Project Islero was an attempted Spanish nuclear program. Named after Islero, the bull which felled the famous bullfighter Manolete, the program was created by Generals Agustín Muñoz Grandes and in 1963. Although Spain possessed the second largest uranium deposits in the world at the time, it was not until the Palomares Incident of 1966 that Spain would focus on plutonium-239 implosion-type designs. Yet, in 1966, Franco paused the military research, shifting efforts to nuclear reactor construction and plutonium production. However, the program was resumed in 1971, with help from France to refine the material and fund the nuclear facilities.

Lasting from the of Francisco Franco's rule into the beginning of the democratic transition, the project was prematurely cancelled due to American pressures in 1981, although it only formally ended in 1987 under Felipe González. Although the project never developed a nuclear weapon, the country possessed the capabilities to both design and manufacture the necessary components, with the Spanish Foreign Minister, José María de Areilza, declaring in 1976 that Spain would be able to manufacture the bomb "in seven or eight years if we set our minds to it."

Background The JIA and JEN

In September 1948, by means of a secret decree, Francisco Franco created the Junta de Investigaciones Atómicas (JIA), or Board for Nuclear Research. Constituted on 8 October,Historia de la minería del uranio en Castilla-León the board was formed by (general director and president until 1974), Manuel Lora-Tamayo, and .Scripta Nova. Revista Electrónica de Geografía y Ciencias Sociales: La cátedra Ferran Tallada (1955–1962). La innovació tecnològica i la formació de l'enginyer, Extraordinary issue dedicated to the II International Colloquium of Geocriticism (Proceedings of the Colloquium) In 1951, the secret phase was declared over, and the JIA was renamed the Junta de Energía Nuclear (JEN), or Board of Nuclear Energy,Agustín González Enciso and Juan Manuel Matés Barco (2007): Historia económica de España, page 906, Editorial Ariel, SA, Barcelona. inaugurated in the University City of Madrid under the presidency of General Juan VigónPatrimonio – Lugares del saber: J.E.N / C.I.E.M.A.T.: historia and with Otero de Navascués as general director. The purpose of the JEN was to work "as a research center, as an advisory body to the Government, as an institute in charge of safety and protection against the danger of ionizing radiation, and as a driving force for industrial development in the field of nuclear energy applications".

The Spanish autarky

On 1 April 1939, the United States lifted the embargoes placed on Spain after the Spanish Civil War, providing limited recognition to the Spanish State, and the embassy to Spain, previously in Barcelona, now headed by a Chargé d'Affaires ad interim, was moved back to Madrid on the 13th. Yet with Spain acting as all but a member of the Axis powers, there was much skepticism among the Western world as to whether or not they should be allowed to join the organs of the new international order such as the United Nations. Public opinion of the Spanish State was low. At the San Francisco Conference, Spain was barred, while prominent Spanish Republican leaders were in attendance, exerting a notorious influence on several delegations, extended to the conditions of entry into the United Nations.

As well, at the Potsdam Conference, the question of how to proceed with post-war Spain was one of the first to be dealt with. In this sense, Stalin was, in a certain way, seeking revenge against the Francoist State, due in part to the fact that the State had sent the Blue Division (volunteers fighting with the German armed forces) to the Soviet Union during World War II, with a joint statement from the Big Three reading:

Atoms for Peace and the American connection Yet as the Cold War began, and with Spanish agents, US military officials, and US businessmen lobbying for the opening of relations, popular opinion shifted. The appointment of a US ambassador to Madrid was announced on 27 December 1950. In July 1951, negotiations started for an alliance which would eventually become the Pact of Madrid, and thus in 1955, amidst warming of relations between Spain and the United States as well as Spanish ascension to the UN, Francisco Franco and President Dwight D. Eisenhower signed an agreement of nuclear cooperation as part of the Atoms for Peace initiative, opening up nuclear research to civilians and countries that had not previously possessed nuclear technology. Eisenhower, a proponent of non-proliferation, sought to stop the spread of military use of nuclear weapons. Although the nations that already had atomic weapons kept their weapons and grew their supplies, the program was designed to prevent other countries from developing similar weapons. The program also created regulations for the use of nuclear power, aiming to shape its use into a solely positive means.

This would not necessarily work towards the desired results; in fact, it would be this very program which enabled General Franco, accompanied by the Minister of the Presidency of the Government, Luis Carrero Blanco, to inaugurate the Juan Vigón National Nuclear Energy Center at its facilities in the University City of Madrid on 27 December 1958. Together with the 1959 Stabilization Plan and the subsequent Spanish miracle, Spain was able to begin shifting away from the previous autarky, allowing for the beginning of a fledgling civilian nuclear sector which would lay the foundations for Project Islero.

