ABC News Exclusive: Inside the US nuclear ballistic missile submarine in South Korea

This week's arrival of a nuclear-capable U.S. Navy submarine in Busan, South Korea, marked the first time that a submarine capable of carrying up to 20 nuclear-tipped ballistic missiles had made a port of call in South Korea.

The rare public visit is intended to demonstrate America's security commitment to South Korea and to deter North Korea.

ABC News Chief Global Affairs Correspondent Martha Raddatz was given exclusive access to the USS Kentucky in Busan, South Korea, on Thursday, the only American journalist allowed to visit the submarine during its port of call in South Korea.

Busan, South Korea's largest port, is located in the most southeasterly point of the Korean peninsula and is more than 200 miles south of the DMZ (demilitarized zone) that serves as the border with North Korea.

More than two football fields in length, the USS Kentucky (SSBN 737) is one of 14 Ohio Class U.S. Navy submarines capable of launching 20 Trident 2 D5 missiles, each armed with multiple targeted warheads capable of striking targets up to 4,000 miles away.

As a standard practice the U.S. Navy does not say if there are nuclear weapons aboard its Ohio-class submarines but it is no secret that the submarines can carry hundreds of nuclear warheads atop those missiles, making up about 70% of the nation's active nuclear arsenal allowed by the New START Treaty.

The deployments of these submarines are highly classified and it is extremely rare for them to make a port of call, but the Kentucky's visit realized a commitment made in April by President Joe Biden and South Korean President Yoon Suk Yeol that an American ballistic missile submarine would visit South Korea in a demonstration of U.S. security commitments.

"It represents our enduring relationship with the Republic of Korea, our security commitment and our extended deterrence. It assures our allies and it deters any potential adversaries," Rear Admiral Chris Cavanaugh, Commander, Submarine Group 7, told Raddatz in an interview aboard the USS Kentucky.

Shortly after the submarine's arrival in Busan on Tuesday, North Korea launched two short-range ballistic missiles in an apparent response to the rare port of call. On Thursday, North Korean Defense Minister Kang Sun-nam warned that the sub's visit posed a threat to North Korea and might fall under that country's conditions for the use of nuclear weapons.

The tensions with North Korea have been evident this week as an American soldier, Army Pvt. Travis King, darted across the DMZ in Panmunjon into North Korea where he is now believed to be in the custody of North Korean authorities.

Cavanaugh said Ohio-class submarines like the Kentucky deter the possibility of a nuclear conflict.

"I am very confident in our own nuclear deterrence. Again, any adversary that would contemplate an attack knows that we have a massive response capability that's untargeted and located by them," he told Raddatz.

That nuclear deterrence is also intended to reassure South Korea's security concerns.

"We do a whole host of things to assure them of our extended deterrence, which means we don't take any capabilities off the table when it comes to defending our allies," said Cavanaugh.

South Korean President Yoon and other senior Korean and American military leaders visited the submarine on Wednesday.

Yoon said the visit "demonstrates the U.S.'s routine deployment of strategic assets and the will of the two countries to defend the ability to execute extended deterrence."

"This means North Korea can't even dream of a nuclear provocation, and it serves as a clear warning to North Korea that such a provocation would spell the end of the regime," Yoon said.

The approximately 150 sailors aboard the USS Kentucky are being commanded during this current deployment by Cmdr. Lee "Randy" Fike who told Fife told Raddatz that his crew took a great amount of pride in serving aboard the first ballistic missile submarine to visit South Korea since 1981.

Daily life aboard Ohio-class submarines during months-long deployments involves a lot of training for the sub's crew and officers, especially in the submarine's missile control center where the crew simulates the launch procedures for the ICBM's it carries.

"It's a large focus of what we do day in and day out training to make sure that we're ready to demonstrate that we have a safe, secure and effective nuclear deterrent," said Fike. " And hope you never have to use it. Absolutely."

The responsibility of being a vital component of America's nuclear triad is not lost on the crew, some of whom have witnessed first-hand the power of a test launch a Trident missile from aboard a submarine.

"It's very, very sobering," Cmdr. Fike told Raddatz. "We go through these training simulations all the time, but nothing can really replicate the feeling that happens when 100,000 pounds of D-5 missile leaves a submarine."

Because ports of call during submarine patrols are so rare, Fike noted that as a veteran submariner the visit to the Busan marked his first-ever port of call during a deployment.

"The gravity of what we've had the opportunity to do and come interoperate with our our allies in the Republic of Korea, it's absolutely amazing," said Fike.

"For most of my crew, this is the first time they've set foot on a foreign country, said Fike. "So it's great opportunity for us. And the host nation has been absolutely welcoming."

Allowed to visit ashore, 24-year-old Missile Technician 2 Ryan Shirley enjoyed a visit to a local shopping mall in South Korea.

"it's a different change of scenery and how they operate over there in South Korea," Shirley told Raddatz.

But the history of the moment was not lost on Shirley who admitted to having thought about the pending visit for days.

"I think it's really good," said Nichols..

Petty Officer 2nd Class Tyler Forner from Savannah, Georgia, who re-enlisted in the Navy on Thursday after having served three years aboard the USS Kentucky.

Forner labeled the port of call "historic" and "a cool experience" for the U.S. Navy's submarine force to be able to make a port of call to Busan.

It is unclear how long the USS Kentucky will remain in port in Busan.