Reload Index (ZRiChK UMCS)

STUDIES ON THE DETERMINATION OF VERTICAL MIGRATION RATE OF BETA-EMITTING 241Pu IN SOIL PROFILES

 

Andrzej KOMOSA, Stanisław CHIBOWSKI, Jolanta ORZEŁ

DEPARTMENT OF RADIOCHEMISTRY AND COLLOID CHEMISTRY

 

 

A major source of plutonium isotopes in the environment was atmospheric nuclear weapon testing which caused dispersion, among other isotopes, large amount of plutonium. A disintegration of nuclear powered satellite in 1964 and Chernobyl accident in 1986 had also a global influence. Alpha emitting plutonium isotopes were intensively studied and their concentration in many places of various components of the environment is known. Among plutonium nuclides 241Pu is the most interesting one. It is almost pure beta radiation emitter of weak energy (Emax = 21 keV), which decays rather quickly (T1/2 = 14.4 yr), producing an alpha radiating 241Am of a long half-life (T1/2 = 432.2 yr) of high radiotoxicity. Therefore, despite its weak radiation, the 241Pu causes a significant hazard to human health. Total amount of this isotope introduced into the environment was about 2·1017Bq, what was several times higher than alpha radiating plutonium. The low beta emission energy of 241Pu makes significant problems in determination of its radioactivity, especially in environmental samples. The investigation of such samples demands the intense radiochemical treatment, leading to separation of plutonium from sample matrix in a pure form, suitable for radioactivity measurement. Low beta radioactivity can be measured directly with excellent result, using the ultra-low-level liquid scintillation spectrometer of Wallac, Quantulus. However, there are not many data in the literature concerning measured concentration of 241Pu in the environment.

The aim of presented investigation was to determine concentration of 241Pu in particular layers of soil profile and calculate a vertical migration rate of this nuclide. The samples taken from uncultivated meadows of alluvial valleys of Bug and Wieprz rivers were a subject of this study. The samples were collected from soil profiles at four points: near Dęblin and Serock (Wieprz valley), and near Zosin and Terespol (Bug valley).

            In the previous work the procedures of radiochemical separation of plutonium for further measuring of 241Pu radioactivity with liquid scintillation spectrometry were elaborated and discussed [1, 2]. In the case of soil samples the most adequate procedure for determination of 241Pu involved separation of plutonium and alpha-spectrometric measurement of electrodeposited source followed by washing the plutonium out from the plate. Next, the plutonium was extracted by means of TOPO solution and transferred to scintillation cocktail for LSC spectrometric. Determination of alpha radiation emitting plutonium was performed spectrometrically after appropriate radiochemical separation that included hydrochloric acid leaching, co-precipitation with ferric hydroxide and calcium oxalate, anion exchange separation, and electroplating. The 242Pu standard solution (AEA Fuel Services, UK) was used as a radiochemical yield monitor.

            Two types of spectrometers were used for radioactivity measurements. One of them was the Canberra alpha spectrometer (equipped with PIPS silicon detector) connected with multichannel analyzer S-100 and Genie-2000 software, which enabled to determine 238Pu and 239+240Pu separately. The second spectrometer was the ultra low-level liquid scintillation spectrometer Quantulus (Wallac) used for measuring the sum of alpha radioactivity (channels 600-805) and the beta activity of 241Pu (channels 2-265).

The radioactivity of 241Pu [Bq/kg] was calculated according the equation:

 

where:

NLSCb = LSC count rate in a 2-265 channel region [cpm],

NSPa = total alpha count rate by alpha spectrometry [cpm],

A242 = activity of the tracer added [Bq],

NLSCa = LSC count rate in the 600-805 channel region [cpm],

m = mass of the sample [kg],

ELSC = LSC counting efficiency calculated using the quenching curve for tritium = 47%, corrected for plutonium,

N242SPα = count rate of the tracer by alpha spectrometry,

 

Obtained results of actual 241Pu activity (determined in 5-cm layers of soil profiles) were recalculated to the date of Chernobyl incident (1986) and to the date of maximum global fallout (1963). Both events were possible sources of the 241Pu. Rate of vertical transport of 241Pu was calculated using the compartment migration model. The results are shown in Table 1. It can be seen that 241Pu from Chernobyl fallout migrate about two times faster than the global one.

 

Table 1. Average values of vertical migration rate [cm/year] of 241Pu in four soil profiles of Wieprz and Bug river valleys, calculated as Chernobyl or global fallout.

 

Sample

Arithmetic mean ± 1σ

[cm/year]

Chernobyl fallout

Global fallout

Profile 1 Dęblin

0.89 ± 0.17

0.44 ± 0.17

Profile 2 Serock

0.73 ± 0.28

0.37 ± 0.21

Profile 3 Zosin

1.04 ± 0.17

0.48 ± 0.13

Profile 4 Terespol

1.32 ± 0.68

0.60 ± 0.24

 

References:

 

[1] A. Komosa, Fizykochemiczne problemy oznaczania i zachowanie się izotopów plutonu w środowisku z uwzględnieniem beta-promieniotwórczego 241Pu, Wyd. UMCS, Lublin 2003, str.178+XII.

[2] A. Komosa, in: LSC 2001, Advances in Liquid Scintillation Spectrometry, S. Möbius, J. Noakes and F. Schönhofer (Eds.), Radiocarbon, Tucson 2002, 363.

 

(Grant KBN 4T09D 05824)