On September 19, 2014, the U.S. Court of Appeals for the Ninth Circuit held that California Senate Bill 990 violates the Constitutional doctrine of "intergovernmental immunity" because it directly regulates the activities of the US Department of Energy in violation of the Supremacy Clause. S.B. 990 prescribes state radioactive cleanup standards at the Santa Susan Field Laboratory, a site which is undergoing extensive cleanup by the federal government. The case is The Boeing Company v. Movassaghi, Acting Director of the California Department of Toxic Substance Control, et al.

The federal government used the Santa Susan Field Laboratory site, located in Ventura County, to conduct nuclear research, operate nuclear reactors, build and test rockets, and conduct other defense-related work. According to the Ninth Circuit, all of these activities over the years "created a terrible environmental mess", which the federal government and Boeing, as its contractor, are addressing. The soil, groundwater and bedrock were seriously contaminated.

California enacted S.B. 990, prescribes stringent cleanup standards, purporting to make the large site suitable for subsistence farming activities. However, the Ninth Circuit notes that the doctrine of intergovernmental immunity provides that the activities of the federal government are shielded by the Supremacy Clause from direct state regulation, and S.B. 990 violates this principle. It concluded that no provisions in the Atomic Energy Act, Resource Conservation and Recovery Act or Comprehensive Environmental Response, Compensation, and Liability Act indicate that Congress envisioned a role for the state as embodied in S.B. 990, and therefore the law is of no effect at this site.