Determination of carbon-14 levels in heavy water and groundwaters

Abstract

The radionuclide carbon-14 is a by-product of nuclear reactors, produced predominantly by the neutron activation of ¹⁷O of the water molecule. The chemical form of carbon-14 in heavy water is bicarbonate (D¹⁴CO₃⁻), and the total inorganic carbon levels (TIC) are expected to be 20–30 μg L⁻¹. A technique was developed primarily for ¹⁴C determination in reactor heavy water (ultra-pure water containing up to ≡10¹² Bq L⁻¹ of tritiated water), and is also applicable to analysis of contaminated groundwater. The method consists in acidifying the sample to convert the bicarbonate into CO₂, which is swept by a stream of nitrogen to an absorbent NaOH solution. An aliquot of this NaOH solution is measured by liquid scintillation for counting ¹⁴C. Isolation of the ¹⁴C from the reactor water matrix was excellent, as tritium removal factors of up to 10⁹ were obtained. The recoveries of simulated ¹⁴C solutions were 94.2 ± 6% (n = 61) for five analysts of various levels of experience. The variations in the recoveries followed a non-systematic random pattern, and were independent of the analyst. The ¹⁴C concentrations in reactor water sampled during operation were in the range 0.1–2 MBq L⁻¹ with a typical variability of ±5% or better among replicates, while the reactor water samples obtained during shutdown and samples of groundwater near a waste management area had values ranging from ≡100 to 1800 Bq L⁻¹, but with a higher variability. The detection and determination limits were 47 and 342 Bq L⁻¹, respectively.

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1 Introduction and background

The radionuclide carbon-14 is a by-product of all nuclear reactors, and is produced predominantly by the neutron activation of ¹⁷O of water molecules.^1,2 In heavy water reactors such as CANDU^® reactors (CANDU^® is a registered trademark of Atomic Energy of Canada Limited), the major portion of the ¹⁴C produced originates from the moderator. The formation of ¹⁴CO₂ is favored, hence the predominant chemical form of ¹⁴C is as bicarbonate (D¹⁴CO₃⁻) at neutral moderator water pH (or pD in heavy water). The levels of D¹⁴CO₃⁻ present in the moderator depend upon the production term, i.e., reactor power output, and the removal term, i.e., the purification rate, which is controlled by ion-exchange columns used for general purification. A potential diagnostic of the performance of the purification system is the ¹⁴C level in the moderator.

Conceptually, measurement of ¹⁴C of moderator water is straightforward. The moderator consists of ultra-pure water, depleted of ions except for those added voluntarily for reactivity control or shutdown. A major complication arises from the presence of tritiated heavy water (predominantly as ²H³HO or DTO) which is present at high levels [≡0.4 × 10⁶–1.5 × 10⁶ MBq L⁻¹ or 10–40 Ci L⁻¹], and originates from the neutron activation of the deuterium atom of the heavy water. The levels of ¹⁴C expected in the moderator of CANDU^® reactors are 0.05–0.1 MBq L⁻¹ or ≡5–6 orders of magnitude lower than DTO. Both tritium and ¹⁴C are β-radiation emitters and direct counting of moderator water for ¹⁴C by liquid scintillation counting (LSC) is nearly impossible, as the signal due to tritium dominates over that of other β-emitters, even if the energies of the other β-emitters do not overlap with tritium. To overcome this problem, a procedure was developed, based on acidifying the sample to release the dissolved ¹⁴CO₂ as gas, stripping the CO₂ from the tritiated water matrix and collecting the CO₂ in a base [NaOH or Carbosorb™ (a product from Packard Instruments, Meriden, CT, USA, consisting of an organic amine; the nominal CO₂ absorption capacity is 4.8 mmol mL⁻¹)].³ An aliquot of the base or the whole absorbent is mixed with a liquid scintillation cocktail and analyzed using a liquid scintillation counter. In the past, various analysts have used different conditions (volumes of solution, absorbents, acids, hardware designs and different protocols on LSC instruments), and the results have varied between analysts. The variations were potentially caused by system leaks, incomplete or inconsistent stripping of the ¹⁴CO₂ and/or similar aspects related to differences in the acidification/ absorption protocols. The results presented in this paper from several analysts show the consistency of the method, the removal of interferences from the matrix and that it can be used successfully by analysts of various degrees of experience.

