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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 reliable (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 and limited distance mobile applications. However, in long distance transportation 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 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 hydrogen, helium, lithium and/or boron isotopes 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, spent 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 U-238 into fissionable Pu-239. With 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 storage time from 400,000 years to about 300 years. FNRs are able to consume spent CANDU and spent light water reactor fuel.

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 temperature and safety shutdown control rod.

A liquid sodium cooled FNR typically operates at a primary liquid sodium temperature of up to about 445 degrees C. In secure stable countries with highly educated work forces liquid sodium cooled FNRs are the best technology for new bulk electricity generation.

Molten Salt Reactors (MSRs) are a special class of fast neutron reactors that use molten salt to suspend the Pu-239 / U-238 fuel in solution. A safety advantage of MSRs is that they are more resistant to water and large sodium/hydrogen fires than FNRs. In principle MSRs can provide heat at higher temperatures (700 degrees C to 1000 degrees C) than pure liquid sodium cooled FNRs, which simplifies implementation of chemical processes such as ammonia production and hydrocarbon fuel reformation. However, lower temperature 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.

Higher temperature MSRs based on NaF are very expensive due to the required exotic high temperature materials and fabrication processes. A compromise methodology is to use NaF salt to keep the fuel in solution and to cool the NaF with floating liquid sodium. This cooling methodology minimizes problems with fission products depositing on or corroding heat exchange surfaces but compromises sodium/hydrogen fire and water resistance.

Further the reactor tank and the heat exchanger of a MSR are more complex and much less durable than the reactor tank and heat exchanger of a liquid sodium cooled FNR. The practical economics of MSRs have 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 produce high temperature heat via MSRs.

Fusion is the power source which drives our sun. However, realizing fusion reactors with the net power gain required for commercial electricty generation on Earth has proved to be very difficult due to the required minimum equipment unit size. It appears that fusion is more immediately useful as a means of breeding U-238 into Pu-239 and Pu-240 as a start fuel for new FNRs than for direct energy production.

Plasma Impact Fusion (PIF) is a method of obtaining nuclear energy via conversion of deuterium-tritium into helium-4. The method involves electrical formation of deuterium spheromak plasmas, use of the spheromak plasmas to form a denser deuterium-tritium random plasma and then 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 currently being investigated by General Fusion Inc. in Burnaby, British Columbia.

Micro Fusion is a technology that provides a practical 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. Micro Fusion involves use of helium-3, which is a by-product of other nuclear processes. However, helium-3 is expensive and is also required for detection of neutrons emitted during illicit transport of fissile materials.

It will take appropriate electricity pricing and major technical and educational efforts to convert Plasma Impact Fusion and Micro Fusion from laboratory curiosities into practical technologies suitable for widespread use.

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 dispose of the existing inventory of spent CANDU fuel.

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. The amount of fission product nuclear waste and the waste toxicity lifetime can be reduced about 1000 fold by fission product extraction and fuel recycling. However, it is imperative to prevent diversion of fissionable material from fuel reprocessing into production of nuclear weapons. In this respect 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 nuclear 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.

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 amount of 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 water 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 decay heat long 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 Chernobyl, Ukrane in 1986 and 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 far safer than water moderated fission in terms of the consequences of a Loss Of Coolant Accident (LOCA).

In summary, this author believes that fission with fuel recycling is more practical than fusion for central electricity generation.

This web page last updated June 22, 2016.

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