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FAST NEUTRON REACTORS - OVERVIEW

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

FAST NEUTRON REACTORS (FNRs):
FNRs are of great importance because today fast neutron reactor technology is the only proven technology that can supply the clean, dependable and sustainable power needed for mitigation of fossil fuel driven climate change. FNRs of various sizes ranging from 800 MWe downwards have been built and operated in Russia, France and the USA. Individual FNRs have operated for more than 30 years. Today more FNRs are under construction in Russia, China and the USA. Major advantages of FNRs over other potentially sustainable nuclear power technologies are that the reactors operate at less than 500 degrees C, can be constructed using readily available materials and all the related technical issues have been resolved. Large scale FNR deployment of FNRs is simply a matter of political will.

A Fast Neutron Reactor (FNR) operates with fast neutrons, as distinct from a thermal neutron reactor which operates with slow neutrons. A FNR cannot be directly water cooled because the hydrogen atoms in water moderate (slow down) neutrons too much. In order to avoid this moderation problem the FNR described herein uses liquid sodium instead of water as a primary coolant. Sodium nuclei are about 23X more massive than hydrogen nuclei.

A FNR is superficially simple. It consists of a pool of primary liquid sodium, comparable in size to a swimming pool, which contains an assembly of passive immersed nuclear heating elements. Each nuclear heating element senses the temperature of the sodium and outputs heat at the rate required to keep the temperature of the sodium close to its setpoint, which is typically about 460 degrees C. If heat is drawn from the pool at a certain rate to produce steam for electricity generation the nuclear heating elements add heat to the primary sodium at an equal rate to keep the average primary sodium temperature at 460 degrees C.
 

FNR SCIENCE:
The Science underlying FNRs is well explained in a 2021 text by Peter Ottensmeyer titled Neutrons At THE Core. The key issue of thermal stability of FNRs is set out on the web page titled: FNR Reactivity

FNR ADVANTAGES:
For stationary power generation FNRs have many practical advantages over existing water cooled reactors including:
a) A 100 fold improvement in natural uranium utilization efficiency;
b) A 1000 fold reduction in long lived solid nuclear waste production;
c) Low pressure primary coolant which enables safe urban reactor siting.
 

FNR POWER PLANT COMPONENTS:
A practical FNR power plant has four main components:
a) A pool type FNR;
b) Many independent heat transfer systems that extract heat from the primary liquid sodium pool and use that heat to produce high pressure steam at remote turbogenerators;
c) Remote turbogenerators which convert high pressure steam into electricity and low grade heat;
d) A district heating system which distributes the low grade heat emitted by the turbogenerators.
 

FNR DEPLOYMENT PROGRESS:
As of 2021 the Russians and Chinese are far ahead of North America in terms of fast neutron power reactor development and deployment. However, in August 2020 a new North American team known as Natrium, which is composed of TerraPower (Bill Gates), Warren Buffett and GE Hitachi, formed to seriously compete in the North American electricity market. This team contemplates use of a pool type sodium cooled reactor with a sodium secondary heat transport loop and a molten nitrate salt tertiary heat transport loop. The Natrium plan is more concerned with capital cost reduction than with fuel sustainability and elimination of decommissioning waste. Natrium plans on providing some molten salt thermal energy storage with each reactor. Its tentative power plant architecture is focused on compliance with US regulatory requirements. The following video outlines the Natrium Plan. A June 17, 2021 news story summarizes a critical view of the Natrium reactor project plan.
 

THIS WEBSITE:
Much of this web site is devoted to the design of a 300 MWe (1000 MWt) modular liquid sodium cooled reactor intended for installation at urban sites to provide both electricity and district heat.

The FNR described on this website consists of 464,192 vertical sealed metal fuel tubes, each 1.27 cm OD X 6.0 m high, centrally immersed within a 15 m deep X 20 m diameter pool of highly thermally conductive liquid sodium primary coolant. This hot sodium pool is protected from oxidation by an atmospheric pressure inert gas (argon) cover. Inside the lower two thirds of each active fuel tube are solid metallic fuel rods plus sufficient liquid sodium to provide a good thermal contact between the fuel rod OD and the enclosing fuel tube wall ID. The upper one third of each active fuel tube is an empty plenum for storing inert gas fission products at an acceptable pressure.

