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

This web page is intended for individuals who know little or nothing about Fast Neutron Reactors (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 atomic fission, in addition to fission products, an atom emits about 200 MeV of thermal energy and between two and four free neutrons, each with a kinetic energy of about 2 MeV.

This property becomes remarkable when there is an assembly of fissile atoms in close proximity to one another. Then neutrons emitted by one atom can be captured by other nearby atoms. When there is a large number of such nearby atoms the number of free neutrons N within the assembly of atoms will exponentially change over time in accordance with the formula:
N = No exp[k(t- to)] where:
No = number of free neutrons in the assembly at time t = to;
k = a material geometry dependent parameter that can be either positive or negative. The parameter k is known as the assembly reactivity.

At any instant in time the fission thermal power emitted by the assembly is proportional to N.

If the the assembly of atoms is shaped like a thin disk 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 k less than zero, which causes N to 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 k can be greater than zero, which causes N to rapidly increase over time. That situation is known as a chain reaction.

If the fissile atoms are distributed like two flat pancakes, then slowly bringing the two pancakes closer together makes their net behaviour change from being disk like to being sphere like. That is, there is a distance between the two pancakes at which k = 0.

We can build such an assembly at room temperature and immerse it in a pond of liquid sodium. Then at time:
t = to
we can move the two solid fissile fuel pancakes slightly closer together which reduces their ratio of outside surface area to volume. As a result k becomes slightly greater than zero and the number of free neutrons 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 which reduces k back to below zero. This reduction in k stops the nuclear fission and hence the heat output. However now the fissile atoms are hotter than they were at time:
t = to

We can gradually decrease the distance between the two solid fissile fuel pancakes until their temperature rises to about 460 degrees C. At that point we have a created a 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 primary liquid sodium, comparable in size to a large swimming pool, which contains fully immersed bundles of sealed vertical, half inch diameter, thin 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. 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 thermal power rating of a FNR is the rate at which heat can 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 fuel rod length and by the maximum allowable difference between the fuel rod centerline temperature and the primary liquid sodium coolant temperature.

This web page briefly describes the appearance of a FNR based nuclear power plant (NPP) that can be safely located in the middle of a city to provide up to 300 MWe of electricity and up to 700 MWt of low grade heat.

Not including the perimeter roads the 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 laneways from 12 surrounding structures. The above grade structure heights vary from 20 m to 50 m.

The primary sodium pool is centrally located, mostly below grade, in the central nuclear structure. Normally there is no personnel access to the 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 primary sodium pool either through a thick window or via a video camera.

When the reactor has cooled down to about 120 degrees C equipment can be transferred in and out of the primary sodium pool space via four truck dock level airlocks.

In the primary liquid sodium pool space there is a circular pool, 20 m in diameter, in the middle of a 25 m X 25 m octagonal shaped room. 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 pool contains liquid sodium, which is a low density metal that melts just below the boiling point of water.

In the middle of the primary sodium pool there is a 12.6 m diameter array of 464 vertical indicator tubes which, during normal reactor operation, project about 0.9 m above the primary sodium surface. These indicator tubes show the actual elevations of the reactor's movable fuel bundles and provide gamma ray intensity and temperature data to an overhead monitoring system that is used to set and monitor the primary sodium pool's operating temperature.

Close examination of the primary 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 one may sense a faint near infrared glow, because normally the surfaces of the pool and the pool enclosure's inside wall operate at about 460 degrees C.

There are 96 horizontal radial pipes which cross over the pool deck and then dip down into the pool. These pipes contain a sodium-potassium alloy (NaK) which conveys heat captured by immersed intermediate heat exchange bundles to NaK-salt heat exchangers located outside the reactor space, in heat exchange galleries located around the upper perimeter of the nuclear building.

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

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

The reactor space is very quiet. In most industrial installations there is a lot of background noise. However, in the FNR crntral nuclear building there are few mechanical moving parts. Within this building liquids move by natural circulation, by smooth electromagnetic 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 perimeter spaces comfortable forced air ventilation with closed circuit cooling is used.

In simple language the FNR passively supplies nuclear heat at the rate required to keep the primary liquid sodium pool surface temperature at 460 degrees C. Near the primary sodium pool perimeter heat is extracted by cooler pumped NaK flowing through intermediate heat exchange bundles. This heat extraction increases the density of the cooled adjacent primary sodium causing it to sink. This cool sodium sinking establishes a natural circulation flow pattern in the primary sodium pool where the primary sodium sinks near the pool perimeter and rises near the pool center. As a result of this cooling the temperature at the pool bottom center can fall to as low as 410 degrees C. The rate of heat generation by the nuclear process is proportional to this temperature drop and to the primary sodium natural circulation flow rate.

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

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

The design maximum power capacity of the 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 by cooling towers. However, in the winter up to 700 MWt of low grade heat are available for comfort heating. Generally this low grade heat should be used as a heat source for high COP terminal heat pumps.

Note that the FNR primary sodium discharge temperature is fixed at 460 degees C and the FNR thermal power output increases with increasing FNR NaK temperature. A three way valve is used to convert the fixed temperature variable flow rate output from an intermediate heat exchange bundle into a variable temperature constant flow rate input for the heat transport system. The heat transport system prevents the primary sodium pool bottom temperature falling below 410 degrees C. This thermal power constraint limits the maximum heat flux through the FNR fuel tubes, which prevents 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 system 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 that must be filtered out of the liquid sodium.

Changes in thermal load cause thermal expansion and contraction of the NaK pipes, nitrate salt 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 long term maintenance considerations related to fluid cleanliness.

Note that there are many 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 mixed NaK temperature setpoints of the three way diverting valves. 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 with some heat transport circuits and some turbogenerators out of service.

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

This web page last updated April 27, 2022

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