Investigation of a precise static leach test for the testing of simulated nuclear waste materials

0191-815X184 $3.00 + .OO Copyright B 1984 Pergamon Press Ltd.

NUCLEAR AND CHEMICAL WASTE MANAGEMENT, Vol. 5, pp. 3-15,1984 Printed in the USA. Al1 rights reserved.

INVESTIGATION OF A PRECISE STATIC LEACH TEST FOR THE TESTING OF SIMULATED NUCLEAR WASTE MATERIALS H. M. Kingston D. J. Cronin ik? S. Epstein Center for Analytical Chemistry, National Bureau of Standards,

Washington, D.C. 20234

ABSTRACT. The precision of the nuclear waste static leach test was evaluated using controlled experimental conditions and homogeneous glass materials. The majority of the leachate components were subjected to simultaneous multielement DCP analysis. The overall precision of the static leach test is determined by the summation of random effects caused by: 1. variante in the experimental conditions of the leaching procedure; 2. inhomogeneity of the material to be leached; and 3. variante of the analytical techniques used to determine elemental concentrations in the leachate. In this study, strict control of key experimental parameters was employed to reduce the tïrst source of variante. In addition, special attention to the preparation of glass samples to be tested assured a high degree of homogeneity. Described here are the details of the reduction of these two sources of variante to a point where the overall test precision is limited by that of the analysis step. Of the elements determined- B, Ba, Ca, Cs, Mo, Na, Si, Sr, and Zn-only Ca and Zn exhibited replicate imprecision significantly greater than that observed in the analysis of the leachate solutions. The imprecision in the Zn was partially attributed to the nonreproducible adsorption onto the leach vessel walls during the 28 day test period. None of the other elements exhibited this behavior.

INTRODUCTION

Present

leach tests do not measure

the fundamen-

tal properties of a material that relate to its durability; rather, they measure the end result of the interaction of a given material and an aqueous environment under a specific set of conditions. The problem then is to describe and measure the leaching characteristics of these materials in a meaningful way. Waste forms are usually ranked according to leach resistance by comparing data from leach tests. Until recently, both the test conditions and materials differed among laboratories. Without a reference material and a reference method, the comparison of results on an inter-laboratory basis is difficult if not impossible. This study describes the establishment of standard materials and methods for such testing. Nuclear waste materials are very complex and it is common for the material to contain 30 or more elements. At the present time, the mechanisms involved in the leaching of such complex materials are not wel1 understood (3,4). Thus the leach tests are largely empirical in nature. In an attempt to establish a uniform basis for obtaining results which can be compared, the Department of Energy established the

In the present scheme for the disposal of high-leve1

nuclear waste materials, the waste wil1 be converted into a glass or ceramic monolith and placed in a carefully engineered repository thousands of feet below the earth’s surface. Once placed into the underground repository, the most probable mechanism by which waste components could be released to the environment and, hence to man, wil1 be through leaching of the waste form by water (1,2). Thus one of the most important qualities sought in the nuclear waste form is its ability to resist leaching by water, thus preventing mobilization of the waste components. RECEIVED3 AUGUST 1983; ACCEPTED 26 NOVEMBER1983.

Acknowledgements-The authors would like to thank William Koch and George Marinenko for the analysis of the pH, fluoride, and nitrate in these samples. The authors express their gratitude to Walter Liggette for his statistical advice in the experimental design and for insight in the evaluation of these results. Certain commercial equipment, instruments. or materiais are identified in this report to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.

Materials 3

Characterization

Center (MCC).

As part of

4

H. M. KINGSTON, D. J. CRONIN AND M. S. EPSTEIN

its duties, MCC has defined a series of waste form leach tests. The most developed of these is the MCC-1 Static Leach Test. This test has been under investigation for approximately three years and a provisional version of the test recently underwent a round-robin study with 25 laboratories participating (5). Two of the summary conclusions made in the round-robin report were that the MCC-1 is a widely applicable test method, and that overall procedural bias between laboratories was a major problem. Certain improvements have since been provisionally incorporated into the current MCC-1 procedure as a result and published in the Nuclear Waste Materials Handbook (6). The purpose of the work described here was to evaluate the precision of the MCC-1 Static Leach Test by using very tight controls over key procedures and experimental conditions. A further objective was to evaluate a potential reference glass material for future use as a means of quality control. Most of the basic test parameters from the current MCC-1 protocol were used. Some changes were made in the procedure when it could be shown that they would significantly reduce the variability of selected parameters. The precision of this final composite procedure was then evaluated using specially prepared glasses. In order to reduce the error caused by variations in sample composition, a homogeneous glass is required. Such a glass, potentially suitable for a simulated nuclear waste reference material, is described. This base glass and 12 additional glasses having compositions that were variations of the base glass composition were tested. The final data were obtained through elemental analysis of the leachate solutions. These durable simulated nuclear waste glasses resulted in leachate element concentrations of a few mg/mL. Due to the number of elements to be determined and the relatively smal1 volume of solution, a multielement method such as inductively coupled plasma (ICP) or direct current plasma (DCP) is desirable and frequently used for the bulk of these analyses by the nuclear waste community. As with al1 instrumental measurements, there is a definitive precision and accuracy associated with the technique. Under the conditions described here this source of variante is the ultimate limitation for interpreting leach test results, since if al1 imprecision is eliminated from the test and the material, the final leachate precision can be no better than that of the analytical measurement of element concentration in the leachate. A comparison of the precision of this modified leach test with the precision of the leachate measurement is described in this paper. The present theoretical limit for the precision in this state-of-the-art leachate measurement is discussed.

