CA95O1022

AECL-10708 COG-92-243

A! ATOMIC ENERGY S O T ' ENERGIE ATOMIQUE OF CANADA LIMITED / ^ * * DU CANADA LIMITÉE

AN EXAMINATION OF THE ANALYSIS OF RADIOSTRONTIUMS IN BIOASSAY APPLICATIONS

EXAMEN DES MÉTHODES D'ANALYSE DES RADIO-STRONTIUMS DANS LES APPLICATIONS DE BIODOSAGE

S.H. LINAUSKAS and J.W. LEON

Chalk River Laboratories Laboratoires de Chalk River

Chalk River, Ontario KOJ 1J0

May 1993 mai

2 7 A 0 t

AECL Research

AN EXAMINATION OF THE ANALYSIS OF RADIOSTRONTIUMS

IN BIOASSAY APPLICATIONS

by

S.H. Linauskas and J.V. Leon

Health Physics Branch Chalk River Laboratories

Chalk River, Ontario KOJ 1JO

1993 May

AECL-10708 COG-92-243

BACL Recherche

EXAMEN DES MÉTHODES D'ANALYSE DES RADIO-STRONTIUMS

DANS LES APPLICATIONS DE BIODOSAGE

par

S.H. Linauskas et J.V. Leon

RÉSUMÉ

Les radio-strontiums figurent parmi les radionucléides les plus importantes sur le plan radiologique dans le domaine des réacteurs nucléaires en raison de leur rendement de fission relativement élevé, de leur longue période, de leur volatilité et de leur mobilité dans les lieux de travail, et de leur longue durée de rétention dans les tissus tels que les os. Les programmes de biodosage efficaces comprennent des processus analytiques qui tiennent compte de la surveillance prospective nécessaire, assurée par les mesures de dépistage, airisi que la surveillance a posteriori nécessaire à la suite d'une incorporation. La chromatographie faisant appel à des éthers cou­ronnes, ainsi que l'utilisation des techniques de spectrométrie à l'aide de compteurs à scintillateur liquide avancés ou de semiconducteurs à barrière de surface, semblent présenter de grands avantages pour les programmes de biodosage.

Service de Radioprotection Laboratoires de Chalk River

Chalk River (Ontario) KOJ 1J0 1993 mai

AECL-10708 C0G-92-243

AECL Research

AN EXAMINATION OF THE ANALYSIS OF RADIOSTRONTIUHS

IN BIOASSAY APPLICATIONS

by

S.H. Linauskas and J.W. Leon

Abstract

Radiostrontiums are among the most radiologically significant radionuclides in the nuclear-reactor environment, due to their relatively high fission yield, long physical half-life, volatility and mobility in the workplace, and long retention times in tissues such as bone. Effective bioassay programs include analytical processes that consider prospective monitoring requirements provided by screening measurements, as well as the retrospective monitoring requirements following an intake. Chromatography using crown ethers, as well as the use of spectrometry techniques with advanced liquid-scintillation counters or solid-state surface-barrier detectors, appear to have significant benefits for Sr bioassay programs.

Health Physics Branch Chalk River Laboratories

Chalk River, Ontario KOJ 1J0

1993 May

AECL-10708 COG-92-243

TABLE OF CONTENTS

Page

1. INTRODUCTION 1

2. LITERATURE SEARCH RESULTS 1

3. DISCUSSION 3

4. CONCLUSION 5

5. BIBLIOGRAPHY 6

LIST OF TABLES

Table 1: Survey by Wilken et al. [1991] on rapid methods for determining 90Sr, 89sr and

90Y 14

Table 2: Recent 90Sr, 89Sr and 90Y analysis procedures and methods not identified by Wilken et al. [1991] 16

LIST OF FIGURES

Figure 1: Urinary excretion-derived investigation levels for 90Sr/90Y, calculated from excretion fractions for the midpoint of the monitoring interval 18

