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

World Nuclear Organization

Long Term Sustainability of Nuclear Fuel

Processing of Used Nuclear Fuel

The remaining known mineable natural uranium resource, presently about 6 million tonnes, sets upper limits on the energy that can be obtained from thermal neutron fission of the rare fissile uranium isotope U-235 and from fast neutron fission of TRU formed by transmutation of the abundant fertile uranium isotope U-238. There is also a corresponding limit on the long term sustainable rate of nuclear energy production (sustainable power) via fast neutron fission.

TRU are isotopes of elements with atomic numbers greater than 92 that gradually accumulate in the fuel of operating thermal neutron (water or heavy water moderated) reactors.

Today, most existing nuclear power reactors rely on thermal neutron fission of fuel containing the rare uranium isotope U-235. However, projected nuclear power reactor construction for climate change mitigation indicates that the known supply of economically mineable U-235 will be depleted within about 40 years. Thereafter it will be practical to produce a limited amount of long term sustainable nuclear power by using Fast Neutron Reactors (FNRs) with TRU based core fuel and U-238 blanket fuel. However, the power output of these FNRs will be limited by the size of the future TRU inventory. That inventory can potentially be increased using neutron spallation based Intense Neuron Generators (INGs).

The size of the future TRU inventory is presently threatened by irrational, uninformed and misguided present day political decisions relating to both reactor type selection and used fuel disposal.

CANDU reactors are more expensive to build and maintain than Light Water Reactors (LWRs) of similar power capacity because CANDU reactors use expensive heavy water for both cooling and moderation. However, on a per unit of natural uranium consumption basis CANDU reactors produce about 2X as much thermal energy and about 4X as much TRU as do LWRs. As the existing rich natural uranium ore bodies are depleted natural uranium will become more expensive and CANDU reactors will have a two fold advantage as compared to LWRs on fuel cost / output kWht and as much as a four fold advantage in terms of TRU production. The sustainable power that future Fast Neutron Reactors (FNRs) can produce will be proportional to the cumulative TRU production. Hence the societal benefit of the CANDU reactor's higher TRU production is enormous.

In summary, the choice at this time of LWR rather than CANDU nuclear power reactors as a present day cost saving measure will result in a severe future TRU inventory shortage that will enormously reduce the dependable and sustainable power available after natural uranium ore depletion makes U-235 based reactor fuel prohibitively expensive. Dependable and sustainable power are essential for electricity grid stability, electricity grid black start and ongoing future displacement of fossil fuels.


The way to maximize the useful energy and long term sustainable power available from natural uranium is to start by using the natural uranium as CANDU reactor fuel. That use yields about 4 grams of TRU per kg of natural uranium. Then: concentrate the TRU formed in the used CANDU reactor fuel, reprocess the concentrates to form Fast Neutron Reactor (FNR) fuel, and then use a FNR to fission the new FNR core fuel within a U-238 FNR blanket.

In addition to producing heat, the FNR will produce new TRU in its core and blanket faster than old TRU is consumed in its core.

Periodically the FNR must be briefly shut down to allow fuel bundle exchange for off-line fuel reprocessing. During reprocessing fission products in the FNR core are rejected to interim storage and are replaced by new TRU atoms drawn from the FNR blanket. Then the FNR blanket is refilled with U-238 atoms and the FNR is restarted.

This heat production process can continue indefinitly as long as there is a supply of U-238. The heat production rate is limited by the TRU inventory.

Another way realize the useful energy and part of the potentially available sustainable power from natural uranium is to first enrich the uranium, form uranium oxide, use the enriched uranium oxide as Light Water Reactor (LWR) fuel, then selectively extract the uranium oxide for use as a CANDU fuel.

This process ultimately yields a similar amount of thermal energy as does the most efficient and effective process. However, due to neutron absorption by light water the TRU production per unit of natural uranium consumed is only about half that of the most efficient and effective process. Since the sustainable power output from a FNR is proportional to its TRU inventory, the sustainable power output is only about one half that of the most efficient and effective process.

The least efficient and least effective way realize energy and power from natural uranium is to first enrich the uranium, form uranium oxide, use the enriched uranium oxide as Light Water Reactor (LWR) fuel, and then to directly reprocess the LWR fuel.

