The day this small country frightened the U.S. with its project to build a nuclear weapon
The secret plan to turn Spain into a nuclear power advanced successfully for decades, but Franco never dared to take the final step.
There are nine nuclear-armed countries in the world. Russia (5,580 warheads), USA (5,044), China (500), France (290), Great Britain (225), India (172), Pakistan (170), Israel (90, not officially confirmed) and North Korea (50). In total, some 12,121 warheads. Of these, 2,100 are on high operational alert, almost all of them by Russia and the USA. That means they are active, loaded, aimed at a target and can be launched in less than 10 minutes. That’s enough to cause massive global destruction. Another 1,800 are deployed on ballistic missiles, bombers, submarines or military bases and can also be launched immediately as soon as a target is designated.
Additionally, around 150 B61 nuclear bombs are currently deployed at air bases in Germany, Belgium, the Netherlands, Italy, and Turkey. Although these weapons are owned by the United States, in the event of a conflict, those countries are authorized to use them aboard their aircraft with U.S. approval. Separately, South Africa developed nuclear weapons in the 1970s, eventually assembling six operational enriched uranium warheads. However, with the end of apartheid in 1989, the South African government chose to dismantle its warheads and terminate its nuclear program.
The Birth of ‘Project Islero’
One of the most intriguing cases of nuclear weapons development is that of Spain. The project emerged in the late 1950s, following the Ifni War (1957–58), with the aim of creating a small nuclear arsenal as a deterrent against Morocco and its claims over Spanish territories in Africa. Spain also possessed rich uranium deposits capable of sustaining a nuclear program without relying on foreign sources.
In 1963, Muñoz Grandes, then Vice President under Franco, commissioned a feasibility report on the project. The report was authored by Guillermo Velarde, a military officer and nuclear physicist trained in the United States, who named it ‘Islero’ after the bull that killed the famous matador Manolete. In his report, Velarde confirmed that building an atomic bomb was feasible if Spain could acquire a nuclear reactor to produce plutonium from the uranium it was already extracting.
The Palomares Incident
However, the initial steps were far from encouraging. The investment required to achieve results was enormous, Spain had no operational nuclear power plants, and no one fully understood the detonation mechanism of an atomic bomb—until January 17, 1966. On that day, a U.S. Air Force tanker collided mid-air with a B-52 Stratofortress carrying four hydrogen thermonuclear bombs over Palomares, in the province of Almería. Three of the bombs fell on land and one into the sea. Two landed intact, but the other two hit the ground with enough force to cause a plutonium leak.
Before U.S. military personnel could recover the debris, Guillermo Velarde’s team analyzed the remains and uncovered a secret previously known only to the United States, the Soviet Union, China, and France: the Ulam-Teller design—the information necessary to trigger a thermonuclear bomb… provided one has plutonium.
Radiation in the Manzanares River
In December 1968, the Zorita nuclear power plant in Guadalajara began operations. That same year, a fast nuclear reactor was built at the headquarters of the JEN (Nuclear Energy Board), located in Madrid’s university district. A year later, it produced the first grams of Spanish plutonium—but not without incident, right in the heart of the capital.
On November 7, 1970, a radioactive accident occurred during the transfer of waste at the facility. Between 40 and 80 liters of water contaminated with strontium-90, cesium-137, ruthenium-106, and plutonium particles were accidentally discharged into the sewage system, eventually reaching the Manzanares River. The contamination spread to the Jarama and Tagus rivers, reaching as far as Lisbon, and affected countless orchards and irrigated farmlands. Radiation levels recorded in some areas were up to 75,000 times above the legal limit.
Despite these alarming setbacks, only one step remained: acquiring the 6 kilograms of plutonium needed to build a bomb—something that could be achieved with help from France. Charles de Gaulle sought to establish a European defense system independent of NATO and aimed to win Spain’s support by providing the technology to launch a nuclear power plant in Vandellós, Tarragona, in 1972. Although civilian in nature, the plant could produce up to 200 kilograms of plutonium annually as a byproduct of irradiated fuel—plutonium that, if authorized, could be diverted for military use. If approval were granted to assemble a nuclear weapon, Spain was ready to produce the plutonium.
The End of the Project
Spain had all the components necessary to build an atomic bomb, but it also faced mounting pressure from the United States and international nuclear oversight bodies. Franco had refused to sign the Nuclear Non-Proliferation Treaty in 1968 and, with France’s support, blocked the IAEA (International Atomic Energy Agency) from inspecting Spanish nuclear facilities. Still, he feared the project could lead to further economic sanctions. While he encouraged theoretical research and development, he consistently refused to authorize actual bomb production.
On December 20, 1973, ETA assassinated Carrero Blanco, a staunch advocate of the nuclear program. It was the final blow to Project Islero. Just one day earlier, the Prime Minister had met with U.S. Secretary of State Henry Kissinger. The contents of that meeting remain classified under Spain’s Official Secrets Act, but some historians speculate that the Spanish nuclear program may have been among the topics discussed. Some authors even suggest possible CIA involvement in the assassination, citing the use of C4 explosives—at the time, exclusively used by the U.S. military.
Franco’s death did not immediately end Project Islero. Work continued during Spain’s democratic transition and under Prime Minister Adolfo Suárez. But in 1987, following Spain’s entry into NATO and the European Economic Community, the country formally renounced the development of nuclear weapons and signed the Nuclear Non-Proliferation Treaty.
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