2 Warning and safety precautions

This procedure is for the determination of ¹⁴C (present as bicarbonate or carbonate) in small volumes (<100 mL) of tritiated water. The potential hazards are ¹⁴C and ³H (tritium or DTO), strong acids and bases used in titrations and magnesium perchlorate, used as a drying agent. The analysts must have proper training for handling of radioactivity and the laboratory must be approved for handling the expected levels of radioactivity. Radioactive wastes must be disposed of according to approved procedures.

3 Methodology

3.1 System parameters and description

A drawing showing the extraction line is shown in Fig. 1. The main parts of the system consist of a supply of inert gas (nitrogen), a reaction flask, a drying trap and an alkaline trap (bubbler).

Fig. 1 Description of the acid stripping line.

The sweep gas used was nitrogen (zero grade), passed through a soda-lime trap to remove CO₂ impurities. An electronic flow meter (Omega Engineering, Stamford, CT, USA) with a separate needle valve allowed precise fine tuning of the gas flow rate to the reaction vessel. The reaction vessel consisted of a 125 mL Erlenmeyer flask, modified with six penetrations. The discharge valve was a standard Pyrex brand, straight bore, PTFE plug stopcock. The carrier gas inlet was made out of an exact, 0.25 in (≡6 mm) glass tube to fit a Swagelok™ PTFE fitting, with the tube running inside the vessel to avoid spillage when filling with liquid. The other inlet lines were from Ace^TM glass, with rubber septa or O-rings, to make gas-tight connections to the addition lines. The drying trap consisted of a borosilicate glass tube (30 cm × 10 mm id), with smaller Labline™ screw-on inlet and outlet tubes to fit 1/16 in (2 mm) PTFE tubing (Ace Glass). The alkaline trap (bubbler) was made of a 50 cm × 20 mm od tubing to hold 140 mL of alkaline solution. The bubbler was equipped with a fritted tip (porosity C) and was vented to a fume hood.

The system was designed to minimize leaks, thus ensuring maximum ¹⁴C recoveries, and minimizing evaporation and potential contact with tritiated water. The pH electrode and the acid inlet line were fed to the reaction vessel by a Tanager (Ancaster, ON, Canada) Model 8901 autotitrator. This was an important component of the system, as the pH of the solution had to be decreased gradually to avoid fast de-gassing. The liquid addition line was fitted to a Luer stopcock for adding various solutions without opening the reaction vessel. The solution in the bubbler was 2 M NaOH. The drying agent consisted of analytical-reagent grade magnesium perchlorate, 4–20 mesh (Fisher Chemicals, Fairlawn, NJ; UN-1475), loosely packed in the trap to minimize the pressure drop. Inconsistent ¹⁴C recoveries were obtained when the drying agent was silica gel.

Some of the items of the hardware are not critical, as follows. Flow meter: a ball rotameter or a soap bubble flow meter can be used. The soap bubble flow meter should be added at the outlet of the bubbler, whereas the ball rotameter could be placed at the same location as the current design. Acid delivery system: a syringe pump can be used instead of an automatic titrator. The acidification should be performed over a period similar to the current titration, ≡30–40 min with an acid of moderate concentration (we used 0.5 M). The use of a pH electrode is recommended for consistency, as opposed to using a pH color indicator. Alkaline bubbler: the key item in the bubbler is the fritted tip. The amount of solution that we used (≡140 mL) featured a long travel path (≡40 cm). The exact concentration of NaOH should not be significant and, although we used 2 M NaOH throughout our work, we tested several solutions from 1 to 4 M without apparent differences.

3.2 Samples and standards

The moderator water samples were withdrawn by operators at the station into 125 mL borosilicate glass bottles (Qorpak; Fisher Scientific, Pittsburgh, PA, USA, Cat. No 03-326-1D), with a PTFE (0.25 mm thick) liner, cut manually to fit inside the cap. The bottles were pre-filled with 100 μL of saturated NaOH solution (19.4 M) to preserve the samples at a high pH. The bottles were filled to the top, without headspace. The samples were stored at ambient room temperature and were analyzed ≡30–180 d after their collection. The groundwater samples were collected from a monitoring well near a waste management area at Chalk River Laboratoreis (CRL), and stored in a 1 L Nalgene bottle, along with 1 mL of saturated NaOH solution. The samples were also filled and capped with no headspace, and were preserved in a refrigerator prior to analysis.

