from a single batch of fissium alloy were analyzed by atomic absorption. Two of these samples were analyzed at Atomics International and treated as described under the section on sample preparation. The other two samples were analyzed at Argonne National Laboratory, Idaho Falls, Idaho, by a similar technique using a Jarrel-Ash instrument (16). The data are summarized in Table 111. Both laboratories made several dilutions from the original sample solution. Mo and Ru are best determined in solutions containing approximately 0.1 % sample by weight; Pd and Rh are determined in solution containing about 1 % sample. Several analyses were performed on each sample at different concentrations simply to confirm earlier statements that the method was applicable over broad ranges of concentration and uranium content. Two sample aliquots were evaporated nearly to dryness and diluted to volume with water in order to determine whether all sample components would remain in solution and to provide solutions containing less acid. Additional uranium was added to a pair of 0.1% (sample by wt) sample solution as additional verification of the observation that uranium concentration below 30 mg/ml was not critical to the analysis. Other sample aliquots were spiked with known quantities of each element and analyzed simply as additional confirmation of the results. The results of the analyses performed at the two sites were in good agreement. No significant differences were apparent due to differences in the handling of sample aliquots. The role of uranium in eliminating interferences among the (16) Earl R. Ebersole, Argonne National Laboratory, Idaho Facilities, private communication, December 1967.
elements of concern here is not clear. The observed interferences are real, and, although variations in instrumental parameters, including flame conditions, influenced the shape of the absorption curves, the interferences as noted remained. Similarly no significant differences were observed in the results obtained with the single slot, premix burner and the 3slot burner except in stability. Since the ionization potentials for Mo, Ru, Pd, and Rh are relatively high, it is not likely that the ionization of these elements in the flame is a significant factor (affected by uranium) which acts to reduce sensitivity or which plays a role in mutual interference. It is more likely that the presence of uranium atoms or complex ions in the flames or in solution serves to stabilize the atom populations of these elements by breaking down or by preventing the formation of stable complex metal-halide ions of Mo, Ru, Pd, and Rh. It is also reasonable 40 suggest that the analysis of other alloys involving one or more of the elements, Mo, Ru, Pd, or Rh may be simplified or improved by the addition of uranium to sample matrices. ACKNOWLEDGMENT
The author is grateful for the kind cooperation of Earle R. Ebersole, ANL, in providing samples of fissium alloy for this work and to the Argonne National Laboratory for the effort expended in performing comparative analyses. RECEIVED for review August 7, 1968. Accepted October 18, 1968. Work supported by the United States Atomic Energy Commission.
Determination of Oxygen-to-Uranium Ratios in Hypo- and Hyperstoichiometric Uranium Dioxide and Tungsten-Uranium Dioxide E. A. Schaefer and J. 0 . Hibbits General Electric Company, Nuclear Systems Programs, Cincinnati, Ohio 45215 Several methods for the determination of oxygen-touranium ratios in hypo- and hyperstoichiometric UOz have been developed and/or evaluated, using NBS 950a (UaOs) and uranium metal as standards. these methods include (a) oxidation to U308+y,(b) a nitric acid dissolution procedure, followed by oxidation to UaOs+N,(c) an oxidation-reduction procedure, whereby the sample is first oxidized (in case the material is hypostoichiometric), then reduced to UOz.oo,(d) direct reduction with hydrogen (of hyperstoichiometric UOz) to UOz.oo, and (e) hydriding, whereby the free uranium metal in hypostoichiometric UOz is first converted to UH, and the hydrogen subsequently determined. I n addition, moisture has been determined on a number of samples in order to evaluate its effect on the oxygen-to-uranium ratios obtained using the above methods. In general, the accuracies obtained are within the expected accuracies of the procedures used. Where applicable, the above methods have been applied to W-UOz*x samples, using UOz control specimens to evaluate the accuracy of the results obtained.
