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INTRODUCTION:
This web page reviews the circumstances in which deployment of Fast Neutron Reactors (FNRs) makes economic sense.
IMPORTANCE OF FNRs:
FNRs provide the only non-CO2 emitting technology that can sustainably and economically fully displace fossil fuels. FNRs, with low pressure coolants and multiple stages of radio isotope and chemical safety isolation, can be safely sited within major cities for supply of electricity, industrial heat, commercial heat and district heat. Such siting almost eliminates electricity transmission costs and potentially reduces the cost of comfort heat about three fold as compared to remotely located water cooled reactors.
FNRs operate by converting abundant fertile isotopes such as U-238 and Th-232 into fissile isotopes such as Pu-239 and U-233 faster than the fissile inventory is consumed. With suitable fuel recycling FNRs can reduce the consumption of natural uranium per kWh output by more than 100 fold as compared to water cooled reactors. This issue will become of increasing importance as existing rich natural uranium deposits are depleted and as nuclear power gradually displaces fossil fuels.
FNRS operate at higher temperatures than water cooled reactors. These higher temperatures increase the efficiency of thermal electricity genertion but introduce additional capital and maintenance costs.
FNR CHALLENGES:
FNR operation is enabled by the use of liquid sodium rather than water as a primary reactor coolant. The sodium, which is necessary for fast neutron nuclear power production, introduces two major challenges as compared to water.
1) Sodium (Na) is highly chemically reactive and will violently react with either water or air if given any opportunity. This issue leads to increased costs in FNRs as compared to water cooled reactors related to isolation, fluid compatibility and chemical safety.
2) A neutron flux through sodium produces the radio isotope Na-24 which has a half life of about 15 hours. That compares to neutron irradiation of water that produces radio isotopes with half lives of less than 30 seconds. In order to maintain a high capacity factor most maintenance on a FNR must be done while the reactor is operating as compared to most maintenance on a water cooled reactor that is done while the reactor is off. This issue causes increased complexity in FNR heat transport system design as compared to water cooled reactors.
3) FNRs operate at higher temperatures than water cooled reactors. While higher temperatures increase the efficiency of thermal electricity genertion these higher temperatues also increase problems with flange type pipe joints and seals. Elastomeric gaskets that can be used with water cooled reactors in general are not suitable for use with FNRs.
4) The higher operating temperatures of FNRs trigger greater equipment complexity relating to compensation for thermal expansion and contraction when the equipment is enabled or shut down.
5) FNRs require core zone start fuel for which the primary source is reprocessing of used CANDU fuel.
FNR MARKETS:
In certain markets and in certain applications FNRs offer features that are not available from water cooled reactors.
1) One market is supply of both heat and electricty for urban district heating in regions where winter temperatures routinely fall below 0 degrees C. In these markets the reactors need to be sited within cities and to have no requirement for an external public safety exclusion zone a is required by most water cooled nuclear reactors;
SUPPLY OF FNR CORE ZONE START FUEL:
In order to successfully function a FNR requres a limitd amount of core zone start fuel which is about 20% TRU (Pu-239 and other high atomic number isotopes). At this time this fuel is not readily available from the market so the financing of any FNR project is contingent upon obtaining a certain source of FNR core zone start fuel. The most practical way of obtaining this FNR core zone start fuel is by reprocessing used CANDU fuel.
1) If the only available used reactor fuel is from light water reactors that fuel needs to be reused in a CANDU reactor that will convert it into used CANDU fuel.
2) About 90% of the mass of used CANDU fuel is removed by selective extraction of uranium. This process, known as recrystallization, requires low grade heat that can be obtained from the condenser section of an existing nuclear power plant.
3) The remaining 10% of the used CANDU fuel mass is subject to a selective electro-chemical process that separates this 10% into four categorys: TRU, uranium, zirconium and fission products.
4) The fission products are placed in 300 year isolated dry storage;
5) New core fuel metal alloy is formed by melting together appropriate weight fractions of TRU, uranium and zirconium;
6) The new core fuel alloy is cast into new metal core fuel rods;
7) Blanket metal fuel rods are cast from depleted uranium.
8) Steel fuel tubes are loaded containing appropriate numbers of properly positioned core fuel rods, blanket fuel rods and sodium slugs.
9) The steel fuel tubes are temporarily heated to over 100 degrees C so that the sodium melts and then solidifies holding the fuel rods in position for future trnssportation.
10) The fuel tubes are assembled into core fuel bundles and blanket fuel bundles.
11) The fuel bundles are stored in suitable shielded and ventilated dry storage until needed for installation in a FNR.
12) The fuel bundles are transported from the reprocessing facility to the FNR in shielded containers mounted on flat deck trucks.
13) The same fuel bundle tranportation equipment is used for returning used FNR fuel bundles to the fuel reprocessing facility.
FNR SAFETY ISSUES:
The public safety issues applicable to a FNR sited in a major city are significantly different from the public safety issues applicable to a remotely sited water cooled reactor surrounded by a public safety exclusion zone. The FNR specific safety issues are extensively addressed on this web site. The over riding issues are that the FNR elevation must be sufficient to ensure that the FNR will never be flooded by water and the FNR's foundation must rest on contiguous bedrock with a long term load bearing capacity in excess of 30 tonnes/ m^2.
FNR FIRST OF A KIND (FOAK) ISSUES:
A major practical issue with FNRs is training continuityof engineering personnel. At various times during the last 60 years FNRs have been deployed by the USA, Soviet Union, Russia, France and China. However, these deployments have been intermittent. That intermittency has led to repeated loss of trained engineering personnel. The consequence of that loss is that each new FNR deployment is in effect a First of a Kind (FOAK) project for the personnel involved, with all the attendant training costs. This training problem applies to both FNRs and FNR fuel production. The result is that until FNRs are deployed in a sustained sequence by a single party, FNR technology will remain expensive as compared to existing water cooled reactor technology.
SUMMARY:
In general FNRs are more complex and more expensive than water cooled reactors of similar thermal capacity. However, FNRs are essential to conserve the world supply of natural uranium, without which economic nuclear power is impossible.
At this time FNRs are only economic in markets where, for varius practical reasons, FNRs provide essential features that water cooled reactors cannot economically provide.
This web page last updated March 22, 2026.
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