In the former URSS there operated 18 nuclear energetic, RBMK reactors, twelve of them operating in Russia. The advantages of these installation are high thermal power 1500 MWt (for one reactor) and the convenience of their operation.
The rupture of coolant circuit in the first loop, however, may cause overheating of fuel elements up to higher temperatures 1500K and their unsealing. At these temperatures (1000-1500 K) the system fuel-cladding-coolant (UO2, UO2 + 20-30% U-Zr, alloy Zr + 1% Nb – water vapour) is not thermodynamically stable.
Exothermal corrosion reactions of Zr and U results in additional overheating of reactor zones, cladding fractures, fuel damage, hydrogen evolution due to Zr and U oxidation and hydrogen accumulation in the first loop of RBMK reactor, the latter giving rise to the formation of explosive hydrogen-oxygen mixture.
The analysis of the interaction of UO2, U, Zr and Zr + 1% Nb alloy with water vapour
At temperature up to 1600K has been made.
The kinetics of urania corrosion over the range 1473-1523 K changes parabolically. The final corrosion product is urania with UO2,20 composition.
At temperatures up to 700 K the law of uranium oxidation is linear, and at temperature above 900 K - parabolic. Thc urania corrosion and uranium oxidation mechanism, structures defects and the electrochemical properties of the formed uranium oxides have been taken into account.
UO2+x (0,005 < x < 0,16) being in contact with Zr (Zr + 1% Nb, Zirckalloy 2;4Nb) at temperature 1273-1573 K the stoichiometry urania effect on the interaction zone thickness was not significant. The oxidation law of Zr and Zr + 1%Nb in water vapour is cubic at 300-1200 K and parabolic at 1300-1500 K.
The comparative analysis has shown that during uranium oxidation. hydrogen evolution rate in RBMK situation is approximately 18 % higher than that for vapour reactions with the outer and inner oxidation of cladding being estimated.
Computation and analysis have shown that for all of elements of the emergency channel the complete crumpling of claddings of the fuel column occurs in the first seconds of the accident.
The part of the unscaling fuel elements during crumpling forms up to twenty percents. In the high power channels (4500 MWt) maximum permissible depth of cladding corrosion (18 percents exceeds since 30 minutes from the beginning of the accident.
After 4x10 seconds from the accident start the part of the cladding material, reacted with the water vapour with the formation of ZrO2 in the emergency channels, forms 30 percents from the total material quantity. The total quantity of hydrogen evaluated in the water vapour – Zirconium reaction, formed during the RBMK-1500 accident, is equal to 196 m under the normal conditions.
Risk of RBMK core accident consists in the probable explosion of hydrogen-oxygen mixture, failure of core with an escape of large amount of radioactive nuclides out of the core.
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