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

This web page provides an overview of an 875 MWt (280 MWe) Fast Neutron Reactor (FNR) facility. This facility includes everything necessary to safely supply electricity and district heat to an urban market. This facility includes the following essential elements:
1) The FNR in a suitable protective enclosure. The FNR provides a pool of liquid sodium that maintains a top surface temperature of 500 degrees C and a bottom temperature of 340 degrees C; 2) Four adjacent argon bladder silos that maintain one atmosphere of absolute argon pressure inside the FNR enclosure, independent of the FNR temperature;
3) Eight heat exchange galleries that safely transfer controllable amounts of heat from the FNR's low pressure primary liquid sodium pool to high pressure non-radioactive steam, that safely contain and suppress any sodium fire and that provide supplementary physical protection for the FNR;
4) Ten metre wide access laneways for delivery and removal of supplies, location of service cranes and vehicle parking;
5) Eight turbogenerator halls, each with two turbogenerators, that convert high pressure steam into electricity and district heat;
6) Four dry cooling towers for on-going rejection of surplus heat;
7) Four water reservoirs for emergency rejection of surplus heat by evaporation;
8) Electrical switchgear, transformers, protective devices and metering as necessary for interfacing with the electricity grid;
9) Pumps, valves and metering as necessary for interfacing with the local district heating system;
10) A local control and monitoring room that allows on-site employees to view the status of all the operating equipment on site, to selectively shut down and restart individual equipment and to communicate planned maintenance related information to central power and maintenance dispatch.
11) Storage space for frequently required files, tools, supplies and spare parts.
12) An on-site meeting space suitable for on site meetings, employee work breaks and visitor presentations.

The primary sodium pool is a 20 m diameter pool of liquid metal with a top surface temperature of 500 degrees C. This pool is contined in a robust 30 m X 30 m concrete and steel enclosure that reliably contains sodium and argon, absorbs gamma ray emissions, and completely excludes air and water, even under extreme storm conditions.

The FNR enclosure's concrete walls surrounding the primary sodium pool are stabilized on the outside by about 17 m of surrounding ground depth and by the 16 1 m thick reinforced concrete shear walls of the heat exchange galleries.

The primary sodium pool enclosure is in the middle of the FNR Site Plan shown below.



The site plan covers an area of 114 m X 114 m which is about one standard city block. It is presumed that this real estate parcel isbordered by 20 m wide public perimeter roads.

On the site plan the green octagons at the corners of the FNR enclosure are argon silos and the blue octagons at the site corners are cooling towers. The black rectangles around the site perimeter are turbogenerator halls each of which contains two turbogenerators and associated condensers. Electrical gear is located above the turbogenerators.

The four 9.5 m X 9.5 m corner spaces adjacent to the FNR enclosure are for argon bladder silos. These silos extend from the foundation surface about 14 m below grade to 16 m above grade.

The enclosed space over the liquid sodium pool is filled with the inert gas argon which will not chemically react with either the liquid sodium or steel. The argon pressure is maintained at one atmosphere by argon filled bladders in the corner silos. This simple argon pressure control system does not require AC power. Fail safe argon space pressure control is essential for FNR enclosure structural integrity.

The argon cover gas volume over the primary sodium pool is about:
26 m X 26 m X 14 m = 9464 m^3.

The argon bladder silos are each 9.0 m inside diameter X 34 m high and collectively contain up to:
4 X Pi(9.0 m / 2)^2 X 34 m = 8652 m^3
of ambient temperature ambient atmospheric pressure argon. This stored argon maintains cover gas pressure during reactor cold shutdowns when the volume of argon cover gas over the reactor thermally contracts about two fold. There is a surplus of stored argon to allow for unplanned emergency events.

Outside the FNR enclosure are the eight heat exchange galleries that extend 9.5 m out from each of the 30 m long FNR enclosure side walls. The heat exchange galleries have shear walls, incorporate large amounts of concrete and steel and serve the secondary function of protecting the FNR from low level airplane attack.

