A number of reasons really. They didn't sign on to the Partial Test Ban Treaty in 1963, so they weren't under any international treaty regulation. France has 75% nuclear energy, so they had fissile material in abundance. Being so heavily invested in nuclear energy is (or was) a huge source of pride for France. Also, Nuclear Weapons Scientist want to test bombs. They don't want to figure out ways to dispose of them or do mock tests. That's not what they trained for and that's not what they dreamed of while studying. It's an interesting case study because there were massive protests against the tests, and it hurt France's reputation internationally, yet they persisted.

I'd have to dig up my syllabus for a class I took on Nuclear Proliferation to get all the sources but you can read more in: The radiance of France: Nuclear power and national identity after World War II G Hecht, M Callon, 2009 books.google.com

One thing worth noting: The modern submarine-based British ballistic missile force, the Vanguard class, uses US-designed Trident missiles. Their predecessors, the Resolution class, used US-designed Polaris missiles. This helps to remove the burden of testing, since the British and Americans could share research during the design and testing of these warheads.

You can read more about the missile-sharing agreement[ here] (http://www.ssp.navy.mil/fb101/ukssp.shtml) and the short-version of the UK's nuclear weapons program [here]. (https://www.fas.org/nuke/guide/uk/doctrine/sdr06/FactSheet5.pdf).

Basically, you could say that you were correct to surmise that the UK required less nuclear testing due to the relationship they have with the United States, whereas France has a very independent nuclear weapons program.

Just read about the reasons that France withdrew from NATO. De Gaulle didn't like playing second fiddle to the Anglo-American alliance, so sought his own defense establishment to enable him to come to independent terms with the Eastern Bloc in the event of WWIII. Obviously the creation of a third Cold War political faction would require a strong nuclear program.

One could suppose that underlying the French position was the Anglo-American stance on the leadership of the Free French during WWII (they were certainly not fond of De Gaulle). From the French leadership's position, they were fighting not only the Germans in WWII, but also the Vichy French, so France deserved to be treated as an equal power on par with the pre-war status quo.

In addition to what has already been posted here, there were political reasons for engaging in so many nuclear tests. At least initially, the French government engaged in nuclear research with no intention of producing a bomb. However, the Mendes-France government in 1954 used the development of a nuclear bomb as political leverage. An essay by Leopoldo Nuti in the Oxford Handbook of Postwar European History (Chapter 16) about postwar nuclear arms in Europe puts it thusly:

That the decision [to develop the bomb] was made shortly after the conclusion of the negotiations for the formation of the WEU [Western European Union] and the rearmament of West Germany clearly shows the implicit rationale behind the French government’s thinking. A French nuclear bomb was meant above all to keep France a step ahead of Germany in the political and military realm, in order to perpetually freeze the balance of power between the two countries. At the same time, at least during the mid-1950s, a French bomb was also conceived as the main tool to participate in the decision-making process of NATO strategy. It was, in other words, still an Atlantic bomb, particularly if the US was willing to share its nuclear secrets with its allies.

However, when France withdrew from NATO in 1966, the development of nuclear weapons took on a different imperative, one that involved de Gaulle's idea of a kind of third way in the Cold War:

With the passing of time, however, this possible multilateralization of the French nuclear programme became increasingly difficult to reconcile with “the imperatives of national sovereignty.” Eventually it was abandoned with the return to power of de Gaulle, who conceived a French bomb as the ultimate guarantee of the country’s more independent attitude in international relations. The force de frappe quickly became the cornerstone of de Gaulle’s project for the transformation of his country’s foreign policy, and in the following years it acquired a symbolic status which went beyond the military and strategic value attributed to it by French nuclear strategists.

There was, in short, a certain status given to nuclear powers in the postwar world that was a strong motivating factor for the development of a nuclear weapons program in France, particularly with the increased pushback from the French to Americanization (derogatorily referred to as "Coca-colization"). Gabrielle Hecht and Michel Callon link the nuclear program and its importance directly to French national identity in The Radiance of France: Nuclear Power and National Identity after World War II, and while most of the book is about the use of nuclear power, it definitely touches on nuclear weapons. Since the two are linked, I would check it out for further info (especially Chapter 7: "Atomic Vintage").

In 1958 the U.S. and the U.K. signed a Mutual Defense Agreement with provisions to share nuclear test data and likely even elements of the nuclear weapons design. Many of the U.S. tests can be seen as a joined US/UK tests.

I know this is also off topic, but I'm currently holidaying in Bora Bora and I'd be very interested to find out more about the longer effects of their 30 years of testing in the region. Have there been any implications of fallout for the locals (bad, unintentional pun)? Is there a map somewhere online of where the tests took place and their radiation areas? Completely fascinating given how gorgeous FP is.

Unlike the U.K. and the U.S., France has been invaded and occupied, and both suffered at the hands of the nazis and had to mightily fight at the political level against a planned Germany-like occupation by the U.S.

Roosevelt had a deep-seated hatred of the French (De Gaulle recalled one of his first meetings with Roosevelt, which happenned under heavy guard of soldiers armed with machine-guns, which gave De Gaulle the impression that Roosevelt looked like a gangster or at least a bully), and wanted to level France's heavy-industry, so it would be dependent on U.S. made goods, prevent it from having an army, and put it essentially under a military occupation régime.

It was only when De Gaulle finally convinced Roosevelt that the Résistance would turn against the U.S. that he finally relented and "allowed" France it's rightful place amongst the victors. Following Roosevelt's death, as nearly no one in the US held his contempt of France, it was a no brainer that France was no longer under threat of occupation.

This is the main reason why many French have no trust whatsoever towards the USA, and the Mers Èl Kébir "incident" also did not help towards the British.

As it saw the world polarize into two superpower camps, the obvious conclusion that the sole guarantor of a country's sovereignty was the nuclear weapon, hence the French insistance of pursuing it's own nuclear military programme, and it made no secrets that it's strike force was to be "tous azimuths", that is, it was not solely aimed at the Soviet Union.

Follow up: why did france keep up testing in the pacific after the PNTBT?