North Korea is expected to begin disabling its nuclear facilities Monday, marking the biggest step the communist country has ever taken to scale back its atomic program. The North shut down its sole functioning nuclear reactor at Yongbyon in July, and promised to disable it by year's end in exchange for energy aid and political concessions from other members of talks on its nuclear program: the US, China, Japan, South Korea and Russia. Disabling the reactor at Yongbyon, north of Pyongyang, would mark a further breakthrough in efforts to convince the North to scale back its nuclear program. The country conducted its first-ever nuclear test in October of last year. "By Monday morning, they will begin their work," US Assistant Secretary of State Christopher Hill said in Tokyo on Saturday, referring to the US team that arrived in Pyongyang on Thursday. "It's a very big day because it's the first time it's actually going to start disabling its nuclear program."