The Moroccan deterrent

In 1956, Morocco declared independence from France under the rule of Mohammed V. Subsequently, they demanded all lands of Greater Morocco, including the Spanish territories in Africa. While Spain did retrocede the Spanish protectorate in Morocco, they retained control of Ceuta, Melilla, Ifni, the Spanish Sahara, and Cape Juby. This led to Morocco declaring war on Spain in 1957, bitterly asserting the young state's proclaimed sovereignty over Spanish West Africa. The fighting continued until 1958, and although under the Treaty of Angra de Cintra Spain would keep Sidi Ifni (until 1969 when the territory was retroceded), both Cape Juby and surrounding parts of Ifni would be lost to Morocco. From then on, the Defense High Command began to consider armed deterrence in the face of any further potential conflict, and in 1963, they put forth the idea of a Spanish atomic bomb, commissioning a secret report on the possibility of constructing it without alerting the international community.

Development Beginnings (1963–1966)

The idea of a Spanish atomic bomb was first envisioned by Agustín Muñoz Grandes. A hardline Falangist, Muñoz Grandes aspired to break from the previous atlanticism promoted during the 50s to make the country independent of both NATO and the United States. In 1963, he approached general director of the JEN, , about preparing a Spanish foray into atomic weaponry. This new project, christened after the bull which felled the famous bullfighter Manolete, would be helmed by Spanish Air Force general , selected for possessing the scientific background and acumen necessary to conduct the project's research. Velarde was a major general and pilot in the Spanish Air Force, as well as president of the Nuclear Fusion Institute of the Polytechnic University of Madrid. He joined the Theoretical Physics Section of the Nuclear Energy Board, where he would remain as Director of Technology until 1981. He had studied Nuclear Energy at both Pennsylvania State University and the Argonne National Laboratory of the University of Chicago.In 1973 he obtained the chair of Nuclear Physics at the Higher Technical School of Industrial Engineering, and in 1980 he proposed the creation of the Institute of Nuclear Fusion.

The work was divided into two phases: the atomic bomb project itself, and the construction of a nuclear reactor, the fuel of which would be extracted for plutonium in order to construct the bombs. While the reactors could be built with help from the French — General de Gaulle supported the idea of an atomic Spain – the matter of what material to use for the bombs themselves, as well as how to actually construct them, was one of consideration. The first results were a fiasco. The JEN specialists, all military, declared themselves incapable of knowing both the technical details for manufacturing the device and, above all, how to obtain the necessary plutonium.

Palomares incident

Their question was answered on 17 January 1966, when (in what would later be known as the Palomares incident) a B-52G bomber from the United States accidentally crashed while carrying four B28FI Mod 2 Y1 thermonuclear (hydrogen) bombs. Three were found on land near the small fishing village of Palomares in the municipality of Cuevas del Almanzora, Almería, Spain. The non-nuclear explosives in two of the weapons detonated upon impact with the ground, resulting in the contamination of a area with radioactive plutonium. The fourth, which fell into the Mediterranean Sea, was recovered intact after a search lasting two and a half months. The Spanish state conducted secret research on the debris zones of the incident, with technicians led by Velarde finding remains of the bomb and the detonators in the area. This enabled to the project to use plutonium-239 — a boon due to its relative cheapness.

Setbacks (1966–1971) Yet later that year, Franco held a meeting with Velarde in which he ordered to indefinitely postpone the physical, but not theoretical, development of the project due to fears that it would be impossible to keep a secret, and with the recent creation of the International Atomic Energy Agency (IAEA), publicity would lead to increased economic sanctions on Spain. He did, however, allow the research to go ahead, albeit detached from the Armed Forces. Franco also pledged not to sign an international agreement being negotiated at the time to prohibit the manufacture of nuclear weapons. Thus on 1 July 1968, when almost fifty countries signed the Treaty on the Non-Proliferation of Nuclear Weapons (NPT), Spain did not number among them. Meanwhile, Spain began to embrace the power of the atom elsewhere. In Guadalajara, the construction of Spain's first nuclear power plant, the José Cabrera Nuclear Power Station, was underway, and JEN installed the first Spanish reactor with capacity to produce plutonium for bombs, Coral-I, able to operate with either plutonium-239 or with 90% enriched uranium (U-235), although with waste still containing almost as much fuel as is consumed. Nevertheless, in 1969, the first grams of Spanish plutonium – the only ones in the world not under IAEA authority – were obtained.

Resumption (1971–1977)