The purpose of the heat exchange galleries is to safely transfer controllable amounts of heat from the primary sodium pool to high pressure non-radioactive steam. The heat exchange galleries have provisions for immediate suppression of sodium fires.

The primary means of sodium fire extinguishing is default drain down of liquid sodium into isolated argon covered tanks.

Floating on top of the primary liquid sodium pool are shallow draught square steel floats that reduce the exposed liquid sodium surface area of by about 99%. These floats have holes through them for the vertical indicator tubes which show mobile aactive fuel bundle status.

If the reactor is to be completely shut down and the argon cover gas potentially removed for an extended period the sodium surface should be flooded with kerosene to prevent sodium oxidation. However, in the presence of air that kerosene is also a potential fire hazard. The number one objective is to keep oxygen out of the enclosed pool space. Any oxygen or water vapor that leaks in will gradually form Na2O or NaOH which must be filtered out of the primary sodium.

The structural concrete normally remains at ambient temperature. The main function of the concrete is to attenuate gamma radiation emitted by sodium pool Na-24 decay while the reactor is operating. Other functions of the concrete include:
1) Exclusion of ground water;
2) Exclusion of rain water;
3) Exclusion of flood water;
4) Protection from physical attack;
5) Reserve fire containment;
6) Supporting the roof structure;
7) Guiding cooling air flow;
8) Gamma radiation absorption.

The functions of the inner sheet stainless steel wall include:
1) Sodium vapor containment;
2) Fiber-frax insulation containment.

The functions of the outer stainless steel wall include:
1) Air exclusion;
2) Argon cover gas containment;
3) Fiber-frax insulation containment.

The functions of the air gap around the sodium pool include:
1) Space for circulation of cooling air flow;
2) Space for inspection and service access;
3) Space for emergency sump collection and pumping of either liquid sodium or water.

The primary function of the FNR's external roof structure is to provide physical protection against violent storms (hurricanes and tornados), long term corrosion and deliberate aerial attack. It must also reliably exclude rain water, house forced air ventilation equipment, and provide structural support for the inner wall, ceiling, gantry crane and electronic monitoring system.

The inner ceiling contains the argon gas and 1 m of fiber frax insulation to contain heat. The outer stainless steel covering on the inner ceiling contins argon and provides secondary protection against a roof rain water leak.

The inner ceiling must be high enough (12 m above the pool deck) to allow fuel bundle repositioning and replacement using the internal gantry crane. The primary sodium pool enclosure must be gas tight and must dependably exclude both air and rain water under the most adverse circumstances.

Between the steel roof structure and the top of the inner ceiling is a 2 m high space to allow easy access to the ceiling mounted reactor monitoring system.

The gantry crane must work reliably after being in the 500 degree C sodium vapor environment for an extended period of time. Temperature sensitive components are removed when the gantry crane is not in use. (check that these components will easily fit through the air lock).

The roof structure supports a line of hangers with thermal breaks (ceramic egg insulators) that in turn support the interior walls and ceiling and the two longitudinal gantry crane tracks. These tracks are further stabilized to the outside wall via thermal breaks.

The gantry crane transverse track is perpendicular to the roof ridge line. The gantry crane transverse track is moved parallel to the roof ridge line by steel cables that run through small holes to a cooler argon filled space containing the gantry crane winches. (Check that these winches will fit within the available 1 m wide space)

THe gantry crane cross trolley movement is also controlled by a similar cable system with a remote winch in a cool argon filled space.(Check that thi9s winch will fit in the available space)

The gantry crane remote manipulator attachment is only used when the primary liquid sodium pool is relatively cool (120 degrees C). When the reactor is operating this remote manipulator is kept in cool storage. The remote manipulator is designed to fit through the air locks. The detail of the connection methodology between the gantry crane remote manipulator and the gantry crane trolley remains to be resolved. Also the detail of how to power the gantry crane remote manipulator needs to be resolved (will teflon insulated cable work?). This remote manipulator must be able to precisely place fuel bundles and bolt fixed fuel bundle corners together 3 m below the liquid sodium surface.

