The paper analyses uncertainty of causes of severe accidents at Fukushima-Daiichi relating to a state of a reactor core and vessels, the nature and mechanisms of steam-gas explosions, efficiency of stuff's actions to decrease pressure in containment and to cool the damaged fuel. It is shown that the main uncertainties are: the state of uncertainty in the core and the reactor pressure during the development of accidents at power units № 1 - 3 determines the possibility of different scenarios, the uncertainty of the nature and mechanisms arising steam explosions uncertainty effectiveness of personnel to reduce the pressure in the containment vessel and the cooling of the molten / damaged fuel.
Keywords: severe accident, containment, hydrogenous steam-air mixture, fuel-containing masses, initial event of accident.
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