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Metal Decontamination Techniques used in Decommissioning Activities

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1 Metal Decontamination Techniques used in Decommissioning Activities
Mathieu Ponnet SCK•CEN

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

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

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

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

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 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 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 Right decontamination technique
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 Soft decontamination process Thorough decontamination process
Some examples about decontamination factor required Primary circuit of BWR and PWR reactors DF Soft decontamination process Outer layer : Fe2O3, Iron rich 1-5 µm 1-5 Intermediate layer (CRUD) : FeCr2O4, Cr2O3, Chromium rich 2-10 µm 5-50 Thorough decontamination process 5 – 30 µm Base alloy : Fe, Cr, Ni 50-10,000 Right decontamination technique

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

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 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 Decontamination Techniques Used in Decommissioning Activities
Objectives and selection criteria Full system Decontamination General consideration Chemical reagents Spent decontamination solutions Guidelines for selecting appropriate FSD The case of BR3 (CORD) Decontamination of components/parts Conclusions

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 Chemical 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 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 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 Chemical reagent MnO4- HNO3 CrIII to CrVI HBF4 Oxidizing agent
for chromium oxide H2C2O4 oxalates anionic species CO2 After destruction Dissolving agent Minimize secondary waste H2O2 Water Destruction agent Minimize secondary waste

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 The BR3 primary loop

24 Full System Decontamination of the primary and auxiliary loops in 1991
CORD®: Chemical Oxidizing-Reducing Decontamination 3 Decontamination Cycles at 80 to 100 °C in 9 days For each cycle : 3 steps oxidation step with HMnO4 Reduction step with H2C2O4 Cleaning step with anionic and cationic IEX resins and removal of excess oxalic acid by oxidation with HMnO4 or with H2O2 on catalysts

25 Chemistry of the process
Oxidation Step with Permanganic Acid HMnO4 at 0.3 g/l For the oxidation of the Chromium from Cr3+ to Cr6+ Temperature 100°C Decontamination step with Oxalic Acid H2C2O4 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 Process steps for each cycle
Ion exchange Resins Chemicals in solution Process Steps Step nr 1: Oxidation Injection of permanganic acid Circulation during several hours MnO4- Step nr 2: Reduction + Decontamination Injection of oxalic acid - Circulation Purification on ion exchange C2O42- Cr, Fe oxalates anionic species Ni2+, Mn2+, Co2+ Fixation on cationic IEX Step nr 3: Cleaning Destruction of organics + purification on ion exchange Water, CO2 Cr, Fe oxalates Fixation on anionic IEX

27 Total activity removed for each cycle

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 Radiological aspects The total dose amounted to only 159 man*mSv
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 135.3 man*mSv man*mSv man*mSv The total dose amounted to only 159 man*mSv The dose saving up to now is over 500 man*mSv

30 Main data and results Contaminated surface treated 1200 m2
Primary system volume 15 m3 Corrosion products removed 33 kg Mean Crud layer removed 5 µm IEX Waste volume produced m3 Final waste volume 8 m3 Dose rate in primary system mSv/h Dose rate purification system mSv/h Mean Decontamination factor ~ 10 Collective Dose exposure man.Sv

31 Lessons drawn from the operation …
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

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 Decontamination Techniques Used in Decommissioning Activities
Objectives and selection criteria Full system decontamination Decontamination of components/parts General considerations Chemical decontamination Electrochemical decontamination Mechanical decontamination Decontamination by melting Guidelines for selecting appropriate decontamination techniques The case of BR3 (ZOE - MEDOC) Conclusions

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 Chemical decontamination
Multi-step processes Same processes : Lomi, Cord, Canderem Processes in one single step (Hard decontamination process) Cerium IV process : SODP, REDOX, MEDOC HNO3/HF HBF4 : Decoha, DfD..

