The Radiolytic Oxidation and Reduction of Plutonium

THE RADIOLYTIC OXIDATION AND REDUCTION OF PLUTONIUM by Sherman Fried, Arnold Friedman, J, C. Sullivan Kenneth Nash, Donald Cohen, and Ruth Sjoblom , ..-* wmieo stales Government _w ^,.,, IHJF erjy agency thereof, not any of trie" emoloyees. makesan\ wa'ranty, express or implied, cv assumes any legal liao'l'tv o' resDonsifaririy for me accuracy. completeness, or usefulness of any information, apparatus, product, c process discfosefl. of represents that its use t w u ' d not infringe privately owned rights. Reference herein to any specific commercial p ' j d u c t , process, or service by trade name, tratfemark, manufacturer, ft' " " * — ' " ~r naue name, tratfemark. not necessarily constitute or imply its e nHnmu-— .Stat« . ^ . ••oxisaniY consiitute or imply its endorsement. recommenOa Rn _... or w any diiy agency agency thereof. thereof. The The views views and and opinions cpinions of States Government of authors i'v state or any agency necessari'v state or or reflect reflect those those of of the the United United States State* Government «™-- Prepared for International Symposium on the Scientific Basis for Nuclear Waste Management Materials Research Society Boston, Massachusetts November 27-30, 1979 UKCAUHUSOOE ARGONNE NAT ONAL LABORATORY, ARGONNE, ILLINOIS Operated under Contract W-31-109-Eng-38 for the U. S. DEPARTMENT OF ENERGY for » u c l e ^ Waste na i o n a l 5 s ium|pfnlo?efby ?h?M^ J Research So-ietv S««+ Materials November 2? - 3 ^ g j * 0 1 1 ' Massachusetts THE RADIOLYTIC OXIDATION AND REDUCTION OF PLUTONIUM Sherman Fried, Arnold Friedman, J. C. Sullivan, Kenneth Nash, Donald Cohen, and Ruth Sjoblom Chemistry Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439 U.S.A. It has been shown on several occasions that all other things being equal, the rate of migration of plutonium through an aquifer is greatly influenced by its oxidation state. It is a reasonable expectation that other actinides exhibiting multiple oxidation states will behave similarly. By and large the more highly charged ions in solution move through rock strata more slowly than those of lesser charge. Thus for comparative experiments on the relative migration rates of Pu(IV) and Am(III) we would expect Pu(III) to move about 10 times faster than Pu(IV)* and it has been shown PuO^ moves about 250 times more rapidly than Pu(IV).* Since one of the more important parameters of safety assessment is radionuclide migration rate it can be seen that it becomes important to know the oxidation states of radionuclides (actinides for the purpose of this report) existing in possible leachates from the material originally emplaced in a repository. It was with this in mind that a study of the "intrinsic stability" of plutonium oxidation states in ground water solutions was undertaken. One of the most prominent proposals for a waste repository in WIPP (Waste Isolation Pilot Project) which intends to utilize bedded salt strata as the isolating medium. Ground water in that context is saturated brine and is made up to specifi*Americium(III) was used instead of Pu(III) for the comparative experiments because of the difficulty of maintaining Pu(III) in that state in air at pH's of about 5-6. It was felt that the behavior of Am(III) would be sufficiently close to that of Pu(III) to serve as a reasonable stand-in. cations issued by Sandia Laboratories and designated as "Solution A." Concomitant with the studies of plutonium oxidation states in "Solution A" parallel studies were made in dilute brines (Copenhagen Seawater) since emplacement of radwastes in seabed sediments is also under consideration (Seabed). Studies were also carried out in distilled water to examine the stability of the oxidation states of plutonium unperturbed by the presence of large concentrations of extraneous ions. Some of these studies have been made under anoxic conditions (to avoid the perturbing effects of oxygen) and others were made in the presence of air since it was not known whether ground water in a WIPP-type repository will contain oxygen or not. In the case of Seabed, bottom waters in some areas of the oceans may contain amounts of oxygen in solution corresponding to 10% of that of surface waters and in other areas, e.g., the Black Sea, the water contains no free oxygen. In some cases the pH was controlled and in others it was allowed to vary as plutonium was converted from one oxidation state to another. It should be made clear that the latter case (of uncontrolled pH) was not one of choice but rather was imposed upon the experiment by virtue of the fact