An international nuclear security expert has warned that South Africa's nuclear facilities are a "cause for concern".
His warning comes in the wake of a third security breach at the Pelindaba nuclear research centre - a top national security keypoint - in seven years.
The centre, west of Pretoria, was used by the apartheid government to research and build nuclear weapons in the 1970s. It is now used to manufacture medical isotopes.
The other nuclear facility, Koeberg power station, is in Western Cape.
According to the international Nuclear Threat Initiative, Pelindaba contains 600kg of weapons-grade uranium, which is sufficient to build 20 nuclear bombs.
The government nuclear agency and regulator have been reluctant to speak about the security breach for over three months now.
However, Micah Zenko, a nuclear security researcher at the Council on Foreign Relations, in the US, has warned the reasons for the breach were irrelevant.
"Even if it is common criminality, there should be concern, especially as the facility has [weapons-grade uranium]. Though the level of sophistication of breaking into a facility by common criminals, and the breaking into a vault and removing [the uranium] by sophisticated criminals, is very different, it is incredibly problematic. The international community should be very concerned," he said.
The latest breach is one of four to have occurred at two of South Africa's nuclear facilities in the past decade, according to the National Nuclear Regulator.
But the regulator has downplayed the latest incident, insisting that it was not overly concerned.
According to a Business Day report this week, the breach occurred on April 28. The Nuclear Energy Corporation of SA reported it to the "relevant" government department nine days later and to the regulator on June 1.
The breach was five years after Pelindaba's emergency control manager, Anton Gerber, was shot after spotting four men stealing a laptop computer in the control room.
The gunmen, described by Nuclear Energy Corporation CEO Rob Adams as "technically sophisticated common criminals", were able to bypass Pelindaba's 10000V perimeter fence and move around the premises undetected for 45 minutes before shooting Gerber and escaping. No one was arrested.
In 2005, another laptop was stolen but, again, no one was arrested.
Police spokesman Captain Dennis Adriao said the break-in and theft took place at a students' residence next to the centre .
He declined to comment on the 2007 incident, saying it was "in the interest of national security as the facility is a national key point".
The Nuclear Energy Corporation, which operates Pelindaba, and the regulator refused to explain the threat assessments of South Africa's nuclear facilities.
Neither would reveal the security measures put in place, the nature of the latest breach, the outcome of the 2007 investigation or whether highly enriched uranium was being stored at Pelindaba.
But Zenko said the incidents at Pelindaba should be investigated.
He said it was troubling that the regulator declined to characterise break-ins and say what it was doing to prevent future breaches.
Zenko said that though regulators tended to minimise problems at nuclear facilities, it was imperative that parliament and the Presidencyinvestigate.
"Even though the [weapons-grade uranium] at Pelindaba is 'locked down', the number of breaches suggests vigilance is needed," he said.
Regulator spokesman Gino Moonsamy said South Africa's nuclear facilities had a "high level of security". He added there had been a "few security incidents" at Koeberg and Pelindaba.
"These have not exceeded four and are not severe. Due to adequate physical protection, no nuclear or radioactive material was accessed, lost or stolen," he said.
Asked about the outcome of the 2007 Pelindaba security breach investigation, Moonsamy declined to comment "due to the sensitive nature of nuclear security".
He said the regulator was not "unduly concerned" about safeguarding nuclear sites.
Van Zyl de Villiers, the Nuclear Energy Corporation's strategy and performance group executive, said the latest incident was an unsuccessful security breach.
"No security systems were disarmed, no shootings occurred and no arrests were made."
Asked why the investigation started so late, De Villiers said: "The issue of deadlines is subject to mutual engagements between the regulator and the operator. The regulator is satisfied with a comprehensive response."
He said a report had been submitted to the regulator with recommendations on preventing similar incidents.
He refused to reveal what happened in the latest incident or whether there were new measures to stop breaches, saying that would be equivalent to revealing the corporation's security plans.