The site of a nuclear reactor in the Santa Susana Mountains that partially melted down more than 50 years ago has soil radiation that exceeds standard limits, according to new data released by the Environmental Protection Agency.

The 2,849-acre site, located in the mountains between Chatsworth and Simi Valley, is mostly owned by Chicago aerospace giant Boeing Co., but federal agencies NASA and the Department of Energy also own portions.

At a public meeting Wednesday in Simi Valley, EPA officials reported that they collected 3,735 soil and sediment samples at the site. About 500 of those contained the radioactive chemicals Cesium-137 and Strontium-90 in amounts that exceeded background levels.

The site has already undergone two cleanup efforts. NASA and the Energy Department have cleaned up their portions to the highest standards, but Boeing has long maintained that a full decontamination of its larger holdings would harm the area’s ecology, including endangered species. The company also says the radiation levels pose no significant health risk, but neighborhood activists disagree.

The site was home to the first nuclear reactor that produced electricity for the commercial power grid, starting in 1957. The reactor had a partial meltdown in 1959 but continued to operate until 1964. The site was later owned and used by Rocketdyne to test rocket engines. Boeing acquired Rocketdyne 1996.