EXPERIMENTAL

Reagents Reagent grade HNO, and HF and high-purity HNO, were used (7). The high-purity acid was prepared by sub-boiling distillation and used only for stabilizing the final leachate solutions (7). Two types of water were used: distilled and highpurity water, the latter prepared by passing distilled water through a modified Milli-Q system. Apparatus The leach vessels and knife edge screen support were fabricated from perfluoralkoxy (PFA) Teflon. These 60-mL sealable containers are specified in the MCC- 1 test procedure (6,8). A set of 48 containers was selected from three lots of containers for this study. Leach Vessel Preparation The PFA leach containers were put into an air oven at 200 “C for eight days to reduce the content of HF contained in the PFA. They were then soaked in 6 M HNO, and 0.2 M HF for 1 hour at room temperature. The containers were rinsed with three container volumes of high-purity water. They were then placed in 6 M HNO, for four hours at 50 “C, rinsed, and soaked by full immersion for 30 min in greater than 60 “C high-purity water. They were then soaked for eight hours in high-purity water at 80 “C. The containers were then boiled for 30 min in fresh highpurity water and then rinsed with three container volumes of high-purity water. This latter container preparation procedure is required in the current version of “MCC-1 Static Leach Test” (6). The large vessels used to hold the containers during cleaning were covered polycarbonate boxes. After carrying out the specified procedure, batches of 20 containers were filled with high-purity water, sealed, placed in a microwave oven, and subjected to 720 watts of microwave radiation for 30 min. The containers were then emptied and dried in a clean air facility (9). Test Chamber The water bath used to keep the leach vessels at a controlled and uniform temperature was a 55-liter sealed water bath (prototype) with a controller (manufactured by Tronac, Model ptc-41). The bath was kept under internal positive pressure by passing Ascarite-treated Argon, via Teflon tubing, from an external tank through the lid of the bath. The leach containers were held in place by two stainless steel grates held together by approximately 15 stainless steel threaded rods with stainless steel nuts. The containers were selected at random, stacked three to a column, and the columns spaced equidistant from one another within the stainless steel holder (Fig. 1).

SIMULATED NUCLEAR WASTE MATERIALS Liquid & Glass Thermometer /

-

lnsulated lid with saai

ne of 5 sensors used

/

stahless steel

Argon

J Data logger Warm water out

Warm water in FIGURE 1. Controlled

Line power cord

leaching chamber and monitoring system.

Ternperature Monitoring The temperature of the bath was continuously monitored for the duration of the test using a HewlettPackard Model 3054 DL Data Logger. The timing function of the system has a battery back-up system and was programmed to auto-restart and update conditions in the case of power interruption. The unit continuously monitored the bath temperature at five locations, provided a visual readout every three seconds, and recorded the temperature on printed paper tape and magnetic tape every hour. A diagram of this system is shown in Fig. 1. Ultra-stable stainless steel encased thermistors were obtained from Thermometrics Inc. Prior to the test and at the 75% point in the test, the temperature measurement system was calibrated by the Temperature and Pressure Measurements and Standards Division at the National Bureau of Standards. Glass Samples A total of 13 glass compositions were investigated, including a base glass composition and 12 glasses that represent controlled composition variations from the base. The base glass was designed to simulate a nuclear waste glass and be used as a reference

material in leach tests. However, certain elements that are routinely found in nuclear waste matrices, such as Fe, Cr, and Ni were omitted to prevent the glass from being opaque. The homogeneity of the samples was checked by optica1 and polariscopic examination. Al1 glasses proved to be essentially free of inhomogeneities. Rare earth ions were added as substitutes for actinides expected in the waste. The base glass composition is shown in Table 1. The base composition was systematically changed to obtain information from the test concerning the effect of composition variation. AI1 changes were based on weight percent and involve a one-to-one substitution with SiO,. The changes were made at two levels. Major constituents such as NazO and B,O, were varied in 5% increments. Al1 other oxides were varied in 1% increments. The variations from the base glass composition for the other twelve glasses are shown in Table 2. The values shown represent a differente from the base glass composition in weight percent. The changes are in absolute weight percent, not relative. For instance a change of +5 Bz03, as in composition no. 4, refers to increasing the B1O, content to 15.96 wt % and decreasing SiO, to 41.10 wt

6

H. M. KINGSTON, D. J. CRONIN AND M. S. EPSTEIN TABLE 1 Base Glass Composition Oxide

ultrasonic wash in high-purity water for five minutes followed by two five-minute ultrasonic washes in absolute ethanol.