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 Cerium IV process T (°C) Acid Regeneration Origin Appli- cation Speed
SODP Amb HNO3 O3 Sweden Closed loop Low REDOX 60-80°C Electro- chemical Japan Open system High MEDOC 80°C H2SO4 Belgium Open and closed loop

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

39 MEDOC : Only one step treatment
Cerium solution O2 Ce 4+ Contaminated Material Ozone gas Ce 3+ Free release Regeneration of cerium IV Decontamination

40 BR3 industrial plant is characterized by three stages
O2 Rinsing loop 2 1 Decon. loop 3 waste treatment O3

41 Effluents are partially treated by SCK and transported to Belgoprocess
< 5 % Asphalt Waste 15 kg/m3 total 4 Gbq/m3 Ph Neutralization Precipitation Filtration Cerium neutralization Nitric acid SCK-CEN Belgoprocess

42 Medoc workshop after installation

43 Control room

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 O3 and H2 detectors with automatic actions on the process two independent ventilation systems

45 Material after decontamination

46 25 tons of contaminated material have already been treated
Treatment capacity is 0.5 ton per treatment (20 m2) 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 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 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 Main circulation loop between SG and MEDOC plant
RBS 87 RBS 86 RBS 84 RBS 82 PCV 02 RBS 85 Decontamination step I RBS81 Treatment gas Medoc ROV 07 R01 ROV 01 ROV 13 RBS83 MEDOC ROV 22 ROV 05 T01 ROV 04 ROV 08 T02 MS01 ROV 09 RBS 80 ROV 03 ROV 21 HV 02 FLT 01 ROV 17 ROV 18 ROV 16 P02 ROV 19 F01 P05

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 Reach the clearance contamination level after melting
10 µm or 42 kg of material were removed on the overall surface. 2.06 Gbq of Co60 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

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 HNO3/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 The efficiency increases with the concentration and the temperature.
HNO3/HF processes 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.

55 Application : Safety HNO3/HF processes
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. Safety Need of special attention to the worker safety due to the presence of HF and fluoride.

56 Decoha or DfD processes
HBF4 processes Decoha or DfD 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.

57 Regeneration of HBF4 Dissolution reaction (Decontamination)
Inlet solution Dissolution reaction (Decontamination) Cathode (-) Anode (+) Fe + 2 HBF4 Fe(BF4)2 + H2 Ion Exchange Membrane Cathodics reactions Fe(BF4)2 + 2e- Fe + 2 BF4 - 2H e-- H2 Anodic reaction H+ H2O 2 H+ + 2e- + ½ O2 Metal Particles Recombination after membrane transfer 2H BF4 - HBF4 Outlet solution to filter

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

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 104 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 Disadvantages for chemical decontamination
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 (H2, HF, …) Chemical decontamination is mostly not effective on porous surfaces

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

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

65 Principle of electropolishing
+ 6000 A at low voltage bath with phosphoric acid - before after H2PO4 Oxide skin chemical or electrochemical Base material

66 Stainless Steel in Acid Bath

67 Stainless Steel after Electropolishing

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 Schematic principle of Regeneration
Dilute acid to concentrated

70 Thermolysis plant for iron oxalate

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 Electropolishing processes
Electro-lyte Conc M Current density A/m2 Elec-trode Time Hours Corro-sion rate µm/h AEA/ Harwell HNO3 1 20-30 Ti 2 NC CEA/ UDIN Basket 1-3 16 – 20 Toshiba H2SO4 0.5 3000 – 10000 60°C <1 60-240 Eldecon ABB/ Sweden Na2SO4 60

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 Disadvantages of Electropolishing
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

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 Typical Mechanical decontamination
Cleaning with ultrasons Projection of CO2 ice or water ice Pressurized water jet Decontamination with abrasives in wet or dry environment Mechanical action by grinding, polishing, brushing

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 Projection of CO2 ice or water ice
CO2 ice pellets are projected at high speed against the surface The CO2 pellets evaporate and remove the contamination The operator works in ventilated suit inside a ventilated room to remove CO2 and contamination Needs some decontamination tests before selecting the process (not efficient for deep contamination)

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

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

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

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 Advantages/Disadvantages abrasive-blasting
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 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 Melting of metals The melting of metal can be considered as a decontamination technique 137Cs 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 Ingot, shield blocks, containers
Melting of metals Country Capacity Material Product Carla Germany 3 t CS, SS, Al, Cu Ingot, shield blocks, containers Studsvik Sweden CS, SS, Al Ingot

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 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 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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