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

This web page focuses on the SM-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 extreme storm conditions.

The space above the liquid sodium pool is filled with the inert gas argon which will not chemically react with either the liquid sodium or steel. This argon pressure is maintained at one atmosphere by an automatic feed system fed by bladders in silos that does not rely on AC power. Fail safe argon space pressure control is essential for reactor enclosure structural integrity.

Floating on top of the liquid sodium are shallow draught square steel floats that reduce the exposed surface area of liquid sodium by about 99%. These floats have holes through them for the indicator tubes.

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.

If the reactor is to be shut down for an extended period the sodium surface should be covered with kerosene to prevent sodium oxidation. However, that kerosene presents a potential fire hazard.

The primary function of the roof structure is to contain the argon cover gas and heat and to keep both rainwater and air away from the liquid sodium. The roof structure must allow fuel bundle and intermediate heat exchange bundle replacement using an external crane. The roof structure must be gas tight and must reliably exclude both air and rain water under the most adverse circumstances, including violent storms (tornados), long term corrosion and deliberate aerial attack. There should be an inner metal ceiling, 1 m of thermal insulation, an outer metal ceiling, at least 1 m of ventilation space and a steel beam supported outer roof. The reactor enclosure outer roof is structurally comparable to a multi-track railway overpass over a major highway.

If an imminent threat to the FNR from an air born object is detected the FNR should be immediately cold shut down and sufficient heat should be extracted from the primary liquid sodium pool to ensure that there will be no spontaneous combustion of sodium with air if there is a major roof failure.

The inner ceiling 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 450 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..

Engineering a roof that can withstand a deliberate direct overhead air attack is one of the most difficult aspects of liquid sodium cooled FNR implementation. If such attacks become reality it may ultimately be necessary to locate FNRs deep underground to provide security against intentional overhead attack.

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 is applied. In such circumstances the liquid sodium pool must be cooled below 200 degrees C and then covered with a kerosene to prevent spontaneous sodium combustion with air. Circumstances that might lead to such a roof failure include a direct bomb, missle or meteorite strike.

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

A 300 MWe FNR has 32 fully independent heat removal systems. This level of independence provides protection in depth against loss of cooling failures.

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 temporarily remove the primary liquid sodium from the primary sodium pool. Hence there must be nearby reserve 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 must also accept hot radioactive fuel bundles transferred from the primary liquid sodium pool. A reserve storage pool can potentially be located underneath each dry cooling tower. When not required for storing sodium the reserve pool can be used for storing emergency cooling water or as underground parking space.

On-site personnel are required to do periodic routine non-nuclear preventive maintenance on the steam generators, turbo-generators, condensers, cooling towers and related mechanical and electrical equipment and to make repairs as necessary. However, this equipment does not involve any radioactivity. 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 reasons for keeping staff on the reactor site 24/7 is compliance with steam power plant regulations and maintenance of site security.

The central core region together with the top and bottom blanket regions involve 640 vertical active fuel bundles. Each fuel bundle is 0.4 m long X 0.4 m wide X 6.0 m high and is supported by 3 m high legs (corner girder extensions on the fuel bundle bottoms). Each active fuel bundle has a removeable 3 m tall chimney and a 7.5 m tall indicator tube.

Each active fuel bundle has a vertically sliding central control portion and a fixed position surround portion. The central control portion is inserted into the surround portion from the bottom. Insertion distance is set using a liquid sodium hydraulic piston actuator. The indicator tube projects above the primary liquid sodium surface to indicate the actual vertical position of the fuel bundle central control portion. 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 1.2 m thick blanket formed from 3 rows of vertical passive fuel bundles.

There is an outer ring of spent 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.

For a 1000 MWt FNR the active fuel bundles are centrally positioned in a 21 m diameter X 15.5 m deep liquid sodium pool. The region 2.8 m wide at the edge the liquid sodium pool is dedicated to intermediate heat exchange bundles and a service access corridor. The service access corridor also forms part of the sodium guard band.

There is an inner steel primary sodium enclosure, an outer steel primary sodium enclosure and a concrete enclosure with a sheet metal liner. As long as at least one of these enclosures maintains its physical integrity the liquid sodium will be sufficiently contained to maintain its minimum required level for safety and fission product decay heat removal following FNR shutdown.

One of the most important aspects of fission reactor design is provision for fission product decay heat removal under extremely adverse circumstances. If some event occurs which causes a reactor shutdown the fission products will continue to produce decay heat at about 5% 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 accident conditions such as shortly after a severe earthquake.

Fission product decay can produce heat at a peak rate equivalent to about 10% of a nuclear reactor's full power rating, even after the nuclear chain reaction is off. Hence there must be certainty about passive removal of this heat. A FNR has multiple independent heat transport systems, proper operation of 10% 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 sodium circulation with just half of the heat transport systems in service. During normal reactor operation no more than one quarter of the heat transport systems may be simultaneously taken out of service for maintenance or repair. In order to meet these requirements at least 20% of the intermediate heat exchange tube length must always be immersed in liquid sodium. This condition effectively imposes geometrical constraints on the FNR facility design.

