J.RADIOANAL.NUCL.CHEM.,LETTERS 165 (3) 185-190 (1992)

CONTRIBUTION TO THE ANALYSIS OF PLUTONIUM ORIGINATED FROM THE CHERNOBYL ACCIDENT IN SOIL

Z. _H61gye, I. Bur6~k

National Institute of Public Health, Center of Radiation Hygiene,

Srob~rova 48, 100 42 Prague, ~SFR

*Laboratory of Applied Ecology, Mlynski 4, 040 01Ko{ice, ~SFR

Received 23 March 1992 Accepted 6 April 1992

Four different procedures have been studied for the extraction of plutonium from soil sampled at Bragin, 40 km from Chernobyl. On extraction by leaching with conc. HNO 3 for 2 or 6 h the concentration of 239,240pu in soil was approximately by 30% lower than on extraction involving treatment with HF or fusion.

INTRODUCTION

In the year 1990 we obtained 220.6 g of soil collect-

ed at Bragin, 40 km from Chernobyl, by an expert group

in the connection with an International Chernobyl pro-

ject. We decided to study the efficiency of different

procedures of plutonium extraction from this soil

sample. This work is a small contribution to the deter-

mination of the degree of contamination of large areas

of USSR by plutonium isotopes.

185 Elsevier Sequoia S. A., Lausanne Akaddmiai Kiad6, BUdapest

HOLGYE, BUR~iK: Pu FROM CHERNOBYL ACCIDENT

EXPERIMENTAL

The soil sample (taken in the depth of 0-5 cm) ob-

tained in powdered form was homogenized before analysis

by thorough mixing. Four different procedures were test-

ed for the extraction of plutonium from the soil. In the

first two cases plutonium was extracted by leaching with

conc. HNO 3. Time of leaching was 2 and 6 h, respective-

ly. A beaker containing 10 g soil, 100 ml conc. HNO 3 and

18.8 mBq 242pu (as chemical yield monitor) was covered

by a watch glass and the sample was boiled under con-

tinuous stirring (and occasional addition of 2 ml

H202) for a given time period. The solution was sepa-

rated from insoluble residue in this and the other

cases by centrifugation. The leaching and washing solu-

tions (25 ml conc. HNO3) were collected and evaporated

to dryness. The residue was dissolved in 100 ml 8M

HNO 3. In the 3rd extraction procedure, 18.8 mBq 242pu

was added to 10 g of the soil in a teflon beaker and

the sample was gradually evaporated to dryness (with

occasional stirring) three times with 50 ml conc.

HNO 3, once with 15 ml tOM HF, 15 ml conc. HF + 15 ml

conc. HNO 3, 15 ml conc. HF and three times with 25 ml

conc. HNO 3. The plutonium was leached from the residue

for 3 hours with 100 ml 8N HNO 3 containing 5 g of

H3BO 3. The liquid was separated from the insoluble resi-

due, the residue was washed (25 ml 8M HNO 3) and the

washing solution was collected with the leaching solu-

tion. In the 4th extraction procedure plutonium was

leached from 10 g soil (after addition of 18.8 mBq

242pu) for 2 h by 200 ml 8M HNO 3 under heating. The

liquid was separated and the insoluble residue was

fused with Na2S207 . After dissolution, the plutonium

was coprecipitated with Fe(OH) 3. The precipitate was

186

HOLGYE. BURCIK: Pu FROM CHERNOBYL ACCIDENT

separated and dissolved in 8M HNO 3. This solution was

collected with the leaching solution.

Plutonium was separated from inorganic and remaining

organic macroeomponents and finally electrodeposited on

a stainless steel disk. Alpha-activity of the plutonium

isotopes on the disk was measured by a-spectrometry.

For each extraction procedure three parallel determina-

tions were carried out. All reagents used were of A. R.

grade.

RESULTS AND DISCUSSION

The results are summarized in Table I. It was found

that there exists no difference between mean values of

239'240pu concentration found in soil using HNO 3 ex-

traction after leaching for 2 or 6 h. No difference be-

tween HF treatment before acid leaching and extraction

by leaching and fusion was found either. The concentra-

tion of 239'240pu determined in the soil using the 3rd

and 4th extraction procedures is, however, by aZout 30%

higher than that obtained with the acid of the first

two procedures.

