Small Modular Reactors: Game changer or more of the same?

Introduction

With current large reactor designs tarnished by their poor record of construction, attention for the future of new nuclear power plants has switched to Small Modular Reactors (SMRs). The image of these portrayed in the media and by some of their proponents is that they will roll off production lines, be delivered to the site on the back of a truck and, with minimal site assembly, be ready to generate in next to no time; they will be easy to site, a much cheaper source of power, be safer and produce less waste than large reactors; as a result, they are being built in large numbers all around the world. But what is the reality?

What are SMRs and AMRs?

In terms of size, the International Atomic Energy Agency (IAEA) defines SMRs as reactors producing 30-300MW of power and defines reactors producing up to 30MW as micro-reactors. In practice, the size of SMRs is increasing and of the seven designs that have received UK government funding, four are at or beyond the 300MW upper limit for SMRs.¹ The vendors of the two micro-reactor designs funded by the UK have both collapsed,² leaving the X-Energy Xe-100 the only reactor design, at 80MW, that is technically an SMR.

The term Advanced Modular Reactor (AMR) is largely a UK invention and denotes reactors using designs other than the dominant large reactor technologies — Pressurised and Boiling Water Reactors (PWRs and BWRs). In other countries, the term SMR covers all reactors in the IAEA’s size range. None of the proposed AMR designs are new, all having been discussed for 50-70 years but not built as commercial reactors. They can be divided into those built as prototypes or demonstration reactors — the Sodium-cooled Fast Reactor (SFR) and the High Temperature Gas-cooled Reactor (HTGR) — and those that have not been built — Molten Salt Reactors (MSRs) and Lead-cooled Fast Reactors (LFRs).

Some designs include a heat storage device so that when demand is high, this heat can be used to generate additional electricity as well as that generated by the reactors. When electricity demand is low, the heat produced by the reactor can be stored for when demand is higher, giving it a generating flexibility. For example, the Terrapower SFR design includes molten salt heat storage to boost the station’s output from 345MW to 500MW at peak times. This is intended to address the issue that operating reactors in ‘load-following mode’ is problematic technologically and economically. It is not clear whether this generating flexibility justifies the substantial additional expense of the heat storage system.

What is the case for SMRs and AMRs?

SMRs and AMRs are presented, not only by the nuclear industry, but also by the media and government, as established, proven, commercial products. The main claims for SMRs and AMRs compared to large reactors are:

1. They will be cheaper to build per kW of capacity and less prone to cost overruns;
2. They will be quicker and easier to build and less prone to delay;
3. They will produce less waste per kW of capacity;
4. Building components on factory production lines will reduce costs;
5. Modular construction, reducing the amount of site-work, will reduce costs and delays;
6. They will be safer;
7. They will generate more jobs.

There have been numerous critiques that demonstrate these claims are at best unproven or at worst simply false.³ The summary of the critiques on each point is as follows.

Construction Cost

The first commercial reactors worldwide were mostly in the SMR size range, but they proved uneconomic and the vendors continually increased their size to gain scale economies, culminating in the 1600MW Framatome European Pressurised Reactor (EPR). Intuitively, a 1600MW reactor vessel will cost less than ten 160MW reactor vessels. While increasing their size was never enough to make the reactors economic, it is implausible that scaling them down will make them cheaper per unit of capacity because of the lost scale economies. It appears that SMRs are struggling to be economically viable. Holtec doubled the electrical output of its design at some point in 2023.⁴ The realistic competitors to SMRs are not large reactors but other low-carbon options such as renewables and demand-side management.

“While increasing their size was never enough to make the reactors economic, it is implausible that scaling them down will make them cheaper per unit of capacity because of the lost scale economies.”

Construction time

There is no clear analysis explaining why reactors are now expected to take longer to build and why they seem more prone to delay.⁵ However, it seems likely that the issue is that the designs have got more complex and difficult to build as they are required to take account of vulnerabilities exposed by events such as the Fukushima disaster. The problems thrown up by the occupation of Ukraine’s Zaporizhia site by Russia have yet to be taken up in new reactor designs. As a result of the 9/11 terrorist attack on New York, new reactor vessels are required to be able to withstand an aircraft impact. The conflict in Ukraine spilled on to the Zaporizhia site causing concerns that a serious accident would result. Analysis suggests that the exterior of other parts of the plant should be toughened. If the issue is complexity rather than size per se, reducing the size of the reactors may do no more than make construction a little easier.

