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

This web page briefly explains what a Fast Neutron Reactor (FNR) is and how it works.

A FNR is simply a pond of liquid sodium, comparable in size to a swimming pool. The surface temperature of this liquid metal pond is automatically maintained at about 500 degrees C by an immersed nuclear fuel bundle that has a particular geometry.

As the name suggests a Fast Neutron Reactor (FNR) operates using the fast neutrons directly emitted by nuclear fission. These neutrons have average kinetic energies of about 2 MeV. A FNR is fundamentally different from a water cooled reactor which operates using slow neutrons. The use of fast neutrons gives a FNR major advantages in terms of public safety, natural uranium utilization efficiency and elimination of long lived nuclear waste.

A modular liquid sodium cooled FNR, such as is described on this web site, consists of a 20 m diameter central sodium pool in which are fuel bundles. Outside the pool are factory fabricated and tested modules. Both the fuel bundles and the modules are individually road truck portable, both before and after long term use.

The FNR fuel assembly contemplated on this web site has both core and blanket zones. The core zone contains Pu-239 atoms and is where the nuclear chain reaction takes place. The blanket zone surrounds the core zone and absorbs excess neutrons emitted by the chain reaction in the core zone. Excess neutrons are absorbed by U-238 atoms in both core and blanket fuel rods and after a short delay spontaneously form new Pu-239 and Pu-240 atoms. These plutonium atoms are either fissioned or are later recovered via fuel rod reprocessing and are used to make new FNR core fuel rods.

The FNR fuel assembly described herein consists of a disk shaped ~ 10.4 m diameter X 0.4 m thick core which is surrounded by an ~ 1.8 m thick blanket. The core contains a mixture of both fissile (potentially neutron emitting) Pu-239 atoms and fertile neutron absorbing U-238 atoms. The blanket contains only fertile neutron absorbing U-238 isotope atoms.

The resulting 4 m thick X 14 m diameter fuel assembly is mounted, disk axis upright, in the middle of a 20 m diameter X 15 m deep pool of liquid sodium and is held in place by a steel structure. There are many thousands of small (~ 10 mm diameter) vertical holes which pass through both the blanket and the core to allow liquid sodium to flow through the fuel assembly for heat removal. The maximum safe thermal output of a FNR is limited by the properties of its fuel tubes.

There is a mechanical means for occasional fine adustment of the core zone thickness while the reactor is operating.

There is a 3 m wide liquid sodium guard band, surrounding the entire fuel assembly, that absorbs any neutrons that escape from the fuel assembly to prevent escaping neutrons causing cumulative neutron excitation and long term physical damage to the sodium pool structure.

A fissile atom such as Pu-239 has the property that if it captures a free neutron it usually fissions and emits an average of G free neutrons per fission, where for Pu-239:
G = 3.1
Each fission reaction also emits about 200 MeV of heat energy.

If the FNR core zone is too thin or the fissile atom concentration in the core zone is too low the probability of a neutron that is emitted by one fissile atom being captured by another fissile atom before that neutron is absorbed by a non-fissile atom is less than (1 / G), so the free neutron concentration in the core zone and the rate of fission heat production remain close to zero. However, if the core thickness is gradually increased while the fissile atom concentration in the core is sufficiently large the probability of a neutron emitted by one fissile atom being captured by another fissile atom eventually reaches (1 / G). At this point, known as reactor criticality, a nuclear chain reaction commences and the concentration of free neutrons and the consequent rate of heat production in the core zone both rapidly rise.

The nuclear heat production in the core zone causes the core zone temperature to rise which, via thermal expansion, increases the average interatomic distance. The increase in average interatomic distance reduces the rate at which neutrons are produced by fission reactions in the core zone and increases the rate at which neutrons diffuse out of the core zone. The net result is that the point of nuclear criticality is core zone temperature dependent, as well as being both core thickness dependent and fissile atom concentration dependent. At core zone temperatures below the point of criticality the free neutron concentration rises which produces more heat in the core zone which causes core zone temperature rise. At core zone temperatures above the point of criticality the free neutron concentration in the core zone falls which stops heat production and hence stops the core zone temperature rise.

