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

Nuclear energy is essential for displacement of fossil fuels and for supply of desalinated water for irrigation.

Nuclear electricity generation is the least expensive non-fossil fuel method of meeting the urban energy load in Ontario because nuclear energy is dependable (not dependent on time varying sunlight and wind speed) and because the related transmission cost per delivered kWh is relatively small as compared to the cost per delivered kWh of wind energy that is generated and stored at remote rural locations and then transmitted to urban locations.

Nuclear electricity can be used to directly displace hydrocarbon fuels in many stationary applications. However, in long distance remote transportation and aircraft applications use of energy dense liquid hydrocarbons is essential to achieve the required vehicle range. For these applications nuclear energy is needed to convert plant carbohydrates and water into energy dense liquid hydocarbons.

Nuclear energy is potentially available via two paths, fission and fusion. Fission energy is released by neutron induced fission of heavy atoms such as uranium, plutonium and thorium. Fusion energy is released when isotopes of hydrogen, helium, lithium and/or boron combine to form helium-4.

Nuclear fission power plants use uranium and thorium as prime energy sources. All fission power reactors have safety issues relating to removal of fission product decay heat and potential nuclear weapon proliferation. Most US fission power reactors are light water moderated and use as fuel uranium enriched in the isotope U-235. CANDU power reactors are heavy water moderated and can be fueled with natural uranium, used light water reactor fuel or thorium/U-233. Both light and heavy water cooled reactors rely on slow neutrons and operate at a cooling water temperature of about 300 degrees C. Both light and heavy water cooled reactors produce large amounts of very long lived high level nuclear waste.

Liquid Sodium Cooled Fast Neutron Reactors (FNRs) are a special class of fission reactors that use fast neutrons to maintain reactor criticality, to fission trans-uranium actinides and to convert plentiful U-238 into fissionable Pu-239. With suitable fuel recycling liquid sodium cooled FNRs realize about 100 fold more energy per kg of natural uranium than do water moderated fission reactors and reduce the required spent fuel isolated storage time from 400,000 years to about 300 years. With fuel reprocessing FNRs are able to consume and thus dispose of used CANDU fuel and used light water reactor fuel.

A major advantage of liquid sodium cooled FNRs is favorable chemistry which prevents corrosion of the fuel tubes, liquid sodium pool walls and the intermediate heat exchangers. The use of sealed fuel tubes prevents fission products forming thermally insulating deposits on heat exchange surfaces. Containment of fission products in sealed fuel tubes simplifies reactor on-site chemistry. Thus from a maintenance perspective liquid sodium cooled FNRs are far simpler than liquid fuel Molten Salt Reactors (MSRs).

A further advantage of liquid sodium cooled solid fuel reactors is that the neutron flux is confined to the replaceable fuel structure, so that with a suitable liquid sodium guard band there is no neutron activation of other components such as the sodium pool structure and the intermediate heat exchangers. Furthermore, the fuel structure metallurgy is such that provided that the component metals are pure and that carbon and nickel are excluded from the fuel bundles, there is no production of long lived low atomic weight radio isotopes other than via a few fission products.

FNRs have the additional power system advantage that their thermal power output is easily modulated to follow rapid changes in the electricity grid load. However, the engineering and safety issues relating to FNRs are quite different from water moderated reactors. In a large FNR each fuel bundle has its own power control system.

A liquid sodium cooled FNR typically operates at a primary liquid sodium temperature of up to 460 degrees C. In secure stable countries with educated work forces liquid sodium cooled FNRs provide the best source of energy for dependable and sustainable electricity generation and district heating.

Liquid Fuel Molten Salt Reactors (MSRs) are a special class of reactors that use molten salt to suspend U-233 / Th-232 fuel in a liquid solution. A safety advantage of MSRs is that they are more resistant to water and fires than FNRs. However MSRs are subject to complex corrosion, fuel tube failure, fission product precipitation and reactor chemistry problems and large liquid fuel MSRs have potentially dangerous power instabilities.

Maintenance of MSRs requires ongoing complex side arm radio chemistry to continuously extract protactinium as it is formed. The costs of maintaining the necessary radio chemistry equipment and expertise at every reactor site should not be underestimated.

In principle MSRs can provide heat at higher temperatures (~ 700 degrees C) than liquid sodium cooled FNRs, which simplifies implementation of chemical processes such as hydrocarbon fuel reformation. However, MSRs based on LiCl, LiF or NaCl are very expensive due to requirements for lithium and chlorine isotope separations. MSRs involving BeF2 create long term radiotoxicity due to formation of Be-10.

Thermal neutron MSRs generally require a graphite moderator that must be replaced about every four years. Fast neutron MSRs generally require molybdenum fuel tubes to restrict free neutrons to the core zone to the center of the reactor. Molybdenum needs at least one isoptope separation and is difficult to fabricate into fuel tubes. These issues tend to make molten salt cooled reactors very expensive.

A further complication with liquid fuel MSRs is that they need an initial charge of concentrated U-235 to start their breeding cycle. There are serious doubts about the long term sustainability of the sources of U-235.

Further the reactor tank and the heat exchanger of a liquid fuel MSR are more complex and much less durable than the reactor tank and intermediate heat exchanger of a liquid sodium cooled FNR. The intermediate heat exchange surfaces of a thermal neutron liquid fuel MSR tend to become coated with thermally non-conducting fission products. All the metal containment and heat exchange surfaces are subject to fluorine corrosion. The practical economics of MSRs has yet to be demonstrated. It may be more economic to run FNRs to generate electricity and to use that electricity to reach high temperatures than to directly produce high temperature heat via MSRs.

