XYLENE POWER LTD.

FNR FACILITY

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

FNR FACILITY OVERVIEW:
This web page provides an overview of a practical urban sited 1000 MWt (300 MWe) Fast Neutron Reactor (FNR) Nuclear Power Plant (NPP). This facility includes the physical equipment necessary to safely supply electricity, high temperature industrial heat (450 degrees C) and low temperature district heat to an urban market of up to 100,000 people. The NPP is designed to fit within one city block to minimize the amount of realestate that must be obtained by expropriation. An additional 20 m wide perimeter access ring road is required.

The various related local control rooms, human support services, offices, spare component storage, maintenance facilities and parking facilities should be located in nearby off-site buildings. The arrangement of these items is generally not critical.

The NPP consists of a 49 m X 49 m robust central nuclear island surrounded by eight steam turbogenerator halls with four small cooling towers located at the site corners. Electrical distribution switchgear is mounted on top of the turbogenerator halls.

Heat moves from the nuclear island to the turbogenerator halls via underground pipe loops containing low pressure Heat Transfer Fluid (HTF) and low pressure molten nitrate salt.

The nuclear island contains a low pressure pool type liquid sodium cooled fast neutron reactor. Heat is transported via liquid NaK from heat exchange bundles immersed in the liquid sodium pool to heat exchange gallery mounted heat exchangers which isolate the NaK from the HTF and the molten nitrate salt. There is sufficient natural NaK circulation and sufficient on-site cooling tower capacity to remove fission product decay heat provided that HTF circulation is maintained.

When the reactor is normally operating off-site cooling towers connected to the buried low temperature district heating mains reject up to 525 MWt of heat.
 

A plan view of the physical plant is shown below.

AIR COOLING OF SERVICE SPACES INSIDE THE NUCLEAR ISLAND:
The nuclear island has service spaces under the dome, in the heat exchange galleries, outside the Na pool side wall and outside the Na pool and argon bladders in the basement.

The Na pool and the various pipes, walls and heat exchangers all leak heat to the service spaces. This heat is transferred to the outside via conventional air conditioning fluid loops which keep the service spaces inside the nuclear island airtight. The benefit of keeping these service spaces airtight is that in the event of a Na pool space ceiling or wall failure the concrete and dome enclosure will still keep airborne radio isotopes confined.

A controlled air port connects the service spaces to the outside. This port allows gradual air pressure balancing while maintaining airborne radio isotope confinement.
 

APPEARANCE:
In plan view an urban FNR physical plant is 114 m X 114 m square that fully occupies one city block. It is bounded by four perimeter roads, each at least 20 m wide. Much of the below grade space under these perimeter roads is occupied by 48 inch diameter district heating main headers and related 24 inch diameter pipe branch connections.

The FNR physical plant has a central nuclear island that is about 49 m X 49 m. Centrally positioned on top of the central nuclear island is a steel dome, ~ 31 m diameter with a radius of curvature of ~ 21.9 m and a maximum height above the the dome perimeter support walls of ~ 8 m.

Spaced along the straight edges of the FNR physical plant property are 8 turbogenerator halls.

At each corner of the FNR physical plant property is a 50 m high cooling tower that is 25 m wide at the base. These on-site cooling towers operate dry in cold weather (< 5 deg C) but wet in warm weather (> 5 deg C).

The nuclear island is separated from the other on-site structures by a ring lane 10 m wide. The nitrate salt and HTF pipes that convey heat from the nuclear island to the turbogenerator halls pass underneath this lane. For safety reasons there is no piped water connection to the nuclear island which contains large amounts of sodium and NaK.

There are 4 X 10 m wide radial lanes between the turbogenerator buildings that provide truck access to the four nuclear island airlocks.

The below grade district heating loop pipes are integral to the reactor heat rejection system. Each district heating loop connects to three distributed remote cooling towers.
 