The reactor enclosure outer roof must be strong enough to safely absorb an aircraft crash. It will likely have a structual steel content comparable to a major highway intersection overpass. There might be some merit in installing additional shock absorbing cables in the roof space.

If an imminent threat to the FNR from an air born object is detected the FNR should be immediately cool shut down. To the extent that time permits sufficient heat should be extracted from the primary liquid sodium pool to prevent spontaneous combustion of sodium with air if there is a major roof failure.

If at any time air leaks into the argon cover gas the immediate requirement is to lower the primary liquid sodium temperature below 200 C to prevent spontaneous sodium combustion. That condition is achieved by reactor cold shutdown which involves by openning the steam generator pressure control valves and flooding the steam generators with water which will lower the secondary sodium return temperature to about 110 degrees C and hence the primary sodium return temperature to about 120 degrees C.

The strong concrete and steel structure of the heat exchange galleries located around the FNR enclosure perimeter, the related shear walls and the corner silos containing the argon bladders make the FNR enclosure very resistant to a airborne physical attack against its sides. Within a few seconds the liquid sodium in the heat exchange galleries can be drained down to argon covered storage tanks to rapidly extinguish almost any sodium fire.

The sodium pool enclosure roof members are structural steel. The roof design is constrained by the use of prefabricated structural steel beams that are limited by road and rail transportation constraints to ~ 15.8 m in length. This objective is achieved by an "A" frame like roof with a rise over run of 0.5. Then the length of the roof rafters is:
14 m (5^0.5 / 2) = 15.65 m
A flat top ridge line provides about 2 m of extra width.

Engineering a roof that can withstand a deliberate direct overhead air attack by a precision guided armour penetrating bomb is almost impossible. If such attacks are a credible risk it may ultimately be necessary to locate FNRs deep underground to provide certain security against such intentional overhead military attacks.

A reasonable compromise is to locate the liquid sodium pool for the new FNR such that in an emergency an argon gas cover can be maintained over the liquid sodium pool while a temporary new roof cover is applied. In such circumstances the liquid sodium pool must be cooled below 140 degrees C and then covered with a kerosene to prevent sodium combustion with air. Circumstances that might lead to such a roof failure include a direct overhead impact by an armour penetrating bomb, missle or meteorite.

There should be a sufficiently large supply of stored argon on-site to prevent sodium combustion while the liquid sodium temperature is being reduced following a sudden major roof failure.

There should be wide rolls of thin sheet metal stored in the roof space together with appropriate tools and supplies sufficient for temporarily blocking of any hole in the ceiling caused by a penetrating missle. In order to execute this fix the gamma radiation in the roof space from Na-24 in the primary sodium pool must be quantified. There may need to be a shielded access route and work cart also used for accessing the reactor monitoring system and roof space mounted ventilation equipment.

A 280 MWe FNR has 16 fully independent heat removal systems connected to four independent cooling towers. This level of independence provides protection in depth against loss of cooling failures.

Unrestricted safe access to the inside roof structure is only available after the reactor has been shut down for a week so that the gamma emission from Na-24 in the liquid sodium is low. Hence provision should be made for safe servicing of the ceiling mounted electronics package. This may involve a shielded route via the stairwell in one of the heat exchange galleries and a shielded cat walk and/or work cart. The monitoring electroncs package might be mounted on a track which moves it from a shielded area to the center of the ceiling over the primary sodium pool. The worst case level of Na-24 gamma radiation in the ceiling space needs to be identified.

Outside the primary sodium pool outer cup wall is a greater than 1 m wide space for air cooling, earthquake shake clearance and for maintenance access to the below pool ventilation space.

There is some space available in the corner spaces between the 28 m X 28 m concrete enclosure inner wall and the 24 m outside diameter primary sodium pool outer cup. In order to safely access this space below the pool deck level it may be necessary to drain the radioactive primary sodium from some of the overhead primary sodium pipes connecting to the intermediate heat exchangers. These spaces should be used by the drives for the induction pumps located in the adjacent heat exchange galleries.