that some of the reaction vassels were sealed jln vacuo and it was not possible to monitor the pH without losing the integrity of the experiment. The plutonium solutions employed were relatively concentrated, approximately 0.02 M. This high concentration enabled the determination of the oxidation state by spectrophotometric observation of optical adsorption peaks corresponding to a specific oxidation state. Monitoring the progress of the change in oxidation state was accomplished by repeated spectrophotometric observations. Plutonium was prepared in pure oxidation states as the chloride. Whenever possible the use of oxidizing or reducing reagents that would leave residues was avoided since the presence of extraneous cations or anions would only complicate the interpretation of the observations. Thus Pu(III) was prepared by electrolytic reduction of Pu(IV) and Pu(VI) was prepared by oxidation of Pu(IV) by ozone. The plutonium isotope used in these experiments was 2^2 In the case of anoxic preparations, a quantity of pure plutoncompound (as the chloride) was introduced into a glass ampoule. To this was added a known volume of "Solution A." The combined solutions were evaporated in a stream of nitrogen at room temperature in order to avoid possible decomposition or oxidation. The ampoule containing the dried mixture of salts was connected to a vacuum line and the system pumped until the pressure was of the order of 1 0 ~ 5 nan Hg. At this point water was distilled onto the dried salt mixture (in the same high vacuum) until enough had been added to reach the original volume of the solution (Pu + Solution A ) . Thus the "Solution A" was reconstituted with the Pu in such a way that it was oxygen free. At this point the glass ampoule was sealed off . and was ready for continued spectrophotometric observation. The foregoing description applies to the preparation of all oxidation states of plutonium in the various solvents when it was desired to study them under highly anoxic conditions. Another method for the preparation of anoxic solutions of plutonium was the use of a controlled atmosphere box. It was possible to lower the oxygen content of its atmosphere to 1-2 parts per million. The use of this box permitted the study of reactions in openable containers (in the box) so that changes in pH and Ei. could be monitored by a pH and E^ electrode in the box or if desired, the pH could be controlled by addition of small amounts of the appropriate reagents. Withdrawal of aliquots from the main batch enables the monitoring of samples by the spectrophotometer. Other solutions of plutonium were prepared in air using stoppered spectrophotometer cells as reaction vessels. These preparations could be monitored for pH from time to time by removing the stopper and insertion of a glass electrode. To anticipate, it should be said at this point that all evidence indicates that the oxidation state of plutonium in solution is affected by radiolysis of the water. This appears to be true whether air is present or the solution is oxygen and whether the solution is acidic or basic or in concentrated brines or pure water. This result is surprising in view of the fact that previous work states that the most common effect of radiation is to decrease the oxidation number.2 The extent or the rate of the reactions may vary from solution to solution but in the main the same effects are observed. Indeed the observations seem to indicate that water radiolysis will be a major factor in determining the oxidation state of plutonium solutions in the near vicinity of the radiation field of a waste repository. In fact it can be postulated that even small quantities of plutonium carried away from the repository by some leaching and transport process and subsequently adsorbed on some mineral in a rock stratum will also be subjected to the oxidation effects of radiolysis from its own alpha activity. The reasons for making the statement given above is as follows:' an anoxic solution of Pu +3 in water made some two years ago was found to oxidize slowly to Fu(IV) and Fu(VI). This was demonstrated by observing the decrease in concentration of Pu(III) in solution spectrophotometrically and then isolating the Pu(IV) and Pu(VI) components by chemical separation procedures. Examination of the kinetics showed that the reaction could be expressed as pseudo first order with a "half life" of Pu + 3 concentration of about 1.3 it 10 3 days. It was considered possible that this oxidation was due to radiolysis. If radiolysis is a factor in these reactions, then the rate ought to correspond to the intensity of