THEPURPOSE of this paper is to attempt to clarify the determination of oxygen-to-uranium (O/U) ratios in UOZ*, and W-UOz,, and, in addition, t o present methods which have 254
ANALYTICAL CHEMISTRY
been developed and evaluated for these determinations. It is all too frequently assumed that the determination of an O/U ratio in UOz*, can be easily made by ignition of the sample to u3os and calculating the O/Uratio on the basis of weight change. This result can be in error because it neglects the effect of adsorbed moisture if present, and some samples attain different final weights depending on the technique used for oxidation. For example, our oxidation technique involves a preliminary oxidation of 470 "C for a minimum of 4 hours followed by 4 hours at 900 "C. On a number of samples that we have analyzed, results are 0.01 O/U unit too high if the 470 "C preliminary oxidation is omitted. Other samples did not reach a maximum weight at 900 "C even after oxidation for 72 hours. Inasmuch as a number of fuel element core materials are being investigated under conditions which could lead to the formation of nonstoichiometric uranium dioxide, it is important that methods be available for accurately measuring oxygen-to-uranium ratios. Depending on the temperature, hypostoichiometric material could contain uranium metal which could be highly reactive with other materials present. On the other hand, hyperstoichiometric material at elevated
temperatures may have an oxygen partial pressure sufficient to cause interaction with other materials present. It has been thoroughly established that the oxygen-touranium ratio in a specimen of uranium oxide is dependent upon its environment and thermal history. Metallic uranium has been identified in specimens of uranium dioxide after heating at temperatures as low as 1600 "C in an atmosphere with a low oxygen partial pressure-viz., dry hydrogen, or high purity argon (1-5). Rothwell(5) has proposed that the free uranium metal appears as a result of precipitation from a hypostoichiometric UOz-, phase during cooling. The hypostoichiometric phase boundary which defines the temperature composition conditions under which metallic uranium exists has been accurately defined (6). Conversely, uranium dioxide has a great affinity for oxygen and, under the proper oxidizing conditions, can ultimately attain the composition UOp (7-11). Throughout this paper, the terms apparent and true OjU are used in discussing results. The term apparent O/U refers t o the OjU one obtains when no correction is made for adsorbed moisture. For example, if a UOz+, sample is converted to a material with known stoichiometry, and the value of x determined on the basis of the change in weight, this result will be in error depending upon the amount of adsorbed moisture present. The term true O / U thus represents a value obtained whereby correction is made for adsorbed moisture. In addition, the term hypostoichiometric UOz as used in this paper refers to a sample of UO? containing U metal (to the best of our knowledge, no sample of hypostoichiometric UOZ is known to exist at room temperature) where the overall composition is UO,-,. Thus, the term hypostoichiometric is not strictly accurate as used in this paper. The following results are obtained in this study: A nitric acid dissolution (N.A.D.) technique has been studied whereby the sample of UOzi, is dissolved in nitric acid, evaporated to dryness, and ignited under a prescribed set of conditions to U30,v. The value of N is very close to 8.00 and was evaluated using NBS sample 950a us08 and high purity uranium metal as standards. This technique yields apparent OjU results accurate to about *0.002 on UOZ+, samples. A standard oxidation procedure, for converting UOZ+, to U 3 0 8 = yhas been selected for samples studied at GE-NSP. This procedure has been found to yield apparent OjU results accurate to about =t0.004 O/U unit on UOz+, samples (by comparison with the N.A.D. procedure described above). True O/U results will be obtained on UOz+, by either of the above methods if no significant amount of moisture is present. If moisture is present, a true OjU result can be obtained by determining the moisture using the electrolytic coulometric (1) E. A. Aitken, H. C . Brassfield, and J. A. McGurty, Amer Nircl. SOC.Trans., 6, 153 (1963). (2) J. S . Anderson, J. 0. Sawyer, H. W. Warner, G. M. Willis, and M. J. Bannister, Nature, 185,915 (1960). (3) R. G. Robins, J . N w l . Mater., 7,218 (1962). (4) E. Rothwell, ibid., 5, 241 (1962). (5) E. Rothwell, ibid., 6, 229 (1962). (6) R . E. Fryxell, D. E. Joyce, and R. Szwarc, J. Nucl. Mater., 25, 97 (1968). (7) K. B. Alberman and J. S. Anderson, J. Chem. SOC.(London), Supplementary Issue No. 2,303 (1949). (8) I. H. M'lne, Amer. Mineral., 36, 415 (1951). (9) E. J. Brooker and E. W. Nuffield, ibid., 37,363 (1952). (10) H. Hering and P. Perio, Bull. SOC. Chim. (France), 1952, 351. (11) P. Perio, ibid., 1953, 256.
