Plastic Scintillators: a Powerful Tool to Reduce Mixed Waste

Wastes containing radioactive and organic compounds (mixed wastes) are difficult to dispose because of the regulations established for nuclear and hazardous wastes. Mixed wastes originate mainly in the emulsions generated in beta emitter determinations by Liquid Scintillation techniques. The use of plastic scintillators instead of liquid cocktails may facilitate the segregation, after measurement, of sample and scintillator without introducing additional wastes in the measurement step. In this study, we compare the capability of Plastic Scintillation (PS) versus Liquid Scintillation (LS) and Cerenkov (C) techniques to determine beta emitters in routine measurements. Results obtained show that high and medium energy beta emitters (Sr-90/Y-90 and C-14) can be quantified in aqueous samples by using PS with similar relative errors (< 5%) as those obtained by LS or C, for any activity level considered. For low energy emitters (H-3), best results using PS are achieved for medium activity levels. Additionally, measurements performed in solutions including alpha (Pu-238) and beta-gamma (Cs-134) emitters confirm the capability of PS to extent the application of this technique to the determination of these types of isotopes. INTRODUCTION The wastes produced by nuclear activities are classified according to their chemical and radionuclide composition. Among them, those containing radioactive and organic compounds (mixed wastes) are difficult to dispose because of the regulations established for nuclear and hazardous wastes [1, 2]. Although their activity can be low, the large amount of mixed wastes makes them difficult to manage. Mixed wastes originate in the manufacture of labelled chemicals [3] and, mainly, in the emulsions generated in beta emitter determinations by Liquid Scintillation techniques in biomedical [4] and environmental studies [5]. In these emulsions, the waste results from mixing the organic liquid scintillation cocktails and aqueous radioactive samples [6, 7]. The use of plastic scintillators instead of liquid cocktails may facilitate the segregation, after the measurement, of sample and scintillator without introducing additional wastes in the measurement step Plastic scintillation (PS), which was first used in 1960 [8, 9], has always been considered a very versatile technique, due to the large number of polymers and shapes that could be synthesized [10]. However, the poor results obtained in its first analytical applications and its good response as a large surface area cosmic particle detector prevented it being widely used for determining beta emitters activity in routine measurements. New plastic scintillators not only overcome some of these drawbacks, but also introduce new possibilities for beta radionuclide quantification. The development of plastic scintillator beads and fibres [11] has led to the design of analytical WM2008 Conference, February 24-28, 2008, Phoenix, AZ Abstract #8225 technology able to determine beta emitter activity by continuous flow [12], remote [13, 14] and routine measurements [15, 16], all of which achieve high detection efficiencies for high-energy beta emitters. The aim of our work is to present Plastic Scintillators as a real alternative to the classical techniques to determine beta emitters: Liquid Scintillation and Cerenkov [17]. The performances of the three methods were compared for the determination of a beta emitter Sr-90/Y-90 in secular equilibrium, a low-energy beta emitter, H-3, an alpha emitter, Pu-238, and a beta + gamma emitter, Cs-134. The capability of plastic scintillation beads and polyethylene vials as a routine technique to determine the activity of three of the most important beta emitters, (Sr-90/Y-90 (high energy), C14 (medium energy) and H-3 (low energy)), in low level aqueous spiked samples was evaluated. Finally, in order to reuse the beads, we establish a cleaning procedure for plastic scintillation beads. This article summarises the content included in the references [18, 19, 20 and 21].