Emergency Protection of the Reactor
In an emergency the fission-reaction must be stopped, by tripping the reactor so as to minimise the heat output. This is achieved by the automatic insertion of all the control rods. Such emergencies arose at a number of junctures in the Chernobyl accident but the operators had switched off the trip arrangements and so the generation of heat by fission was not halted.
The emergency protection systems were explained in some detail, including the various levels of protection afforded. The nature of these systems is such that certain signals do not produce full shutdown, but rather permit continued operation at lower power levels. The way the coolant circuit functions was illustrated and all of the operating parameters given. Natural circulation has been shown to provide satisfactory cooling when the main pumps are turned off at power levels up to 30%. Tests have been made to establish this on operating plant, on special experimental rigs and also by calculation. There was a description of the three train emergency core cooling systems and the provision of multiple safety relief valves to guard against overpressure in the coolant circuit.
Quantitative Description of the Actual Sequence of Events
In attempting to provide an explanation of why the operators made such a series of errors, two points were emphasised. First of all, the accident occurred at a very bad time psychologically. It was the end of the working week and in the early morning. Although not mentioned it is also worth noting that it was the eve of the May holiday. In addition it was suggested that the psychological state of the operators could have been affected by the fact that this unit had been "top of the league" for availability.
By taking the data which had been recorded by the plants own data processing and using it to validate a computer model of the reactor system, a very detailed and accurate simulation of the course of events leading to the catastrophe was produced.
Some emphasis was placed upon the fact that it was not actually necessary to have the reactor operating at power in order to perform these tests. The Russians argue that it was done at power so that if the first attempt had proved unsuccessful, then there would still be enough steam in the operating reactor to restart the turbine, enabling the experiment to be repeated. Preparations for the test, in terms of reactor safety, were minimal. If the reactor presented problems during the experiment, the advice to the operators was only concerned with operating procedures.
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