New satellite photos show that Kim Jong Un is continuing to develop his nuclear weapons program, and U.S. intelligence sources say they believe North Korea has increased its production of nuclear fuel at multiple sites. This wasn’t supposed to happen after the Donald Trump-Kim summit last month in Singapore.

According to an analysis by experts at the Stimson Center in Washington, North Korea has improved the cooling system of its plutonium-producing reactor at Yongbyon. Activity also continues at a nearby building where plutonium...