The Chernobyl Tragedy

Where? Ukraine
When? Chernobyl Unit 4 reactor at about 1.24 a.m. on 26 April 1986


How??

The accident at the Chernobyl nuclear power station occurred during a low-power engineering test of the Unit 4 reactor. Safety systems had been switched off, and improper, unstable operation of the reactor allowed an uncontrollable power surge to occur, resulting in successive steam explosions that severely damaged the reactor building and completely destroyed the reactor.

Actions taken during this exercise resulted in a significant variation in the temperature and flow rate of the inlet water to the reactor core (beginning at about 1.03 a.m.)

The relatively fast temperature changes resulting from the operators actions weakened the lower transition joints that link the zirconium fuel channels in the core to the steel pipes that carry the inlet cooling water

Other actions resulted in a rapid increase in the power level of the reactor, which caused fuel fragmentation and the rapid transfer of heat from these fuel fragments to the coolant (between 1.23:43 and 1.23:49 a.m.)



This generated a shock wave in the cooling water, which led to the failure of most of the lower transition joints. As a result of the failure of these transition joints, the pressurized cooling water in the primary system was released, and it immediately flashed into steam.


The steam explosion occurred at 1.23am. It is surmised that the reactor core might have been lifted up by the explosion, during which time all water left the reactor core. This resulted in an extremely rapid increase in reactivity, which led to vaporization of part of the fuel at the centre of some fuel assemblies and which was terminated by a large explosion attributable to rapid expansion of the fuel vapour disassembling the core. This explosion, which occurred at about 1.24 a.m., blew the core apart and destroyed most of the building. Fuel, core components, and structural items were blown from the reactor hall onto the roof of adjacent buildings and the ground around the reactor building. A major release of radioactive material into the environment also occurred as a result of this explosion.



The reactor:


The RBMK-1000 is a Soviet-designed and built graphite moderated pressure tube type reactor, using slightly enriched (2% U-235) uranium dioxide fuel. It is a boiling light water reactor, with two loops feeding steam directly to the turbines, without an intervening heat exchanger.

Water pumped to the bottom of the fuel channels boils as it progresses up the pressure tubes, producing steam which feeds two 500 MW turbines. The water acts as a coolant and also provides the steam used to drive the turbines. The vertical pressure tubes contain the zirconium alloy clad uranium dioxide fuel around which the cooling water flows.

The extensions of the fuel channels penetrate the lower plate and the cover plate of the core and are welded to each. A specially designed refuelling machine allows fuel bundles to be changed without shutting down the reactor.

One of the most important characteristics of the RBMK reactor is that it it can possess a 'positive void coefficient', where an increase in steam bubbles ('voids') is accompanied by an increase in core reactivity. As steam production in the fuel channels increases, the neutrons that would have been absorbed by the denser water now produce increased fission in the fuel. There are other components that contribute to the overall power coefficient of reactivity, but the void coefficient is the dominant one in RBMK reactors.

The void coefficient depends on the composition of the core – a new RBMK core will have a negative void coefficient. However, at the time of the accident at Chernobyl 4, the reactor's fuel burn-up, control rod configuration and power level led to a positive void coefficient large enough to overwhelm all other influences on the power coefficient.


Effects:

Chernobyl tragedy dramatically affected people’s life, economy, science and culture of Belarus as it both suffered from the radioactive devastation and lost historic and cultural values of the Belarusian Palesse, a distinctive natural area in the South of the country.

Economic damage of the Chernobyl accident is estimated at $235 billion for 30 years on after the
explosion, making up 32 national budgets as of1985.

Chernobyl disaster vastly damaged the agricultural sector of the Belarusian economy,which is worth over $700 million annually. Due to radioactive fallout, Belarus lost one fifth of all agricultural lands. It also led to contamination of around a quarter of the Belarusian forests, 132 deposits of mineral resources and nearly 350 industrial enterprises.

Human damage of the accident was 2 million people who suffered from its consequences, with over 1.3 million people, including almost half a million of children and adolescents, still living in the contaminated areas.

Rehabilitation of affected areas is still ongoing. Since 1986, Belarus has spent some $18 billion on this purpose. Annually, public spending of Belarus on rehabilitation totals around 5 per cent of the GDP.



Childbirth deformities Deformities among animal due to DNA mutation

HIROSHIMA	NAGASAKI
Gravity bomb known as "Little Boy", a gun-type fission weapon with 60 kilograms (130 lb) of uranium-235, took 43 seconds to fall from the aircraft flying at 31,060 feet (9,470 m) to the predetermined detonation height about 1,900 feet (580 m) above the city.	"Fat Man" was an implosion-type device using plutonium-239.
The design used the gun method to explosively force a hollow sub-critical mass of uranium-235 and a solid target spike together into a super-critical mass, initiating a nuclear chain reaction. This was accomplished by shooting one piece of the uranium onto the other by means of chemical explosives. It contained 64 kg of uranium, of which less than a kilogram underwent nuclear fission, and of this mass only 0.6 g was transformed into energy.	A subcritical sphere of plutonium was placed in the center of a hollow sphere of high explosive. Thirty-two pairs of detonators located on the surface of the high explosive were fired simultaneously to produce a powerful inward pressure on the core, squeezing it and increasing its density, resulting in a supercritical condition and a nuclear initiation.