DE2619089A1 - METHOD OF DIGITATING PLUTONIUM DIOXYDE - Google Patents

Description

US ENERGY RESEARCH AND DEVELOPMENT ADMINISTRATION Washington, DC 20545, USA

Process for dissolving plutonium dioxide

The invention relates generally to the dissolution of plutonium dioxide in the preparation of nuclear fuel, in particular on the nitric acid dissolution of plutonium dioxide under the catalytic action of hydrofluoric acid .

One of the most commonly used nuclear fuels is plutonium in the form of dioxide. The dioxide is suitable for use in nuclear fuel because it has high fire resistance and is inherently inert. For nuclear fuel applications, PuO_ is generally mixed _{with UO_ in compositions of 15-30% PuO 3.} In nuclear reactors, particularly those of the breeder type, the spent fuel is periodically removed and reprocessed in order to separate the waste materials from the usable products. There are

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Methods available to recover and purify the plutonium after the material is dissolved in an aqueous nitrate solution. However, PuO ₂ is relatively difficult to dissolve in pure concentrated nitric acid.

The difficulty of dissolution depends on the fuel preparation process. Fuels that experience high sintering temperatures result in a solid solution of the mixed oxides and are soluble in 10-12 M nitric acid. Mechanically mixed fuels of high PuOp content that have been sintered at a relatively low temperature contain separate PuO ^ and UOp phases and are sparingly soluble in 10-12 M nitric acid. The preferred dissolution of U0_ normally takes place with these fuels, which leaves behind poorly soluble residues with a high PuO ~ content. In addition, certain forms of PuO _{2 have been} fired at very high temperatures in order to obtain maximum density, such as microspheres with a theoretical density of 96-98% and are therefore only slowly dissolvable by known means. The accumulation of residues containing plutonium in fuel processing plants is extremely undesirable and critical. The present invention is particularly applicable _{to the dissolution of PuO 2 that is difficult to dissolve.}

The problem of dissolving plutonium dioxide is well known in the field of nuclear fuel processing. Known processes can generally be divided into two groups: oxidative dissolution and non-oxidative dissolution. In the oxidative dissolution process, the main mechanism by which the plutonium dioxide dissolves is the oxidation of Pu (iv) in solid PuO ₂ to a more soluble oxidation state

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such as Pu (VI). In the non-oxidative dissolution, plutonium dioxide goes into solution without oxidation from Pu (IV) / such as through the formation of a soluble Pu (IV) complex.

An example of oxidative or oxidative dissolution is contained in U.S. Patent 3,005,682. The process described there uses the addition of cerium ions (strongly oxidizing agents) to 4 M nitric acid above 115 ° C. The addition of cerium ions supports the dissolution of solid PuO ₂ by oxidizing the poorly soluble PuO ₀ into the soluble one

+2

Plutonyl ion (PuO ₂ ), in which the plutonium shows the oxidation state of (VI). A disadvantage of this oxidizing dissolution process in the preparation of nuclear fuel is the observed behavior of ruthenium. Ruthenium is always present as a fission product in irradiated fuels. For example, a typical 1,000 kg batch of irradiated liquid metal from a fast breeder reactor (after 10% burn-up) contains 200-250 kg plutonium (as PuO ₂ ) and approximately 6 ^ kg ruthenium, with the remainder being uranium and other fission products. Due to its variety of available oxidation states and its tendency to form complex ions, the behavior of ruthenium (especially in the presence of oxidizing or reducing agents) is largely unpredictable and has always been a major concern in fuel preparation processes. During the oxidative dissolution with cerium ions, ruthenium is also oxidized (to RuO.) And volatilized, which results in extremely serious handling problems for the radioactive materials. In addition, some of the volatile ruthenium types condense and are reduced _{back to RuO 2} , re-enter the dissolving solution and disrupt the PU oxidation, with the reduction of Ce (IV) to Ce (III)

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is catalyzed, whereby the Pu (IV) oxidation is prevented. The fear of the volatility of the Ruthenium speaks against dissolution processes, which Use oxidation processes.

