With emissions regulations tightening, nuclear power is experiencing renewed interest, both on shore and for maritime use. However, many still think nuclear power is unsafe. SWZ editor Björn von Ubisch discusses what this perception is based on and explains the workings of the new generation of nuclear reactors.

Björn von Ubisch, SWZ editor
Björn von Ubisch.

This article was written by SWZ editor Björn von Ubisch MSc (pictured on the right) and serves as a background article to SWZ|Maritime’s March 2023 issue. This issue features an energy transition special.

In 2019 about 4.3 per cent of the total world energy is produced by nuclear power. About ten per cent of the world’s electricity production is produced by nuclear power. When looking at the accidents statistics and fatalities in a hypothetical study produced in 2007 for 2014, the fatalities caused by nuclear power are less than 0.1 per cent of all fatalities in the energy sector. The Oil and Gas production account for about 25.5 per cent of all fatalities but represents 57 per cent of all energy produced.

Source: Lancet and IEA Energy Statistics – OurWorldInData.org/what-is-the-safest-form-of-energy/
Source: Lancet and IEA Energy Statistics – OurWorldInData.org/what-is-the-safest-form-of-energy/.
Mortality rate in relation to produced kWh in the Energy Sector, compared to Energy Production by Source. Wind Energy + Solar Energy has 5 x higher mortality rate than Nuclear Energy.
Mortality rate in relation to produced kWh in the Energy Sector, compared to Energy Production by Source. Wind Energy + Solar Energy has 5 x higher mortality rate than Nuclear Energy.

For some reason, nuclear power is not regarded as safe. The statistics above indicate that however you look at it, nuclear energy is much safer than any other sort of energy.

Often, three nuclear accidents are frequently referred to as examples of how unsafe nuclear power is. These accidents are: the Three Mile Island, the Chernobyl and the Fukushima accidents. If you look closer at each event, you will find it was the human factor that failed combined with poor design and poorly trained operators. In the meantime, much has been learned and it must be assumed that future nuclear power plants will be even safer and that the power plant operators will be better trained in simulators prior to being let loose on the actual plant.

Three Mile Island disaster, March 1979

The Three Mile Island disaster was caused by poor design of the control and alarm system, malfunctioning equipment and poor operator emergency training. The accident was triggered when the operators tried to free a clogged filter with compressed air. This caused an air pocket in the cooling water system because of a faulty check valve (this type of valve was well known for its un-reliability) and the air pocket caused the circulation of cooling water to cease and various pumps stopped. It took eleven hours for the operators to realize things were wrong. This all resulted in a partial meltdown of the reactor.

Chernobyl disaster, April 1986

The accident occurred during a safety test, meant to measure the ability of the steam turbine to power the emergency feedwater pumps in the event of a simultaneous loss of external power and major coolant leak. During a planned decrease of reactor power in preparation for the test, the operators accidentally dropped power output to near-zero, due partially to xenon poisoning. While recovering from the power drop and stabilising the reactor, the operators removed several control rods, which exceeded limits set by the operating procedures.

Upon test completion, the operators triggered a reactor shutdown. Trying to restart the reactor, they withdrew the graphite moderator rods. All this resulted in a very high heat, the cooling water separating into oxygen and hydrogen and a huge hydrogen explosion. The reactor itself was not contained in a separate strong reactor casing. The reactor was housed in an ordinary building and the complete building with its contents was destroyed. Radiation from the accident was first picked up in Forsmark, Sweden, some 1200 km away.

Fukushima disaster, March 2011

The disaster was triggered by an earthquake and a tsunami. The reactor design did not take into account the possibility of a tsunami with an extremely high wave height, on a coast likely to encounter tsunamis. The Japanese authorities refused to take advice from outside expertise during the design phase.

On detecting the earthquake at the time of disaster, the active reactors shut down. Because of these shutdowns and other electrical grid supply problems, the reactors electricity supply failed, and the emergency diesel generators automatically started. The emergency diesels were to feed the cooling water pumps to keep the reactor temperature under control.

The earthquake had also generated a tsunami with a fourteen metres high wave that arrived shortly afterwards, swept over the plant’s seawall and then flooded the lower parts of the reactors. This flooding caused the failure of the emergency generators and loss of power to the circulating pumps. This resulted in three nuclear meltdowns, three hydrogen explosions and the release of radioactive contamination.

