South Africa stores nearly a quarter-ton of uranium that could be readily fashioned into an atomic bomb at the Pelindaba Nuclear Research Center. (Douglas Birch/Center for Public Integrity)

Enough nuclear explosive to fuel half a dozen bombs, each powerful enough to obliterate central Washington or most of Lower Manhattan, is locked in a former silver vault at a nuclear research center near the South African capital.

Technicians extracted the highly enriched uranium from the apartheid regime’s nuclear weapons in 1990, then melted the fuel down and cast it into ingots. Over the years, some of the cache has been used to make medical isotopes, but roughly 485 pounds remains, and South Africa is keeping a tight grip on it.

That gives this country — which has insisted that the United States and other world powers destroy their nuclear arsenals — a theoretical ability to regain its former status as a nuclear-weapons state. But what really worries the United States is that the nuclear explosives here could be stolen and used by militants to commit the worst terror attack in history.

Senior current and former U.S. officials say they have reason to be concerned. On a cold night in November 2007, two teams of raiders breached the fences here at the Pelindaba research center, set in the rolling scrubland a half-hour’s drive west of Pretoria, the country’s administrative capital. One group penetrated deep into the site unchallenged and broke into the site’s central alarm station. They were stopped only because a substitute watch officer summoned others.

The episode remains a source of contention between Pretoria and Washington because no suspects were ever charged with the assault, and officials here have dismissed it as a minor, bungled burglary. U.S. officials and experts — backed up by a confidential South African security report — say to the contrary that the assailants appeared to know what they were doing and what they wanted: the bomb-grade uranium. They also say the raid came perilously close to succeeding.

The episode still spooks Washington, which as a result has waged a discreet diplomatic campaign to persuade South Africa to get rid of its large and, by U.S. reckoning, highly vulnerable stock of nuclear-weapons fuel.

Graphic: Global stock of weapons uranium

But South African President Jacob Zuma, like his predecessors, has resisted the White House’s persistent entreaties and generous incentives to do so, for reasons that have partly baffled and enormously frustrated the Americans.

President Obama, in a previously undisclosed private letter sent to Zuma in August 2011, went so far as to warn Zuma that a terrorist nuclear attack would be a “global catastrophe.” He proposed that South Africa transform its nuclear explosives into benign reactor fuel, with U.S. help.

If Zuma agreed, the White House would trumpet their deal at a 2012 summit on nuclear security in South Korea, Obama wrote, according to a copy of the letter. Together, he said, the two nations could “better protect people around the world.”

Zuma was unmoved, however, and in a letter of his own, he insisted that South Africa needs its nuclear materials and was capable of keeping them secure. He did not accept a related appeal from Obama two years later, current and former senior U.S. officials said.

Differing points of view

The United States says there are reasons to be concerned about South Africa’s nuclear explosives. (Jim Watson/Agence France-Presse/Getty Images)

Washington may bear a special responsibility for ensuring that South Africa’s materials do not wind up in the wrong hands.

Over nine years ending in 1965, it helped South Africa build its first nuclear reactor under the Atoms for Peace program and then trained scientists to run it with U.S.-supplied, weapons-grade uranium fuel. Washington finally cut off the fuel supply in 1976, after becoming convinced the apartheid regime had used nuclear research to create a clandestine bomb program, fueled by its own highly enriched uranium.

The apartheid regime hatched the bomb program at a time when it faced sabotage at home, wars on its borders and increasing international isolation. But by the end of the Cold War, the government realized that its whites-only rule would have to be scrapped, and so its leaders ordered the weapons destroyed and the production facilities dismantled, while holding onto the explosive fuel.

In interviews, top officials in both countries made clear that they see the issue through different prisms. Zuma’s appointees assert that it is absurd for the United States to obsess over the security of the country’s small stockpile while downplaying the starker threat posed by the big powers’ nuclear arsenals.

Raising the threat of nuclear terror, officials here say, is an excuse to restrict the spread of peaceful and profitable nuclear technology to the developing world, and to South Africa in particular.

