WO2000051135A1 - Traitement de metaux contamines par une radioactivite - Google Patents

Traitement de metaux contamines par une radioactivite Download PDF

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Publication number
WO2000051135A1
WO2000051135A1 PCT/GB2000/000685 GB0000685W WO0051135A1 WO 2000051135 A1 WO2000051135 A1 WO 2000051135A1 GB 0000685 W GB0000685 W GB 0000685W WO 0051135 A1 WO0051135 A1 WO 0051135A1
Authority
WO
WIPO (PCT)
Prior art keywords
resultant solution
solution
metal
calcium hydroxide
decontamination
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
PCT/GB2000/000685
Other languages
English (en)
Inventor
Timothy N. Milner
Rebecca A. Robbins
Robert G. G. Holmes
Stuart Shealy
Edward Morren
Colin R. Austin
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Sellafield Ltd
Original Assignee
British Nuclear Fuels PLC
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by British Nuclear Fuels PLC filed Critical British Nuclear Fuels PLC
Priority to EP00906489A priority Critical patent/EP1155417A1/fr
Priority to AU28149/00A priority patent/AU2814900A/en
Publication of WO2000051135A1 publication Critical patent/WO2000051135A1/fr
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
    • G21F—PROTECTION AGAINST X-RADIATION, GAMMA RADIATION, CORPUSCULAR RADIATION OR PARTICLE BOMBARDMENT; TREATING RADIOACTIVELY CONTAMINATED MATERIAL; DECONTAMINATION ARRANGEMENTS THEREFOR
    • G21F9/00—Treating radioactively contaminated material; Decontamination arrangements therefor
    • G21F9/001—Decontamination of contaminated objects, apparatus, clothes, food; Preventing contamination thereof
    • G21F9/002—Decontamination of the surface of objects with chemical or electrochemical processes
    • G21F9/004—Decontamination of the surface of objects with chemical or electrochemical processes of metallic surfaces
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
    • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions

