Kim Jong Un’s apparent inspection of a nuclear-powered submarine last month suggests that the endeavor is not unserious, even if Pyongyang is likely far from prototyping a safe, effective and reliable naval nuclear reactor.

The gold standard for any robust nuclear force is survivability and North Korea has many reasons to seek improvement here. The military modernization plan, which should mostly conclude by the end of next year, has prioritized survivability through various means, including dispersal, mobility, responsiveness and concealment.

With the exception of naval propulsion and multiple reentry vehicles for intercontinental ballistic missiles