Figure 2: Urinary excretion-derived investigation levels for 89Sr, calculated from excretion fractions for the midpoint of the monitoring interval 19

Figure 3: Analysis scheme for 90Sr/89Sr in a bioassay program 20

1. INTRODUCTION

Radiostrontiums are among the most radiologically significant radionuclides in the nuclear-reactor environment, due to their relatively high fission yield, long physical half-life, volatility and mobility in the workplace, and long retention times in tissues such as bone. The most prevalent isotopes, 89Sr, and 90Sr with its progeny 9 0Y, have little or no gamma emissions accompanying their beta decay. Consequently, direct measurements by whole-body counting are not suitable for monitoring for internal intakes, and urine bioassay and beta counting must be used to quantify intakes. This report reviews the literature on radiostrontium analysis, and examines the procedures for their suitability in bioassay applications. The underlying objective is to improve the monitoring practice in place at Chalk River Laboratories (CRL) and the CANDU Owners Group (COG) utilities.

Improving the sensitivity and accuracy in 89Sr and 90Sr measurements produces a gain in identifying the existence of an intake that is important with the reduction in dose limits proposed by the International Commission on Radiological Protection (ICRP) in publication 60 [1991], and provides more reliable feedback on the performance of radiation-protection programs, particularly when intakes are not expected to occur. More practically, improvements in sensitivity allow for longer sampling intervals in the routine monitoring of workers. Combined with reduced complexity in analysis, tracer radionuclides detected by whole-body counting (for example, 137Cs) do not have to be relied upon in establishing the existence of 90Sr intakes.

Unfortunately, measurement improvements in sensitivity and accuracy are also accompanied by increases in costs and resources. For work environments such as those in nuclear generating stations, having a very low probability of significant intakes, consideration must be given to the benefits obtained in increasing the overhead arising from more complex analytical procedures. This concept is recognized in many internal-dosimetry programs [Boeker et al., 1991], and screening bioassay measurements with less stringent purity requirements are accepted for routine monitoring programs. If the screening assay is easily adapted to the further isolation of interfering radionuclide constituents, once activity is detected, the analytical process for both routine monitoring and special monitoring becomes streamlined, resulting in a more practical means of conducting bioassays.

2. LITERATURE SEARCH RESULTS

Many different radiochemical procedures are used in strontium analysis. A literature search of recent (since 1986) published scientific literature turned up 195 published articles dealing primarily with 89Sr, 90Sr, and 9 0Y measurements. Sixty of the articles concerned actual chemical separations and analysis in a variety of sample matrices, ranging from environmental samples (vegetation, water, and soil) and biological samples (tissues, urine, and milk) to high-level reactor wastes. A complete list of the articles is included in the bibliography.

A review of rapid methods for 89Sr and 90Sr analysis in environmental samples was recently conducted by Wilken et al. [1991]. A summary is given in Table 1, with each of the major analytical steps of pretreatment and preconcentration, chemical isolation, and counting identified. The conclusion of the authors that, among the methods available, no single analytical technique showed significant superiority, was not unexpected. Considering the matrices involved in environmental sampling—soils and bulk quantities of biomass—much of the effort in the analysis must be directed to pretreatment and preconcentration. Furthermore, the counting protocol, and some steps to remove interfering radionuclides, are dictated by the decay characteristics of 90Sr.

Indirect determinations of 90Sr by separating 90Y and beta counting the 9 0Y are common to many of the methods listed in Table 1. This separation may be accomplished by extraction using di-2-ethylhexyl phosphoric acid (HDEHP), by trybutyl phosphate (TBP), or by a ferric-hydroxide co-precipitation. This presents problems when 90Sr is not in secular equilibrium with 9 0Y, or when 89Sr is present. Both situations may occur in samples derived from routine bioassay programs. In the former case, large differences exist in the strontium and yttrium excretion fractions, and equilibrium in a urine sample is unlikely even if the intake material was in equilibrium. With uncertainties in the time of excretion, and delays until analysis begins, two to three weeks of decay would be necessary to ensure 90Sr + 9 0Y equilibrium and a reliable 9 0Y result.