The fuel rods are arranged within the fuel tubes so that the there is a 0.40 m to 0.60 m thick pancake shaped central core zone containing fissile fuel which is sandwitched between 1.8 m thick blanket zones containing fertile fuel. The fission reactions take place in the core zone and emit surplus neutrons which are absorbed by fertile atoms in both the core and blanket zones. Over time each such absorbed neutron causes formation of a new fissile atom.

There is a mechanical arrangement for adjusting the thickness of the core zone to which sets the FNR temperature setpoint.

The fuel tubes and their contained liquid sodium prevent contamination of the primary liquid sodium coolant by both fuel and fission products. The fuel tube alloy is chosen for its nuclear properties and for its resistance to swelling when exposed to a sustained fast neutron flux. The metallic fuel alloys are chosen for their nuclear properties and their fuel tube and sodium compatibility.
 

REACTIVITY:
At any instant in time t the number of free neutrons N in a FNR can be expressed as:
N = No Exp[R (t - to)]
where:
No = N|(t = to)
and reactivity R is a function of FNR average temperature T. Thus:
R = R(T)
where:
T = average FNR temperature.
 

FNR TEMPERATURE STABILITY:
A fundamental FNR design requirement is choice of FNR physical parameters such that at the desired FNR operating temperature To the reactivity R as a function of temperature T satisfies the equations:
R(To) = 0
and
{[dR(T) / dT]|(T = To)} < 0

The equation:
R(To) = 0
implies that
{[dN / dt]|(T = To} = 0
which says that the number of free neutrons in the FNR core zone is constant over time and hence is neither growing nor shrinking.

{[dR(T) / dT]|T = To} < 0
says that if
T > To
then:
R < 0
so the number of free neutrons in the FNR shrinks over time causing the nuclear fission to stop and if
T < To
then:
R > 0
so the number of free neutrons in the FNR grows over time causing an increase in the rate of nuclear fission heat generation.

Note that for a pancake shaped FNR with thick blanket zones, to a good approximation:
N = Integral from Z = - infinity to Z = + infinity of:
n(Z) A dZ
where:
n(Z) = free neutron concentration as a function of vertical position Z,
and
A = cross sectional area of FNR core zone

In the FNR described herein the required temperature stability is achieved by use of a pancake shaped reactor core zone with a geometry such that with new core fuel about half of the fission neutrons produced diffuse out of the core zone and into the surrounding reactor blanket zone.

The FNR reactivity is temperature dependent because the atomic concentrations of the FNR components change with temperature. Significant issues are the effect of the high thermal coefficient of expansion of sodium and the sodium volume fraction required to provide sufficient cooling with natural circulation of liquid sodium.

From a reactor safety and stability perspective it is essential that the reactivity R have a strong negative temperature coefficient under all possible operating conditions.

When a FNR has no thermal load and there is no fission product decay heat the primary liquid sodium coolant temperature T gradually rises to the FNR setpoint temperature To at which point nuclear heat generation stops and the primary liquid sodium coolant temperature T remains at:
T = To.
During FNR commissioning the thickness of the reactor core zone should be slowly adjusted so that at a constant thermal load:
To = 460 degrees C.
 

NORMAL FULL POWER OPERATION:
Note that:
To = 460 degrees C
is the average temperature setpoint of the FNR fuel. The thermal load is limited by the secondary sodium flow rate to limit the maximum primary sodium coolant temperature rise to 50 degrees C. Due to limited core fuel thermal conductivity, at maximum thermal load the fuel centerline temperature is 50 C above the adjacent coolant temperature. Thus at maximum rated thermal load:
Maximum fuel centerline temperature near fuel bundle discharge = 510 C
Minimum fuel centerline temperature near fuel bundle inlet = 460 C
Average fuel centerline temperature = (510 C + 460 C) / 2 = 485 C
Average fuel temperature = 485 C - (50 C / 2) = 460 C
Coolant inlet temperature = 460 C - 50 C = 410 C
Coolant discharge temperature = 510 C - 50 C = 460 C

Thus in normal opertion the FNR coolant discharge temperature remains stable at 460 degrees C independent of the thermal load. The normal maximum coolant temperature rise is 50 degrees C.

