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INTRODUCTION:
Elsewhere on this website Fast Neutron Reactors (FNRs) have been identified as the main source of sustainable and dependable power in the future. However, one important aspect of sodium cooled reactor design is total exclusion of air and water from the sodium.
NaK FIRE:
NaK fires are usually small and are primarily extinguished by individual heat transport loop drain down. The remaining or leaked NaK is extinguished with argon or Na2CO3./MgCO3. An important issue in suppressing a NaK fire is that only the heat transport loops directly invoved should be drained down. The remaining heat transport loops must be left operational to remove fission product decay heat.
NaK PIPE LEAKS:
Exposed NaK pipes only exist in the heat exchange galleries. They operate at a negative pressure with respect to atmosphere. Any NaK pipe leak increases the pressure in its vacuum space which should automatically trigger NaK drain down into the NaK dump tank for that heat transfer loop. Hot liquid NaK has a low viscosity and on release of supporting Ar pressure will immediately naturally drain into its Ar covered dump tank. Detection of a fire in a heat exchange gallery should within a few seconds trigger automatic drain down of NaK into its dump tanks. Then use Na2CO3 to extinguish the much smaller fire related to the NaK splashes that leaks out instead of flowing down into the NaK dump tank.
There should be clearly labelled manual Nak dump tank pressure relief valves outside the space(s) where NaK fires might occur.
The likely potential NaK leak spots are at gaskets, at the welds around the intermediate heat exchange bundles, at the NaK/steam generator, at the NaK dump tanks and around the induction pumps.
Liquid NaK reliably runs downhill like water into an Ar covered sump tank. The problem is not the NaK. The problem is ensuring continuation of Ar cover of the NaK in adverse circumstances such as following an attack on a FNR by a jihadist with a large aircraft or following an attack with a military missle.
PHYSICAL PROTECTION:
Physical Protection of the sodium pool and the NaK dump tanks against malicious assult is a matter of sufficient tonnes of protective concrete and dry sand/rock fill. For the required amount of such fill Look at the largest crater ever made by an aircraft diving into the ground. The sand/rock fill must be gravity drained to ensure that it never becomes saturated with water. Hence a FNR needs to be built into a hill, either natural or man made.
FIRE SUPPRESSION CHEMICAL OVERVIEW:
Sodium fires primarily involve the chemical reactions:
4 Na + O2 = 2 Na2O
2Na + H2O = Na2O + H2 gas
Na2O + H2O = 2 NaOH
Airborne Na2O and NaOH are toxic, and in the case of a FNR,, are also radioactive.
Na vapor and H2 both burn violently in air.
Both sodium carbonate and bicarbonate have been used for extinguishing small sodium fires. The relevant chemical reactions are:
Na2CO3 = Na2O + CO2
NaHCO3 = NaOH + CO2
NaOH melts at 318 deg C so it may be less effective at air exclusion if the underlying surface is hotter than 318 degrees C.
Na2O + H2O = 2 NaOH
This reaction is terrible if it occurs in either your lungs or eyes. Suitable protective apparatus is essential.
At high temperatures Na will react with CO2, probably yielding CO. In terms of fire fighting none of CO, NaOH or Na2O is human friendly. Thus sodium carbonate/bicarbonate is OK for extinguishing small fires but for larger fire suppression engineered gravity based oxygen exclusion systems with Ar are better. The main issue is to reliably exclude O2 and H2O.
SODIUM POOL FIRE:
In normal operation the sodium pool space contains hot sodium vapor that will self ignite on exposure to air. This web page section assumes that the inner structural wall has been breached by a beyond design basis event and focuses on the design and operation of the FNR's sodium fire extinguishing system that then must reliably operate to prevent/minimize escape of airborne radio isotopes.
The sodium pool inner structural wall is robust and is able to withstand a major tornado, hurricane or earthquake. However, a meteorite or a malcontent with a large military missile might still cause a potentially serious sodium fire by breaching both this inner stuctural wall and the less robust thermal wall. The resulting breach would likely vent argon and admit air and precipitation into the sodium pool space. Likewise, an extremely a very violent earthquake might rupture the inner structural wall with the same overall effect. In either case it is essential that the resulting sodium fire be promptly extinguished to prevent emission of both toxic Na2O and airborne radio isotopes such as Na-24.
