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

This web page sets out the sustainable fuel cycle that raises the efficiency of energy harvesting from natural uranium from about 1% for a water moderated thermal reactor up to nearly 100% for a suitable combination of thermal and fast neutron reactors with supporting fuel reprocessing.

The reactor types are CANDU and FNR. The FNR core zone is surrounded by six concentric blanket rod layers, where the TRU concentration in the blanket rods decreases with increasing distance from the core zone.

The sustainable fuel cycle sequence involves breeding TRU in the blanket rods and periodically transferring that TRU from the blanket rods to core rods while simultaneously rejecting fission products from the core rods.

The operating sequence is:
1) Optional initial U-235 enrichment;
2)Initial fissioning of U-235 with thermal neutrons in a CANDU reactor or LWR to create an initial inventory of TRU in the thermal reactor fuel;
3) Concentration of the TRU;
4) Send surplus UO2 to interim storage;
5) Extraction of fission products;
6) Initial FNR fuel rod and fuel bundle fabrication;
7) Divide the FNR's active fuel bundles into six nearly identical lots, 5 pie shaped and one in the perimeter storage ring;
8) Divide the FNR's passive fuel bundles into five nearly identical pie shaped lots;
and passive fuel bundles into five nearly identical pie shaped lots;
9)After 6 years of FNR operation transfer active fuel bundles from FNR cool perimeter fuel bundle storage ring to fuel reprocessing site;
10) Transfer active fuel bundles Lot #1 to the perimeter storage ring;
11) Move new active fuel bundles from reprocessing to active fuel bundles Lot #1 previous position;
12) Move passive fuel bundles Lot #1 from the FNR to remote reprocessing;
13) Move new passive fuel bundles to the previous passive fuel bundle Lot #1 position;
14) Run the FNR for another six years;
15) While the FNR is running sort the fuel rods at the remote reprocessing site;
16) Mechanically recycle the weakly irradiated blanket rods;
17) Reprocess the core fuel rods and the strongly irradiated blanket rods;
18) Reject fission products to interim storage;
19) Form new core rods;
20) Add U drawn from interim storage;
21) Form new blanket rods;
22) Form new active fuel bundles;
23) Form new passive fuel bundles;
24) Return to Step #9 and increment the Lot #;
25) Execute Steps #9 to #24 five times;
26) Reset the Lot # to 1;
27) Continue until there is no more depleted U available to feed the process.

Note that blanket fuel rods from the outer portion of the blanket do not need to be processed. They are simply mechanically sorted and reused. Hence the TRU concentration in the inner portion of the blanket is usually higher than in the outer portion of the blanket.

Note that the total required number of active fuel bundles is the nominal number plus (1 / 5) to account for active fuel bundles in the perimeter storage ring + (1 / 5) to allow for active fuel bundles in reprocessing = 1.4 X nominal number of active fuel bundles.
Note that the total require number of passive fuel bundles is the nominal number plus (1 / 5) to allow for passive fuel bundles in reprocessing = 1.2 X nominal number of passive fuel bundles.

The working life of each fuel bundle is estimated to be 30 years based on 15% burnup of the core fuel at 0.5% per year. During the fuel bundle working life the core fuel TRU fraction drops from about 20% to about 12%.

Natural uranium can be fed into the nuclear fuel flow either at enrichment or the CANDU reactor. Consider 1,000,000 natural uranium atoms at the input to the enrichment process. Natural uranium consists of 0.7% U-235 and the balance is U-238. Hence at the input to the enrichment process there are:
7000 U-235 atoms
= 993,000 U-238 atoms

The enrichment process divides these atoms into two piles, a small pile of enriched uranium and a large pile of depleted uranium. The enriched uranium is used as fuel for Light Water Reactors. The depleted uranium can ultimately be used as feedstock to make blanket fuel for Fast Neutron Reactors (FNRs).

For example, asssume that the pile of depleted uranium contains 4000 U-235 atoms so the pile of enriched uranium contains:
7000 - 4000 = 3000 U-235 atoms.
If the pile of depleted uranium contains 0.43% U-235, then the total number of atoms in the depleted uranium pile is:
(100% / 0.43%) X 4000 atoms = 930,232 atoms.

Hence the number of U-238 atoms in the depleted uranium pile is:
930,232 - 4000 = 926,232 atoms

Hence the total number of atoms in the enriched uranium pile is:
1,000,000 - 930,232 = 69,768

Hence in the enriched uranium pile the fraction of atoms that are U-235 is:
3000 / 69,768 = 4.3%

However, to achieve this level of enrichment about 93% of the natural uranium input was depleted making it unsuitable for immediate use as a fuel for use in water moderated reactors.

The enriched uranium can be fissioned first in a Light Water Reactor (LWR) and then in a CANDU reactor to bring the U-235 concentration down to about 0.4%. Hence the number of U-235 atoms fissioned is about:
[(4.3% - 0.4%) / 4.3%] X 3000 = 2721

However, if the same amount of natural uranium is directly fed into a CANDU reactor the number of U-235 atoms fissioned is:
7000 atoms X (0.7% - 0.4%) / 0.7% = 3000

Hence a CANDU reactor is at least as efficient with natural uranium as is a LWR fed with enriched fuel. The CANDU reactor avoids the fuel enrichment step. The LWR has the apparent benefit that produces about twice as much TRU per unit of fuel processed. However, that benefit is an illusion because to produce approximately the same amount of thermal energy the LWR uses:
1,000,000 / (69,768) = 14.33
times as much natural uranium. Thus a CANDU reactor is about:
14.33 / 2 = 7.15 X as effective as a LWR at TRU production.

After some years in cooling used CANDU reactor fuel is fed to a TRU Concentrator which extracts about 90% of the fuel weight in the form of uranium oxide. The uranium oxide is stored for future use as FNR blanket fuel feedstock. The fuel residue containing TRU concentrated about 10 fold is fed to the reduction process.

In the reduction process TRU concentrates are changed from oxides to metallic form.
Interim stored uranium oxide is changed into metallic uranium.

Pyro processing separates the metallic form TRU concentrates into:
zirconium, uranium, uranium plus TRU and fission product components. The pyroprocess gets material inputs from TRU Concentration and from recycling of used FNR fuel. The pyroprocess rejects fission products to interim storage.

Fuel rod fabrication gets material inputs from interim uranium oxide storage and from the reduction process and from the pyro process and is used to fabricate FNR blanket fuel rods and FNR core fuel rods.

The fuel bundle fabrication process gets fuel inputs from both direct blanket fuel rod recycling and from Fuel Rod Fabrication.

Initialy FNRs obtain their fuel via TRU Concentration of used CANDU fuel. However, once TRU concentration of the available used CANDU and LWR fuel is complete the FNRs obtain their new fuel by gradually drawing down depleted uranium oxide from interim storage and reducing it to form new blanket fuel rod material. Over time this blanket material is progressively richer in TRU. Then the inner layer of the blanket is converted into core fuel that replaces fission product mass that flows to multi-century interim storage.

This web page last updated April 29, 2023

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