The problem of volatilization of the ruthenium is avoided in a non-oxidative dissolution process. Some success has been achieved with essentially non-oxidative fluoride catalyzed dissolution. In this method, PuO ₃ is dissolved by contacting it with aqueous HNO 3 (about 8-16M) in the presence of an effective catalytic amount of fluoride (about 0.05-0.3oM). The fluoride ion catalyzes the PuO ₂ dissolution without the oxidation of plutonium. The fluoride ion is preferably added as HF in order to avoid foreign ions. Of course, regardless of the fluoride added, it behaves essentially as HF in an acidic medium and is _{hereinafter referred to as an HNO 3} -HF dissolving agent. Although this fluoride-catalyzed, non-oxidative dissolution process is very effective in avoiding the volatilization of ruthenium, it has certain disadvantages. For example, after a period of time the rate of dissolution or velocity decreases and almost comes to a halt. Obviously, the fluoride ion is consumed in the dissolution reaction and the constant addition of fluoride is required to maintain the rate of dissolution. In some cases, the refractory PuO "is not completely dissolved, even after lengthy periods of work-up and the stoichiometric addition of fluoride. It would of course be useful in any fuel _{preparation process to minimize the concentrations of HNO 3} and the fluoride ion in order to reduce the corrosive nature of the dissolving solution.

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The present invention has set itself the goal of a method for dissolving PuO "in nitric acid to be provided, this dissolution taking place essentially completely. The invention also aims at the rate of dissolution of PuOp in a mixed nitric acid-hydrofluoric acid to increase. The invention also provides for the fluoride concentration of the dissolving solution in order to reduce the corrosive effect of the solution. The invention also aims the consumption of fluoride during the dissolution of PuO_ in the mixture of nitric and hydrofluoric acids.

According to the invention, these and other objects are achieved by a method for dissolving PuO ₀ in an aqueous dissolution mixture which has PuO, HNO ₃ and an effective catalytic amount of fluoride, the invention providing that the dissolution rate is increased by oxidizing conditions be provided in the dissolution mixture to oxidize the dissolved Pu ions. It is believed that when the dissolved plutonium is oxidized, the fluoride ions are released from a soluble plutonium-fluoride complex, and necessary for further catalytic action. With this method, the fluoride is no longer consumed during the dissolution process and the dissolution rate no longer decreases significantly over time. As expected, the dissolved ruthenium does not evaporate and neither does it interfere with the oxidation of the dissolved plutonium ions. According to the present invention, the oxidation can be carried out in any effective manner that does not interfere with the catalytic action of the fluoride; electrolytic oxidation is preferably used.

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Further advantages, objectives and details of the invention emerge in particular from the claims and from the description of exemplary embodiments with reference to the drawing; in the drawing shows:

FIGURE 1 - a graphical representation of the PuO dissolution rate as a function of the Pu (IV) / F molar ratio, where the decrease in the rate of dissolution as the concentration of the dissolved Pu (IV) increases;

FIGURE 2 - a graph of the rate of PuO dissolution as a function of time, showing the effect of the addition of Ag2O ₂

FIGURE 3 - a graph of total plutonium ion concentration in the solution, depending on the time for the solution with and without periodic addition of O;

FIGURE 4 - a graph of the total plutonium ion concentration in solution as a function of time for two acid solutions, with and without electrolytic oxidation.

In the context of the invention, kinetic studies of the PuO_ dissolution in an aqueous HNO ₃ -HF solution were carried out. An investigation of the PuO ₃ dissolution rates in solutions with different ENT and HF concentrations showed that the initial PuO dissolution rate is higher for higher ENT concentrations and for higher HF concentrations. Furthermore, double logarithmic representations showed the initial dissolution rates as a function of the initial fluoride concentration in solutions of constant acidic activity, that the kinetics of the dissolution reaction in the ENT - HF system were the first

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~ 7 -

Order with regard to the HF concentration follows.

An experimental demonstration of the change in the initial dissolution rate with respect to time is shown in FIG. This experiment was carried out as follows: half a gram of PuO ~ microspheres of a refractory / difficult to dissolve type (these microspheres are considered to be no less difficult to dissolve than the PuC ^ residues from fuel rods of the liquid metal fast-breeder reactor) were at 100 ° C dissolved in a solution of 10 M in HNO ₃ and 0.02 M HF in a 30 ml polytetrafluoroethylene container.