Thus the core zone temperature is a function of the core zone thickness and the core zone fissile atom concentration. As the core zone thickness increases so also does the core zone temperature. As the fissile atom concentration decreases so also does the core temperature. In the power FNR described herein as fissile fuel is consumed the core thickness must be periodically mechanically increased so that the average core zone temperature remains at about 520 degrees C.

When this fuel assembly is immersed in cool liquid sodium that liquid sodium naturally rises through the vertical coolant holes in the fuel assembly and is discharged from the top of the fuel assembly in the temperature range 490 degrees C to 520 degres C, depending on the liquid sodium flow rate. This high temperature liquid sodium then flows through an intermediate heat exchanger, where it cools down to about 340 degrees C, before flowing back to the bottom of the primary sodium pool. The heat removed from the flowing liquid sodium by the intermediate heat exchanger is used to make high pressure steam for electricity generation.

When the reactor thermal load is at its rated maximum the average core zone fuel temperature remains at ~ 520 degrees C, but there is an ~ 40 degree C temperature difference between the core fuel rod center line and the core fuel rod outside surface. Hence the core fuel rod centerline temperature becomes ~ 540 degrees C and the core fuel rod surface temperature becomes ~ 500 degrees C. There is a further ~ 10 degree C temperature drop across the fuel tube wall and the adjacent sodium films, leading to a full load liquid sodium discharge temperature of about 490 degrees C.

The FNR thermal power output is controlled by controlling the rate of heat extraction from the intermediate heat exchanger via the secondary sodium circulation rate.

If there is no thermal load the primary liquid sodium pool temperature at the core fuel level will gradually rise to 520 degrees C and then stop.

If due to nuclear heat release the temperature of the materials increases, thermal expansion of the materials in three dimensions increases the fraction of fission neutrons diffusing from the core zone into the blanket zone and decreases the rate at which Pu atoms in the core zone capture neutrons and then fission. Hence the free neutron concentration in the core falls and the nuclear chain reaction stops. Similarly, if the core temperature decreases the chain reaction restarts. By this mechanism the FNR maintains a nearly constant fuel temperature in the core zone.

During a switch from reactor at zero power to reactor at maximum power the liquid sodium stratification level rises by about 3.8 m.

If the chain reaction is initially off due to high sodium pool temperature the amount of heat that must be extracted from the primary liquid sodium to bring the reactor to full power is:
3.8 m X Pi (10 m)^2 X 927 kg / m^3 X 150 deg C X 1.27 KJ / kg-deg C X 1 KWt-s / kJ
= 210,818,062 KWt-s
= 210,818 MWt-s

Assume that the power transition is caused by a 840 MWt load step. Then the time for the FNR to fully change state in response to this load step is:
210,818 MWt-s / 840 MWt
= 251 seconds
= 4.2 minutes

Provided that the fuel geometry remains stable and the primary sodium remains clean and is completely isolated from both air and water this energy production process, which involves no moving mechanical parts and no ongoing chemical changes is extremely stable. One nuclear fuel load can power the reactor for about 30 years during which time about 15% of the reactor core fuel mass is converted into short lived fission products. In order to expedite fuel reprocessing a partial fuel change is contemplated every six years.

In the core zone the rate of loss of Pu by fissioning is partially offset by the rate of production of Pu via neutron capture by U-238. Reactor criticality at the desired operating temperature is maintained through the operating life of fuel bundles via small incremental changes in core zone thickness. These geometry changes are accomplished by using liquid sodium hydraulic piston actuators to slightly change the vertical overlap between each mobile fuel bundle and its corresponding adjacent fixed fuel bundles.

During normal FNR operation most of the surplus neutrons are absorbed by U-238 isotope atoms which naturally transmute into Pu-239 isotope and Pu-240 isotope atoms. After a reactor fuel change the removed core and blanket fuel rods should be reprocessed to extract the plutonium atoms, which should then be used to make new core fuel rods. The Pu-240 isotope formation slightly reduces the Pu-239 isotope output but has the benefit that it makes the reactor generated plutonium undesirable for military use.

If there is any unanticipated problem the reactor defaults into a walk away safe warm, cool or cold shut down mode, depending on the circumstances and the availability of station power.

This web page last updated August 22, 2020

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