The US Navy claims to have encountered zero incidents with their nuclear plants onboard nuclear submarines, starting in 1954. The Soviet Union and later the Russian Federation (RF), did have some nuclear incidents onboard their nuclear submarines, about fourteen some records indicate (nuclear incidents meaning release of radiation – Wikipedia).

The claim that nuclear energy is unsafe does not correspond with the statistics. Renewables like wind and solar energy has five times the mortality rate of nuclear energy and the oil and gas industry has 355 times the mortality rate of nuclear energy based on energy produced.

Also read: Hyundai jumps on board development of nuclear power for ships

Nuclear waste

Waste storage is another issue, often brought up as a huge problem with nuclear energy.

The low-level radioactive waste is as radioactive as the uranium ore mined out of the ground and can be put back into the ground. The high-level radioactive waste must be stored for some time in order to be at safe levels (as the ore mined long time ago).

Decay of radioactivity of high-level waste resulting from reprocessing 1 Tonnes of fuel. The horizontal line represents the radioactivity of the Uranium ore needed to produce 1 Tonnes of fuel. (Radioactivity is measured in Becquerels (Bq). 1 GBq equals 109 Bq.)  Source Peter Saunders NEA / OECD
Decay of radioactivity of high-level waste resulting from reprocessing 1 Tonnes of fuel. The horizontal line represents the radioactivity of the Uranium ore needed to produce 1 Tonnes of fuel. (Radioactivity is measured in Becquerels (Bq). 1 GBq equals 109 Bq.) Source: Peter Saunders NEA/OECD.

Oak Ridge National Laboratory molten salt reactor

In 1964 the Oak Ridge Laboratory built an experimental molten salt reactor (MSR) that was operated from 1965 to 1969, when the project was shut down, due to the US Government deciding to continue with pressurised water reactor (PWR). The experiment was successful, and many lessons were learned during the five-year period.

Also read: How nuclear power can be used on ships safely

Present development

The new reactor types of the future (post 2022) are called the fourth generation of reactors. Many companies are looking at small modular reactors (SMRs) with a capacity of less than 100 – 500 MW and some companies are looking at micro modular reactors with a capacity of around 10 MW.

There are quite a few companies that claim to be working with SMRs, MSRs or liquid metal cooled reactors/liquid metal fast reactors (LMFRs). There are at least 24 different SMR designs on the market. Many of the companies claiming to be working on nuclear technology development are apparently just lobby groups promoting the technology while other companies are producing components, designs and are also actually planning to build a prototype. The most significant of the latter type are:

Characteristics of some of the MSR systems under development by commercial companies. Note: Transatomic Power, Thorizon, Seaborg Technologies and Copenhagen Atomics intend to use nuclear waste, mainly depleted uranium, as fuel. Transatomic Power stopped operations in 2018 claiming they got it wrong. Copenhagen Atomics are producing components for MSRs like heat exchangers, valves and pumps together with Alpha Laval. SNF – spent nuclear fuel, U – uranium, Th – thorium, LEU – low enriched uranium, Pu – plutonium, MA – minor actinides, ZrH – zirconium hydride.

Also read: ‘Nuclear only option that decarbonises and lowers cost when compared to marine e-fuels’

Generation IV reactor types

Below are some types of generation IV reactors listed. They are all inherently safe with passive safety features like if the temperature gets too high, the reaction stops. “Walk away safety”. These reactors operate at much higher temperatures than the conventional light water reactors or PWRs, therefore the efficiency is higher (45 per cent compared to 36 per cent for existing types). They also operate at almost atmospheric pressure, unlike traditional reactors. This reduces the risk of leakage. A thermal reactor uses slow or thermal neutrons. A moderator reduces the speed of the neutrons. Moderator can be graphite of water. Heavy water does not absorb neutrons. A fast reactor has no moderator.

  • High-temperature gas-cooled reactor (HTGR)
    – Gas can be used directly in a gas turbine driving a generator
    – Fuel is in the form of pebbles inside rods and can be fed on-line
  • Very-high-temperature reactor (VHTR)
    – Helium or molten salt cooled
    – Pebble bed technique can be utilised (see below)
    – Gas can be used directly to generate H2 and O2 from water due to high heat
  • Molten salt reactor (MSR)
    – Regarded as the most inherently safe of all generation IV reactors
    – Fuel thorium and/or uranium is in the salt
    – Fitted with freeze plug which drains the salt containing the fuel to dump tanks
  • Supercritical-water-cooled reactor (SCWR)
  • Gas-cooled fast reactor (GFR)
  • Sodium-cooled fast reactor (SFR)
  • Pebble bed reactor
    – Pellets Ø 60 mm contains small fissile material Ø 0.6 mm, impeded in graphite
    – Fuel can be thorium and/or uranium
    – Pellets can be fed online and spent pellets can be drained out
    – Too high temperature will cause the reaction to stop
  • Lead Cooled Fast Reactor
    – Lead does not moderate the neutrons
    – Lead does not react with water or air
Very-high-temperature reactor (VHTR).
Molten salt reactor (MSR).
Pebble bed reactor.
Pebble bed reactor.

