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ORIGINAL ARTICLE
Year : 2015  |  Volume : 38  |  Issue : 1  |  Page : 39-44  

Evaluation of alkali fusion and acid leaching methods for the determination of insoluble plutonium and americium in fecal samples


1 Internal Dosimetry Section, Radiation Safety Systems Division, BARC, Trombay, Mumbai, India
2 Environmental Survey Laboratory, TAPS Colony, TAPP (PO), Tarapur, Thane, Mumbai, Maharashtra, India

Date of Web Publication14-Aug-2015

Correspondence Address:
D D Rao
Internal Dosimetry Section, Radiation Safety Systems Division, BARC, Trombay - 400 085, Mumbai, Maharashtra
India
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Source of Support: Nil., Conflict of Interest: None


DOI: 10.4103/0972-0464.162815

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  Abstract 

Monitoring of radiation workers of fuel reprocessing plant and fuel fabrication facility for internal contamination is carried out by analyzing their urine and fecal samples. Assessment of intake is made from these results using standard biokinetic and dosimetric models. In the analysis of fecal samples, there is a need for improvement in the pretreatment technique for complete dissolution of the sample. Conventional alkali fusion method has yielded recovery of plutonium in the range of 12–60% with a mean of 36%. Newly adopted acid leaching followed by hydrogen fluoride treatment method achieved recovery in the range of 43 to 90% with a mean of 70%. Conventionally, the separation of "Pu" and "Am" was carried out at Bioassay Lab., Tarapur by alkali fusion followed by the anion exchange separation for Pu and cation exchange separation for Am. This paper deals with an alternative method in which initially the entire ash of the sample spiked with 236Pu tracer (3–11 mBq) and 243Am tracer (2.8–14.5 mBq) was acid leached and Pu was separated by anion exchange method and Am by using TRU resin.

Keywords: Acid leaching, alkali fusion, hydrogen fluoride treatment, plutonium in fecal samples, Type M, Type S, 239Pu


How to cite this article:
Raveendran N, Rao D D, Baburajan A, Yadav J R. Evaluation of alkali fusion and acid leaching methods for the determination of insoluble plutonium and americium in fecal samples. Radiat Prot Environ 2015;38:39-44

How to cite this URL:
Raveendran N, Rao D D, Baburajan A, Yadav J R. Evaluation of alkali fusion and acid leaching methods for the determination of insoluble plutonium and americium in fecal samples. Radiat Prot Environ [serial online] 2015 [cited 2019 Jun 20];38:39-44. Available from: http://www.rpe.org.in/text.asp?2015/38/1/39/162815


  Introduction Top


Occupational radiation workers of fuel reprocessing and fuel fabrication facilities are monitored for the assessment of intakes of radionuclides (e.g. plutonium, americium, uranium) and evaluation of committed effective dose. The determination of the internally deposited radionuclides is carried out either by using in vivo technique and/or by analyzing the excreta like urine and feces. Generally, urine samples are analyzed for readily soluble (Type M) Pu/Am isotopes while fecal samples are analyzed for the determination of insoluble forms of Pu/Am isotopes (solubility Type S). The strategy involves analysis of both fecal as well as urine samples for making an attempt to assess the possible nature of compound based on the respective excretion rates. This paper presents the comparison of analytical methods for fecal sample processing and sequential separation of Pu and Am. Generally, separation of Pu present in the fecal sample is carried out by alkali fusion followed by standard radioanalytical technique[1] and cation exchange separation for Am.

An effort has been made to improve the radiochemical recovery and thereby the detection limit for Pu/Am estimation by using acid leaching method and 236Pu as a tracer. The method adopted for separation has improved the minimum detectable activity (MDA) to 0.14 mBq/sample compared to conventional methods which yielded a MDA of 0.3 mBq/sample. The measurements were made with eight channel alpha spectrometer having passivated ion-implanted planar Si detectors (PIPS), and samples were counted for a duration of 3,50,000 s.


  Materials and Methods Top


Ashing of sample

Complete fecal sample collected over 24 hours from radiation workers was dried, ashed in a muffle furnace at 450°C, and the ash was spiked with appropriate 236Pu and 243Am tracer. Concentration of 236Pu and 243Am tracers used were in the range of 2–13 mBq, which is about 10 times the detection capability of the alpha spectrometry system at the lower band.

Alkali fusion method

The ash was fused with fusion mixture (NaOH: NaNO3:Na2CO3= 5:2:1) on the burner. The temperature of fusion mixture reaches about 900°C, which could assist in dissolving any oxide form of plutonium present in the sample.[2],[3],[4] Fused lump is taken in distilled water from which carbonate residue was separated. The carbonate is separated by centrifugation and further dissolved in 1:1 HNO3, evaporated to dryness twice, and taken in 8N HNO3 for further loading to the anion exchange column after the addition of ~30 mg of NaNO2.

