Jointly published by Elsevier Science S. A., Lausanne and Akad[miai Kiad6, Bttdapest

J.Radioanal.Nucl. Chem.,Letters 200 (6) 545-550 (1995)

A FAST METHOD FOR THE DETERMINATION OF PLUTONIUM IN SOIL AND SEDIMENT SAMPLES

Godoyl J.M. , M.L. Godoy, Z.L. Carvalho

Comiss~o Nacional de Energia Nuclear, Instituto de Radioproteg~o e Dosimetria,

Caixa Postal 37750 - CEP 22780-190, Barra da Tijuca, Rio de Janeiro, RJ, Brazil

Received 16 September 1994 Accepted 2 May 1995

A so-called hour method for the determina- tion of plutonium based on extraction chro- matography is validated by its application to several certified soil and sediment sam- ples from the International Atomic Energy Agency's Seibersdorf Laboratory. The results are in good agreement with the reference values. Chemical yields range between 47%- 66%, with a mean value of (56• The possible use of this method for 241pu de- termination is also demonstrated.

During the first Coordinated Research Program (CRP)

meeting on "Rapid Instrumental and Separation Methods

for Monitoring Radionuclides in Food and Environmental

Samples", some needs and requirements related to these

rapid methods, mandatory for the intermediate and late

post-accident phase, were identified I . Concerning

e-emitters, in particular, the need for a so-called

Present address: Departamento de Qufmica, Pontiflcia Universidade Cat6lica do Rio de Janeiro , Rua Marqu@s de Sao Vicente 225, G[vea, Rio de Janeiro, RJ, Brazil, CEP 22453-900.

0230-5731/95/US $ 9.50 Copyright �9 1995 Akad~miai Kiad6, Budapest All rights reserved

545

GODOY et al.: DETERMINATION OF PLUTONIUM IN SOIL

"hour" method applicable to water and soil samples was

stressed. An hour method was defined as a method with

a processing time of 6 h or less and a desired accuracy

between • 20% and a factor of 2. Assuming 2 h for count-

ing and evaluation, there are 4 h to perform the chemi-

cal work from sample dissolution to counting source

preparation. Since a counting time of 100 min should

not be exceeded, soil sample aliquots of at least 10 g

were recommended. As a consequence of this 4-h period

for the whole chemical procedure, it became evident

that time-consuming steps such as evaporations should

be avoided and, whenever possible, one single step

should he used for two simultaneous purposes, e.g. micro-

preparation with LaF 3 for Pu/U separation and counting

source preparation.

Two different procedures were tested and the one

based on extraction chromatography with tri-n-octylphos-

phine oxide (TOPO) was chosen, since it involved fewer 2

steps than the one using ion exchange . In previous work

the method was tested only with NBS 4353 Rocky Flats

Soil Number I, showing good results. The present work

summarizes new results on the application of the same

method to a larger number of certified soil and sediment

samples, also demonstrating its usefulness for the deter- 241

mination of Pu, which requires only a few additional

steps.

EXPERIMENTAL

The detailed procedure used for the determination of

239+240pu in soil and sediment samples has been described

previously 2. It can be summarized as follows. To a 10 g

sample are added 74 mBq 242pu {purchased from NIST/USA).

546

GODOY et al.: DETERMINATION OF PLUTONIUM IN SOIL

Leaching is performed with 50 ml (7.2M HNO 3 + 0.1M HF)

and 50 ml (7.2M HNO 3 + 1.0M AI(NO3)3), with boiling

for 15 min in each step. The solution obtained is

diluted with water (1:1), and percolated into a 5 x I -I

cm TOPO/Silicagel column at a flow rate of 2-3 ml min

After washing the column with 20 ml 3.6M HNO 3 and 10 ml

6M HCI, plutonium is eluted with 50 ml (6M HCI + 0.2M

ascorbic acid). The eluate is washed with 10 ml CCI 4,

and the plutonium co-precipitated with 50 ~g La 3+ as

described by Sill 3.

In order to determine 241pu, after the e-spectro-

metric measurement, the LaF 3 precipitate on the count-

ing disc was dissolved in 10 ml IM AI(NO3)3+0.1M HNO 3,

using an ultrasonic bath. Plutonium was extracted with

10 ml 0.1M TOPO in toluene and the organic solution

mixed with 10 ml scintillator solution (120 g naphtha-

lene, 4.0 g PPO and 0.05 g POPOP). The B-activity of

241pu was measured for 400 min in the 3H channel of a

liquid scintillation spectrometer (Beckman 5801). The

counting efficiency was assumed to be the same as for

tritium (ca. 35%) at the same quenching level. Data ob-

tained with 241pu supported this assumption 4'5

The soil and sediment samples were obtained from the

IAEA Seibersdorf Laboratory/Austria. Because of the low

activity of these samples, not typical of accidental

situations, a counting time of 1000 min was used for the

~-spectrometric measurements.

