Advanced Nuclear Technologies

How Advanced Nuclear Technologies can solve problems

Introduction

This is the second of a series articles looking at developments in Small Modular Reactors (SMRs) and Advanced Nuclear Technologies. The first looked at the designs that have been short-listed in the Government’s SMR programme. This article will look at advanced nuclear technologies and how new companies are looking to exploit these technologies in new advanced reactors.

Conventional Nuclear Reactors

Conventional nuclear reactors are almost always Pressurised Water Reactors (PWRs) and sometimes Boiling Water Reactors (BWRs), collectively known as Light Water Reactors (LWRs). There are also Heavy Water Reactors, for example Canada’s CANDU rectors, which use Deuterium Oxide as moderator and coolant. The main strength of LWRs is that they can produce safe and steady baseload power for decades. However, they have limited capability to flex output as demand varies. Moreover, the output temperature is relatively low at around 325^oC, which limits the efficiency of electricity generation and is not suitable for many process heat applications.

Advanced Nuclear Reactors

Advanced reactors are under consideration for several reasons. First, they typically operate at higher temperatures than conventional reactors, so they can improve the efficiency of the steam turbines used to generate electricity. Second, the higher temperatures can be used for a wider range of process heat applications to replace natural gas in industry. Third, fast reactors can ‘burn’ spent fuel from conventional LWR reactors. Fourth, some designs are better able to “load-follow” meaning they can increase or decrease power output in response to demand fluctuations, making them more useful on the grid. Finally, many of these advanced designs incorporate passive-safety features to make them even safer than existing reactors.

Advanced reactors can be characterised by the type of coolant and whether they employ fast or slow neutrons in the fission process. The types of coolant can be gas (for example Helium or Carbon Dioxide), Liquid Metal (for example, Sodium or Lead) or Molten Salts.

Gas-Cooled Reactors

The UK has been running some form of Advanced Gas Cooled Reactor (AGR) since 1963. Four reactors that were commissioned in the 1980’s are still operating at Hartlepool, Heysham (1 and 2) and Torness. These use Carbon Dioxide as the coolant. Hartlepool and Heysham 1 are due to be decommissioned in 2026 with Heysham 2 and Torness due to close in 2028. These reactors operate at around 640^oC and have a thermal efficiency of ~41% compared to the typical 325^oC and 34% for PWRs.

The technology lead for gas-cooled reactors (GCRs) has now largely passed to China, with their HTR-PM which entered commercial operation in December last year. This plant uses Helium as the coolant and generates high temperature steam at 500^oC. The reactor uses High-Assay Low Enriched Uranium (HALEU) with the fuel enriched to 8.5% U₂₃₅ encased in small balls, known as TRISO fuel. The two 250MWt reactors drive a single 210Mwe steam turbine for an efficiency of 42%, although it also supplies high temperature steam to the petrochemical industry in China.

There are some western designs of GCRs such as the GTHTR300C from Japan. This is being designed with a thermal capacity of 600MWt and electrical output of 274MWe. It is being designed to operate at temperatures of 850-950^oC and turbine efficiency of 47%. The higher temperature variants are being positioned as being able to cogenerate hydrogen or produce high temperature process heat. This is targeted for commercialisation around 2030. It appears that a version of this reactor has been chosen to participate in Phase B of the UK’s Advanced Modular Reactor (AMR) programme.

Another GCR design is the Xe-100 being developed by X-energy, cooled by Helium at an operating temperature around 750^oC and a thermal efficiency of 40% with a thermal output of 200MWt and electrical output of 80MWe. The company has held talks with and signed several agreements to develop its technology. It was recently announced that the UK Government has awarded a multi-million pound grant to Babcock to explore the feasibility of developing a fleet of X-energy’s units in Hartlepool. The company raised £235m in additional funding late last year and hopes to deploy its first units in 2027.

Both the HTR-PM and the Xe-100 claim to be able to flex output and load follow to match changes in demand.

Additionally, there are conceptual designs for fast gas-cooled reactors such as the GA-Framatome Fast Modular Reactor, GA’s Energy Multiplier Module (EM²) and the European Allegro design.

