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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).

A FNR is simply a pool of primary liquid sodium, comparable in size to a swimming pool, which near its center contains fully immersed bundles of vertical, half inch diameter, thin wall chrome-steel fuel tubes. Sealed inside the fuel tubes are metallic fuel rods formed from a uranium-plutonium-zirconium alloy. 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 fuel tubes. Inside the fuel tubes, above the top of the fuel rods, is a sealed 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 surface at a constant temperature. During normal reactor operation that temperature is 460 degrees C.

The thermal power of a FNR is the rate at which heat is drawn from the liquid sodium pool by immersed heat exchange bundles located near 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 fuel rod centerline 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 50 m X 50 m central structure separated by laneways from four corner structures. The structure heights above grade vary from 20 m ____ to 50 m.

The FNR is centrally located in the central structure. You cannot go into the reactor space because it is very hot, there is gamma radiation and there is no oxygen to breath. However, you can view the top of the reactor either through a thick window or via a video camera.

If you look through a window you will see a circular pool, 20 m in diameter, in the middle of a 24 m X 24 m open space. The pool surface is about 1 m below the surrounding pool deck and the ceiling is about 14 m ___above the pool deck. The pool is filled with liquid sodium. Sodium is a low density metal which melts at a temperature just below the boiling point of water.

In the middle of the primary sodium pool there is an array of vertical tubes that project about 1 m above the sodium surface. These tubes indicate the actual fuel geometry and are used to properly set the primary sodium pool operating temperature.

If you look carefully at the pool surface you may notice 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 you turn out the lights you may be able to sense a faint near infrared glow, because the surfaces of the pool and the pool enclosure's inside walls operate at 460 degrees C, which is cooler than red hot.

There are many (112) horizontal radial pipes which cross over the pool deck and then dip down into the pool. These pipes contain secondary sodium which conveys heat captured in immersed heat exchange bundles to sodium-salt heat exchangers located outside the reactor space, around the upper perimeter of the reactor building.

Other pipes containing hot molten nitrate salt convey heat from the sodium-salt heat exchangers to electricity generation equipment in the other buildings. That equipment converts about 30% of the transported heat into electricity. Note that the heat transport pipes are mounted so that they can thermally expand or contract without causing significant material stress.

Near the inside corners of the primary sodium pool enclosure are trap doors in the pool deck leading to below pool deck air locks.

You may be impressed with how quiet the facility is. In most industrial installations there is a lot of noise. However, in the central FNR building there are few mechanical moving parts. Within the central building liquids move either by natural circulation, by smooth electromagnetic pumping or by external differential gas pressure.

If you listen carefully you will likely hear fan or blower noise from the ventilation system. The sodium pool and the 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 is used.

You might ask how the reactor works. In simple language a nuclear reaction maintains the primary liquid sodium pool surface temperature at 460 degrees C. Heat extraction from the hot primary liquid sodium increases the primary sodium density causing it to sink and thus reduces the pool bottom temperature to as low as 410 degrees C. The rate of heat extraction from the primary liquid sodium pool is proportional to this temperature difference and to the primary liquid sodium discharge flow rate from the assembly of fuel tubes.

The faster that the secondary heat transport fluid circulates the faster heat is extracted from the primary sodium pool and hence the more electric power that can be generated.

The nuclear behavior of the fuel assembly is such that it maintains the pool surface at a fixed temperature of about 460 degrees C. If the rate of heat extraction from the primary sodium pool increases the nuclear reaction rate increases sufficiently to keep the pool surface temperature at 460 degrees C. If the rate of heat extraction decreases the rate of nuclear heat supply also decreases to again keep the pool surface temperature at 460 degrees C. If heat extraction stops then the heat emitting nuclear reaction also stops.

The 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. However, in the winter up to 700 MWt of low grade heat are available for comfort heating. Generally this low grade heat is 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 falling FNR primary sodium inlet temperature. However, the thermal load must be limited to prevent the primary sodium temperature falling below 410 degrees C. This thermal power constraint limits the maximum heat flux through the FNR fuel tubes, which protects the fuel from 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 goes into designing the system to safely tolerate both 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 slag that must be filtered out of the liquid sodium.

Changes in thermal load cause thermal expansion and contraction of the feed 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.

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 flow rate setpoints of the electromagnetic secondary sodium pumps. If something in a heat transport circuit fails the simple solution is to turn that heat transport circuit off and drain it to its dump tanks until such time as a competent maintenance person can attend to the problem. There is enough equipment redundancy that the FNR NPP can continue safe operation with some heat transport circuits and some turbogenerators out of service.

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

This web page last updated October 14, 2021

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