reactors, unit 2 restarted May 11, 1999 and unit 3 restarted July 1, 1998. Unit 1 was never restarted as the cost of remedying numerous safety issues was too high. In 2002 Millstone's operators were fined $288,000 for failing to have properly accounted for two uranium fuel rod components that had been misplaced 30 years earlier in 1972. On April 17, 2005, Millstone unit 3 safely shut down without incident when a circuit board monitoring a steam pressure line short-circuited, which caused the board to malfunction and indicate an unsafe drop in pressure in the reactor's steam system, when in reality there was no drop in steam pressure. The cause was attributed to tin whiskers on the board. In response to this event, Millstone implemented a procedure to inspect for these whiskers at every refueling outage, or 18 months. In September, 2009, unit 2 shut down when an electrical storm caused power fluctuations. When workers tried to restart the unit, they discovered a small leak in the reactor coolant pump. On December 21, 2009 the Unit 3 reactor tripped and shut down for longer than 72 hours. On July 27, 2009, the Unit 2 reactor tripped and shut down for longer than 72 hours. On August 9, 2013, the Unit 3 reactor tripped and shut down due to a malfunction. In 2016, unit 3 was shut down following a hydrogen gas leak. Surrounding population The Nuclear Regulatory Commission defines two emergency planning zones around nuclear power plants: a plume exposure pathway zone with a radius of , concerned primarily with exposure to, and inhalation of, airborne radioactive contamination, and an ingestion pathway zone of about , concerned primarily with ingestion of food and liquid contaminated by radioactivity. The 2010 U.S. population within of Millstone was 123,482, an increase of 29.5 percent in a decade, according to an analysis of U.S. Census data for msnbc.com. The 2010 U.S. population within was 2,996,756, an increase of 9.5 percent since 2000. Cities within 50 miles include Hartford (41 miles to city center) and New Haven (43 miles to city center). Seismic risk The Nuclear Regulatory Commission's estimate of the risk each year of an earthquake intense enough to cause core damage to the reactor at Millstone was 1 in 90,909 for Reactor 2 and 1 in 66,667 for Reactor 3, according to an NRC study published in August 2010. See also List of