Gas Cooled Reactors are thought to be potentially useful for providing limited amounts of heat and power for displacing diesel generation at remote sites. The paper:
A Compact Gas Cooled Reactor with an Ultra-Long Fuel Cycle outlines a typical gas cooled reactor design.

A typical 15 MWt (5 MWe) gas cooled reactor has low density ceramic pebble fuel which is used to heat a flow of helium gas. The helium gas exchanges heat with a molten salt which serves as both a heat transfer fluid and an energy storage medium. The ceramic pebble fuel relies on its own thermal expansion and a negative power versus temperature characteristic for fuel temperature control. A feature of helium gas is that it does not become radioactive. Thus a gas cooled reactor has the potential advantage of inherit simplicity. However, its power density is low which implies that it is impractical to make small modular reactors with large power ratings. Another complication is provision for certain removal of fission product decay heat.

A serious economic problem with gas cooled reactor technology is ultimate disposal of spent irradiated fuel. The irradiated ceramic pebble fuel has a low average density but a high residual radiation output which makes it extremely expensive to safely transport away from the reactor site. On-site fuel reprocessing is extremely difficult due to the chemically stable nature of the ceramic fuel. The irradiated spent fuel transportation issue is further aggravated if the fuel contains thorium, because neutron activated thorium has a decay product which emits high energy gamma photons. The only economic way to deal with the used ceramic pebble fuel is to bury it close to the reactor site. That issue has serious long term public safety and property use implications.

Past practical experience in the UK seems to indicate that the power density of gas cooled reactors is so low that they are not economic for large scale public electricity generation. However, currently Rolls-Royce is challenging that assumption.

FUSION is the power source which drives our sun. However, realizing fusion reactors on Earth with the net power gain required for economic commercial electricty generation has proved to be very difficult.

The D-T (H-2 plus H-3) reaction liberates 13.6 MeV neutrons which are equipment damaging and which contribute little to fusion plasma temperature maintenance. There are many practical difficulties relating to these high energy neutrons. These neutrons must be used to produce more H-3 via the reactions:
n + Li-6 = H-3 + He-4
n + Li-7 = 2 n + Li-6

The D-He-3 reaction liberates some high energy protons that can assist in maintenance of the fusion plasma temperature. However, He-3 is rare on Earth. A D-He-3 reactor might be practical in the future if we can economically harvest He-3 from the surface of the moon.

Plasma Impact Fusion (PIF) is a pulsed method of obtaining nuclear energy via fusion. The method involves electrical formation of ordered deuterium spheromak plasmas, partial compression of the spheromak plasmas to form a denser deuterium-He-3 random plasma and then further compression of the random plasma to fusion conditions by spherically convergent high velocity liquid lead fired from precisely synchronized flywheel guns.

PIF is in some respects similar to the Magnetized Target Fusion (MTF) process investigated by General Fusion Inc. in Burnaby, British Columbia.

A fundamental issue with PIF is use of liberated high energy protons from the D-He-3 reaction for plasma heating.

Micro Fusion is a technology that provides a means of generating low temperature nuclear heat in relatively small amounts. Micro Fusion is a distributed heat production technology that is potentially useful for enhancing on-farm methanol production. However, Micro Fusion net consumes helium-3. Helium-3 is rare and expensive and is also required for other applications such as low temperature physics and detection of neutrons emitted during illicit transport of fissile materials.

It will take major technical and educational efforts to convert Plasma Impact Fusion and/or Micro Fusion from laboratory curiosities into practical energy source technologies suitable for widespread use. Even when He-3 becomes more available there is a serious questions as to whether fusion reactors can ever economically compete with fast neutron fission reactors.

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. 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).

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 input thorium into U-233, to either dispose of the existing inventory of spent CANDU fuel or to expand the reactor fleet.

An informative two hour video lecture on Fission Fuel Reprocessing

Nuclear fission in water cooled reactors produces radioactive waste products that, absent fuel recycling, take hunderds of thousands of years to naturally decay. Safe long term storage of spent fission fuel requires dedicated vaults in stable hard rock mountains where there is certainty regarding long term exclusion of ground water from the used fuel. The amount of fission product nuclear waste and the waste toxicity lifetime can be reduced over 1000 fold by fission product extraction and fuel recycling.

CANDU nuclear reactors are attractive because they can operate with used fuel from light water nuclear reactors, thus reducing the contained potential energy in spent light water reactor fuel. The spent CANDU fuel can then be converted into fuel for liquid sodium cooled fast neutron reactors. The fast neutron reactor fuel can be repeatedly recycled to consume all of the available U-238 and the long lived transuranium actinides.

A major advantage of fission power over fusion power is that the parasitic power needed to operate a fission power plant is much less than the corresponding parasitic power needed to operate a fusion reactor with the same net electricity output. The lower parasitic electrical consumption of a fission power plant as compared to a fusion power plant substantially reduces both the capital cost and the cooling requirement per kWh of net plant electricity output.

However, a disadvantage of fission as compared to fusion is that the fission products continue to emit significant decay heat for months after the chain reaction is turned off. This unwanted fission product decay heat is orders of magnitude greater than the corresponding fusion product decay heat. The fission product decay heat has led to serious Loss of Coolant Accidents (LOCA), such as occurred at Fukushima Daiichi, Japan in 2011.

When a fission fuel bundle is no longer useful for electricity generation, it is removed from the nuclear reactor but absent fission product extraction and fuel recycling it continues to emit fission product decay heat and dangerous ionizing radiation for about 400,000 years. Hence fusion is believed to be safer than water moderated fission in terms of the consequences of a Loss Of Coolant Accident (LOCA).

In summary, due to lack of He-3 on Earth this author believes that fission in Fast Neutron Reactors (FNRs) with fuel recycling is presently much more practical than fusion for sustainable and dependable electricity generation.

This web page last updated January 21, 2022.

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