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

Abstract

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 melt's 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.

Synthesis, characterisation and corrosion behaviour of simulant Chernobyl nuclear meltdown materials

“…During the meltdown of the reactor core, temperatures reached in excess of 1600°C, which caused the uranium nuclear fuel to melt with the zirconium cladding. 1,2 This molten material interacted with structural reactor building materials, including steel, concrete, serpentine and sand, forming~100 tons of a glasslike lava that was transported under its own weight to sub-reactor rooms, solidifying in large masses, such as the widely reported "elephants foot". 1,[3][4][5] Known as lava-like fuel-containing materials (LFCM), these highly crystalline, radioactive, glass-like slags, were found in two main forms, known as Brown and Black lavas.…”

“…1,2 This molten material interacted with structural reactor building materials, including steel, concrete, serpentine and sand, forming~100 tons of a glasslike lava that was transported under its own weight to sub-reactor rooms, solidifying in large masses, such as the widely reported "elephants foot". 1,[3][4][5] Known as lava-like fuel-containing materials (LFCM), these highly crystalline, radioactive, glass-like slags, were found in two main forms, known as Brown and Black lavas. 1 Crystalline phases observed in both types of LFCM include nonstoichiometric UO 2 and ZrO 2 from fuel and cladding materials, a U-Zr-O mixed oxide phase and high uranium-zircon (Zr 1-x U x )SiO 4 , referred to as "chernobylite".…”

“…1,[3][4][5] Known as lava-like fuel-containing materials (LFCM), these highly crystalline, radioactive, glass-like slags, were found in two main forms, known as Brown and Black lavas. 1 Crystalline phases observed in both types of LFCM include nonstoichiometric UO 2 and ZrO 2 from fuel and cladding materials, a U-Zr-O mixed oxide phase and high uranium-zircon (Zr 1-x U x )SiO 4 , referred to as "chernobylite". 1,[6][7][8][9] The glassy material in which these crystalline phases reside is a Mg-bearing (from serpentine minerals used in the reactor construction) Ca-aluminosilicate glass, 1,6 although the actinide and minor fission product content of the glass is not well known.…”

“…1 Crystalline phases observed in both types of LFCM include nonstoichiometric UO 2 and ZrO 2 from fuel and cladding materials, a U-Zr-O mixed oxide phase and high uranium-zircon (Zr 1-x U x )SiO 4 , referred to as "chernobylite". 1,[6][7][8][9] The glassy material in which these crystalline phases reside is a Mg-bearing (from serpentine minerals used in the reactor construction) Ca-aluminosilicate glass, 1,6 although the actinide and minor fission product content of the glass is not well known. The main differences between Brown and Black lavas are the extent of crystallisation (greater in the Brown LFCM), the presence of a significant proportion of Febearing phases (Brown LFCM only) and the uranium content (higher in the Brown LFCM).…”

“…While analysis of samples taken from Chernobyl LFCM was performed over 20 years ago, [1][2][3][4][5][6][7][10][11][12][13][14][15][16] the highly radioactive nature of the materials limited the types of characterisation performed. Samples of the material are limited due to the difficulties and dangers associated with their collection (unstable building, high radiation levels), and only a few specimens of the lava that were sampled from within the reactor building have been studied.…”