Chernobyl Before dawn on April 26, 1986, less than two miles south of what was then a city of 50,000, the Chernobyl Nuclear Power Plant's number four reactor exploded.

Events Leading up to the disaster of April 26, 1986 Plant power decreasing, signifying the start of the reactor shutdown 13:05 Emergency Core Cooling System disconnected, automated regulation system is shut off, power decreased to 30 mwt. 14:00 Power reduction Resumed 23:10

April 26, :00 Increased power to 200mwt by withdrawing rods. 1:07 Two additional recirculation pumps- running. Decreased void All but 6 rods withdrawn Emergency Protection signals blocked by operators.

Leading up to the Accident Increased feed water flow to stream drums Control room printout of core reactivity showed the excess reactivity required immediate shutdown- warning ignored and tests initiated. 1:19 Feed water flow to stream drums decreased to very low value. 30 seconds later reactor inlet temperature begins to rise. 1:22:30

Things continuing to decline 1:23:04 Turbine feed valves closed Power went up 1:23:04 Test was over Operator saw rods moving on core map 1:23:40 Emergency scram initiated by button.

The Moment before and of the Explosion Power increasing rapidly due to positive void coefficient 1:23:43 Explosion occurs followed by a second explosion a few seconds later. This explosion blew the 1,000 ton steel and cement filled biologic shield off the top of the reactor, destroying the roof and exposing the hot core to the atmosphere 1:23:48

The Moments after 1:28 All firemen arrived on the scene 2:00 The largest fires on the roof of the reactor hall were brought under control by a group of Firefighters called in from Pripyat 5:00 Most of the fires were put out, but graphite fire had started, Causing the dispersion of radionuclides high in the atmosphere.

What Went Wrong The immediate cause of the Chernobyl accident was a mismanaged electrical-engineering experiment. Engineers with no knowledge of reactor physics were interested to see if they could draw electricity from the turbine generator of the Number 4 reactor unit to run water pumps during an emergency when the turbine was no longer being driven by the reactor but was still spinning initially.

What Went Wrong The experiment was delayed due to an electrical demand. The experimenters tried to make up for lost time by lowering the power level rapidly. That mistake caused a rapid buildup of neutron-absorbing fission by products in the reactor core, which poisoned the reaction. To compensate, the operators withdrew a majority of the e reactor’s control rods, but even with the rods withdrawn, they were unable to increase the power level

What went wrong This made the system increasingly unstable and led to the loss of more control rods. They also bypassed most of the safety system which included the emergency core- cooling system. They disconnected the backup electrical system and the diesel generator.

Explosion! This led to an explosion of reactor Number 4. The explosion was caused by a simple test of the emergency systems to be used in a power failure. Anyone who had any knowledge of nuclear technology would have known that the test program was inherently unsafe.

Contributing Factors Control rods had a design flaw that now proved deadly: their tips were made of graphite. The graphite tip went in first, which rather than reduce the reaction, they increased it. The control rods displaced water from the rod channels as well, increasing reactivity further.

Faulty Reactor Design The reactor had no sort of containment structure which was a huge design flaw. If there would have been some sort of containment structure, like the ones in the United States, then most likely none of the radioactivity would have escaped, and there would have been no injuries or deaths.

The Days Following: Pripyat evacuated April 27 Announced to the world the reactor was damaged. April 28 Chernobyl evacuated. May 2

Radiation Fallout

Radiation Pattern The radiation field held the strength for the 10 days that the Chernobyl fire continued, slowly falling off to near background levels by the end of three weeks.

Radiation Patterns Within a few weeks the radioactive effluents had both diffused and decay to undetectable levels. While the radiation field intensity eventually fell back to normal levels, exposure due to internally deposited radioactive effluents continued to provide a threat to the population.

References Hyper Physics Chernobyl section, C.R. Navy, hosted by department of Physics and Astronomy of Georgia State University, D. Marples, “the Social Impact of the Chernobyl Disaster.” London, 1988, pp Glenn Alan Cheney, “Chernobyl: The Ongoing Story of the World’s Deadliest Nuclear Disaster, “Macmillan, 128pp. New York, A.P.Hill, “Dose Estimates from the Chernobyl Accident,” ANS Transactions, Winter R.A. Schlenker, “Internally Deposited Fallout from the Chernobyl Accident, “ANS Transactions, Winter 1987

References: Stone, Richard. "THE LONG SHADOW OF CHERNOBYL." National Geographic (2006): 32. MasterFILE Premier. EBSCO. Web. 28 Feb Images provided by Google. travail/radiation/dosim/res-centre/glossary-lexique- eng.php ction/readings/chernobyl.html