Standards consisted of Na₂¹⁴CO₃ (≡50000 Bq mL⁻¹), prepared by diluting a primary NaH¹⁴CO₃ solution (50 μCi in a 1 mL sterile ampule; New England Nuclear, Life Sciences, Boston, MA, USA, Cat. No. NEC-0865) in 2 M NaOH solution. Aliquots of our standard were analyzed repeatedly by LSC on several instruments, and compared with a NIST-traceable standard (EG&G Wallac, Gaithersburg, MD, USA) for accuracy. We used glass scintillation vials and Hionic Fluor cocktail (Packard Instruments, Meriden, CT, USA), in a volume ratio of 1∶9 (mL∶mL 2 M NaOH to cocktail). All solutions mixed with the cocktail were 2 M NaOH, or brought to 2 M NaOH to minimize matrix effect differences during counting. All the samples and standards were shaken and allowed to stand for at least 1 h prior to counting, to allow chemiluminescence to decrease (the chemiluminescence should be negligible after 20 min, according to the manufacturer). We tested a waiting time of ≡16 h (overnight) without noticeable difference. Each sample or standard was counted three times, which contributed to decreasing the noise due to chemiluminescence, if significant.

3.3 Procedure for ¹⁴C stripping

After filling the drying trap and the alkaline bubbler, the tubing connections were attached between the reaction vessel, the trap and the bubbler. The carrier gas inlet line (without flow), the acid inlet and liquid addition lines were also attached and the discharge valve was kept closed. A magnetic stirrer, 2 mL of 2 M KNO₃ solution (for ionic conductivity) and 2 mL of 0.7 M Na₂CO₃ carrier solution were added through the pH electrode opening of the reaction vessel. The pH electrode was then inserted and secured (gas-tight). Using a 60 mL disposable syringe, 30 mL of heavy water were withdrawn from the sample bottle and the syringe was carefully attached to the liquid addition line and pushed into the reaction vessel through the line. Using a different syringe, a slightly alkaline de-ionized water (DIW) solution (prepared by adding a few drops of saturated NaOH solution to DIW) was carefully added to bring the level of solution up to the pH electrode junction.

The carrier gas was then opened at a flow rate of 38 ± 2 mL min⁻¹, and the connections were checked for leaks with a soap solution. The magnetic stirrer was turned on and the titration was started only after successful completion of these checks. Titration gradually decreased the pH to 2 using 0.5 M HCl, over a 30–40 min period. At the end of the titration, a syringe filled with 0.1 M HCl was attached to the liquid addition line and 10–30 mL of acid were added to decrease the volume of headspace in the reaction vessel to a minimum. The system was allowed to purge for an additional 30 min. The same procedure was used for process blanks (DIW only) and process standards (100 μL of Na₂¹⁴CO₃ solution, ≡50000 Bq mL⁻¹, in DIW) for recovery checks.

At the end of the acid-stripping procedure, the NaOH solution in the bubbler was recovered first (to avoid the potential pick-up of airborne DTO from the next steps) and poured into a high density polyethylene bottle, which was capped immediately. The reaction vessel was then emptied and its contents were collected for disposal or recovery (heavy water). The ¹⁴C sample in the NaOH solution is stable indefinitely.

The groundwater samples were processed similarly to above, except that the flow rate of carrier gas was 30 mL min⁻¹.

3.4 Counting procedure

3.4.1 Carbon-14.. Counting was performed in batches: each batch consisted of one to three samples (each one in triplicate); one process blank and one process standard per sample; and one or two NaOH blanks and one to three standards, for direct counting. The ratio of NaOH solution to cocktail was 1∶9, as above. The instrument was a Wallac 1414 WinSpectral liquid scintillation instrument (EG&G Wallac). The samples were counted with wide open windows (all channels open), with dual label (¹⁴C and ³H) and DOT correction (digital overlay templates) for overlapping energies and for automatic quench correction. The instrument software contains a library produced at the factory by measuring the spectra of 60–80 standards of a given isotope, at various quench levels (both chemical and color quench). One subset has only chemical quench, another subset has only color quench. The instrument built-in counting efficiency is determined from a surface function of the color and chemical quenches. In addition to this efficiency determination, we counted our own standards to compare the efficiency.

