Determination of 63Ni and 55Fe in nuclear waste samples using radiochemical separation and liquid scintillation counting
Introduction
In decommissioning of nuclear facilities, the radioactivity inventory of various materials needs to be assessed for the classification and repository of the nuclear waste. A part from the reactor cores, the main radioactivity in the reactor comes from the graphite and the construction materials. The radioactivity of the irradiated reactor graphite comes from many nuclides such as 3H, 14C, 36Cl, 41Ca, 55Fe, 60Co, 63Ni, 90Sr, 133Ba, 137Cs, 152Eu, 154Eu and some transuranics. In the concrete shield, the main radionuclides include 3H, 14C, 36Cl, 41Ca, 55Fe, 60Co, 63Ni, 133Ba, 152Eu, and in the metal materials, e.g. aluminum alloy, steel and lead, 55Fe, 63Ni, 60Co and 152Eu are the main sources of radioactivity [1]. Of these radionuclides, the gamma emitters such as 60Co, 152Eu, 154Eu, 133Ba, and 137Cs can be easily determined by gamma spectrometry. However, the determination of various beta and alpha emitters such as 3H, 14C, 36Cl, 41Ca, 63Ni, 55Fe, 90Sr and the transuranics requires complete separation of the individual radionuclides from the matrix and from the other radionuclides due to poor energy resolution of beta spectroscopy and the high self-absorption of alpha particles in the samples. A rapid analytical method for 3H and 14C in graphite and concrete has been developed [2]. This work aims to develop an accurate, sensitive and simple radiochemical analytical method for the determination of 63Ni and 55Fe in different construction materials.
Both 63Ni and 55Fe are neutron activation products. 63Ni is produced by two neutron reactions with Ni and Cu: 62Ni(n, γ)63Ni, 63Cu(n, p)63Ni, and 63Ni mainly exists in steel materials because of the high concentration of Ni in these materials. Other reactor materials such as graphite, concrete, lead, and Al alloy also contain 63Ni, because trace amounts of Ni and Cu always exist in these materials. 63Ni is a pure beta emitting radionuclide with maximum beta energy of 66.95 keV and half-life of 100.1 years. It is therefore an important radionuclide in the view of nuclear waste repository. Due to its low beta energy, the measurement by windowless gas flow proportional counter or ion implanted silicon detector [3], [4], [5] gives low counting efficiencies (2.6–20%). Liquid scintillation counting (LSC), which has a high counting efficiency, especially for low energy beta emitters, is therefore suitable for the determination of 63Ni. In this work, LSC is used for the detection of 63Ni. Since nuclear waste contains many different radionuclides, and the radioactivity concentration of some of the radionuclides, such as 152Eu, 154Eu, 60Co, 55Fe and 133Ba were very high in the first few years after reactor operation has stopped, a chemical separation procedure of 63Ni from all other radionuclides with a high decontamination factor is required. Many chemical separation procedures for the determination of 63Ni have been reported previously. The procedures are mainly based on the formation of a complex of Ni with dimethylgloxime (DMG), and precipitation or organic extraction of the Ni–DMG complex. Chelation and anion chromatography as well as hydroxide precipitation using ammonium hydroxide have also been used for the chemical separation of Ni [3], [4], [5], [6], [7], [8], [9], [10], [11]. However, anion or chelation chromatography alone cannot separate Ni from isotopes of Cr, Cs, Sr, Eu and other rare earth elements, and therefore it has to be combined with other separation methods. In this work, a new procedure is developed that combines hydroxide precipitation, anion exchange and extraction chromatography yielding an effective separation of Ni from the matrix and interfering radionuclides.
