Metal Decontamination Techniques used in Decommissioning Activities

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 Pool, Tank Open system Pipe Line System Decontamination Closed 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. 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. 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. 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 • 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 HBF4 CrIII to CrVI 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 • 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 6.4 man*mSv 16.9 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 1.35 m3 • Final waste volume 8 m3 • Dose rate in primary system 0.08 mSv/h • Dose rate purification system 0.06 mSv/h • Mean Decontamination factor ~ 10 • Collective Dose exposure 0.16 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

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

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

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

41. Effluents are partially treated by SCK and transported to Belgoprocess 10 T 0.3 T < 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 decontaminationin 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 13 ROV 01 ROV 05 RBS83 ROV 22 MEDOC T01 ROV 04 T02 ROV 08 MS01 ROV 09 ROV 03 RBS 80 ROV 21 ROV 17 ROV 18 ROV 16 ROV 19 FLT 01 HV 02 P02 P05 F01

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.