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XYLENE POWER LTD.

FAST NEUTRON REACTORS

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

IMPORTANT FNR FEATURES:
If a nuclear reactor's neutron spectrum contains primarily fast neutrons it is known as a Fast Neutron Reactor (FNR). In a FNR the trans-uranium actinides, instead of simply capturing neutrons as in a water moderated reactor, preferentially fission. With appropriate fuel reprocessing a FNR yields 100 fold more energy per kg of natural uranium than does a heavy water moderated CANDU reactor. About 95% of the resulting fission products decay to safe levels in 300 years. Hence on a per kWh basis the rate of FNR spent fuel long term waste production is aboout 2000 fold less than for a CANDU reactor.

The best method of CANDU reactor spent fuel disposal is to reprocess the CANDU spent fuel into new FNR fuel and consume it in a FNR.

Another important feature of FNRs is efficient load following. Unlike the almost fixed thermal output of a water moderated nuclear reactor the thermal output of a FNR can be rapidly increased or decreased to balance electricity grid net load changes arising from rapid variations in unconstrained generation and load.

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.
 

FNR OPERATION PRINCIPLE:
When a Pu-239 atom absorbs a fast neutron and fissions on average it emits 3.1 fast neutrons. If the probability of other Pu-239 atoms absorbing these fast neutrons and fissioning is greater than (1 / 3.1) then a rapid nuclear chain reaction will occur liberating a large amount of thermal energy. Otherwise the thermal energy emission by spontaneous fissioning of Pu-239 atoms is extremely small.

A Fast Neutron Reactor (FNR) consists of an active pancake shaped core zone containing uniformly distributed U-238 - Pu-239 - Zr alloy fuel rods sandwitched between two 1.2 m thick passive blanket zones containing uniformly distributed U-238 - Zr alloy fuel. The Pu-239 in the core zone fissions and emits fast neutrons. The U-238 in the blanket zones absorbs fast neutrons. Hence there is an ongoing flux of fast neutrons flowing from the core zone into the blanket zones.

The plutonium concentration in the core zone, the core zone thickness and the blanket zone thicknesses are chosen so that slightly more than (1 / 3.1) of the neutrons emitted by fission of Pu-239 atoms in the core zone are captured by other Pu-239 atoms. Hence a chain reaction occurs because the probability of an emitted neutron being captured by a Pu-239 atom is slightly greater than (1 / 3.1).

However, if due to nuclear heat release the temperature of the materials increases thermal expansion of the structure in three dimensions causes the fraction of fission neutrons diffusing from the core zone into the blanket zones to increase, decreasing the probability of Pu-239 atoms in the core zone capturing fission neutrons. Hence the chain reaction will stop.

Liquid sodium has a high thermal coefficient of expansion (TCE) which enhances this effect. An increase in primary liquid sodium temperature can turn the chain reaction off. A subsequent decrease in primary liquid sodium temperature will increase the primary liquid sodium density enough to restart the nuclear chain reaction. The TCE of the other reactor core materials are smaller but also enhance this effect.

At some primary liquid sodium temperature the nuclear chain reaction will stop. Below that temperature the chain reaction will restart. Hence, a liquid sodium pool type nuclear reactor can be built that automatically maintains the pool of liquid sodium at a chosen fixed temperature. A reactor operating in this manner is known as a liquid sodium cooled fast neutron reactor (FNR).

Note that in a liquid sodium cooled FNR each fuel bundle has a vertically mobile portion known as the control bundle with controlled insertion/withdrawal that provides control of the primary liquid sodium discharge temperature set point. The reactor thermal power is set by the rate of extraction of heat from the primary liqud sodium pool, which is a function of the secondary liquid sodium flow rate.

The pressure in the steam generator is controlled by a motorized steam discharge valve which maintains a constant pressure (9.5 MPa) in the steam generator. That pressure, via the pressure-temperature relationship for saturated steam, determines the water temperature in the steam generator (315 C). The difference between the liquid sodium primary discharge temperature and the water temperature in the steam generator, less two heat exchange wall temperature drops, determines the change in temperature across the secondary sodium loop. Thus controlling the secondary sodium flow rate controls the reactor thermal power and the amount of steam delivered to the turbogenerator.
 

LIQUID SODIUM COOLED FAST NEUTRON BREEDER REACTOR DESCRIPTION:
Liquid sodium coolant enters the bottom of a FNR fuel tube bundles at about 335 C, flows upwards through many small inter-tube gaps between the active fuel tubes, and emerges from the top of the active tube bundles at 445 C. The liquid sodium discharge temperature setpoint of each fuel bundle is controlled by the amount of control bundle insertion. Withdrawing a control bundle into the fuel bundle support pipe reduces the fuel bundle discharge temperature setpoint.