PHYSICAL PLANT:
The FNR physical plant includes the following essential elements:
1) The FNR sodium pool in a double wall reinforced concrete octagonal protective enclosure. The sodium pool maintains a top surface temperature of 460 degrees C;
2) Eight heat exchange galleries around the perimeter of the FNR sodium pool enclosure that safely transfer controllable amounts of heat from the FNR's pressurized NaK loops to atmospheric pressure molten nitrate salt and heat transport fluid (HTF) loops. The heat exchange galleries safely contain and suppress any NaK fire and provide supplementary physical protection for the FNR;
3) Nine basement level argon bladders that maintain one atmosphere of argon pressure inside the FNR enclosure, independent of the FNR temperature;
4) Four air locks located between the heat exchange galleries that enable replacement of: fuel bundles, intermediate heat exchange bundles and other equipment in the reactor space while keeping air out of the reactor space.
5) A 31 m diameter constant external radius dome shaped steel roof over the primary sodium pool which protects the primary sodium pool from both extreme weather and missile attack and which contains airborne radio isotopes.
6) Four stairwells at the corners of the nuclear island that enable personnel access to the argon bladders, the heat exchange galleries, the polar gantry crane and equipment mounted in the roof space under the dome.
7) Perimeter and radial laneways, each 10 m wide, for delivery and removal of supplies, positioning of service cranes and short term vehicle parking;
8) Eight turbogenerator halls, that use heat from molten nitrate salt and HTF to produce high pressure steam, expand the steam through turbogenerators to produce electricity and condense the low pressure steam for district heating or heat rejection;
9) Four on-site cooling towers located at the site corners that together with twelve remote cooling towers are used for rejection of surplus low grade heat;
10) Four water reservoirs located under the cooling towers for emergency rejection of surplus heat by evaporation of water;
11) Electrical switchgear, protective devices and metering as necessary for interfacing the turbogenerators and parasitic loads with the external electricity grid, located in and above the turbogenerator halls;
12) Valves and metering located within the turbogenerator halls for interfacing with four district heating loops;
13) Control and monitoring systems that allow on-site or off-site personnel to view the status of all the operating equipment on FNR site, to selectively shut down and restart individual components and to communicate planned maintenance related information for power and maintenance dispatch.
14) Redundant cryogenic facilities for on-site liquid argon production.
15) Dual emergency power generators. At all times when the primary FNR sodium is above 120 degrees C there must be sufficient emergency power available to remove fission product decay heat from the primary sodium pool by natural circulation of NaK, pumped circulation of HTF and then evaporation of water by the HTF.
 

ELECTRICAL:
Each of the eight turbogenerators operates independently of all the others. There is a split station bus. The external electrical grid connect breaker goes to a common point which connects to breakers to each half of the station bus, breakers to each of two standby generators and two breakers to the two independent shutdown systems of the nuclear island. This common point is always energized except after a prolonged reactor shutdown.

Each half of the station bus contains four breakers to turbogenerator outputs, and four breakers to the associated non-nuclear parasitic loads.

Thus either one of the emergency generators can be used to start any one of the turbogenerators, then power from that turbogenerator can be used to start the remaining turbogenerators. At any stage during this process the station bus can be connected to the external electrical grid.

During normal operation any combination of turbogenerators can be taken out of service for maintenance provided that two remain in service to meet the station parasitic load if the external grid goes down.

If there is loss of the external grid the two standby generators should be started so that the station bus remains powered if something causes shutdown of the operating turbogenerators. Each of the two standby generators must have sufficient capacity to safely remove the reactor's fission product decay heat.
 

ELECTRICAL EQUIPMENT LOCATION:
The electrical switchgear associated with each electricity generator is located on the turbine hall roof above that generator. If a major transformer is required to interface the station bus voltage to the grid voltage that transformer may have to be located off-site.
 

HUMAN AND OTHER SUPPORT SERVICES:
Due to limited space on the NPP site the control rooms, human support and maintenance services are located in buildings across the road or otherwise as close as practical to the plant site. The space required for these support services is much larger during the construction period than during the subsequent operating period. At times when the FNR is shut down for major maintenance additional human support facilities will likely again be required.

