26 APRIL, 1986
Bharat Bhushan | Intro to Nuclear Engineering | June 20, 2013
Chernobyl disaster, a man made catastrophe, is the world’s worst Nuclear Accident till date. The Chernobyl nuclear power plant, located in Ukraine, had four RBMK-1000 reactors in service and two under construction. A safety system test was planned for Unit 4 before shut down for maintenance on April 25th, to determine how long turbines would spin and supply power to the main circulating pumps following a loss of main electrical power supply. This type of test was conducted the previous year, but the power delivered from the running down turbine fell off too rapidly, so it was decided to repeat the test using the new voltage regulators that had been developed. Four of the eight main circulating pumps were supplied power by unit service power and other four by station service power for forced circulation which ensured that at shutdown four pumps would be in operation.
The RBMK-1000 is a direct cycle (boiling water), graphite moderated pressure tube type reactor which uses 2% U-235, uranium dioxide fuel. There were two loops feeding steam directly to turbine (without an intervening heat exchanger). Water was being pumped to the bottom of fuel channels. Pressure tubes contain zirconium alloy clad uranium dioxide fuel around which the water flows. Water acted as a coolant as it flows around the cladding and starts boiling as it moves up in pressure tubes thereby generating steam which in turn is used to run 500Mwe turbines. A specially designed refueling machine allows fuel bundles to be changed without shutting down the reactor.
Pressure tubes were surrounded by graphite moderator for slowing down neutrons to make them more efficient in producing fissions. A mixture of helium and nitrogen was circulated between graphite blocks to prevent oxidation of graphite. There were four main coolant circulating pumps in each of the two loops. Power of reactor was controlled by moving up or down the control rods (211 in number), which, reduced the power when lowered by absorbing neutrons. Various safety systems, such as an emergency core cooling system, were incorporated into the reactor design.
As RBMK reactors are water cooled, there’s a certain amount of steam present in the core due to which steam bubbles (also known as ‘Voids’) makes a certain proportion of coolant volume known as “Void Fraction”. Any change in void fraction will change core reactivity and ration of these changes is called void coefficient of reactivity. It can be positive or negative depending upon the reactor design. A positive void coefficient means that increase in void fraction causes less neutrons to be absorbed ( compared to if there were only liquid phase), resulting in increased fission in the fuel, thereby increase fuel temperature and a further increase in void fraction. In case of RBMK reactors, void coefficient plays a significant role. The void coefficient depends upon a number of factors like composition of core, fuel burn up, fuel enrichment level, control rod configuration and power level etc. A new RBMK core will have a negative void coefficient. However, at the time of accident at Chernobyl 4, everything led to a positive void coefficient large enough to overwhelm all other influences on the power coefficient.
Control Rod Design
There were a total of 211 boron carbide absorber rods. Out of which 24 are for emergency protection, 12 local automatic controls and 24 automatic controls. A total of 139 were manual rods that are manipulated by operator’s w.r.t. to reactor conditions. These rods were fitted with graphite followers separated by a distance of 1.25m. These followers were shorter than the length of the core. When a rod is inserted from fully withdrawn position, initial –ve reactivity insertion is minimized and at the bottom a +ve reactivity insertion occurs. This is known as...
References: http://www.world-nuclear.org/info/Safety-and-Security/Safety-ofPlants/Appendices/Chernobyl-Accident---Appendix-1--Sequence-of-Events/ http://institutionalfailure.com/?page_id=115
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