On-site personnel are required to do periodic routine non-nuclear preventive maintenance on the intermediate heat exchangers, induction pumps, steam generators, turbo-generators, condensers, cooling towers and related mechanical and electrical equipment and to make repairs as necessary. However, this maintenace work should not involve any exposure to dangerous radiation. There is sufficient redundancy in the FNR support equipment that some of the heat transport systems can be shut down for maintenance or repair while others remain in operation. Thus the only reason for keeping staff on the reactor site 24/7 is compliance with steam power plant regulations and maintenance of site security.

One of the most important aspects of fission reactor design is provision for fission product decay heat removal under adverse circumstances. If some event occurs which causes a reactor shutdown the fission products will continue to produce decay heat at up to 8% of the reactor's full power rating. It is essential to have a 100% reliable means of ensuring ongoing removal of the fission product decay heat under adverse conditions such as shortly after a severe earthquake.

There must be certainty about passive removal of this heat. A FNR has multiple independent heat transport systems, proper operation of 8% of which is sufficient for safe fission product decay heat removal. In the event of loss of station power fission product decay heat removal should be achieved by natural fluid circulation with just half of the heat transport systems in service. However, the steam generators will be flooded with water at 100 degrees C which provides good natural circulation of the secondary sodium.

During normal reactor operation for safety certainty at any instant in time at least 2 of the 8 heat exchange galleries should be operational.

For a liquid sodium cooled FNR all heat removal is via primary liquid sodium, so it is essential that:
1) Under no circumstances will the primary liquid sodium level ever fall to the point that the fuel rods are not fully immersed in liquid sodium or where primary sodium thermal siphon operation is lost.
2) The liquid sodium pool walls are designed such that if the inner and middle nested cup walls fail and the primary liquid sodium leaks into the space between the walls, the leakage into the space between the walls will not lower the primary liquid sodium level below the tops of the fuel tubes or below the primary sodium pipe inlets.
3) Even if the intermediate loop sodium induction pumps fail when the steam generators are at 100 degrees C there is enough intermediate liquid sodium natural circulation to ensure safe removal of the fission product decay heat.
4) The intermediate liquid sodium dumps its heat into steam generators. During a reactor cold shutdown the pressure in the steam generators is released to the atmosphere so that the steam generator injection water pump does not face a pressure load.
5) There must be enough clean water in on-site storage such that in an emergency the fission product decay heat can be removed by evaporating that water. The cooling towers normally operate dry with pumped steam condenser cooling water circulating through the coils at the bottom of the cooling tower.

The main chemical threat from a power FNR is the 3770 m^3 of liquid sodium contained in the primary sodium pool. If this liquid sodium contacts water there will be an explosive chemical reaction which liberates hydrogen that will spontaneously ignite in an air atmosphere. Hence one of the main issues in FNR application is choice of a reactor site where the sodium will NEVER be exposed to flood water.

Even if the reactor enclosure floods 14 m deep the outer stainless steel wall around the primary sodium pool should prevent any contact between water and the primary sodium.

The other main potential threat is a sodium fire. Quite apart from the release of Na2O and NaOH the big threat is melting of the fuel tubes leading to potential release of air borne plutonium and fission products. It is essential that the reactor be designed and sited such that a large sustained sodium fire cannot occur. In order to extinguish a sodium fire the oxygen concentration over the sodium must be minimized and heat must be extracted from the sodium. Under no circumstances can water be allowed to contact the primary sodium.

The soil and bedrock outside the concrete enclosure should be sufficiently dry, dense and stable to safely contain the liquid sodium in the unlikely event that a major earthquake ruptures the inner, middle and outer stainless steel walls of the liquid sodium pool and cracks the enclosing concrete wall.

It is equally important that there be an effective non-water based fire suppression system. The local fire department must be trained that water should NEVER be used to fight a FNR fire. Inappropriate use of water carried by a fire truck could change a minor fire into a major disaster. The FNR is provided with excess argon in silo storage for emergency use.