This claim of being singled out is similar to that made by another emerging nuclear power: Iran. And for good reason: Both countries defiantly constructed facilities to enrich uranium in the past, over foreign opposition, and want the rest of the world to agree they have a right to do it in the future. They have long been diplomatic friends and trading partners and have discussed helping one another’s nuclear research.

But this demand for enrichment rights — which Tehran wants enshrined in an agreement with six great powers — is hardly theirs alone. Although the Obama administration has tried to discourage uranium enrichment everywhere, leaders in Brazil, Argentina, Egypt, Saudi Arabia, Vietnam, Jordan and South Korea say they see nuclear power, along with the ability to enrich uranium, as their right.

By most accounts, Iran doesn’t have significant amounts of weapons uranium, only the means to make it. But it stands accused by the International Atomic Energy Agency (IAEA) — and behind it, by the United Nations Security Council — with failing to come clean about past nuclear work with weapons applications. That’s why Iran has been hit with sanctions.

South Africa, in contrast, was praised by the IAEA in 1995 for “transparency and openness” in discussing its weapons program. The agency also declared it had no reason to suspect that South Africa’s inventory of fissile materials was incomplete or that the program had not been completely stopped and dismantled.

Unlike Iran, however, South Africa already possesses highly enriched uranium — nearly a quarter-ton of it, which the United States has tried but failed to pry loose. That’s why current and former U.S. officials say that South Africa is now the world’s largest uncooperative holder of nuclear explosives, outside of the nine existing nuclear powers.

Few outside the weapons states possess such a large stockpile of prime weapons material, and none has been as defiant of U.S. pressure to give it up.

Told what this story would say, the South African government responded Friday with a statement reaffirming its view that the November 2007 break-in was a run-of-the-mill burglary and asserting that the weapons uranium is safe.

“We are aware that there has been a concerted campaign to undermine us by turning the reported burglary into a major risk,” said Clayson Monyela, spokesperson for the country’s foreign ministry, called the Department of International Relations and Cooperation. He said the IAEA had raised no concerns, and that “attempts by anyone to manufacture rumors and conspiracy theories laced with innuendo are rejected with the contempt they deserve.”

A crime problem

Experts consider highly enriched uranium the terrorists’ nuclear explosive of choice. A bomb’s worth could fit in a five-pound sack and emit so little radiation that it could be carried around in a backpack with little hazard to the wearer. Physicists say a sizable nuclear blast could be readily achieved by slamming two shaped chunks of it together at high speed.

Several months before becoming responsible for White House nonproliferation policies last year, arms control expert Jon Wolfsthal told the Center for Public Integrity in an interview that the U.S. motives for seeking to clean out South Africa’s weapons uranium were straightforward and that they focused on the stockpile held at Pelindaba.

Graphic: Break-in at Pelindaba Nuclear Research Center

“The bottom line is that South Africa has a crime problem,” Wolfsthal said. “They have a facility that is holding onto material that they don’t need and a political chip on their shoulder about giving up that material. That has rightly concerned the United States, which is trying to get rid of any cache of HEU [highly enriched uranium] that is still out there.”

Thanks in part to U.S. efforts, just nine nonnuclear-weapon states besides South Africa still have enough enriched uranium to build a nuclear weapon, although mostly not in a readily usable form, according to Miles Pomper, senior research associate at the James Martin Center for Nonproliferation Studies: Germany, Japan, Canada, Belgium, Kazakhstan, Poland, Italy, the Netherlands and Belarus.

Each has been similarly asked by Washington and its allies to reduce or eliminate their stocks of highly enriched uranium. Canada, Japan, Kazakhstan, Italy and Poland promised publicly at the 2014 White House nuclear security summit to reduce their holdings in the next few years. Belgium said it would eliminate its stocks “in time.”