The samples were counted in three cycles of 10 min each. The counter and the Na₂¹⁴CO₃ solution (50000 Bq mL⁻¹) were checked periodically against NIST-traceable ¹⁴C and blank solutions (traceability provided by Wallac), and a 2822 dpm mL⁻¹ (47.0 Bq mL⁻¹) NIST-traceable secondary standard (solution prepared on October 6, 1996, from [¹⁴C]hexadecane, NIST SRM 4222C). The latter was also used in comparisons with other instruments at CRL. The comparisons with the other instruments suggested that the Wallac 1414 instrument could have a ≡0–3% low bias, but the results were still within instrument and standards specifications, and no correction for LSC accuracy was considered to be needed.

3.4.2 Tritium in heavy water.. The levels of tritium in heavy water were measured for an order of magnitude value only. The values were taken as reported and corrrected by the instrument. A 100 μL aliquot of original sample was diluted sequentially in calibrated flasks, to obtain a dilution factor of 10⁷. A 100 μL aliquot of this diluted water was mixed with 0.9 mL of 2 M NaOH and Hionic Fluor, as above. No blanks, standards, etc., were processed for accuracy; the built-in efficiency determination of the instrument was used.

4 Results and discussion

4.1 Precision and discrimination between ¹⁴C and ³H

The original samples of heavy water contain known, relatively high levels of tritiated water and unknown amounts of ¹⁴C. The ¹⁴C analyses of reactor water are shown in Table 1. With a few exceptions, the typical precision for samples having 0.1 MBq L⁻¹ or more was ≡5% or better. Most of the variations could be explained by statistical variations in the efficiencies of the acid-stripping process. On the other hand, the results for which the ¹⁴C concentrations were below 0.002 MBq L⁻¹ were limited by the precision of LSC analysis. The process blanks, reported as a percentage of the ¹⁴C found in the same set of runs, were usually less than 1%, except for the low ¹⁴C solutions. This indicates that memory effects and cross-contamination between samples are negligible.

Table 1 Results of ¹⁴C analyses, with sample RSD, and the amount of tritium (as tritiated water) present in the original sample

Sample	^14C concentration in original 30 mL sample MBq L^−1	RSD (%)	Process blank (% of ^14C recovered)	H concentration in the 30 mL sample/10^5 MBq L^−1	Reported ^3H in bubbler (total)/Bq^a
a Value reported by the LSC instrument. This signal may not be for tritium (see text).b Samples not analyzed; the values given on the shipping manifests are reported.
A1I	1.3	4.3	0.5	7.4	337
A2I	3.8	3.7	0.6	10.0^b	3069
A3O	0.023	20.0	1.1	7.2	7
A4I	3.3	3.1	0.2	16	1593
B1I	0.17	12.9	0.9	3.8	81
B2I	1.7	5.6	0.6	3.7^b	613
B3I	1.8	4.2	0.4	3.0^b	1372
B4I	3.3	4.6	0.5	3.7	1617
C1I	0.14	2.2	0.3	19.0	75
D1I	0.0016	15.0	16.0	7.5	33
D1O	0.0008	25.9	33.0	6.1	24
D2I	0.0012	3.7	<0.1	5.2	18
D3I	3.7	4.3	0.6	3.6	2071
D4O	4.2	4.9	0.7	5.7	3132
D5I	4.2	2.7	0.8	4.6	2634
D5O	4.0	4.7	0.7	5.2	2801
D6I	3.4	4.1	0.4	4.5	1595