Each fuel tube has four adjacent cooling channels. If two cooling channels adjacent to the same active fuel tube are blocked the coolant temperature can rise by another 50 degrees C to 560 C which allows the adjacent fuel centerline temperature to reach:
510 C + 50 C = 560 C.

Thus the design is for a coolant discharge setpoint of 460 C and a normal maximum coolant differential temperature of 50 C. However, each fuel tube will tolerate blockages of up to two adjacent primary sodium cooling channels.

The initial core zone thickness is designed to be 0,44 m. However, as years go by the reactor core fuel fissile atom concentration will slowly decrease causing the setpoint temperature To to slowly decrease. To compensate every few months the thickness of the core zone should be slightly increased to keep the fuel setpoint temperature To at:
To = 460 degrees C.
 

FNR HEAT EXTRACTION:
Heat is extracted from the FNR primary liquid sodium coolant at a controlled rate and is used to generate electricity. When the reactor is at steady state as fast as heat is extracted an equal amount of heat is generated by fission reactions in the core zone metallic fuel rods. This heat flows from the core zone fuel rods through the metal fuel tube walls and into the primary liquid sodium coolant. The warmed coolant spontaneously rises. This convective heat flow keeps the primary liquid sodium pool surface temperature close to temperature To.

When the reactor is under maximum rated thermal load there are temperature drops along the fuel rod length, across the fuel rod radius and across the fuel tube wall which collectively result in the primary liquid sodium pool surface temperature operating at about 460 degrees C.

The chosen average fuel temperature setpoint:
To = 460 degrees C
prevents the peak core fuel rod centerline temperature exceeding 560 degrees C at the FNR maximum rated thermal load and in the presence of two coolant channel blockages adjacent to the same fuel tube.

Also immersed in the primary liquid sodium at the perimeter of pool are intermediate heat exchange bundles which transfer heat from the primary liquid sodium to isolated circulating secondary liquid sodium.

At the maximum rated thermal load the circulating secondary sodium discharges from the intermediate heat exchange bundles at temperature Tsd where:
Tsd = 450 degrees C
and conveys heat through secondary sodium pipes at mass flow rate Fs towards the sodium-salt heat exchangers.

The salt discharge from the sodium-salt heat exchangers conveys heat to the corresponding steam generators which are located in another building and transfers heat to water / steam in the steam generators via the isolating heat exchange tube bundles within the steam generators. The recirculating salt then flows back to the sodium-salt heat exchangers.

The secondary sodium is circulated by induction pumps at mass flow rate Fs and at return temperature Tsr, where at full load temperature Tsr is about 20 degrees C above the boiling point of super heated water in the steam generator. The resulting net thermal power P flowing from an intermediate heat exchange bundle to the steam generator is:
P = Cp Fs (Tsd - Tsi)
where:
Cp = the heat capacity of the secondary sodium.

Hence the thermal output power from a FNR power plant is proportional to the secondary sodium mass flow rate Fs and the temperature difference
(Tsd - Tsi).
This temperature difference is only weakly thermal load dependent.

At low thermal loads:
Tsd ~ 460 C
and
Tsr ~ 320 C

At high thermal loads:
Tsd ~ 450 C
and
Tsr ~ 340 C
in order to maximize the steam thermal power output suitable for electricity generation while preventing deposition of NaOH on cool heat exchange surfaces. At the FNR's maximum rated thermal load the secondary sodium return temperature Tsr is about 20 degrees C above the water temperature corresponding to the satuation steam pressure in the steam generator. The saturation steam pressure is controlled at a fixed setpoint of about 11.25 MPa by a pressure regulating valve which regulates the flow of discharge steam from the steam generator. The discharge steam is fed to a steam turbine where it expands to generate electricity.

Extraction of heat from the primary sodium by the intermediate heat exchange bundles cools the primary sodium so that it becomes more dense and sinks to the lower part of the primary sodium pool, where it flows underneath the fuel tube assembly. There it rises through the fuel tube assembly due to its decrease in density as it absorbs nuclear heat.

