanical decontaminations are often less aggressive than the chemical ones but they are a bit simpler to use.Mechanical and chemical techniques are complementary to achieve good resultsThe two basic disadvantagesThe contaminated surface needs to be accessibleMany methods produce air bone dust.
76Typical Mechanical decontamination Cleaning with ultrasonsProjection of CO2 ice or water icePressurized water jetDecontamination with abrasives in wet or dry environmentMechanical action by grinding, polishing, brushing
77Cleaning with ultrasons The cleaning in ultrasonic batch is only applicable for slightly fixed contaminationDoes not allow to remove the fixed contaminationThis technique is used in combination with detergent (Decon 90, …)However, it is mainly used to enhance the corrosion effect in chemical decontamination processes (Medoc,…)
78Projection of CO2 ice or water ice CO2 ice pellets are projected at high speed against the surfaceThe CO2 pellets evaporate and remove the contaminationThe operator works in ventilated suit inside a ventilated room to remove CO2 and contaminationNeeds some decontamination tests before selecting the process (not efficient for deep contamination)
79Pressurized water jet Low pressure water Jet : 50 – 150 bar Pre-decontamination techniqueRemoval of sludge or deposited oxideDecontamination of toolsMedium pressure water Jet : 150 – 700 barUsually used for the decontamination of equipments or large surfaces (pool walls,…)Large water consumption (60 – 6000 L/h) and contaminated aerosolsRequires a suitable ventilation system and a recirculation loop with filtration (recycling of water)
80Decontamination with abrasives Uses the power of abrasives projected at high speed against the surfaceWet environment: fluid transporter is waterDry environment: fluid transporter is airImperative to ensure the recycling of the abrasive to reduce the secondary waste productionNeeds a suitable ventilated system to remove contamination and aerosols.
81Abrasives in wet environment at BR3 Roof opening for large piecesOperator at work
82Abrasives in dry environment Working in enclosed areaDecontamination (Metal, plastics, concrete…)DecoatingCleaningDegreasing
83Abrasives in dry environment at Belgoprocess Automatic process in batch treatmentDeclogging filter (ventilation)Load of material
84Comparison of the wet and dry sandblasting Choose an abrasive with a long lifetime (recycling)Minerals (magnetite, sand,…)Steel pellets, aluminum oxideCeramic, glass beadsPlastic pelletsNatural productsWet and dry techniques allow to recycle the abrasive by separationFiltration or decantation in wet sandblastingOn declogging filter (ventilation) in dry sandblastingThe air contamination in dry sandblasting is much more important (cross contamination…)
85Advantages/Disadvantages abrasive-blasting Effective and commercially availableRemoves tightly adherent material (paint, oxide layer…)DisadvantagesProduces a large amount of secondary waste (abrasive and dust…)Care to introduce the contamination deeper in porous material.
86Mechanical action by grinding, polishing, brushing Large range of abrasive belts or rollers available on the marketIdeal to remove small contaminated surfaceDue to the production of dust, used in a ventilated enclosure, the operator wears protection clothes
87Melting of metalsThe melting of metal can be considered as a decontamination technique137Cs are eliminated in fumes and dustHeavy elements coming from oxide are eliminated in slag (radioactive waste)The melting technique is used forThe recycling of material in nuclear field (container,..)The clearance of ingots after melting (measurement of activity easier …)
89Advantages of meltingAdvantages of redistributing of radionuclides in ingots/slag and dust: decontamination effectEssential step when releasing components with complex geometries (allows the measurement after melting)
90ConclusionsSelection criteria of decontamination techniques for metalsThe geometry and size of piecesThe objectives of the decontamination (dose rate or waste management…)The nature and the level of contaminationThe state of the surface and the type of materialThe availability of the process
91Needs for decommissioning For decommissioning we need several complementary techniquesTo reduce the dose rate before dismantlingFSDTo treat materials with complex geometriesChemical decontaminationTo treat materials with simple geometriesSand blasting or electrochemical decontaminationTo decontaminate tools or slightly contaminated piecesHigh pressure jetManuel cleaningOther mechanical techniquesTo remove residual ‘hot spot’ after decontaminationMechanical techniques : grinding, brushingTo help in the evacuation route of materialsMelting of metals