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1 Metal Decontamination Techniques used in Decommissioning Activities Mathieu Ponnet SCKCEN.

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Presentation on theme: "1 Metal Decontamination Techniques used in Decommissioning Activities Mathieu Ponnet SCKCEN."— Presentation transcript:

1 1 Metal Decontamination Techniques used in Decommissioning Activities Mathieu Ponnet SCKCEN

2 2 Objectives and selection criteria Full System Decontamination Decontamination of components/parts Conclusions One detail example (BR3 case) in each part Summary

3 3 Definition Decontamination is defined as the removal of contamination from surfaces by Washing Heating Chemical or electrochemical action Mechanical action Others (melting…)

4 4 3 main reasons To remove the contamination from components to reduce dose level in the installation (save dose during dismantling) To minimize the potential for spreading contamination during decommissioning To reduce the contamination of components to such levels that may be Disposed of at a lower category Recycled or reused in the conventional industry (clearance of material)

5 5 Decontamination for decommissioning In maintenance work, we must avoid any damage to the component for adequate reuse In decommissioning, decontamination techniques can be destructive, the main goal being the removal of as much activity as possible (high DF)

6 6 Decontamination before Dismantling Objectives : Reduction of occupational Exposure Pipe Line System Decontamination Closed system Pool, Tank Open system Chemical Method Mechanical method Hydro jet Method Blast Method Strippable coating Method

7 7 Decontamination after Dismantling Objectives : Reduction of radioactive waste or recycling Pipes, Components Open or closed system Chemical Immersion Method Electrochemical Method Blast Method Ultrasonic wave Method Gel Method

8 8 Decontamination Of Building Objectives : Reduction of radioactive concrete waste or Release of building Concrete Surface Mechanical Method Scabbler Shaver Blast Method Concrete Demolition Explosives Jackhammer Drill &Spalling

9 9 Selecting a specific decontamination technique Need to be considered Safety: Not increase radiological or classical hazards Efficiency: Sufficient DF to reach the objectives Cost-effectiveness: should not exceed the cost for waste treatment and disposal Waste minimization: should not rise large quantities of waste resulting in added costs, work power and exposure Feasibility of industrialization: Should not be labour intensive, difficult to handle or difficult to automate.

10 10 Parameters for the selection of a decontamination process Type of plant and plant process Operating history of the plant Type of components: pipe, tank Type of material: steel, Zr, concrete Type 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 surface Decontamination factor required Destination of the components after decontamination Time required for application Capability of treatment and conditioning of the secondary waste generated

11 11 Some examples about the type of material Stainless steel: Resistant to corrosion, difficult to treat, needs a strong decontamination process to remove several µm Carbon 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.

12 12 Some examples about the type of surface Porous: 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

13 13 Some examples about decontamination factor required Primary circuit of BWR and PWR reactors Outer layer : Fe 2 O 3, Iron rich Intermediate layer (CRUD) : FeCr 2 O 4, Cr 2 O 3, Chromium rich Base alloy : Fe, Cr, Ni DF 1-5 5-50 50-10,000 Soft decontamination process Thorough decontamination process 1-5 µm 2-10 µm 5 – 30 µm Right decontamination technique

14 14 Pipes, tanks, pools… Some examples about the type of components Right decontamination technique 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…

15 15 Some 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.

16 16 Overview of decontamination process for metals Chemical process In 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 process Physical process (open system) Wet or dry abrasives, Ultrasonic cleaning, HPW, CO2 ice blasting, others…

17 17 Decontamination Techniques Used in Decommissioning Activities

18 18 Full system and closed system Decontamination Objectives Reduce the dose rate and avoid spreading of contamination during dismantling Typical decontamination factor 5 to 40 Application Decontamination of the primary circuit (RPV,PP, SG and auxiliary circuits) directly after the shutdown of the reactor Decontamination of components in a closed loop Practical objectives Remove the crud layer of about 5 to 10 µm inside the primary circuit

19 19 Chemical process Chemical process commonly used 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 solution NITROX or CITROX based on the use of nitric or citric acid. EPRI DFD (Decontamination For Decommissioning) based on the use of fluoroborique acid. Siemens England Russia Westinghouse EPRI

20 20 Multi-step decontamination process Oxidation step Oxidation of the insoluble chromium with permanganate in alkaline or acidic media, Nitric acid or fluoroboric acid Decontamination step A dissolution step is carried out with oxalic acid to dissolve the crud layer The reduction / dissolution step is enhanced by complexing agent Purification step The excess of oxalic acid is removed using permanganate or hydrogen peroxide The dissolved cations and the activity are removed using Ion Exchange Resins.

