By Yeganeh Torbati
MINOT AIR FORCE BASE, N.D. (Reuters) - In this flat, windy expanse just south of the Canadian border, U.S. Air Force pilots fly the same bombers their grandfathers flew, using mid-20th century cables and pulleys.
Each spring, the airmen and airwomen must clear melting snow from the steel and concrete doors of the silos that house 150 nuclear-tipped ballistic missiles scattered across thousands of square miles of the upper Great Plains.
The U.S. nuclear arsenal, patched, welded, and re-skinned countless times, was built between 25 and 62 years ago when the United States found itself locked in an arms race with a rival superpower, the Soviet Union. Now, its future is an issue in the campaign for the Nov. 8 presidential election.
Democratic candidate Hillary Clinton has said she would call for a nuclear posture review, last completed in 2010, as one of her first acts upon taking office. Republican candidate Donald Trump has said he would be open to reversing decades of U.S. policy and allowing allies such as Japan and South Korea to acquire their own nuclear weapons to deter a strike from North Korea, which carried out its fifth and largest nuclear test this month.
On Monday, in his first visit to Minot Air Force Base in North Dakota as U.S. Secretary of Defense, Ash Carter said America's nuclear deterrence was the "bedrock" of its security, and the Pentagon's No. 1 priority.
"If we don't replace these systems, quite simply they will age even more, and become unsafe, unreliable, and ineffective," Carter said, speaking at a lectern in front of a B-52 bomber loaded with cruise missiles.
"The fact is, most of our nuclear weapon delivery systems have already been extended decades beyond their original expected service lives. So it's not a choice between replacing these platforms or keeping them - it's really a choice between replacing them or losing them," Carter said.
Russia, he said, had built new nuclear weapons systems, raising questions as to whether its leaders were cautious enough when it came to atomic weapons. And North Korea presents a sustained threat, Carter said.
Carter's speech coincides with a growing realization among defense officials and experts that budget constraints almost certainly will force the next president to decide whether and how quickly to proceed with the Obama administration's plans to maintain and modernize the U.S. nuclear arsenal.
The crunch comes in the next decade as American ballistic missile submarines, bombers, and land-based missiles – the three legs of the nuclear triad – reach the end of their useful lives.
The Congressional Budget Office estimates the total cost of nuclear forces through 2024 at $348 billion, but that does not include some of the costliest upgrades, scheduled for the latter half of the next decade. Independent estimates have put the cost of maintaining and modernizing the arsenal at about $1 trillion dollars over 30 years.
"There's a bipartisan commitment to doing that upgrade, so we have to assume those funds will come through," Energy Secretary Ernest Moniz told Reuters on Sept. 20. "But it will be a significant budget increase, especially in the next decade."
The Energy Department shares responsibility with the Pentagon for the nuclear arsenal, and some of its research and production facilities are 73 years old.
The next administration could abandon or delay some aspects of modernization to cut costs. Or it could raise taxes, increase the budget deficit, or cut domestic programs, all unpopular steps with American voters.
The most vulnerable elements of the modernization plans are a long-range standoff weapon, or LRSO – a nuclear-capable cruise missile launched from an aircraft – and new land-based intercontinental ballistic missiles (ICBMs).
Ten U.S. senators, including Elizabeth Warren of Massachusetts and Bernie Sanders of Vermont, called on President Barack Obama in July to cancel the LRSO, saying it "would provide an unnecessary capability that could increase the risk of nuclear war."
Some Pentagon officials and defense experts have said the cruise missile would be a hedge against improved air defenses that are difficult for even a stealthy bomber to penetrate.
A Clinton or Trump administration also could cut the number of land-based ballistic missiles below the 400 currently planned or delay a new missile by extending the life of the current Minuteman IIIs, which each carry a warhead with an explosive yield of at least 300 kilotons, 20 times more powerful than the atomic bomb dropped on Hiroshima, which killed an estimated 140,000 people.
The United States is one of five nuclear weapons states allowed to keep a nuclear arsenal under the Nuclear Non-Proliferation Treaty. The others are Russia, Britain, France and China.
The U.S. Air Force has estimated the cost of a new ground-based system – including missiles, command-and-control systems and launch control centers – to be more than $60 billion.
Some former U.S. officials, including former defense secretary William Perry, have argued that land-based missiles are not essential and should be phased out; proponents say they are more important in the face of potential nuclear threats by Russia and North Korea.
Much of the planned modernization is nearly locked in because of the need for new weapons and because some of it is so far along, said Evan Montgomery, a senior fellow at the Center for Strategic and Budgetary Assessments, a Washington-based research group.
The B-21 long-range strike bomber and the replacement for the Navy's 14 Ohio-class ballistic missile submarines are "the two most expensive items, and they're arguably the two safest in a lot of ways," Montgomery said. The bomber can deliver both conventional and nuclear weapons, and the submarine is considered a priority because it would survive any first strike by an adversary.
The Navy plans to replace its Ohio-class submarines, first commissioned in 1981 and armed with up to 24 Trident missiles with multiple independently targeted warheads, with 12 boats costing about $100 billion.
In his speech on Monday, Carter said that most people do not realize that spending on the nuclear program is a small percentage of total defense spending. At its peak, nuclear spending would make up about five percent of the Pentagon's budget, which is now around $600 billion annually, defense experts said.
Still, the funding gap means that despite the Pentagon's massive budget, the next president will face an unavoidable dilemma.
"There's no doubt in my mind that we're tens of billions of dollars short in that time frame from what it takes to execute a global strategy," said James Miller, a former undersecretary of defense for policy in the Obama administration. "It's not a rounding error anymore."
(Reporting by Yeganeh Torbati; additional reporting by Timothy Gardner; Editing by John Walcott and Grant McCool)