The data are shown in FIG. 1, the proportion of the maximum dissolution rate depending on the Pu (IV) / fluoride molar ratio (the concentration of dissolved Pu (IV) in relation to the initial fluoride concentration) is. The resulting curve has a slope of minus. One, which indicates that the decrease in the rate of dissolution is directly proportional to the increase in the Pu (IV) / F molar ratio. The fraction of the maximum rate of dissolution is defined as the rate of dissolution at a given time divided by the initial rate of dissolution (which results as the maximum rate of dissolution). This linear relationship with a slope of minus one shows the formation of a Pu (IV) fluoride complex at a 1: 1 molar ratio, causing the rate of reaction to decrease when the fluoride ion complexes subject. In practice it follows that the formation of this complex the total amount of Pu that can be dissolved in a given amount of HF. It can therefore be concluded that that the mechanism of dissolution of Pu0_ in ENT-HF-

+3

Solutions involves the formation of a soluble PuF complex. It should be noted that Pu (IV) in PuO _ in this

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is not oxidized by fluoride catalyzed dissolution processes.

In accordance with the present invention, it has been discovered that the fluoride catalyzed dissolution of plutonium dioxide in ENT_ in a significant way in the rate or Speed is increased by oxidizing the dissolved plutonium ions. Based on kinetic Investigations are believed that the oxidation of dissolved plutonium from the soluble fluoride ions PuF -complex releases, with the same-

+2 ongoing formation of the soluble plutonyl ion PuO ₀ Unexpectedly, the dissolved ruthenium ions did not interfere with the plutoniome oxidation. It should be noted that the process of the invention is still essentially a non-oxidative dissolution process since the "plutonium is not oxidized during the dissolution reaction, but only after the dissolution.

The oxidation conditions have very little effect on the rate of dissolution of PuO ₂ in ENT in the absence of fluoride. For example, only 1.2 milligrams of PuO "were dissolved after 1.0 gr. Of PuO- had been worked up at 100 ° C. in 50 ml of 8.0 M HNO _{3 for} 24 hours with electro-lytic oxidation. The electrolytic oxidation was carried out with an applied potential of 2 V with a current of 100 milliamps. The total dissolved Pu concentration was 0.001 mol / liter.

To carry out the method according to the invention, it is only necessary to bring PuO ₂ into contact with an aqueous HNO ₃ dissolving solution, the latter containing an effective catalytic amount of fluoride under oxidizing conditions. The correct oxidizing conditions can be electrolytic in the dissolving solution

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or by some non-interfering oxidizing agent. Electrolytic Oxidation can by any conventional means of passing an electrolytic current through the Solution are carried out with corrosion-resistant electrodes, such as platinum electrodes.

For the dissolution of irradiated fuels by electrolytic By oxidation it is advisable to set the standard potential of the anolyte to a value between the standard potentials to regulate the following reactions:

Pu ⁺⁴ + 2H ₂ O --v PuO ₂ ⁺² + 4H ⁺ + 2e (-1.04V) RuO ₂ + 2H ₂ O - *. RuO ₄ + 4H ⁺ + 4e (-1.39V)

By maintaining the cell potential between approximately -1.15 V and -1.25 V (with respect to a hydrogen electrode), dissolved plutonium is oxidized without volatilization of dissolved ruthenium. The correct applied potential can be routinely determined by measuring the standard cell potential. For the purposes of the present invention, a non-interfering oxidizing _{agent is one (such as Ag 2} O ₃ ) which oxidizes the dissolved plutonium without complexing available fluoride ions. According to the present invention, fluoride is no longer required in stoichiometric amounts with respect to Pu. Although the rate of dissolution can be increased by adding larger amounts of fluoride, the only thing required is an effective catalytic amount, since the fluoride ion is now brought back into the solution and is no longer bound in the PuF complex ion. For the purposes of the present invention, an effective catalytic amount is that amount which is independently

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Depending on the Pu stoichiometry, the PuO ₂ dissolution for the specific HNO ₃ concentration used is effectively brought about and can be routinely determined for the desired application. In accordance with the present invention, sustained dissolution rates no longer require high initial fluoride concentrations, and the necessary fluoride ion concentration can now be reduced to less than 0.01 M to match the corrosion resistance of the dissolution vessel.

To illustrate the process according to the invention, the following examples are given. The required oxidation of the dissolved plutonium is achieved in Examples I, II by an oxidizing agent and in Examples III and IV by electrolytic oxidation. The dissolutions were carried out on refractory PuO ₃ microspheres of 98% theoretical density, ie a material which is representative of the most difficult to dissolve PuO ₉ , which is likely to occur in the preparation of nuclear fuel in a breeder reactor.

