On July 21, 1959, former US first lady Mamie Eisenhower smashed a champagne bottle against the towering hull of the NS Savannah before the newly built ship slid into the Delaware River, carrying with it the lofty promise that shipping would be changing forever.

Rather than having a conventional diesel engine in its machine room, the Savannah was powered by a nuclear reactor. Between 1962 and 1970, at a time when nuclear power was considered the future of energy, the merchant vessel shipped goods and people around the world to showcase the peaceful use of splitting atoms.

Today, only a few countries still operate nuclear-powered ships, and mostly for military uses in aircraft carriers and submarines. Russia continues to employ a small fleet of nuclear icebreakers on the so-called Northern Sea Route in the Arctic, for example. 

Cargo vessels, let alone passenger ships, that run on nuclear fuel have all but disappeared. But some people are convinced the time has come to bring them back.

Shipping's carbon emissions problem

Merchant ships carry about 80% of all internationally traded goods, which makes them indispensable to keeping the global economy running.

But the majority still run on bunker oil, a thick, tar-like fuel made from crude oil, and have smokestacks that spew toxic pollutants into their air. Collectively, they emit as much climate-changing CO2 as the entire country of Japan.

The International Maritime Organization, which is responsible for global shipping, wants the sector to reach net zero emissions by around 2050.

But none of the technologies touted to substantially curb greenhouse gas emissions — for example batteries or alternative fuels like methanol and ammonia — are able to achieve this goal all by themselves.

Building a case for nuclear-powered ships

As global efforts to reduce CO2 emissions intensify, the issue of nuclear energy in shipping is getting more and more attention.

Jan Emblemsvag, an engineering professor and project lead at the Norwegian research consortium NuProShip, names the most obvious advantage, telling DW that nuclear-powered ships "would have no emissions."

Inside NS Savannah, back in the 1960s, a so-called pressurized water reactor was using nuclear fission to heat water, which then produced steam to turn turbines and power the ship's propeller and electrical generators.

The advantage of using nuclear fuel, back then and now, is that huge amounts of energy can be packed into a relatively small space, enabling vessels to traverse the oceans for years without refueling.

"And that, of course, gives a tremendous range," said Emblemsvag

NS Savannah, for example, could make 14 trips around the world on a single fuel load, while current container ships typically running on oil don't even manage one.

Occasionally, they even need to reduce speed to save fuel. By contrast, nuclear ships wouldn't run into refueling problems, so they could go faster and save money at the same time.

What killed the NS Savannah?

Ultimately, it was the business case behind the NS Savannah that spelled her end in 1970. Built at a cost of $46 million (€39.6 million) — today about $500 million in purchasing power parity — the ship required large annual government subsidies to operate, around $2 million, which was not financially sustainable, particularly during periods when oil was cheap. 

Apart from that, the ship was hamstrung by a limited cargo capacity of only around 10,000 tons, the need for a specially trained crew and limited access to ports around the world due to nuclear safety concerns.

Additionally, the ship's design was costly, with thick shielding around the reactor and even extendable fins to keep the vessel steady in stormy waters.

The NS Savannah was officially retired in 1970, with its nuclear reactor being shut down and defueled in 1971. 

After it, only three other nuclear cargo ships were built — Germany's Otto Hahn, the Japanese Mutsu and the Russian Sevmorput. The first two were later converted into diesel ships, and the Sevmorput is nearing retirement.

New reactor types for a technological breakthrough?

The current discussion about a nuclear revival in shipping is mainly fueled by technological advances in developing next-generation nuclear reactors, said Mark Tipping from the global shipping classification society Lloyd's Register,

"The technologies that we're looking at today for maritime are very different to what was applied in the 1960s and '70s," he told DW.

Often referred to as "Generation IV" reactors, their biggest promise is that they will be safer than those of the past. Pressurized water reactors (PWR), the most common type of reactor today, rely on active intervention when things go wrong, meaning that additional pumps would have to be switched on, for example.

The new reactors would "take that out of the equation," Tipping said. "If something goes wrong, they're fail-safe. It doesn't need people to achieve that."

The hope is that this will convince port authorities to allow nuclear ships to dock more readily.

Emblemsvag's Norwegian consortium — including the Norwegian University of Science and Technology and shipbuilder Vard — looked into 80 different new reactor designs to single out the three they consider most promising for propelling ships.

Apart from questions of nuclear safety, they also looked at commercial aspects.

"For one of the reactor technologies that we're working on, we've already done some cost calculations. They indicate that the fuel costs will be roughly 40% cheaper than heavy fuel oil," said Emblemsvag, adding that the selected reactors are rather small and can be mass-produced, pushing down costs further.

But none of the reactors scrutinized by the consortium have actually been built yet, let alone production capacities established to build them in large numbers.

When asked about the potential of nuclear ships today, some of the world's biggest shipping companies — MSC, CMA CGM and Maersk — didn't even reply to DW's request for comment.

Will nuclear-powered ships remain a sailor's yarn?

In June, the International Maritime Organization (IMO) agreed to update its rules governing civilian nuclear vessels, dating back to 1981 and covering PWR reactors only.

Ricardo Batista, technical officer for maritime safety at the IMO, said the authority first wants to understand the hazards and the safety requirements. 

"And then from there, [we can] develop the relevant mitigation measures that can be incorporated in the new code," he told DW.

IMO expects this process to take years, with serious questions arising from the operation of nuclear ships: How can we prevent nuclear fuel from leaking when a ship sinks? How can we stop it from falling into the hands of terrorists? And what happens to the nuclear waste?

Beyond this, classification societies, like Lloyd's Register in London or US-based ABS that establish and enforce technical rules, need to update their guidelines. And port authorities as well as ship insurers would have to revise their rules, too.

So sorting out all the legal aspects of nuclear-powered ships will probably take longer than building them. Taking them to sea in the early 2030s, as Tipping and Emblemsvag hope, seems rather ambitious.

Edited by: Uwe Hessler