technique (12) and correcting the initial sample weight for moisture. Accurate apparent OjU results can also be obtained on UOZ+,using an oxidation-reduction technique, whereby the sample is first oxidized, then reduced to UOz. Accurate OjU results both true and apparent, can be obtained on W-UOn-, samples using the oxidiation-reduction technique (described later) as indicated by the agreement between these results and those obtained by the hydridingdehydriding technique of Hibbits and Schaefer (13). Presumably accurate OjU results can be obtained on sintered W-UO2.0~,and sintered W - U 0 e z samples, as indicated by the agreement between the OjU result obtained on these materials and those obtained on UOz control samples sintered simultaneously. EXPERIMENTAL
Apparatus. A Solids Moisture Analyzer based on the coulometric electrolytic principle (12) or its equivalent. The reduction of uranium and/or tungsten oxide was carried out in a 1-inch diameter Vycor combustion tube. Materials. High purity uranium metal and uranium oxide, NBS 950a, certified to contain 99.94 U308 after ignition for 1 hour at 900 "C. The remaining 0 . 0 6 z has not been characterized. Assuming the remainder to be oxygen (no major impurities present), this corresponds to a composition u 3 0 8 . 0 3 3 . The types of UOziz specimens investigated were as follows. The starting materials were powders, either ceramic grade (as-received) UOn or agglomerated UOz. The latter was obtained by isostatically pressing ceramic grade UOz at 70,000 psi. The pressed cake is crushed and sieved. The -100 +200 particles are fired at 750 "C in helium for 1 hour. Basically, this treatment produces UOz of larger particle size. If a pressed compact of either fuel type is subjected to high temperature firing, at least 1400 "C, in either an inert or a reducing atmosphere for extended periods of time the resulting material is referred to as sintered. In the absence of high temperature firing, the specimen is referred to as nonsintered. Procedures A. O/U RATIOBY THE HYDRIDING-DEHYDRID. ING TECHNIQUE.Utilizing the technique employed by Hibbits and Schaefer (13), uranium metal in a UOn-. or W-U02-. specimen was converted to the hydride U H 3 by soaking the sample in dry hydrogen at 225 "C. The hydride was then decomposed at 600 "C and the released hydrogen determined by gas chromatography. The calculation of the OjU ratio is based upon the assumption (5) that:
uoz-,
(at temperature)
-
uoz,oo + u
(1)
(at room temperature)
Obviously, the method is restricted to the measurement of hypostoichiometric ratios and, as the authors have indicated (131, the "accuracy of the determination is limited by the location of the metal phase present. In general, metal located along a grain boundary will react with hydrogen and subsequently be determined, whereas metal located within a grain will probably not hydride and, thus, will escape subsequent detection." For details of this procedure one is referred to the paper cited. B. o/u RATIOBY DISSOLUTION IN NITRICACIDFOLLOWED BY OXIDATION TO u308 (N.A.D. PROCEDURE). Five-gram samples were weighed in tared 50-ml platinum crucibles (with lids). Seventeen ml of water and 2 ml of acid were added. The samples were then placed under infrared lamps. After 3-4 hours, another 3 ml of nitric acid was added to complete dissolution of the sample. The samples were heated under (12) E. A. Schaefer and J. 0. Hibbits, Talanta, 15,129 (1968). (13) J. 0.Hibbits and E. A. Schaefer, ANAL. CHEM., 38,1687 (1966). VOL. 41,NO. 2, FEBRUARY 1969
255
Table I. Evaluation of N in U3O.y by N.A.D. Treatment of Uranium Metal N,
Uranium metal, grams 4,6797 5,2808 4.8683 3.2623 5.8932 3.7287 3.9310
after N.A.D. treatment 5 hours at 900 "C 8.001 8.013 8.007 7,992 7.989 8,001 7.995 Av 8.000
-
