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


Light water and heavy water moderated and cooled nuclear reactors rely on fission of uranium-235 (U-235) for criticality, power production and power control. The isotope U-235 is only 0.7% of natural uranium, which is mainly uranium-238 (U-238). The U-235 neutron fission cross section is greatly enhanced by use of either light water or heavy water as a moderator which absorbs kinetic energy from the fast neutrons liberated by fission of U-235. The neutron capture cross section of heavy water is less than for light water, which allows CANDU reactors to operate with natural uranium whereas light water reactors require enriched uranium.

The ability of CANDU reactors to operate with natural uranium gives Canada political independence from the USA. After the terrorist attacks of 9/11 the USA banned exports to Canada of highly enriched uranium. This issue rendered useless several hundred million dollars of Canadian investment in Maple reactors for production of medical isotopes.

Water cooled and moderated nuclear reactors gain additional power via slow neutron capture by U-238 which breeds plutonium-239 (Pu-239) and higher atomic number actinides, a small portion of which fission. The actinides are responsible for most of the long lived radio toxicity of spent CANDU fuel bundles. Much more Pu-239 and actinides would fission if the ratio of fast neutrons to slow neutrons was increased.

If the neutron spectrum contains primarily fast neutrons the actinides, instead of simply capturing neutrons, preferentially fission. The fission process yields much more energy and more fast neutrons. Most of the fission products have half lives that are short (< 30 years) as compared to the half lives of actinides in spent CANDU fuel bundles (25,000 years). The corresponding fission product toxicity decay lifetimes are short (300 years) as compared to the toxicity decay lifetime of spent CANDU fuel bundles (400,000 years).

Neutron capture by thorium in a CANDU reactor breeds U-233 in sufficient quantities that U-235 is no longer required to sustain operation in a suitably designed nuclear reactor fleet. However, U-233 fission does not produce sufficient extra fast neutrons, beyond those required for breeding thorium into U-233, to dispose of the actinides in the existing inventory of spent CANDU fuel.

1. The cost of natural uranium has been sufficiently low that the cost of fueling a water moderated power reactor with freshly mined uranium has not been an issue;
2. Utilities using nuclear reactors for electricity generation have not been forced to face either the technical issues or the full costs of disposal of spent fuel and decommissioning waste;
3. Concerns about nuclear weapon proliferation have caused politicians to avoid facing FNR fuel reprocessing issues;
4. Utilities using nuclear reactors for power generation have usually chosen the simplest power generation technology that meets their needs. They have had little economic incentive for achieving efficient natural uranium utilization or high thermal efficiency or for minimizing consumption of cooling water or for minimizing waste heat output. Their compensation has been almost entirely based on cents per electrical kWh delivered to the electricity grid. They generally have not had to pay for cooling water or for the full costs of spent fuel disposal;
5. For an electricity utility the dominant nuclear reactor related costs are capital cost amortization and staff compensation related to operation and maintenance. An electricity utility needs reliable equipment with minimum capital cost that is operable and maintainable by the lowest quartile of engineering, science and technology graduates with minimum additional training;
6. The fuel bundles in a FNR are designed and fabricated so that as the fuel temperature increases the fuel bundle reactivity decreases. This arrangement provides fuel bundle discharge temperature control, operating power stability and safe passive reactor shutdown in the event of loss of coolant flow. These two safety features do not exist in a water moderated reactor. However, these fuel bundle properties must be maintained through the entire operating life of every fast neutron reactor fuel bundle. Hence the fuel bundle dimensions, strength, uniformity and constituant control are all important and must be 100% tested prior to reactor fueling and then retested again after fuel bundle discharge. Maintenance of these fuel bundle parameters in a fast neutron power reactor requires greater staff and supplier expertise than is the case for fuel bundles in a water moderated reactor;
7. The technical name for the fuel bundle complexity in FNRs is positive reactivity. As the fuel bundle ages it can go through a period when its reactivity increases. To compensate for positive reactivity the control rods should be automatically repositioned as a fuel bundle ages. Regulatory authorities are presently poorly equiped to deal with positive reactivity issues;
8. FNRs generally require a liquid metal primary coolant, usually liquid sodium. Use of liquid sodium coolant introduces design complexity, material and fabrication quality control, maintenance complications, personnel training and safety issues that electricity utilities will generally try to avoid if given the choice.
9. Use of liquid sodium minimizes the required parasitic pumping energy. However, liquid sodium is highly flammable and on contact with water liberates hydrogen which in air usually self ignites. Thus additional staff training is required to address sodium leaks, water penetration prevention and related potential fire containment and fire suppression issues;
10. Fast neutron reactors must be designed, operated and maintained so that there is no way for the liquid sodium to escape to the extent that some of the nuclear fuel is no longer immersed in liquid sodium while the control rods are withdrawn. This issue is critical and requires additional engineering and staff training;
11. The issue of human resources for on-going reactor maintenance and operation is not trivial. These persons must fully understand the electrical, mechanical, chemical, nuclear and safety aspects of fast neutron power reactor operation. However, their employment mobility is extremely limited. They frequently must work outside major urban areas. They may have to work rotating shifts for much of their lives. If a nuclear accident occurs, even a minor one within a plant, they may be exposed to radiation that adds to their risk for long term cancer and having children with mutations. For all these reasons nuclear plant operation and maintenance are usually not the first choices of the top three quartiles of engineering, science and technology graduates.
12. Most nuclear plant workers choose that life, in spite of its known negatives, because the average compensation is significantly better than these persons can earn working elsewhere. However, from a utility executive perspective, staff compensation and staff training are a major costs. To minimize these costs utility executives choose the simplest suitable nuclear power generation technology.
13. One of the major means of simplifying nuclear power system maintenance with water moderated reactors is to keep the water temperature under 300 degrees C, which allows use of elastomeric gaskets and O-ring seals. However, keeping the water temperature under 320 degrees C limits the efficiency of conversion of heat into electricity.
14. Up until recently electricity utility executives have not concerned themselves about the temperature limitations of water moderated nuclear power reactors because there have been no restrictions on use of natural gas for large scale production of hydrogen and ammonia and because there has been no fossil carbon emissions tax. It is only recently that long term utility decisions have been significantly influenced by global warming issues.
15. A large inventory of spent light water reactor fuel must be suitably processed to obtain the fuel load required for starting a FNR. Thereafter the FNR breeds its own fuel from depleted uranium.