Definitions

  • This invention relates to the treatment of radioactively contaminated metals.
  • the present invention may be applied to the treatment of metal components which have been used in the nuclear power industry and have become radioactively contaminated.
  • Such components can be cleaned using a combination of physical and chemical techniques following which they may be released onto the scrap metal market or melted and formed into plate or billets for recycling.
  • a chemical decontamination process utilises a series of immersion tanks to remove fixed radionuclides from the surface of metal components. Acid solutions contained in these immersion tanks dissolve scale, corrosion products and some base metal to effect removal of surface contamination. The various metal species and radionuclides accumulate in the immersion tanks and, as a result, performance deteriorates and the decontaminant must be changed.
  • the present invention provides a flexible, robust decontamination and waste treatment process enabling high throughput in a minimal cost operating regime.
  • the process of the invention is capable of generating a significant volume of radioactive scrap metal (RSM), ranging from P WR Inconel components, which are contaminated with activation and fission products, to aluminium components contaminated with transuranics.
  • RSSM radioactive scrap metal
  • a method of treating a radioactively contaminated metal object comprising contacting the object with an acid solution to cause dissolution of a surface layer of the metal object, and raising the pH of the resultant solution with calcium hydroxide and a magnesium containing compound to cause dissolved metal to separate from the solution in solid form.
  • the acid is a mixture of nitric and hydrofluoric acids.
  • the calcium hydroxide and magnesium containing compound are provided in the form of the mineral Dolomite.
  • sodium hydroxide is additionally used to raise the pH of the resultant solution. More preferably the sodium hydroxide is added after the calcium hydroxide and until the pH is raised to from 9 to 11, more preferably to about 9.5.
  • caesium removal from the resultant solution is effected by addition of nickel hexacyanoferrate. It is preferred that the nickel hexacyanoferrate is prepared "fresh in situ", that is to say immediately or shortly before addition to the resultant solution. This avoids degradation of the nickel hexacyanoferrate which decomposes on storage.
  • a polymer is then added to assist settling of solids. More preferably, the polymer is an anionic polymer.
  • the resultant solution is subjected to UV oxidation, more preferably by adding sodium hypochlorite and subjecting the solution to UV radiation.
  • the resultant solution is subjected to ion exchange using the material clinoptillolite, a natural zeolite, in order to remove caesium not already removed by the nickel hexacyanoferrate treatment.
  • the resultant solution is subjected to mechanical filtration prior to disposal of the supernatant liquid.
  • FIG. 1 of the accompanying drawings is a flow diagram of a decontamination process in accordance with the present invention.
  • the process involves various unit operations.
  • the incoming radioactive scrap (RS) is subjected to segregation and size reduction. It is then sentenced for physical chemical decontamination. Processing takes place through the decontamination line and a radionuclide contamination survey of the processed metals is carried out.
  • the decontaminated metals are sent for recycling, disposal as LL W or sale into the scrap market.
  • the spent decontamination liquor is treated and this results in solid/liquid separation.
  • the process liquors are treated to meet the local sewer discharge limits. Process sludges are immobilised for disposal as non-RCRA dry active waste.
  • the chemical decontamination stage or stages provide a safe and cost effective process for the decontamination of RSM.
  • the chemical decontamination should be capable of accepting a wide variation in feedstock and offer a cost saving over direct disposal of RSM. There should be produced a minimal volume of non RCRA secondary waste and the process liquors should be suitable for discharge to the local sewer.
  • the acid solution providing the decontamination of liquor comprises nitric and hydrofluoric acids which enable decontamination to be carried out on a varied or variable feedstock at high throughput levels.
  • the aggressive nature of the nitric/hydrofluoric acid decontamination process requires careful control of the process parameters to achieve an optimum surface corrosion rate for a given substrate.
  • Such optimising has various advantages. Base metal corrosion is kept to a minimum. The lifetime of the decontamination tank or tanks is prolonged. Uniform surface corrosion takes place without pitting of the base metal. The volumes of secondary waste are minimised by avoiding dissolution of uncontaminated base metal. There is also an avoidance of uncontrolled dissolution reactions which gives rise to off gases as the acids break down.
  • Ni 2+ + 2HF NiF 2 + 2H +
  • the metal dissolution rate of the acids is directly proportional to the concentration of the acids. Accordingly, for a high throughput of metal in the decontamination line, two factors have a significant impact on decontamination performance. First, the overburdening of the acids with dissolved metals results in insufficient unreacted acid to achieve significant dissolution of the metal substrate. Secondly unreacted acid in the spent liquor has to be neutralised during waste treatment and this generates a significant volume of secondary waste.
  • the operation of the decontamination process of the present invention has involved a balance which has been struck between metal loading of the acids and dissolution rates, taking into account the potential for surface pitting and off gas production. It is important to minimise the production of off gasses that require scrubbing, thereby resulting in the generation of a secondary waste form and depletion of the acid capacity. This is particularly important when decontaminating the more reactive metals such as carbon steel and aluminium.
  • the process is operated using a plurality of decontamination tanks in a manner referred to as "lag" basis.
  • This procedure involves designating the decontamination tanks as low, medium and high dissolved metal content, each fulfilling a particular duty within the overall decontamination process.
  • the low tank is used for final polishing of the decontaminated metal as this tank has the lowest radionuclide inventory and the highest dissolution kinetics.
  • the high tank is used for initial decontamination where the highest levels of radionuclides, associated with the outermost surface, are removed.
  • the medium tank is a transitional tank which is additionally used for "special" materials, which are substantially different from other materials of the feedstock. The use of the medium tank is optional.
  • the tanks are rotated from low to high based on a predetermined dissolved metal and radionuclide inventory.
  • the high tank is taken off line and designated as spent once decontamination has become ineffective.
  • a regular sampling and analysis procedure to measure and record trends in tank performance.
  • the procedure may be simplified with the cost of analysis reduced by using the experience gained from operating the process to devise a "finger print" which can be used to determine when a tank should be reclassified.
  • the tank lag system allows decontamination to free release to be performed in the low activity tank and decontamination for recategorisation to LLW to be performed initially in the high and subsequently in the medium activity tank.
  • An additional advantage of the lag system is the degree of uniformity, in terms of dissolved metal and radionuclide content, that is introduced to the spent tank waste treatment process.
  • the spent acid resulting from the decontamination stage or stages may be subjected to a number of treatment steps.
  • Treatment of the spent acids produces two waste streams.
  • One is an alkaline wastewater which, after final polishing by chelant oxidation followed by ion exchange, is discharged to the local sewer.
  • the other is a sludge containing the major metal species (iron, chromium, nickel) and radionuclides (cobalt, cesium) which may be directly solidified by grouting with, for instance, Portland cement to meet the appropriate waste acceptance criteria.