If both fl9Sr and 90Sr are present, HDEHP and TBP extractions must be combined with other steps to isolate and count the strontium immediately. Determining the 9 0Y content after secular equilibrium is reached allows 89Sr and 90Sr to be calculated from the combined measurements.

Wilken et al. [1991], and recent publications since his review (Table 2), discuss separation of strontium using macrocylic crown ethers [Strzelbicki and Bartsch, 1981]. With a high specificity for strontium and decontamination factors typically exceeding 500 for other radionuclides (X37Cs> 60 C O j 9 5 Z r > 9 5 Nb^ th e c r o w n ethers (dicyclohexyl-18-crown-6, DC18C6) appear to be useful in eliminating additional separation procedures for the removal of interfering radionuclides. Originally used in liquid-liquid extractions [Kimura et al., 1979], most of the recent applications employ thin-layer and paper chromatography, or column-extraction chromatography. Some convenience in gross-beta counting using gas-flow proportional counters with the thin-layer and paper chromatography methods is evident, by virtue of the simple geometry and readiness to count. Strontium isolation procedures with the crown ethers are not significantly influenced by Na or Ca cations, often present in the natural urine matrix or pretreated samples through the use of carriers. Other advantages are that only small volumes of eluent are needed; typically, water or very dilute nitric acid and ashing of the initial urine is unnecessary. This greatly minimizes the toxicity and volume of wastes generated.

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One aspect of the radiostrontium analytical process discussed in more recent publications (detailed in Wilken et al.'s [1991] review) is the counting technique followed in the 90Sr analysis. It is likely that the best means of improving the speed, accuracy, and detection limit is beta-particle spectrometry. Most of the traditional counting methods involve gross beta counting with gas-flow proportional counters. These have poor beta energy resolution capabilities. Knowledge of the beta spectrum would be useful where the degree of equilibrium between 90Sr and 9 0Y is not known, or the ratio between 89Sr and 90Sr is uncertain. If the spectrum helped resolve the individual components, it would be possible to reduce the number of analytical steps in 90Sr and 9 0Y separations, or eliminate 9 0Y ingrowth after preliminary isolation of radiostrontiums from 9 0Y.

Spectrum-unfolding techniques in liquid-scintillation (LS) spectrometry are being used increasingly with the technological developments available in modern LS spectrometers. Dietz et al. [1991] have combined the use of LS spectrometry and extraction chromatrography to estimate B9Sr and 90Sr in urine samples, obtaining results in one day. Similar reductions in time by LS spectrometry were achieved by Tait et al. [1989] and Amano and Yanase [1990].

Solid-state beta-particle detectors have the potential to provide similar benefits. Although their energy resolution is poor compared to liquid-scintillation counters (LSCs), they have some advantages, particularly when "near zero" detection limits are needed. Background count rates with silicon surface-barrier detectors are a small fraction (10-2 or 10"3) of those available with even ultra low-level LSCs. The very low backgrounds achieved would allow much smaller sample volumes to be used to meet sensitivity requirements [ICRP 54, 1988]. Small sample volumes, in turn, reduce sample preparation time. This technology is not extensively developed, and is susceptible to energy-resolution problems in assays of large matrices, where sample mass may degrade the beta energy spectrum. If it is necessary to obtain better energy resolution from a sample, the possibility exists to re-process the sample for LSC spectrometry.