21 21 Chemical reagent MnO 4 - HNO 3 HBF 4 H 2 C 2 O 4 oxalates anionic species H2O2H2O2 Cr III to Cr VI CO 2 After destruction Water Oxidizing agent for chromium oxide Dissolving agent Minimize secondary waste Destruction agent Minimize secondary waste

22 22 The Full System Decontamination of the primary system of the BR3-PWR reactor with the Siemens CORD Process Objectives Reduce the radiation dose rate by a factor of 10 Remove the surface contamination, the so-called CRUD to avoid dispersion of contamination during dismantling of contaminated loops with a particular attention to: Minimize the amount of secondary wastes Minimize the radiation exposure of the workers Minimize the modifications to be done to the plant for the decontamination operation.

23 23 The BR3 primary loop

24 24 Full System Decontamination of the primary and auxiliary loops in 1991 3 Decontamination Cycles at 80 to 100 °C in 9 days For each cycle : 3 steps oxidation step with HMnO 4 Reduction step with H 2 C 2 O 4 Cleaning step with anionic and cationic IEX resins and removal of excess oxalic acid by oxidation with HMnO 4 or with H 2 O 2 on catalysts CORD ® : Chemical Oxidizing-Reducing Decontamination

25 25 Chemistry of the process Oxidation Step with Permanganic Acid HMnO4 at 0.3 g/l For the oxidation of the Chromium from Cr 3+ to Cr 6+ Temperature 100°C Decontamination step with Oxalic Acid H 2 C 2 O 4 at 3 g/l Dissolution step for the hematite dissolution and the activity dissolution Temperature 80°C to 100°C Cleaning step: Destruction of the excess oxalic acid by oxidation with permanganic acid or with hydrogen peroxide on a catalyst Combined with fixation of corrosion products on Ion Exchange resins Temperature: 80 to 60°C (last cycle)

26 26 Process steps for each cycle Step nr 1: Oxidation Injection of permanganic acid Circulation during several hours Step nr 2: Reduction + Decontamination Injection of oxalic acid - Circulation Purification on ion exchange Step nr 3: Cleaning Destruction of organics + purification on ion exchange Process Steps Chemicals in solution Ion exchange Resins MnO 4 - C 2 O 4 2- Cr, Fe oxalates anionic species Water, CO 2 Ni 2+, Mn 2+, Co 2+ Fixation on cationic IEX Cr, Fe oxalates Fixation on anionic IEX

27 27 Total activity removed for each cycle

28 28 Primary loop Decontamination factors ! In some points, still some hot spots due to redeposition in dead zones horizontal line of the pressurizer, dead zones in heat exchangers..

29 29 Phase I : Preparatory phase manual closure of the reactor pressure vessel maintenance of the components modifications to the circuits Phase II : Decontamination operation hot run 3 decontamination Cycles Phase III : Post decontamination operations evacuation of the liquid wastes evacuation of the solid wastes The total dose amounted to only 159 man*mSv The dose saving up to now is over 500 man*mSv 16.9 man*mSv 6.4 man*mSv 135.3 man*mSv Radiological aspects

30 30 Main data and results Contaminated surface treated1200m 2 Primary system volume15m 3 Corrosion products removed33kg Mean Crud layer removed5µm IEX Waste volume produced1.35m 3 Final waste volume8m 3 Dose rate in primary system0.08mSv/h Dose rate purification system0.06 mSv/h Mean Decontamination factor~ 10 Collective Dose exposure0.16man.Sv

31 31 Expected... Smooth process, minor operational problems Careful and detailed preparation is a must Requires a reactor in full satisfactory conditions To be performed shortly after the operation Man-Sv savings for future dismantling justify the operation Unexpected... More ion exchange resins needed and higher liquid waste volume Pollution 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 filtration Internals of RPV remarkably clean facilitating inspection and dismantling and allowing to evacuate waste in a lower category LAW vs MAW Lessons drawn from the operation …

32 32 Guidelines for selecting appropriate FSD Objectives in terms of Decontamination Factor Type of material: Acidic solution is not appropriate for carbon steel Volume 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 CRUD Capability of treatment and conditioning of the secondary waste generated (Evaporation, IEX, Precipitation, filtration…)

33 33 Decontamination Techniques Used in Decommissioning Activities

34 34 Decontamination of components/parts To reduce the contamination of components to such levels that they may be Disposed of at a lower category - decategorization Recycled 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.