  • the spent acids from the chemical decontamination process contain a high concentration of dissolved metals and radionuclides, as illustrated in the following table:
  • the aim is to produce a minimal volume of non- RCRA solid waste. Since the composition of the metal component which is treated in the chemical decontamination stage affects the ratios of metals in the spent acid, a highly flexible approach is required.
  • the waste treatment process may be either a batch process or a continuous process although reference will be made hereinafter to a batch process.
  • the waste treatment process may involve various steps as illustrated in Figure 2 of the accompanying drawings.
  • the spent acids solution is partially neutralised and conditioned using calcium hydroxide.
  • the pH is then adjusted to a value in the range of from 9 to 10 using sodium hydroxide.
  • Flocculation is then effected by seeding with inorganic adsorbents causing resultant coprecipitation of metals and radionuclides.
  • Polymers are then added to aid gravity settling of solids.
  • the solids and liquid are then separated and the solids are subjected to grouting.
  • the wastewater liquid is then treated by means of UV/sodium hypochlorite to cause oxidation of chelate.
  • the waste water is polished by ion exchange using clinoptillolite as the resultant liquid is discharged to a local sewer.
  • the above described process steps may be performed sequentially over a 24 hour period and the acid tank, when empty, is replenished with fresh acid and returned to service.
  • the above described treatment process is both robust and simple to operate and uses readily available, low cost chemicals and process equipment.
  • Calcium hydroxide is used principally to condition the metal hydroxide floe by assisting in the binding of leachable metals, principally chromium.
  • Use of the magnesium containing compound improves the removal of hexavalent chromium.
  • the calcium fluoride formed in the partial neutralisation forms the body of the floe and improves sludge settling characteristics.
  • the calcium hydroxide is not used for pH control as this may result in the formation of an excessive amount of calcium sludge requiring disposal as a secondary waste product. Accordingly pH adjustment is carried out using sodium hydroxide, which is highly soluble and results in a sharp inflection point in pH, thereby allowing greater process control.
  • the calcium hydroxide and the sodium hydoxide may be added
  • the calcium from the calcium hydroxide forms insoluble calcium fluoride, making downstream processing of the supernatant liquid easier since the fluoride has been removed.
  • the presence of dissolved iron assists in chromium removal due to the Fe (II)- ⁇ -»Fe (III) couple ensuring the chromium is present in reduced Cr (III) state.
  • additional ferrous iron may be required to reduce chromium to the less soluble form. This can also be controlled by management of the processing of nickel/chromium alloys and stainless or carbon steels in order to supply the appropriate iron ratio needed for chromium reduction.
  • HCF nickel hexacyanoferrate
  • a solid HCF ion exchanger is added in the amount of 300 ppm as a dilute slurry directly into the batch treatment tank once the pH has been adjusted.
  • Caesium removal is aided by isotopic dilution by adding 10 ppm of non-radioactive caesium chloride.
  • the HCF is used immediately after it has been formulated. Lower caesium removal and higher residual nickel resulted if the HCF is aged.
  • the kinetics of the cesium complexation with HCF is very rapid and the complex formed is removed from solution by adsorption and agglomeration with the metal hydroxide floe.
  • a decontamination factor of 600 has been achieved for cesium and the supernant liquor was free of all metals except a few ppm of nickel and chromium and trace levels of caesium.
  • Final caesium removal was accomplished by adsorption on clinoptillolite, a natural zeolite.
  • Separation of the floe from the supernatant can be achieved by the use of mechanical separation devices.
  • mechanical separation devices are either costly, generate a secondary waste in the form of a filter media or require frequent manual intervention. Accordingly it is preferred to make use of gravity settling and decanting.
  • several hundred ppm of an anionic polymer was added to increase the rate of settling and the clarity of the supernatant. More preferably, the tank content are rotated using a stirrer device.
  • the supernatant from the HCF step was decanted and processed through the waste water treatment polishing step.
  • the solution was dosed with 0.25 to 0.5% sodium hypochlorite and treated with UV light to oxidise the soluble heavy metal complexes (nickel and cobalt-60). Insoluble compounds are produced.
  • the treatment time was from 6 to 18 hours.
  • the particulate is removed by mechanical filtration by passing the material through a bed of sand and charcoal followed by cartridge filters.
  • the concentration of heavy metals primarily nickel, is reduced from 10 to 20 mgL to the required discharge limit of not greater than 1.5 mg/L.
  • the 60 Co was similarly reduced from 20 to 50 pCi/L to the required discharge limit of less than 1 pCi/L.
  • caesium removal may be achieved by mixing the decant liquor with laundry water from the final effluent tank. This dilution lowered the concentration of nitrate and fluoride salts in the wastewater and improved the efficiency of the caesium removal by ion exchange using two columns packed with clinoptillolite.
  • the clinoptillolite column had a residence time of 60 to 80 minutes and the resulting wastewater met local discharge criteria.
  • the capacity of the ion exchange column was found to be between 80,000 to 90,000 bed volumes and the sand bed filter did not require to be changed out.
  • the second waste stream from the WTS comprises the radionuclide and metal contaminated sludge from the settling and decanting step.
  • This sludge or metal floe was directly solidified in a cementicious grout. This was achieved by pumping the settled solids from the batch treatment tank into 55-gallon drums. A helical drum mixer was used to incorporate 0.75 to 1.0 pounds of portland cement per pound of sludge. This was accomplished with 20 to 30 minutes mixing and there was only a small heat rise during this process. The grout was then pumped from the drums into moulds that were designed to produce a final waste form meeting local acceptance criteria for size requirements for debris. The resulting grouted waste was produced with a minimal volume increase of 20%. It was cured overnight with no bleed and had a strength of over 50 psi.
  • a quantity of radioactively contaminated ferrous iron was loaded into a bath contain nitric and hydrofluoric acids and the acids were allowed to react with the surface of the iron until a desired metal loading in the bath had been achieved.
  • Dolomitic lime calcium hydroxide containing magnesium
  • the bath was then agitated for 30 minutes following which caustic soda (sodium hydroxide diluted with water) was added to give a pH in the bath of 9.5.
  • the bath was agitated for a further 30 minutes following which caesium removal is effected by the addition of nickel hexacyanoferrate (HCF).
  • the HCF was prepared "fresh in situ" to avoid degradation in storage, that is to say, it is added to the bath immediately after its preparation.
  • an anionic polymer (Nalclear) was added to the bath in order to promote the settling of the solids over a period of about 12 hours.
  • the resultant solid sludge was separated from the supernatant and was immobilised in grout.
  • the resultant liquid may be treated by clinoptillolite ion exchange to remove further caesium, following which the liquor may be discharged to a sewer.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • General Chemical & Material Sciences (AREA)
  • Metallurgy (AREA)
  • Mechanical Engineering (AREA)
  • Materials Engineering (AREA)
  • Organic Chemistry (AREA)
  • Electrochemistry (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Food Science & Technology (AREA)
  • Physics & Mathematics (AREA)
  • General Engineering & Computer Science (AREA)
  • High Energy & Nuclear Physics (AREA)
  • Treatment Of Water By Ion Exchange (AREA)