Acid leaching method

The ash of 2–7 g was treated with 4 mL conc. HNO3 and evaporated to dryness and this process was repeated twice. Again, it was treated with 4 mL conc. HCl and evaporated to dryness and the process was repeated twice. Then, it was leached thrice with 5 mL 8N HNO3 and 8N HCl separately. After centrifugation, the separated residue from the supernatant was treated with 1 mL conc. HNO3 and 0.5 mL hydrogen fluoride (HF) and evaporated to dryness. To remove HF interferences, 1 mL conc. HNO3 was added twice and evaporated to dryness. The residue was leached with 8N HNO3 and was added to the earlier supernatants. The preconcentration of plutonium and americium was carried out by co-precipitation with 50 mg of Ca carrier and ammonia to form the precipitation of calcium phosphate. The Ca3(PO4) precipitation was dissolved in 1:1 HNO3, evaporated to dryness, and taken in 8N HNO3 for further loading to the anion exchange column after the addition of ~30 mg of NaNO2.

The loading solution from both the methods containing "Pu' and "Am" was loaded on amberlite 1 × 8 anion exchange resin preconditioned with 8N HNO3. The plutonium adsorbed on the resin was eluted with 1.5 M hydroxylamine hydrochloride and taken for electrodeposition. The loading effluent and washings are taken for "Am" estimation.

In the present work, the extraction chromatography method[5],[6],[7],[8],[9] using TRU resin which contains N-N-di isobutyl carbamoyl methyl phosphine oxide as extractant, tri-n-butyl phosphate as diluent absorbed on inert polymeric support has been used for the separation of Am from fecal sample. The 8N HNO3 effluent from Pu separation step was dried and the residue was dissolved in 10 mL 1M Al (NO3) 3 in 3M HNO3 and pinch of ascorbic acid was added and loaded on a TRU resin column (dia ~ id 4 mm, height 60 mm; 3–4 mL/min flow rate), preconditioned with 30 mL 1M Al (NO3)3 in 3 MHNO3. The column was washed with 5 mL 3M HNO3 and 5 mL 2M HNO3. The nitrate concentration was lowered using the addition of 10 mL 0.05 M HNO3. Am was eluted with 3 mL 9M HCl and 20 mL 2M HCl. The elute was dried and electrodeposited[10] on a SS planchet in NH4(SO4) 2 solution at pH 2.2 for 2 h. Pu and Am activity was estimated by counting in PIPS detector coupled to 8 K channel alpha spectrometer. The samples were counted for a duration of 3,50,000 s.

In case of eight fecal samples, Fe1 to Fe8, (1.9–6.9 g), the analysis was carried out after pretreatment by alkali fusion. Fe9 to Fe17 fecal samples (2.2–4.8 g) were analyzed after pretreatment by acid leaching. Seven fecal samples, Fe18 to Fe24 (3.2–8.4 g) were divided into two halves; one-half was taken for alkali fusion and other half for acid leaching method. The work was carried out to determine the suitable pretreatment method for better recovery. The analysis of urine sample was carried out to determine the soluble fraction of "Pu." Analysis of urine sample was carried out by Ca3PO4 precipitation, anion exchange, and electrodeposition.[11] Fecal samples Fe29 to Fe53, whole ash was taken for sequential analysis.

Quantification of results

The activity (A) of 236Pu, 243Am (mBq) tracer is calculated by:



The activity of239+240 Pu, 238Pu, 241Am (mBq/sample) are calculated by:



C = peak area counts of respective isotope; T = time of counting; EF%: Percentage efficiency


  Results and Discussion Top


[Table 1] gives results of activity concentration of 236Pu determined by the conventional method in fecal samples (Fe1–Fe8) by the dissolution of samples with alkali fusion method. Recoveries varied from 12% to 60% with a mean of 36% and standard deviation (SD) of 15%.
Table 1: Percentage radiochemical recovery of 236Pu by alkali fusion method

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[Table 2] gives activity concentration of 236Pu determined in fecal samples Fe9–Fe17 by acid leaching method. The recovery was improved due to the increased extent of dissolution of the sample. Recovery varied from 71% to 88% with a mean of 79% and SD 5%.
Table 2: Percentage radiochemical recovery of 236Pu by acid leaching and HF method