To mitigate the fire threat the primary liquid sodium is covered by floating steel covers, an argon cover atmosphere, a sodium vapor resistant inner metal ceiling, and a gas tight outer metal ceiling. In the event of air penetration into the argon cover gas the reactor should be immediately shut down and heat dumped from the primary liquid sodium pool to lower the primary liquid sodium temperature below 200 degrees C, to prevent spantaneous combustion of sodium in air. As the argon temperature over the primary sodium pool decreases stored argon from bladders in the argon storage silos is added to the cover gas to maintain the 1 atmosphere pressure in the argon cover gas.

Once the liquid sodium temperature is below 140 degrees C the surface of the liquid sodium can be flooded with a thin layer of kerosene to prevent the liquid sodium oxidizing during prolonged work such as roof repair.

Similarly if there is an enclosure roof failure the immediate objective is to extract heat from the sodium to reduce its temperature to the point where kerosene can be safely used to prevent sodium oxidation. Until the heat is removed from the sodium argon must be used to exclude oxygen from the sodium surface. That heat extraction might easily take half an hour, depending on the available heat sinking capacity. The fastest way to emergency cool the system is to directly vent steam from the steam generators. It is important to have enough water in on-site tank storage to remove the fission product decay heat by latent heat of vaporization. Then the limiting factor is the maximum safe heat transfer capacity of the intermediate heat exchanger tube bundles and the steam generator tube bundles. If there is a FNR roof failure it is essential to prevent this steam condensing and falling onto the exposed liquid sodium surface. This issue highlights the importance of FNR enclosure roof and ceiling integrity and immediate availability of material for temporary exclusion of rain or other water falling from overhead.

Sooner or later it will be necessary to do maintenance work on the primary sodium pool inner wall. To do such work it will be necessary to remove all the fuel bundles and the primary liquid sodium from the primary sodium pool. Hence there must be nearby sodium storage with sufficient capacity to hold the entire volume of the primary liquid sodium while the aforementioned maintenance work is being carried out. The reserve sodium storage might must also be able to accept hot radioactive fuel bundles transferred from the primary liquid sodium pool. The best place to store these hot fuel bundles is in another nearby liquid sodium cooled reactor. The transport truck may need special provisions for fuel bundle cooling while driving.

During the winter much of the reactor waste heat output from electricity generation is dumped to a water based district heating system. The highest temperature of this water is about 120 degrees C. If the district heating load is larger than can be met with waste heat from enabled electricity generation the electricity generators can be additionally loaded by heat pumps that further extract heat from the district heating water. If the district heating load is smaller than the waste heat from electricity generation the surplus heat is rejected via roof top fan coil units installed at the remote load locations and via the cooling towers at the reactor site. The remote fan coil units are powered via dedicated circuits that run along side the district heating pipes from the reactor location. Thus the remote building owner is not responsible for the electricity load imposed by remote fan coil units that are operated for the benefit of the reactor owner rather than for the benefit of the building owner.

However, in the event of a loss of power at the reactor location this remote heat rejection methodology will cease operating because both the fans of the fan coil units and the circulation pumps of the district heating system will not operate. In these circumstances the reactor must be able to reject fission product decay heat at the reactor location using local natural draft cooling towers and stored cooling water.

At the reactor site there are four independent natural draft dry cooling towers. Each such cooling tower is sized and piped to safely reject the fission product decay heat by natural circulation even if the other cooling towers are out of service. Assuming that all cooling towers remain in service the additional heat rejection capacity will enable system black start which will include energizing the district heating system circulating pumps and the remote fan coil units. Once there is remote heat rejection capacity the reactor power can be increased.

The minimum required continuous dry cooling capacity per cooling tower is:
(2 / 3) X 875 MWt X (1 / 4 towers) = 146 MWt / tower.

In an emergency the dry cooling towers can be operated as wet cooling towers. The resulting steam plume may not thrill the neighbours, but it is far better than a thermal meltdown resulting from inability to safely reject fission product decay heat.

It is prudent to have on the reactor site a certain source of water sufficient to remove fission product decay heat by evaporation in emergency circumstances when the normal heat sink such as a dry cooling tower is unavailable. The reserve water storage reservoirs can be located underneath each cooling tower.

