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

This web page is intended to introduce Fast Neutron Reactor (FNR) issues to individuals who have a basic science education but who know little or nothing about FNRs.

Fissile atomic isotopes such as U-233, U-235 and Pu-239 have the interesting property that when they capture a free neutron they usually fission. During each nuclear fission, in addition to fission products, a fissioning atom emits about 200 MeV of thermal energy and two to four free neutrons, each with a kinetic energy of about 2 MeV. These are known as fast neutrons. Fast neutrons have desirable properties in terms of efficient utilization of natural uranium, recycling of used nuclar fuel and disposal of nuclear waste.

Nuclear fission becomes remarkable when there is a sufficient concentration of fissile atoms in close proximity to one another. Then neutrons emitted by one fissile atom can be captured by other nearby fissile atoms. Then the number of free neutrons N within the assembly of atoms will exponentially change over time in accordance with the formula:
N = No exp[R (t- to)] where:
No = number of free neutrons in the assembly at time t = to;
R = a material, geometry and temperature T dependent parameter that can be either positive or negative. The parameter R is known as the fuel assembly reactivity.

At any instant in time the fission thermal power (heat per unit time) emitted by the assembly is proportional to N which governs the fission rate.

Note that if R > 0 the emitted thermal power rapidly increases, if R < 0 the emitted thermal power rapidly decreases and if R = 0 the emitted thermal power remains constant.

Thus power reactors are designed such that in normal operation:
R ~ 0.

An FNR has the additional requirement that:
[dR / dT] < 0

Hence, when the fuel assembly temperature T rises above the setpoint temperature To the reactivity R becomes less than zero causing the fuel assembly to soon stop emitting heat. Assuming that there is a thermal load constantly removing heat from the sodium pool the fuel assembly will then start cooling. As the fuel assembly temperature T falls below the setpoint temperature To the reactivity again becomes positive causing the fuel assembly to soon emit much more heat. That heat will raise the fuel assembly temperature back toward To. These effects combine to cause the fuel assembly temperature to quickly converge to the setpoint temperature To.

If the the assembly of fissionable atoms is shaped like a pancake the outside surface area of the assembly is large compared to its volume, so most of the emitted neutrons escape from the assembly without being captured by other atoms, making R less than zero, which causes N to exponentially decrease over time.

If the assembly of atoms is shaped like a sphere, which has a relatively small outside surface area as compared to its volume, then R can be greater than zero, which causes N to exponentially increase over time. That situation is known as a rapid chain reaction.

If the fissile atoms are distributed like two parallel flat pancakes sharing a common axis, then slowly bringing the two pancakes closer together makes their net behaviour change from being like two separate disks to being more like one single sphere. That is, there is a distance between the two pancakes at which R = 0.

We can build such an assembly at room temperature and immerse it in a pool of liquid sodium. Then at time:
t = to
we can slowly move the two solid fissile fuel pancakes slightly closer together which reduces their ratio of outside surface area to volume. As a result R becomes slightly greater than zero and the number of free neutrons exponentially increases causing a brief large increase in N and hence in the thermal power emitted by fission reactions. This thermal power heats the solid fissile fuel causing it to thermally expand, so that the fissile atoms move further apart. In effect the ratio of surface area to volume of the assembly of fissile atoms increases with increasing temperature which reduces R back to below zero. This reduction in R stops the nuclear fission and hence the heat output. However now the fissile atoms are hotter than they were at time:
t = to

This behaviour is achieved by designing the fuel assembly such that at:
R = 0:
[dR / dT] < 0

We can gradually decrease the distance between the two fissile fuel pancakes until the assembly setpoint temperature rises to about 460 degrees C. At that point we have a created an elementary Fast Neutron Reactor (FNR). We can harvest its thermal energy output by extracting heat from the highly thermally conductive liquid sodium pool.

A FNR is simply a pool of liquid sodium, comparable in size to a large swimming pool, which contains fully immersed bundles of sealed vertical, (3 / 8) inch diameter, 0.036 inch wall chrome-steel fuel tubes. Inside the fuel tubes are metallic fuel rods formed from uranium-zirconium and uranium-plutonium-zirconium alloys. A small amount of liquid sodium fills the narrow gap between the outside surface of the metallic fuel rods and the inside wall of the chrome-steel fuel tubes, providing a good thermal connection between the fuel rods and the steel tube. Inside the fuel tubes, above the top of the fuel rods, is a gas space known as the fuel tube plenum.

The geometry of the fuel assembly enables a passive nuclear process which keeps the liquid sodium pool top surface at a nearly constant temperature. During normal FNR operation that temperature is typically set at 460 degrees C. The exposed top surface of the sodium pool is almost entirely covered by floating hollow stainless steel spheres which reduce the exposed liquid sodium surface area and assist in air exclusion during sodium fire suppression.

