XYLENE POWER LTD.

FNR IMPLEMENTATION

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

INTRODUCTION:
This web page reviews the practical challenges to implementation of Fast Neutron Reactors (FNRs).
 

IMPORTANCE OF FNRs:
FNRs provide the only non-CO2 emitting technology that can sustainably and economically fully displace fossil fuels. Unlike water cooled reactors, FNRs with low pressure coolants can be safely sited within major cities for supply of electricity, industrial/commercial heat and district heat. Such siting reduces electricity transmission costs and via district heating can reduce the cost of comfort heat about three fold as compared to supply of comfort heat using remotely located water cooled reactors.

FNRs provide heat output in the temperature range 300 degrees C to 500 degrees C. This temperature range compares to 280 degreesw C to 320 degrees C for water cooled reactors. The higher operating temperatures of FNRs enable certain industrial chemical processes, improve the efficiency of electricity generation and permit use of dry cooling towers at sites where water for evaporative cooling is either uneconomic or unavailable.

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 isotope inventory is consumed. In concert With nuclear fuel recycling, FNRs can reduce the consumption of natural uranium per kWhe 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 displaces fossil fuels.

FNRs offer the benefit of complete fissioning of high atomic weight fuel. By so doing they reduce the used nuclear fuel waste problem about 100 fold by mass and more than 1000 fold by required isolation time.

FNRS operate at higher temperatures than water cooled reactors. These higher temperatures increase the efficiency of thermal electricity genertion and enable use of dry cooling towers for rejection of waste heat. A FNR is more expensive than a lower cost water cooled reactor of the same power. Hence the present market for FNRs is concentrated in circumstances where the performance advantages of FNRs justify the extra capital cost per kWe of electrity generation capacity.

For remote sites that have large electricity loads (over 1 GWe) and unlimited water cooling capacity, on a simple capital cost per kWe of electricty generation capacity, FNR technology is not price competitive with BWR (Boiling Water Reactor) and PWR (Pressurized Water Reactor) technology. The market for FNRs is in applications where large BWRs and large PWRs either cannot function or cannot economically compete. These markets include urban district energy systems, communities with populations less than 300,000, communities with little or no cooling water, industries that need process heat in the temperture range 300 degrees C to 500 degrees C and nations that lack natural nuclear fuel and hence value nuclear fuel sustainability.

Howeever, the superior technical features of FNRs are not free of cost. FNRs generally have two thermal isolation loops as compared to none for a boiling water reactor (BWR) or one for a presurized water reactor (PWR). FNRs require maintenance of an argon atmosphere. Hence, a FNR is economically uncompetitive against a BWR or a PWR unless circumstances are such that a BWR or PWR either cannot perform or economically cannot do the required task.
 

MAJOR FNR IMPLEMENTATION ISSUES:
1) FNRs need fuel typically consisting of 20% TRU, 70% U-238 and 10% zirconium. The 20% Tru is typically composed of at least 12% Pu-239 and Pu-241, with the remaining 8% being a mix to transurnium element isotopes. The most practical way to initially obtain this fuel is by reprocessing of used CANDU fuel. The major issue is that automated fuel reprocessing has to be fully funded prior to any serious discussion regarding funding of FNR deployment.

2) The next FNR implementation issue is mechanical pipe joints. The use of mechanical pipe joints, typically involving bolted flanges and a gasket mateial, is key to minimizing future maintenance costs. The major problem is the available gasket material. Available elastomeric gaskets are suitable for containing water at up to 320 degree C. However, FNRs require containment of NaK at up to 500 degrees C. NaK is chemicaly aggressive. In the past soft copper gaskets plated with nickel have been used for this purpose, but they lack the flexibility of elastomeric mateials. As a result the pipe and flange dimensions become more critical. To minimize costs we really need a better flange/gasket material.

3) The next issue is compensation for thermal expansion. In a FNR the pipe diameter required to move a specific thermal power is about twice as large as in a comparable water cooled reactor. Meanwhile the thermal expansion in a FNR is also about twice as large as in a comparable water cooled reactor due to use of stainless steel alloys and a higher operating temperature. The combination of these two factors makes compensation for themal expansion in FNRs about 4X as difficultas in water cooled reactors.

4) A FNR involves three concentric gas tight biosafety barriers. One of these barriers must operate at 500 degrees C. The other two barries must pass 16 inch diamete pipes containing NaK at up to 500 degree C. Maintaining these gas tight feed through seals over 500 degree C temperure changes is a challenge.

SODIUM COOLED FNR CHALLENGES:
The FNRs described herein use liquid sodium rather than water as a primary reactor coolant. The sodium, which is necessary for fuel sustainable 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 fluid 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.
 

SUPPLY OF FNR CORE ZONE START FUEL:
In order to successfully function a FNR requres a limited 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 April 21, 2026.

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