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

This web page focuses on the FNR enclosure. The primary sodium pool must have a robust enclosure that reliably contains sodium and argon, contains gamma ray emissions, and excludes air and water, even under extremely adverse conditions.

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 contained within a robust 30 m X 30 m concrete and steel enclosure.

The FNR enclosure's 1 m thick reinforced concrete walls surrounding the primary sodium pool are stabilized on the outside below grade by about 19 m of surrounding ground depth and both above and below grade by the 16 X 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.


A side elevation of the FNR Enclosure is shown below:

Surrounding the primary sodium pool is a square reinforced concrete wall 28 m X 28 m inside, 30 m X 30 m outside, which extends from the foundation top surface 17 m below grade to 17 m above grade. This wall provides protection against storms and external attack, supports the roof structure and its loads, absorbs above pool level gamma emissions from Na-24 and forms one wall of the heat exchange galleries.

Immediately inside this concrete wall is a 1 m wide access space which also provides an air flow path for cooling and space for relative lateral FNR primary sodium pool movement in an earthquake.

The inner wall (shown in faint purple) is a 1 m thick layer of rigid fiber-frax insulation covered on both the inside and outside by sheet stainless steel. This inner wall is supported from the top so that it can flex to allow thermal expansion and contraction. The pool deck is slightly sloped toward the pool so that under normal circumstances sodium vapor condensation on the inside walls drains back into the sodium pool. At the edge of the sodium pool is a safety rail to prevent a suited worker slipping into the sodium pool.

Radiating out from the edge of the primary sodium pool are 112 secondary sodium pipes going from the intermediate heat exchangers to the equipment in the adjacent heat exchange galleries.

On the site plan the green octagons are argon silos and the blue octagons are cooling towers. The black rectangles around the diagram perimeter are turbogenerator halls each containing two turbogenerators.

The main components of the FNR enclosure height are:
Footings = 2 m
Ventilation space under pool = 1 m
Supporting Fire brick = 2 m
Primary sodium depth = 15 m
Sodium surface to pool deck = 1 m
Pool deck to inner ceiling = 12 m
Ceiling insulation = 1 m
Flat open ceiling space for ventilation, instrumentation and roof structural access = 2 m
Roof rise = 7 m

The sodium pool is mostly below grade. The top 2 m of the pool walls are above grade to prevent surface flood damage and to enable easy horizontal transfer of irradiated fuel bundles and intermediate heat exchangers via an airlock between the space over the sodium pool and flat deck truck mounted horizontal shielded fuel bundle shipping containers.

The pool deck is a 24 m X 24 m almost flat plate with the 20 m diameter primary sodium pool at its center.

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 located in corner silos. This simple argon pressure control system does not require AC power. Fail safe argon pressure control is essential for reactor 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 30 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 pressure argon. This stored argon maintains cover gas pressure during reactor cold shutdowns when the volume of argon cover gas normally over the reactor shrinks more than two fold. There is a surplus of stored argon to allow for unplanned emergency events.

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 to allow passage of the vertical indicator tubes which show reactor status.

If the reactor is to be 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, that kerosene is a potential fire hazard and is difficult to remove. 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 concrete portion of the FNR enclosure 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, the ceramic fiber insulation ceiloing and wall, the gantry crane and the fuel bundle electronic monitoring systems;
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 and outside the inner wall 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 extermal roof structure is to provide physical protection against violent storms (hurricanes and tornados) and aerial attack. It must also exclude rain water, house forced air ventialtion equipment, and provide structural support for the inner wall, ceiling, gantry crane, gantry crane load 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 contains the argon and provides secondary protection against a roof rain water leak.

The inner ceiling must be high enough to allow fuel bundle and intermediate heat exchanger 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 open space to allow easy access to the ceiling mounted reactor monitoring system.

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 subsequent major roof failure.

If at any time a significant amount of 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 of an alarm the liquid sodium in the heat exchange galleries can drained down into below grade argon covered dump 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 fission product decay cooling.

It is anticipated that the reactor enclosure inner ceiling will have an inside height above the pool deck of 12.0 m. Assuming that the gantry crane I beam rails are supported by the roof structrure. These rails are almost 26 m long. These I beam tracks will need to be fabricated by field joining of two shorter I beam lengths. The transverse I beam track may have to be delivered by helicopter due to its almost 24 m length. There is a further complication related to mounting the monitoring electronics package. The electronics package will consist of two parts, one which illuminates the fuel bundle indicator tubes and one which receives and processes data from the indicator tubes. The electronics packages will require continuous cooling. Loss of this cooling must trigger a reactor cool shutdown.

The inner ceiling over the primary sodium pool should be supported by hangers attached to the the outer ceiling and outer structural roof via thermal breaks. The inner ceiling is made from sheet stainless steel and normally operates at about 500 degrees C. On top of the inner ceiling is a 1 m thick layer of high temperature rated fiber ceramic insulation so that the space between the outer metal ceiling and the structural roof is relatively cool. This space is normally kept cool by a forced air flow which enables service access to the ceiling mounted reactor monitoring system.