3. DISCUSSION

Basic sensitivity requirements for bioassay are outlined in ICRP 54 [1988] for the dose-limit recommendations given in ICRP 26 [1977] and ICRP 30 [1981], These are given as Derived Investigation Levels (DIL) and Derived Recording Levels (DRL) for activity concentrations in urine as a function of monitoring intervals. Figures 1 and 2 give DILs for routine and special monitoring programs for 90Sr and 89Sr based on the dose limits and annual limits on intake given in ICRP 60 [1991] and ICRP 61 [1991]. These were calculated using the excretion fractions computed using Genmod-PC [Johnson and Carver, 1981; Dunford and Johnson, 1987] and the Alkaline Earth Model [Johnson and Myers, 1981]. Class D material was assumed, because the existence of other solubility classes in a reactor environment is unlikely. The most restrictive measurement requirement occurs for 90Sr at a monitoring interval of about 120 days. The values of 4 Bq/L and 1.3 Bq/L for the DIL and DRL cannot be detected by direct beta counting of urine samples, even with recently developed ultra-low background LSCs.

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Similarly, the less restrictive DRLs and DILs, which occur at 10- to 14-day monitoring intervals, cannot be detected. Therefore, urine volumes larger than 5-10 mL--which can be directly beta counted (by LSC)—and concentrations of strontium are needed to meet routine monitoring sensitivity requirements.

The minimum sample volumes needed are about 100-200 mL, depending on the counting system (LSC, gas-flow proportional counter (GPPC)), background count rates, and counting time used in the analysis process. Typically, GFPCs have detection efficiencies for 90Sr and 90Y between 30-40%. With background count rates at about 1 cpm and moderate count times (< 1 hour), measurement sensitivities of 0.1 Bq/sample can be readily achieved. Similar sensitivities are obtained using LSC systems.

The implementation of monitoring programs introduces constraints other than the elementary sensitivity requirements. Samples obtained as follow-up to established intakes usually comprise the full 24-hour output, to be more representative of longer-term metabolic processes. The long-term biokinetic behaviour of the radioactive material establishes the committed effective dose.

With relatively large volumes, sample pretreatment and preconcentration steps (as in the case of environmental sampling) have a significant impact on the processing period and the resources needed for analysis. By emphasizing operational radiation-protection needs (namely, identifying intakes), minimum sample volumes can be used and limitations of screening measurements can be accepted. The latter are characterized by low or variable chemical'recoveries, poor decontamination factors, the potential for interfering radionuclides to contribute to the measurement, and less reliability in radionuclide identification. Apart from the often conservative nature of the screening measurements themselves, 90Sr or 89Sr retention and excretion patterns of typical monitoring intervals (60-120 d) indicate that activity concentrations in resamples obtained within a week [ICRP 54] of an original sample would be almost the same in magnitude. This provides ample opportunity to use a more resource-intensive and accurate 90Sr/89Sr bioassay, required for internal dose assessments.

Figure 3 illustrates an analysis scheme for radiostrontiums, which contains elements to satisfy screening bioassay objectives as well as dosimetry objectives.

Two characteristics dominate:

(1) preconcentration, by precipitating the radiostrontiums and conducting a screening measurement; and

(2) isolating the 90Sr/89Sr from the precipitate and counting by LSC spectrometry.

The literature reviews, as well as experience in this laboratory, indicate good strontium recovery (70-95%) with calcium oxalate co-precipitations [Kramer and Davis, 1981]; and calcium phosphate co-precipitation [Dietz et

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al., 1991], with a minimum of sample pretreatment. The presence of interfering radionuclides in the precipitate matrix, which could exist with mixed fission/activation product intake, is an advantage in the screening measurement.

As a preconcentration step, precipitation methods are relatively fast, and the geometry and mass of the product are easy to count and re-process, if necessary.

The single most important factor, which dominates the bioassay process after the initial screening, is the removal of interfering radionuclides from the screening-sample matrix. Chromatography methods—in particular, those employing the crown ether resins—appear to be most suited for removing, in a few steps, radionuclides that could exist in urine samples of mixed fission/activation product intakes. Liquid-scintillation spectrometry offers the ability to resolve 90Sr and 89Sr, if they co­exist, and provides a means to establish the presence of unexpected interferences from information in the beta spectrum.