The FNR geometry is chosen so that about (1 / 3) of the neutrons are absorbed by plutonium in the core zones to sustain the nuclear chain reaction and the remaining (2 / 3) of the neutrons are absorbed by depleted U-238 located in both the core and blanket zones for the purpose of breeding more Pu-239 for future use.

As the reactor temperature rises to its setpoint the material density decreases allowing a larger fraction of fission neutrons to escape from the core zone into the adjacent blanket zones, which locally turns off the nuclear chain reaction.

Similarly, as the reactor temperature falls below its setpoint the material density increases confining a larger fraction of neutrons in the core zone, which turns on the nuclear chain reaction.

If the reactor's external heat load is less than the reactor thermal power output the excess heat is absorbed by the liquid sodium causing the liquid sodium temperature to increase, turning off the chain reactions and hence causing the reactor's thermal power output to drop to zero.

If an external heat load removes heat from the liquid sodium the liquid sodium temperature decreases and the nuclear chain reactions restart.

Thus when the control rod bundles are correctly positioned the FNR thermal power output automatically tracks the external thermal load.

As FNR fuel ages its plutonium concentration slowly changes, its concentration of neutron absorbing fission products slowly increases and the liquid sodium flow rate through the reactor slowly decreases. Compensation for these long term changes is achieved by fine adjustment of the positions of the control rods to maintain a discharge temperature setpoint of 450 degrees C.

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

Most of the neutrons that are not consumed by the fission chain reaction are absorbed by U-238 which over time converts to Pu-239. Periodically the reactor fuel rods from both the core and blanket zones are reprocessed to extract fission products and to move newly bred Pu-239 from blanket rods into core rods.

The extracted fission product mass is replaced by an equal mass of depleted U-238. The extracted fission products should be safely stored in isolation for about 300 years to allow their radio toxicity to naturally decay to the level of natural uranium.
 

PRACTICAL FNR DESCRIPTION:
A practical power FNR has an octagonal assembly of vertical steel fuel tubes 6.0 m high X 12.8 m in diameter that is centrally positioned within a rectangular pool of liquid sodium.

The liquid sodium pool is 25.4 m long X 18.4 m wide X 13.0 m deep. The liquid sodium surface is 1 m below grade. The depth from the liquid sodium surface to the top of the steel fuel tubes is nominally 3.0 m. There is nominally 3.0 m of liquid sodium depth below the bottom of the fuel tubes. There is 2.8 m of liquid sodium between the fuel tube bundle sides and the nearest side wall of the liquid sodium pool and the between the fuel tube bundle sides and the nearest intermediate heat exchange tube bundle.

The concept is to surround the reactor fuel tubes by a 2.8 m thick guard band of liquid sodium that absorbs all neutrons that escape from the reactor blanket. Hence the pool walls, pool floor, overhead structures and the heat exchange components are not subject to neutron activation or cumulative neutron damage. The aforementioned liquid sodium pool dimensions also prevent neutron emission from the liquid sodium top surface.
 

The naturally circulating liquid sodium rises through gaps between the steel fuel tubes where it is heated and emerges from the top of the fuel tube bundle. The hot liquid sodium rises to the top surface of the liquid sodium pool, flows across the top of the liquid sodium pool and at the pool ends is cooled by the intermediate heat exchange bundles, causing the circulating primary sodium to sink. The cooler primary liquid sodium flows along the bottom of the pool to a point underneath the reactor fuel tube bundle and then rises again through the reactor fuel tube bundle.

The FNR has one core zone 0.375 m high X 9.6 m in diameter that is surrounded by breeding blanket that is 1.2 m thick. Each fuel tube has a 3.2 m high plenum which also serves to enhance liquid sodium natural circulation.

The reactor fuel tube assembly consists of thousands of vertical 0.5 inch OD steel tubes X .065 inch wall, 6.0 m high that form a square lattice spaced (5/8) inch center to center. These steel fuel tubes are closed at both ends and contain the reactor core and blanket rods as well as liquid sodium to enhance thermal contact between the core and blanket rods and the steel tubes and to chemically absorb fission product and by product gases such as F, Cl, Br and I. For structural purposes the steel fuel tubes are asssembled into square tube bundles. Each such tube bundle, including its shroud, is nominally 15.50 inches face to face X 20 feet high and contains 472 X 0.5 inch OD HT-9 steel tubes. The tube to tube spacing is maintained by wound .0625 inch thick steel spacer wire. The steel tube lattice spacing is fixed by the fuel bundle top and bottom gratings which support and position the fuel tubes and permit vertical liquid sodium coolant flow.

