US not planning nuclear explosions at this time, energy secretary says

"I think the tests we’re talking about right now are system tests," Wright said in an interview with Fox News. "These are not nuclear explosions. These are what we call non-critical explosions."

The testing involves all the other parts of a nuclear weapon to make sure they are functioning and can set up a nuclear blast, said Wright, whose agency is responsible for testing U.S. nuclear weapons.

The tests will be carried out on new systems to help ensure replacement nuclear weapons are better than previous ones, Wright said on Fox News' "The Sunday Briefing."

Just before his meeting with Chinese leader Xi Jinping in South Korea on Thursday, Trump said he ordered the U.S. military to immediately restart the process for testing nuclear weapons after a halt of 33 years, a move that appeared to be a message to rival nuclear powers China and Russia.

He reaffirmed his comments on Friday but did not answer directly when asked whether that would include underground nuclear tests that were common during the Cold War.

The United States conducted nuclear test explosions in the 1960s, 1970s and 1980s, Wright said, and collected detailed information and measurements on the blasts.

"With our science and our computation power, we can simulate incredibly accurately exactly what will happen in a nuclear explosion," Wright said.

"Now we simulate what were the conditions that delivered that, and as we change bomb designs, what will they deliver?"