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By Charles Rhodes, P.Eng., Ph.D.

For additional information this author recommends the text:
Handbook of Small Modular Reactors by Igor Pioro
and the text:
Overview of Generation IV Reactor Designs
and the thesis titled:
A Conceptual Framework For The Comparison of Small Modular Reactors Based on Passive Safety Features, Proliferation Resistance and Economic Potential
Small Modular Reactors: Challenges and Opportunities

During the period 1970 to 2020 most nuclear reactors intended for public electricity generation had rated electrricity outputs in the range 700 MWe to 1500 MWe. The motivation for choosing this size range was "bigger is better" in terms of economies of scale. However, practical experience has shown that beyond a certain size there are no economies of scale. Small Modular Reactors (SMRs) are more economic. There are a number of reasons for this phenomena including:
1) Smaller generation projects are less risky to finance;

2) Smaller reactors are less expensive to back up with reserve generation;

3) From a labor training perspective it is much more efficient to execute a series of nearly identical small projects on a single site or on nearby sites than one large project.

4) In general when a large reactor is first commissioned the purchasing utility only needs a fraction of the reactor capacity. The balance of the reactor capacity is usually a provision for meeting future load that must be financed now. Hence from a cost of financing perspective smaller is better.

5) Major Component transportation costs:
When individual reactor components are too large to be moved down a public highway these components generally have to be moved by barge and special crane. The costs of the barges, special cranes, support crews and related facilities skyrocket. It is much less expensive to move multiple smaller components down a highway on conventional trucks and to lift these smaller components with conventional cranes such as are used for constructing high rise buildings.

6) Large individual reactor components such as calendrias, steam generators, turbogenerators, etc. are custom made for a particular reactor. Any problem anywhere in the supply chain or in the related labor force holds up the entire project which adds to the project financing cost. If individual reactors are smaller the consequent financial losses are smaller and to a large extent use of custom made components can be avoided.

7) From time to time reactors must be shut down for refulling or maintenance. While the reactor is shut down its load must be met by standby generation. The larger an individual reactor the more standby generation a utility must own to permit reactor shutdowns without affecting load customers.

8) As a reactor size increases the efficiency of on-site work decreases. On a large reactor site tradesmen can spend more than half their working day simply walking too and from their work locations with necessary tools and supplies.

9) If a power station is assembled from many identical smaller components the components can be economically stocked in a warehouse which gives the purchaser certainty as to price and delivery. Similarly at a multi-reactor site it becomes practical to stock spares at the reactor site.

10) If the components are smaller they can be built and fully tested in a factory where the material quality control is better and conditions are optimum for labor efficiency.

11) The combination of these factors indicates that a smaller reactor assembled out of standard highway truck transportable modules is much more economic than a large reactor which requires barge and special crane transport of its major custom components and requires a lot of skilled field labor.

12) Small modular reactors are not restricited as to reactor type. In principle water cooled reactors, liquid metal cooled reactors, gas cooled reactors and molten salt cooled reactors can all be executed in SMR form. However, due to neutron conservation issues fuel sustainable reactors cannot be made in very small sizes. For fuel sustainability it appears that the optimum reactor size is in the range 250 MWe to 300 MWe.

13) In the future large nuclear power stations will likely be assembled from a multiplicity of Small Modular Reactors. The installed capacity can be increased over time as required. The individual reactors should be connected so that at any instant in time any two reactors can be out of service without impacting load customers.

Practical SMRs are appearing in four categories:
1) Very small SMRS (5 MWe to 50 MWe) as required by remote communities and mines; eg NuScale
2) Water cooled SMRs (~ 300 MWe) whose main features are simplicity, low cost and ease of regulatory approval as required to meet immediate electricity grid load growth; eg GE Hitachi BWRX-300
3) Gas cooled SMRs (~ 80 MWe) whose main feature is high temperature output as required by the chemical industry; eg X-energy-100
4) Sodium cooled SMRs (~ 300 MWe) intended for long term bulk electricity and heat generation whose main features are long life, fuel sustainability and fuel waste disposal. eg FNR Power

It may be that category (1) cannot survive due to financing and support costs that exceed available revenue.

This web page last updated December 22, 2023.

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