Each 0.5 inch OD steel fuel tube contains either 4 X .600 m long blanket (B) rods and 1 X .35 m long core (C) rod or 5 X .600 m long blanket (B) rods. For the 532 active bundles the fuel rod types from stack bottom to stack top are B, B, C, B, B. For the 272 perimeter passive tube bundles the fuel rod types from stack bottom to stack top are: B, B, B, B, B. During prolonged reactor operation the core fuel rods swell from 0.35 m long to about 0.400 m long. Each fuel tube contains a measured amount of liquid sodium. The top 3.2 m of each steel fuel tube are nominally empty to provide sufficient gas plenum volume and sufficient liquid sodium plenum volume to relieve pressure stress and to accommodate fuel tube material swelling.

Each fuel tube bundle is structurally supported by a 12.750 inch OD steel support pipe. The vertical insertion/withdrawal of each control tube bundle is controlled by liquid sodium pressure applied to a piston in the support pipe. The control bundle's vertical position is indicated by a vertical indicator tube attached to the top of the bundle. This indicator tube projects above the surface of the liquid sodium. At the bottom inside of the indicator tube is a mercury pool that maintains a mercury vapor pressure inside the indicator tube corresponding to the liquid sodium temperature at the bottom of the indicator tube. This temperature is that fuel bundle's liquid sodium discharge temperature.

The core fuel rods are initially 10% zirconium; 20% plutonium, U-235 and fissionable transuranium actinides; and 70% U-238 by weight.

The blanket rods are 10% zirconium and 90% U-238.

The purpose of the zirconium in both the core and blanket rods is to prevent plutonium from forming a low melting point eutectic with the steel fuel tube material.

The support pipe is mates with a pointed vertical rod mounted on the steel frame on the bottom of the liquid sodium pool. This steel frame position stabilizes the fuel bundles and provides the controlled liquid sodium pressure that raises or lowers each control bundle.

Heat is removed from the fuel tube assembly by primary liquid sodium which flows upwards via natural convection through the fuel tube support grids and then through the gaps between the HT-9 steel fuel tubes.
 

FNR THERMAL OUTPUT LIMITS:
There is a core zone height constraint required to maintain the FNR at the threshold of fission criticality. This constraint sets the nominal core zone height at 0.35 m.

The reactor core zone maximum outside diameter is a function of the liquid sodium pool width. That width is constrained by practical structural issues related to reactor roof construction. With a 9.6 m diameter reactor core the sodium pool width is about 18.4 m . It is necessary to have a roof covered 5 m wide perimeter strip around the liquid sodium pool for the insulating lava rock, access and air cooling, and gamma ray absorbing concrete.

The total core zone height and fuel tube assembly diameter establish the active heat transfer area of the fuel tubes.

There is also a heat transfer limit relating to the rate at which liquid sodium will naturally circulate between the fuel tubes.

There are other practical limitations related to modular component transport that also come into play at a FNR core zone diameter of over 9.6 m. At a smaller FNR core zone diameter the economies of scale related to the required liquid sodium pool volume are lost. At core zone diameters above 9.6 m the FNR roof component transportation costs quickly become prohibitive.
 

LIQUID SODIUM COOLED FAST NEUTRON REACTOR OPERATION:
During reactor operation the reactivity of the core zone is at its critical point so there is ongoing fission in the core zone and the core zone acts as a net source of neutrons. During reactor operation the blanket zones act as net neutron sinks.

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 gradually spontaneously converts into Pu-239 and Pu-240.

After the reactor has run for some time the accumulated fission products in the core fuel rods reduce the core zones' reactivity and the fuel tubes may swell reducing the primary liquid sodium flow. The fuel tube bundle is then moved to the perimeter zone of the liquid sodium pool to allow dissipation of short lived fission product decay heat. Over a fuel cycle period of about 30 years the fuel and blanket bundles are gradually replaced and the fuel bundle material is reprocessed and recycled.

After fission product decay heat dissipation the tube bundles are removed from the liquid sodium pool, the fuel tubes are extracted and disassembled. The fuel tube material, the core rods, the blanket rods, the liquid sodium and the sodium salts are each reprocessed differently. The fuel reprocessing involves selective removal of uranium, selective removal of fission products, and reforming the remaining fuel rod residue into new core rods. The selectively removed uranium plus some new U-238 is formed into new blanket rods. New steel tube bundles are assembled and the entire process is repeated. The steel tubes are formed from an iron-chromium alloy known as HT-9 that has a low nickel and low carbon content. The fuel tube material recycling process involves selective titanium and chromium extraction.