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We obtained a high tritium removal factor [defined as total activity (reaction flask)/total activity (bubbler), where total activity (reaction flask) is concentration (MBq L⁻¹) × 0.030 L of initial sample]. The data in Table 1 show that the ³H signal reported in the bubbler ranged from 7 to ≡3100 Bq total, indicating that the removal factor of the stripping process plus the drying trap is at least 5 × 10⁶–3 × 10⁹. The actual removal factor, however, could be higher because the ³H signals, as reported by the Wallac 1414 instrument, are questionable, as we found a positive correlation between the ¹⁴C signal and the ³H signal (see second and last columns in Table 1). The cause(s) of this is (are) not clear; it appears that the ³H signal reported is a ¹⁴C signal falsely reported in the ³H energy window. We tested and dismissed other causes: for example, we tested with different batches of Hionic Fluor LSC cocktail, chemiluminesence and interferences due to low-energy γ-emitters produced in reactors, such as ⁴¹Ar, ¹⁶N, ¹⁹O and ¹⁷F because of their short half-lives, combined with a 1–6 month period prior to sample analysis. Our carbonate standards containing known amounts of ¹⁴C gave a signal in the ³H window, but this signal was inconsistent among individual measurements. Conversely, samples containing a single ³H spike [83000 disintegrations per minute (dpm)] did not give a false positive ¹⁴C signal. Hence, the actual ³H signal in our samples is most likely lower than reported, which would indicate a higher removal factor than mentioned above.

Table 2 gives the results for the process standards and their associated process blanks, analyzed using the same procedure as for the reactor water. In all but two cases (see Table 2), no tritiated water was present. The precision, given as relative standard deviation (RSD), was generally similar to that of the actual samples. The presence of tritiated water did not affect the precision.

Table 2 Precision of triplicate analyses of spiked solutions, prepared by adding a known amount of [¹⁴C]carbonate standard to DIW. In two cases, tritiated water was added to the spike solution in the reaction vessel. The last column refers to the reported ¹⁴C value of the process blank in the NaOH bubbler compared with that of the spiked solutions processed in the same batch

Processing date (1998)	Total ^14C in spikes and test solutions/Bq	Total ^14C recovered/Bq	Precision (RSD) (%)	Process blank (% of ^14C recovered)
a Process blank not applicable to this batch.
27 Jan	9623 Bq ^14C test solution, prepared Dec. 23, 1997	9024	5.4	0.4
29 Jan	9623 Bq ^14C test solution, prepared Dec. 23, 1997 + 30.7 MBq ^3H	8935	2.5	0.7
3 Feb	4811 Bq ^14C spike in DIW	4493	1.5	N/A^a
24 Feb	4811 Bq ^14C spike in DIW	4253	3.7	0.2
27 Feb	4811 Bq ^14C spike + 30.7 MBq ^3H in DIW	4531	2.2	0.1
7 Apr	9623 Bq ^14C test solution, prepared Dec 23, 1997	9210	0.6	0.2
28 Apr	5236 Bq ^14C spike in DIW	5601	4.0	0.3
4 Jun	5172 Bq ^14C spike in DIW	4583	8.7	0.4
17 Jun	5172 Bq ^14C spike in DIW	4395	5.2	0.1
23 Sep	5092 Bq ^14C spike in DIW	4833	0.5	N/A^a
17 Nov	9623 Bq ^14C test solution, prepared Dec. 23, 1997	9443	1.1	N/A^a

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Table 3 gives the results for groundwater samples and additional details of some low ¹⁴C reactor water. One could note an improved precision when a larger 2 mL aliquot is used in LSC counting at these low levels. The method, as used, has the potential to analyze solutions reaching drinking water limits (200 Bq L⁻¹).⁴ In comparison, the total carbon corresponding to this value is ≡1.4 × 10⁻⁹ g L⁻¹ if it was pure ¹⁴C. For solutions (reactors or groundwater) containing μg L⁻¹ to mg L⁻¹ levels of total carbon, the radioactive carbon is a small fraction of the total.

Table 3 Results for groundwaters around the Chalk River site and some low ¹⁴C reactor heavy water. For comparison, the Canadian drinking water guideline⁴ gives a maximum allowable concentration (MAC) of 200 Bq L⁻¹ for ¹⁴C