For South Africa, maintaining a grip on its bomb fuel is tangled up with its national pride, its suspicion of big power motivations and its anger over Washington’s past half-measures in opposing apartheid. “It’s a technical issue with an emotional overhang,” said Donald Gips, the U.S. ambassador to South Africa from 2009 to 2013.

Some of its top officials complained privately, Gips said, that Washington’s pressure stems from a conviction that Africans “cannot be trusted to keep nuclear materials.”

Other South Africans have said that by refusing to let go of its uranium, the country retains the higher political and scientific stature of a country such as Japan, which is considered “nuclear weapons-capable” while possessing none.

The chief obstacle to achieving one of the White House’s top arms control priorities, according to U.S. officials, is Zuma, the president since May 2009. He led the ruling African National Congress (ANC) to another victory last year with 62 percent of the vote and could serve at least through 2019.

Zuma, a former ANC intelligence chief, is a shrewd populist and one of the most influential figures in the Non-Aligned Movement representing 120 mostly developing nations. That’s why Washington thought swift action by Zuma could set a valuable precedent.

Obama’s election was celebrated here, and the two presidents seemed to forge a personal bond at their first meeting in July 2009, raising White House hopes for progress. A team of senior Energy and State department officials traveled to Pretoria a month later to sell the idea of relinquishing the explosive materials.

Obama invited Zuma to a series of White House summits on nuclear security and dispatched scientists from U.S. nuclear-weapons labs and FBI antiterrorist experts to help protect the 2010 World Cup in Johannesburg against nuclear-related threats.

After Zuma nonetheless rejected Obama’s 2011 plea, Obama raised the issue again, during a trip to Pretoria in June 2013.

This time, he privately asked Zuma to relinquish a different trove of weapons-usable uranium — still embedded in older reactor fuel that by U.S. accounts is lightly guarded — in exchange for a free shipment of 772 pounds of fresh, non-weapons-usable reactor fuel, valued at $5 million.

Obama followed up with a three-page letter in December 2013, two days after he spoke with Zuma at Nelson Mandela’s memorial service in Soweto. According to a copy of the letter, he urged Zuma to seal this new deal at a March 2014 nuclear summit in the Netherlands.

Although technical experts held preliminary talks, Zuma never accepted the swap and didn’t bother to attend that summit, sending Foreign Minister Maite Nkoana-Mashabane in his place.

There, the South African emissary told reporters that the summits should “wrap up” their work and leave nuclear security to the IAEA, which considers the expansion of civilian nuclear power a key mission.

Fear of “what could go wrong” with nuclear technology, she said, should not violate the “inalienable rights” of countries to use enriched uranium for peaceful purposes. “We have no ambition for building a bomb again. That is past history,” Nkoana-Mashabane said. “But we want to use this resource.”

South Africa has used some of the former bomb fuel to make medical and industrial isotopes — generating $85 million in income a year. But about six years ago, South Africa started making the isotopes with low-enriched uranium that poses little proliferation risk — a decision that robbed it of its long-standing rationale for keeping the materials.

Now officials here say they’re retaining their weapons uranium partly because someday someone may find a new, as-yet-undiscovered, commercial application. If and when one is found, a senior South African diplomat said in an interview, “it’ll be like OPEC to the power of 10,” where states without the material would be at the mercy of a cartel of foreign suppliers.

Ambassador ‘No’

As a member of the International Atomic Energy Agency’s governing board from 1995 to 2011, Abdul Samad Minty was considered a leader of the Non-Aligned Movement of 140 mostly developing nations, many of them skeptical of nonproliferation programs. (Douglas Birch/Center for Public Integrity)

Pretoria’s determination to keep its weapons uranium dates to the apartheid era, but the most vocal advocate in democratic South Africa has been Abdul Samad Minty, who served for most of the past two decades as his country’s top nuclear policymaker.

Gary Samore, the White House coordinator on weapons of mass destruction from 2009 to 2013, called Minty “a worthy adversary for me in all of the nuclear security summits,” who was “deeply, emotionally opposed to giving up their HEU.”

