XYLENE POWER LTD.

FNR FUEL CYCLE

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

PREREQUISITE STUDY:
Students are encouraged to first study the web pages titled:
FNR POLITICS
FNR CONCEPT
FNR DESCRIPTION
FNR FEATURES
FNR OPERATION
so that they have a good understanding of what a Fast Neutron Reactor (FNR) is, how it works and its purpose.
 

FUEL CYCLE:
The object is to implement a nuclear reactor fuel cycle that will simultaneously achieve all of the following important objectives:
1. Optimally utilize the inventory of used water moderated reactor fuel to make FNR fuel;
 
2. Achieve a major increase in energy yield by breeding U-238 in the used water moderated reactor fuel into Pu and other fissionable trans-uranium actinides and then fissioning these actinides;
 
3. Effectively utilize existing water moderated reactors for interim electricity generation;
 
4. Do all necessary to prevent the used fuel reprocessing being used as a vehicle for nuclear weapon proliferation;
 
5. Achieve a FNR fuel composition that meets the required performance and safety requirements. In this respect metallic Pu-U alloy core fuel is desirable for safety reasons because:
a) metallic Pu has a much larger thermal coefficient of expansion than does metallic U which ensures a certain fuel assembly negative coefficient of reactivity with temperature;
b) Pu has a much lower melting point than U which means if the fuel gets too hot the core fuel Pu-U alloy will melt. The resulting liquid fissile alloy will flow downwards past the blanket fuel rods reducing the Pu concentration in the fuel which will stop the nuclear reaction;
c) In the event of sudden prompt neutron criticality the liquid sodium adjacent to the core fuel will boil causing a local vapor pressure which will tend to blow the fuel rods in the fixed fuel bundles toward the fuel tube plenums, which will stop the nuclear reaction;
 
6. Produce sufficient Pu-239 during each fuel cycle to sustain the reactor reactivity during the following fuel cycle and to provide yet more Pu for starting additional FNRs;
 
7. Use an interim underground dry storage facility, comparable to the Jersey-Emerald mine, to store the extracted fission products in engineered containers. The extracted fission products should be safely and securely stored in isolation for about 300 years to allow the fission product radio toxicity to naturally decay to below the radio toxicity level of natural uranium. Use another part of the same mine to safely store the tiny fraction of long lived nuclear waste for about one million years.
 

BASIC CANDU FUEL RECYCLING CONCEPT - THE OTTENSMEYER PLAN:
The concept of recycling spent CANDU fuel through a fast neutron reactor was originally developed circa 2010 by Professor Peter Ottensmeyer and a group of students at the University of Toronto. The process relies on a combintion of physical, chemical and electrolytic processes that first selectively extracts UO2 from used CANDU fuel, then makes the residue metallic and then separates low atomic weight atoms (fission products) from high atomic weight atoms (remaining uranium, plutonium and trans-uranium actinides).

The low atomic weight fission product atoms are placed in isolated dry storage where over 300 years their radio toxicity naturally decays to below the radio toxicity level of natural uranium. Then, subject to selective radio isotope chemical extraction, this low atomic weight waste can either be recycled as rare earth elements or buried in existing depleted uranium mines.

The mass of the fission product atoms removed from the core fuel is replaced by an equal mass of transuranium actinides extracted from the spent CANDU fuel inventory. New core rods and blanket rods are then fabricated and are used to fuel a fast neutron reactor.

This fuel cycle is repeated over and over again until the entire inventory of spent CANDU fuel is exhausted. During each fuel cycle about 15% of the core fuel rod weight is converted from high atomic weight atoms to low atomic weight atoms.

It is estimated that the Ottensmeyer Plan realizes about 100 fold more energy per kg of mined natural uranium than does the present CANDU fuel cycle that operates with natural uranium in a slow (thermal) neutron environment with no fuel recycling. Hence the existing inventory of spent CANDU fuel should be sufficient to power Canadian Fast Neutron Reactors for centuries to come.

During each fuel cycle sufficient Pu is produced to sustain the reactor reactivity during the following fuel cycle and to provide yet more Pu for starting other FNRs. A storage facility comparable to the Jersey-Emerald mine is needed to securely and safely store the fission products for about three centuries and to store the tiny fraction of long lived nuclear waste for about one million years.