Assume that emergency cooling water is stored in four on-site below grade cylindrical tanks, each 20 m high by 25 m diameter, located directly below the cooling towers. Then the volume of water immediately available on-site for emergency cooling is:
4 X Pi (25 m / 2)^2 X 20 m
= 39,270 m^3
= 39.270 X 10^6 kg


The latent heat of vaporization of water is:
22.6 X 10^5 J / kg

Hence evaporation of this stored water requires: 22.6 X 10^5 J / kg X 39.270 X 10^6 kg
= 887.5 X 10^11 J

Without any use of the cooling tower dry cooling capacity that amount of water is sufficient to remove maximum fission product decay heat of:
0.08 X 875 MWt = 70 MWt for:
887.5 X 10^11 J / (70 X 10^6 J / s)
= 12.68 X 10^5 s
= 12.68 X 10^5 s / (3600 s / h)
= 352 hours.

Hence no matter what the disaster, it is essential to either replenish the stored emergency cooling water or restore 50% operation of at least one of the four cooling towers within 352 hours of the disaster.

The primary sodium pool enclosure is a square 30 m / side. Along each 30 m face are two heat exchange galleries, each with external dimensions 14.5 m long X 9.5 m wide. The four 9.5 m X 9.5 m corner squares are reserved for argon bladder silos. There is a 1 m wide space for the air lock. At the outer ends of the heat exchange galeries are stairwells to allow access to the bottom of the adjacent heat exchange galleries and to the reactor roof space.

Outside each heat exchange gallery face is a 10 m wide laneway allowance for heavy equipment delivery and removal, so before considering the turbogenerator halls, cooling towers, electrical switch gear, water reservoirs and laneway space for trucks approaching air locks the minimum required land area is a square 69 m X 69 m. This space includes some employee parking under the steam pipes connecting the heat exchange galleries to the turbogenerator halls.

It is reasonable to contemplate a total FNR power plant land area of 114 m X 114 m, which is approximately one city block. That space allows for up to 8 turbogenerator halls, each with outside dimensions of 22.5 m X 27.0 m and four cooling towers, each with outside dimensions 25 m X 25 m. The emergency cooling water tanks are located directly under the cooling towers.

There are four 10 m wide X 22.5 m long access passages between pairs of turbine halls. A truck delivering or removing a fuel bundle will block one of these passages. Note that the passages are wide enough to permit delivery of the argon silos before the steam pipes are installed. The turbogenerators should be oriented so that the generator portion faces the access passage. Then the rotor pull allowance on both sides of the access passage will provide space for lifting the turbogenerators and rotating them into place.

All other FNR facilities such as control rooms, argon cryosystems, electrical switch gear, administration, etc. are located above the 8 turbine halls. Each turbogenerator hall has internal dimnsions of about 21.5 m X 26.0 m that must be shared by 2 X 20 MWe turbo generators, condensers, injection water pumps and associated equipment. An issue of outstanding concern are details of the required provisions for turbogenerator and condenser installation. It is presently assumed that in each turbogenerator hall the turbines are installed in parallel and the space allowance for condenser tube pulling is shared by the two turbines.

The real estate requirement is one square city block 114 m X 114 m. A road allowance is 20 m. Then 12 city blocks have a length of:
12 (114 m + 20 m) = 1608 m
= 5275.6 feet.
There are 5280 feet / mile.

There may be some tolerance in the turbogenerator hall dimensions depending on the exact equipment used.

The length of a line through the center of the reactor is:
2 (15 m + 9.5 m + 10 m + 22.5 m) = 114 m

The length of a line along one property face is:
2(25 m + 27.0 m) + 10 m = 114 m

Note that with these dimensions there is no allowance for setback from the adjacent public roads or sidewalks and that a truck delivering or picking up a fuel bundle will have to back into an access passage. While lining up the truck with the access passage it will block the public road.

Note that some main road allowances, instead of being 66 feet (20.11 m) are instead 86 feet (26.2 m) or 100 feet (30.48m). Such wider road allowances, if adjacent, could substantially reduce the generation capacity that can be installed at a particular FNR site.

This web page last updated August 30, 2020

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