RESULTS AND DISCUSSION

The results are presented in Tables I and 2. The num-

ber of aliquots analyzed was a function of the sample

amount received from the IAEA Laboratory. In addition to

547

GODOY et al.: DETERMINATION OF PLUTONIUM IN SOIL

TABLE I

Plutonium results for soil and sediment reference samples obtained by the 'hour method'

(Values in mBq g-l)

Sample 239+240pu determ.

(N*)

239+240pu ref.

(conf. interval)

IAEA Soil 6

IAEA Sed. 367

IAEA Sed. 368

IAEA Sed. 306

B 11"*

B 12"*

1.01-+0.09 (4)

35.0+-1.0 (2)

25.5-+1.0 (3)

6.0-+0.4 (3)

0.60• (3)

3.7+-0.7 (2)

1.04 (0.96-1.11)

38 (34.4-39.8)

31 (29-34)

5.7 (5.5-6.3)

0.6

3.4

*Number of aliquots analyzed. **Bulgarian soils analyzed only once by the IAEA

Seibersdorf Lab.

TABLE 2

Results for 238pu and 241pu (Values in mBq g-l)

Sample 238pu determ. 238pu ref. 241pu determ. 241pu ref.

IAEA Sed. 368 (8.0+0.6) 8.5 (7.6-8.9) -

IAFA Sed. 367 - - (170-+8) 170 (159-190)

239+240pu, 238pu in IAEA Sediment sample 368 and 241pu in

IAEA Sediment sample 367 were also determined, since ref-

erence values for these radionuclides and samples were

available. They show that the extraction-chromatographic

method can be applied to different soils and sediments

with reliable results, even when the plutonium concen-

tration is quite low, as in the case of soil sample B11.

The variation of the chemical yield was also meas-

ured. The mean value was (56• with a range of

548

GODOY et al.: DETERMINATION OF PLUTONIUM IN SOIL

47%-66%, which is in good agreement with previously re-

ported results 2. Based on this mean chemical yield and

the above conditions, the typical detection limit for 241pu is 10 mBq g-1

It is necessary to point out that differences in the

purity of TOPO from different suppliers as well as

variations between TOPO impregnated silicagel batches,

used as column fillings, may alter the chemical yield.

Therefore, it is recommended to prepare large TOPO im-

pregnated silicagel batches and to measure the elution

curve for each new batch. The LaF 3 micro-precipitation

basically followed procedure described by Sill 3, but in

order to obtain a coprecipitation yield higher than 95%

for plutonium directly from the eluate, the volume of

HF (40% v/v) added had to be increased to 10-15 ml.

CONCLUSIONS

Results for 239+240pu obtained from different soil and

sediment samples demonstrate that the proposed extraction

chromatography method is reliable and shows good accuracy -I

and precision at levels as low as 0.5 mBq g . A few sim-

ple additional steps after s-spectrometric measurements

of these Pu isotopes permit to determine 241pu by liquid

scintillation counting with good results.

The authors thank Dr. Nobert Miekeley for the numer-

ous suggestions and to Dr. J. LaRosa, IAEA Seibers-

dorf Lab, for the samples.

549

GODOY et al.: DETERMINATION OF PLUTONIUM IN SOIL

REFERENCES

I. International Atomic Energy Agency (IAEA) - Report of the First Coordination Meeting on the Coordinated Research Program: Rapid Instrumental and Separation Methods for Monitoring Radionuclides in Food and En- vironmental Samplesr IAEA/AL/043, 1989.

2. J.Mo Godoyt M.L~ Godoy, ZoL. Carvalho, DoC. Lauria~ J. Radioanalo Nucl. Chem.~ 177 (1994) 19.

3. CoWo Sill, Analytical Chem.~ 51 (1981) 412o

4o Jo Paatero, Jo Jaakkola~ Radiochim~ Acta, 64 (1994) 139.

5o J.M. Godoy~ Ho Sch~ttelkopf~ M. Pimpl, Die Bestim- mung von 241pu durch F l~ssigszintillationspektro- metrie in der Umgebung des Kernforschungszentrums Karlsruhe, KfK-report-3552, 1983.

550