as on the development of the High Temperature Reactor at the Kurchatov Institute.
11.00 to 13.00 hours: Overview of the Accident
Legasov now presented a video of the sequence of events in the Chernobyl accident. The reactor power had been teetering on the brink of dangerous thermal hydraulic and neutronic instabilities because the operators had turned off vital safety systems and had too few absorber rods in the core. When they diverted steam from the turbine this was the last straw. The reactor power rocketed up, steam pressure burst the reactor and the overheated fuel then gave off many millions of curies of radioactivity. Within a day or so the 100,000 or so people living up to 30 km away were evacuated.
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He went on to describe the RBMK reactor. This has a graphite moderator pierced by holes or channels, lined with zirconium-niobium tubes and containing the uranium dioxide fuel. Water in the channels is boiled by the fuel. produces the steam needed to drive the turbo-alternators and it also keeps the fuel from overheating, by continuously removing the heat generated by nuclear fission. The rate of heat-generation is controlled by inserting or withdrawing neutron-absorbing rods. There are pumps to pump water into the bottom of the channels and it boils as it rises up the channels. A mixture of hot water and steam emerges from the top and passes through pipes to steam-separators. Here the steam collects above the water and is led by pipes to the turbines whilst the water is drawn off and pumped back through the channels to be boiled again. The steam from the turbines is condensed and it, too, is pumped back through the channels, completing the cycle.
Legasov summarised the conditions of coolant flow, level, temperature or steam-content which could, if allowed to persist, lead to an accident and which therefore normally automatically trigger a "trip" or cessation of heat-generation due to fission. If there are fewer than 15 neutron control rods inserted in the reactor then the rules require it to be tripped by the operators. They judged that the probability of the operators failing to trip it in such a case was lower than the probability of failure of a purely automatic trip system. In the event it was precisely this error that the operators made. They had fewer than 15 rods but did not trip the reactor, leaving it critical and poised on a knife-edge.
Legasov went on to describe the safety systems which take the heat away should an accident commence: the emergency core cooling systems. Then he described the containment philosophy: the steam separators, the pumps and the pipes leading to and from the channels are separately contained each in its own
Tubes from each cell are immersed in a
concrete cell or box.
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