Above the pool deck there are both inner and outer sheet stainless steel walls separated by 1 m of compressible ceramic fiber thermal insulation. The outer sheet stainless steel wall must be continuously edge welded and bubble tested to be gas tight. There is a gas tight connection between the pool deck and the inner steel wall of the primary sodium pool.

The wall must flex to allow thermal expansion and contraction. Above the pool deck there is considerable side wall flexing due to thermal expansion and contraction of the secondary sodium pipes and their argon filled jackets.

The space containing the fiber frax ceramic insulation is filled with argon. The inner sheet stainless steel wall is used to contain the neutron activated sodium vapor. The outer sheet stainless steel wall is used to contain the argon and exclude air. The inner sheet stainless steel wall normally operates at about 500 deg C and the outer sheet stainless steel wall normally operates at ambient temperature.

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 safe access 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.

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 this 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 6 m below the liquid sodium surface.

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 wall. These spaces should be used by the drives for the induction pumps located in the adjacent heat exchange galleries.

There is an inner steel primary sodium steel cup, a middle steel primary sodium steel cup and an outer primary sodium zteel cup. As long as at least one of these steel cups maintains its physical integrity the liquid sodium will be sufficiently contained to maintain its minimum required level for fission product decay heat removal following FNR shutdown.

During normal reactor operation there is no requirement for personnel to enter the argon filled pool enclosure. Such entry should only occur after the reactor has been in cold shut down for at least a week to allow Na-24 gamma emission and the pool temperature to subside. Even so such entry requires personal protectrive equipment against the ~ 120 degree C temperature and closed circuit breathing equipment.

The fuel bundles are centrally positioned in a 20 m diameter X 15 m deep liquid sodium pool. A pool 1.7 m wide band of liquid sodium at the perimeter of the liquid sodium pool is dedicated to intermediate heat exchangers and secondary sodium pipes. The remaining primary sodium guard band serves as a fuel bundle service access corridor.

The central core region of the reactor together with the top and bottom blanket regions involve 640 _____vertical active fuel bundles. Each fuel bundle is 6.0 m high and has an additional 1.5 m bottom projection. Fixed fuel bundles are supported by 1.5 m high legs (corner girder bottom extensions on the fixed fuel bundles) that plug into sockets fixed to the open steel lattice located near the bottom of the primary sodium pool. Each mobile fuel bundle has a 1.5 m bottom probe and a removeable 7.5 m tall top indicator tube.

The mobile fuel bundles slide into the fixed fuel bundle matrix from the bottom. The insertion distance is set by a liquid sodium hydraulic piston actuator with 1.2 m of travel. For each mobile fuel bundle an indicator tube projects above the primary liquid sodium surface to indicates the mobile fuel bundle's actual vertical position. This vertical position is constantly monitored using an overhead device similar to a laser measuring tape.

The reactor core region is surrounded on its outer perimeter by a 1.33 m thick blanket formed from 4 rings of passive fuel bundles.

There are two further outer rings of used fuel bundles in which natural decay of fission products occurs over a six year period before the fuel bundles are removed from the primary liquid sodium pool.

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 that there be 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 station power may be lost.

To provide certainty regarding fission product decay heat removal a FNR has multiple independent heat transport systems. 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. This objective is achieved by flooding the steam generators with water at 100 degrees C which causes strong 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 connected to two different cooling towers 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 adequate heat transfer via the intermediate heat exchangers is no longer possible.
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 more than 4 m, so 2 m of intermeidate heat exchanger tube length is still immersed in the primary sodium.
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 fuel bundles are repositioned and/or replaced from time to time using the overhead gantry crane and remote manipulation. Note that the ceiling height must be sufficient to allow extraction and replacement of individual fuel bundles and individual intermediate heat exchangers. During the extraction process used fuel bundles are lifted 0.5 m to clear the mounting sockets and then are then moved horizontally to the reactor perimeter zone of the primary sodium pool where the irradiated fuel bundles are stored until they lose most of their fission product decay heat before being removed from the primary sodium pool.

The FNR fuel bundle can be thought of as being centrally located in a 15 m deep liquid sodium pool with a liquid sodium top surface which is 1 m below the pool deck.

In an earthquake with a 1.5 g horizontal component the liquid sodium might slosh right up to the ceiling. The inside walls must withstand the related forces.

The concrete walls surrounding the primary sodium pool are stabilized on the outside by the about 17 m of surrounding ground depth and by the reinforced concrete shear walls of the heat exchange galleries.

The main chemical threat from a power FNR is the 4700 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 18 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.

This web page last updated August 30, 2020

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This web page last updated May 26, 2020

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