President-elect Donald J. Trump has picked Brandon Williams, a former Navy officer and one-term congressman, to become the keeper of the nation’s arsenal of thousands of nuclear bombs and warheads.

Mr. Trump’s selection is a shift from a tradition in which the people who served as administrator of the National Nuclear Security Administration typically had deep technical roots or experience in the nation’s atomic complex. What’s unknown publicly is the extent of Mr. Williams’ experience in the knotty intricacies of how the weapons work and how they are kept reliable for decades without ever being ignited.

Terry C. Wallace Jr., a former director of the Los Alamos weapons laboratory in New Mexico, expressed surprise at Mr. Trump’s pick.

Dr. Wallace said he had “never met him or had a meeting” with Mr. Williams and characterized him as having “very limited experience” with the N.N.S.A.’s missions, based on his own decades of work in and around the nation’s atomic complex.

Hans M. Kristensen, the director of the Nuclear Information Project at the Federation of American Scientists, said Mr. Williams “will be facing an incredibly complex, technical job.”

Mr. Williams did not return calls for comment on his selection by Mr. Trump or his credentials.

The credentials and credibility of whoever becomes N.N.S.A.’s new leader may face close scrutiny because advisers to Mr. Trump have suggested that the incoming administration may propose a restart to the nation’s explosive testing of nuclear arms. That step, daunting both technically and politically, would end U.S. adherence to a global test ban that sought to end decades of costly and destabilizing arms races.

From 2023 to early this year, Mr. Williams, a Republican, represented New York’s 22nd Congressional District, an upstate area that includes the cities of Syracuse and Utica. He was defeated by a Democrat in the November election.

Mr. Williams joined the U.S. Navy in 1991 and served as an officer on the U.S.S. Georgia, a nuclear submarine, before leaving the service as a lieutenant in 1996.

In his congressional biography, Mr. Williams said he made a successful transition during his Navy career into nuclear engineer training, calling it “a very steep learning curve” that he met “against significant odds.” The program is widely considered one of the U.S. military’s most demanding.

Mr. Trump announced his choice of Mr. Williams as the nation’s nuclear weapons czar in social media posts on Thursday morning, calling him “a successful businessman and Veteran of the U.S. Navy, where he served as a Nuclear Submarine Officer, and Strategic Missile Officer.”

According to his congressional biography, Mr. Williams founded “a software company that now helps large industrial manufacturers modernize their production plants, secure their critical infrastructure from cyberattacks, and paves the way for reduced emissions through advances in artificial intelligence.”

Chris Wright, Mr. Trump’s nominee for secretary of energy, the cabinet-level post that oversees the N.N.S.A., called Mr. Williams “a smart, passionate guy” who wants to “defend our country and make things better,” according to an interview on Wednesday with the website Exchange Monitor.

A lengthy 2022 profile of Mr. Williams described him as a multimillionaire who starts each morning by reading a section of the Bible. After high school, it said, Mr. Williams went to Baylor University, a private Christian school in Waco, Texas, and then transferred to Pepperdine University in Malibu, Calif.

His congressional biography says he earned a bachelors from Pepperdine in liberal arts, and later an MBA from the Wharton School, a contrast with the advanced degrees in physics or engineering that typically dot the résumés of weaponeers who end up in senior positions of the nation’s atomic complex.

The outgoing administrator of the National Nuclear Security Administration, Jill Hruby, offers a striking contrast with Mr. Williams in terms of technical background and nuclear experience. Before her 2021 nomination to the post, she had a 34-year career at Sandia National Laboratories, retiring in 2017 as director. By training, she is a mechanical engineer.

Sandia is one of the nation’s three nuclear weapons labs, with its main branch located in Albuquerque. It is responsible for the nonnuclear parts of the nation’s arsenal of atomic bombs and warheads.

Other N.N.S.A. administrators have had backgrounds in national security, nuclear operations, the military or scientific fields related to nuclear technology. The first was an Air Force general and a former deputy director of the C.I.A.

The overall responsibilities of the N.N.S.A. include designing, making and maintaining the safety, security and reliability of the nation’s nuclear arms; providing nuclear plants to the Navy; and promoting global atomic safety and nonproliferation. In Nevada, the agency runs a sprawling base larger than the state of Rhode Island, where the United States in the latter years of the Cold War tested its weapons in underground explosions.

Dr. Wallace, the former Los Alamos director, said he had tracked Mr. Trump’s search for an agency leader and found that “any candidate will be making a pitch for resumption.” He added, “That more or less disqualifies any recent director of any nuclear weapons lab.”

Many experts see a restart as unnecessary given the depth and breadth of the nation’s nonexplosive testing program, which the N.N.S.A. runs at an annual cost of roughly $10 billion. Experts argue that the program’s decades of analyses have led to deeper understanding of nuclear arms and greater confidence in weapon reliability than during the explosive era.

Dr. Wallace said Mr. Trump was aided in his hunt for a nuclear czar by Robert C. O’Brien, his national security adviser from 2019 to 2021. Last year in Foreign Affairs magazine, Mr. O’Brien, a lawyer, argued that Washington “must test new nuclear weapons for reliability and safety in the real world.” He added that the freshly tested arsenal would be a deterrent to China and Russia.

Republicans have long criticized the test ban and urged a testing restart. President Bill Clinton, a Democrat, signed the accord in 1996. In 1999, however, he suffered a crushing defeat when the Senate refused to ratify the test ban treaty.

In spite of the treaty’s defeat, successive administrations have informally abided the terms of the test ban. That position began to come under fire during Mr. Trump’s first administration.

In 2018, the Defense Department declared that “the United States must remain ready to resume nuclear testing.” John R. Bolton, Mr. Trump’s national security adviser from 2018 to 2019, reportedly argued for a restart but made little headway.

In 2020, when Mr. O’Brien was the national security adviser, the Trump administration reportedly discussed whether to conduct nuclear test explosions in a meeting with national security agencies.

Opponents of a restart see the nonnuclear tests as more than sufficient to ensure arsenal reliability. “We have more confidence today than when we stopped explosive testing,” Victor H. Reis, the program’s architect, said in an interview.

Siegfried S. Hecker, a former Los Alamos director, argued that a restart would probably start a chain reaction of testing among the world’s atomic powers and perhaps among the so-called threshold states. Like Iran, they’re considered close to being able to build a bomb.

Dr. Hecker noted that during the Cold War, China conducted 45 test explosions, France 210, Russia 715 and the United States 1,030. He said that Beijing, which in recent years has rebuilt its base for nuclear tests, had a major incentive to design and explosively test a new generation of nuclear arms. He argued that the arms could make its expanding missile force more lethal.

“China,” Dr. Hecker added, “has much more to gain from resumed testing than we do.”