Classifications

• • G—PHYSICS
  • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
  • G21C—NUCLEAR REACTORS
  • G21C19/00—Arrangements for treating, for handling, or for facilitating the handling of, fuel or other materials which are used within the reactor, e.g. within its pressure vessel
  • G21C19/42—Reprocessing of irradiated fuel
  • G21C19/44—Reprocessing of irradiated fuel of irradiated solid fuel
  • G21C19/48—Non-aqueous processes
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E30/00—Energy generation of nuclear origin
  • Y02E30/30—Nuclear fission reactors
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02W—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO WASTEWATER TREATMENT OR WASTE MANAGEMENT
  • Y02W30/00—Technologies for solid waste management
  • Y02W30/50—Reuse, recycling or recovery technologies

Definitions

• the present invention relates to a method of separating uranium from at least fission products in irradiated nuclear fuel.
• the Purex process includes an initial stage or x head-end' in which the fuel rods first are cut into shorter lengths and then are exposed to hot nitric acid which leaches out the irradiated nuclear fuel from inside the Zircaloy (tradename) cladding.
• the unreacted Zircaloy cladding is collected and disposed of as Medium Activity (MA) waste.
• MA Medium Activity
• the head-end' is an extraction stage, in which the nitric acid solution which contains the uranium and plutonium as well as fission products is subjected to a solvent extraction cycle to separate the uranium and plutonium from the fission products. In subsequent stages the uranium and plutonium are separated and purified. It remains desirable to improve aspects of the Purex process. For example, it would be advantageous to simplify the cutting and dissolution steps in the head-end' . It would also be desirable to reduce the volume of acid and solvent used and thereby the volume of waste generated.
• the present invention aims to overcome these problems by way of an alternative method to the Purex process.
• a reprocessing method has been disclosed some time ago in US 3'012'849 and US 3'145'078 comprising converting the uranium and zirconium in the irradiated fuel and cladding respectively to uranium and zirconium fluoride complexes and then separating the uranium complex.
• the fluoride complexes are formed by reaction of the fuel and cladding with a mixture of HF and either NOF or metal fluoride (denoted MF) .
• NOF NOF
• MF metal fluoride
• ammonium bifluoride has been proposed to treat Zircaloy to convert it to a convenient storage form as part of a nuclear waste storage scheme. Also previously, in Chem. Eng. Prog. 5_0 (1954) 230, ammonium bifluoride has been used in a process for fabricating uranium metal nuclear fuel.
• DD 301,016 discloses use of aqueous solutions of HF and/or ammonium fluoride and nitric acid for etching to achieving a corrosion resistant layer on the surface of fuel elements.
• US 383 2439 discloses use of an aqueous solution of ammonium fluoride for dissolving zirconium cladding.
• a method of separating uranium from at least fission products in irradiated nuclear fuel comprising reacting said irradiated nuclear fuel with a solution of ammonium fluoride in hydrogen fluoride, fluorinating said reacted irradiated nuclear fuel to form a volatile uranium fluoride compound and separating said volatile uranium fluoride compound from involatile fission products.
• the solution of ammonium fluoride in hydrogen fluoride further details of which are given below, will hereinafter be designated as NH 4 F/HF.
• the present invention is also capable of reacting, and breaking down Zircaloy cladding and stainless steel assembly components.
• the cutting steps as used in the conventional Purex process may be simplified or may not be required in the present invention.
• the invention also provides a way of dissolving the whole fuel element (fuel, cladding and assembly components such as stainless steel grids and structural components of the elements) thereby providing a simplified ⁇ head-end' .
• the NH 4 F/HF solution is capable of being readily recycled by an evaporation-condensation process.
• the total volume of the solution consumed can be much less than the volumes of acid and solvent consumed in Purex methods, leading to lower volumes of waste being generated.
• the NH4F/HF is produced by dissolution of ammonium fluoride, NH 4 F, in anhydrous hydrogen fluoride, HF. It is believed that the NH 4 F reacts with the HF in solution to form the so-called bifluoride ion HF 2 ⁇ .
• the precise identity of the fluorinating species is not known with certainty and so the foregoing description of how NH 4 F behaves in HF should be understood as not limiting the invention in any way.
• the chemical species in the HF may be written either as NH 4 F.HF or NH 4 .HF 2 .