In the case of 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 thermal contact with the intermediate heat exchange bundles is lost.
2) The liquid sodium pool walls must be designed such that if the inner wall fails and the primary liquid sodium leaks into the space between the inner and outer pool walls, the leakage into the space between the two walls will not lower the primary liquid sodium level below the tops of the fuel rods or below the intermediate heat exchange bundle tubes. Ideally this condition should also be met if the outer steel wall also fails. This condition restricts the volume of the air space around the reactor. Viewed another way this condition sets a minimum on the reactor size if reasonable service access clearances are to be maintained and if double wall failures are required to be tolerated.
3) Even if the intermediate loop sodium pumps fail there must be enough secondary liquid sodium natural circulation to ensure safe removal of the fission product decay heat.
4) The secondary liquid sodium dumps its heat into steam generators. Hence on a reactor shutdown the pressure in the steam generators must be released so that the system that injects water into the steam generators does not face a pressure load.
5) There must be enough clean water in storage, above the elevation of the steam generators, such that the steam generators can be gravity fed and the fission product decay heat can be removed by evaporating that water and condensing the resulting steam in the cooling towers.

The steam generators are located at an elevation higher than the reactor and on the return side of the intermediate sodium loop so that the intermediate sodium will natuarally circulate even if the intermediate sodium induction type circulation pump loses power. Each steam generator has a dedicated recirculation pump for thermal stress mitigation. The equipment should be sized so that the natural circulation rate is sufficient to safely remove fission product decay heat after the reactor is shut down.

The fuel bundles are repositioned and/or replaced from time to time using an overhead gantry crane and remote manipulation. Note that at the air locks the ceiling height must be sufficient to allow extraction and replacement of individual fuel bundles. During the removal process spent fuel bundles are lifted 3 m to clear the vertical square support tubes 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 is designed to safely withstand earthquake induced horizontal acceleration of up to 0.38 g. At a sustained 0.38 g horizontal acceleration the surface of the liquid sodium could adopt an angle that is 0.36 radians to the horizontal. Under these circumstances the liquid sodium height on one edge of the pool could theoretically reach up to 4.0 m above the normal liquid sodium surface.

The gantry crane is located at this maximum liquid sodium height. The bricks forming the pool thermal insulation must have surrounding structural steel elements that firmly stabilize the brick wall to the surrounding concrete walls and to upper level steel walls to prevent a structural failure in severe earthquake conditions. Hence the FNR fuel bundle can be thought of as being centrally located in a 15.5 m deep liquid sodium pool with a liquid sodium top surface which is 1 m below the pool deck.

The concrete walls surrounding the primary sodium pool are stabilized on the outside by earth/rock embankments with a one unit rise for each two horizontal units. These embankments should be drained at the bottom, should have grass growing on top with sets of concrete stairs. Rain should either run off the surface or should drain out via a sloped drains near the bottom. Even if there is a violent earthquake causing a total pool rupture this embankment must contain the sodium and must prevent any rain or flood water reaching the sodium.

The main chemical threat from a power FNR is the 4900 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 design is choice of a reactor site where the sodium will NEVER be exposed to flood water.

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 release of air borne plutonium and fission products. It is essential that the reactor be designed and sited such that a 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 anywhere near the sodium. Perhaps a high molecular weight inert gas such as radon, xenon, krypton could be used to extinguish a sodium fire. The key issue is to not have any makeup fresh air flow that could blow the high molecular weight inert gas away.

The soil and bedrock around the liquid sodium pool must be sufficiently dry, dense and stable to safely contain the liquid sodium in the unlikely event that a major earthquake ruptures both the inner and outer stainless steel walls of the liquid sodium pool and crecks 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 other chemical threat is a spontaneous reaction between hot liquid sodium and air. To mitigate this threat the liquid sodium is covered by floating steel covers, an argon cover atmosphere, a gas tight suspended inner metal ceiling, and a gas tight suspended 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 temperatre below 200 degrees C, the threshold for spantaneous combustion of sodium in air. As this heat is dumped stored argon molecules from bladders in storage silos must be added to the cover gas to maintain the 1 atmosphere pressure in the argon cover gas.

Once the liquid sodium temperature is down to about 120 degrees C the surface of the liquid sodium can be flooded with a thin layer of low density oil such as kerosene to prevent the liquid sodium oxidizing during work such as roof repair or replacement of an intermediate heat exchange tube bundle.

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 cooling capacity. The fastest way to emergency cool the system is to directly vent steam from the steam generator discharges. It is important to have enough water in tank storage in or near the steam generator building 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 ceiling integrity.

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 temporarily remove the primary liquid sodium from the primary sodium pool. Hence there must be nearby reserve 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 must also accept hot fuel bundles transferred from the primary liquid sodium pool. A reserve storage pool can potentially be located underneath each dry cooling tower. When not required for storing sodium the reserve pool can be used for storing emergency cooling water or as underground parking space.

Normally most of the reactor waste heat output from electricity generation is dumped to a water based district heating system. The discharge temperature of this water ranges from 80 degrees C in the summer to 120 degrees C in the winter. 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 electric boilers that further heat 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. These 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 disposal 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.

At the reactor site there must be at least two independent natural draft dry cooling towers. Each such cooling tower must be sized and piped to safely reject the fission product decay heat by natural circulation even if the other cooling tower is out of service. Assuming that both 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.

At the base of the local natural draft cooling towers are inward openning air admittance doors which are normally closed for energy conservation purposes but which are held closed by electromagnetic locks. On loss of electric power the electromagnetic locks release and the natural draft opens these doors allowing air flow through the cooling tower. When system power is restored maintenance personnel must manually close these doors to reduce the thermal load imposed by the cooling towers. The doors are fitted with closure sensing switches which trigger a maintenace alarm when the door opens.

This web page last updated March 24, 2019

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