the radiation field. Accordingly very small amounts of 21fIfCm were added, increasing the level of alpha activity from ten to one hundred times that of the plutonium alone. The amount of 21fItCni required was only a few micrograms and it is unlikely that any chemical effect of curium would be observed since the predominant if not sole oxidation state of curium is plus three. It was found that the oxidation of the Pu + 3 was speeded up in amounts roughly proportional to the level of added alpha activity. Thus the question of intrinsic stability of plutonium oxidation states is probably moot since all plutonium is radioactive and indeed in a real repository where substantial radiation fields exist radiolysis effects on oxidation states will be of paramount importance. In view of the foregoing it was decided to carry out all experiments in the presence of enough 2lflfCm to raise the level of alpha activity one hundred fold over the original alpha activity of solution due to 2lf2Pu alone. In this way changes could be observed in reasonable time spans and the prior demonstration of proportionality of redox rate to alpha activity justified these experiments. It is proper to point out at this time that the effect of radiolysis is obviously mainly on the water rather than the plutonium directly since the concentration of the water is so much greater than the plutonium. It is observed that the overall, effect of radiolysis is to oxidize Pu(III) to Pu(IV) and thence to Pu(VI). After a time Pu(VI) appears to be reduced and a cycle of oxidationreduction can be initiated. The fact that the radiolytic oxidation of Pu exhibits pseudo first order reaction kinetics is not surprising. The reaction is probably between the Pu ion and OH radical. The OH radical is maintained at a constant concentration by its replenishment from the radiolysis of the water. The only other reactant is apparently the Pu. The rate will then depend only on the concentration of the Pu and hence the reaction appears to be first order. As the Pu(III) is depleted, the OH radicals can accumulate more rapidly than they are consumed and they may combine to form H2O2. In any event the concentration of OH radicals will now change as the reaction proceeds and the rate will no longer be first order. The apparent paradox of radiolysis acting both to oxidize and reduce plutonium can be explained if certain reactions are postulated. They are: 1. Water is decomposed by radiation into OH radicals and hydrogen atoms or hydrogen atoms plus hydrated electrons. 2. The hydroxyl radical is an oxidizing agent oxidizing Pu(III) and its products successively to Pu(VI). 3. The Pu(III)-hydroxyl radical oxidation reaction is fast, (the second order rate constant is of the order of 1 0 8 ) . The Pu(IV) to Pu(V) is slow while the Pu(V) to Pu(VI) is fast. 4. When all the Pu(III) is oxidized, the OH radicals accumulate (for lack of substrate) and begin to react with each other to form hydrogen peroxide. I ] Hydrogen peroxide reacts with PuO 2 [Pu(VI)] to reduce it according to the equation 5. 2Pu0 2 (0H) 2 + H 2 0 2 •* 2PuO 2 + 0 2 + 2H 2 0 + 20H~ . (1) Thus the Pu(VI) is reduced after all the Pu(III) is depleted. The reaction is made plausible by observing that the addition of H2O2 to PuC>2 results in a precipitate and an increase in pH as required by equation (1) and Pu(V) has been spectrophotometrically observed during the reaction of Pu(VI) with H 2 0 2 at pH 8-8.5. Furthermore on standing for a time some of the Pu(VI)' reappears. This is consistent with the known disproportionation of Pu(V) into Pu(VI) and Pu(lV) according to the equation 2Pu(V) -• Pu(IV) + Pu(VI) . (2) The results of our experimental observations are summarized in the following set of graphs. These graphs show the changes of pH, the oxidation of the various plutonium species in various solutions as well as the final reduction of Pu(VI). They display the reactions under anoxic conditions, and in the presence of air as well as in various media. The data shown in the various graphs are only for comparison and cannot in themselves be used to predict the concentrations of various plutonium species in the indicated solutions in a real repository. These calculations can only be made when the rate laws governing the reactions are completely elucidated. Likewise the changes observed in E^ and pH in the laboratory sized samples cannot be directly translated into the corresponding changes in very dilute solutions such as would result from the leaching of the radioactive source emplaced in a repository. Clearly the reaction rates will change over many orders