Sample ID	Volume of bubbler solution counted/mL	Process run ID	Count rate/ dpm	Error (RSD) (%)	Concentration in sample/Bq L^−1	Notes
a Three values from three counting cycles are usually reported.
Groundwater—
C-112	July	2	DEC16R3	13.5	0.7	405	2 count vaues reported^a
Aug.	2	DEC16R4	4.5	16	99	2 count values reported
1	DEC16R4	4.4	48	193	2 count values reported
Sept.	2	DEC16R5	9.1	24	245	2 count values reported
C-265	July	1	DEC17R2	1.9	82	90
2	DEC17R2	3.1	11	73	2 count values reported
Aug.	1	DEC17R5	5.1	58	197
2	DEC17R5	5.5	33	102
Sept	1	DEC17R4	17.4	23	1047
2	DEC17R4	25.2	3.3	757
C-267	July	1	DEC16R2	22.7	0.9	1101
2	DEC16R2	38.6	5.9	943
Aug.	1	DEC15R1	4.3	53	163
2	DEC15R1	9.7	22	187
Sept	2	DEC15R2	2.8	7.1	67
Reactor—
D1I	2	FEB17R1	49.5	3.1	1860
FEB17R2	40.2	1.8	1540
FEB17R3	25.5	6.1	1290
Average	1563
D1O	1	FEB23R2	9.1	44	676
FEB23R3	5.1	48	392
FEB23R4	6	29	460
Average	566
2	FEB23R2	25.8	8.6	960
FEB23R3	14.2	17.4	547
FEB23R4	15.9	6.8	606
Average	804
D2I	1	MAR4R1	15.5	13.5	1200
MAR4R2	16.1	7.4	1230
MAR4R3	14.5	7.2	1130
Average	1187

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4.2 Recoveries and robustness between analysts

Fig. 2 shows the recoveries of ¹⁴C process standards for sequential runs and the recoveries among different analysts. The method gave an average recovery of 94.2 ± 6% (n = 61) for the various process standards. The system does not have a bias among analysts. Three of the analysts were experienced (Nos. 1, 2 and 5), whereas the other two analysts were relatively inexperienced.

Fig. 2 Recoveries of ¹⁴C analyses for process standard runs, also showing the recoveries for several analysts with various levels of experience.

The percentage recovery data match reasonably well the pattern of the probability density function, as shown by the frequency distribution of our recoveries compared to the normal distribution (Fig. 3). The median (94%) is very close to the average value for all these points. We conclude, therefore, that the analysis method as presented does not have systematic errors or user bias. The variations of the recoveries can be explained by the normal statistical model.

Fig. 3 Comparison of the recoveries with a probability density function (PDF).

4.3 Detection and quantification limits

The instrument background noise for the full ¹⁴C spectrum is 55.0 ± 2.2 counts per minute (cpm) over a 10 min period. Using the Student’s t-test (two-sample case, large number of data points, i.e., n > 30⁵), the minimum detectable limit of a sample for LSC is 2.5 cpm above the background (95% confidence limit), which corresponds to ≡3 dpm at 90% efficiency for ¹⁴C. Using 140 mL as the volume of NaOH solution in the bubbler and a volume of groundwater or reactor water of 75 mL in the reaction vessel, the calculated detection limit is 93 Bq L⁻¹ if 1 mL of the bubbler solution is analyzed by LSC, or 47 Bq L⁻¹ for a 2 mL analysis.

A generally accepted value for the quantification limit is when the signal is approximately 10 times the standard deviation associated with the background noise.⁵ We set this level at 20 cpm, or 22 dpm above the background (at 90% efficiency). Using the same numbers as above, the quantification limit is set at 685 and 342 Bq L⁻¹ for 1 and 2 mL aliquots counted by LSC, respectively.

The results given in Table 3, when revisited, show that only two groundwater samples (C-265, September, 2 mL, and C-267, July) and two reactor water samples (D1I and one of D1O, 2 mL) display a value above the quantification limit of 22 dpm. The values for most of these samples are between the detection and the quantification limits.

4.4 Sample stability

A synthetic 10 kBq solution total (DIW with 100 μL of saturated NaOH and 200 μL of nominal 50000 Bq mL⁻¹ solution) was prepared on December 23, 1997, in a Qorpak bottle with a PTFE-lined cap. Four different bottles were prepared and left at ambient temperature until processing at various times, up to ≡320 d after preparation. The ¹⁴C recoveries ranged from 92.8 to 98.2%, which is well within the recoveries found with process standards throughout the study. The samples were stable for this period (≡1 year), which was important, as many of the samples were stored for 3-6 months prior to processing.

4.5 Key variables

This work has overcome several fine-tuning problems that were encountered in the past by several different analysts. In principle, the method is simple, but in practice, several aspects of the hardware and the procedure can produce inconsistent results.

