On April 26, 1986, a sudden surge of power during a reactor systems test destroyed Unit 4 of the nuclear power station at Chernobyl, Ukraine, in the former Soviet Union. The accident and the fire that followed released massive amounts of radioactive material into the environment. Emergency crews responding to the accident used helicopters to pour sand and boron on the reactor debris. The sand was to stop the fire and additional releases of radioactive material; the boron was to prevent additional nuclear reactions. A few weeks after the accident, the crews completely covered the damaged unit in a temporary concrete structure, called the "sarcophagus," to limit further release of radioactive material. Chernobyl's three other reactors were subsequently restarted but all eventually shut down for good, with the last reactor closing in 1999. The Chernobyl accident's severe radiation effects killed 28 of the site's 600 workers in the first four months after the event. Another 106 workers received high enough doses to cause acute radiation sickness. Two workers died within hours of the reactor explosion from non-radiological causes. Another 200,000 cleanup workers in 1986 and 1987 received doses of between 1 and 100 rem (The average annual radiation dose for a U.S. citizen is about .6 rem). The last completed nuclear power plant built in the US is Watts Bar Plant (Rhea County, Tennessee, near Spring City) completed in 1990 and went into commercial operation in 1996 (there are two reactors under construction). There are currently 60 commercially operating nuclear power plants with 98 nuclear reactors in 30 US states. The US is the world's largest producer of nuclear power, accounting for more than 30% of the worldwide nuclear generation of electricity. The country's nuclear reactors produce about 20% of total electrical output. In 2018 US electrical generation was 35% from gas, 27% from coal-fired plants, 19% nuclear, 6.99% from Hydro, 6.6% from Wind, 1.6% from Solar, 1.51% from Biomass, and 1.43 % from Geothermal and other sources (US Energy Information Administration data).