The human support facilities include:
1) The control room for the nuclear island.
2) Four independent control rooms for the heat to electricity conversion equipment. This equipment is divided into four independent portions, each portion serving a different quadrent of the surrounding district heating system. In effect there are four independent 75 MWe power plants sharing the same common FNR primary sodium pool.
3) Washrooms;
4) Office space for employees and subcontractors;
5) Storage space for frequently required files, tools, supplies and spare parts.
6) A larger space suitable for on-site meetings and visitor presentations.
7) Cafeteria space;
8) Site security space;
9) Employee and visitor parking space;
10) Mobile crane parking space;
11) Construction material storage space;
12) Parking space for contractor trailers, subcontractor vehicles, etc.
 

FNR ENCLOSURE:
The primary sodium pool is a 20 m diameter pool of liquid sodium with a top surface temperature of about 460 degrees C. This pool is centrally located within an octagonal enclosure within a square enclosure. The main octagonal wall is 1 m thick reinforced concrete. This concrete enclosure inside dimension is 29.0 m face to face. There is a metal sheathed inner octagonal wall containing fiber ceramic thermal insulation. This inmer wall is 25 m inside face to face. The two walls absorb gamma ray emissions and completely exclude air and water, even under extreme storm conditions.

The octagonal concrete wall surrounding the primary sodium pool is stabilized on the outside by 12 X 1 m thick reinforced concrete shear walls, each extending radially 9 m from 17 m below grade to __ m above grade. These shear walls form the end walls of the heat exchange galleries.

The FNR sodium pool is further protected from low angle aircraft impact by the 1 m thick outside concrete wall of the heat exchange galleries and a by the ring of heat exchangers, dump tanks and induction pumps contained in the heat exchange galleries.

The FNR sodium pool is protected from overhead attack by a structural steel dome which rises 8.0 m above the top of the octagonal concrete wall.
 

SITE PLAN DESCRIPTION:
The physical plant site plan covers an area of 114 m X 114 m which is about one standard city block. This real estate parcel must be bordered by perimeter public roads, each at least 20 m wide. Much of the space under the four perimeter roads is dedicated to district heating pipe mains.

On the site plan the triangles at the corners of the nuclear island are the stairwells. The blue octagons at the site corners are cooling towers built over buried water tanks. The black rectangles around the site perimeter are steam generator/turbogenerator halls each of which contains steam generator(s), one 37.5 MWe turbogenerator and associated condenser, nitrate salt circulation pump and HTF circulation pump. Significant amounts of space in the turbogenerator halls must be reserved for district heating water pipes.

Electrical switchgear is located above the turbogenerator halls.
 

ARGON BLADDERS:
The liquid sodium pool cover gas is 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 that are surrounded by air at atmospheric pressure. As the argon over the sodium pool expands argon flows into the bladders via gas coolers. This simple argon pressure control system does not require AC power. Fail safe argon space pressure control is essential for FNR enclosure structural integrity.
 

HEAT EXCHANGE GALLERIES:
Abutting the FNR enclosure are the eight heat exchange galleries that extend 9.0 m out from each of the octagonal FNR enclosure side walls. The heat exchange gallery end walls provide 1 m thick shear walls, incorporate large amounts of concrete and steel and serve the secondary function of protecting the sodium pool from low angle airplane or artillary attack. The outside dimensions of the central nuclear building are:
49 m X 49 m

The purpose of the heat exchange galleries is to safely transfer controllable amounts of heat from 0.2 MPa to 0.8 MPa NaK to atmospheric pressure molten nitrate salt and atmospheric pressure HTF. The heat exchange galleries have provisions for immediate suppression of NaK and HTF fires.

The inside width of each heat exchange gallery is 8.0 m. Each heat exchange gallery contains 6 NaK-salt heat exchangers, 6 NaK-HTF heat exchangers and associated equipment spaced on 1.5 m centers.

The nitrate salt and HTF pipes are reverse return connected to the NaK-salt heat exchangers and Nak-HTF heat exchangers and are routed under the 10 m wide laneway to steam generator(s) located in the nearest steam generator/turbogenerator hall. The diagram below shows the piping for one of 48 identical heat transfer circuits.

The NaK-salt heat exchanger and NaK-HTF heat exchanger manifold end covers are removeable for service access and for interior welding inspection, so these end covers are free of pipe connections.

The high point of the molten nitrate salt circuit is at its upper connection to the NaK-salt heat exchanger shell. This high point is vented to the atmosphere via a ball check. Under accident conditions this vent can discharge hot molten salt, possibly containing NaOH, so the vent must have a protective shield.