Because the prospect of powdering sintered specimens was not considered attractive, the authors have limited the utilization of this technique to those specimens in which reduction by hydrogen will readily occur in the compact form (nonsintered specimens). For reasons which will become apparent in Procedure E, it was found preferable t o measure the water formed using a moisture analyzer based upon the electrolytic coulometric principle (12). The OjU ratio is then calculated on the basis of the amount of moisture evolved. E. O/U BY OXIDATION-REDUCTION. If one examines the reaction occurring in Equation 3, it is apparent that the value of x can also be determined by measuring the amount of UOn,ooo formed. Utilizing this approach, the reaction can be rewritten to accommodate hypostoichiometric UOz also. UOn*,
Table 11. Evaluation of N in U30.y by N.A.D. of NBS 95Oa Uranium Oxide
Treatment
N,a
Grams NBS 950a, 1 hour at 900 "C 4.9775 5.0242 4.7688 5.3593 5.0403 4.9298 4.9966 4.8949 5.0248
after N.A.D. treatment 5 hours at 900 "C 8.004 7.998 7.998 8.007 8,004 8.001 8.007 8.016 7.986 Av 8.002 Calculation based on the assumption that NBS 950a is U308.033 (O/U 2.678). Q
infrared lamps until evaporation to dryness was completedapproximately 24 hours (with lids on crucibles at all times). The crucibles were heated on a hot plate for several hours and then preheated a t 470 "C for 4-12 hours. The samples were then ignited at 900 "C for 5 hours. The OjU ratio was calculated from:
This procedure differs from direct oxidation (Procedure C) t o the extent that all specimens are converted to a common origin-a uranyl nitrate solution-before ignition to U308. While the technique is applicable to both hypo- and hyperstoichiometric UOz, it is unsuitable for W-U02 because the composition of the ignited oxides can vary with temperature and time of ignition. C. OjU RATIO BY OXIDATION TO U308.Five-gram samples were weighed in tared 50-ml platinum crucibles and preheated at 470 "C for a minimum of 4 hours (with lids o n crucibles). The samples were then ignited at 900 "C for 4 hours. The OjU ratio was calculated from reaction ( 2 ) . As in the N.A.D. Procedure B, the method is not applicable t o W-U02 specimens. D. OjU BY REDUCTION IN HYDROGEN. I n 1965, Bright et al. (14) proposed the technique of reducing powdered U02+, specimens in a hydrogen atmosphere at 900 "C to what was believed to be U 0 2 . ~ oand o measuring the water formed by absorption on anhydrous magnesium perchlorate. The calculation of the OjU ratio was then based upon the equation : UOn,,
+xH~
+
UOz.aoo
+ xHz0
(3)
(14) N. F. Bright, L. G. Ripley, J. F. Rowland, and R. H. Lake, Research Report No. MD207, Dept. of Mines and Technical Surveys, Canada, July 1965. 256
ANALYTICAL CHEMISTRY
air
H1
uoz+z 900
c
u0z.000
(4)
Because tungsten oxide is readily reduced to tungsten metal a t 900 OC in hydrogen, the above procedure is also applicable to W-U02+z (if there is no tungsten oxide present in the original sample). In order to utilize the above reaction, the following procedure was used. The sample, contained in a 40-ml platinum crucible, was placed in a muffle furnace maintained a t 470 "C. After several hours, the oxidized powder was removed from the furnace and transferred to a nickel boat, 43/8r1X 5 / ~ K X 3 / ~ " . (In some cases, in order to obtain the maximum amount of data possible from the same material, the sample, after heating at 470 "C, was oxidized to U 3 0 8and this result used in the oxidation study-Procedure C. After weighing, the oxidized powder was transferred to a nickel boat.) The boat was then placed in a 1-inch diameter Vycor combustion tube. With hydrogen passing through the tube, the specimen was heated to 900 "C and this temperature maintained for at least 2 hours. The sample was then isolated in a hydrogen atmosphere and cooled to room temperature. The partial vacuum created when the hydrogen gas cooled was replaced with dry argon and the system kept sealed until the sample was weighed (in air). Under ideal conditions, the weight change from the initial starting weight represents the amount of oxygen deviation from stoichiometry. However, freshly reduced UOe shows considerable reactivity in air (with water and oxygen), a fact