With North Korea's growing nuclear threat and rising tensions with Russia, the U.S. administration of President Donald Trump may be beefing up its readiness to restart underground nuclear tests that have been suspended since 1992, a recent report by the National Nuclear Security Administration and sources close to the matter suggest.

The annual NNSA report titled "Stockpile Stewardship and Management Plan (SSMP)," published in November, proposes a relatively new concept for nuclear tests, under which an experiment called a "simple test" would be conducted within "6-10 months" once the U.S. president decides to resume nuclear testing.

The NNSA, a specialized branch of the Energy Department, is in charge of development and production of nuclear weapons as well as their technical maintenance.

Through its "Stockpile Stewardship Program," a comprehensive package of modern nuclear technologies including "subcritical nuclear tests," the NNSA has certified the technical capabilities of U.S. nuclear stockpiles for more than 20 years.

"A 'simple test' is one for which the diagnostics for the test would be minimal or non-existent, such as for an underground nuclear explosive test conducted for political purposes," an NNSA official told Kyodo News through recent email exchanges.

Linton Brooks, former administrator of the NNSA under the George W. Bush administration, recently told Kyodo News that the purpose of conducting a "simple test" is to demonstrate "political resolve." He also indicated that an adversary's excessive nuclear activities, such as drastic nuclear expansion by Russia, could be a trigger for a simple test.

Under the administration of President Barack Obama, the same type of nuclear stockpile report, called SSMP, was published annually by the NNSA. "Past SSMPs have been completely consistent on these points, to include the timeframes required," the NNSA official said.

However, Kyodo News could not find any words or phrases relating to a "simple test" in past reports publicized on the NNSA website.

"During the Obama administration there were no discussions about a 'simple test' that I recall," Andy Weber, a former assistant secretary of defense for the Nuclear, Chemical and Biological Defense Program told Kyodo News. "I think a 'simple test' is a new formulation," he added.

"The claim that this is consistent with previous SSMPs sounds strange because the new SSMP says the NNSA has reassessed the requirement (of nuclear test readiness)," said Hans Kristensen, a nuclear weapons expert with the Federation of American Scientists.

Kristensen was the first specialist to raise this issue in his recent analytical paper titled "NNSA's New Nuclear Stockpile Stewardship and Management Plan." In the paper, he criticized the NNSA's approach, saying "this reassessment of the test readiness requirement appears to erode its U.S. commitment to the testing moratorium."

Since the administration of President Bill Clinton, who decided to extend the nuclear testing moratorium in 1993 and signed the Comprehensive Test Ban Treaty, U.S. nuclear scientists have maintained technical conditions to resume "a fully instrumented test" for diagnosing and fixing the technical problems of nuclear warheads in "24-36 months" after the president formerly decides to do so.

"The 'simple test' timeframe is not in the same sanction of the previous SSMP. So unless it was included somewhere else in the previous version then it seems to be new," said Kristensen.

In the new SSMP, the NNSA stipulates a "simple test" to be conducted "with waivers and simplified processes."

"What this refers to is short-circuiting the lengthy environmental and safety assurance process that would entail a 'normal' resumption of nuclear testing, if that were to ever occur," said John Harvey, a former senior official of the NNSA and the Department of Defense, explaining about "waivers and simplified processes."

"That does not necessarily mean that the test would be unsafe or negatively affect the local environment -- assuming it were an underground nuclear test. If a test was needed to be done quickly, to 'fix' a technical problem with a warhead or for political reasons, then the president might authorize accelerating or removing some of these procedural obstacles," Harvey added.

The U.S. Republican Congress has continued to oppose and prevent ratification of the CTBT since the late 1990s, even though Democratic administrations led by Clinton and Obama feverishly tried to persuade Congress to approve the ratification.

The current Trump administration will conclude an ongoing Nuclear Policy Review as early as the end of this year.

"I don't think the mention of the term 'simple test' has any clear relationship to the upcoming NPR," Harvey said.

On the other hand, Harvey added. "It is conceivable that the NPR might recommend a strengthened 'test readiness posture' as did the 2001 NPR. That does not mean a return to testing, but rather spending some dollars at the Nevada test site to ensure that we could reasonably promptly conduct a test if that were required."