The drawback to the process in Figure 3 is the need for several counting systems: a low-background GFPC or surface-barrier detectors and LSC spectrometers. However, the demand on resources, space cost, training, etc., for such systems is low in the long term, compared to processes that do not incorporate an intermediate screening process.

4. CONCLUSION

Effective bioassay programs include analytical processes that consider prospective monitoring requirements provided by screening measurements, as well as the retrospective monitoring requirements following an intake. Chromatography using crown ethers, as well as the use of spectrometry techniques with advanced liquid-scintillation counters, or solid-state surface-barrier detectors, appear to have significant benefits for Sr bioassay programs.

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- 12 -

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- 14 -

Table 1: Survey by Wilken et al. [1991] on rapid methods for determining 90Sr, 89Sr and 90Y.

Reference

Wilken et al. [1987]

Butler [1962]

Baratta et al. [1969]

Borcherding et al. [1986]

Suomela and Vallberg [1989]

Bunzl et al. [1991]

Borus-Boszormenyi et al. [1982] and Kramer et al. [1982]

Mundschenk [1979]

Koprda et al. [1983] and Scasnar [1984]

Kimura et al. [1979]

Mikulaj et al. [1986]

Pretreatment

dry ash

dry ash

dry ash

Na2C03 co-precipitation

Cation exchange concentration

dry ash

dry ash

dry ash

extractions with dicarbo-lide-H+

dry ash

dry ash

Isolation

nitric acid method

extraction with HDEHP

extraction with TBP

extraction with HDEHP

extraction with HDEHP

oxalate co-precipitation

extraction with HDEHP

extraction with HDEHP-soaked filter material

extractions with dicarbolide-H+ and PEG 300

extraction with crown ether chloroform solution

emulsion membrane extraction

Counting

count 90Y by GFPC

count 90Y by GFPC

count 90Y by GFPC

count 90Y by GFPC

LSC spectrometry

count 90Y by GFPC

count 90Y and 90Y+90Sr by GFPC

count 90Y by GFPC

LSC spectrometry

count 90Y and 90Y+90Sr by GFPC

counting method not specified

continued . . .

- 15 -

Table 1 (concluded): Survey by Wilken et al. [1991] on rapid methods for determining 90Sr, a9Sr and 90Y.

Reference

Vaney et al. [1989]

Wai and Du [1990]

Stanley et al. [1956]

Bryant et al. [1959]

So et al. [1983]

Stadlbauer et al. [1988]

Knapstein [1960]

Pretreatment

batch treatment with cation exchange resin

dry ash

load sample on cation exchange resin column

co-precipitation of ammonium salts

dry ash

carbonate co-precipitation

dry ash

Isolation

extraction with crown ether DC18C6 in solution

paper chromato­graphic separation with crown ether impregnated paper

elution of Y with 5% ammonium citrate solution

chromatographic separation of Y with cation exchange resin

chromatographic separation of Y with hydrous manganese dioxide resin

chromatographic separation of Sr with HPIC-CS2 cation exchange resin

chromatographic separation of Sr and Y with Dowex 50X12 cation exchange resin

Counting

count 90Y and 90Y+90Sr by GFPC

count 90Y and 90Y+90Sr by GFPC

count 90Y by GFPC

count 90Y by GFPC

count 90Y by GFPC

count 90Y and 90Y+90Sr by GFPC

count 90Y by GFPC

Table 2: Recent 90Sr, 89Sr and 9 0Y analysis procedures and methods not identified by Wilken et al. [1991]

Reference

Tait et al. [1989]

Bern et al. [1991]

Bjornstad et al. [1992]

Lapid et al. [1984]

Dietz et al. [1991]

Testa et al. [1990]

Separation Technique

Liquid extraction

Liquid extraction

Liquid extraction

Liquid extraction

Extraction chromatography

Extraction chromatography

Description

Chelation of divalent cations. Carbonate coprecipitation. Liquid extractions with crown ethers. Precipitation of SrC03 and dissolve. P LSC-spectrometry of 90Sr, a9Sr and 9 0Y.