The thermal power rating of a FNR is the rate at which heat can continuously be safely withdrawn from the liquid sodium pool by immersed heat exchange bundles located adjacent to the pool walls. Since the sodium is heated by thermal conduction through the fuel tube walls the maximum thermal power rating of a FNR is limited by the total active core fuel rod length and by the maximum allowable temperature difference between the fuel rod centerline and the liquid sodium coolant.

This web page titled: FNR Facility briefly describes the appearance of a FNR based nuclear power plant (NPP) that can be safely located in the middle of a population center of about 100,000 people to provide up to 300 MWe of electricity and up to 700 MWt of low grade district heat.

Not including perimeter roads and nearby human support facilities, the FNR NPP above grade structures fully occupy one city block (a space 114 m X 114 m). The NPP footprint consists of a 49 m X 49 m central structure with a central steel dome roof separated by 10 m wide laneways from 12 surrounding structures. The above grade structure heights vary from 20 m to 50 m.

The sodium pool is centrally located, mostly below grade, in the central structure. Normally there is no personnel access to the sodium pool space because it is very hot, there is gamma radiation and there is no oxygen to breath. However, one can view the top of the sodium pool of an operating FNR either through a thick window or via a video camera.

When the FNR is off and has cooled down to about 120 degrees C equipment can be moved in and out of the sodium pool space via four truck dock level airlocks.

The sodium pool is circular, 20 m inside diameter X 15 m deep, located in the middle of an octagonal space which is 25 m face to face. The liquid sodium surface is about 1 m below the pool deck and the ceiling is about 14.5 m above the pool deck. The liquid sodium is a low density metal that melts just below the boiling point of water.

In the middle of the sodium pool there is a 11.3 m diameter array of 464 vertical indicator tubes which, during normal reactor operation, project about 0.9 m above the sodium surface. These indicator tubes show the actual elevations of the reactor's movable fuel bundles and also provide gamma ray intensity and temperature data to an overhead monitoring system that is used to monitor the FNR operation. The indicator tubes also have an emergency safety shutdown role.

Close examination of the sodium pool surface reveals a slight ripple because, when the reactor is producing power, natural circulation causes liquid sodium to rise in the center of the pool and to sink near the pool walls.

If the overhead lighting is extinguished during normal reactor operation one may sense a faint near infrared glow, because normally the exposed surface of the pool and the pool enclosure are at about 460 degrees C.

There are 96 X 12 inch dia radial pipes which cross horizontally over the pool deck and connect to the intermediate heat exchange bundles immersed in the sodium pool. These pipes contain a sodium-potassium alloy (NaK) which conveys heat captured by the intermediate heat exchange bundles to NaK-salt and NaK-HTF heat exchangers located outside the reactor space, in heat exchange galleries located around the upper perimeter of the reactor building.

Other pipes containing hot molten nitrate salt and HTF convey heat from the NaK-salt and Na-HTF heat exchangers to turbogenerator halls in nearby buildings. The turbogenerator related equipment converts about 30% of the transported heat into electricity and rejects the balance of the transported heat to district heating pipes. Note that the nitrate salt pipes, HTF pipes and the NaK pipes are all mounted so that they can thermally expand and contract without causing significant material stress.

Just above the pool deck level are four radial trays that are used, in combination with the air locks and an overhead polar gantry crane, to move fuel bundles and other equipment in and out of the reactor space. The inner air lock doors are located behind removable sections of the insulated side walls.

The reactor building is very quiet. In most industrial installations there is a lot of background noise. However, in the FNR central nuclear building there are few mechanical moving parts. Within this building liquids move by natural circulation, by smooth induction pumping or by differential gas pressure.

If you listen carefully you may hear fan or blower noise from the ventilation system. The sodium pool and the various radial pipes are hot. In spite of good thermal insulation part of that heat leaks through the enclosure walls, so to keep the human occupied service spaces comfortable forced air ventilation with closed circuit cooling is used.

In simple language the FNR passively supplies nuclear heat at a variable rate up to 1000 MWt. Near the sodium pool perimeter heat is extracted from 460 degree C surface sodium by cooler pumped NaK flowing through intermediate heat exchange bundles. This heat extraction increases the density of the adjacent sodium causing it to sink. This cooler sodium sinking establishes a natural circulation flow pattern in the sodium pool. The sodium sinks near the pool perimeter and rises near the pool center. As a result of this natural circulation the temperature at the pool bottom center can fall to as low as 400 degrees C. The rate of heat generation by the nuclear process is proportional to the temperature difference between the fuel tube bottom and the fuel tube top and to the sodium natural circulation flow rate.

In principle the faster that the pumped NaK removes heat from the sodium the more electric power can be generated.

If the NaK flow stops the cooling of the sodium stops. Hence the sodium natural circulation flow stops which causes the nuclear heat production process to stop.