The second part of this fuel recycling process is outlined in the paper titled:
"Super-Size Me" The Role of Sodium-Cooled Fast Reactors in a Large-Scale Nuclear Economy.
 

IMPORTANT FNR FUEL CYCLE PERFORMANCE TARGETS:
In FNR core fuel the trans-uranium actinides preferentially fission, instead of simply capturing neutrons as in a water moderated reactor. With appropriate periodic fuel reprocessing a FNR yields about 100 fold more energy per kg of natural uranium consumed than does a heavy water moderated CANDU reactor where burial is used for disposal of used CANDU fuel.

During FNR fuel reprocessing the fission products are extracted from the fuel. About 95% of the extracted fission products decay to safe levels within 300 years.

After several months of fission product decay the dominant radioactive fission product species are:
Strontium-90
Iodine-131
Cesium-137
Of these the Cesium-137 is a particular problem because it has a high vapor pressure at electrolytic fuel processing temperatures.

On a per kWh basis the rate of FNR long lived spent fuel waste production is about:
100 X 20 = 2000 fold
less than for a CANDU reactor.

The best method of spent CANDU fuel disposal is to reprocess the spent CANDU fuel into FNR fuel and then use a FNR to convert that fuel into fission products.

IMPORTANT FNR FEATURES:
In summary, liquid sodium cooled power FNRs can provide sufficient energy to sustainably displace fossil fuels with almost no production of long lived nuclear waste. FNRs can also be used to safely dispose of spent fuel from CANDU and other water moderated nuclear reactors.
 

FUEL CYCLE OVERVIEW:
The liquid sodium cooled fast neutron reactors (FNRs) contemplated herein are primarily fuelled by the abundant uranium isotope U-238. The main FNR nuclear process converts the uranium isotope U-238 into the plutonium and then fissions the plutonium. Most of the resulting fission products have short half lives. There are excess fission neutrons that:
a) if absorbed by the abundant U-238 atoms in the blanket transmute into fissionable Pu atoms;
b) if absorbed by a Pu-239 atom in the blanket may yield a Pu-240 atom which plays an essential role in preventing proliferation of nuclear weapons.

At about six year intervals 20% of the fuel in a FNR is removed, trucked to a central shared reprocessing facility and reprocessed to:
a) extract fission products;
b) to transfer Pu and other transuranium actinides from the used blanket fuel rods to new reactor core fuel rods;
c) replace the extracted fission product mass by an equal mass of new U-238 added to new blanket rods;

The time required for one complete reactor fuel replacement is known as a fuel cycle and is typically about 30 years. It takes about two complete fuel cycles to double the Pu inventory.

During each FNR fuel cycle the Pu fraction in the core fuel rods decreases from 20% to 12.5% by weight corresponding to 15% core fuel burnup and 7.5% conversion of U-238 into Pu in the core fuel. With U-238 blanket fuel the fuel recycling process yields excess core fuel rod material which enables ongoing expansion of the FNR fleet.
 

FNR NUCLEAR FUEL CYCLE DETAIL:
The FNR geometry is chosen so that about (1 / 3) of the fission neutrons are absorbed by plutonium in the core zone to sustain the nuclear chain reaction and the remaining (2 / 3) of the fission neutrons are absorbed by U-238 located in both the core and blanket zones for the purpose of breeding more Pu-239 for future use. A small fraction of the fission neutrons are absorbed by steel in the fuel tube assembly and by the cooling liquid sodium.

During FNR operation there is ongoing fission in the core zone so the core zone acts as a source of neutrons. During reactor operation the blanket zone acts as a net neutron sink.

As the reactor runs there is a gradual accumulation of fission products, primarily in the core fuel rods. Neutrons emitted by fission of Pu-239 are absorbed by U-238 in both the core and the blanket rods. The resulting U-239 spontaneously converts via Np-239 into Pu-239 within about one week. Further neutron absorption by Pu-239 that does not fission causes formation of Pu-240. This Pu-240 spontaneouly fissions which prevents FNR fuel from being used for atom bomb production.

As FNR fuel ages its core zone plutonium concentration gradually decreases and its core zone concentration of fission products gradually increases. Compensation for this fissionable isotope concentration change is achieved by adjustment of the insertion depth of the mobile fuel bundles into the matrix of fixed fuel bundles.

