E. B. Anderson

Secondary Uranium Minerals on the Surface of Chernobyl “Lava”

The formation of uranium minerals is still continuing in Chernobyl Unit No. 4. Yellow products of alteration that stain the surface of Chernobyl “lava” have been examined by SEM and X-ray diffraction methods. Secondary minerals of uranium identified are: UO 4 ·4H 2 O studtite; UO 3 ·2H 2 O epiianthinite; UO 2 ·CO 3 rutherfordine; also, Na 4 (UO 2 )(CO 3 ) 3 was identified together with the sodium carbonate phases Na 3 H(CO 3 ) 2 ·2H 2 O and Na 2 CO 3 ·H 2 O. These minerals formed due to the interaction between fuel-containing masses or “lava”, water and air. The matrices of the “lava” do not contain significant amounts of sodium. The source of sodium may be water that has penetrated into the “Sarcophagus”. All identified secondary minerals of uranium are highly unstable, and their continued formation can seriously endanger the radiological situation of the 4 th Unit.

Behavior of 238 Pu-Doped Ceramics Based on Cubic Zirconia and Pyrochlore under Radiation Damage

Crystalline ceramics based on durable actinide host phases such as cubic zirconia and titanate pyrochlore have been suggested for the immobilization of weapons grade plutonium and actinide wastes. Samples of crystalline ceramic based on the gadolinia-stabilized cubic zirconia, (Zr,Gd,Pu)O2, structure doped with 9.9 wt.% 238Pu were synthesized and characterized in comparison with samples of pyrochlore-based ceramic, (Ca,Gd,Hf,U,Pu)2Ti2O7, doped with 8.7 wt.% 238Pu. It was found that a resistance of cubic zirconia to self-irradiation is much higher than that of pyrochlore. At the cumulative dose l.lx1025alpha decays/m3, cubic zirconia retained its crystalline structure. No swelling or cracking were observed in the ceramic matrix. At the same cumulative dose the titanate pyrochlore became nearly amorphous and the density decreased by approximately 10 % in comparison with the initial, unaltered sample. Under self-irradiation, both ceramics demonstrated an increase of normalized Pu mass loss in deionized water depending on cumulative doses, but this increase is significantly greater for the pyrochlore-based ceramic.

The Behavior of Nuclear Fuel in First Days of the Chernobyl Accident

Various types of Chernobyl fuel containing masses named black “lava”, brown “lava”, porous “ceramic” and “hot” particles that formed during first days of the accident at the Chernobyl Nuclear Power Plant 4th Unit were studied by methods of optical and electron microscopy, microprobe and x-ray diffraction. Data about their chemical, phase and radionuclide composition are summarized. The products of interaction between fuel, zircaloy and concrete, produced under experiments in laboratory were examined for comparison with samples of Chernobyl “lava” and “hot” particles. The behavior of nuclear fuel in first days of the Chernobyl accident was a three-stage process. The first stage occurred before the moment of the Chernobyl explosion and was exceptionally short-lasting, perhaps, less than a few seconds. It was characterized by reaching a high temperature, ≥2600 °C, in the epicenter of accident and formation of a Zr-U-O melt in a local part of the core, which is estimated to be not more than 30% of whole core volume. The second stage lasted for about 6 days since the explosion, during which there was interaction between uranium products of the destroyed reactor: UOx, UOx with Zr, Zr-U-O, with the environment and silicate structural materials of the 4th Unit. The third stage, after 6 days involved the process of final formation of the radioactive silicate melt or Chernobyl “lava” at one of the sections of the destroyed 4th Unit. During this stage the melts lamination occurred, followed by a break-through of the “lava” reservoir on the 11 th day of the accident and penetration of the “lava” into space under the reactor.

Behavior of 238 Pu-Doped Cubic Zirconia under Self-Irradiation

To investigate the resistance of cubic zirconia to accelerated radiation damage, which simulates effects of long term storage, 238 Pu-doped polycrystalline samples of cubic zirconia, (Zr,Gd,Pu)O 2 , were obtained and studied using X-ray diffraction analysis (XRD), electron probe microanalysis (EPMA), optical and scanning electron microscopy (SEM), and modified MCC-1 static leach test. The ceramic material was characterized by the following chemical composition (from EPMA in wt.% element): Zr = 50.2, Gd = 15.4, Pu = 12.2. This corresponds to the estimated formula, Zr 0.79 Gd 0.14 Pu 0.07 O 1.99 . The content of 238 Pu estimated was approximately 9.9 wt.%. The XRD measurements were carried out after the following cumulative doses (in alpha decay/m 3 × 10 23 ): 3, 27, 62, 110, 134, 188, 234, and 277. Even after extremely high self-irradiation, cubic zirconia retained its crystalline structure. All XRD analyses showed no phases other than a cubic fluorite-type structure. The following results of normalized Pu mass loss ( NL , in g/m 2 , without correction for ceramic porosity) were obtained from static leach tests (in deionized water at 90°C for 28 days) for 4 cumulative doses (in alpha decay/m 3 × 10 23 ): The results obtained confirm the high resistance of cubic zirconia to self-irradiation. This allows us to consider zirconia-based ceramic as the universal material that is suitable for actinide transmutation and geological disposal.