Current SMR manufacturers, near term construction

A number of companies are planning to build anSMR in the near future. The list below is sampled from the Norwegian research organization IFE in November 2022:

  • Westinghouse eVinci – 2027
    – Heat pipe technology, with most likely, helium gas as coolant
    – Uses fuel pellets (TRISO – Tri-Structural ISOtropic particle fuel)
    – Up to 5 MWe plus 8 MW thermal energy from residual heat
    – Nuclear demonstrator 2025
  • Seaborg CMSR – 2028
    – Molten salt technology (MSR)
    – 200-800 MWe – floating system
  • GE Hitachi BWRX – 300 – 2028
    – Boiling water reactor (BWR)
    – 300 Mwe
  • Ontario Power Generation (Darlington)
  • NuScale Voygr – 2029
    – Pressurised water reactor (PWR)
    – 77 Mwe per unit, scalable from 4 to 12 units
    – USNRC certified design for safety requirements
  • ColdLead SEALER – Blykalla – (Swedish Advanced Lead Reactor) – 2032
    – Molten lead technology
    – SEALER-55 – 55 MW SMR, 25-year life cycle

The US company ThorCon is planning to set an MSR complex in Indonesia, at an uninhabited island close to the main island of Sumatra. The reactor(s) will be built into a barge at a Korean shipyard and towed to the location. Fuel exchange will not take place on site, but one complete reactor unit will be removed and exchanged with a fully fuelled unit. The MSR complex will consequently consist of at least two units. One on-line and one offline as required. The reactor units are expected to be fully operational in 2030.

Copenhagen Atomics claim to have their first 100 MW MSR online in 2028.

The costs of the various types of energy are indicated below:

  • ThorCon nuclear energy (MSR): USD 0.0324/kWh (source ThorCon)
  • Coal energy: USD 0.0592/kWh (source ThorCon)
  • Vattenfall large BWR: USD 0.0220/kWh (source Vattenfall)
  • ColdLead – Blykalla – SEALER: USD 0.0620/kWh (source Montel)
  • Onshore wind energy: USD 0.0330/kWh (source IRENA)
  • Offshore wind energy: USD 0.0750/kWh (source IRENA)
  • Solar panel energy: USD 0.0480/kWh (source IRENA)

Also read: Core Power and MIT secure funding for floating nuclear research project

Mass production

The strategy with SMR/MSR is that they can be mass produced at a specialised factory and transported to the site for final fueling and start up. Mass production in one factory has several advantages compared to a one-off purpose-built reactor built on site as it happens today. The modern industry is full of examples of advantages with mass production, starting with among others Henry Ford and his automobile production.

Smaller units can be coupled together to form one large unit. Large units are important for the stability of the distribution of power in large national distribution nets. SMRs can also be built into a barge for easy transportation and later removal.

Molten salt reactor

The MSR belongs to the fourth generation of reactors, ready to be commercially operational in 2030. The MSR has several features that make it simple to operate and inherently safe. The inherently safe means that no action is required from outside to shut down the reactor when things tend to go wrong.

This type of reactor uses salt as coolant and the fuel for the fission reaction is mixed into the salt. The coolant/fuel is pumped around to distribute the generated heat to a secondary circuit with no radioactive material, which in turn heats for example water or helium that drives a turbine to produce electricity. The working temperature of the coolant in this type of reactor can be between 500°C and 700°C. This makes the reactor much more thermally efficient than a traditional water reactor.

Another feature is that the reactor has no or very little overpressure, which means coping with eventual leaks is much simpler. Refuelling can be during operation and does not require reactor shutdown. The containment of the reactor core can be small as the system pressure is low and the heat capacity high.

If for any reason, the reactor tends to overheat, there is a freeze plug at the bottom of the reactor vat. This plug will then thaw and dump the coolant with the reactor fuel into dump tanks below the reactor. The critical mass required for a chain reaction is eliminated and the salt is spread into several tanks, which are cooled by the surroundings and the salt will eventually solidify.