This method yields about half of the thermal energy per unit of natural uranium as does the most efficient and effective process and yields about 25% as much TRU per unit of natural uranium as does the most efficient and effective process. The reductions in both thermal energy and ultimate sustained power output have huge social implications on the future of mankind, especially considering that today (2023) LWRs are the dominant form of power reactor in existence and that present US regulators are opposed to US adoption of CANDU technology.

New CANDUNatural uranium oxide 0.7% U-235, 99.3% U-238
Used CANDU0.125% U-235, 0.25% Pu-239, 0.15% Pu-240, 1.0% FP, 98.4% U-238
New LWREnriched uranium oxide 3.5% U-235,96.5% U-238
New FNR CoreMetallic 20% TRU, 70% U-238, 10% Zr
Used FNR CoreMetallic 12% TRU, 15% FP, 63% U-238, 10% Zr
New FNR BlanketMetallic 89.8% U-238, 10% Zr
Enrichment TailingUF6 0.3% U-235, 99.7% U-238


Typically FNR blanket fuel is made by alloying the nearly pure uranium rejected during TRU concentration of used CANDU fuel with 10% zirconium. Hence the blanket fuel initially contains the 0.125% U-235 remaining in the U-238 after the uranium extraction during TRU concentration. If the blanket fuel is made of uranium enrichment tailings the U-235 fraction is about 0.3%.

The most convenient source of FNR core fuel is TRU concentrates obtained from used CANDU fuel. CANDU reactors are natural uranium fuel efficient sources of both heat and TRU. In the long term the TRU contained in used CANDU fuel has much more economic value than the heat initially harvested by the CANDU reactor.

The usual source of CANDU fuel is natural uranium.

In principle used LWR fuel can also be used to fuel CANDU reactors. Various proceesses may be used to adjust the used LWR fuel reactivity to achieve reactor compatibility. Typically, in used LWR fuel the U-235 concentration is about 1.25%. One complication in implementing this LWR fuel recycling process is that the CANDU reactors used must be fitted for safe loading of hot fuel. A second complication is that used CANDU fuel resulting from recycled LWR Fuel has a higher trace U-232 concentrtion in the U-238 than does used CANDU fuel resulting from natural uranium. This trace U-232 increases the biosafety complications involved in making and storing new FNR blanket fuel.

LWRs use light water instead of heavy water for neutron moderation and reactor heat removal. Light water molecules have a much higher neutron absorption cross section than do heavy water molecules. To achieve the necessary reactivity in a LWR enriched fuel initially consisting of about 3.5% U-235 and 96.5% U-238 is used.

The fuel enrichment process involves forming UF6 gas. The fluorine atoms all have an atomic weight of 19. The UF6 molecules containing U-235 have a molecular weight of:
235 + 6(19) = 349
and the UF6 molecules containing U-238 have a molecular weight of:
238 + 6(19) = 352

This small difference in UF6 gas molecule weight is used to concentrate the lower molecular weight molecules using a centrifuge cascade with hundreds of stages. Once suitably isotopically concentrated, the UF6 is converted to UO2 for use in LWRs.

As compared to CANDU reactors LWRs are only about 70% as efficient as in terms of use of natural uranium for production of heat and LWRs are only about 25% as efficient as CANDU rectors in terms of direct production of TRU. These inefficiencies flow in part from the uranium enrichment process during which 8 kg of natural uranium is used to make 1 kg of LWR fuel. ie 7 kg of U are depleted from 0.7% to 0.3% U-235 and one kg of U is enriched from 0.7% U-235 to:
0.7% + 7 (0.4%) = 3.5%

The fuel enriched to 3.5% U-235 gets burned down to about 1.25% U-235 so the burned down amount per kg of natural uranium is:
(3.5% - 1.25%) / 8 = 2.25% / 8 = 0.28%
as compared to:
0.7% - 0.125% = 0.575 % in a CANDU reactor.
The production of TRU in a LWR is about twice that of a CANDU on a per kg of LWR fuel basis but is only about 25% of a CANDU on a natural uranium consumption basis.

