Title: RATIONALE FOR THE LONG-TERM STRATEGY OF THE DESTROYED CHORNOBYL UNIT IV CONVERSION INTO ECOLOGICALLY SAFE SYSTEM
Authors: Yu.Kostenko, K.Rudya (Ministry for Environmental Protection and Nuclear Safety of Ukraine)

Overview of the factors affecting long-term strategy decisions on the conversion of the ruined unit 4 of the Chornobyl nuclear power plant is given. The essence of the strategy is a complete and final removal of the hazards peculiar to the "Shelter". The total amount of fuel residuals in the "Shelter" comes to 200 tons of uranium dioxide. The fuel masses physical and chemical state is subject to increasing change with time, thus leading to the negative modification of their nuclear physical characteristics. Cooling down, cracking and erosion of the vitrified fuel-containing masses increase their water penetrability and lead to the gradual growth of the system reactivity. Instability of the damaged unit 4 structures, as well as of the "Shelter" itself creates the hazard of local and major collapses. In these conditions the probability of the criticality cannot be excluded. Moreover, real events that took place in 1990 and 1996 demonstrated a very uncertain situation with safety parameter monitoring and control. The listed factors provide a solid basis for the strategy of fuel removal, isolation and disposal in accordance with international and national regulatory requirements, a process to be started as early as possibly, after preliminary stabilization of the existent structures and parallel implementation of the local isolation and localization measures, preparation of the relevant waste management infrastructure, etc.

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