Analysis of the Chernobyl accident from 1:19:00 to the first power excursion

Many researchers have reported that the root cause of the Chernobyl accident has not been clarified still now. Since many of them discussed the accident without a precise thermal-hydraulic investigation, thermal-hydraulic calculations coupled with neutronic calculations have been done on the basis of the recorded result at the Chernobyl Unit-4. Plant configurations and operational conditions were given to the code on the basis of reported result and published papers. Calculation could trace plant parameters from 1:19:00 to the first power excursion without any discrepancies measured at the Chernobyl Unit-4. Reactivity slightly smaller than 1β by the positive scram is concluded as a possible direct cause of the accident, which acts as a trigger to increase the reactor power. Other possibilities as a trigger of the accident such as cavitation in pumps and pump coast-down were investigated. The importance of the calculation from the stable condition is also described in this paper in order not to bring unnecessary assumptions into the calculation.