There are a number of issues that will within the coming years force large scale adoption of fast neutron power reactors:
1. A future increase in the price of newly mined uranium. India, which has few natural uranium resources, but is reliant on nuclear electricity, is already pursuing a reactor program involving breeding thorium into uranium-233. France, which has had a nuclear fuel recycling program in operation for some years, is presently having difficulty economically justifying the program in the current environment of low cost newly mined natural uranium. However, as nuclear power is increasingly used for displacement of fossil fuels, U-235 will increasingly be in short supply and its cost will become prohibitive.
2. As power utilities are forced to meet the costs of nuclear waste disposal CANDU reactors will be replaced by FNRs which produce far less spent nuclear fuel waste and far less radioactive decommissioning waste.
3. A shortage of cooling water. Some nuclear reactors, particularly in the interior of continents, rely on river or lake water for cooling. Global warming is reducing the supply of river and lake water. Electricity load increase, in part due to global warming, is forcing development of even more electricity generation. In these circumstances it is important to increase the electricity generation efficiency which is the ratio:
(electrical kWh produced) / (thermal kWh produced in the nuclear reactor).
Increasing this ratio requires increasing the reactor primary coolant temperature. With water at 480degrees C the pressure is unreasonably high, which makes use of a liquid metal primary coolant much more attractive.
4. Safe disposal of spent fuel such as CANDU fuel bundles that contain a range of long lived radio toxic transuranium actinides including Pu-239. It is only recently that a viable plan using FNRs for reducing the radio toxicity lifetime of spent CANDU fuel has been developed.
5. The first step in permanent disposal of spent light water reactor fuel is to reuse it in a CANDU reactor to fission the remaining U-235 and to convert a portion of the U-238 into Pu-239. Thereafter the fuel should be recycled multiple times through a FNR to fission both the Pu-239 and the residual actinides, while continuing to convert more U-238 into Pu-239.
6. At each fuel cycle U is selectively removed from the blanket rods. The remainer is new core fuel rod material. The selectively removed U is used to fabricate new blanket rods.
7. At each fuel cycle low atomic weight fission products with short half lives are separated from the core fuel rods and replaced with an equal weight of new core fuel rod material obtained from reprocessing the blanket rods.
8. To maximize the long term supply of energy light water moderated reactors should eventually be eliminated altogether because the U-235 enrichment process that they require discards the U-238 which contains over 99% of the fuel's potential energy.
9. A major economic trigger for adoption of fast neutron power reactor technology would be a fossil carbon tax sufficient to make off-peak nuclear power preferable to natural gas for large scale production of hydrogen.

In the 1960s, as a result of work at Atomic Energy of Canada Limited (AECL), Canada was a world leader in fast neutron technology. An unfortunate consequence of a series of technically inept federal and provincial governments has been that Canada lost its lead position in this important technology. India, China, South Korea, France, Russia and the USA have all surged ahead of Canada.

Nuclear power provides more than 60% of the electricity in the province of Ontario, yet the government of Ontario does not have the confidence, technical competence or moral fortitude to support its own nuclear industry. The government of Ontario is not even serious about addressing global warming. The government of Ontario currently does not allow economic use of surplus non-fossil electrical energy to displace furnace oil for space and domestic hot water heating in rural Ontario where there is no natural gas service. The government of Ontario also does not allow economic use of surplus non-fossil energy for production of synthetic liquid hydrocarbon fuels. The government of Ontario purchases wind and solar energy that delivered to Ontario consumers costs several times as much as the cost of equivalent nuclear energy. There is no understanding in the Ontario government that the base metal and forest product commodity industries and all their related downstream jobs are dependent upon the extraction sector being able to purchase off-peak electricity at a reasonable price.

Recent repeated delays in the Ontario nuclear electricity generation program will force extremely imprudent decisions on future Ontario governments. The economic way to build nuclear reactors is one at a time so that the work force and the supply chain are efficiently trained and constantly employed. Instead Ontario will be forced into a very inefficient, expensive and time constrained new build construction cycle as current nuclear reactors reach the end of their useful working lives.

The Nuclear Waste Management Organization is contemplating spending over $20 billion on a new long term storage facility for spent CANDU fuel bundles when a much safer and much less expensive toxic waste storage facility already exists at Jersey Emerald in British Columbia. Further, adoption of fast neutron power reactors would almost eliminate the requirement for long term spent fuel storage and would yield about 100 fold more useful energy per kilogram of mined natural uranium.

The aforementioned problems are all aggravated by an obsolete electricity pricing model in Ontario based on fossil fuel rather than non-fossil fuel energy sources.

This web page last updated June 23, 2016

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