Example I.

This example illustrates the dramatic increase in the rate of dissolution of PuO ₂ as a result of the _{addition of Ag 3} O ₃ , as shown graphically in FIG. Half a gram of refractory (refractory) PuO ₃ microspheres was worked up at 100 ° C. in a polytetra-fluoro-ethylene equipment in 30 ml of a 10 M HNO ₃ -0.02 M HF solution. The initial rate of dissolution for the sample was approximate

-4
10.4 10 moles per liter and per hour. After 24 hours, enough plutonium had dissolved to produce a

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Pu (IV) / f "* molar ratio of 0.75. The rate of dissolution had then decreased to about a quarter of the initial rate. At this point, 0.5o grams of Ag ₀ O _{2 were} added to the solution Plutonium happened over the next three hours as determined by analysis for Pu (IV) and Pu (VI), the molar ratio of Pu (IV) in solution to the initial fluoride decreased to 0.25 and the rate of dissolution increased more than 85% of the initial rate.

Example II

This example provides for a comparison of the PuO ₂ resolutions both with and without the addition of the oxidizing agent. One half gram samples of ₀ Pu0 -Mikro spheres was heated at 100 C in a polytetra-fluoro-Äthylenausrüstung in 40 ml volumes of 8 M HNO ₃ - edited 0, o2 M HF solution with and without Ag ₀ O ₀ addition. The data in the table show that after 174 hours of dissolution in 8 M HNO ₃ -O, O ₂ M HF 51, ο% more total plutonium is dissolved with the addition of _{Ag 2 O 2 than without this addition.} The data in the table show that resolution with Ag ₀ O ₀ resulted in large proportions of the

+2 dissolved plutonium are oxidized to the plutonyl (PuO ₀ ) state. Although some oxidation of the dissolved plutonium occurs without the addition of Ag 0, the significant increase in the rate of dissolution corresponds to the greatly increased amount of Pu (VI) in the solution with the addition of _{Ag 0 O.}

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51 65.5 75 204.3 129 247.8 174 325.0

Effect of the addition of Ag ₃ O on plutonium dissolution in 8 M HNO ₃ -O, O2 M HF solvents at 10O ⁰ C

Percentage of Increase Percentage of Increase

Time of the Pu0_ ⁺ 2 concentration of the total Pu concentration

(Std) ^a ^ ~ ^{s F} ° l9® ^ ^er addition tion owing to the addition

of Ag ₂ O of Ag O ₃

'8.0

10.1 28.7 51.0

Add 100 mg of Ag "O" every 24 hours

The increased plutonium dissolution rates resulting from the Ag ₃ O ₃ additions are shown in FIG. 3, where plutonium concentrations of the dissolving agents are shown as a function of the dissolution time for dissolutions with and without Ag »O_ additions. The curves of Figure 3 show that the initial dissolution rates are the same with and without Ag _? 0 "additions, but the rate without the _{addition of Ag 3} O decreases much faster than does the rate with the addition of _{Ag 3 O.}

Example III

This example demonstrates the effect of electrolytic oxidation on the PuO dissolution in HNO-HF solutions. 1.2 gr. Samples of Pu0 ₃ microspheres were processed at 100 ⁰ C in 30 ml volumes of HNO ₃ -HF solutions with and without electrolytic oxidation. The results are shown in FIG. The electrolytic oxidations in this example were

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- 1 "J _

Ij ~

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led that one dipped platinum wire electrodes (0.2 _c m 0) about 1.8 - 2, ο can from each other at a distance, specifically directly into the dissolving solutions to a depth of 8 cm, within a 2.3 cm Polytetrafluoroethylene cylinder diameter. The applied potential was 2.0 V and the current flow was 200 itiA. The cathode was enclosed in a polytetrafluoroethylene sleeve and ventilated separately. The electrolytic oxidation was carried out for 28 hours in an 8.0 M HNO-O, 85 M HF dissolving agent and resulted in a 75% increase in the PuO ₂ dissolution. The relative effects were even more pronounced in a 4, ο M HNO ₃ -O, 15 M HF dissolving agent, because there a more than 800% increase in the Pu0 “dissolution was found after 72 hours. There was no sharp drop in the rate of dissolution even after 100 hours.