Weight 070

SiO,

46.10 10.96 15.32 5.75 3.43 2.31 0.45 2.10 1.96 0.31 1.36 0.66 1.41 0.57 0.63 5.78 0.63 0.25

BG Na,0 Zn0 TiO, Ca0 Sr0 ZrOl MoO, TeO, Cs,0 Ba0 CeO, p,os La203 Nd&), pr.011 YzO,

Preparation of the Leachate One of the parameters that was deemed important is the concentration of dissolved gases in the leachant. Control of this parameter was most easily accomplished by elimination of these gases. To remove the gases, the leachant was prepared by plating highpurity water (distilled and treated with a Milli-Q System) into a 2 liter FEP bottle fitted with a twoport PFA tap and a tube reaching to the bottom of the container. The water was heated for 20 min in a modified microwave oven at full power (720 watts), while Ascarite-treated argon was bubbled through the water. This procedure was repeated three more times and the fourth quantity was used as the bulk leachant.

Vo. Al1 other components of composition no. 4 are at the levels given in Table 1. Al1 glasses were made from reagent grade oxides and carbonates. The initial batch was mixed and then melted in a platinum crucible using an electric resistance-heated furnace. To assure homogeneity the melts were stirred with a platinum stirrer. Following homogenization the melts were tast into metal molds and the resultant glasses annealed to relieve any residual strain. Samples to be tested were cut from the tast blocks of glass using a diamond impregnated saw blade. Since surface condition is critical to obtaining reliable leach test results, the cut specimens were each hand-lapped using 600 grit SiC in a water slurry on a glass plate. This procedure gave a uniform, reproducible surface finish (10). Prior to testing, the specimens were cleaned by an

Glass Composition Composition Number

SiOl

“Base” 2 3 4 5 6 7 8 9 10 11 12 13

0 +10 0 -5 0 - 10 -5 -5 -5 -5 -4 -4 -4

B1O,

Na,0

0 -5 -5

0 -5

5 5 5 0 0 0 0 0 0 0

5 0 -5 5 5 0 0 0 0 0 0

Leaching Procedure The containers were assembled in numerical order. Each container and Teflon specimen support screen was weighed on a top-loading balance. The balance was contained in a Plexiglass box slightly pressurized with argon gas to prevent the adsorption of gasses. The amount of leachant added to each container was directly dependent upon the surface area of the glass sample to be added to that particular container. The ratio of the geometrie surface area to the volume of leachant (SA/V) was 0.01 mm-’ as specified in the MCC-1 protocol (6,8). Approximately 35 g of the leachant was weighed directly into the leach container to within *O.O2g. The glass was then placed in the container directly onto the knife-edge screen. The container threads were wrapped with Teflon tape and the container lid was screwed on firmly.

TABLE 2 Vnrintions (9’0 Wt Change)

TiOl

Sr0

MoO,

CsrO

CeO,

Nd203

0 0 0 0 0 0 0 1 1 1 1 0 0

0 0 0 0 0 0 0 1 1 1 0 1 0

0 0 0 0 0 0 0 1 1 1 0 0 1

0 0 0 0 0 0 0 0 1 1 1 1 1

0 0 0 0 0 0 0 1 0 1 1 1

0 0 0 0 0 0 0 1 1 0

1

1 1 1

7

SIMUJATED NUCLEAR WASTE MATERIALS

The sealed containers were then placed in an air oven and heated to 90 f 1 “C for 1 hr. The samples were removed and the containers hand-tightened a second time. They were then mounted in 15 columns of three containers each in the stainless steel grid and placed in the constant temperature bath filled with distilled water at 90 “C. The duration of the test was 28 days plus the additional time the glass was in contact with the solution during preparation and disassembly. Approximately one hour for both preparation and initiation of the experiment translates into less than a 0.3% error. Its effect is much less than that due to the differente between ambient temperature and the temperature of the bath (approximately 67 “C). Upon completion of the test the rack was removed from the water bath and the containers were removed from the rack and placed in numerical order under a clean air hood. Each container was disassembled in the Plexiglass weighing box and the glass, the leach solution, and the container were weighed. A 5 mL aliquot was removed and placed in pre-treated 7 mL polyethylene bottles. Prior to the transfer, the bottles were filled with argon and the transfer took place in a stream of argon. The samples for pH analysis were then set aside in an argon-filled box overnight. The glass was removed with Teflon tweezers, high-purity HNO, was added (1% by volume) and the container was sealed and placed in the air oven at 90 “C overnight. Several blanks and new containers (not subjected to the cleaning procedures) were not opened prior to elemental analysis but were taken directly for pH, nitrate, and fluoride analyses. The pH and Anion Measurements A combination glass/saturated-calomel electrode system was used for the pH measurements. The samples were measured in random order. The nitrate and fluoride were surveyed in selected samples by ion chromatography using direct injection. Identification of the anion species was by retention time and quantitation was by peak height. Leachate Element Analysis Quantification of the 12 target elements in leachate, leachant, and blank solutions were performed using three analytical spectroscopic techniques. Table 3 summarizes these techniques and the most significant parameters related to them. Analyses were performed by introducing the solutions directly from the leaching vessels into the analytical sources to minimize contamination. Eleven of the 12 elements were determined using a direct-current plasma/echelle spectrometer (DCP); 10 of the elements were determined simultaneously (except sodium) with a 10 Channel multielement cassette. Cesium was determined by atomic emission

TABLE 3 Technique Used for Determiaation of Elemental Concentratioas in the Leachate rad Leachant

x Element

Technique

(nm)

L.O.D.a (ng/mL)

Precisionb (“70RSD)

