Small modular reactors (SMRs) might be the future for nuclear power and right now there are a lot of designs under consideration. In fact the latest IAEA SMR Booklet lists more than 70 mooted SMR models at various stages of design, with just one in commercial operation. There is little commonality among the designs in terms of size (physical, thermal and electrical) or in their approach to modularity. In this article we provide a definition of an SMR and examine the benefits of a truly Small Modular Reactor.

The IAEA defines an SMR as any reactor with an electrical output of up to 300 MW but this definition seems to be based on the simple fact that this is smaller than the numbers provided by currently operating reactors. At 470 MWe, the Rolls-Royce SMR provides electrical output far greater than this value, yet still has a smaller footprint than reactors that are currently in operation or under construction; a single Rolls-Royce SMR requires a site roughly one-ninth of the size of Hinkley Point C. So what is a more appropriate definition of an SMR?

The focus on electrical output does a disservice to the flexibility that is inherent to a great many modern reactor designs. As well as electricity generation so called co-generation can use the reactor’s thermal output for a variety of purposes including district heating, hydrogen generation or industrial processes. Some designs of molten salt reactors incorporate thermal storage which allows for a degree of load following in response to fluctuations in the grid thus overcoming one of the traditional disadvantages of inflexible ‘big nuclear’.

The measure of a reactor’s size, or indeed capability, as the instantaneous electrical output simply doesn’t tell the whole story. We propose that a measure of thermal output would allow like-for-like comparisons of reactor designs. A focus on thermal output would also open up the market to co-generation and would mean that a comparison would be independent of the efficiency of the turbine. If, however, a fixation on electrical output is retained then introducing a capacity factor similar to that used for wind generation would allow for a comparison of instantaneous and long term electrical output.

Modular construction typically means that self-contained parts can be manufactured off-site in a production line environment then shipped to the construction site where they can be ‘bolted together’. There seem to be various degrees of modularisation among SMR offerings with some looking more like scaled-down versions of traditional NPPs than truly modular plants. But modularisation can extend far beyond the construction of the plant to include the design itself. A truly modular design would allow for a nuclear thermal unit that was agnostic of both site conditions and the use to which the thermal energy was put.

This has enormous benefits: instead of having to adapt every site design to account for the vagaries of ground water levels orheatsink characteristics, a common design would allow for a safety case to bound likely site conditions for a given region.

With a plant that has been built again and again to the same design, the focus of the construction phase can be on actually getting the SMR up and running rather than grappling with the latest raft of design modifications.

Uniformity and repeatability breed efficiency so a truly modular common design would also lead to cost savings. It’s well known that building a first of a kind is expensive. With the traditional approach of creating a bespoke design for every site even a ‘fleet’ of power plants ends up being a muddle of prototypes with many similarities but just as many differences. True modularisation means n^th of a kind comes more quickly. It also means that the unit cost is more secure when moving to a new host country.

Exporting standardised, home grown SMR technology would require a little bit of planning to truly reap the benefits. A standardised design can be manufactured in centralised hubs which would shift the skills base away from far flung construction sites to established manufacturing heartlands. This makes sense: given the choice a lot of people would choose to stay close to home to develop their skills while also putting down roots rather than relocating to a new construction site every few years. A central manufacturing hub would build a community of highly skilled people who intimately understand the technology they are working with. This would provide a manufacturing legacy for the host nation and would allow highly developed kills to be exported which would have a benefit to that nation’s economy.

Other industries have become great at standardisation and it allows them to operate across national borders. Car manufacturers know how to standardise powertrains and chassis platforms in innumerable different configurations. The aerospace industry is perhaps the best example of standardisation operating across national borders: a given airframe can accept a number of different engine configurations yet remain sufficiently common to be maintained at every major airport in the world. Using common part and a common maintenance manual by people trained to the same standard makes this an effective way of working.

Following the model from aerospace, standardised SMR designs would mean that operator training would be the same, as would maintenance which would build a workforce that can learn from and support each other. It would also revolutionise how spare parts are managed. Any reactor will have spare parts in stock to use as the need arises but many parts have a shelf life: an expensive item can end up as scrap without ever being used. Standardised design will allow intelligent central spare part management which would reduce the need for scrapping spares.

It’s likely that building a fleet of SMRs in the UK will require skills to be imported. There is a renaissance of nuclear in Europe so we can capitalise on the skills that are already being developed abroad. We can then learn from these imported skills and continue to maintain them so rather than building our own national programme from scratch, we can collaborate with others. Again, this comes back to development of specialists who really focus on one aspect of an SMR. Trying to do this all ourselves would lead to slow pace of work, an increased amount of rework, and an associated increased cost as a new industry is effectively built from scratch.

Agreeing on a robust definition of a small modular reactor can help the industry move forwards. Capacity can be defined in many ways and basing the definition of a Small Modular Reactor on thermal capacity would allow meaningful comparisons to be made. This refocus then opens up the playing field for truly modular designs that can be put to a variety of uses. A definition of modular should also incorporate a standardised design that can be put in any location, meet any nation’s regulatory requirements, and be independent of the final use of the thermal energy, whether that’s for electricity generation or other industrial processes.