The vents are designed to prevent molten nitrate salt and HTF hammer damage that might otherwise occur on the rupture of a steam generator heat exchange tube.

There is a 12.75 inch OD schedule 40 pipe that conveys NaK from each intermediate heat exchange bundle to the top manifold of each NaK-salt heat exchanger.

Directly under this pipe about 5 m further down is the induction pump. It pumps cooler secondary liquid NaK from near the bottom of the NaK-HT heat exchanger shell back toward the intermediate heat exchanger. The induction pump inlet pipe has provision for selective freezing to permit expulsion of liquid NaK from the companion intermediate heat exchanger. Just above the pool deck level there is a tee in the NaK circuit. The tee branch goes to the intermediate heat exchanger NaK inlet. The bottom port on the tee goes down to the NaK dump tank.

The bottom of the NaK-salt heat exchanger shell is connected to a nitrate salt dump tank. The purpose of this dump tank is to lower the nitrate salt level below the level of the NaK-salt heat exchanger tubes in the event of a tube leak. This lowering must occur even if the salt in the extended pipe from the steam generator is frozen. There must be certainty that when the NaK pressure drops there is no salt or water in the NaK-salt heat exchanger that could potentially back flow through a tube rupture into the secondary sodium circuit.

This level lowering in the nitrate salt circuit is achieved by releasing the air charge from the salt dump tank.

The NaK-salt and NaK-HTF heat exchangers must be located high enough above the top surface of the sodium pool to ensure that fission product decay heat can be safely extracted by natural circulation of NaK.

The equipment in the heat exchange galleries is supported by ~ 10 m long horizontal I beams that are embedded in the walls and by vertical I beam columns under the heat exchange gallery that take the load weight down to the basement level.

At an elevation below the sodium pool surface are dump tanks into which the NaK, nitrate salt and HTF can be transfered to permit equipment or pipe service.
 

FIRE PREVENTION:
The main means of sodium fire extinguishing is oxygen exclusion and argon cover to keep the sodium isolated from air. If there is a NaK fire it will be the result of NaK leaking out of a NaK circuit. The immediate remedy is to release argon pressure from the appropriate NaK dump tank and let gravity transfer that circuit's NaK into a dump tank. This transfer will asphyxiate a NaK fire very quickly.

Floating on top of the primary liquid sodium pool are spherical steel floats that reduce the exposed liquid sodium surface area. These floats are sized to permit the vertical indicator tubes to show the movable active fuel bundle status. The upper manifolds of the intermediate heat exchange bundles also cover part of the top surface of the primary sodium.

When the reactor is completely shut down, before the sodium argon cover gas removed, sodium surface should be flooded with kerosene to prevent oxidation. However, in the presence of air (oxygen) that kerosene is a potential fire hazard. The number one objective is to keep oxygen out of the enclosed primary sodium pool space. Any oxygen or water vapor that leaks in will likely eventually form Na2O or NaOH which must be filtered out of the primary sodium.
 

ROOF FUNCTIONS:
The main function of the FNR's dome roof structure is to exclude air and provide physical protection against precipitation, violent storms (hurricanes and tornados), long term corrosion and missle attack. The roof structure must also house forced air cooling equipment, and provide structural support for the inner walls, ceilings, gantry crane and electronic monitoring system.

The interior walls and ceilings contain the argon gas, contain heat and exclude air. The outer stainless steel covering on the ceiling also provides secondary protection against a dome rain water leak.

The inner ceiling must be high enough (14.5 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 and air cooling equipment.
 

GANTRY CRANE:
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 equipment transfer air locks).

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

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 equipment transfer 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 reliably connect and disconnect fixed fuel bundle corners together 6 m below the liquid sodium surface.
 

DEFENSE AGAINST A POTENTIAL AERIAL ATTACK:
The FNR dome 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. The dome contains two horizontal layers of sand bags.

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 enclosure failure.

If at any time air leaks into the argon cover gas the immediate requirement is to rapidly lower the primary liquid sodium temperature below 200 C to prevent spontaneous sodium combustion.