Abstract

La présente invention concerne le traitement d'objets contaminés par une radioactivité. En l'occurrence, on traite ces objets au moyen d'une solution acide de façon à provoquer la dissolution d'une couche superficielle des objets. On fait alors remonter le pH de la solution au moyen d'hydroxyde de calcium et d'un composé à base de magnésium de façon à amener le métal dissout à se séparer sous forme solide de la solution. On peut compléter le traitement par l'adjonction d'hydroxyde de sodium et d'hexocyanoferrate de nickel, par une décantation des solides sous polymères, par une oxydation sous UV, par un échange d'ions et une filtration.
PCT/GB2000/000685 1999-02-26 2000-02-28 Traitement de metaux contamines par une radioactivite Ceased WO2000051135A1 (fr)

Priority Applications (2)

Application Number Priority Date Filing Date Title
EP00906489A EP1155417A1 (fr) 1999-02-26 2000-02-28 Traitement de metaux contamines par une radioactivite
AU28149/00A AU2814900A (en) 1999-02-26 2000-02-28 Treatment of radioactively contaminated metals

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
US12185499P 1999-02-26 1999-02-26
US60/121,854 1999-02-26
US12283399P 1999-03-04 1999-03-04
US60/122,833 1999-03-04

Publications (1)

Publication Number Publication Date
WO2000051135A1 true WO2000051135A1 (fr) 2000-08-31

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PCT/GB2000/000685 Ceased WO2000051135A1 (fr) 1999-02-26 2000-02-28 Traitement de metaux contamines par une radioactivite

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EP (1) EP1155417A1 (fr)
AU (1) AU2814900A (fr)
WO (1) WO2000051135A1 (fr)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP3244418A1 (fr) * 2016-05-12 2017-11-15 RWE Power AG Décontamination chimique de surfaces métalliques radioactives

Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2612528A1 (fr) * 1987-03-18 1988-09-23 Chimiderouil Procede de traitement de surface de pieces, notamment metalliques

Patent Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2612528A1 (fr) * 1987-03-18 1988-09-23 Chimiderouil Procede de traitement de surface de pieces, notamment metalliques

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
HUSAIN A: "A PROCESS FOR DECONTAMINATING STAINLESS STEELS TO RELEASE LIMITS", NUCLEAR TECHNOLOGY,US,AMERICAN NUCLEAR SOCIETY. LA GRANGE PARK, ILLINOIS, vol. 85, no. 1, 1 April 1989 (1989-04-01), pages 66 - 73, XP000008933, ISSN: 0029-5450 *

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP3244418A1 (fr) * 2016-05-12 2017-11-15 RWE Power AG Décontamination chimique de surfaces métalliques radioactives
DE102016208202A1 (de) * 2016-05-12 2017-11-16 Rwe Power Aktiengesellschaft Chemische Dekontamination von radioaktiven Metalloberflächen

Also Published As

Publication number Publication date
AU2814900A (en) 2000-09-14
EP1155417A1 (fr) 2001-11-21

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