• the reaction of the irradiated nuclear fuel, Zircaloy or stainless steel with the NH 4 F/HF may be enhanced by the presence of elemental fluorine, F 2 , dissolved in the NH 4 F/HF solution. It has been found that the higher the fluorine pressure, the higher the dissolution rate of the irradiated fuel, Zircaloy and stainless steel.
• the most saturated solution of ammonium fluoride is preferred.
• the solubility limit is 326g of ammonium fluoride per litre of HF. Elevated temperatures may be used. If the temperature increases above room temperature, the HF begins to boil off. If the concentration of the ammonium fluoride exceeds the solubility limit at a given temperature, a solid complex initially forms which may be written as NH 4 F.HF or NH 4 .HF 2 or may be termed ammonium bifluoride.
• the HF may boil off to leave molten NH 4 F.HF or NH 4 .nHF complexes.
• references herein to reaction of the irradiated fuel, Zircaloy or stainless steel with the solution of ammonium fluoride in HF shall be understood to be references to reaction with the molten NH 4 F.HF.
• NH 4 F.HF melts at around 125°c and boils at around 239°. Pressurisation may enable higher temperatures to be used with molten NH 4 F.HF.
• the dissolution rates of the irradiated fuel and Zircaloy increase with increasing temperature up to the boiling point of the NH 4 F.HF.
• the reacted irradiated fuel, Zircaloy, stainless steel solid products indicated in equations (1) - (3) may be separated from the NH4F/HF solution.
• the separation maybe effected by filtering the solid products or by evaporation of the NH 4 F/HF solution.
• the reacted irradiated fuel, Zircaloy and stainless steel are then fluorinated so that the uranium forms a volatile uranium fluoride.
• the volatile uranium fluoride is then separated from involatile forms of fission products, zirconium and iron.
• the volatile uranium fluoride is uranium hexafluoride .
• the separated volatile uranium fluoride gas contains one or more impurities such as a volatile plutonium fluoride, it may be purified in known ways if desired.
• the irradiated fuel 10 together with any fuel cladding and/or fuel assembly components is broken down by reaction with the NH 4 F/HF in a Dissolver 14.
• a gas sparge 12 which typically may be dry air, is passed through the Dissolver 14 to prevent the build up of gases such as HF, H 2 I 2 , Xe, Kr, C0 2.
• the gas sparge 12 may cause considerable HF loss which may require more HF to be added to keep the solvent mobile.
• the off-gas 16 passes through a Condenser 18 to condense the HF and trap the I 2 as solid iodine.
• the iodine is then filtered from the HF by an Iodine filter 20 and the HF is recycled to the Dissolver 14. If fluorine is used in the fuel dissolution then iodine will form complex iodine fluoride compounds and this will probably result in the iodine remaining in solution during dissolution.
• the solid reacted irradiated fuel, Zircaloy and stainless steel in the Dissolver 14 is then separated from the NH 4 F/HF solution in the Evaporator 22.
• the solution is heated to a temperature of the order of 240°c to remove the HF and NH 4 F and leave the solid product comprising reacted irradiated fuel, Zircaloy and stainless steel. Lower temperatures could be used under vacuum.
• the evaporated NH 4 F/HF may be recycled back to the Dissolver 14.
• the evaporation step it may be possible to filter the uranium reaction product, and possibly the plutonium reaction product, directly from the reaction solution and hence provide a quicker way of separating it from the solution than evaporation and there would also be some purification from "soluble" elements, such as strontium and caesium.
• the solution could be cycled straight back into the reactor after addition of fresh ammonium fluoride. However, repeated recycling would eventually result in a build up of fission products in the solvent and so some solvent would have to be periodically drawn off for purification (purification involves evaporation and condensation) .
• the plutonium product is soluble in solution while the uranium product is mostly insoluble (plutonium is normally present in much smaller amounts than uranium) then an immediate uranium- plutonium separation would be possible (direct recycling of solvent would not be used in this case as this would lead to a build up of plutonium and eventually to the precipitation of plutonium) .
• the solid product is then fluorinated in the Fluorinator 24 by thermally reacting the solid product with fluorine gas. Flame fluorination is a preferred method. It may be necessary to heat the solid before fluorination to 500°c to convert the uranium to uranium tetrafluoride to make it easier to fluorinate.
• the NH 3 and NH 4 F evolved by the conversion to uranium tetrafluoride may be recycled back into the solvent stream.
• the fluorination produces a volatile fluoride of uranium, UF 6 , together with minor amounts of volatile products of plutonium and fission products.
• the OFe is separated from any other volatile products by known means, e.g. laser separation.
• the bulk of the fission products remain after the fluorination step as an involatile solid which is treated as waste.