Also read: C-Job: Ships could sail on nuclear power in the future

The nuclear reaction generates very small amounts of impurities like Xenon and Krypton, which need to be filtered out. These impurities can be collected and sold or just stored or let loose like helium gas.

Moderator can be graphite rods or heavy water. Heavy water does not absorb neutrons required for the fission process. The fuel can be thorium, which is found everywhere on earth and is present in, for example tailings from mining operations. Thorium may not always require mining as it is already mined and at present dumped as a waste product. Thorium itself is not radioactive and is much more abundant than uranium. Thorium is mixed with lithium and fluoride to form the salt FLiTh and uranium to form FLiThU (Copenhagen Atomics).

Various radioactive waste products can also be mixed in with the fuel/coolant salt. The MSR is also quick to respond to load changes, which makes it suitable for example ship’s propulsion.

The thorium process. Neutrons are required to start off the process and those can be from uranium, 233U.
The thorium process. Neutrons are required to start off the process and those can be from uranium, 233U.

Another salt that can be used as fuel carrier and coolant is FLiBe, which is a mixture of lithium fluoride (LiF) and beryllium fluoride (BeF2), forming Li2BeF4. This mixture has a melting point of 459°C, a boiling point of 1430°C, and a density of 1.94 g/cm3. FLiBe is also an effective neutron moderator. Slow neutrons make the fission process more efficient.

Fluorine (F) in the salt causes corrosion and Sodium (Na) prevents corrosion.

Limited nuclear waste

The nuclear waste from thorium-fuelled MSRs is very limited compared to the conventional water reactors (PWRs/BWRs). The Chinese scientists, working on the Chinese MSR, claim that the nuclear waste is one per mille of the waste of a conventional uranium-fuelled reactor. A conventional PWR/BWR starts with 250 tonnes of natural uranium. This is turned into 35 tonnes of enriched uranium and 215 tonnes of depleted uranium. The radioactivity of the depleted uranium is the same as the ore mined and should not be regarded as radioactive waste. In the end, 35 tonnes of spent fuel is received and must be stored some 104 to 106 years.

A thorium reactor of the same capacity starts with one tonne of natural thorium plus a small amount of transuranic and finally, 83 per cent of the spent fuel contains stable products and can be separated and sold. The remaining seventeen per cent must be stored for 300 years.

Nobel laureate Carlo Rubbia of CERN (European Organization for Nuclear Research), estimates that one tonne of thorium can produce as much energy as 200 tonnes of uranium, or 3,500,000 tonnes of coal. Mining thorium is also safer than mining uranium as the thorium ore (monazite) is almost pure and thorium mining is open pit mining. Uranium mining is underground and high levels of radioactive radon gas, found together with the uranium ore, are a real hazard.

Det Norske Veritas estimates that a commercial MSR will be available in 2028 – 2030.

Also read: The three key criteria for new-nuclear in maritime

Challenges with an MSR

Corrosion is a big challenge and corrosion was the reason for the MSR in the USS Seawolf to fail. The heat exchangers were constantly cracking due to corrosion and steam was polluting the molten salt mixture. In the 1960s, Oak Ridge National Laboratories (ORNL) asked the US metal industry to come up with a heat resistant alloy. The industry declined, so the ORNL developed its own heat resistant alloys. One is called Hastelloy-N and is now widely used in the oil and gas industry for well stimulation systems.

The ASME (American Society of Mechanical Engineers) Boiler and Pressure Vessel Code governs materials certified for use in nuclear reactors in the code “Rules for Construction of Nuclear Facility Components, Division 5, High Temperature Reactors”. There are currently only five materials approved for use in Division 5: 2¼Cr-1Mo and 9Cr-1Mo steels, Type 304 and Type 316 stainless steel and Alloy 800H. The materials are qualified for a maximum use, temperature and time.

A number of companies involved with MSR and SMR concentrate on material development that can withstand the heat and corrosive environment. One company claims that its salt is so pure that they will have no corrosion problems and will make use of conventional commercial materials for their MSR design.


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Picture (top): Samsung Heavy Industries (SHI) has received approval in principle from ABS for a conceptual design of the CMSR Power Barge, a floating offshore nuclear power plant. The barge was developed together with Seaborg. SHI plans to commercialise it by 2028 after completing detailed design of all power generation facilities (picture by Seaborg).

Also read: Samsung Heavy moves ahead with floating nuclear power barge