35 35 Multi-step processes Same processes : Lomi, Cord, Canderem Processes in one single step (Hard decontamination process) Cerium IV process : SODP, REDOX, MEDOC HNO 3 /HF HBF 4 : Decoha, DfD.. Chemical decontamination

36 36 Cerium 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.

37 37 Cerium IV process T (°C)AcidRegene ration OriginAppli- cation Speed SODPAmbHNO3O3SwedenClosed loop Low REDOX60-80°CHNO3Electro- chemical JapanOpen system High MEDOC80°CH2SO4O3BelgiumOpen and closed loop High

38 38 The MEDOC process has been selected for its high decontamination efficiency Clearance of material Objectives MEDOC process at BR3

39 39 MEDOC : Only one step treatment Regeneration of cerium IV Ce 4+ Ce 3+ Cerium solution O2O2 Ozone gas Contaminated Material Free release Decontamination

40 40 BR3 industrial plant is characterized by three stages O3O3 O2O2 waste treatment Decon. loop Rinsing loop 1 2 3

41 41 Effluents are partially treated by SCK and transported to Belgoprocess Waste SCK-CENBelgoprocess Ph Neutralization Precipitation Filtration 15 kg/m 3 total 4 Gbq/m 3 Cerium neutralization Nitric acid 10 T < 5 % 0.3 T Asphalt

42 42 Medoc workshop after installation

43 43 Control room

44 44 Safety precautions taken in the MEDOC installation Due to the combined radioactive and chemical hazards construction materials selected to resist to the aggressive process unreacted ozone thermally destroyed before release O 3 and H 2 detectors with automatic actions on the process two independent ventilation systems

45 45 Material after decontamination

46 46 25 tons of contaminated material have already been treated Treatment capacity is 0.5 ton per treatment (20 m 2 ) Average corrosion rate 2.5 µm/h The treatment time is about 4 to 10 hours Very low residual contamination < 0.1 Bq/g Specific activity of material after decontamination in 200 Liters drums

47 47 Steam generator and pressurizer decontamination in May 2002 Main goal Make the demonstration of large components decontamination using MEDOC Reach the clearance contamination level after melting Steam generator characteristics (primary loop - SS) 30 tons of mixed stainless and carbon steel Number of tubes 1400 in stainless steel Total length of tubes 15 km Total surface 620 m2 Volume 2.7 m3 7,94 m

48 48 Handling of the SG before decontamination The SG has been removed and placed horizontally to allow the total filling up of the primary side

49 49 Main circulation loop between SG and MEDOC plant T01 ROV 01 ROV 17 HV 02 ROV 21 ROV 18 ROV 04 ROV 03 F01 ROV 16 ROV 22 T02 ROV 05 ROV 19 ROV 13 P05 P02 MS01 ROV 07 ROV 08 R01 ROV 09 FLT 01 RBS 82 RBS81 PCV 02 RBS 85 RBS 80 RBS83 Treatment gas Medoc RBS 86RBS 84 RBS 87 Decontamination step I MEDOC

50 50 Workload 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.

51 51 10 µm or 42 kg of material were removed on the overall surface. 2.06 Gbq of Co 60 The 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 Reach the clearance contamination level after melting

52 52 Conclusions on MEDOC Contaminated 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 success It will easy the post-operation dismantling (HPWJC), It will avoid the evacuation of huge components in a waste category.

53 53 HNO 3 /HF processes The sulfonitric mixture is commonly used for the etching of stainless steel in batch process in pulverization solution The 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.

54 54 The 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. HNO 3 /HF processes

55 55 Application : Not very attractive in batch treatment due to the consumption of reagent Good result in pulverisation process at low temperature followed by rinsing with pressurised water jet. HNO 3 /HF processes Safety Need of special attention to the worker safety due to the presence of HF and fluoride.