After an active fuel bundle has operated for some time (~ 30 years) there is an accumulation of fission products in the core fuel rods and there is a decrease in Pu-239 concentration in the core zone fuel rods. There is also an accumulation of Pu-239 and some Pu-240 in the blanket fuel rods.

In addition, after many years of operation the fuel tubes will have swollen reducing the primary liquid sodium coolant flow.

Full withdrawal of the mobile fuel bundles from the matrix of fixed fuel bundles causes a total shutdown of fission chain reactions regardless of the liquid sodium temperature.

The used fuel tube bundles are then moved to the perimeter of the liquid sodium pool where they rest for 6 years to allow decay of fission products with short half lives. Over a fuel cycle period of 30 years about 20% of the active and passive fuel bundles are replaced every six years.

After decay cooling the fuel tube bundles are removed from the liquid sodium pool and are transported in a shielded container to a reprocessing facility where the fuel bundles are disassembled. The inert gases, fuel tube material, core rods, blanket rods, liquid sodium and sodium salts within each fuel tube are each reprocessed differently.

The fuel rods from both the core and the blanket zones are reprocessed to extract fission products and to selectively extract uranium. The extracted fission product mass is replaced by an equal mass of new core rod alloy obtained by reprocessing blanket rod material. Then the consumed blanket rod material is replaced by an equal mass of depleted uranium-zirconium alloy.

The extracted fission products should be safely stored in isolation for about 300 years to allow their radio toxicity to naturally decay to below the level of natural uranium.

The fuel tube reprocessing sequence involves venting the inert gases, selective removal of sodium and sodium salts, selective extraction of uranium, selective separation of low atomic weight fission products from the high atomic weight elements, selective extraction of zirconium from the low atomic weight elements and reforming the remaining fuel rod residue into new core fuel rods. The selectively removed uranium plus some new U-238 plus some recycled Zr is formed into new blanket fuel rods. New steel tube bundles are assembled and the entire FNR fuel cycle process is repeated. The steel fuel tubes are formed from an iron-12% chromium alloy known as HT-9 that has a low nickel and low carbon content.

Due to neutron activation of the fuel bundle materials the fuel reprocessing is nearly all carried out by robotic equipment.

The reprocessing of the used fuel rods and blanket rods yields more Pu and other trans-uranium actinides than the FNR needs for replacing its core fuel rods. The excess Pu and actinide inventory is accumulated and is used to provide fuel for starting other FNRs.

This fuel cycle is repeated over and over again until the entire inventory of spent CANDU fuel is exhausted. It is estimated that during each fuel cycle about 15% of the core fuel rod weight will be converted from high atomic weight atoms to low atomic weight atoms. It is estimated that the Ottensmeyer Plan realizes about 100 fold more energy per kg of mined natural uranium than does the present CANDU fuel cycle that operates with natural uranium in a slow neutron environment with no fuel recycling. Hence the existing inventory of spent CANDU fuel should be sufficient to power Canadian Fast Neutron Reactors for centuries to come.

A major constraint on the rate of implementation of liquid sodium cooled FNRs is the available supply of FNR start fuel containing 20% (plutonium plus actinides). This start fuel may be obtained by reprocessing spent water cooled reactor fuel or by plutonium breeding by FNRs. It may take as much as 67 years to double the amount of available FNR start fuel via breeding. Thus in the near term the maximum rate of FNR deployment will be limited by the available supply of spent water cooled reactor fuel. It appears that due to this FNR start fuel constraint the world will have to live with a mixed fleet of water cooled reactors and FNRs for at least the next century.
 

TECHNICAL CONSIDERATION:
A major practical issue with liquid sodium cooled FNRs is the necessity for full automation of fuel rod, fuel tube and fuel bundle production and reprocessing. The required processes have been proven in a laboratory which can process several kg of fuel per day. This processes needs to be scaled up to tonnes per day for supporting a fleet of power FNRs.

A 840 MWt FNR has about 247,000 fuel tubes containing about 3,500,000 fuel rods. In order to make FNRs economic all aspects of FNR fuel rod, fuel tube and fuel bundle production and reprocessing must be fully automated. This automation requires a major capital commitment.
 

