The future of the nuclear industry could lie with small reactors, argues Professor Juan Matthews.
In the beginning, all reactors in nuclear power stations were small. Calder Hall, Britain’s first power station which went on-line in 1956, consisted of four reactors each generating just 50 MW of electricity.
The next generation of Magnox reactors averaged 200 MWe each. Then, in the 1970s, the Advanced Gas Cooled Reactors (AGRs) were 600 MWe. The last reactor built before the nuclear programme was snuffed-out by the ‘dash for gas’ – Sizewell B, our only Pressurised Water Reactor (PWR) – was completed in 1995 and is about 1200 MWe. This trend of increasing reactor size is reflected around the world. Hinkley Point C will contain two French EPRs (European PWRs), each rated at 1600 MWe.
Since the start of the nuclear age, we have seen the length of time to build a reactor increase from three years to eight or 10 years, as was pointed out in a recent article by Professors Tim Abram and Gregg Butler. Meanwhile, the cost of nuclear power – particularly the capital cost of reactor construction – has become a major issue. Experience has not enabled us to build reactors quicker and cheaper. The larger reactors have become very complex and the safety systems needed for reactors to be cooled and fission products contained have added significantly to construction cost and timescale.
A possible solution emerged in the late 1980s in response to the Chernobyl disaster. Smaller reactors are easier to contain and easier to cool. Small reactors can be designed to be cooled passively by natural convection and directly by conduction. This led to a flurry of small reactor designs, including a collaboration between the USA and the UK called the Safe Integral Reactor, or SIR. This potential revolution happened at a time of decreasing oil and gas prices and the loss of interest in new nuclear power everywhere except Asia.
This situation has now changed and the need to have sustainable and affordable electricity with low emissions of CO2 has led to a large number of Small Modular Reactor (SMR) concepts being developed. ‘Small’ means that they are below a generating capacity of about 300 MW and ‘modular’ that they can be built in modules, i.e. blocks of factory build systems that are assembled on the reactor site with a minimum of fuss. They can be installed in clusters of two to 12 reactors to make a ‘large’ nuclear power station, if needed, but with spend committed sequentially.
SMRs are one of the areas identified in the UK’s Nuclear Industrial Strategy for future development and international partnership, alongside the more sustainable GEN IV reactors, which could also be SMRs. As a result a feasibility study has been published by the National Nuclear Laboratory, to which The University of Manchester’s Dalton Nuclear Institute has contributed.
There are two sorts of SMR. Really small ones can be built as packages and delivered to a site as a unit and removed when not needed with little decommissioning. An example is the U-Battery, developed as a concept by The University of Manchester and the University of Delft for URENCO. The first reactor could be running at Capenhurst in 2023.
U-Battery is a very small high temperature gas-cooled reactor, cooled by helium, with the fuel made from small particles of enriched uranium dioxide coated in carbon and silicon carbide, embedded in a graphite moderator. This fuel is very resistant to overheating and the reactor has many passive safety features. At just 4 MW of electrical capacity, the reactor uses a secondary nitrogen circuit to drive a low maintenance gas turbine. A U-Battery could be used at a remote location off-grid, or as part of an industrial complex. Off-grid it would compete with more expensive alternatives like diesel generators. Waste heat from the gas turbines can be used in industrial processes, or for heating local buildings.
The main line of SMR development is for reactors producing 50 to 200 MW of electricity. Large reactors are popular because of their economies of scale, so new SMRs have to be less complex and capable of more efficient construction.
The most promising development is an integral PWR. This is like a conventional PWR, except the main primary circuit containing steam generators, re-circulating pumps and pressuriser is squeezed into the reactor pressure vessel. This simplifies the layout of the reactor systems and has an additional safety advantage by replacing the large primary circuit penetrations with smaller non-active steam outlets and feed water inlets.
Some of the smaller designs, such as the NuScale reactor, do away with the main re-circulating pumps, using natural convection. NuScale is a small integral PWR of just 45 MWe that would be built in clusters of 12 in a water pond, each cocooned in a containment vessel that can be flooded with water in the event of an accident. NuScale is promoted by Fluor Corporation, while Rolls-Royce has supported applications for US Department of Energy funding.
Closer to realisation are projects in China, where ACP100 reactor construction is about to start. The next stage of development, ACP100+, could be one of the first projects at the recently announced UK-China research centre to be operated by the Chinese National Nuclear Corporation and the UK National Nuclear Laboratory.
The UK now has an opportunity to develop its own SMR design, or to partner an existing project. The objective is to develop economic SMRs that will give UK industry the opportunity to regain a role in reactor manufacture.
The UK is currently limited in the number of licensed sites suitable for large reactors and SMRs may allow us to use some smaller licensed sites, as highlighted in a recent Energy Technologies Institute report. However, the main attraction of SMRs is to open up new nuclear power markets in countries, especially where finance is limited or where the power grid is not robust enough to take large reactors.
There is an opportunity also to design the fuel and the fuel cycle for these reactors to simplify non-proliferation measures. That and the development of advanced manufacturing are the challenges our nuclear research centres and industry need to meet to establish a true renaissance.