B Ba Ca Ce CS

DCP DCP DCP DCP FES DCP DCP DCP DCP DCP DCP DCP DCP FAAS

249.173 455.403 3%.847 394.275 852.124 455.535 374.825 588.995 430.358 251.611 407.771 365.350 213.856 213.856

0.06 0.001 0.002 0.2 0.2 2 0.01 0.006 0.04 0.06 0.0004 0.05 0.02 0.008

1.2 3.3 1.7

Mo Na Nd Si Sr Ti Zn

0.7 3.2 2.8 1.8 2.9 1.1 1.4

DCP = direct-current plasma; FES = flame emission spectroscopy (air-acetylene flame); FAAS = flame atomic absorption spectroscopy (air-acetylene flame). a Lower limit of detection based on analyte signal = 3 x standard deviation of the blank. bEstimated precision of the analytical method under conditions limited by analyte emission fluctuations for the determination of that specific element under the experimental conditions used for the analysis of the simulated nuclear waste leachates. Eight replicate determinations are performed over a 4 hr period and each determination is the average of three consecutive 5 sec integrations. The listed percent RSD’s are the average of the RSD’s obtained on two separate days.

in an air-acetylene flame, since its most sensitive emission wavelength is beyond the range of the DCP spectrometer. Zinc was also determined by atomic absorption in an air-acetylene flame in order to confirm DCP analysis results. RESULTS AND DISCUSSION Twelve of the 13 different glasses tested were variations on the base glass composition. However, the evaluation of the leach test precision can be accomplished by comparing the results of the seven base glass replicate samples. Several test conditions were significantly different from those specified in the original MCC-1 protocol; changes were made where a suspiciously large tolerante in parameter conditions was specified by the MCC-1 protocol. It is recognized that there is a need for a greater degree of precision in a system used for calibration than in the system being calibrated. The MCC-1 protocol specifies several test parameters each with its own tolerante. Selected parameters were chosen for tighter tolerante levels. The selection was based on both laboratory data and intuition involving these theoretical concepts. The altered tolerante parameters included surface

8

H. M. KINGSTON, D. J. CRONIN AND M. S. EPSTEIN

roughness, Eh, dissolved OZ, dissolved CO1, temperature, specimen homogeneity, leachant losses and some procedural measures. Instrumental analysis precision was also closely defined as a point of reference for test precision. Some of the tighter specifications add complexity to a test, while others are only different procedures of approximately the same complexity. The ratio of truc surface area of the sample to the volume of leachate (SA/V) is recognized as a significant parameter (3,4,6,8). Surface roughness and uniformity are not isolated parameters but have interactions with other parameters. The specific influence of this parameter alone on precision cannot be extracted, however, the synergistic effects of surface roughness with pH and other parameters are significant to the point of influencing mechanistic changes in the leachate characteristics (11). Presently, a saw cut surface is specified in MCC-1 surface preparation. This method results in a true surface area that differs from the geometrically calculated surface area because of varying degrees of roughness. These physical variations are a result of differences in the condition of the tools and the cutting technique. Therefore, a 600 grit silicon carbide abrasive powder was used to produce much more uniform sample surfaces. Although the exact true surface area to geometrie surface area ratio is stil1 specifically unknown, it is very uniform and reproducible. The Eh or oxidizing potential of the leachate solution is a parameter which becomes increasingly difficult to evaluate as the leachate solution becomes more complex. However, in this system using distilled deionized water, the Eh is determined chiefly by the amount of dissolved oxygen in the initial solution and the amount entering the sample by diffusion through the walls of the leach container. At the present time the MCC-1 protocol suggests equilibrating the leachate with air at ambient temperature prior to starting the test. The amount of dissolved oxygen and carbon dioxide present in the leachate can vary from laboratory to laboratory depending on the laboratory temperature. The role of Eh in this test is not clearly understood. Thus, to eliminate the effect of various quantities of dissolved oxygen and minimize Eh effects, the solution was saturated with argon. The containers were also prevented from diffusion of 0, through the Teflon container walls by saturating the water bath with argon and keeping it under positive pressure. The leaching characteristics are closely linked to the pH produced by the glass in solution. It has been mentioned that varying quantities of CO, may cause significant variations in pH and therefore in mechanisms and leachate concentrations (4,11). The same technique used to eliminate dissolved oxygen