POLITICAL OBSTACLES:
Political obstacles to FNR implementation are outlined on the web page titled:
FNR POLITICS
Obstacles to immediate implementation of FNRs are political resistance to transportation, storage and processing of material derived from spent CANDU nuclear fuel bundles. The spent fuel, instead of gradually diminishing in radioactivity as the years pass, would be chemically/electrolytically reprocessed and reused about every 36 years. It is contemplated that the initial FNR fuel reprocessing site would be at Chalk River, Ontario, which is far from any urban center. Another suitable site might be Trail, BC. Ideally the fission product storage facility should be located close to the fuel rod reprocessing facility. One of the lessons to be learned from nuclear fuel reprocessing experience in France is that if the fuel reprocessing and storage facilities are not properly located highly radioactive materials wind up being transported to and fro across the country.

The scientific issues related to FNRs are well understood. However, due to governmental corruption by the fossil fuel industry, in North America there is little power FNR operating experience and North American electricity utilities have no pressing financial motivation to adopt FNRs.
 

BACKGROUND INFORMATION:
The FNR geometry is chosen so that about (1 / 3) of the fission neutrons are absorbed by plutonium in the core zone to sustain the nuclear chain reaction and the remaining (2 / 3) of the fission neutrons are absorbed by U-238 located in both the core and blanket zones for the purpose of breeding more Pu-239 for future use. A small fraction of the fission neutrons are absorbed by steel in the fuel tube assembly and by liquid sodium.
 

Pu DOUBLING TIME:
1) A CANDU reactor takes about 1.7 years to convert 1% of its fuel weight into fission products.

2) One FNR fuel cycle time is the time period between successive reprocessing of FNR fuel bundles. During one ~ 30 year fuel cycle time a FNR converts about 15% of its core fuel weight or 75% of its plutonium into fission products. With each fission it generates about 0.6 atoms of plutonium in addition to the two atoms of plutonium required to usstain its own operation.

3) The Pu-239 doubling time is the time required to double the total available Pu-239 mass. Since each Pu-239 fission in a FNR produces about 0.6 new Pu-239 atoms in addition to those necessary to sustain the FNR operation in one fuel cycle the amount of net new plutonium produced is:
0.75 (number of original plutonium atoms in core zone)(0.6) = 0.45 (number of original plutonium atoms in core zone)

Hence the Pu-239 doubling time is given by:
(fuel cycle time) / 0.45 = 2.22 fuel cycle times

Since one fuel cycle time is about 30 years the time required to double that amount of available Pu-239 is about:
2.22 X (30 years) = 67 years

A relevant reference is: Effect of TRU fuel loading on core performance and plutonium production
 

GOVERNMENTAL CONCERNS:
1) Presently there is no recognition by either the Canadian or US governments that the Pu-239 shortage threatens the very existence of the human species. Without sufficient Pu-239 there is no sustainable substitute for fossil fuels.
2) Making Pu-239 from U-235 requires consumption of one atom of U-235 for every 1.5 atoms of Pu-239 produced. It takes much more natural uranium to start a FNR than it does to fuel a CANDU reactor of similar thermal output capacity.
3) If we contemplate quadrupling the the world nuclear reactor capacity over the next 40 years using breeder reactors to achieve sustainability we are committing the entire known mineable natural uranium resource. If we fail to do so as fossil fuels are exhausted there will be no economic fuel source left but renewable energy.
4) The only strategy that can mitigate these problems is conservation of Pu-239 and U-235. The present practise of consuming Pu-239 in water moderated reactors or burying spent water moderated reactor fuel containing these isotopes in the ground is worse than stupid.
5) All the new reactor designs that do not net breed new fuel should simply be discarded as a waste of critical resources. The regulatory authorities should do all necessary to to accelerate approval and funding of new breeder reactor designs.
6) From an electricity market perspective all new breeder reactor capacity should have the highest priority for electricity grid access. The existing market mechanisms will just have to be changed to make that happen. The high school core curriculum should have a section that discusses the crucial role of Pu in future energy production and that sufficient Pu will not exist unless breeder reactors are both funded and operated at maximum capacity today irrespective of present natural gas prices.
7) The above observations are dictated by the laws of physics. There are all kinds of claims based on market models that have no foundation in physical reality. The human species as we now know it will live or die in accordance with the natural physical laws.
8) There is a possibility of harvesting natural uranium from the oceans. However, the concentration of uranium in sea water is only about 3 parts per billion so the cost of uranium recovery from the ocean is very high. Possibly there might be some marine life species that naturally concentrates uranium in the ocean which might make harvesting of uranium from the ocean more practical.
 

This web page last updated January 22, 2022