The overhead steel dome, 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. Within a few seconds the liquid NaK in the heat exchange galleries can be drained down to argon covered dump tanks to rapidly extinguish almost any NaK fire.
 

FNR DOME:
The sodium pool enclosure dome shaped roof is formed from structural steel. The dome 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.

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. The roof should have a constant external curvature to permit suchpatching. 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 preformed sheets of curved roof patch9ng material stored on top of the roof 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.

The dome interior space should have a liquid tight sloped floor that will guide any released liquid aircraft fuel to a steel pipe drain. That drain must go to a sewage dump tank that is nearly sealled.

A 300 MWe FNR has 8 fully independent heat removal systems connected to four independent cooling towers. This level of independence provides protection in depth against loss of cooling failures.
 

SAFE ACCESS TO ROOF SPACE:
Unrestricted safe access to the inside of the dome 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 packages.

This may involve a shielded route via the stairwells adjacent to 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.
 

SAFE ACCESS TO BELOW POOL DECK SPACE:
Outside the primary sodium pool outer cup wall is a greater than 1.0 m wide space for air cooling and for maintenance access to the bottom of the nested steel cups.
 

NON-NUCLEAR MAINTENANCE:
On-site personnel are required to do periodic routine non-nuclear preventive maintenance on the induction pumps, NaK-salt heat exchangers, NaK-HTF heat exchangers and vents in the heat exchange galleries.

There is also required maintenance on steam pressure regulating valves, lower manifold drain valves, water pumps, turbo-generators, condensers, cooling towers and related mechanical and electrical equipment in the surrounding turbogenerator halls. However, this maintenance work should not involve any potential exposure to dangerous radiation. There is sufficient redundancy in the intermediate heat exchangers that some of the heat transport systems can be shut down for later skilled maintenance or repair while others remain in operation. Thus the only reason for keeping staff on the FNR site 24/7 is compliance with steam power plant regulations and requirements for site security.
 

FISSION PRODUCT DECAY HEAT REMOVAL:
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 when there may be no externally suppled AC power or after a roof failure due to a military armour piercing bomb or jihadi attack.

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 secondary sodium circulation.

If the molten salt pumps fail the molten salt should immediately drain to its sump thank and be replaced by water. That water will lower the temperature in the shell of the sodium/salt heat exchanger to about 100 degrees C. The high temperature difference between the primary sodium and the 100 degree C water will drive a rapid natural circulation of secondary sodium, which will remove heat from the primary sodium by formation of steam in the salt circuit. This steam is vented via the salt circuit vents. In a real life emergency 48 of these steam vents would operate simultaneously. An important issue is that we do not want this steam to locally condense and fall back through a hole in the FNR roof to react with the primary liquid sodium. For this reason the primary sodium pool has a sloped roof so that liquid water cannot accumulate on top of it. The roof insulation should be poor such that snow and ice cannot accumulate on the roof.

During normal reactor operation for safety certainty at any instant in time at least 2 of the 8 heat exchange galleries associated with 2 different cooling towers should be kept 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.
2) The intermediate heat exchangers must always be at least partially immersed in liquid sodium. 3) The liquid sodium pool walls are designed such that if the inner and middle nested steel cup walls fail and the primary liquid sodium leaks into the space between the walls, the sodium 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) If the NaK induction pumps fail there must be enough natural NaK circulation to ensure safe removal of the fission product decay heat.
4) The NaK dumps its heat into NaK-salt and NaK-HTF heat exchangers. During a reactor cold shutdown the salt is used to remove heat until the salt approaches its freezing point at which time the NaK-HTF heat exchangers continue to remove heat provided that HTF circulation continues and the pressure setting in the steam generators is reduced.
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 evaporation of water.
 

CHEMICAL THREATS:
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 Na-24, Na2O and NaOH the big threat from a prolonged fire 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 primary 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 steel cup 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 and NaCl in silo storage for emergency use. The default means of stopping a NaK fire is NaK drain down into a dump tank. In each heat exchange gallery this drain down mechanism should be automatic. However, in no circumstance can all the heat transport capacity be taken out of service.