Minty, 75, now South Africa’s ambassador to U.N. agencies headquartered in Geneva, sipped green tea in his office as he explained that it is the United States that is recalcitrant. Even as it campaigns to halt the spread of nuclear weapons, he says, it refuses to part with its own.

“The problem is you can’t have nuclear-weapons states who feel they can have nuclear weapons and have as many as they want,” he said.

Stocks of fissile materials held by countries outside the small club of nuclear-weapons states, he said, are just “not that important” a threat, compared with the thousands of nuclear weapons held by the bigger powers.

As an ANC activist for 30 years, Minty successfully pushed to have the regime expelled from the IAEA’s Board of Governors. Named South Africa’s top representative to the IAEA in 1995, Minty became a regular thorn in the side of the West. He abstained in 2005 and 2006 on resolutions referring Iran’s nuclear program to the U.N. Security Council, arguing the resolutions were procedurally flawed or premature.

The IAEA, the 75-year-old diplomat said, cannot be used as a tool to undermine the “basic right” of nonnuclear countries to develop their own nuclear industries, by setting expensive and restrictive security standards.

He also harshly criticized the 1970 Non-Proliferation Treaty — in which the members of the U.N. Security Council agreed to get rid of their nuclear arsenals if the rest of the world promised not to acquire them — for not pressuring the major powers to disarm.

“Yes they are reducing, not disarming,” Minty said. “Now if you say you need nuclear weapons for your security, what stops another country from saying at another time, in another situation, I also need nuclear weapons for my security?”

“People who smoke can’t tell someone else not to smoke,” Minty said.

Bitterness and resentment

U.S. officials reject this reasoning. “Nuclear disarmament is not going to happen,” Samore said he told Minty, and waiting for it is a dangerous excuse for inaction. “It’s a fantasy. We need our weapons for our safety, and we’re not going to give them up.”

According to U.S. officials and experts, South Africa uses only about 16.5 pounds of its remaining stock of weapons uranium to make isotopes annually, out of a total stockpile estimated by foreign experts at around 485 pounds. And it need not use it at all.

Some American officials say they think Minty still bears a grudge from vigorous U.S. opposition to his bid to replace Mohamed ElBaradei as director general of the IAEA in 2009. Minty fought hard, but he had angered U.S. officials by making supportive comments about Iran, including an assertion early in 2008 that “there is increasing confidence in the Iranian enrichment program.”

Getting beyond the impasse

Waldo Stumpf, a longtime atomic energy official in South Africa who presided over the dismantlement of the apartheid-era bomb program, said in an interview that handing over the highly enriched uranium “was never part of the thinking here. Not within Mr. [Frederik W.] de Klerk’s government. Not afterwards, when the ANC took over. Why would we give away a commercially valuable material that has earned a lot of foreign exchange? Why would we do that?”

In fact, South Africa intends not only to keep its existing enriched uranium, officials here say, but also insists on the right to make or acquire more. “Our international legally binding obligations . . . allow for the enrichment of uranium for peaceful purposes only, irrespective of the enrichment level,” Zuma said at the 2012 nuclear security summit in Seoul.

Asked about South Africa’s policy, a former senior Obama administration official who spoke on the condition of anonymity because of diplomatic sensitivities said that after U.S. officials pressed their arguments “at every level possible,” he became convinced that South Africa would not give up its nuclear explosives so long as Zuma remains in power.

Xolisa Mabhongo, who served from 2010 to 2014 as South Africa’s ambassador to the IAEA and last year moved to a senior executive post at the South African Nuclear Energy Corp., confirmed this assessment.

“I don’t think there is any incentive that can be offered” that South Africa would trade for its weapons uranium, Mabhongo said. “It’s our property. We do not see the need to give it to anybody else. [President Thabo] Mbeki explained this to Bush, and Zuma explained this to Obama. So I don’t think this position is ever going to change.”

Birch reported from Washington and South Africa; Smith reported from Washington. This article comes from the Center for Public Integrity, a nonpartisan, nonprofit investigative news organization.