Rapid determination of 90Sr/90Y in water samples by liquidscintillation and Cherenkov counting

Jennifer M. Olfert • Xiongxin Dai •

Sheila Kramer-Tremblay

Received: 30 August 2013

� Atomic Energy of Canada Limited 2014

Abstract Strontium-90 (90Sr) is a ubiquitous contaminant

at nuclear facilities, found at high concentrations in spent

nuclear fuel and radioactive waste. Due to its long half-life

and ability to be transported in groundwater, an accurate

method for measuring 90Sr in water samples is critical to

the monitoring program of any nuclear facility. To address

this need, a rapid procedure for sequential separation of Sr/

Y was developed and tested in groundwater samples col-

lected from an area of riverbed affected by a 90Sr

groundwater plume. Sixteen samples, plus spike and water

blanks, were analyzed. Five different measurements were

performed to determine the 90Sr and yttrium-90 (90Y)

activities in the samples: direct triple-to-double-coin-

cidence ratio (TDCR) Cherenkov counting of 90Y, liquid

scintillation (LS) counting for 90Sr following radiochemi-

cal separation, LS counting for 90Y following radiochem-

ical separation, Cherenkov counting for 90Y following

radiochemical separation and LS counting of the Sr sam-

ples for 90Y in-growth. The counting was done using a low-

level Hidex 300SL TDCR counter. Each measurement

method was compared for accuracy, sensitivity and effi-

ciency. The results following Cherenkov counting and

radiochemical separation were in very good agreement

with one another.

Keywords Radiostrontium � Liquid scintillation

counting � Cherenkov counting � Water � Monitoring

Introduction

Strontium-90 (90Sr) is a ubiquitous contaminant at nuclear

sites, resulting from nuclear fission of plutonium and ura-

nium. As a result, it is found in high concentrations in spent

nuclear fuel and radioactive waste. World-wide nuclear

weapons testing in the 1950s and 1960s also resulted in

widespread distribution of 90Sr [1]. With its long half-life

of 28.79 years, 90Sr persists in the environment and if

released into soil, can form subsurface groundwater plumes

that could eventually discharge to surface waters [2, 3].90Sr is mobile in groundwater, with studies at Atomic

Energy of Canada Limited (AECL)’s Chalk River Labo-

ratories (CRL) showing that it moves at about 10 % of the

groundwater speed [4]. It is biochemically similar to cal-

cium and can therefore be accumulated in the bones if

taken up, potentially leading to cancers of the bone marrow

and the soft tissues surrounding the bone [5]. Thus, it is

important that effective monitoring techniques exist for90Sr in water samples, both as part of the regular moni-

toring program at nuclear sites and in the case of a radio-

logical emergency.

Although numerous radioanalytical procedures for 90Sr

in environmental water samples have been reported [6–11],

many of them require a large sample volume and are time-

consuming. Therefore, a more rapid method for measuring90Sr in environmental water samples would be beneficial.

To address this, rapid procedures for radiochemical sepa-

ration of 90Sr/90Y were developed and tested in ground-

water samples collected from an area of riverbed at the

CRL site affected by a 90Sr groundwater plume. The

measurements obtained from these procedures were also

compared to the measurements obtained from direct triple-

to-double-coincidence ratio (TDCR) Cherenkov counting.

The use of direct Cherenkov screening has increased [12];

J. M. Olfert (&) � X. Dai � S. Kramer-Tremblay

Chalk River Laboratories, Atomic Energy of Canada Limited,

Chalk River, ON K0J 1J0, Canada

e-mail: olfertjm@aecl.ca

123

J Radioanal Nucl Chem

DOI 10.1007/s10967-013-2913-0

however, questions remain about how potential interfer-

ences (e.g., 40K, 210Pb/210Bi and other high energy beta-

emitting radionuclides) may affect the accuracy of its

measurements. Details of all the methods tested are

described and compared for accuracy, efficiency and

sensitivity.

Experimental

Reagents and standards

All of the reagents used in the procedures were analytical

grade or better. Hydrochloric and nitric acids were pur-

chased from Fisher Scientific Canada (Ottawa, ON, Can-

ada). Deionized water was obtained from a Millipore

Direct-Q5 Ultrapure water system.

Sr and DGA-N resins (50–100 lm) in 2 ml pre-packed

cartridges were obtained from Eichrom Technologies Inc.

(Lisle, IL, USA). The TraceCERT� stable Sr standard

(1,000 mg/l) was obtained from Sigma-Aldrich Canada

(Oakville, ON, Canada). The stable Y standard (2,000 mg/l

in 1 M HCl) was made using yttrium(III) nitrate tetrahydrate

obtained from Sigma-Aldrich Canada. The stable standards

were used to monitor chemical recovery. The radioisotope

standard for 90Sr was obtained from the National Institute of

Standards and Technology (Gaithersburg, MD, USA).

Ultima Gold AB liquid scintillation (LS) cocktail is avail-

able from PerkinElmer Canada (Woodbridge, ON, Canada).

Sample collection

Sixteen groundwater seep samples were collected from the

discharge zone of a groundwater plume containing 90Sr.