To mitigate the fire threat the liquid sodium pool surface is covered by floating steel balls, an argon cover atmosphere, a sodium vapor resistant and gas tight inner metal ceiling, and a gas tight middle and outer metal ceilings. In the event of air penetration into the argon cover gas the reactor should be immediately shut down and heat dumped from the liquid sodium pool to lower the liquid sodium temperature below 200 degrees C so as to prevent spantaneous combustion of sodium in air. As the argon temperature over the 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. Note that there is a small air leak between the bladder surround space and the outside.

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 liquid 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.

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 heat transport system. If there is a FNR roof failure it is essential to use dry cooling to prevent vented 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 roof repair material for temporary exclusion of rain or other water falling from overhead.
 

RESERVE STORAGE FOR FUEL BUNDLES AND SODIUM:
Eventually it may be necessary to do maintenance work on the 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. Ideally the reserve sodium storage might must also be able to accept hot active fuel bundles transferred from the liquid sodium pool. The best place to store these hot fuel bundles is in another nearby liquid sodium cooled reactor. The fuel bundle transport truck may need special provisions for fuel bundle cooling while driving.
 

COOLING TOWERS:
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 district heating water is about _____ degrees C. Generally there is a heat pump in every building to raise the temperature of the building heating water.

If the district heating load is smaller than the waste heat from electricity generation the surplus heat is rejected via distributed cooling towers.

However, in the event of a loss of power at the reactor location the circulation pumps of the district heating system may not operate. In these circumstances the reactor must be able to reject fission product decay heat at the reactor location using just two of its four on-site cooling towers.

In an emergency the cooling towers can be operated wet. 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.

At the reactor site there are four independent natural draft cooling towers. Each such cooling tower is sized and piped to safely reject at least half of the fission product decay heat, 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 cooling tower pumps. Once there is remote heat rejection capacity the reactor power can be increased.

The minimum required continuous cooling capacity is:
700 MWt / 16 cooling towers = 43.75 MWt / tower.

Each remote cooling tower should be installed on a 38 m X 38 m remote property so as to provide a 6.5 m setback from the property line to the 25 m diameter base of the cooling tower. These cooling towers should be spaced along and close to the district heating main distribution pipe routes. Typically the cooling tower sites must be obtained by purchase/expropriation of two adjacent 19 m X 38 m single family home residential properties.
 

WATER RESERVIORS
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 capacity 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 dry cooling tower 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.

That is sufficient for disposing of fission product decay heat.

During the course of this disaster situation it is necessary to lift:
39.270 X 10^6 kg water / 352 hours = 111.56 tonnes / hour of water from the storage location to the top of the steam generators. That requires absolutely reliable standby power generation. The required pumping power is:
111.56 X 10^3 kg X 40 m X 9.8 m / s^2 X 1 / 3600 s
= (111.56 X 40 X 9.8) / 3.6 kg m^2 / s^3
= 12141 (kg m^2 / s^2) / s
= 12,141 J / s
= 12.141 kW

Allowing for motor and pump inefficiencies redundant 25 kW pumps are required.
 

FNR FACILITY FOOTPRINT:
The nuclear building is 49 m X 49 m. There are 10 m wide laneways 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.

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 small cooling towers, each with base 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. 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.

Most of the other FNR facilities such as argon cryosystems, electrical switch gear, etc. are located above the 8 turbogenerator halls. Each turbogenerator hall has internal dimensions of about 21.5 m X 26.0 m that must be shared by six steam generators, a 37.5 MWe turbo generator, condenser, injection water pumps, condenser cooling pumps, district heating loop isolation valves and associated equipment. An issue of outstanding concern are details of the required provisions for turbogenerator and condenser installation. Note that the district heating water circulation pumps are remotely located at the thermal loads.
 

The site real estate requirement is one square city block 114 m X 114 m. The perimeter road allowance width 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.

Note that the 48 inch diameter district heating loop headers are under the perimeter roads. The district heating pipes entering the turbogenerator halls are each 24 inch diameter. It is likely that the existing buried services under the perimeter roads will have to be rearranged.

The length of a line through the center of the reactor is:
2 (15.5 m + 9.0 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 road allowance 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, will increase the total required property area.
 

This web page last updated May 31, 2023