It was not a nuclear explosion. Read the article.

"They use chemical explosives to blow up bits of nuclear materials designed to stop just short of erupting into a nuclear chain reaction, also known as a criticality."

They are testing to make sure the stockpiles of nuclear materials are not going to blow up or do something unexpected. The genie is out of the box so we have responsible scientists doing what is necessary to prevent it from freeing itself.

Ironic this is condemned by Japan on Pearl Harbor day [at least still in my time zone]. Sad that what they did to the US necessitated us letting that genie out.

Peace... but probably not gonna happen for a while anyway.