Waste

For SMRs, there is a clear consensus that they will produce more waste per unit of capacity than a large reactor. For example, Nuclear Waste Services, the UK body responsible for waste disposal said: “It is anticipated that SMRs will produce more waste per GW(e) than the large (GW(e) scale) reactors on which the 2022 IGD data are based.”⁶ Alison MacFarlane, former chair of the US Nuclear Regulatory Commission (NRC) wrote: “The low-, intermediate-, and high-level waste stream characterization presented here reveals that SMRs will produce more voluminous and chemically/physically reactive waste than LWRs, which will impact options for the management and disposal of this waste.”⁷  The AMRs will produce an entirely different cocktail of waste varying according to the type of reactor.

“SMRs will produce more voluminous and chemically/physically reactive waste than Large Light Water Reactors”

Factory production lines

In principle and in general, production lines, which have high set-up costs, can reduce costs with high-volume items with a fixed design and a full order book. But, if demand is not sufficient to fully load the production line or the design changes requiring a re-tooling, the fixed costs might not be fully recoverable. The production lines proposed for SMRs will produce less than a handful of items per year — a long way from a car or even an aircraft production line — and the market for SMRs is uncertain, so guaranteeing a full order book is impossible. There is also a ‘chicken and egg’ issue that the economics of SMRs will only be demonstrated when the components are produced on production lines, but production lines will only be viable when the designs are demonstrated sufficiently to provide a flow of orders.

Modularity

Modularity is a rather vague term, and all reactors will be made up of components delivered to the site and assembled there, any difference between designs being down to the extent of site work. The Westinghouse AP1000 design is said to be modular but this did not prevent all eight orders suffering serious delays and cost overruns. Framatome now describes the successor design to the EPR, the 1600MW EPR2, as modular.⁸

Safety

Some of the SMRs and AMRs rely on ‘passive’ safety, in other words, they do not require the operation of an engineered system to bring the reactor back under control in the event of an accident. A common assumption is that because it is passive, it is fail-safe, and will therefore not require back-up safety systems and so will be cheaper. None of these assumptions is true and, for example, the UK Office of Nuclear Regulation (ONR) has said for the 20MW PWR design from Last Energy: “ONR advised that it is philosophically possible to rely entirely on two passive safety systems, providing there is adequate defence in depth (multiple independent barriers to fault progression)”.⁹ Some designs rely on being built underground but the Nuward and NuScale designs that use this have struggled to win orders with Nuward being abandoned and NuScale losing its only major order prospect because of rising costs.¹⁰

Job creation

A key selling point for SMRs is that they will require much less site work and that implies fewer jobs. More of the work will be done in factories but the business model for SMRs requires that, globally, as few factories be built as possible to maximise scale economies, so if, for example, the factory is not in the UK, neither will the jobs be.

What is the experience with SMRs?

Many reactors that fall into the size range of SMRs were built in the 1960s including 24 reactors in the UK. By the mid-60s, almost all new orders were for reactors larger than 300MW. This century, only two SMR projects have been completed^11, one in China and one in Russia, but neither design appears to have any firm follow-up projects. Two projects are under construction, one in Russia and one in China, but neither design appears to have any further firm order prospects. There is one micro-reactor under construction in Argentina (see Table below).

The most advanced project using a commercially available design is for a GE Vernova BWRX-300 reactor to be built at the Darlington site in Canada. There appears to be a firm order for this reactor although by January 2026, construction had not started. The Canadian safety regulator will assess the design during the construction period, not before construction starts as would be required in most jurisdictions; this gives rise to a risk of delays and cost escalation if a design issue requiring additional cost emerges during construction.

“Many reactors that fall into the size range of SMRs were built in the 1960s including 24 reactors in the UK. By the mid-60s, almost all new orders were for reactors larger than 300MW”

There are several other projects with a named site and design, often presented in the media as being under construction, but these have yet to receive regulatory approval for the design, they do not have construction permits and a firm reactor order has not been placed. Those listed in Table 1 are the ones that appear most advanced in terms of regulatory approvals. Numerous other projects have been publicised, invariably with ambitious completion date targets, but they are some distance from a firm order being placed. Up to this point, historically, a high proportion of nuclear projects of all sizes announced do not proceed and there is no reason to believe this will not be the case with these projects. Once a firm reactor order has been placed, the project is more likely to go ahead because the cost of abandonment is high.