also reported by Bright e f al. (14). Thus, ideal conditions d o not exist. To correct for this chemical reactivity, the nickel boat and specimen, immediately after its weight was recorded, was placed in the combustion tube of a moisture analyzer (12) and the moisture on the sample determined by heating the specimen to 500-600 "C in a stream of dry argon. The argon atmosphere was then replaced with a stream of hydrogen and the excess oxygen on the specimen determined by heating to 850 "C. The weight of moisture and/or oxygen determined was then subtracted from the weight in air of the reduced specimen. Applying this correction, a reasonably accurate measurement of the oxygen gain or loss for the overall reaction U02*, --t UOn.0o was obtained, regardless of the chemical reactivity in air of the specimen. That this procedure is necessary (to correct for reactivity of the reduced material) is discussed later. RESULTS AND DISCUSSION U02,,. An evaluation of methods for the measurement of OjU ratios in UOz and more complex samples suffers from the lack of adequate standards-[.e., a lack of U 0 2 of known OjU ratio. The simplicity of the oxidation technique (Procedure C) recommends its utilization as a means of providing secondary standards for the evaluation of the more sophisticated procedures (A, D, and E). Therefore, it was found necessary to provide bias and accuracy figures for this determination. The ignition of a UO,,, sample results in a weight gain, which, assuming the final oxide t o be exactly U308, can be
used t o calculate the OjU ratio in the sample. Obviously, the degree of accuracy depends largely (assuming sufficient sample t o minimize weighing errors) upon the exactness of stoichiometry of the final oxide. According t o Booman and Rein (1.5)"the preparation of stoichiometric U308 depends o n such factors as starting compound, surface area-to-volume ratio of the sample, temperature, and time for ignition." In addition, the partial pressure of oxygen in the atmosphere has also been cited as a factor which could lead t o variable results. The evaluation of the oxidation technique was accomplished according t o the following scheme: a)
Uranium Metal
b)
NBS 950a oxide
N.A.11. (Procedure B) 5 tirs-900 ' C 1 hour N A n. A U308.033 hours-9OO .c'U30s
I 4
standard
c)
UO?,.
( P r o c ~ d i i r eC )
U3OY
N.A.D. 6 hoiirs-900 'C:
/
Steps a and b involve two materials accepted as standards for the study. One is high purity uranium metal. The other is a uranium oxide-National Bureau of Standards 950a. These two standards were subjected t o the nitric acid dissolution procedure (N.A.D.) in which the dissolved sample is evaporated to dryness, pre-ignited a t 470 OC for 4-12 hours, and ignited at 900 "C for 5 hours. O n the basis of weight change, the oxygen content (N) can be determined in the resultant oxide U 3 0 N . With this established, evaluation of the standard ignition method can be carried out in accordance with Step c. The primary purpose of the N.A.D. method is to put Steps a, b, and c on the same basis, in which the final conversion t o oxide in each instance begins with hexavalent uranium in aqueous solution. Thus, differences in mechanism of oxidation and resultant stoichiometry among these three materials are avoided. The final evaluation of Step c is thus to calculate Y from N a n d the weight changes. This in turn determines the value of x in U02=,. The data presented in Tables I and I1 indicate that the N.A.D. procedure gives an N value in U30Nof 8.00 =t 0.01 (corresponding to an OjU ratio of 2.667 =t 0.003). Having established the value of N in U30.v, samples of UO,,, were ignited in the manner outlined in Procedure C and the OjU ratio was calculated assuming the Y in the oxide U 3 0 y to be 8.000. The oxide was then dissolved in nitric acid and the N.A.D. treatment (Procedure B) carried out. The O/U (15) I. M. Kolthoff and P. J. Elving, "Treatise on Analytical Chemistry,'' Part 11, Vol. 9, Interscience Publishers, New York, N. Y . , 1962; p 74.
