As reported by the New York Times: President Biden requested the intelligence community complete the assessment shortly after taking office, and his administration has made fighting domestic terrorism a priority.
University of Chicago Professor Kathleen Belew has warned, on C-SPAN's "Q&A," that far right, white supremacist extremists and Neo-Nazis have threatened in the recent past to attack nuclear power plants in order to unleash catastrophic mass casualty events. See the C-SPAN "Q&A" interview with Belew, here.
Belew is referring to the domestic terrorist group, Atomwaffen Division. Atomwaffen means nuclear weapons in German. See the PBS Frontline special "Documenting Hate: New American Nazis," that aired on Nov. 20, 2018. It documents an apparently disrupted Atomwaffen Division plot to attack the Turkey Point nuclear power plant in southern Florida.