Example IV

This example shows that the oxidation of dissolved plutonium according to the invention can be carried out in the presence of ruthenium without volatilization of ruthenium. 0.5o gr. Samples of Pu0 ₂ microspheres were worked up in 40 ml volumes of 8.0 M HNO ₃ -O, 10 M HF-0.10 M ruthenium with and without electrolytic oxidation as in Example III. The amount of plutonium dissolved with electrolytic oxidation was more than 100% greater than without oxidation at the end of 24 hours, and was 45% in the plutonyl state. The ruthenium in the solvents has been marked with Ru. Based on gross gamma and gamma scan analyzes, the ruthenium concentration in the dissolvents did not change significantly during

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ü * r

of the dissolutions, indicating that no ruthenium volatilization occurred.

The beneficial effect of the oxidation states on the non-oxidative fluoride-catalyzed nitric acid dissolution from PuO- was completely unexpected, and in particular also because of the experience of the inventor with various oxidation agents with this system Addition of sodium dichromate, a strong oxidizing agent, to a dissolution mixture of refractory PuO_ microspheres (approximately 98% theoretical density) in 8 M HNO-O, O2 M HF this was already oxidized in solution

2+

located Pu to PuO-, but did not increase the rate of dissolution of the solid PuO ". In a similar way the addition of Ce (IV) ions, as in the oxidative dissolution process according to the prior art, to one Similar dissolution solution not only delayed the complete dissolution of the microspheres, but rather delayed it actually the resolution. It was even more unexpected that the oxidation would increase the rate of dissolution in the presence of ruthenium ions such as those found in the dissolution mixtures for irradiated fuels will. Previous experience with cerium in ENT dissolving agents containing dissolved ruthenium, was that the dissolution rates were not increased. It is believed that this is likely occurred as a result of the fluoride complex formation, and as a result the tendency of ruthenium to be oxidized in preference to plutonium.

An additional advantage of the method according to the invention consists in preventing plutonium precipitation by oxidizing the dissolved plutonium ions will. It was observed experimentie11 that the

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higher fluoride concentrations (more than about 0.1 M), which are favorable for higher dissolution rates, and in the absence of oxidation, the formation of an un-

+2
soluble PuF complex occurs, which retards the rate of total plutonium dissolution. When dissolved piutonium ions are oxidized according to the invention, the insoluble complex does not occur.

The invention has now been illustrated by means of experimental demonstrations which demonstrate the feasibility of the method. These laboratory examples can easily be used industrially by those skilled in the art. The ENT concentration has been shown not to be critical to the feasibility of this procedure (good results can be obtained even at 4, o M ENT) and the procedure can now be tailored according to the corrosion resistance of the equipment. Electrolytic oxidation appears to be the most attractive for large-scale applications, but chemical oxidizing agents such as Ag ₉ O ₅ can also be used without departing from the inventive concept. It is believed that the reason the cerium ions and sodium dichromate do not aid HNO-HF dissolution is that the Ce and Cr ions complexed with fluoride ions by themselves. Other metal ions -which have been found to be the complex formation-

+2 +3

effect the formation of fluoride in HNO ₃ -HF are Zn, La,

Ca, Al, Th and Zr. The equivalents of Ag ₂ O "envisaged in this process are those oxidizing agents which are capable of oxidizing Pu ions dissolved in HNO_-HF solutions to increase the PuO_- * dissolution rate without essentially forming complexes themselves to effect the fluoride ions.

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At the increased rates of dissolution available in accordance with the invention, dissolution can be carried out efficiently, even below the boiling point of the acid, with higher rates of dissolution being achievable at elevated temperatures. The preferred temperature of the mixture resolution is now approximately 10O ⁰ C.

Regarding the state of the art, see US Patent 3,005,682 and Volume 2 of the "Plutonium Hand Book" by 0.J.Wick, Science Publisher, New York, London, Paris 1967, p. 732.

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Claims (4)

Expectations 1.) A method for dissolving PuO_ in an aqueous dissolution mixture which holds PuO ₃ , HNO and an effective catalytic amount of fluoride, characterized in that oxidation conditions are provided in the dissolution mixture in order to increase _{the dissolution rate of PuO 9.} 2.) Method according to claim 1, characterized in that the oxidation conditions are provided electrolytically are. 3.) The method according to claim 1, characterized in that the oxidation conditions are provided by adding Ag ₅ O_ to the dissolution mixture. 4.) The method according to claim 1, characterized in that the dissolution mixture contains dissolved ruthenium. 609848/0603