But there are suggestions, reinforced by official confirmation that all relevant documentation had been shredded, that he was economical with the truth. In particular these refer to his contention that South Africa, which built - and recently destroyed - six Hiroshima-sized bombs to deter the perceived threat of Soviet expansionism, had not co-operated with other countries in the development of its nuclear technology.
Such doubts have fuelled fears, in turn, that nuclear materials and expertise might have been supplied, officially or unofficially, to countries considered dangerous by the West. International experts on the nuclear weapons business have expressed scepticism at some of Mr de Klerk's claims, arguing that it is inconceivable South Africa, given the level of its domestic technology, could have developed its own programme without outside assistance.
Abdul Minty, director of the world campaign against military and nuclear collaboration with South Africa, said logic alone told one that outside help had been provided. 'South Africa in fact had no choice but to pick up foreign scientists. For example, when prime minister John Vorster announced in 1970 that South Africa had developed its own unranium-enriching process he was not telling the truth. The process was actually German and it required German know-how.'
Mr Minty added that a foreign-assisted commercial nuclear facility outside Cape Town, the Koeberg power plant, had provided a front for the development of nuclear weapons.
Martin Navias, a lecturer in the Department of War Studies at King's College, agreed with Mr Minty. 'There is no great difference between the kind of expertise required for nuclear energy production and that needed to develop the bomb.'
As to international assistance, Mr Navias said natural uranium, which might have been partially enriched, had been sold to South Africa by Switzerland, Belgium, China, Germany and France, which also supplied South Africa with civil nuclear reactors. In the mid-Seventies, the United States had sold South Africa 45 per cent enriched uranium.
Yesterday's Johannesburg Citizen newspaper carried a report quoting an expert from the International Institute for Strategic Studies, Marie-Helene Labbe, saying Israel had supplied nuclear experts in exchange for South African uranium and permission to conduct nuclear tests on South African territory. This allegation has often been made but Mr de Klerk roundly denied it in his speech.
Mr Navias was inclined to give Mr de Klerk the benefit of the doubt. Evidence, he said, certainly existed that South Africa had intended to conduct a nuclear test. But after pressure from President Carter and other Western leaders it was called off. 'If South Africa only had six nuclear devices, and these were under 20 kilotons, then tests would not have been absolutely necessary. They could have relied on other people's information.'
One question which Mr de Klerk was not prepared to answer on Wednesday was how much weapons-grade enriched uranium South Africa still possessed. Mr Minty said that, according to leaks from International Atomic Energy Agency inspection teams that visited South Africa last year, the stockpile could be 350kg - enough for a dozen nuclear warheads.
The South African Foreign Minister, Pik Botha, met senior US officials in Washington last week to discuss his country's nuclear programme, and to negotiate the sale of the entire enriched uranium stock. Mr Botha, it was reported, sought to boost Pretoria's bargaining position by seeking to persuade US officials of the dangers implicit in an ANC-dominated government controlling the country's nuclear capacity. The ANC enjoys close relations with the likes of Libya and the PLO.
Western officials have cause to ponder whether Pretoria has beaten the ANC to the count in sharing their nuclear secrets with undesirables.
Even if the West is loath to doubt Mr de Klerk's personal integrity, his assurances on Wednesday were based largely on information from officials in the arms establishment. They, in turn, are intricately tied in with the military who, Mr de Klerk acknowledged last year, had long withheld information from him.