• it may be converted to U0 2 e.g. by known routed such as reaction of UF 6 with water-hydrogen at high temperature.
• the reactions were carried out in a reactor comprising a closed fluorinated ethylene polymer (FEP) tube which was shaken except for the reactions at high concentrations of NH 4 F/HF or when heating above room temperature or when using pressures exceeding Ca .6 atmospheres.
• FEP closed fluorinated ethylene polymer
• the reaction of the Zircaloy was independent of the NH 4 F concentration.
• a series of experiments were carried out to establish whether any variation in rate associated with changes in the NH 4 F: Zircaloy molar ratio was actually a concentration effect.
• NH 4 F and Zircaloy were reacted in a x:y ratio solvated in either 5 or 10 cm 3 of anhydrous HF (AHF) under 2.86 atmospheres of fluorine gas.
• AHF anhydrous HF
• the rates of Zircaloy dissolution were identical indicating that the variation in rate is associated with the relative molar ratios and is not associated with a variation in the concentration of NH 4 F in solution.
• Figure 2 shows the dissolution rates from the experiments performed with increasing molar ratios of NH 4 F: Zircaloy (experiments performed at 2.86 atm F 2 ) .
• the plot shows that the dissolution rate is approximately quadratically dependent, therefore, increasing the amount of NH 4 F to a maximum, based on the maximum solubility in AHF (32.6g in lOOg AHF) at room temperature, will afford data on the maximum attainable dissolution rate under these conditions.
• the maximum measured rate of dissolution for unoxidised and oxidised Zircaloy under these conditions was 14.07 x 10 ⁇ 6 mol hr -1 and 4.39 x 10 ⁇ 6 mol hr -1 , which corresponds to 1.284 mg hr "1 and 0.400 mg hr -1 respectively, both were at 6:1 molar ratio NH 4 F: Zircaloy, and 2.86 atm F 2 pressure.
• the dissolution rate for the oxidised Zircaloy is 3.2 times slower than that for the unoxidised Zircaloy.
• An idea of the attainable dissolution rates can be obtained if the two maximum measured dissolution rate are considered together to calculate the maximum attainable dissolution rates based on these results. This maybe done by calculating the amount by which the rate increased upon increase in F 2 pressure and applying it to the maximum rate obtained by varying the NH 4 F: Zircaloy molar ratios.
• Example 3 Variation of the NH 4 F: uranium dioxide molar ratio
• NH 4 F uranium dioxide molar ratio
• Figure 4 shows the dissolution rates of U0 2 in the experiments performed with increasing molar ratios of NH ⁇ F: U0 2 .
• the plot shows that the dissolution rate is approximately quadratically dependent on the molar ratio of NH 4 F: U0 2 , as observed for Zircaloy.
• the maximum measured rate of dissolution achieved for U0 2 under these conditions was 1.567 x 10 "4 mol hr "1 , which corresponds to 42.32 mg hr "1 , at a 6: 1 molar ratio NH 4 F: U0 2 , and 2.86 atm F 2 pressure.
• the uranium dioxide reacted considerably faster than Zircaloy with NH 4 F/F 2 /AHF.
• the uranium dioxide would be exposed and then react rapidly. From this work, even under moderate conditions, the reactivity of U0 2 is approximately over 100 times greater than both forms of Zircaloy.
• Figure 6 shows the results from experiments performed at higher concentrations undertaken in a Monel autoclave, which demonstrates that rapid dissolution is possible.
• the maximum measured rate of dissolution achieved was 1.05 x 10 " 2 mol hr "1 which corresponds to 0.283 g hr "1 at an ammonium fluoride concentration of 3.85 M.
• the present invention is efficient in breaking down both irradiated nuclear fuel and Zircaloy fuel cladding. Therefore, it can be seen that the present invention provides a method of reprocessing in which (1) the fuel cladding and any stainless steel assembly components may be dissolved together in a simple step, (2) the NH 4 F/HF solution may be recycled and (3) the uranium may be separated by a simple fluorination step, all of which are advantages over the known Purex process.

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• Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Plasma & Fusion (AREA)
• General Engineering & Computer Science (AREA)
• High Energy & Nuclear Physics (AREA)
• Inorganic Compounds Of Heavy Metals (AREA)

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Publications (2)

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• 1999
  • 1999-11-27 GB GB9928035A patent/GB9928035D0/en not_active Ceased
• 2000
  • 2000-11-14 AU AU12915/01A patent/AU1291501A/en not_active Abandoned
  • 2000-11-22 JP JP2001540789A patent/JP4410446B2/ja not_active Expired - Fee Related
  • 2000-11-22 US US10/130,906 patent/US6960326B1/en not_active Expired - Fee Related
  • 2000-11-22 WO PCT/GB2000/004434 patent/WO2001039209A2/en not_active Ceased
  • 2000-11-22 EP EP20000974700 patent/EP1243000B1/en not_active Expired - Lifetime

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