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STUDY ON ANALYTICAL METHODS FOR BETA RADIATING 241Pu DETERMINATION IN ENVIRONMENTAL SAMPLES

 

Andrzej KOMOSA

DEPARTMENT OF RADIOCHEMISTRY AND COLLOID CHEMISTRY

MARIA CURIE-SKLODOWSKA UNIVERSITY

 

 

Among plutonium isotopes introduced into the environment as a result of nuclear weapon tests and Chernobyl accident the 241Pu is the most interesting one. It is almost pure beta radiation emitter of weak energy (Emax = 21 keV) which decays rather quickly (T½ = 14.4 years). Global inventory of 241Pu is about 1.751017 Bq (together with the Chernobyl releases), what exceeds more than 10 times the amount of alpha emitting plutonium nuclides. Despite of its weak radiation the 241Pu causes a significant hazard to human health decaying to alpha emitting 241Am of a long half-life and a high radiotoxicity. The amount of americium, ingrowing successively during the plutonium decay, can be estimated from the known concentration of 241Pu in the environment. Unfortunately, a low energy of beta radiation makes significant problems in the determination of 241Pu activity. Investigation of environmental samples demands an intense radiochemical treatment. A suitable liquid scintillation spectrometer, which enables to measure a beta activity of low energy, is also necessary.

The aim of this work is to create and test a new radiochemical method of plutonium separation for 241Pu determination in environmental soil samples of various origins.

An alpha spectrometer (Canberra 7401) was used for alpha emitting plutonium determination (what is necessary for yield determination in other measurements) and a standard liquid scintillation counter - LSC (Beckman 5000TD) for beta plutonium measurement.

The following procedures for 241Pu determination were established and investigated:

Procedure (I), which consisted in plutonium separation and alpha-spectrometric measurement followed by washing out of plutonium from the plate by means of 8M HNO3. After evaporation to dryness extraction of plutonium with the TOPO solution (from 4M HNO3) was performed, and organic phase was transferred to scintillation vial for LSC measurement.

Procedure (II), a simplified one, consisted in pre-concentration of plutonium by leaching, co-precipitation with ferric hydroxide and calcium oxalate (this is a part of plutonium separation procedure for alpha spectrometry) followed by liquid extraction with TOPO from 4M HNO3, washing of organic phase with 4M HNO3, then 6M HCl, stripping of plutonium using 6M HCl / 0.1M NH4I, evaporation and re-extracting to the TOPO phase. The organic phase is transferred into scintillation cocktail and measured by LSC.

Procedure (III) based on division of the solution after leaching with 6M HCl into two fractions. In the first one, plutonium activity was measured (after radiochemical separation) by means of the alpha spectrometry. In the second one, plutonium was separated according to procedure (II) and its activity determined by LSC.

In procedures (I) and (III) a radiochemical separation yield was calculated from the amount of 242Pu added as a tracer. Procedure (II) was applied to samples of known concentration of alpha emitting plutonium nuclides, determined earlier. Thus, the yield was calculated with regard to the total alpha activity.

The experiments with soil samples, influenced by Chernobyl (Bragin, Byelorussia) and global fallout (Lublin, Poland), as well as being a result of weapon testing (IAEA-384, French Polynesia) were also performed. Determined concentration of 241Pu ranged from 0.90.5 Bq/kg in the case of Lublin soil to 4510 Bq/kg in IAEA-384 sample, and 940300 Bq/kg in Bragin soil.

Procedures both (I) and (II) show the comparable result with samples of high activity of beta radiating plutonium. Uncertainty in activity determination is relatively high as a result of error propagation during multistage processing of the sample. Inhomogenity of the sample material is not meaningless. In Chernobyl influenced samples the ratio of 241Pu to 239,240Pu ranged from 40 to 60, which is consistent with reported literature data. The samples of much less radioactivity level of beta plutonium (French Polynesia) analyzed according to procedure (I) showed better reproducibility and higher yields than did by procedure (II). Obtained mean value of 241Pu concentration (4510 Bq/kg) was slightly lower than the information value reported by IAEA equals to 66 Bq/kg (with confidence interval 48-188 Bq/kg). Concentration of beta radiating plutonium in the grassland soil from Lublin region ranges from 0.2 to 1.6 Bq/kg, and in forest soil is about 5.5 Bq/kg. Soil samples influenced by Chernobyl fallout, taken from deeper part of the profile revealed the same level of radioactivity as samples of Lublin region. What means, that plutonium in deeper layer of soil was rather under the influence of the global fallout.

Three investigated procedures applied to samples of relatively high 241Pu concentration give comparable result, which are spread within 20%. Procedure (III) shows however the lowest mean value of 241Pu activity whereas procedure (II) the highest. Results obtained with procedure (II) are characterized by the lowest relative uncertainty. In the case of samples, which reveal lower concentration of 241Pu, only procedure (I) gives an acceptable result.

Validation of studied methods of 241Pu determination was confirmed by taking part in the Proficiency Test measurements (PT002) organized by IAEA.

The method is in the course of improvement by application of a new low-level liquid scintillation spectrometer Quantulus (Wallac, Perkin-Elmer). This enables to measure extremely low beta radioactivity with low uncertainty of results.