56 56 HBF 4 processes The fluororic acid is able to dissolve both the oxide layer and the base alloy on stainless or carbon steel This process is used in batch treatment or in pulverization process The fluoroboric acid can be regenerated by electrodeposition of the metal. Decoha or DfD processes

57 57 Regeneration of HBF 4 Cathode (-)Anode (+) Ion Exchange Membrane H+ Inlet solution Outlet solution to filter Metal Particles Dissolution reaction (Decontamination) Fe + 2 HBF 4 Fe(BF 4 ) 2 + H 2 Cathodics reactions Fe(BF 4 ) 2 + 2e-Fe + 2 BF 4 - Anodic reaction H2OH2O2 H + + 2e- + ½ O 2 Recombination after membrane transfer 2H + + 2 BF 4 - HBF 4 2H + + 2 e- - H2 H2

58 58 Application Compared 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 137 Cs which is not deposited has to be eliminated in IEX or by added chemical treatment.

59 59 Advantages for chemical decontamination Chemical decontamination allows the treatment of complex geometry material (hidden parts, inside parts of tubes,…) With strong mineral acids, DF over 10 4 can be reached allowing the clearance of material With proper selection of chemicals, almost all radionuclides may be removed Chemical decontamination is a known practice in many nuclear plants and facilities (experience…)

60 60 The main disadvantage is the generation of secondary liquid waste which requires appropriate processes for final treatment and conditioning The safety due to the chemical hazard with high corrosive products (Acid, gas,…) and by- products (H 2, HF, …) Chemical decontamination is mostly not effective on porous surfaces Disadvantages for chemical decontamination

61 61 Electrochemical decontamination Electrolytic polishing is an anodic dissolution technique Material to be decontaminated is the anode, the cathode being an electrode or the tank itself Objectives : removed hot spot lowered dose rate decategorisation of material

62 62 + - chemical or electrochemical bath with acid or salt Electrochemical decontamination Phosphoric acid Nitric acid Sulfuric acid Sodium sulfate Electrolyte High current density at low voltage

63 63 Application Electropolishing can be used for the treatment of Carbon steel, Stainless steel, Aluminum Electropolishing requires conducting surfaces (the paint must be removed) Not really adapted for small or complex geometry material with hidden parts (current density inside pipes, …)

64 64 Special technique at KRB A plant in Gundremmingen Decontamination of stainless steel parts with phosphoric acid quick processing time reliability less secondary waste maximal recycling effect Electropolishing

65 65 Principle of electropolishing Oxide skin Base material before after + - H 2 PO 4 6000 A at low voltage chemical or electrochemical bath with phosphoric acid

66 66 Stainless Steel in Acid Bath

67 67 Stainless Steel after Electropolishing

68 68 Regeneration of Phosphoric Acid Recycling of Phosphoric acid by Reuse acid for decontamination Thermolysis 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

69 69 Schematic principle of Regeneration Dilute acid to concentrated

70 70 Thermolysis plant for iron oxalate 190°C

71 71 Example Secondary Steam Generator Vessels of three Steam Generators decontaminated from 20,000 Bq/cm² to free release producing only 1,5% radioactive waste

72 72 Electropolishing processes Electro- lyte Conc M Current density A/m2 Elec- trode Time Hours Corro- sion rate µm/h AEA/ Harwell HNO 3 120-30Ti2NC CEA/ UDIN HNO 3 2100-300Basket Ti 1-316 – 20 ToshibaH 2 SO 4 0.53000 – 10000 60°C NC<160-240 Eldecon ABB/ Sweden Na 2 SO 4 0.51000- 6000 NC<160

73 73 Advantages of Electropolishing Commercially available and relatively inexpensive Large panel of material and geometry (water box of SG, tanks, large pieces,…) can be treated with this technique High corrosion rate and quick treatment Low volume of secondary waste.

74 74 Electropolishing does not remove fuels, sludge or any insulating material Inside parts of tubes or hidden parts are treated poorly Like 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 process The access to contaminated parts and free space are required when an electrode (pad) is used Handling of components may lead to additional exposure to workers Disadvantages of Electropolishing

75 75 Mechanical 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 results The two basic disadvantages The contaminated surface needs to be accessible Many methods produce air bone dust.