also eliminates dissolved COZ. The diffusion of COZ into the container was prevented during the 28 day period by saturating the sealed bath with argon and keeping it under positive pressure. By taking this precaution, the composition of the glass is the principal determinant of the final pH of the leachate. Improvement in the control of the temperature was necessary to achieve maximum precision. An estimate of the precision of the elemental concentration values was obtained from a comparison of MCC-1 leach results at 70 “C and 90 “C for PNL 76-68 glass (12). The relationship between elemental concentration and temperature was assumed to be linear. The leachate elemental concentrations at these two temperatures were used to calculate the percent change in the concentration for f 1 “C (the range allowed by the MCC-1) at 90 “C for B, Ba, Ca, Cs, Mo, Na, and Si. The values obtained were 6%, 20%, 16%, 7(r/o, 4070,4%, and 2%, respectively. Thus the best possible precision which could be expected for Cs would be 7% using the MCC-1 temperature protocol. It should be noted here, however, that this argument is based solely on a kinetics (linear) model. Recent studies have shown that Ba and Ca concentrations are solubility-limited and not amenable to such a calculation. The estimate is, however, that a 2 “C variation could introduce an error approximately equal to or greater than the analytical error (13,14). In addition, air ovens normally used can often produce significant losses of leachate solution during the test. In the present MCC-1 procedure up to 10% solution loss is tolerated before the sample must be discarded. The sealed water bath system employed avoided solution losses and controlled the temperature to 90 f 0.04 “C for the duration of the test. The containers did not lose any detectable quantity of leachate during the 28 day experiment (determined to within 0.06%). The water bath, temperature measurement apparatus and data recording system functioned extremely wel1 as a unit. The repetitive temperature readout was used extensively to make minor adjustments to the controller. The temperature measurement system was calibrated prior to the start of the experiment and again 5 weeks later. Both calibrations were identical. During the experiment a total of 8 liters of 90 “C water was added to replace that which was lost due to escaped vapor. This addition was accomplished by adding 2 liter portions of water through a port in the bath. Great care was taken to protect the initial leachant from contamination not only prior to the test, but also during the transfer of smal1 aliquots of the final leachate for pH, fluoride and nitrate analysis. The fluoride and nitrate analyses were survey determinations with relative errors estimated to be 20% and 50070, respectively. The results of the fluoride and

9

nitrate measurements were examined to determine if the container contributed to the total concentrations and might therefore affect the leach mechanisms. Results of the pH, fluoride, and nitrate analyses of the 7 replicates, blanks, and new containers are given in Table 4. It was suspected that HF from the Teflon could influence the pH of the leachate during the test (11). There are two possible sources of fluoride contamination in this system. One source could be the hot molding of the original containers where molecular changes may occur (15). The second source could be the nitric acid and hydrofluoric acids used in the preparative cleaning procedures of the Teflon leach vessels. The microwave cleaning procedure was used in addition to the specified MCC-1 leach container preparation procedure to help eliminate these sources of contamination. The neutrality of the solutions in the blank containers suggest that the trace quantities of fluoride and nitrate have little affect on the pH of the leach solutions. The amount of fluoride in the new (untreated) containers and the accompanying pH show the necessity of stringent container TABLE 4 The

pH, Fluoride, and Nitrate of Selected Leachate Samples

Sample Number

PH

Fluorideb tppm)

Nitrateb (ppm)

0.99

< 0.08

1.04

0.11

1.07

0.18

Base Glas?

1 6 11 16 21 26 31 x S

Blank Containers 32 33 34 35 36 37 38 x New Containers 41 42 43 44 K S

9.36 9.33 9.37 9.27 9.39 9.35 9.34 9.34 0.04C 7.12 6.67 6.60 6.69 6.74 6.59 6.54 6.71 0.19 3.81 3.81 3.87 3.90 3.85 0.05

0.85 0.78 0.65 0.74 0.80 0.82

3.41 3.45 3.50

0.09

0.13

0.26 0.33

0.40

0.14 c 0.08

aBa.sed on the repetitive measurements of three NBS buffers the standard deviation of these pH measurements is 0.03 pH, however, due to the differente in ionic strength the accuracy cannot be assured beyond 0.1 pH units. bThe accuracy for these fluoride and nitrate survey analyses are no better than 20% and 50% respectively. cs is the standard deviation.

preparatory steps. The fluoride value for the original leachant material was 0.09 pg/mL, which is appreciably smaller than any of the fluoride levels detected in the leachant (Table 4). The pH and nitrate values as determined in the blank containers are assumed to be representative of those in a carefully prepared leachant solution. Al1 45 samples were analyzed in a random order. Comparisons of the pH values show essentially the same precision that was established for the NBS buffers. This is important since certain elemental concentrations and ultimately some leach mechanisms are linked directly to the leachate pH. Similar agreement was found between the precision of the buffers and that of the other 12 pairs of glasses. In order to evaluate the uncertainty associated with the leach test procedure, it was necessary to characterize the uncertainty of the analytical data resulting from the instrumental methods used. Al1 three instrumental analytical techniques used in this study fellow the genera1 trend of precision characteristics illustrated in Fig. 2, which is a plot of relative standard deviation versus concentration for the determination of zinc using the direct current plasma (DCP). Precision values are given as the relative standard deviation of nine replicate 7 sec integrations. Four sets of the nine replicates were averaged to obtain the values in Fig. 2. Using the DCP at concentrations near the detection limit, the dominant sources of noise are generally background emission from the plasma and/or photomultiplier dark current. At higher concentrations, the variability in the analyte emission intensity due to changes in sample transport, plasma fluctuations, or wavelength drift becomes a dominant sources of noise. This type of noise defines a lower limit of 0.7% R.S.D. in Fig. 2. The curve turns upwards at very high concentrations as a result of the non-linear relationship between intensity and concentration caused by self-adsorption and self-reversal in the DCP. The standard deviation (in concentration units) is therefore no longer linearly related to emission intensity (17). The precision values in Fig. 2 represent a “shortterm” variability of the analytical method. The precision values in Table 3, however, represent a more realistic picture since they were obtained under conditions identical to those used in the analysis of the simulated nuclear waste leachates (one set of determinations on two separate days). These values were obtained from a composite leachate sample (supplied by Battelle Pacific Northwest Laboratories [PNL] to the participants of the MCC-1 Round Robin). The instrument was calibrated using aqueous standards and in a manner identical to the calibration procedure used during actual analysis of the individual leachate samples. The precision of the zinc determination in Table 3