Part of the inefficiency in TRU production arises from use of light water for moderation and cooling. The light water absorbs neutrons that in a heavy water moderated reactor would contribute to TRU formation.

In the future this inefficiency in TRU production by LWRs will be a major issue because it will lead to a serious TRU shortage. Mitigating that TRU shortage will require building and operating many INGs and more CANDU reactors.

Mankind is facing future shortages of both mineable natural uranium and TRU that will severely bite within 40 years. Those shortages will be impossible to mitigate unless appropriate measures are taken now to build INGs and more CANDU reactors, to harvest TRU and to immediately apply that TRU to fuel sustainable Fast Neutron Reactors (FNRs). The use of LWRs instead of CANDUs aggravates both the natural uranium and the the TRU shortages.

Under the most optimistic reactor choice circumstances the miniimum consequences of the nuclear fuel shortage are indicated by the following. Absent immediate corrective action reality will likely be much worse.

Climate change is mainly a result of the rising atmospheric CO2 concentration. In Canada, in 2023, climate change is enabling record wildfire damage, violent storm damage, permafrost melting, sea level rise and ocean acidification. The climate change consequences in the USA and Europe are comparable.

Today, preventing further rise in the atmospheric CO2 concentration would require about 20,000 GWt of new dependable and sustainable clean (non-fossil) thermal power to displace the thermal power that is presently supplied by world wide combustion of fossil fuels. Due to increasing electrification in developing countries, if present trends continue, by 2070 the world thermal power load is projected to be about 40,000 GWt.

Due to its intermittant output renewable electricity generation cannot supply dependable electrical power and can economically provide only about 25% of the required clean energy. Meeting the world thermal power load with dependable power and clean energy would require both maximum economic renewable energy generation and a world nuclear reactor fleet with a total thermal power output of about:
0.75 X 20,000 GWt = 15,000 X 1 GWt,
which could potentially provide a total electric power output of about:
15,000 X 300 MWe.

CANDU reactors usually use natual uranium (0.7% U-235, 99.3% U-238) as fuel.

If CANDU reactors, that require fuel replacement every 1.5 years, are used to meet the projected thermal load and the CANDU fuel requirement is 50 tonnes of natural uranium per 300 MWe there would be an installed fuel requirement of:
(50 tonnes / 300 MWe reactor) X 15,000 X 300 MWe reactors
= 750,000 tonnes natural uranium /fueling
or 500,000 tonnes / year.

The present total economically mineable world natural uranium resource is estimated to be about:
6,000,000 tonnes.

Hence the present known mineable natural uranium resource would be consumed in only 12 years.

TRU are elements with atomic numbers greater than 92. TRU is produced when low kinetic energy (thermal) neutrons are absorbed by the abundant uranium isotope U-238. In CANDU ractors the TRU production rate is 4 grams / kg of natural uranium.

Hence the resulting used CANDU fuel could provide about:
6,000,000 tonnes natural uranium X (.004 tonne TRU / tonne natural uranium)
= 24,000 tonne TRU.

Each 0.2 tonne of TRU recovered from used nuclear fuel can be applied to make one tonne of fuel sustainable Fast Neutron Reactor (FNR) core fuel.

Hence the potentially recoverable TRU would be sufficient to produce:
24,000 tonne TRU X (1 tonne FNR core fuel / 0.2 tonne TRU)
= 120,000 tonne FNR core fuel.

Each 300 MWe FNR needs at least 102 tonnes of core fuel (85 tonnes active, 17 tonnes cooling)
which should allow fueling:
(120,000 tonne FNR core fuel) X (300 MWe / 102 tonnes core fuel)
= 1200 X 300 MWe fuel sustainable FNRs.

Thus, while on the basis of present fossil fuel usage we reasonably project a near term need for:
15,000 X 300 MWe thermal power capacity,
the present known mineable natural uranium resource is only sufficient to enable construction of:
1200 X 300 MWe of fuel sustainable FNRs.

Thus the maximum fuel sustainable nuclear power level is:
[1200 / 15,000] X 100% = 8% of the original projected fossil fuel thermal power.

