President Donald Trump’s surprise announcement last month that the United States would resume its testing of nuclear arms produced a blur of condemnations and contradictory statements. Lost in the uproar, some nuclear experts say, is a forgotten type of atomic testing. It produces no mushroom clouds and poses little radioactive risk to the public. But it can reframe the recent debate.

The tiny explosions are known to nuclear weaponeers as hydronuclear tests. They appear to lie behind the Trump administration’s claims that Moscow and Beijing have violated the global treaty that bars nuclear tests, and led to Trump’s statement that Washington should respond “on an equal basis.”

Neither the federal Department of Energy nor its subordinate unit, the National Nuclear Security Administration,

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