XYLENE POWER LTD.

SMALL MODULAR REACTORS (SMRs):

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

SMALL MODULAR REACTORS (SMRs):
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
and
Small Modular Reactors: Challenges and Opportunities

During the period 1970 to 2020 most nuclear power 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 potentially more economic. There are a number of reasons for this economy including:
1) Smaller nuclear power projects require less capital and less construction time and hence are easier to finance;

2) Smaller reactors require less reserve generation for back up;

3) From a labor training perspective,it is usually more efficient to execute a series of nearly identical small projects than one large project;

4) When a large reactor is first commissioned, usually the purchasing utility only needs a fraction of that reactor's 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 using special cranes. The costs of the barges, special cranes, support crews and related facilities skyrocket. It is usually much less expensive to move multiple smaller components down a highway on conventional trucks and to lift these smaller components into place with conventional cranes.

6) Large individual reactor components such as calendrias, steam generators, turbogenerators, etc. are often 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 refuelling 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 nuclear power station is assembled from many identical smaller components the components can be economically stocked in a nearby warehouse which gives the purchaser certainty as to price and delivery.

10) If the components are smaller they can be built and fully tested in a factory where 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 potentially 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:
Practical SMRs are appearing in four categories:
1) Very small SMRS (5 MWe to 50 MWe) as required by off-grid communities and mines. The economic viability of very small SMRs is questionable;
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, low pressure for urban siting, potential air cooling, 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 12, 2025.