Determination of Pu 238-242 and Am 241 contents by liquid scintillation spectrometry in samples of Chernobyl origin

Nuclear accidents cause release of not only fission products, but also activation products of fuel nuclei. Of these, plutonium isotopes occupy a special place because they are the main contributions to the mass of activation products of fuel nuclei. The most toxic from the ecological point of view are alpha-emitting isotopes Pu 238, 239, 240, 242 whose ingress into the human body causes malignant tumours. Therefore, the determination of alpha-emitting isotopes of plutonium in radioactive fallout is a priority task, although the specific activity of the beta-emitting isotope Pu 241 is much higher than in the above alpha-emitting isotopes. Technically, the determination content of alpha-emitting plutonium isotopes is a complicated task, especially in the cases where determination of not only the total activity, but activities of individual isotopes as well, is needed. The situation is less complicated when we speak of accident reactor fallout. In this case, theory permits making reliable estimates of masses of plutonium isotopes accumulated in the reactor core depending on the burn up depth W characterizing the amount of fission nuclei in 1 kg of fuel. Moreover, is can be shown that for the fuel of the Chernobyl RBMK reactor at burn up depths of 7 MW day/kg and more the ratio of Pu 241 beta-activity to the total alpha-activity of the other plutonium isotopes p=3DA(β)/A(α) is almost constant and equal to 50 with an accuracy of 15 %. This result practically coincides with analogous estimates obtained in other work for given values of the average fuel burnup depth in the reactor core: 54,6 (=3D9,5), 54,5 (=3D10,9), 59,4 (=3D12,8), 56,6, 56,1. The obtained activity ratio p=3DA(β)/A(α) can be also practical importance. The beta-decay energy of Pu 241 is 21 KeV, so its activity can be measured by beta-radiometers designed for measuring tritium activity. Measurement off Pu 241 beta-activity of the order of 5 Bq (an easy task from the technical viewpoint) will correspond to measurement of the total activity of the other plutonium isotopes of the order of 0,1 Bq (a far more complicated task from the technical point of view). Simultaneously, it the average burnup depth is known, it is easy to calculate also the contribution of each alpha-emitting plutonium isotope. For instance, according to our estimates, at =3D 10.9 the activity ratio A(Pu 238):A(Pu 239): A(Pu 240) =3D 1:1:1.6. In turn, it is not difficult to determine the average burn up depth. This can be done on the basis of measurement data on the activity ratio, for example, A(Cs 134)/A(Cs 137) and A(Ru 103)/A(Ru 106). For Chernobyl samples varies from 8 to 13. Determination of Pu 241 content (activity) permits solving the Am 241 problem as well. By the moment of the accident in the reactor core of the Chernobyl NPP RBMK 1,48 kg of Pu 238, 412,7 kg of Pu 239, 176 kg of Pu 240, 49,1 kg of Pu 241, 14,2 kg Pu 242 and 1,08 kg of Am 241 hat been accumulated. The accident nuclear fallout of Chernobyl origin should also have an analogous relation between the masses of plutonium isotopes. Since the half-life of Pu 241 is relatively small (T1/2 =3D 13,2 years), the Am 241 isotope resulting from the beta-decay of Pu 241 will soon exceed in mass the Am 241 isotope accumulated in the accident reactor core. For example, to date, 13 years after the Chernobyl accident due to the beta-decay of Pu 241 24,3 kg of Am 241 have been formed, which is 22,5 times more than the mass of Am 241 accumulated in the reactor core. It can easily be shown that the alpha-activity of the Am 241 isotope resulting from Pu 241 is equal approximately to the total activity of Pu 238, 239, 240, 242 isotopes. In other words, due to the beta-decay of Pu 241 we have now another 'Chernobyl' as to alpha-emitters. And in prospect the activity of alpha-emitters should be trebled due to the Pu 241 decay. Therefore, control over the content of Pu 241 is also important for the general evaluation of contamination of areas with alpha-emitting radionuclides. The obtaining of such information is quite possible. Consider one of the variants of practical implementation of this idea. The Laboratory of nuclear spectroscopy of the Institute of Physic has a beta-radiometer 'Beta-2' designed for measuring tritium and C 14 activities. For this device method have been developed and certificated for quick analysis of Sr 90 content on the basis of strontium extraction by means of dicyclohexyl-18-crown-6. Therefore, it this device turns out to be suitable for measuring Pu 241 activity as well, a possibility will appear to determine in samples the content of isotopes of plutonium and strontium, ecologically most dangerous nuclides. This problem is most easily solved by the method of liquid scintillation spectrometry. It is easy to separate pulses from beta-particles of Pu 241 and alpha-particles of the other plutonium isotopes. To do this, it is necessary to have Pu 241 standards with activities from fractions of Bq to hundreds and thousands of Bq. But Pu 241 standards are not available. It is therefore necessary to use plutonium standards containing all the above listed isotopes: Pu 238-242. It the content of individual plutonium isotopes in the standards is known, one can try to establish experimentally the dependence between the particle counting rate in the tritium channel and the Pu 241 concentration in the sample being analyzed. Analysis of the spectra was carried out by means of the base software GENIE-2000 and the program of interactive adjustment of peaks. The Pu 241 activity was determined on the analytical line of 148.57 keV. As samples, plutonium extracted radiochemically from old depleted fuel assemblies was used