Table 111. O/U Ratio of U0ziz by Ignition a t 900 "C Direct oxidation After Sample after N.A.D.b Absolute No. prebum treatment errorc s-1 2.003 2,003 0.000 s-2 1.966 1.966 0.000 Duplicate 1.966 1.965 +o. 001 s-3 1.915 1.914 +o, 001 Duplicate 1.917 1.918 -0.001 s-4 1.905 1.908 -0.003 Duplicate 1.904 1.903 +0.001 a Calculated assuming the oxide after oxidation was U308.000. On rare occasions the resulting oxide will contain massive particles resembling coal sinters. When this occurs, the O/U ratio obtained by the oxidation procedure ( C ) should not be considered reliable. Calculated assuming oxide after N.A.D. treatment was U308.000. Error-difference between O/U obtained after N.A.D. treatment and after 4 hours at 900 "C (after usual preburn).
ratio of the sample was then recalculated assuming the N value in the final oxide UjONwas 8.000. The results shown in Table I11 indicate that the value of Y (see flow diagram) is indeed very close t o 8.000. If this were not the case, the O j U value obtained by the oxidation procedure would not be in agreement with the N.A.D. value. Consequently, including the error involved in establishing N , the determination of the OjU ratio in UOzl, by the Oxidation Procedure (C) is definitely accurate to within i 0 . 0 0 4 O/U units. The latter statement is predicated upon the assumption that the moisture content of the specimen is negligible. If the moisture content is appreciable, the Oxidation and the N.A.D. Procedures yield apparent O/U results. Having established that Procedures B and C are capable of yielding accurate apparent OjU ratios in UO,,, specimens, the Reduction Procedure (D) and the Oxidation-Reduction Procedure (E) were evaluated by comparison with O/U values obtained by the former methods. OjU values obtained on sintered UO, specimens by the Oxidation-Reduction Procedure (E) are shown in Table IV. The results indicate the following: 1) The chemical reactivity in air of sintered uranium dioxide compacts is negligible (footnote a). 2) Freshly reduced UOz powder produced from a sintered uranium dioxide compact exhibits limited chemical reactivity in air. 3) The reduction of powdered U308 proceeds rapidly in hydrogen at 850 "C until a value of about UO?.oa?is reached. Then the reduction proceeds very slowly. Because of inherent weighing errors and possible electronic drift in the electrolytic coulometric measurement
Table IV. Oxygen-to-Uranium Ratios in Sintered Uranium Dioxide O/U Ratioa Oxidation-Reduction Hydriding Sample Fuel Oxidation Corrected forb No. type to U308 N.A.D. Uncorrected chemical reactivity methodc SU-1 As recd. 2.000 ... (1.998) 1.998 su-2 As recd. 1.998 (1.996) 1.997 su-3 Agglom. 2.005 2:%4 (2.002) 2.003 su-4 Agglom. 2.005 2.005 (2.001) 2,002 SU-5 Agglom. 1,984 1.982 (1 ,979) 1.981 1.982 SU-6 Agglom. 1.938 1.938 (1.934) 1.935 1.913 a Repeat determinations over a period of several months had no effect on indicated values. Electrolytic coulometric measurement for moisture and oxygen. Test terminated after 2-3 hours in HPat 850 "C. Reduction was still proceeding very slowly. Reference (13).