76 76 Cleaning with ultrasons Projection of CO 2 ice or water ice Pressurized water jet Decontamination with abrasives in wet or dry environment Mechanical action by grinding, polishing, brushing Typical Mechanical decontamination

77 77 Cleaning with ultrasons The cleaning in ultrasonic batch is only applicable for slightly fixed contamination Does not allow to remove the fixed contamination This technique is used in combination with detergent (Decon 90, …) However, it is mainly used to enhance the corrosion effect in chemical decontamination processes (Medoc,…)

78 78 Projection of CO 2 ice or water ice CO 2 ice pellets are projected at high speed against the surface The CO 2 pellets evaporate and remove the contamination The operator works in ventilated suit inside a ventilated room to remove CO 2 and contamination Needs some decontamination tests before selecting the process (not efficient for deep contamination)

79 79 Pressurized water jet Low pressure water Jet : 50 – 150 bar Pre-decontamination technique Removal of sludge or deposited oxide Decontamination of tools Medium pressure water Jet : 150 – 700 bar Usually used for the decontamination of equipments or large surfaces (pool walls,…) Large water consumption (60 – 6000 L/h) and contaminated aerosols Requires a suitable ventilation system and a recirculation loop with filtration (recycling of water)

80 80 Decontamination with abrasives Uses the power of abrasives projected at high speed against the surface Wet environment: fluid transporter is water Dry environment: fluid transporter is air Imperative to ensure the recycling of the abrasive to reduce the secondary waste production Needs a suitable ventilated system to remove contamination and aerosols.

81 81 Abrasives in wet environment at BR3 Roof opening for large pieces Operator at work

82 82 Abrasives in dry environment Decontamination (Metal, plastics, concrete…) Decoating Cleaning Degreasing Working in enclosed area

83 83 Abrasives in dry environment at Belgoprocess Automatic process in batch treatment Declogging filter (ventilation) Load of material

84 84 Comparison of the wet and dry sandblasting Choose an abrasive with a long lifetime (recycling) Minerals (magnetite, sand,…) Steel pellets, aluminum oxide Ceramic, glass beads Plastic pellets Natural products Wet and dry techniques allow to recycle the abrasive by separation Filtration or decantation in wet sandblasting On declogging filter (ventilation) in dry sandblasting The air contamination in dry sandblasting is much more important (cross contamination…)

85 85 Advantages/Disadvantages abrasive-blasting Advantages Effective and commercially available Removes tightly adherent material (paint, oxide layer…) Disadvantages Produces a large amount of secondary waste (abrasive and dust…) Care to introduce the contamination deeper in porous material.

86 86 Mechanical action by grinding, polishing, brushing Large range of abrasive belts or rollers available on the market Ideal to remove small contaminated surface Due to the production of dust, used in a ventilated enclosure, the operator wears protection clothes

87 87 Melting of metals The melting of metal can be considered as a decontamination technique 137 Cs are eliminated in fumes and dust Heavy elements coming from oxide are eliminated in slag (radioactive waste) The melting technique is used for The recycling of material in nuclear field (container,..) The clearance of ingots after melting (measurement of activity easier …)

88 88 Melting of metals CountryCapacityMaterialProduct CarlaGermany3 tCS, SS, Al, Cu Ingot, shield blocks, containers StudsvikSweden3 tCS, SS, Al Ingot

89 89 Advantages of melting Advantages of redistributing of radionuclides in ingots/slag and dust: decontamination effect Essential step when releasing components with complex geometries (allows the measurement after melting)

90 90 Conclusions Selection criteria of decontamination techniques for metals The geometry and size of pieces The objectives of the decontamination (dose rate or waste management…) The nature and the level of contamination The state of the surface and the type of material The availability of the process

91 91 Needs for decommissioning For decommissioning we need several complementary techniques To reduce the dose rate before dismantling FSD To treat materials with complex geometries Chemical decontamination To treat materials with simple geometries Sand blasting or electrochemical decontamination To decontaminate tools or slightly contaminated pieces High pressure jet Manuel cleaning Other mechanical techniques To remove residual hot spot after decontamination Mechanical techniques : grinding, brushing To help in the evacuation route of materials Melting of metals

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