What is the silver ball disclosed by North Korea as a “miniaturized nuclear warhead”?

According to the March 9 edition of the Rodong Sinmun, First Secretary of the Workers' Party of Korea Kim Jong-un said, "This is a nuclear warhead designed with a reasonable structure that can instantaneously facilitate a thermal nuclear reaction," as he stood near the device placed before a KN-08, an inter-continental ballistic missile.

The device in the picture released by North Korea looked similar in its appearance to the “Fat Man,” a nuclear bomb that the U.S. dropped on Nagasaki, Japan in August 1945. It is a nuclear bomb using an antiknock detonator, and it is shaped like a soccer ball with plutonium, the nuclear fission material, in the center, surrounded by high explosives. Fat Man had 32 high explosive lenses, but the detonator released by the North was covered with approximately 72 high explosive lenses.

Dr. Jungmin Kang, a nuclear expert at the Natural Resources Defense Council (NRDC) in Washington D.C. said, "It appears they have improved the performance compared to the Fat Man, but it still seems to follow the early models of U.S. nuclear weapons development." The antiknock detonator, which the U.S. developed in the early days was also a soccer ball shape covered with 72 high explosive lenses

It is difficult to tell whether North Korea has miniaturized the detonator enough to load it on a KN-08 missile just from the photo. But the North has already conducted four underground nuclear tests and given that they have tested high explosives for a long time now, many believe it is difficult to dismiss the latest announcement as a bluff.

Karl Dewey, a CBRN analyst at IHS Jane’s, a British military intelligence firm, said, "This picture clearly shows that North Korea has made something that they can load onto the KN-08. This silver round ball may be a simple antiknock weapon or it may be an amplified (nuclear) weapon."

On March 9, the U.S. Department of Defense said that they have not witnessed the North's ability to miniaturize a nuclear warhead and mount it on an inter-continental ballistic missile and said that their evaluation remained the same.