H. M. KINGSTON, D. J. CRQNIN AND M. S. ERSTEIN

10

Background f luctuation limit of detection

Analyte

emission fluctuation limit

Z inc

concentration

(yg/mLJ

FIGURE 2. Precision plot for zinc atomic emission from the D.C. plasma.

is significantly greater (1.1070) than that illustrated in Fig. 2 (0.7%). This is a result of long-term intensity drift which becomes more significant over the longer time periods encountered in the actual leachate analyses. The particular plasma observation region chosen was a compromise among the best positions for the elements listed in Table 3. The plasma observation region was chosen to optimize the signal-tobackground ratio for boron, and this element also exhibits the best precision under compromise conditions for simultaneous multielement analysis. Wavelength drift did not appear to be of major significante, since our spectrometer is thermally regulated (18). One objective of this study was to evaluate the measured overall sample to sample uncertainty versus the uncertainty due to the instrumental measurement alone. One approach would be to obtain sufficient replicate instrumental measurements for each sample to perform an analysis of variante. However, since the individual sample volumes were too smal1 to permit enough repetitive measurements to obtain a good estimate of instrumental error, the instrumental

uncertainties were estimated using the following method. The actual leachate concentrations fa11most often in the concentration range influenced by both noises which are dominant at the detection limit and those due to analyte emission fluctuations. In order to determine the uncertainty expected from the analytical technique alone it was therefore necessary to convolute the two dominant sources of variante. This was done using the following equation: s = .\I(oUpc)’+ (LOD/3)’ P where: standard deviation bg/mL) at concentration “C”, CY= analyte noise factor (high concentration noise), dihition factor (dilution of the leachate P= sample), C= concentration of leachate @g/mL), LOD = limit of detection bg/mL) S=

11

SIMULATED NUCLEAR WASTE MATERIALS

regarding the performance of waste forms using a similar test. At the present time, the relative withinlaboratory precision of 6 to 11% and relative betweenlaboratory precision of 25 to 40% was reported for the MCC round-robin of 25 laboratories (5). The standard deviation for both Ca and Zn in the seven solutions markedly exceed the uncertainty of the instrumental measurement. Zinc was determined by two techniques to assure the accuracy of the concentration measurement. In an effort to identify other sources of error contributing to the test the stability of the leachate was tested. Samples 6 and 16 were split prior to the last step in the MCC-1 protocol. Both samples were acidified in the original leach container. The second half of each sample (6b and 16b) was acidified in an acid-cleaned polyethylene (CPE) vessel. Analysis of the original containers (6 and 16) showed concentrations of 0.0365 f 0.0007 and 0.0310 f 0.0007 g/m2 respectively. Analyses of the splits in the polyethylene (6b and 16b) gave 0.0045 f 0.0007 and 0.0091 f 0.0007. In addition to these experiments, irradiated glass which had been subjected to the MCC-1 procedure showed that 40 f 5% and 73 f 5% of the Zn remained in the leach solution prior to acidification, with the remainder adsorbed on the container walls (17). Both of these experiments point out the instability of Zn in the unacidified leach solution. Continued adsorption and instability is not a problem for the leachate after it is acidified. This was demonstrated using tracers from irradiated glass; the Zn remained in solution for at least three months (19). However, during leaching the concentration of Zn in solution must affect the total quantity of Zn liberated from the glass.

which combines (quadratically) high and low concentration noise contributions (17). These values could then be compared to the variante of the reference leachate sample to determine if a statistically significant differente was observed. The accuracy of the leachate analyses was monitored by the use of standard additions, by the analysis of NBS-SRM 1643a (trace elements in water), and by the analysis of MCC simulated leachates SL-1 and SL-2. Table 5 presents a comparison of results. In genera1 the results agreed reasonably well, with a few differences which are unexplained at this time. To evaluate overall test precision, for a given element, the standard deviation of the determinations in seven replicate samples is listed in Table 6. This standard deviation is compared to the instrumental measurement standard deviation as calculated using Eq. (1) (also listed in Table 6) for each element. The values themselves are in grams of the element in solution per geometrie square meter of sample area. For the elements Ba, Cs, Mo, Si, and Sr, the instrumental uncertainty and the precision of the seven replicates are essentially identical. For these elements the test behaved reproducibly within the ability to measure the concentration of the elements in the leachate. This means that errors due to the variability of the test procedure or inhomogeneity of the base glass are too smal1 to be measured using the analytical technique employed in this study. These data were unexpected in light of the MCC-1 round-robin study of 25 laboratories (5). These results are important to the nuclear waste community where precision within and between laboratories is essential to the decisions being made now and in the future

TABLE 5 Comparison of Analytical Results

SRM 1643a Element

This Work

B Ba Ca Ce CS Mo Na Nd Si Sr Ti Zn (DCP) Zn (FAAS)

<0.06 0.046 <0.2 0.091 9.9 <0.04 <0.06 0.240 <0.05 0.072 0.075

‘Reference-private communication-D. . . . Leach Test Method. b Information only value.

