The spectre of Chernobyl 30 years later

At 1: 23 am on April 26, 1986, at the Soviet- run Chernobyl Nuclear Power Plant in northern Ukraine, a test of emergency operation procedures went horribly wrong, resulting in a sudden, massive explosion in Reactor No. 4, igniting the world’s deadliest nu

1 An experiment had been set p to test methods for managing the reactor during a drop in external electrical power delivered to its control systems, a blackout or similar grid problem. On April 25, engineers began the test, which involved running the reactor core at very low power. But the crew was unaware that this RBMK-model Soviet reactor had a tendency to become unstable at such low core power levels. To facilitate the experiment, operators turned o an automated emergency core- cooling system, despite strict safety regulations to the contrary.

2 Power in Reactor No began to fall slowly from the normal level of 10,000 thermal megawatts. Test preparations continued throughout the day. At a m April the core s power le el drop accelerated. When the power level fell so low, to 30 megawatts, that it threatened stability, operators withdrew all but 18 of the total 211 control rods, to raise core power, sidestepping automatic systems again. They also left the reactor unprotected by backup cooling systems. Power stabili ed brie y at 200 megawatts.

3 As the final phase of the test began, operators expected reactor power to rise slowly, but in this design, any rise while running at such low power levels could trigger an exponentially bigger rise, in an unstable e ect called positive reactivity feedback. As this began to occur, they tried to reinsert control rods, but they rapidly overheated, fractured and jammed in their channels. Power spiked. Within seconds, steam and hydrogen gas built up and exploded, blew the reactor lid o and sparked a huge re.

4 Most of the reactor b ilding s roof and the upper parts of the north and south walls were blasted away see explosion detail below . Radioactive dust and burning graphite rubble were hurled into the air, Much of it landed on the roof of the next building, and threatened Reactor No. 3. An estimated 10 tons of uranium dioxide fuel and radionuclides such as Cesium 137 and Iodine 131 were scattered across the compound and into the air, as the roof re raged.