Chernobyl Nuclear Disaster: Why Chernobyl nuclear reactor accident happened? On 26th of April 1986, massive explosion happened in unit 4 of Chernobyl nuclear reactor. This accident damaged the reactor components, reactor buildings and resulting in the spread of huge amount of radioactive substances in the environment. Salient features of Chernobyl RBMK reactor has been discussed earlier. An experiment was planned in unit 4 before its planned maintenance related shutdown. Aim of the experiment was to make the reactor more safe if the station power fails, when core cooling is must to remove decay heat from the fission products, otherwise core meltdown can happen. It was planned to investigate whether slowing down turbine can be used to generate sufficient power for running vital components like cooling pumps.   Planned power level for the experiment was 700-1000 MWth. Series events on 25-26.04.1986 has been explained earlier. During full power (3200MWth), the reactor configur...

Chernobyl Nuclear Disaster: Series of events on 25-26 April 1986 Deadline was set by May day celebration on 1st May , that showcases the major scientific and technical updates every year. Minimum requirement for the test: The reactor power should be in between 700-1000 MWth.The maximum power was 3200MWth.  Difficulty of the test: RBMK reactor is unstable at lower power. RBMK reactors have very high positive reactivity coefficient. Reactivity increases with increase in temperature of the reactor. This makes the reactors unable at lower power .  As per USSR version , Shutdown was planned for Unit 4 of Chernobyl RBMK reactor on 25.04.1986 for a planned maintenance work.   Photo credit: ge.com 25.04.1986 Planned shutdown for was scheduled for maintenance. An experiment was planned in case of emergency situation and loss of station power, before the diesel generators gets switched on, whether the slowing turbine can be utilized for the generation of power so that vital c...

Part 3: Salient features of Chernobyl RBMK Reactor Chernobyl Nuclear Power Plant Unit 4: Soviet RBMK Reactor: Its salient features Reactor (RBMK):  Reaktor Bolshoy Moshchnosty Kanalny in Russian or High Power Channel Reactor (RBMK) in English, is a graphite moderated  boiling water (light water, H2O) reactor.     Statistics about RBMK:  RBMK Reactors were designed, constructed and operated only in the USSR. There were 15 RBMK reactors were operational in the USSR at different point of time between 1974-1996. Unit 4 of Chernobyl Nuclear Power Plant (NPP):  29 September 1967: The design specifications were approved by the USSR Ministry of Power.   April 1979: The construction started and  March 1984: C ommercial operation started. Fuel:  Enriched uranium as fuel (2% 235U), 114.7kg U.  Peak fuel temp.: 2100 oC.  Peak clad temp. In/out: 325 oC/ 295 oC. Sub assembly and fuel element 18 fuel clad tube/ sub assembly. 2 Sub as...

Problems of Chernobyl RBMK reactor Large positive void ( steam ) coefficient.  In case of coolant loss due to increase in temperature, when coolant becomes steam, reactivity changes differently for RBMK and PHWR/ PWR/ LWR reactors.  In case of PHWR/ PWR/ LWR, where both coolant and moderators are water, either light water or heavy water, in case of increase in temperature, water becomes steam. Moderator thermalises neutron i.e. reduced energy by collisions. Since, thermalisation of fast neutron is reduced, which decreases availability of thermal neutron and hence fission process gets reduced. So reactivity of the reactor decreases. Reactor goes towards subcriticality.  In case of RBMK reactor, light water was used as coolant and graphite was used as a moderator. In case water turned into steam, no. of atoms of light water decreases, which increases no. of neutron due to the fact that light water has higher absorption crosssection than steam. But unlike wate...

Advantages of Chernobyl RBMK reactor As fuel, low enriched Uranium is required (2% 235U), which makes the reactor more economical. By enrichment process, % of 235U content in natural uranium increases from 0.75% to higher content. Cost of building a RBMK reactor is very less due to no containment building. Containment building is made of a reinforced steel, concrete or lead structure which encloses a nuclear reactor. Building reactor is less time consuming also due to the above two reasons. As per design, graphite displacers were placed in bottom of the boron carbide absorbers/ control rod. During normal operation of the reactor, graphite portion of the control rod should be inserted into the core, so that coolant natural water (H2O) shall be displaced by graphite. Graphite is a good moderator than natural water. Neutron absorption cross section is less for graphite than water. This allows reactor to remain critical with very less enrichment, e.g. 2%. Light water moderated reactor nee...