The samples were filtered through a 0.45 lm filter and

acidified with concentrated nitric acid to a final concen-

tration of 1 %.

TDCR Cherenkov counting

Three spike samples were prepared by adding known

amounts of the 90Sr standard to 20 ml of 1 % nitric acid.

The sixteen water samples, three spike samples and a

deionized water blank were counted on a low-level Hidex

300SL liquid scintillation (LS) counter (Hidex Oy, Fin-

land) for 30 min per sample. The TDCR ratio was used for

the correction of the counting efficiency by the following

formula:

Corrected counting efficiency ¼ 0:9� TDCR0:75

Details of the TDCR Cherenkov method have been

described in [13, 14].

Gamma analysis

Gamma spectrometry analysis was performed on five

samples representing the concentration ranges observed in

TDCR Cherenkov counting (SS1, SS7, NS2, NS8, NS9) to

determine if there were any significant contributions from

high energy beta-emitters that might interfere with the

direct TDCR Cherenkov results. A 500-ml polypropylene

bottle containing approximately 125 ml of sample was

placed directly on the detector face of a high purity ger-

manium detector (Ortec, USA) for up to 4 h.

Radiochemical separation

After the TDCR Cherenkov counting, all the groundwater

seep samples, spike samples and a water blank sample were

processed for simultaneous separation of 90Sr and 90Y,

followed by Cherenkov and LS counting. A 20 ml aliquot

of the original sample was first acidified by adding 20 ml

of concentrated nitric acid for a final concentration of

approximately 8 M. To measure the chemical recovery,

1 mg of stable strontium and 1 mg of stable yttrium were

added to each sample.

In order to sequentially separate the Sr/Y from the water

and any interfering substances, the Sr resin and DGA-N

resin cartridges were stacked with the DGA-N cartridge on

top. A 12-hole vacuum box (available from Eichrom

Technologies Inc.) system was used to help draw the

samples through the cartridges. A 60-ml plastic syringe

was attached to the top of the Sr resin cartridge with a two-

way luer lock valve (available from Cole-Parmer) to use as

a sample reservoir. Prior to separation, the columns were

pre-conditioned with 10 ml of ultrapure water followed by

10 ml of 8 M HNO3, both at a flow rate of 3.0 ml/min. The

samples were then passed through the cartridges at a flow

rate of 1.0 ml/min to extract Y onto the DGA-N resin and

Sr onto the Sr resin. After rinsing with 10 ml of 8 M

HNO3, the two resin cartridges were split for elution. The

Y was eluted off the DGA-N cartridge with 6 ml of 0.05 M

HCl into a pre-weighed 20 ml plastic scintillation vial. The

eluate was immediately counted on the low-level Hidex

LSC using the TDCR Cherenkov counting protocol for

direct determination of 90Y. After the Cherenkov counting,

0.2 g of the Y eluate was transferred to a 50-ml pre-

weighed centrifuge tube and diluted with 50 ml of 0.1 M

HNO3 for analysis of stable Y by ICP-MS analysis to

determine the Y chemical recovery. The remainder of

eluate was then mixed with 14 ml of Ultima Gold AB LS

cocktail for LS counting.

The Sr extracted was eluted off the Sr resin cartridge

with 6 ml of ultrapure water into a pre-weighed 20 ml

plastic scintillation vial. About 0.2 g of the Sr eluate was

transferred to a 50-ml pre-weighed centrifuge tube and

J Radioanal Nucl Chem

123

diluted with 50 ml of 0.1 M HNO3 for analysis of stable Sr

by ICP-MS to determine the Sr chemical recovery. The rest

of the eluate was mixed with 14 ml of Ultima Gold AB LS

cocktail for LS counting.

LS counting

The samples were counted for 90Sr or 90Y on the Hidex

300SL TDCR LS counter. For both LS and Cherenkov

counting, samples were counted for 30 min each. To

measure 90Y in-growth, the samples containing the 90Sr

fraction were recounted 8 days later.

Results and discussion

Minimum detectable activity and concentration

The minimum detectable activity (MDA) for each sample,

spike and blank was calculated using the Currie equation

[15] as described by Dai et al. [16]. The average MDAs of

five blank samples were 0.12 ± 0.02 Bq for 90Sr and

0.18 ± 0.06 Bq for 90Y. Considering the sample volume of

20 ml, the minimum detectable concentrations (MDC)

were calculated to be 5.7 ± 0.9 Bq/l for 90Sr and

9.1 ± 2.8 Bq/l for 90Y.

Based on the ICP-MS analysis, the average chemical

recovery for 90Sr was found to be consistently high at

92 ± 5 %, while that of 90Y was lower at 52 ± 15 %. The

low chemical recovery for 90Y is due to eluting the column

with an insufficient volume of HCl. The procedure has

been modified so that elution would be done with 9 ml of

0.05 M HCl instead of only 6 ml, with an increased

recovery of [90 %.