INTERACTION OF UO2 AND ZIRCALOY DURING THE CHERNOBYL ACCIDENT

A summary of the results collected during the studies of the products of a chemical interaction between uranium oxide fuel and Zircaloy cladding in the Chernobyl accident is presented in this paper. The reaction products are mainly Zr-U-containing phases with different U/Zr ratio and are described on the basis of electron microprobe and X-ray diffraction (XRD) analyses. The Zr-U-bearing phases were discovered among the inclusions in different types of Chernobyl fuel-containing masses (”lava”) inside the destroyed 4th Unit and in hot particles collected up to 12 km from the 4th Unit along the West Plume. A correlation of data on the chemical composition and phase interrelations obtained in investigated samples with a phase diagram of Zr(O) - UO 2 shows, that a temperature >1900 °C was reached in a part of the core before the explosion. The detection of hot particle with segregated morphology points out that liquid immiscibility existed between U-rich and Zr-rich melts. This and other observations indicate that the core temperature locally was above 2400–2600°C.

General Classification of “Hot” Particles from the Nearest Chernobyl Contaminated Areas

The morphology and composition both chemical and radionuclide of the main types of the solid-phase hot particles formed following the accident on the Chernobyl NPP have been studied by SEM, electron microprobe and gamma-spectrometry methods. Differences in many isotopes including: {sup 106}Ru, {sup 134}Cs, {sup 137}Cs dependent upon the hot particle matrix chemical composition was observed. The classification of hot particles based upon the chemical composition of their matrices has been done. It includes three main types: (1) fuel particles with UO{sub x} matrix; (2) fuel-constructional particles with Zr-U-O matrix, (3) hot particles with metallic inclusions of Fe-Cr-Ni. Moreover, there are more rare types of hot particles with silicate or metal matrices. It was shown that only metallic inclusions of Fe-Cr-Ni are concentrators of {sup 106}Ru, which caused this nuclides assimilation in the molten stainless steel during the initial stages of the accident. Soils contamination of non-radioactive lead oxide particles in the Chernobyl NPP region were noticed. It was supposed that part of metallic lead, dropped from helicopters into burning reactor during first days of accident, was evaporated and oxidized accompanying solid oxide particles formation.

Principal Features of Chernobyl Hot Particles: Phase, Chemical and Radionuclide Compositions

The accident at the Chernobyl Nuclear Power Plant (ChNPP) 4th Unit on 26 April 1986 was accompanied by the destruction of a reactor core and the release of solid and gaseous radioactive products. As a result of the accident, a part solid radioactive materials of the 4th Unit was dispersed by the explosion. The hot particles released settled on the surface of soil hundreds kilometers from ChNPP in the Sweden [1], Germany [2,7], Poland [2,3,7], Belorussia [4] and in particular, Ukraine [5,6,8]. The size of hot particles vary from one to hundreds microns. The bulk radioactivity of a single particle based on the initial activity calculated for 26th April 1986 might differ by hundreds of kBq. While particle size tends to decrease with increasing distance from the 4th Unit, some relatively large particles of 100–300 micron in size were collected 12 km West of ChNPP. Phase, chemical and radionuclide compositions of hot particles are essentially heterogeneous [1–8]. We have suggested dividing all hot particles into two main groups: 1) fuel particles — with relatively homogeneous matrix consisted of uranium oxides, UO2+x; 2) fuel-constructional particles — with a complex chemical matrix and/or multi-phase composition that is a result of high-temperature interaction between nuclear fuel, (UO2+x), and cladding materials such as zircaloy and stainless steel composed of Fe-Gr-Ni. The temperature could have exceed 2600°C. In some places of Western Plume in Chernobyl region these particles achieve up to 40 % of all hot particles [8]. Radionuclide composition of hot particles depends on the chemical and phase composition of their matrices [1,2,7,8].

A Search for Optimal Forms for Solidifying High-Level Radioactive Waste which are Geologically Compatible with Granitic Host Rocks

Various wasteforms for immobilising high-level radioactive waste (HLW) are considered in this paper with respect to their geochemical compatibility with granites of the Nizhnekansky massif, in the Krasnoyarsk region of Russia. The geochemical system of this granitic rock is assumed to be at, or close to, equilibrium, having existed in this state for billions of years. Construction of a shaft- or well-type HLW disposal facility within the massif will disturb existing geochemical conditions, however, although such a perturbation may be insignificant or relatively quickly cancelled out if materials with chemical and thermodynamic behaviour similar to that of the host rock (granite) are used as wasteforms for HLW immobilisation.