SL-1

Certified

SL-2

This Work

PNLa

This Work

PNL

52.0 0.005 0.027 2.1 5.32 1.91 79.6 1.96 19.0 4.97 <0.05 <0.02 0.019

47.1 0.02 2.01 4.46 1.92 79.5 1.60 18.9 5.13 -

26.2 0.003 0.009 4.0 4.87 1.93 38.2 2.74 <0.06 2.48 < 0.05 <0.02 <0.009

23.5 0.01 4.16 4.6 1.92 38.3 2.43 0.02 2.55 _ -

0.046 0.095 (9)b 0.239 0.072 0.072 M. Strachan,

Battelle (PNL) and PNL-4249,

Summary

Report

1.35 1.37 1.35 1.40 1.39 1.40 1.34 1.37

1 6 11 16 21 26 31 x

SD

0.02a 0.02 0.02 0.02 0.02 0.02 0.02 0.03b

B

0.0077 0.0077 0.0076 0.0078 0.0082 0.0076 0.0074 0.0077

X

SD 0.0004 0.0004 0.0004 O.CKlO4 0.8004 0.04 O.OtXkI 0.0003

Ba

0.135 0.135 0.132 0.137 0.143 0.137 0.128 0.135

X

Ca

0.002 0.002 0.002 0.902 0.003 0.002 0.002 0.005

SD 0.421 0.433 0.423 0.428 0.435 0.433 0.445 0.431

X

Cs

0.007 0.007 0.007 0.007 0.007 0.007 0.007 0.008

SD 0.527 0.531 0.518 0.533 0.529 0.508 0.516 0.523

X

Mo

0.017 0.017 0.017 0.017 0.017 0.017 0.017 0.009

SD 4.42 4.32 4.40 4.36 4.32 4.52 4.80 4.45

X

Na

0.12 0.12 0.12 0.12 0.12 0.12 0.13 0.17

SD 5.42 5.47 5.31 5.62 5.49 5.53 5.38 5.46

X

Si

0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10

SD 0.0308 0.02% 0.0298 0.0315 0.0323 0.0317 0.0315 0.0310

X

Sr

SD 0.0009 0.0009 0.0009 0.0009 0.0009 O.ooo9 0.0099 0.0010

aX is the concentration in g/mz, SD is the average standard deviation of the instrumental measurements calculated from Eq. (1). bSample standard deviation of the seven samples.

X

Sample No.

TABLE 6

Components Leached from the Base Glas in g/rnl

0.0807 0.0007 0.0007 O.CGO7 0.9007 0.0045

0.0286 0.0212 0.0192 0.0294 0.0245

SD 0.0240

X

Zn

N

13

SlMULATED NUCLEAR WASTE MATERIALS

Al1 of the other elements measured in samples 6, 6b, 16, and 16b were insignificantly different from the other five replicates analyzed. It is significant that the irreproducibility of Zn is due to a particular test condition which is not a specifically controlled parameter and that the other elements do not behave similarly under these same test conditions. Thus, no special improvement can be offered in the system for Zn and none is needed for the other elements. For these elements, the test procedure is as precise as the ability to perform compositional analysis. The standard deviation is only a few percent of the mean for eight of the nine elements measured: B-1.8, Ba-3.9, Ca-3.4, Cs-1.9, Mo-1.7, Na-3.8, Si-1.8, Sr-3.2, Zn-18, for the homogeneous base glass tested. These results wil1 be compared with the overall conditions of the other glass compositions tested. While the other 12 pairs of glasses behaved in a similarly precise manner to the base glass, the information gained from their varied composition wil1 be the subject of a separate discussion (20). They do, however, address the question of precision. During the statistical analysis of the leachate concentrations an empirical equation was used to fit the leach data (20). These concentration data were fit to a function with independent variables including glass composition and weight loss of the samples. All of the data was subjected to statistical evaluation using “OMNITAB” (21,22), an extensive computerized group of statistical programs. One of the tests is for conformity to normality and another is to determine relative standard deviation of all 31 samples which was based on 18 degrees of freedom (7 replicates and 24 pairs) in this system. Table 7 compares the instrumental uncertainty for the base glass, the uncertainty in the seven individual measurements, and the residual standard deviation for the fit of al1 31 samples. It would be expected that the residual standard deviation should be larger than these observed for the seven replicates, since its magnitude is directly proportional to the leachate concentration. Several of the 12 pairs of glasses were much less durable and gave larger elemental leachate concentrations. However, an examination of the standard deviation for al1 samples shows that the uncertainty for Na and B is actually identical to that of the theoretical instrumental measurements for the base glass. Thus, seven of the nine elements tested demonstrate that the leach test for homogeneous samples does not add significant error compared to that of the instrumental analysis. The limiting factor in the precision for these elements (excluding Zn) is the instrumental determination of the leachate concentration and not the material tested or the variability of these leach test conditions. These results demonstrate that this set of closely controlled parameters is capable of reproducible leach data in a test similar to the MCC-1 static leach test procedure.

