France's First Naval Reactors Were HEU

Following Submarine Matter's articles France’s Future SSBN “SNLE 3G” Starts Production of March 28, 2024; France's New K22 reactor for Carrier & 3G SSBN of December 27, 2023; and French LEU Reactor & SSN Implications of March 24, 2023: 

The USN, and partly the UK RN, rely on the US' massive HEU naval reactor industrial base to develop new reactor technology and to create the 94+% HEU fuel.

In contrast France developed LEU naval reactors. HEU requires a specialized and expensive naval reactor fuelling program, while dual civilian-naval LEU fuel is less expensive. 

But it was not always so. Alain Tournyol du Clos in a Federation  of American Scientists (FAS) study of December 2016, reported https://uploads.fas.org/2016/12/Frances-Choice-for-Naval-Nuclear-Propulsion.pdf on page 1:

"In 1959, a French delegation went to the United States in order to negotiate some technology transfer in [page 2] naval reactors; it was a failure. The United States refused any cooperation with France; however, the US agreed to sell a limited amount of highly enriched uranium (HEU), provided that it should only be used in a land-based installation. Hence, the first naval propulsion reactor, conceived by French scientists, was a land-based prototype (PAT: prototype à terre) set up near Cadarache in the southeast of France. It was designed and built in less than five years. The French Government decided — unlike other countries, which started using nuclear submarines — to start with SSBNs."

"[The Redoubtable-class SSBNs always used HEU reactors. They] were equipped with nuclear plants identical to" [France’s land based prototype HEU reactor known as “PAT”]. 

“Yet, very soon after the start of the first SSBN, the scientists working in CEA (the French Atomic Energy Commission) who were in charge of developing nuclear propulsion, realized that [French nuclear submarine classes after the Redoubtables] could use LEU, in dioxide form, to produce cores which would provide energy to the SSBNs four to five times (20 to 25 years) greater than with the first generation of cores. This was possible, on the one hand, because the conception of the first generation cores included high margins and, on the other hand, because SSBNs are low consumers of energy due to the nature of the slow, steady speed of patrolling.”

“After [the Redoubtables], the French Navy decided to adopt cores using low-enriched uranium dioxide for all the following classes of nuclear ships: SSN Rubis class, SSBN Le Triomphant class, SSN Barracuda class, and CVN Charles de Gaulle."

Alain Tournyol du Clos further reports: 

"In 1996, France decided to stop enriching uranium to HEU levels for weapons purposes [A secret - France by 1996 then mainly relied on Pu and Tritium for its all 2-stage thermonuclear weapon arsenal]. If the Navy had wanted to use HEU fuel, it would have had to invest significant money to have its own HEU enrichment facility. By choosing to only use LEU fuel with enrichments much less than 20 percent in the fissile isotope uranium-235, France has saved money by purchasing from the commercial market. Moreover, France’s decision to use LEU fuel for naval propulsion has not degraded the operational performance of the ships." 

France’s new K22 220 MW (thermal) about 37 MW (electrical?) naval reactor

Regarding France’s latest naval reactor, the K22 French Anonymous commented (along the lines) the K22 is not a major problem. It has been tested in land based prototype form at the new RÉS facility https://inis.iaea.org/search/search.aspx?orig_q=RN:38002931. This K22 prototype went critical in 2018 at France’s major nuclear reactor research centre at Cadarache.

Cadarache's location on the red dot above. Map courtesy CIA World Factbook. Cadarache is just up the road from the French naval base at Toulon (French naval bases map below).

The K22’s engineering may have been completed in 2023 with actual construction of the sea based K22 being started at Naval Group’s Nantes-Indret complex https://www.navalnews.com/naval-news/2021/06/naval-group-signs-first-industrial-orders-for-future-french-navys-third-generation-ssbns/ . K22s will initially go into France’s future SSBNs (SNLE 3Gs).

More interesting on the Naval Group video [Alas Pierre (Pete) cannot find it!?] is the computerized thick steel plates water jet cutting in a swimming pool sized facility (very high precision and no thermal stresses).

Two x K22s will later go into France’s future nuclear powered aircraft carrier PA-NG (see video here and below) with construction beginning in France’s Saint Nazaire shipyard in the mid 2020s to early 2030s.

Significantly the K22 is a development of the K15 with 50% more power and perhaps a 25% increase in size. The K22 will use the same LEU refuelling and inspection processes as the K15 – that is using robots.

A Nuclear Threat Initiative 2010 paper https://www.nti.org/analysis/articles/expanding-nuclear-propulsion-challenges/  makes some interesting points about LEU "Caramel" fuel in French Navy reactors including the K15 and presumably the K22: 

"...French nuclear vessels are able to extract more energy from LEU than are U.S. vessels by taking advantage of a more LEU-efficient fuel design. The French navy uses a uranium-dioxide composite embedded in a zirconium alloy grid, an arrangement known as "caramel" fuel.

CARAMEL fuel increases the efficiency of the burn-up of uranium-235 so that lower enrichment levels and/or smaller reactor volumes can be employed with a greater energy yield. Studies have shown that a design in which small spheres of uranium dioxide are embedded in a zirconium matrix can boost the efficiency of the fission reaction even further.[32]"