Reload Index (ZRiChK UMCS)

 

 

STUDY ON METHOD OF PLUTONIUM ISOTOPES DETERMINATION IN AEROSOLS AND DUST COLLECTED ON AIR FILTERS BY THE ASS-500 STATION

 

ST. CHIBOWSKI, A. KOMOSA, J. ORZEŁ

DEPARTMENT OF RADIOCHEMISTRY AND COLLOID CHEMISTRY

 

            Plutonium isotopes presence in the environment are the result of global fallout caused by testing of nuclear weapons in the atmosphere. Chernobyl incident had also a small contribution. These isotopes emits alpha radiation, which is highly hazardous due to its big ionization ability. At present a main source of plutonium isotopes entering the human body is the surface soil. Because a transfer factor of plutonium radionuclides from soil to plants is very small, a human diet contains only negligible amount of these isotopes. The largest fraction of plutonium, cumulating in the body is inhaled together with the air, which bears small particles of eroded soil.

            In Poland monitoring of gamma emitting isotopes concentration in the air is carrying on by using of ten ASS-500 stations. They collect aerosols and dust from a ground layer air continuously [1]. The filters are changed weekly and stored after gamma measurements. These filters were subjected to radiochemical treatment with the aim to separate of plutonium isotopes for alpha-spectrometric determination. The base material of the Petrianov filter is a chlorinated polyvinyl chloride. This material liberates an aggressive, gaseous HCl during thermal mineralization, a typical stage in plutonium separation procedures. This is why a direct method of acid leaching of the comminuted filter was applied instaed of ashing. Concentrated acids: HCl, HNO3, HF, and H2O2, and their mixtures were used. The aim of the study was to find the efficient method for leaching and separation of plutonium isotopes from such kind of difficult matrix as the Petrianov filter at a small level of plutonium concentration.

            Stored filters from a period of Oct. ’93 to Oct.’94 were used as a testing samples. The best radiochemical yield of separation from the matrix was found, when HCl (1:1) was used as a leaching agent. It was observed, that a simplified procedure of plutonium separation (i.e. co-precipitation with ferric hydroxide and anion exchange) led to no complete removing of interfering radionuclides. A final separation product was contaminated with 210Po. This isotope is a daughter of 210Pb, which is formed by decay of 222Rn, present in the air. The 210Pb contaminates also a leaded petrol. A presence of relatively high polonium activity makes difficult quantitative determination of plutonium isotopes, because of close energy of alpha radiation emitted. Hence, a typical method of polonium isolation from the solution was tested [2]. It is based on spontaneous deposition of polonium on copper or silver plate from hydrochloric acid solution. Nevertheless, if this method is applied, further separation of plutonium from that solution was unsuccessful. On the other hand, repeating of an anion exchange separation step markedly diminishes the yield of plutonium.

            The best results of separation were obtained when a calcium oxalate co-precipitation stage was added to the plutonium procedure.

            The preliminary results of plutonium determination in the air filters are presented in Table 1. Every filter sample contains about 3 ÷ 5 grams of a dust, what corresponds to 50.000 ÷ 80.000 m3 of air passed through the filter. Presented results are of the same level as found by other authors in the ground level air [3].

 

Table 1. Preliminary results of plutonium isotope determination in air filters.

 

Sample No.

239,240Pu

[mBq/g of dust]

239,240Pu

[nBq/m3]

Yield

[%]

7+8+9

0.47 ± 0.02

23

67

10

0.22 ± 0.07

9

21

11

0.02 ± 0.03

1

30

12

0.57 ± 0.05

57

35

13

0.10 ± 0.03

5

41

 

References:

[1]. J. Jagielak, A. ¯ak, M. Biernacka, K. Isajenko, P. Lipiñski, Raport CLOR, No. 132, Warszawa 1998.

[2]. B. Skwarzec, Anal. Chem., 42 (1997) 107.

[3]. A. Pietruszewski, R. Bojanowski, Proc. Int. Symp. on Post-Chernobyl Environmental Radioactivity Studies in East European Countries, Kazimierz 1990, 118.