Due to neutron activation of the materials the entire fuel recycling process is carried out by robotic equipment.

The reprocessing of the used fuel rods and blanket rods yields more Pu-239 and trans-uranium actinides than the reactor needs. Hence the excess Pu-239 and actinide inventory can be accumulated and eventually used to start another FNR.

A major constraint on the rate of implementation of liquid sodium cooled Fast Neutron breeder Reactors is the available supply of FNR start fuel. This start fuel may be obtained from enriched uranium, plutonium and other actinides in spent water cooled reactor fuel and by plutonium breeding by FNRs and H2 - H-3 fusion power equipment. It may take over 70 years for one FNR to breed enough surplus plutonium to start another identical FNR. Thus in the near term the maximum rate of FNR deployment is primarily a function of the available supply of spent water cooled reactor fuel. It appears that due to the FNR start fuel constraint the world will have to live with a mixed fleet of both water cooled reactors and FNRs for at least the next century.
 

FNR FEATURES:
In liquid sodium cooled Fast Neutron Reactors (FNRs) the thermal power output is proportional to the fast neutron flux incident upon the fuel, as compared to water moderated thermal neutron nuclear reactors in which the thermal power output is proportional to the slow neutron flux incident upon the fuel. Fast neutron reactors are characterized by rapid fuel bundle thermal power output changes near each core zone's set point. For safe power control Fast Neutron Reactors rely on thermal expansion to reduce each core zone's reactivity as its temperature increases. The mobile active fuel bundle positions are adjusted so that all the active fuel bundles operate at the same discharge temperature. These mobile fuel bundles are also used to achieve a reactor cold shutdown.

One of the issues in FNR design is ensuring that no matter what adverse circumstances occur in an emergency the mobile fuel bundles will always fall into a safe cold shutdown position.

The major advantages of liquid metal sodium cooled fast neutron reactors (FNRs) over CANDU reactors are:
1. FNRs yield about 100 fold more energy per kg of mined uranium;
 
2. FNRs reduce nuclear waste storage time about 1000 fold;
 
3. FNRs can easily track rapid changes in grid load:
 
4. The primary liquid sodium coolant in a FNR typically runs at 335 degrees C to 445 degrees C as compared to the 260 degree C to 300 degree C primary coolant temperature in a water moderated reactor. The higher primary coolant temperature allows efficient use of evaporative water cooling and hence requires much less cooling water per kWhe generated than does a direct lake water cooled CANDU reactor;
 
5.The reduced cooling water requirement of an FNR reduces its impact on marine ecology.
 
6. Due to the reduced requirement for cooling water a FNR can be sited much further above the local water table and surrounding bodies of water than a direct water cooled reactor, thus enhancing system safety in rare but severe events such as earthquakes, hurricanes, meteorite strikes, tsunamis, etc.
 
7. The primary liquid sodium coolant in a FNR operates at a low pressure rather than a high pressure, which simplifies many reactor design, construction, operation and maintenance issues;
 
8. In a liquid sodium cooled FNR the structural components such as the sodium pool walls and the heat exchangers are not exposed to the neutron flux, enabling a reactor service life of hundreds of years rather than just 60 years as with a CANDU reactor. The secondary sodium flow velocity within the heat exchangers is chosen to minimize heat exchange tube internal erosion while maintaining a sufficiently turbulent liquid sodium flow to enhance heat transfer.
 
9. The long reactor service life and the liquid sodium guard band minimize the rate of formation of radioactive decommissioning waste.
 
10. The intermediate heat exchange bundles, which are subject to internal pressure stress and internal erosion, are not exposed to the neutron flux. Hence the service life of the heat exchange bundles is enhanced and when they do need replacing a simple exterior surface cleaning will remove all radio activity allowing economic material recycling;
 
11. The steel components of a FNR that are exposed to a high neutron flux are replaced and recycled at the same time as the reactor fuel. Part of the iron is transmuted into chromium.
 
12. A FNR should be designed with multiple independent heat transport systems so that there is absolute certainty relating to removal of decay heat after fission reaction shutdown;
 
13. Unlike water cooled and moderated reactors a liquid sodium cooled FNR will not produce high pressure radioactive steam. Hence a FNR does not need a pressure rated enclosure for radioactive steam containment. If the pressure in the steam generator gets too high the steam can be safely vented to the atmosphere because it is not radio active. The liquid sodium pool must enormously over heat before the sodium vapor pressure becomes structurally significant.
 
14. A major non-obvious advantage of FNRs is a nearly infinite fuel supply. Since a FNR consumes natural uranium at less than 1% of the consumption rate of a CANDU reactor an FNR can economically utilize natural uranium resouces that have very low uranium concentrations. That feature greatly increases the total available natural uranium resource.
 