55Fe is produced by neutron activation reactions of two major stable iron isotopes: 54Fe(n, γ)55Fe and 56Fe(n, 2n)55Fe. Since iron is the main component of steel, and the concentration of iron is relatively high in many reactor materials, 55Fe is a main contributor to the radioactivity of nuclear waste in the first few years after reactor operation has stopped. 55Fe (t1/2 = 2.7 years) decays via electron capture to stable 55Mn with the emission of Auger electrons and low energy X-rays (5.89 keV, 16.9%). Measurement of 55Fe can be carried out by low energy gamma and X-ray detector or gas flow proportional counter, but their counting efficiencies are normally very low (<1%). The most common and more sensitive technique is liquid scintillation counting. Due to very low energy of the Auger electrons emitted from 55Fe, Fe has to be separated completely from other radionuclides before the counting. Chelating and anion exchange chromatography combined with solvent extraction has been used to separate Fe from other nuclides [12], [13]. The common used extraction reagents are ethyl acetate, isobutyl ketone or isopropyl ether, in which iron has a high partition coefficient in ethyl acetate [14]. However, the solvent extraction is not very specific for Fe, and therefore the decontamination of other metals is not satisfactory. To improve the purification of Fe, an extraction chromatography based on di-isobutyl-keton has been developed [14]. In this work, a simple procedure based on anion exchange and hydroxide precipitation is investigated for the simultaneous determination of 55Fe and 63Ni.
Due to the large volumes of graphite and heavy concrete, these two materials comprise considerable amounts of low-medium radioactive waste. Calculations of radionuclide inventories in DR-2 have shown that graphite was the main source of radioactivity at time of dismantling [15].
The reported analytical methods for the determination of 55Fe and 63Ni have mainly focused on the metal materials of the reactor [8], the sludge [5], and the biological and environmental samples such as algae, soil, sediment, and biota [3], [4], [6], [10], [13], [16], [17]. Few methods for the determination of 55Fe and 63Ni in graphite and heavy concrete have been reported [21]. Graphite is normally difficult to dissolve, because graphite is highly resistant to high temperatures as well as concentrated acids and bases. Heavy concrete (barites) is normally used in nuclear facilities as a shielding material due to high absorption of gamma radiation. It mainly consists of Ba2SO4 and silicates, and standard acid digestion with HF cannot be used for the decomposition of heavy concrete. In this work a method for the decomposition of graphite and concrete is investigated for the determination of 55Fe and 63Ni. In addition, method for the determination 55Fe and 63Ni in lead and aluminium used in a nuclear reactor is also investigated.
Section snippets
Equipment and chemicals
The Quantulus™ 1220 liquid scintillation counter is from PerkinElmer Inc. (PerkinElmer Inc., Wallac Oy, P.O. Box 10, FIN-20101 Turku, Finland). A high purity germanium detector with Genie 2000 gamma spectroscopy analysis software (Canberra Industries, USA) was used to acquire and analyse gamma spectra. Inductively coupled plasma optical emission spectrometry (ICP–OES, Varian Vista Pro, Varian Inc., Palo Alto, CA 94304-1030, USA) is used for the determination of stable Ni and Fe. Ultima Gold LLT
Results and discussion
The distribution of 55Fe and 63Ni in two concrete cores is shown in Fig. 3. The radioactivity was decay corrected to the shutdown date of the reactor DR-2 in 1975. It can be seen that the concentration of 55Fe was originally much high than the concentration of 63Ni in the concrete. However, at the present time the radioactivity of 55Fe is 5–10 times lower than that of 63Ni due to the rapid decay of 55Fe(t1/2 = 2.7 years). The average concentrations of stable Fe and Ni in the concrete samples from
Conclusion
A variety of decomposition and separation procedures were investigated for the separation and determination of 55Fe and 63Ni in graphite, heavy concrete, lead and aluminum alloy dismantled from a nuclear reactor for decommissioning. Acid leaching with acqua regia can release more than 92% of Fe and Ni from heavy concrete, but less than 80% of Fe and Ni from graphite. An ashing at 800 °C for less than 3 h should be used for the decomposition of graphite sample. In this case more than 96% of Fe and
Acknowledgements
This work was supported by Danish Decommissioning. The samples were collected and prepared by Dr. Knud Brodersen and Dr. Anne Sørensen. Mr. Steen M. Carugati gave a great support to this work. The authors appreciate their help.
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