VOL. 41, NO. 2, FEBRUARY 1969
257
there is no advantage t o be gained in extended reduction periods (>3 hours). It is sufficient that one be cognizant of the probability that OjU values obtained by the oxidationreduction technique may contain a slight negative bias. 4) Companion tests performed o n other portions of the sintered specimens shown in Table IV indicated that the initial moisture content was negligible. The tests also indicated that the direct reduction of sintered UOztz compacts in hydrogen a t 850 "C will not proceed rapidly enough t o be of analytical value. I n order t o lend additional support t o the data presented in Table IV, a number of sintered UOz samples were chosen at random and subjected t o the Oxidation-Reduction Procedure. Because the laboratory manipulation time required to adjust the reduced oxide weight for chemical reactivity represents a major undertaking when a large number of specimens are involved, and because the correction as indicated in Table IV would not exceed 0.002 OjU units, the electrolytic coulometric measurement for moisture and oxygen was not performed. Typical values obtained are shown in Table V. Oxygen-to-Uranium Ratios in Sintered Uranium Dioxide O/U Ratio Oxidation Oxidationn Sample to UIOa reduction A No. 1.919 -0.001 su-7 1.920 1.917 O.Oo0 su-8 1.917 1.936 -0.003 1.939 su-9 1 ,940 -0,004 su-10 1.944 1.935 -0.003 su-11 1.938 1.872 0.001 su-12 1.871 2.006 0.000 2.006 SU-13 2.002 -0.003 2.005 SU-14 2.007 0.000 SU-15 2.007 1.998 -0.002 2.000 SU-16 1.997 -0.003 SU-17 2.000 1.999 -0.003 su-18 2.002 1.994 -0.005 SU-19 1.999 1.990 0.000 1.990 su-20 1.989 0.000 1.989 su-21 1.998 -0.005 su-22 2.003 1.952 -0.007 1.959 SU-23 a Values are not corrected for chemical reactivity of the reduced UO,. Data reported in Table IV indicates that the maximum correction would be 0.002 O/U units (to be added to these results). Table V.
Table VI.
In direct contrast t o sintered uranium dioxide, nonsintered specimens (that is, UO, that has not been subjected t o high temperature firing-at least 1400 "C-for extended periods of time) possess a high degree of chemical reactivity in air for both moisture and oxygen. The extent of the chemical reactivity is such that a direct measurement of moisture and subsequently, reducible oxygen, performed by the electrolytic coulometric technique, Procedure D, is t o be preferred over the oxidation-reduction technique. The results shown in Table VI indicate that extreme care must be exercised when using OjU values obtained on nonsintered specimens. The following points must be understood: First, the OjU ratio of a prefired specimen (for example, 15 minutes in hydrogen at 1000 "C) begins to increase immediately upon removal from the furnace. The analytical value reported by the laboratory represents the OjU value only at the time of determination. Second, a true OjU value (on a nonsintered specimen) is obtained only if the specimen weight has been corrected for adsorbed moisture. W-U02+,. Because the oxides of tungsten are readily reduced in hydrogen to the metal, we can postulate that the same OjU ratio would be obtained on a W-UO, sample as would be obtained on UOz, if both samples were sintered under identical conditions. Consequently, the applicability of the oxidation-reduction technique has been evaluated by comparing the results obtained on the W-UO?.,, samples with those obtained on UO, control samples (the two materials sintered simultaneously). The composition selected for testing was 54z W-46z UO?. Typical results are shown in Table VI1 and indicate the applicability of the technique. It should be noted that the correction for chemical reactivity in air of reduced sintered W-UO, specimens is generally greater than that required (see Table IV) for reduced sintered UO, specimens (even though the UO, content has been reduced by one half). This suggests that the reduced W powder is also chemically reactive in air. This activity would explain the failure of nonsintered W-UO,,, compacts to correlate with their UO? control specimens (see Table VIII). Companion tests performed on W powder confirmed this chemical reactivity in air for moisture and oxygen. Reactivity of Freshly Reduced UO, and W-UO2. The results presented in this paper stress the need to correct all reduced U 0 2 and W-U02 weights for chemical reactivity in air. The extent of this reactivity is indicated in the accom-
Oxygen-to-Uranium Ratios in Nonsintered Uranium Dioxide Apparent-~ O/U ratio Oxidation reduction Corrected for Direct reduction Oxidation Sample Z H?O N.A.D. Uncorrected chemical reactivity o/u to UIOS No. ... (2.057) 2,064 ... 2.068 P1 2: 081 (2.069) 2.078 2.056 0.146 2.080 PI -A ... (2.073) 2.083 2.060 0.147 2,086 PI-B 2.033 (2.02oj ... 2.021 0.051 2.033 P2 ... (2.007) ... 2,010 0.034 2.017 P3 Notes: 1) P1, PI-A, PI-B represent determinations performed on portions of the same specimen at intervals of 1 Neek, 1 month, and three months after removal from a low temperature (1400 "C)reduction furnace. 2) The results reported above are all self-consistent despite the apparent contradictions. For example, if the apparent 0,'U ratios obtained on specimen PI-A are corrected for moisture (specimen contains 0.146 % HsO), the OjU ratios become: Oxidation to UaOe 2.055 N.A.D. 2.056 Oxidation-reduction with chemical reactivity correction 2.054 Direct reduction 2.056
258
ANALYTICAL CHEMISTRY
Table VII. Oxygen-to-Uranium Ratios in Sintered W-U02 O/U Ratios Oxidation reduction Corrected for
Sample No.