The two operating SMRs (in China and Russia) have a very poor record in terms of construction time and operating performance, but authoritative construction costs are not known. Completion of the three under construction is also behind schedule. While these projects are not for commercial designs, this provides no evidence that the ambitious claims for SMRs will be met.

Conclusions

The perception that SMRs are being built in large numbers is untrue and the claims made for them in terms of, for example, cost, safety, and waste are at best unproven and at worst false.

The image of them being much smaller than existing reactors is incorrect. The IAEA’s size range is arbitrary but the clear trend for SMRs to increase in size does put a question mark against the claims made for them such as reduced cost per kW due to small size, ease of siting and mass production. Most of the designs that have realistic order prospects are at or beyond the 300MW upper limit of the IAEA range for SMRs. This is illustrated by the Holtec design which, for more than a decade was being developed as a reactor, SMR160, designed to produce 160MW of electricity. In 2023 and with no publicity, the output of the reactor was doubled to become the SMR300 and projects using this technology are foreseeing 340MW of power. The idea that siting and building them will be easy is not credible; a reactor of more than 300MW will need to be carefully sited so it is not vulnerable to sea-level rise or to seismic issues and will require substantial on-site work including foundations, suggesting that the claim that these projects would be largely factory built is implausible. It would also mean that either the modules would be very large making them difficult to transport or would require a larger number of modules increasing the amount of site-work.

“The perception that SMRs are being built in large numbers is untrue and the claims made for them in terms of, for example, cost, safety, and waste are at best unproven and at worst false.”

This increased size also means that the image of a rolling production line producing large numbers of reactors is inaccurate. Rolls Royce, whose design has increased to 470MW, is anticipating its production lines would produce components for only two reactors per year.

The UK, along with Canada and the USA is in the vanguard of development of SMR designs. The history of nuclear power shows that developing new reactor designs is an expensive venture with a high probability of failure. The UK’s chosen design is the largest SMR design on offer and is being developed by a company with no experience designing or building civil nuclear power plants. Submarine reactors have very different design priorities and the reactors built by Rolls Royce use US designs. There is huge scope for the UK to build much cheaper offshore wind and to carry out energy efficiency measures which would have the double dividend of reducing emissions and tackling fuel poverty. It would make much more sense for the UK to let other countries make the investments and take the risk and only if SMRs are shown to fulfil the claims made for them to then adopt them as part of the UK’s generating mix.

Endnotes

1. Rolls Royce, GE Vernova, Westinghouse LFR and Holtec.

2. Ultra Safe Nuclear Corporation and U-Battery.

3. S D Thomas & M V Ramana (2022) ‘A hopeless pursuit? National efforts to promote small modular nuclear reactors and revive nuclear power’ WIRES Energy & Environment.

4. Holtec is not willing to specify when this occurred, just saying ‘late 2023’. Email to the author.

5. When the EPR was launched, it was expected it could be built in less than 5 years. For its successor design, EPR2, EDF has a target to reduce construction time from 105 months to 70 months, but it only expects it to be able achieve this by the end of a 6-reactor programme. See: ‘Le ambitious plan of EDF: to build its EPR 2 nuclear reactors in just 70 months’, Julien, ino industry, 14 December 2024.

6. ‘2022 Inventory for Geological Disposal: Key Changes Report’, Nuclear Waste Services, July 2025; see also, ‘Nuclear waste from small modular reactors’, L.M. Krall, A.M. Macfarlane, & R.C. Ewing, Proceedings of the National Academy of Sciences (journal of the National Academy of Sciences), USA, 2022.

7. Ibid. “Large Light Water Reactors (LWR) are reactors generating at least 700 MWe using ordinary water as coolant. This includes boiling water reactors (BWRs) and pressurized water reactors (PWRs), the most common types of operating reactors in the United States.”, ‘Large Light Water Reactors’, US Nuclear Regulatory Commission.

8. ‘Alfa Laval Extends Strategic Partnership With EDF To Accelerate Nuclear Energy Projects In France’, Power Online, 5 November 2025.

9. ‘Last Energy PDR Summary Report, Office for Nuclear Regulation’.

10. ‘NuScale ends Utah project, in blow to US nuclear power ambitions’, Timothy Gardner and Manas Mishra, Reuters, November 9, 2023.

11. Four reactors of 220MW have been completed in India, but this is a design first ordered in 1960 and is no longer being pursued.