A disadvantage of a liquid sodium cooled FNR is that the sodium reacts violently on contact with water and above 200 degrees C burns in air. The liquid sodium pool requires both a floating steel cover and an argon gas cover to be safe. The biggest issues in safe siting and operation of liquid sodium cooled FNRs are certain exclusion of water and continuous maintenance of an argon gas cover.

A significant security issue with FNRs is that they operate with a large Pu inventory. During FNR operation first-in first-out fuel bundle replacement should be followed to maintain a sufficient Pu-240 concentration that the plutonium cannot be used to fabricate fission bombs.

The scientific issues related to FNRs have been well understood since the late 1960s. The practical engineering issues were resolved by the mid 1990s. However, North American electricity utilities have failed to adopt liquid sodium cooled FNRs for various reasons, most of which are no longer valid.
 

FUEL CYCLE:
The object is to implement a nuclear reactor fuel cycle that will simultaneously achieve several important objectives:
1. Optimally utilize the inventory of spent water cooled reactor fuel to start future FNRS;
 
2. Achieve a major increase in energy yield by breeding U-238 in the spent CANDU fuel bundles into Pu-239 and fissionable trans-uranium actinides and then fissioning these actinides;
 
3. Effectively utilize existing CANDU reactors for interim on-going electricity generation;
 
4. Achieve a fuel cycle supervision, chemistry, transportation and storage arrangement that will prevent to nuclear weapon proliferation;
 
5. Achieve a fast neutron reactor fuel composition that meets the safety requirements.
 

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

The low atomic weight atoms are placed in isolated storage where over 300 years their radio toxicity naturally decays to the level of natural uranium. Then, subject to selective Se-79 and Sn-126 chemical extraction, this low atomic weight waste can either be recycled or buried in existing depleted uranium mines.

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

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 20% 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 more than 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 may be sufficient to power Canadian Fast Neutron Reactors for centuries to come.

During each fuel cycle more Pu-239 is bred which sustains the reactor reactivity during the following fuel cycle and which provides yet more Pu-239 for starting other FNRs. A storage facility comparable to the Jersey-Emerald mine is needed to securely store the fission products for about three centuries and to store the small fraction of long lived nuclear waste for about one million years.

Obstacles to immediate implementation of FNRs are political willingness to accept 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 and mechanically reprocessed and reused about every thirty years. Hence on-going access to a secure naturally dry spent fuel storage facility, such as the Jersey-Emerald mine complex in British Columbia, is an important part of FNR implementation. To minimize transportation of highly radioactive material the blanket rod chemical/mechanical processing and refabrication facilities should be located at or near the reactor site and the core rod processing facility should be located far from any urban center. Ideally the fission product storage facility should be located close to the core rod processing facility. One of the lessons to be learned from experience in France is that if the facilities are not properly located highly radioactive materials wind up being transported to and fro all over the country.

Suitable potential sites for the core rod processing facility are Chalk River, Ontario and Trail, British Columbia where the local populations have for generations been accustomed to large scale management of highly toxic materials.

Once all of the reprocessing issues related to the spent fuel bundles have been resolved there is no obvious reason why analogous techniques could not also be applied to recycling medium level nuclear waste that OPG currently contemplates burying in yet another deep geological repository.
 

NUCLEAR WEAPON NON-PROLIFERATION:
A blunt reality that humans must face is that fossil hydrocarbons must be left in the ground. Replacement of fossil hydrocarbons requires fast neutron power reactors. Fast neutron power reactors require about 20% Pu in their core fuel rods to sustainably operate. Hence any treaty, legislation or regulation that only permits lower fractions of Pu in nuclear fuel is not compatible.

The important issue in prevention of nuclear non-proliferation is maintianing a sufficient Pu-240 to Pu-239 ratio in the fuel to prevent the plutonium being chemically extracted and used to make fission bombs. This ratio is maintained by doing all necessary to ensure that FNR fuel bundles are reprocessed in a first in-first out sequence.
 

USA PROGRESS:
In the USA a 20 MWe fully functional prototype liquid sodium cooled FNR known as the EBR-2 was built and successfully operated from about 1964 to 1994. Under the Bill Clinton administration the USA took a huge step backwards when it cancelled funding of its fast neutron reactor program.
 

RUSSIAN PROGRESS:
In Russia a 600 MWe fully functional prototype liquid sodium cooled FNR known as the BN600 was built and successfully operated from about 1984 to 2016. See 600 MWe LIQUID SODIUM COOLED POWER REACTOR
 

This web page last updated February 21, 2017

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