Approximately 70% of plutonium content in the studied

soil behaves during the leaching with conc. HNO 3 analo-

gously as the plutonium originating from atomospheric

nuclear tests. When the monitoring of plutonium in soil

and sediment samples originating from atmospheric nuclear

tests was started, different laboratories have found I-4

that the plutonium is extractable from these matrices by

leaching with 8-15M HNO3, 8-12M HCl or with the mixture

of these acids. From 16.9-19.3 mBq.g -I 239'240pu in soil

determined with the use of the Ist and 2nd extraction -I

procedures only ~0.5 mBq.g can be expected 5 to origi-

187

HOLGYE, BURC~K: Pu FROM CHERNOBYL ACCIDENT

TABLE I

Concentration of 239'240pu in soil by different extraction procedures

Extraction procedure Concentration of

239,240pu in soil, mBq.g-1

I. Leaching with conc. HNO 3

for 2 h

2. Leaching with conc. HNO 3

for 6 h

3. Treatment with HF

4. Leaching with 8M HNO 3

and fusion

16.9-+I .8

19.3+2.1

17.7-+I .9 WW

mean 18.0+-2.4

16.0+_2.9

19.1-+2.5

18.2-+2.2

mean 17.8-+3.1

20.8-+2.2

22.4-+I .8

33.0-+I .3

mean 25.4-+7.4

21 .2_+2.3

27.7+2.0

27.1_+I .8

mean 25.3+4.3

W

WW I S.D. - counting error. I S.D. - total error.

nate from the atmospheric nuclear tests. The majority

of plutonium has its origin in the Chernobyl accident

and it is present probably as a component of dispersed

fuel p a r t i c l e s 6.

Approximately 30% of plutonium in soil behaves dur-

ing the leaching with conc. HNO 3 as if the plutonium

were present in soil in the form of PuO 2, which is

188

HOLGYE, BUR~[K: Pu FROM CHERNOBYL ACCIDENT

hardly soluble in nitric, hydrochloric and sulfuric

acid. The dissolution of PuO 2 in these acids decreases

with increasing temperature of its formation 4'7'8. Con-

version of PuO 2 to the form soluble in acids on treat-

ment with hydrofluoric acid leads, according to some

authors 4, to erroneous results. The source of the error

is probably the incomplete dissolution of PuO 2, and the

incomplete destruction of siliceous matrix, which sub-

sequently causes incomplete isotopic exchange between

plutonium present in the sample and that added as the

chemical yield monitor. The procedures described for

the extraction of plutonium, however, differ signifi-

cantly with regard to the amount of conc. HF used for

I g of soil, the mode and time of treatment, the use of

other acids, etc. It can be expected that the use of

more consistent treatment with HF leads to better con-

version of PuO 2 to acid soluble form. Sill 4 has de-

scribed that on the evaporation of 10 g soil containing

PuO 2 first with 50 ml conc. HNO 3, then with 50 ml conc.

HF and subsequently with 50 ml 8M HNO3, and leaching

plutonium from the residue with dilute nitric acid con-

taining boric acid, complete isotope exchange between

plutonium present in the sample and that added as the

yield monitor is attained. Complete equilibration be-

tween the plutonium in soil in form of PuO 2 and that

used as yield monitor on the extraction procedure simi-

lar to that applied in the 3rd case of extraction was 9 found recently in our laboratory

PuO 2 for the experiments mentioned 4'9 was prepared

at 1000 ~ The correspondence of mean concentration of

239'240pu in soil in the 3rd and 4th extraction proce-

dures indicates that the conversion of PuO 2 originating

from the Chernobyl accident (its formation as a volatile

product of release 6 takes place probably at far higher

189

HOLGYE, BUR~K: Pu FROM CHERNOBYL ACCIDENT

temperatures) to acid soluble form occurs in the 3rd

extraction procedure, too. Owing to the small number of

parallel determinations and only one site of sampling,

the validity of this conclusion is in our opinion lim-

ited to the PuO 2 present in the soil sample obtained.

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2. K.M. Wong, Anal. Chim. Acta, 56 (1971) 355.

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190