Liquid Metal Fast Reactors

When the fuel of a conventional LWR is spent it still contains around 90% of fissile material. Fast reactors were originally conceived to burn Uranium more efficiently and extend the world’s Uranium resources. However, when it was found that Uranium is not particularly scarce, development of fast reactors stalled. Fast reactors are attractive in that they offer an opportunity to close the nuclear fuel cycle. Without going into all the technical details, fast reactors can destroy the longest-lived nuclear waste from conventional reactors and so reduce the scale of the nuclear waste problem. However, this can come at the cost of producing Plutonium in fast breeder reactors. But not all fast reactors are breeders: they can be designed to burn up fuel.

Sodium-Cooled Fast Reactors

Several experimental designs of sodium cooled reactors have been built over the years but have never achieved mainstream adoption. Russia did attempt to construct the BN-1200 sodium-cooled reactor but it was put on indefinite hold in 2015. However, Rosenergoatom has more recently announced that a pilot reactor would be built by 2035.

The most promising western designs appear to be the GE-Hitachi PRISM design and TerraPower’s Natrium concept, which includes features from PRISM. Natrium is a sodium-cooled fast reactor operating at around 500^oC, with the addition of a molten salt heat store so the nominal output of 345Mwe can be increased to about 500MWe for up to five hours for load-following. The lower operating pressures and the use of passive safety features are claimed to increase safety.

GE-Hitachi has proposed the potential use of PRISM to dispose of the UK’s plutonium stockpile. TerraPower is backed by Bill Gates, who chairs the corporation and raised $830m in funding in 2022. In 2021, they announced plans to build a demonstration unit on a retired coal plant site in Wyoming, but announced a two-year delay to the project in December 2022 because of difficulties in obtaining specialist fuel.

Lead Cooled Fast Reactors

The IAEA lists about a dozen lead or lead-bismuth cooled reactors on its database, but none are in an advanced state of development. However, Anglo-Italian Newcleo is not listed on the IAEA site, and they are advancing their lead-cooled fast reactor design. Newcleo had over £350m on its balance sheet at the end of 2022 and launched a further equity raise of up to €1 billion in March last year.

Molten Salt Reactors

Molten Salt Reactors (MSRs) reactors use molten fluoride salts as the primary coolant at low pressures. However, even though the US built a prototype reactor in the 1960’s, it is thought that licensing MSRs is a major challenge because there is so far extremely limited experience in design or operation.

Most MSR concepts use a molten mixture of lithium and beryllium fluoride salts with dissolved enriched uranium as both coolant and fuel. Some could even use Thorium as a fuel. These designs are attractive because of low waste production; high burn-up of fuel; high operating temperature, being suitable for process heat and increasing the efficiency of electricity generation and passive safety features.

Fast MSRs could represent the holy grail of reactor design in that they could burn up existing waste, vary output to match demand and operate at high temperatures to produce process heat or make hydrogen. However, there is a long way to go before these designs are commercialised.

Interesting MSR designs include the Moltex SSR and the ThorCon MSR. Moltex is a UK company that also has operations in Canada. The main reactor design, the Stable Salt Reactor – Wasteburner (SSR-W) is a fast reactor that uses recycled nuclear waste as fuel, operating at around 590^oC. This reactor can be coupled with their GridReserve which is a series of tanks containing molten salt that can be used to vary output to match grid demand. In 2021, the Canadian Government announced a C$50m to support the initial deployment of an SSR-W reactor and New Brunswick Power’s site and Point Lepreau.

Martingale in the USA is designing the ThorCon MSR. This design uses a mix of uranium and thorium salts dissolved in sodium-beryllium fluoride as fuel. There is interest in Indonesia is developing this design for either power generation or marine propulsion.

Conclusions

Comparison of Conventional Reactors and Selected Advanced Reactors

The main drawbacks of existing LWR designs are that they use very little of the fissile material during their life and have limited operating flexibility. We should start to consider the spent fuel from LWRs as a resource that can be reprocessed and used in fast advanced reactors.

Although not very well advanced yet, there is a significant opportunity for new types of reactor to solve the nuclear waste problem, replace natural gas in the production of process heat and respond better to demand fluctuations on the grid. GCRs look most promising for very high temperature applications, but fast reactors with either liquid metal or molten salt coolants look to be good options for burning spent fuel and load following. There are no doubt many challenges ahead with materials specifications and licensing, but this is an opportunity that needs to be grasped with both hands.

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