We used a 0.5 M acid, compared with undiluted or 2–4-fold diluted commercial concentrated acids, as often used previously and in early tests. With stronger acids, a sudden release of CO₂ was often clearly visible in the NaOH bubbler, thus increasing the potential for losses through leaks or inefficient contact with the alkaline solution. The use of a titrator for gradual acidification through the S-shaped part of the titration was also a factor that contributed to a controlled release of CO₂.

The bubbler had a long contacting path between the impinger and the top of the solution (≡40 cm), and a frit to divide the gas bubbles finely. Tests carried out with ≡10 MBq of ¹⁴C in the reaction flask going to two bubblers in series showed no significant amounts of ¹⁴C in the second bubbler, hence the ¹⁴C recovery is quantitative in the first bubbler (results not shown).

The design of the reaction flask was important. The conical shape of the reaction vessel and the addition of 0.1 M HCl provided a maximum displacement of the headspace to ≡5% of the flask volume. This resulted in a combined flushing time of ≡3.1 min in the flask, which corresponds to a first-order flushing rate of 3.7 × 10⁻³ s⁻¹. In comparison, the conversion reactions to CO₂ were much smaller:⁶

 H⁺ + HCO⁻₃ → H₂CO₃  k₁ = 4.7 × 10¹⁰ l mol⁻¹ s⁻¹ (1)

H₂CO₃ → H₂O + CO₂   k₂ = 11.9 s⁻¹ (2)

The rate of reaction (1) in our solutions, with the 2 mL of Na₂CO₃ carrier (≡0.014 M in the reaction vessel) is three orders of magnitude faster than that of reaction (2), hence the latter is the rate-limiting step. The other parallel reaction:

H⁺ + HCO⁻₃ → H₂O + CO₂  k₃ = 7 × 10⁴ l mol⁻¹ s⁻¹  (3)

is negligible. The flushing time of 30 min, after the end of the titration, allowed a flushing rate ≡10 times longer than the slowest rate, i.e., the flushing time of the aqueous solution in the reaction vessel. The addition of the Na₂CO₃ carrier solution ensured complete recovery of the TIC and ¹⁴C. Without carrier, the expected TIC levels (20–30 μg L⁻¹ in the original solutions) would mean that reaction (1) could dominate and inconsistent recoveries may result unless a sufficient flushing time is used.

The detection and quantification limits could be improved using the same apparatus by increasing the counting times, using 2 mL of bubbler solution, and potentially using half of the volume of NaOH solution in the same bubbler design. Increasing the counting times to 30 min would bring the noise level down to the equivalent of 1.5 dpm, thus decreasing the quantification limit by a factor of two. Hence the overall detection limit could be lowered to 12 Bq L⁻¹ and the quantification limit to ≡90 Bq L⁻¹; this was not tested. We further tested the procedure by precipitating the dissolved CO₂ of the bubbler solution as CaCO₃,^7,8 which could lower the detection limit to <1 Bq L⁻¹ (to be published separately).

5 Conclusion

Our test method allowed for carbon-14 determination in a difficult matrix consisting of highly tritiated heavy water, taken from reactor systems. Removal of tritium was quasi-quantitative, while recoveries of carbon-14 were ≡94%, based on several runs with process standards and standards in simulated solutions. The method was tested with five analysts, and showed no operator bias. Key items were the choice of desiccant and the design of the reaction vessel, which could easily be custom-made at a low cost.

The detection limit is sufficiently low to analyze contaminated groundwaters down to levels near the Canadian drinking water guideline of 200 Bq L⁻¹. The reactor water samples had ¹⁴C levels sufficiently higher than this, and reliable quantification was possible.

6 Acknowledgements

The authors thank J. Torok, R. Rao, J. Young, L. Jones, AECL, W. Watson, who was on term at AECL, and A. O’Connor, Cambrian College, Sudbury, ON, for various aspects of discussion and laboratory help with liquid scintillation techniques. The work was supported by COG (CANDU^® Owners Group) Working Project Information and Release WPIRs 1524 and 2727. The authors thank M. Totland, J. Gulens (AECL) and R. Laporte (Hydro-Québec, Gentilly nuclear station) for reviewing the manuscript. The main author received the IUPAC/Gendron award, given by the Canadian Society for Chemistry to support the presentation of this work at SAC 99.

7 References

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Footnotes

† Presented at SAC 99, Dublin, Ireland, July 25–30, 1999.

‡ © Copyright Government of Canada.

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