The FNR thermal output power and hence the generated electrical power are controlled by varying the secondary sodium mass flow rate Fs with variable flow rate induction pumps. Subject to thermal stress issues and steam turbine constraints this control methodology allows a rapid electric power ramp rate between 10% and 100% of reactor plate rating.
 

FNR SAFETY:
As compared to other nuclear reactor types a liquid sodium cooled FNR has several huge safety advantages:
a) The radioactive species are confined to fuel tubes in the primary sodium pool;
b) The primary sodium pool operates at atmospheric pressure;
c) Subject to stability of the fuel geometry the primary sodium coolant temperature cannot exceed temperature To;
d) In the event of a secondary sodium leak in any heat transfer loop a secondary sodium fire can be suppressed by immediately dumping the remaining secondary sodium in that loop into a dedicated below grade secondary sodium dump tank.
e) The volume of secondary sodium in each heat transfer loop is limited to:_____ m^3.
f) The fuel and fuel tube design suppresses any minor excursions into prompt neutron criticality that might be caused by sabotage or a control system defect.
 

FNR PERFORMANCE ADVANTAGES:
One of the main features of FNRs is that during operation they decrease the fissile atom concentration in the nuclear fuel. This feature allows FNRs to operate in a manner in which the fission product fuel waste decays to safe radiotoxicity levels in about 300 years as compared to 400,000 years for thermal neutron reactors.

A very important feature of a liquid sodium cooled FNR is that it can continuously convert the abundant fertile isotope uranium U-238 into the isotopes plutonium Pu-239 and Pu-240. After fuel reprocessing the FNR can fission the plutonium isotopes to realize about 100X more energy per kg of natural uranium than can a water cooled reactor.

A FNR produces more fissile atoms than it consumes, so the surplus fissile atoms can be used to expand the fissile fuel inventory available to start other FNRs.

Another benefit of a FNR is that it produces higher temperature and drier steam than a light water reactor enabling increased efficiency of electricity generation and decreased steam turbine wear.
 

FNR DISADVANTAGES:
A FNR contains no low atomic weight elements such as hydrogen or carbon to moderate its neutrons. A consequence of the resulting high neutron speed is that the neutron fission cross section in a FNR is substantially less than in a thermal neutron reactor, so to maintain a chain reaction the required initial fissile fuel atom inventory must be much larger than in a thermal neutron reactor of similar rated power.

The coolant sodium is incompatible with water, so for public safety a FNR must be sited where it will never be subject to flooding by water. This constraint may mean that electricity generation waste heat must be rejected via cooling towers instead of via direct lake or sea water cooling.
 

FAST NEUTRON REACTOR SIDE ELEVATION:


For diagramatic simplicity, in the above diagram the air locks, the open steel lattice supporting the fuel bundles and the steel columns supporting the intermediate heat exchange bundles are not shown.
 

FNR POWER STABILITY:
In a FNR the neutron concentration and hence the reactor thermal power could potentially grow or decay very quickly. the FNR coolant discharge temperature is maintained by a passive control loop. There is external control system to set the FNR thermal power. Delayed neutrons in a FNR prevent large thermal power excursions when the fuel geometry is slowly changed. The reactor is protected against rapid return coolant temperature changes by a large liquid sodium pool thermal mass.

FNRs derive their safety by operating with a reactor thermal power versus average fuel temperature characteristic that has a strong negative slope. This safety characteristic is near optimal when about half of the fission neutrons formed in the FNR core zone diffuse out of the core zone and are absorbed in the adjacent blanket zone. The design of a FNR fuel assembly should adhere to this safety principle.

The fuel setpoint temperature To should always be raised very slowly to prevent the core zone fuel center line temperature and/or the fuel tube heat flux exceeding their design limits.

FNRs should be always be operated with a primary coolant temperature boiling point far above the highest local fuel temperature. Any coolant boiling would indicate a FNR fuel temperature far outside the reactor design limits.

A FNR should have control system features that constrain the setpoint temperature setting To and the maximum thermal load to safe values regardless of operator or programming error.
 

