PERRIS, Calif. -- A section of a major Southern California freeway was shut down and 170 homes were under evacuation orders Friday as a chemical reaction inside a railroad tank car threatened to cause an explosion, authorities said.
The tank car was parked on a spur off a main rail line along Interstate 215 in Riverside County, about 57 miles (92 kilometers) east of Los Angeles.
County fire officials identified the chemical as styrene, which is used in making foam products. It wasn't immediately clear what caused the reaction, which dramatically raised the temperature inside the tanker car.
However, the tanker was filled in Texas 50 days ago, and one possibility was that a stabilizing chemical in the tanker had broken down, said Mark Scoville with the California Department of Forestry and Fire Protection Riverside County.
The evacuation zone extended a half-mile (0.8 kilometers) in all directions and included 170 residences. The area has a mix of businesses and large plots of undeveloped land between the city of Perris and March Air Reserve Base.
John Crater, a fire division chief, said he had consulted experts across the country.
“What they are telling me is this can resolve itself in two days; it could get worse before it gets better," he said.
The heat in the rail car builds pressure "and it could have a release, meaning some kind of violent explosion," he said.
Drones equipped with infrared sensors were monitoring the temperature, which appeared to be dropping but remained too high for firefighters to safely approach, authorities said at an afternoon news conference.
The chemical was effectively boiling inside the car, which brought a concern that the tank might rupture, fire officials said.
When conditions are safe, firefighters planned to move away a dozen tanker cars surrounding the chemical container and then eventually move it away to a safer location, officials said.
The problem was discovered when a plume was seen coming from the rail car about 7:40 p.m. Thursday. Authorities said the plume spread over Perris before dissipating.
Authorities said the chemical’s normal temperature is 85 degrees (29.4 Celsius) and the tank’s temperature had reached 323 degrees (161.6 Celsius) before declining to 304 degrees (151.1 Celsius) Friday morning.
Authorities said they hoped it would further fall to about 100 degrees (37.8 degrees Celsius).