X-RAY PHOTOELECTRON STUDY OF CRYSTALLINE NEW FORMATIONS OVER MELTED FUEL-CONTAINING MASSES FROM THE 4TH BLOCK OF THE CHERNOBYL NUCLEAR-POWER-PLANT

Three samples of ''new formations'' I-III, i.e., yellow outgrowths tha t overlaid the melted fuel-containing masses of the fourth block of th e Chernobyl NPP in late 1990, were studied by X-ray photoelectron spec troscopy. Quantitative elemental analysis yielded the following relati ve elemental compositions for these samples: U1Na69S9K1.5C11O88 (I), U 1Na27S1C9O47 (II), and U1Na10K1.5C6O39 (III). Analysis of the fine str ucture parameters of spectra of valence and core electrons showed that uranium occurs in the samples as U(VI), with the uranyl group UO22+ a s the main structural unit. The studied samples of ''new formations'' are shown to be essentially the hydrate of the double uranyl salt Na4U O2(CO3)(3) probably mixed with Na2CO3, Na2SO4, and NaOH impurities, wi th Na atoms partially (n(K) < 2.5 at. %) substituted by K atoms. The s amples have the following stoichiometric compositions: Na4UO2(CO3)(3) . 8Na(2)CO(3) . 9Na(2)SO(4) . 31NaOH . nH(2)O (I), Na4UO2(CO3)(3) . 6N a(2)CO(3) . Na2SO4 . 9NaOH . nH(2)O (II), and Na4UO2(CO3)(3) . 3Na(2)C O(3) . nH(2)O (III). A structure of the nearest surrounding of the U6 ion is proposed, and the lengths of bonds between the U6+ ions and ox ygens in the uranyl group and ligands (oxygens) in the equatorial plan e RU-O are estimated at 0.174 and 0.239-0.260 nm, respectively. The '' new formations'' are enriched in uranium as compared to the ''lavas'' (19.3 and 4.0 wt %, respectively). It is noted that under ambient cond itions the melted fuel-containing masses from the fourth block of the Chernobyl NPP form hydrothermal flows containing soluble uranyl specie s which crystallyze out at certain sites (crystallization sites) as do uble uranyl salts, which can indefinitely grow in size.