Inside the US nuclear ballistic missile submarine in South Korea.

The arrival this week in Busan, South Korea of ​​a nuclear-capable US Navy submarine marked the first stopover in South Korea by a submarine capable of carrying up to 20 ballistic missiles with nuclear warheads.

The rare public visit is intended to demonstrate the US security commitment to South Korea and deter North Korea.

On Thursday, ABC News international affairs correspondent Martha Raddatz was given exclusive access to the USS Kentucky in Busan, South Korea, becoming the only American journalist allowed to visit the submarine during its stopover in South Korea.

Busan, South Korea’s largest port, is located at the most southeastern point of the Korean peninsula and is more than 200 miles south of the DMZ (demilitarized zone) that borders North Korea.

Measuring more than two football fields in length, USS Kentucky (SSBN 737) is one of 14 Ohio-class Navy submarines capable of launching 20 Trident 2 D5 missiles, each armed with multiple nuclear warheads capable of hitting targets up to 4,000 miles away.

As is standard practice, the US Navy won’t say if there are nuclear weapons aboard its Ohio-class submarines. Still, it’s no secret that the subs can carry hundreds of nuclear warheads atop those missiles, constituting about 70% of the nation’s active nuclear arsenal allowed under the New START Treaty.

Deployments of these submarines are highly classified, and stopovers are extremely rare. Still, Kentucky’s visit fulfilled a commitment made in April by President Joe Biden and South Korean President Yoon Suk Yeol that a US ballistic missile submarine would visit Korea for the South demonstration of the security commitments of the United States.

“It represents our enduring relationship with the Republic of Korea, our commitment to security, and our expanded deterrence. It secures our allies and deters any potential adversaries,” Rear Admiral Chris Cavanaugh, director of the US Pacific Fleet Maritime Headquarters, told Raddatz in an interview aboard the USS Kentucky.

Shortly after the submarine arrived in Busan on Tuesday, North Korea launched two short-range ballistic missiles in an apparent response on a rare scale. On Thursday, North Korean Defense Minister Kang Sun-nam warned that the submarine visit posed a threat to North Korea and could fall within North Korea’s conditions for the use of nuclear weapons.

Tensions with North Korea have come to the fore this week when a US soldier, Army Private Travis King, plunged through the Panmunjon demilitarized zone into North Korea, where he is now believed to be in the custody of North Korean authorities.

Cavanaugh asserted that Ohio-class submarines, like the Kentucky, deter the possibility of nuclear conflict.

“I am very confident in our own nuclear deterrent. Again, any adversary contemplating attack knows that we have a massive response capability with no targets they can locate,” he told Raddatz.

That nuclear deterrent is also meant to reassure South Korea on security.

“We do a whole host of things to ensure our extended deterrence for you, which means we don’t take any capabilities off the table when it comes to defending our allies,” Cavanaugh said.

On Wednesday, south Korean President Yoon and other top Korean and US military commanders visited the submarine.

Yoon said the visit “demonstrates the routine deployment of US strategic assets and the willingness of the two countries to defend the ability to execute extended deterrence.”

“This means that North Korea cannot even dream of a nuclear provocation, and it serves as a clear warning to North Korea that such a provocation would mean the end of the regime,” Yoon said.

The approximately 150 sailors aboard the USS Kentucky are commanded during this current deployment by Commander Lee “Randy” Fike, who told Raddatz that his crew was very proud to serve aboard the first ballistic missile-launching submarine to visit South Korea since 1981.

During months-long deployments, daily life aboard the Ohio-class submarines involves extensive training for the crew and submarine officers, especially in the submarine’s missile control center, where the crew simulates launch procedures for the ICBMs they carry.

“It’s an important part of our daily training to make sure we’re prepared to demonstrate that we have a safe and effective nuclear deterrent,” says Fike. “And we hope we never have to use it. Of course”.

The responsibility of being a vital component of the US nuclear triad is not lost on the crew, some of whom have witnessed firsthand the power of a Trident missile test launch from aboard a submarine.

“It’s very, very sobering,” Commander Fike told Raddatz. “We do these training drills all the time, but nothing can really replicate the feeling you get when a 100,000 pound D-5 missile comes out of a submarine.”

Since ports of call during submarine patrols are so infrequent, Fike noted that, as a veteran submariner, the visit to Busan marked his first stop during a deployment.

“The severity of what we’ve had the opportunity to do and come to interoperate with our allies in the Republic of Korea is absolutely incredible,” Fike said.

“For most of my crew, it’s the first time they’ve set foot in a foreign country,” Fike said. “It is a great opportunity for us. And the host country has been absolutely welcoming.”

Ryan Shirley, 24-year-old missile technician 2, who was allowed to go ashore, enjoyed a visit to a local shopping mall in South Korea.

“It’s a change of scenery and how they operate there in South Korea,” Shirley told Raddatz.

But the story of the moment was not lost on Shirley, who admitted to thinking about the pending visit for days. Petty Officer 2nd Class Tyler Forner, from Savannah, Ga., reenlisted in the Navy Thursday after serving three years aboard the USS Kentucky.

Forner called the stop “historic” and “a great experience” for the US Navy submarine force to call at Busan. It is unclear how long the USS Kentucky will remain in Busan port.

Martha Raddatz and Luis Martinez