TABLE 7 Standard Deviation of the Leachate Measurement in g/m’

Element

Instrumental Uncertainty for the Base Glass

Standard Deviation for the 7 Base Glasses

Standard Deviation for al1 31 Samples (df = 18)

B Ba Ca Cs Mo Na Si Sr Zn

0.02 0.0004 0.002 0.007 0.017 0.12 0.10 0.0009 0.0007

0.03 0.0003 0.005 0.008 0.009 0.17 0.10 0.0010 0.0045

0.02 0.0003 0.009 0.009 0.011 0.12 0.13 0.0014 0.0047

It is impossible to evaluate the significante of the contribution of a single parameter to the variability of these data. Further study is required to alter individual procedures to determine their actual affect on the overall reproducibility and bias in the test. In addition to the comparative relative waste form performances, data is being sought to attempt to isolate and understand the mechanisms controlling the leaching of these complex materials. The comparison of subtle mechanism parameters usually results in minor shifts in leachate concentrations. The ability to obtain measurements of higher precision is becoming more important in these investigations. One of the objectives of the study was the evaluation of the performance of potential homogeneous simulated nuclear waste glass materials. The use of a reference material would provide a means by which data from various laboratories could be evaluated in a condition dependent test such as a leach test where relative performance is a large part of the criteria testing of waste forms. The homogeneity of a potential reference material is an important parameter. Using the residual standard deviation for the whole experiment (df = 18) the percent of the base glass surface area was calculated which, when substituted by composition number 6 (a glass 10% different in composition from the base glass, Table 2), would result in a detectable leachate concentration change (at the 2s level). The results, given in Table 8, display several important trends. First, the ability to determine the element is important; the greater uncertainty for Zn and Ca permit large differences in glass composition which cannot be distinguishable based on leachate analyses. The ability to distinguish diferences in the leach solution also depends on the extent of the concentration difference of the two leach solutions resulting from the different glasses; in general, the greater the differente the more easily they are distinguished. However, the main point is that a substitution of only a few percent

14

H. M. KINGSTON, D. J. CRONIN AND M. S. EPSTEIN TABLE 8 Influence of Glass Homogeneity on the Test

Element

Mean Leachate Conc. for Base Glass in g/m’

Mean Leachate Conc. for Comp. 6 in g/m2

Residual SD (df = 18) in g/m’

Percent of Comp. tos be Significant (2s)

B Ba Ca Cs Mo Na Si Sr Zn

1.37 0.0077 0.138 0.431 0.523 4.45 5.46 0.0310 0.027

6.16 0.0020 0.046 1.17 1.71 18.6 9.08 0.0076 0.041

0.02 0.0003 0.009 0.009 0.011 0.12 0.13 0.0014 0.0047

20 1.9 1.5 2.3 5 7.7 78

7

aThe percent of the base glass surface area that when substituted by composition number 6 (10% different) would be found to give a detectible different concentration in the leachate.

of the glass surface area wil1 result in an observable differente in leachate concentration. While composition bands or stria are analytically indistinguishable when bulk analyses of the material are used, a dramatic change in leachate concentration can result from relatively smal1 composition variability on the surface of the glass. Inhomogeneous bands and stria in the glass could cause dramatically different leachate concentrations depending on the plane of the surface and the ratio of exposure of various compositions. This source of error has been eliminated in the glasses tested in this study. The future reference material must possess this same high degree’ of homogeneity so that surface composition variability wil1 not lead to high uncertainty for the test. The computer fitted the data and determined the number of factors that were significant in changing the chemical durability of the glasses. The magnitude of the effect of these factors was also obtained. The maximum number of significant factors possible was 13. The ability to determine the relevante and significance of leach factors is affected by the ability to obtain repeatable results with a known uncertainty. A comparison was made between these factors at the present uncertainty level, and what these factors would be if the uncertainty in the leach data were twice the leve1 of this study. Table 9 compares the differente in the number of factors and demonstrates that much of the information would be lost if the uncertainty in the data were increased. This is significant especially when the data are to be used in mechanistic evaluation or modeling when understanding the subtleties of the reactions is the object of the test. Even a moderate increase in these uncertainties would reduce the information obtained from the test significantly. Thus, the uncertainty of the test and the information gained are inversely related. A point can be reached where very little information is obtained from a leach test due to the uncertainty of the results.

CONCLUSIONS

This set of experiments has shown that with closely defined parameters and a high degree of control, precise results are possible in leaching tests of this type. The reproducibility of these leach results are comparable to the uncertainty in the analytical concentration analysis technique. While more work is needed, these results verify that the leach test can be made to perform in a highly reproducible manner by careful adherente to appropriately specified control of key parameters. Al1 of the imprecision contributed by the test parameters and the glass samples combined was found to be insignificant compared to the analytical technique used for leachate analysis under these conditions. An investigation of the defined controlled parameters should be initiated to understand each of their exact relationships to the test. It is possible that many of these controlled conditions could be relaxed to a degree in the interest of simplifying the test without significantly increasing the imprecision. The glasses used in this test have proven to be of sufficient homogeneity to allow precise, reproducible results. It is critical that any material used as a reference in the leach test give reproducible results TABLE 9 The Number of Significant Factors-Fit to an Empiricai Equation at the Current Leve1 of Measurement Uncertainty and if That Uncertainty Were Twice as Great (17)

Element B Ba Ca Cs Mo Na Si Sr Zn

At the Current Leve1 of Uncertainty 12 13 7 10 13 9 9 12 3

If the Uncertainty Were Twice as Much 9 7 4 10 9 8 5 9

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