Performance evaluation of spiked water samples

Three water samples were spiked with known amounts of90Sr (1, 3.5 and 10 Bq), resulting in concentrations of 51.7,

175 and 546 Bq/l. The results for the spiked water samples

are shown in Fig. 1 and Table 1. The measurements in

each of the methods tested agreed well with the expected

Fig. 1 A comparison of the expected concentration of the spike

samples versus the five measured concentrations. Error bars represent

combined errors. The solid line is the 1:1 reference line

Table 1 Results of the expected concentration of the spike samples compared to the actual concentration

Low spike

51.7 Bq/l

Medium spike

175 Bq/l

High spike

546 Bq/l

Measured (Bq/l) Variation (%) Measured (Bq/l) Variation (%) Measured (Bq/l) Variation (%)

90Y:TDCR Cherenkov 55 5.9 182 4.0 577 5.590Sr:LSC 53 2.5 205 16.8 550 0.690Sr:Y in-growth 61 18 160 -8.9 614 12.490Y:LSC 44 -15.9 198 12.8 556 1.690Y:Cherenkov 48 -7.2 192 9.4 600 9.8

Fig. 2 Diagram showing an overview of the methodology used to

determine 90Sr and 90Y in groundwater samples

J Radioanal Nucl Chem

123

value. The chemical recovery for 90Sr was very good,

ranging from 87 to 90 % and averaging 88.7 ± 1.5 %.

There was much more variability in the chemical recovery

of 90Y, which ranged from 36 to 71 % and averaged

47.7 ± 7.2 %. Again, this is a result of insufficient elution

volume.

Gamma spectrometry analysis

The primary high energy beta-emitters that would be

expected to interfere with Cherenkov counting are 40K,137Cs, 60Co and 210Bi. Gamma analysis of selected samples

revealed no appreciable contribution from 40K, 137Cs or60Co. Determining the concentration of 210Bi requires more

extensive analysis and has not been completed at this time.

However, as the daughter product of naturally-occurring

210Pb, 210Bi is not expected to be present in these water

samples at a level of [5 Bq/l and thus would not interfere

with Cherenkov counting of 90Y.

Comparison of five measurements

The complete methodology is shown in Fig. 2.

In total, five different measurements were conducted:

direct TDCR Cherenkov counting of 90Y, LS counting for90Sr following radiochemical separation, LS counting for90Y following radiochemical separation, Cherenkov

counting for 90Y following radiochemical separation and

LS counting for 90Y in-growth of the Sr samples. Each

measurement has its own advantages and drawbacks.

Direct TDCR Cherenkov counting is very rapid, as there is

no sample pre-treatment or radiochemical separation

a

b

Fig. 3 The concentration of90Sr as determined by the five

different measurements is

shown in a. Note that the scale

is logarithmic. The

concentration of 90Sr

determined by direct Cherenkov

counting compared to the

averaged concentration of the

other four measurements is

shown in b

J Radioanal Nucl Chem

123

required, and scintillation cocktail does not need to be

added. Another advantage is that it is independent of

chemical quenching. It provides a good screening method

to quickly determine if there are any high-energy beta-

emitters (including 89Sr) in the samples. However, Cher-

enkov counting is subject to interference from other high-

energy beta/gamma-emitters (i.e. 40K, 210Pb/210Bi, 60Co

and 137Cs). Although radiochemical separation requires

sample pre-treatment and overall takes longer than direct

Cherenkov counting, results are still achievable in 1 day.

Doing a radiochemical separation removes impurities or

interferences from the sample, thus giving results specific

to Sr or Y without the concern of interferences. The DGA

and Sr resin columns can be stacked during the separation,

allowing Sr and Y radionuclides in a sample to be simul-

taneously separated.

Figure 3a shows the 90Sr concentration determined by

each measurement on a logarithmic scale. The results from

each method agreed very well with one another above the

MDC. The concentration of 90Sr determined by direct

TDCR Cereknov counting was also plotted against the

average 90Sr concentration of the other four measurements

following radiochemical separation (Fig. 3b). As shown in

Figs. 3a and 3b, all five measurements agreed very well

with one another, demonstrating that results obtained by

TDCR Cherenkov counting are just as reliable as those

obtained following radiochemical separation.

Conclusions

A rapid procedure for sequential separation of Sr/Y was

developed and tested. The method involved sequential

separation of 90Sr and 90Y, followed by Cherenkov and LS

counting. Following radiochemical separation, the activity

of the 90Sr fraction was counted immediately on a Hidex

300SL TDCR counter. The samples were recounted 8 days

later to measure 90Y in-growth. The activity of the 90Y

fraction was measured immediately after radiochemical

separation by Cherenkov counting and then by LS count-

ing. The results of the four measurements following

radiochemical separation were compared to those obtained

from direct TDCR Cherenkov counting. A set of water

samples collected from the discharge zone of a ground-

water plume containing 90Sr were analyzed. The mea-

surements obtained from counts following TDCR

Cherenkov counting and radiochemical separation agreed

very well. This confirmed that direct TDCR Cherenkov

counting can serve as a rapid screening method with reli-

able results. However, the radiochemical separation

method may yield more accurate results in cases where

other interfering radionuclides are present in the samples.

Acknowledgments The authors acknowledge the financial support

from Atomic Energy of Canada Limited (AECL).

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