wu-1
Uncorrected (2,004)
wu-2 (2.001) wu-3 ( 1 ,952) wu-4 (1.960) wu-5 ( 1 ,926) a Reference (13).
chemical reactivity 2.007
Hydriding method"
2.005
1,960 1.986 1.939
Table VIII. Oxygen-to-Uranium Ratios in Nonsintered W-UOz by Direct Coulometric Measurement Sample
UOZ control 2.005
UOz control ~7-z H20
oiu
zHzO
o/u
PWU-1
2. I16 2.052
0.115 0,040
2.056 2.021
PWU-2
2,005
1.961 1.980 1.939
w-U02"
No.
0.146 0.051
Total reducible oxygen assumed to be associated with the uranium.
1.960 1.984 1.938
Table IX. Oxygen-to-Uranium Ratios in Fuel Materials after Subjection to Successive Firing Cycles
Specimen fired in Hz at 2 . 1 1 6 ~(direct reduction1000 OC for 15 min: moisture correction) Cooled in H?. Specimen fired in Hz at 1 ,986 (ox-red correction for chemical reactivity) 2000 "C for 2 hours? Cooled in He. Specimen fired in H? at 1 ,939 (ox-red correction for 2400 OC for 3 hours. chemical reactivity) Cooled in He. Specimen fired in wet H? 2.005 (ox-red correction for (dewpoint = 40 "F) chemical reactivity) a t 1400 "C for 6 hours. Cooled in Hz. Specimen fired in wet Hz 2.007 (ox-red correction for (dewpoint = 40 OF) chemical reactivity) at 1400 "C for 12 hours. Cooled in H?. a At this point, the specimen is not sintered. * After this treatment (and for subsequent treatments), the specimen is considered a sintered body. Ratio at time of analysis. All reducible oxygen assumed to be associated with the UOz present.
panying Tables. That chemical reactivity can be nearly instantaneous has been demonstrated by reducing a specimen in the coulometric apparatus and, after cooling t o room temperature in argon, exposing the specimen t o air for 30 seconds. The specimen was then reinserted in the electrolytic coulometric apparatus. Upon heating this specimen in argon, moisture was evolved, indicating that even with this limited exposure, moisture had been adsorbed by the specimen. Upon switching t o a hydrogen atmosphere, additional moisture was evolved, indicating that the O/U of the reduced specimen had increased by reaction with atmospheric oxygen. Typical Application of Methods Presented. The flow diagram in Table I X illustrates the changes that occur in the O/U ratio of a fuel core material when it is subjected to a series of successive firing cycles. The diagram also indicates
2.056 (direct reductionmoisture correction)
+
1 .984 (ignition to UlOs)
+
1.938 (ignition to U308)
+
2 005 (ignition to UaOs)
+
2.005 (ignition to U308)
how a laboratory can determine the O/U values in materials tested in different ways by employing a combination of the various methods discussed in this paper. ACKNOWLEDGMENT
The authors thank D. E. Burgbacher for specimen preparation, s. Kallmann for advice on hyperstoichiometric UOz reduction in hydrogen, and M. R. Menke for some of t h e analyses. RECEIVEDfor review August 26, 1968. Accepted October 14, 1968. This paper originated from work sponsored by the Fuels and Materials Branch, U S . Atomic Energy Commission, under Contract AT(40-1)-2847.
VOL. 41,NO. 2, FEBRUARY 1969
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