The design maximum power capacity of the FNR NPP is 300 MWe electrical, 700 MWt low grade thermal or 1000 MWt high grade thermal. In practical application during the summer most of the low grade heat rejected by electricity generation is discarded. However, in the winter up to 700 MWt of low grade heat are available for district heating. Generally this low grade heat should be used as a heat source for high COP water source heat pumps.

Note that the FNR sodium coolant discharge temperature is fixed at 460 degees C and the FNR thermal power output increases with increasing coolant flow. Induction pumps set the normal NaK flow rate. The heat transport system design prevents the sodium temperature at the bottom of the fuel tubes falling below 400 degrees C. This thermal power constraint limits the maximum heat flux through the FNR fuel tubes, which prevents metallic nuclear fuel center line melting and protects the fuel tubes from excessive heat flux.

A FNR has the virtue of rugged simplicity. However, a lot of work has gone into designing the FNR to safely tolerate both peripheral equipment failures and extreme events such as earthquakes, tsunamis, hurricanes, airplane impacts and terrorist attacks.

A practical FNR must be maintainable. Eventually the nuclear fuel will be consumed and it will be necessary to reconfigure and/or recycle the nuclear fuel. If air leaks into the reactor space over time the oxygen and water vapor in the air will react with the liquid sodium forming a sludge with a melting point of 318 degrees C that should be filtered out of the liquid sodium.

Changes in thermal load cause thermal expansion and contraction of the NaK pipes, nitrate salt pipes, HTF pipes and heat exchange bundles which cause long term wear. Internal pipe scouring causes further long term wear, eventually leading to equipment repair or replacement. Any suspended particulates in the heat transport fluids will aggravate long term scouring of the insides of pipes, fittings and heat exchange bundles. Hence, in spite of the apparent equipment simplicity, there are important ongoing maintenance considerations related to fluid cleanliness.

Note that there are 48 heat transport circuits, so various combinations of them can be shut down for service without impacting the performance of the remainder.

Most of the system maintenance is non-nuclear in nature and is related to the heat transport system, steam turbines, condensers, cooling towers, electricity generators, cooling water pumps and other non-nuclear electrical and mechanical equipment.

Normally the FNR can operate for months with minimal maintenance attention. The reactor power can be remotely adjusted by changing the induction pump set NaK flow rate. If anything in a heat transport circuit fails the simple solution is to turn that heat transport circuit off and drain its fluids to its dump tanks until such time as competent maintenance personnel can attend to the problem. There is sufficient equipment redundancy to permit the FNR NPP to continue operation at reduced power with some of its heat transport circuits and some of its turbogenerators out of service.

The liquid sodium pool operates at atmospheric pressure with argon cover gas. Unlike water cooled reactors there is nothing to blow up. This reactor will passively default to its setpoint temperature. The main concern is fire prevention by continuous exclusion of both air and water from the sodium pool space.

A FNR has five shutdown mechanisms. One is passive and is used for normal ongoing autonomous temperature control. Two are active and independent and are used to for normal operating temperature adjustment and for scheduled reactor service. Two are intended for emergencies likely involving an unplanned fuel geometry change, malicious sabotage or a military attack.

FNR: Fast Neutron Reactor;
NPP Nuclear Power Plant;
Fission: Splitting of an atomic nucleus;
Fissile Fuels: Materials that readily liberate nuclear energy via neutron capture induced fission;
Fission Thermal Power: Rate of heat generation caused by fission reactions in a nuclear reactor;
Reactivity R: A mathematical parameter which indicates the rate of change number of free neutrons and hence fission thermal power;
Power Reactor: A nuclear reactor intended for producing electricity and/or heat;
Sodium: A highly thermally conductive highly reactive metal that melts at 98 degrees C;
Sodium Pool: A pool of liquid sodium about 20 m in diameter X 15 m deep which captures both the heat and radiation emitted by centrally contained nuclear fuel;
Indicator Tubes: Vertical metal tubes which project above the surface of the liquid sodium pool to transmit reactor status information to overhead electronic monitoring equipment;
Hollow Stainless Steel Spheres: Buoyant stainless steel balls that float on the surface of the liquid sodium to reduce its exposed surface area and assist in fire asphixiation;
NaK: An alloy of the metals sodium and potassium that is liquid at a low pressure from room temperature up to over 700 degrees C;
Intermediate heat exchange bundle: A device which transfers heat from the hot liquid sodium to slightly cooler but fully isolated NaK;
Induction Pump: A non-contact electromagnetic pump type used for safe circulation of NaK;
Molten Nitrate Salt: A liquid used for heat transfer in the temperature range 280 C to 500 C;
HTF: A synthetic Heat Transfer Fluid used for heat transfer in the temperature range 20 C to 335 degrees C;
1000 MWt: One thousand million watts thermal (sufficient power to meet all the power requirements of about 100,000 North American people)

This web page last updated November 4, 2023

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