Thirty-five years ago, on April 26, 1986, during a test meant to simulate an electrical outage, power to the No. 4 reactor at the Chornobyl Nuclear Power Plant unexpectedly dropped to a near-zero level.  Reactor operators, who were not scheduled to work and were not properly prepared to run the test, tried to restore power.  The core of the reactor, which had become unstable and suffered from various design flaws, exploded twice, spewing radioactive material across a large swath of Europe.