Dry ash and dissolve. Liquid extractions with TBP. Decontamination extractions with HN03. 7 coprecipitation with ferric hydroxide. P counting with a proportional counter.

Dry ash and dissolve. Calcium oxalate coprecipitation. Dry ash the precipitate and dissolve. Extract Y with HDEHP-toluene and add scintillator. p count by LSC activity.

Add sample, Tiron, NH4C1 and NH40H to a separatory funnel. Extract with TTA/T0P0 in CH. Wash with pH 10.5 buffer and strip Sr.

Calcium phosphate coprecipitation.

Wet ash precipitate. Dissolve in (Al N0 3) 3. Chromatographic extraction with Sr.Spec crown ether column. p LSC spectrometry of 90Sr, 89Sr and 9 0Y.

Dry ash and dissolve. Chromatographic extraction on HDEHP loaded microthene column. Y elution and oxalate coprecipitation. p counting with a proportional counter.

Table 2 (concluded): Recent 90Sr, 89Sr and 9 0Y analysis procedures and methods not identified by Vilken et al. [1991]

Reference

Kremliakova et

al. [1990]

Horwitz et al. [1991]

Horwitz et al. [1992]

Àmano and Yanase [1990]

Ryabukhin et al. [1991]

Elchuk et al. [1992]

Separation Technique

Extraction chromatography

Extraction chromatography

Extraction chromatography

Cation exchange chromatography

Thin-layer chromatography

Dynamic ion exchange chromatography (HPLC)

Description

Prepare Sr selective extraction resin, using the crown ether di-cyclohexyl-18-crown-6, and loading TVEX.

Load columns. Use a chromatographic method, i.e., Dietz et al. for bioassay

samples with the prepared columns.

This method is the same as Dietz et al. using Sr-Spec for bioassay samples.

For bioassay samples, this paper references Dietz et al., using Sr-Spec.

j Dry ash and dissolve in HN03. Precipitation of Sr(N03)2. Coprecipitation with ferric hydroxide. SrC03 precipitations and dissolving in HCl. Pass through a chromatography column of Amberlite CG-120. SrC03 precipitation and dissolving. p LSC spectrometry of 90Sr, 89Sr and 9 0Y.

Dry ash and dissolve in HN03. Liquid extraction in crown ether and chloroform. "Silufol" thin-layer plate chromatography. /J counting with a proportional counter for 90Sr and 89Sr. Remove Sr from the disk with CHCl3 seven to ten days later. fi counting with a proportional counter.

Heat sample with HN03 and cool to R.T.

Pass through AG 50WX8, H+ cation exchange resin. Wash and elute Y and rare earths with citric acid.

- 18 -

1000

c O

CD o X <D

G0

100

10

V S . B V 19 &

100 200 300

MONITORING INTERVAL (days) -B-ICRP3O-0-ICRP 61

400

Figure 1. Urinary excretion-derived investigation levels for 90Sr/90Y, calculated from excretion fractions for the midpoint of the monitoring interval.

- 19 -

10000

1000 c o <D l _ o X 0)

cr CO LU 100 -

_J Q

100 200 300

MONITORING INTERVAL (days) - B - I C R P 3 0 - ^ - I C R P 6 1

400

Figure 2. Urinary excretion-derived investigation levels for 89Sr, calculated from excretion fractions for the midpoint of the monitoring interval.

- 20 -

Prospective

Monitoring

Strontium Isolation Igroup isuiauou oy co-precipitation)

i r

Count Product (gas flow proportional counter/ surface-barrier

detector)

Intake Identified

Remove Interfering Nuclides solvent extraction/crown ether

chromatography

Intake Identified

Record Result

A \.

Retrospective

Monitoring

Figure 3. Analysis scheme for 90Sr/89Sr in a bioassay program.

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