MR 31 : The hasty, heavy, inefficient, warm and monstrous first missile warhead of the french nuclear stockpile

 

A MR 31 warhead being loaded on its missile in a silo. A fireman with a hose stands ready. Photo from Franck Laidin's collections

 

 

 

 

Because of the very sensible aspect nuclear weapons have, it is very complicated to find detailed informations about them. In France, archives whose divulgation could help concieve, build, use or locate weapons of mass destruction are definitely off limits (Art L.213-2,  French code of patrimony). However, there are some ways to get open source limited insights on these topics

About the Munition Radiologique 31, France's first strategic missile nuclear warhead, used on top of the S2 IRBM from 1971 onwards, some information has been able to get on the surface. This warhead is a device only using pure fission of its plutonium core to get a yield of about one hundred kilotons. A particulary heavy warhead (about 750 kg, without thermal protection for reentry), the phyics package seems to have been tested in In Ecker, in the Algerian desert on February 27, 1965 (Saphir test, underground) as well as over Mururoa on September 11, 1966, in front of president Charles de Gaulle (Bételgeuse test, atmospheric)

The captive balloon of the Bételgeuse nuclear test. Source : ECPAD

 

This warhead is representative from the french need to get material as fast as possible to equip its strategic nuclear forces, even as research on thermonuclear weapons is slow. While only 32 months separate the first chinese test from its first thermonuclear one, eight years separate Gerboise Bleue (the first french fission test) to Canopus (the first thermonuclear test made by France). And even then, the first french nuclear warheads only came to service in the second half of the 1970s, onboard the M20 missiles equipped by the SSBNs of the Force Océanique Stratégique. However, the Albion Plateau missile complex is put in service in 1971 (with some delay due to the development of the rockets)

The MR 31 contains a considerable amount of plutonium for a weapon of its yield, especially when compared with later designs (especially the warheads of the MIRVed french missiles). This brings issuesabout heat coming from disintegration, which can, if left unchecked, alter the explosives around the fissile core, making them dangerously sensible. This means the warheads have to be temperature-controlled. Transport vehicles were equipped with a cooling unit (which aren't seen after 1980). Silos were also fitted with temperature control equipment. Those elements had to be restarted after storms, as lightning strikes would trigger circuit breakers to protect the installations. The S2, being buried in sturdy silos, however had an advantage over the Jupiter missiles based in Italy ten years before, which were struck by lightning several times.   

A S2 missile in its silo. Credits: SIRPA Air. 

Among the most interesting clues on the MR 31, the testimony of Pierre Billaud (La Grande Aventure du nucléaire militaire français, p.233),  who worked as a physician to the french nuclear program, allows to pinpoint the safety concerns of the early french nuclear warheads. Billaud explains shortly that the MR 31 had a "monstruous" package of plutonium, with its difficulties in terms of safety. The TN 60 and 61, lighter but also safer, with a far smaller amount of plutonium, are presented as considerably safer, certainly by having taken advantage of the U.S. Secret Assistance to the French Nuclear Program, of which nuclear safety was one of the main focuses.

What can we learn about this hasty warhead ? One could think that a recent nuclear program, like the Iranian one, or an hypothetical Japanese, south-Korean or Polish program, would give birth to a pure fission device to get quickly a way to lay nuclear hellfire through ballistic missiles. These warheads would be heavy and inefficients, with a yield of a hundred of kilotons at most. Safety concerns would also be brought up, because of the considerable mass of fissile material. A two tons warhead would however be somewhat adaptable to an Iranian missile, as the ballistic program of the Islamic Republic does have some experience.

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BILLAUD, Pierre, La grande aventure du nucléaire militaire français Des acteurs témoignent, Editions L’Harmattan, 2017. 

VAÏSSE, Maurice, Armement et Ve République Fin des années 1950 - fin des années 1960, Paris, CNRS Éditions, 2013. 

BURR William, « U.S. Secret Assistance to the French Nuclear Program, 1969-1975: From "Fourth 
Country" to Strategic Partner » [en ligne], Wilson Center [consulté le 28/08/2024]. Disponible sur 
https://www.wilsoncenter.org/publication/us-secret-assistance-to-the-french-nuclear-program-1969
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