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[Table 3] summarizes the comparison of results obtained using the two methods namely alkali fusion method and acid leaching method for the two equal portions of ash of the same sample (Fe18–Fe24). One-half analyzed by alkali fusion method showed239+240 Pu activity <0.3 mBq/sample whereas the other half analyzed by acid leaching method showed detectable Pu activity of 0.24 mBq/sample due to higher recovery. Ranjeet et al. 2008[12] have also reported that the average percentage recovery by acid extraction has improved over alkali fusion from 48% to 65% for plutonium. The 239+ 240Pu activity concentration values in the samples (Fe18–Fe24) for both the methods are comparable although the tracer recovery obtained was in the range of 22–44.7% for the alkali fusion method and 70–90% for the acid leaching method. The increase in the recovery by acid leaching method could be due to the complete oxidation of plutonium which remains as insoluble fraction even after the repeated extraction. In the acid leaching technique, the insoluble fraction of plutonium is converted to acid-soluble form by HF treatment.[13],[14] Sill et al.,[15] concluded that aqua regia acid leaching is inadequate for sample requiring plutonium separation. A study by Haque and Nakanishi[16] has showed that an appreciable fraction of Pu remains insoluble even after the two-stage extraction in sediment sample.
Table 3: Comparison of radiochemical recovery between alkali fusion and acid leaching method

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To assess the solubility class, (M or S type) of the "Pu," both the urine and fecal samples of the occupational worker were collected and analyzed. The results of the analysis are given in [Table 4]. The activity concentration in urine samples is found to be in the range of <0.3 - 0.6 mBq/sample. Whereas the corresponding fecal samples (Fe25-Fe28) showed activity well above the detection limit of 0.3 mBq/sample. The correlation between these two activity concentrations gives rise to an indication of type S material. In case of insoluble plutonium, the fecal analysis is the most suitable method for assessment of intake.[17]
Table 4: Activity concentration in urine/fecal samples of occupational workers

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[Figure 1] is the typical plutonium spectrum in urine samples counted using alpha spectrometry system. The spectrum indicated low-level detection of plutonium in the subject while [Figure 2] is the typical plutonium spectrum of detectable level of activity in a fecal sample of the same subject counted in the alpha spectrometry.
Figure 1: Typical plutonium spectrum for low-level activity in urine sample by alpha spectrometry

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Figure 2: Typical plutonium spectrum for detectable level of activity in fecal sample by alpha spectrometry

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[Figure 3] shows scatter graph for recoveries of 236Pu tracer against the number of samples by alkali fusion method. Average recovery was 36% with a range of 12–60%. Variation of 236Pu tracer recoveries obtained within ± 2 σ except one value.
Figure 3: Recoveries of 236Pu tracer by alkali fusion method

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Twenty-five fecal samples were (Fe29–Fe53) analyzed for Pu and Am simultaneously. The preconcentration for sequential separation for plutonium and americium is carried by acid leaching method. Plutonium separation is carried out by anion exchange and Am separation is by TRU resin.[18],[19],[20],[21] The [Figure 4] and [Figure 5] give scatter graph of analytical recoveries of 236Pu and 243Am tracer with respect to the number of samples. In [Figure 4], the Pu tracer recovery varied from 43.6% to 90% with a mean of 69.9% and SD of 13.4%. Observed recoveries in the maximum number of samples are above the average. In [Figure 5], Am tracer recovery was in the range of 20–89.4% with a mean of 54.7% and SD of 21.3%. It is observed that 243Am recovery by conventional cation exchange separation was in the range of 30–60 with a mean of 45%. The typical spectrum of 239+ 240Pu and 241Am in the fecal sample along with respective tracers are shown in [Figure 6] and [Figure 7].
Figure 4: Recoveries of 236Pu tracer by acid leaching method

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Figure 5: Recoveries of 243Am tracer by acid leaching method

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Figure 6: Typical Pu spectrum after separation from anion exchange resin

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Figure 7: Typical Am spectrum after separation from TRU resin

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  Conclusions Top


Acid leaching is the simplest method and gives better chemical recovery for fecal samples containing a high proportion of refractory oxides. The method adopted for the extraction has improved the recovery of Pu up to 70% by complete dissolution of the sample with acid leaching followed by HF treatment. The TRU resin was found to be highly selective for Am. The Am separation with TRU resin column can be completed within 4–5 h compared to 2–3 days required for the cation exchange separation method. Multiple number of samples can be analyzed by using an array of TRU resin columns as the number of steps involved in the separation are a few.

Acknowledgments

The authors would like to express their sincere thanks to Dr. Pradeepkumar K. S., Associate Director, HS & EG of BARC, Dr. R. M. Tripathi, Head, HPD, and Dr. P. M. Ravi, Head, ESS for their encouragement during the course of the work. They would also like to thank other staff members of Environmental Survey Laboratory for their support during the course of the work.



 
  References Top

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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4]


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