FNR OVER TEMPERATURE PROTECTION:
It is necessary to do all necessary to ensure that a FNR will not have an uncontrolled increase in average fuel temperature setpoint due to a change in fuel geometry caused by any credible earthquake, aircraft impact, overhead crane failure or overhead structural failure.

A FNR should use a fuel designed such that in a prompt neutron critical condition the core fuel will instantaneously disassemble within its fuel tube to suppress the prompt critical condition.
 

CRITICAL IMPORTANCE OF FAST NEUTRON REACTORS (FNRs):
FNRs are far superior to existing water moderated nuclear reactors for public electricity and thermal power generation because per unit of energy production properly designed and operated FNRs consume about 100X less natural uranium and produce about 1000X less long lived nuclear waste than CANDU reactors and have technical features which allow them to be safely sited inside major cities where they can supply both electricity and district heat.

FNRs operate by transmuting abundant U-238 into Pu-239 and then fissioning the Pu-239. Excess fission neutrons that are not required to maintain the Pu-239 fission chain reaction enable the U-238 transmutation.

If mankind is to survive people must face the reality that FNR core fuel will always contain a significant fraction of plutonium, typically in the range 12% to 20%._____ The plutonium in FNR core fuel is kept unsuitable for nuclear weapon use by continuously denaturing it with Pu-240.

Those that oppose use of Pu in FNRs are in effect choosing certain extinction of mankind through lack of dependable and sustainable clean power. These persons should instead do all necessary to prevent fabrication, storage or transport of nuclear weapons.
 

REFERENCE FILE:
The future of FNRs is further discussed at an elementary level in the file titled:
The Role of Sodium-Cooled Fast Reactors in a Large-Scale Nuclear Economy

A more technically advanced but older general reference is:
Sodium - NaK Engineering Handbook Vol III.
and
Sodium Fast Reactor with once through depleted uranium breed and burn blanket
 

INITIAL FNR STUDY:
It is recommended that students should initially study the web page titled: FNR CONCEPT so that they gain an elementary understanding of what a Fast Neutron Reactor (FNR) is and how it works before attempting to study other FNR related material.
 

FNR OVERVIEW:
There are several FNR overview web pages:
The web page FNR INTRODUCTION is intended for persons who have no knowledge about FNRs.

The web page FNR POLITICS summarizes present political constraints relating to the deployment of FNRs and explains why Russia and China are far ahead of North America in terms of FNR development and deployment.

The web page FNR CONCEPT indicates conceptually how the passive nuclear portion of a FNR works.

The web page FNR DESCRIPTION describes how a FNR power plant is physically realized.

The web page FNR FEATURES summarizes the advantages and limitations of FNRs.

The web page FNR OPERATION discusses practical installation, operation, maintenance and safety matters related to FNRs.

The web page FNR FUEL CYCLE discusses FNR fuel issues including: sources, concentration, transport, processing, interim storage, recycling and long term storage.

The aforementioned subjects are explored in much greater detail on numerous web pages that are accessible via the Nuclear Power Table of Contents which is located at the bottom of the web page titled: NUCLEAR POWER
 

FNR POWER PLANTS:
Fast Neutron Reactor (FNR) based power plants provide the only dependable and sustainable source of clean power that is sufficient to meet human energy and dependable power needs. Other sustainable non-fossil energy sources, such as solar, wind and run of river hydro can only economically meet a fraction of human energy requirements and cannot meet dependable power requirements. No other nuclear reactor type has demonstrated fuel sustainability, although theoretically a thorium fueled molten salt reactor or a deuterium-tritium fusion reactor might eventually be capable of doing so.
 

FNR POWER PLANT CONCEPTUAL DESCRIPTION:
A FNR power plant consists of:
1. A pool of primary liquid sodium that by a passive nuclear process maintains a nearly constant surface temperature of about 460 degrees C.

2. Numerous (56) independent isolated heat transport systems which extract heat from the primary sodium pool at controlled rates and deliver that heat to steam generators to make steam.

3. Turbogenerators and condensers that convert steam into electricity, condensate water and reject heat. The electric power produced is proportional to the steam flow delivered to the turbogenerators.

