Reload Index (ZRiChK UMCS)

 

 

STUDY ON DETERMINATION METHOD OF BETA EMITTING PLUTONIUM-241 IN ENVIRONMENTAL SAMPLES

 

ANDRZEJ KOMOSA, MARTA TYMCIO

DEPT. OF RADIOCHEMISTRY AND COLLOID CHEMISTRY

 

            A major source of plutonium isotopes were atmospheric nuclear weapon tests, which caused rather uniform dispersion of about 1.3·1016 Bq 239,240Pu, 3·1014 Bq 238Pu and 1.7·1017 Bq 241Pu, mostly above the Northern hemisphere. This uniformity was disturbed by additional amount of 238Pu (6.3·1014 Bq) being a result of disintegration of a nuclear powered satellite, and by Chernobyl releases. This latter accident caused introducing to the atmosphere about 7.2·1013 Bq 239,240Pu, 3.5·1015 Bq 238Pu and 5.5·1015 Bq 241Pu [1].

            Among the plutonium isotopes the 241Pu has different properties. It decays rather quickly in comparison with all other plutonium isotopes (E½ = 14.4 years) emitting of a beta radiation of Emax = 21 keV. Global inventory of 241Pu is about 570 Bq/m2, and alpha emitting Pu about 43 Bq/m2. Despite of its weak radiation 241Pu causes a significant hazard to human health - it decays to alpha emitting 241Am of a long half-life (432.2 years) and high radio-toxicity. Unfortunately, weak energy of beta radiation makes significant problems in determination of 241Pu activity. Knowing the concentration of 241Pu in soil (which is a main source of human body contamination) one can estimates a concentration of 241Am, which ingrows successively during the plutonium decay.

            In the literature concerning determination of beta emitting plutonium there is a limited number of known methods and laboratories dealing with them [2-4].

            We continue the work on the beta-plutonium determination using solvent extraction with TOPO for separation, liquid scintillation counting (LSC) for beta activity determination and alpha spectrometry with silicon detector for alpha activity measuring. We have tested five different procedures for sample preparation before LSC measurement of 241Pu.

            The first procedure consisted in:

- a total separation of plutonium isotopes for alpha-spectrometric determination (acid leaching, co-precipitation with ferric hydroxide, next with calcium oxalate, anion exchange and electrodeposition on the support),

- washing out the radioactive material from a support and transferring to a solution (boiling with nitric acid),

- liquid extraction of plutonium by means of TOPO solution in cyclohexane,

- LSC measurement (organic phase mixed with liquid scintillation solution).

            The second procedure was more simplified and included only acid leaching, liquid extraction combined with washing of organic phase, back extraction of plutonium to water phase and repeated extraction to organic phase, next liquid scintillation counting.

            In the third method the initial separation step was more developed (a leaching step was followed by co-precipitation with iron hydroxide and calcium oxalate).

            The fourth method consisted in acid leaching and dividing of the solution obtained in two parts (at a ratio of 1:3). The smaller part was treated according to standard procedure for alpha isotope determination [5], the larger one - according to the third method.

            The fifth procedure was almost the same as the first one, but repeated alpha-spectrometric counting was the initial step. The samples underwent to this procedure were stored for about 2 years. Such treatment enable to calculate the beta plutonium activity from ingrowing of 241Am.

Soil and sediment samples from Lublin region and Byelorussia, both influenced by Chernobyl fallout, and French Polynesia sediment (affected by weapon testing) was used as a testing material. The obtained results of sample analysis are showed in Table 1. Activity of 241Pu was recalculated on the date of Chernobyl incident or, in the case of Polynesia sample, on the reference date (Aug., 1996). Combined uncertainty of 241Pu activity determination (at 1s level) is connected with a concentration value as well as with a kind of procedure used. In a case of strongly polluted soil the lower uncertainty was obtained with procedure 1st and 3rd. A main factor of uncertainty control at small concentrations is rather error of activity determination than a kind of procedure used.

 

Table 1. The 241Pu activity in soil and sediment samples measured with different separation procedures.

 

Kind of sample

No. of parallels

Procedure

241Pu ± 1s

[Bq/kg]

241Pu / 239,240Pu

Polynesia sediment

8

4

1st

3rd

44 ± 10

134 ± 131

0.4

0.5 ¸ 2.7

Byelorussia soil 1

1

3

6

6

1st

3rd

4th

5th

1150 ± 25

1060 ± 45

810 ± 295

940 ± 270

63

58

44

52

Byelorussia soil 2

2

1st

4.3 ± 2.8

7.4

Lublin soil

 (8 different samples)

8

1st

1.4 ± 0.3

6.3

Lublin soil

(organic soil)

2

1st

5.4 ± 0.5

3.4

References:

[1]. B.F. Myasoedov, F.I. Pavlotskaya, Analyst 114 (1989) 255.

[2]. Lee, Lee, Radiochim. Acta, 84 (1999) 177.

[3]. J.W. Mietelski, J. Dorda, B.Was, Appl.Radiat.Isotop. 51 (1999) 435.

[4]. M. Buzinny, I. Los’, N. Tsigankov, S. Soroka, Advances in Liquid Scintillation Spectrometry 1994, G.T. Cook (Ed), Radiocarbon 1996, p.97.

[5]. A. Komosa, Polish J. Environm. Studies, 7 (1998) 89.