of magnitude from laboratory conditions to field conditions and possibly even the mechanism will change. It is to be expected that the rock strata themselves through which the radionuclide migrate will modify and "buffer" the pH and E, of the solutions. The net result of these series of reactions is that plutoniua may undergo a cycle of oxidation and reduction. This is a composite effect since it is the sum of radiolytic oxidation and reduction and disproporti aticn and reproportionation reactions. The period of the cycle must depend on concentration of Pu, radiation field intensity, pH of solution, temperature, and formation of complexes. Thus the migration characteristics of all of the oxidation states of Pu (with the possible exception of Pu(VII)) will have to be considered. It should also be pointed out that these radiolysis effects are probably not confined to plutonium. Neptunium, uranium, technetium and iodine are probably among the multivalent ions that will be affected and their various migration characteristics will have to be determined and will also have to be factored into the safety assessment of a repository. 1. 2. S. Fried and A. Friedman, Retention of Plutonium(VT) on Los Alamos Tuff, Waste Management 76, P. 206, CONF-761020, Proceedings of the Symposium on Waste Management, Tucson, Arizona, Oct. 3-6, 1976. National Technical Information Service, U.S. Department of Commerce, Springfield, Virginia 22161. J. M. Cleveland, The Chemistry of Plutonium, pp. 38-46, published by the American Nuclear Society, LaGrange. Park, Illinois 60520, 1979. > - • CQ O- li_ CO O o O o 2: s o —' II X o UN nna i v AIISN^Q RADIOLYTIC REDUCTION OF Pu +6 BY Y'S ANAEROBIC - Pu CONC, - ,017 M 1 MIN Co 60 - 33,000 RADS 50 o n z:u a: *io 0 80 120 160 240 180 200 MINUTES OF Co 60 280 300 •n +6 GROWTH OF Pu BY RADIOLYTIC OXIDATION OF Pu+i| IN WATER ANAEROBIC - Pu CONC, * ,017 M 0.1 0,3 as CO V) zHi a » o 0,1 j. 0 60 90 120 150 180 210 TIME IN DAYS 270 300 330 360 o CO cc 111 o CD O a: CD to CL. CD CM en o o rH X CO II II oo sr sr w O o O •CNJ UJ CD Qu CD CD CM _]«•£> CD CS1 Jo ! X fc.-iaafefrJJE'-Niy.tfv MM 7C/ IV A1ISN3Q IVDUdO CD CD X CD CD X CD CD CD CD .. '.y i'J'y , '••,;•'.. CHANGE IN PH OF P U * 3 IN AIR DURING a RADIOLYTIC OXIDATION IN SEA WATER Pu CONC, - ,017 M CM CONC, - 1.8 x 1O 9 D/MIN/HL 0 8 10 12 1M TIME (DAYS) 16 18 20 22 i CHANGE IN PH OF Pu +3 IN AiR DURING « RADIOLYTIC OXIDATION IN WATER Pu CONC, -..Ol/M CHCONC. - 1.8 x 10 9 D/MIN/ML. 3.0 1,0 0 8 10 12 M TIME IN DAYS 16 18 20 22 CHANGE IN PH OF Pu +3 IN AIR IN SOLUTION A DURING a RADIOLYTIC OXIDATION Pu CONC, - .017 H CM CONC, = 1.8 x 10 9 D/MIN/ML. 6 8 10 12 TIME .(DAYS) 16 18 20 22 OXIDATION OF Pu + 3 BY <* RADIATION IN COPENHAGEN SEA WATER Pu CONC. « .017 N CM CONC. -1.8 x 10 9 D/MIN/ML. Pu +3 + CM - ANAEROBIC TJ./2 " IS DAYS PU* 3 + CM IN AIR = 3.5 DAYS 56 TIME IN DAYS 0.9 •N _ • 0.8 • ' *' I OXIDATION OF Pu +3 BY ALPHA RADIATION IN WATER Pu CONC, * .017 M CM-CONC. - 1.8 x 10 9 D/HIN/ML. » 0,7 0.6 z 30.5 o - \ 5M to Pu +3 + CM + AIR z O DAYS \ D 0.2 — + CM - ANAEROBIC Ty2 » 2 DAYS 0.1 - 0 8 10 12 14 TIME IN DAYS 16 18 20 22 OXIDATION OF Pu + 3 BY ALPHA RADIATION IN SOLUTION A Pu CONC. - .017 M CM CONC. = 1.8 x 10 9 D/MIN/ML. 0.6 0.5 Pu +3 + CM IN AIR 1/2 ~ 8 DAYS Pu +3 + CM - ANAEROBIC T|/2 = 12 DAYS 0 6 8 10 12 TIME IN DAYS 16 SUCCESSIVE APPEARANCE OF Pu PEAKS A .900 .600 DURING a RADIOLYSIS Pu CONC. = .017 N \ Pu +3 SPECTRA (STARTING MATERIAL) ;3oo .900 Pu+i| COLLOID SPECTRA (20 DAYS) 1 .600 c5 -300 .750 PEAK DAYS) .150 4600 5500 6400 A 7300 8200 9400 10,000 '1 l 600 .80 OXIDATION OP PU+3 BY ALPHA RADIATION IN AQUEOUS SOLUTION .70 •v 3 days .60 ,50 .40 .30 Contaxns 10 Times a activity of P u 2 4 2 as Cm 2 4 4 .20 *Contains 100 times a activity of Pu 2 4 2 as cm 2 4 4 .10 20 40 60 80 100 120 TIME IN DAYS 140 160 180 200 220 240 RATE OF DISAPPEARANCE OF 2li2?\i FROM DISTILLED WATER SPIKED WITH 1.5 x 10 9 DPM/ML 2 ^ C M UNDER ANAEROBIC CONDITIONS B ,015- PI t O Pu 3+ PH = 6 .69 ±0 .16 T (DAYS) RATE OF DISAPPEARANCE OF FRON SOLUTION A SPIKED WITH 1.5.x 10 9 DP«/ML UNDER ANAEROBIC CONDITIONS 0.015- 0 Pu3+ V MONOMERIC Pui|+ FOUND IN SOLUTION INITIALLY ALL Pu 3+ pH - 7.15 ± 0.15 Q s 0101- EPu] 0.005- 15 10 T (DAYS) 25 RATE OF DISAPPEARANCE OF 2£f2Pu FROM COPENHAGEN SEAWATER SPIKED WITH 1.5 x 10 9 DPM/ML 2 ^ C M UNDER ANAEROBIC CONDITIONS 3.015 - PH )-.Q10r CPU] T (DAYS) - 8.65 ± 0,10 OXIDATION OF Pu +3 BY 3 RADIATION PU +3 + Ca 4 5 IN H 2 0 l/2 O.fr days 0.40.20 o Pu +3 + Ca 4 5 IN SOL. A 0.6T o in l/2 = 270 days 0.40.2- •a 0 Pu +3 + Ca 4 5 IN SEAWATER 0.6T l/2 = 320 days 0.20 25 time (days) 300