SEOUL—North Korea is upgrading its nuclear research center at a rapid pace, new satellite imagery analysis suggests, despite Pyongyang’s commitment to denuclearization at a summit with the U.S. this month.

The analysis from 38 North, a North Korea-focused website published by the Stimson Center in Washington, found that Pyongyang, in recent weeks, appears to have modified the cooling system of its plutonium-production reactor and erected a new building near the cooling tower. New Construction could also be observed at the...