This is a very serious FNR capacity shortfall caused by a best case TRU shortage. For parties that rely on light Water Reactors (LWRs) the situation is 4X worse. Meeting the dependable power demand with fuel sustainable FNRs will require a combination of:
a) A major increase in the price of natural uranium;
b) Finding new mineable natural uranium ore bodies;
c) Deployment of CANDU reactors instead of other reactor types for near term efficient production of both clean energy and TRU;
d) Efficient TRU recovery from used CANDU reactor fuel;
e) Deployment of a sufficient fleet of fuel sustainable FNRs;
f) Major per capita energy conservation;
g) A major world population reduction;
h) A new method of TRU production, involving large scale deployment of Intense Neutron Generators (INGs).

The projected shortage of TRU and hence the corressponding shortages of sustainable and dependable clean power capacity will take decades to correct via fuel breeding. In the meantime, if fossil fuel consumption is not promptly halted, much of our present human society will be extinguished by CO2 driven climate change.

Canada presently has an inventory of 60,000 tonnes of used CANDU fuel containing 0.4% TRU. This inventory can be used to make about:
(60,000 tonnes U) X (.004 tonne TRU / tonne U) X (1 Tonne FNR core fuel / 0.2 tonne TRU)
= 1200 tonnes FNR core fuel
which would allow immediate making of up to:
(1200 tonnes FNR core fuel) / 102 tonnes core fuel per 300 MWe FNR)
= 11.76 X 300 MWe fuel sustainable FNRs.

By 2070 Canada's projected human population will be about 60,000,000 and to continue meeting its present per capita energy expectation Canada would need about:
450 X 300 MWe fuel sustainable FNRs.

By enlarging the Canadian CANDU reactor fleet about 20X, during the coming 50 years Canada could potentially form enough TRU to enable meeting a 50% reduced dependable power load from fuel sustainable FNRs.

However, delays in promptly addressing the shortage of inexpensive natural uranium, deployment of CANDU reactors, production of TRU based sustainable FNR core fuel and deployment of fuel sustainable FNRs will result in serious dependable power shortages within a few decades. The most important near term relief measures are:
1) Prompt deployment of more CANDU reactors, many of which are configured for fissioning used Light Water Reactor (LWR) fuel;
2) Prompt implementation of efficient TRU recovery from used CANDU fuel;
3) Prompt fleet deployment of fuel sustainable FNRs with TRU based core fuel.

However, these fuel sustainable FNRs will not exist if the federal government fails to promptly enable efficient recycling of used nuclear fuel or if the provinces fail to meet the required deployment schedules for both CANDU reactors and fuel sustainable FNRs.

Provincial adoption of Small Modular Reactors (SMRs) that require enriched uranium fuel is a total diversion from the main goal of fuel sustainable climate change relief. Reactors that need enriched uranium fuel are 2X less efficient at energy production and are 4X less efficient at TRU production than are heavy water moderated CANDU reactors.

As of 2023 the Canadian federal government continues to squander tens of billions of dollars on new fossil fuel infrastructure instead of investing in CANDU reactors, efficient nuclear fuel recycling and fuel sustainable FNRs to supply clean, dependable and sustainable nuclear power.

A fossil carbon tax provides little CO2 emission reduction benefit if fossil fuel consumers do not have an available economic alternative clean power source.

Permanently accessible safe interim used nuclear fuel component storage is required to enable economic nuclear fuel recycling.

A necessary immediate federal government policy change is for the Nuclear Waste Management Organization (NWMO) to seek permanently accessible dry storage for used nuclear fuel components, as provided by stable granite rock formations high above the local water table, instead of future inaccessible used nuclear fuel storage in unstable limestone or salt formations, far below the local water table. In this respect the geology of southern British Columbia is more favourable for safe interim storage of used nuclear fuel components than is the geology of Ontario.

Absent prompt deployment of the required CANDU reactors, Intense Neutron Generators (INGs), nuclear fuel recycling and fuel sustainable FNRs the propect for continued human existence on planet Earth is poor. At the heart of the problem is that many life safety systems on which humans rely need dependable and sustainable power rather than intermittent power.

This web page last updated February 23, 2024

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