See glossary pages 33 to 36 Chernobyl, the ultimate disaster The Chernobyl accident resulted from a convergence of events combining human errors and faults in the design of the NPP. A test sequence on the emergency electrical power supply of reactor 4 was to turn into a major catastrophe and raise worldwide awareness of the risks associated with nuclear power. What happened on 25 April 1986 in the building of reactor 4 (RBMK reactor) of the V.I. Lenin NPP situated 18 km from Chernobyl, in Ukraine? A test was to be carried out to check the possibility of energising the reactor recirculation pumps via a turbogenerator set if the electrical power supply failed. This test was to be carried out at about 20% to 30% of the nominal power level. 25 April 1986 The operators started the procedure to lower the power to the level required for the test. However, at the request of the electrical power distribution centre, the reactor was maintained during the day at a higher power level than that required for the test. At 23:00 (11 pm), the operators started to reduce the reactor power level to attain the test conditions but could not stop the power reduction. They therefore decided to withdraw the control rods, beyond the authorised limits, in order to raise the power level. At 01:00 (1 am) on 26 April, the reactor power stabilised at a level significantly below the required level. The team nevertheless decided to perform the test. 25 april 1986 Test linked to the electrical power supply of reactor 4. The safety conditions were not observed. 26 april 1986 The uncontrolled increase in power led to an explosion of the reactor and a graphite fire. Réacteur 4 Réacteur 4 THE ACCIDENT The RBMK reactor This is a Soviet-designed highpower reactor. It is a graphitemoderated reactor that uses boiling light water as the coolant. The fuel is uranium oxide enriched with uranium-235. Each fuel assembly is contained in a “pressure tube” within which the coolant fluid circulates. The major drawbacks of this type of reactor are the complexity of the cooling fluid distribution and collection system, the large build-up of thermal energy in the metal structures and in the graphite, the absence of reactor containment and the difficulty in controlling the reactor core. Eleven RBMK reactors were still in operation in 2023, all located in Russia. 16 • Les cahiers Histoire de l’ASN • November 2023
RkJQdWJsaXNoZXIy NjQ0NzU=