4. A heat rejection system, consisting of a combination of 4 on-site cooling towers, local and remote pumps, buried district heating piping, remote heat loads and 12 remote cooling towers. Typically a FNR power plant produces about two energy units of low grade comfort heat for every energy unit of electricity produced.

5. Pumps which inject high pressure condensate water into the steam generators.

6.Electrical switchgear and transformers necessary for interfacing the turbogeneators to the electricity grid.

7. On-site equipment, material storage and personnel facilities necessary to support FNR power plant operation and maintenance.

8. On-site cooling water storage sufficient for removal of fission product decay heat by evaporation in the extreme circumstances of sudden reactor cold shutdown, loss of grid power and loss of city water service.

9. On-site argon production and storage sufficient to safely manage FNR cold shutdowns, maintenance and fire suppression incidents.
 

FNR POWER PLANT DESCRIBED ON THIS WEB SITE:
The FNR power plant described on this web site has a design output capacity of 1000 MWt (300 MWe), sufficient for meeting the total power and energy needs of a North American population of about 100,000 people. This power plant can be sited within a major city at an elevation sufficient to ensure that it will never be subject to flooding by water.

The 300 MWe power plant occupies a single city block (114 m X 114 m) with 20 m wide perimeter streeds and requires a 50 m long X 50 m wide X 20 m deep excavation from grade to bedrock. The four corner cooling towers on the reactor site extend 48 m above grade. In order to dependably generate maximum rated electrical power in the summer 12 additional similar remote cooling towers should be connected to the district heating piping. Four emergency water tanks occupy the spaces underneath the on-reactor site cooling towers.

The supporting mechanical and electrical equipment is assembled from factory fabricated and tested road truck portable modules.

Nuclear temperature maintenance of the primary liquid sodium pool is passive, autonomous and extremely dependable. The power plant's modular mechanical/electrical equipment design permits thermal and electrical power production at reduced levels while non-nuclear mechanical and/or electrical maintenance is underway. Total power plant shutdowns of about one month duration are recommended at 6 year intervals to permit nuclear fuel bundle rearrangement and intermediate heat exchange bundle service.

An important secondary sodium fire suppression feature is gravity drain down to a dedicated below grade dump tank for each heat transport circuit.
 

WORK BY OTHERS:
An early FNR was the Sodium Reactor Experiment.
This experiment suffered fuel tube failures due to:
a) Lack of sufficient gap between the fuel rods and the inside fuel tube wall to permit normal fuel swelling caused by fission product gas bubble formation;
b) Lack of sodium inside the fuel tube to efficiently transfer heat from the fuel rods to the fuel tube wall;
c) Lack of plenum space inside the fuel tubes for safe storage of compressed inert fission product gases;
d) Too thick a core zone for reactor power stability.

A later reactor was the EBR-2 which operated successfully for 30 years and was only taken out of service due to lack of continuing funding. The EBR-2 fuel design addressed all of the aforementioned fuel tube failure issues.

A later large sodium cooled reactor was the French:
Phenix
Phenix Guidez Statement.

A even larger sodium cooled reactor was the French:
Super Phenix
which once fully operational was closed for irrational political reasons.

A still later "paper FNR design" was the GEH PRISM (300 MWe). This reactor design was outlined in the papers: S-PRISM specs and TRIPLETT LOEWEN DOOIES-2012-PRISM. This "paper FNR" may have potential maintenance and power instability issues.

A still later "paper FNR design" is the proposed ARC-100 (100 MWe) from ARC Energy. Known fundamental problems with this "paper reactor" are lack of capacity and lack of fuel sustainability. The power stability, maintainability and waste disposal issues are not known to this author.

Another "paper FNR" is the Moltex SSR . The Moltex concept is to use molten salt fuel in fuel tubes and a different molten salt coolant instead of sodium. There are many related material challenges.

Major real Russian liquid sodium cooled power reactors include the
BN-600 (600 MWe) which has operated for over 30 years,
BN-800 (800 MWe) which has operated since late 2015
and
BN-1200 (1200 MWe) which is in developmental design review.

Today in 2021 the Chinese are building two CFR-600 (600 Mwe) sodium cooled power reactors in Xiapu County, Fujian province with the objective of operating with metallic fuel.
 

This web page last updated October 10, 2021

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