In normal operation the entire space over the sodium pool is be filled with sodium vapor at vapor pressure corresponding to the sodium pool temperature. The sodium vapor pressure as a function of temperature is as follows:
| Temperature(K) | Pressure (MPa) | Pressure (atmos) |
|---|---|---|
| 400 | 1.8 X10^-10 | 1.78 X10^-9 |
| 500 | 8.99 X 10^-8 | 8.87 X 10^-7 |
| 600 | 5.57 X 10^-6 | 5.49 X 10^-5 |
| 700 | 1.05 X 10^-4 | 1.04 X 10^-3 |
| 800 | 9.41 X 10^-4 | 9.28 X 10^-3 |
| 900 | 5.147 X 10^-3 | 5.080 X 10^-2 |
| 1000 | 1.995 X10^-2 | 0.1969 |
| 1100 | 6.016 X10^-2 | 0.5937 |
at temperatures above 200 degrees C sodium vapor will burn violently if mixed with air. If there is any rain water present it will combine with sodium to emit hydrogen gas, that will then burn viooently at any temperature.
In order to extinguish a sodium pool fire it is necessary to exclude both air and water, fully withdraw the movable fuel bundles and then extract heat from the bulk liquid sodium so as to reduce its temperature and hence its vapor pressure.
SODIUM FIRE SUPPRESSION:
Liquid Na fires cannot be extinguished by the drain down method because of the necessity of keeping the reactor fuel tubes immersed in liquid Na to absorb fission product decay heat. The Na fire suppression sequence is:
1) Attempt to asphixiate fire with argon;
2) Attempt to asphixiate the fire with 900 m^3 of buoyant hollow steel spheres;
3) Attempt to asphixiate fire with Na2CO3. This method has the advantage that it minimizes damage to stainless steel. Further, the reaction product Na2O will tend to form NaOH which is readily separated from liquid Na;
4) Asphixiate fire with a NaCl crust.
The NaCl is in powder form with an embedded a MgCO3 additive to give the crust additional CO2 bubble buoyancy in liquid Na. The NaCl melts at 801 degrees C, which is below the sodium boiling point of 883 degrees C at a pressure of 1 atmosphere. The buoyant steel spheres are designed to withstand heating up to 900 degrees C.
As heat penetrates the NaCl granule the MgCO3 decomposes between 350 C and 900 C liberating CO2. The granules should have a treatment to stop them sticking together in storage. This use of NaCl for fire suppression should only be used in the event of a breach of the inner structural enclosure that threatens public safety because this fire suppression method will likely cause large scale equipment damage.
MAJOR SODIUM FIRE SUPPRESSION STRATEGY:
The liquid sodium surface at 460 degrees C does not actually burn in air. However, when the temperature of liquid sodium exceeds about 200 degrees C the sodium vapor pressure over the liquid sodium is sufficient to self ignite in air. The resulting heat will vaporize more sodium, so the resulting fire is extremely difficult to extinguish except by denying air. Any water present will instantly combine with sodium to generate hydrogen that will burn violently until the water is exhausted.
Minor fires can be extinguished with argon.
The major fire extinguishing strategy advocated herein is to asphixiate fire with a NaCl crust supported by floating hollow buoyant steel spheres. The first step is to cover the liquid sodium surface with several layers of buoyant steel spheres, each about 20 cm in diameter. Then the spaces between the spheres will fill with sodium vapor but there will be no air except in the upper sphere layers. Actual sodium vapor combustion will occur near the top layer of spheres. The lower layers of spheres will prevent radiant heat from the sodium vapor combustion from vaporizing more sodium.
It is essential to immediately withdraw all the movable fuel bundles and then to cool the bulk liquid sodium as fast as possible. The cooling rate will be limited by the heat tranport capacity of the 48 cooling loops.
It is important to understand that the it is sodium vapor and hydrogen, not liquid sodium, that burns. When the liquid sodium surface is covered by multiple layers of hollow buoyant steel spheres sodium vapor diffuses through the layers of steel spheres. If there is any oxygen in the sodium pool space that vapor will spontaneously burn, raising the temperature of the upper layers of buoyant steel spheres. Then if an overhead sprinkler like device discharges powdered NaCl onto the balls the NaCl melts forming a crusty sealing layer. This crusty layer will reduce contact beween sodium vapor and air while the sodium pool is cooling.
. The NaCl melts at 801 degrees C, which is below the sodium boiling point of 883 degrees C at a pressure of 1 atmosphere. The argon filled hollow buoyant steel spheres are designed to withstand heating up to 900 degrees C.
However, this is only a temporary fire suppression solution. To prevent fire re-ignition it is is necessary to remove sufficient heat from the liquid sodium pool to lower its temperature below 200 degrees C using NaK heat transfer, so that the sodium vapor pressure falls below the point of spontaneous ignition in air.
Since inner structural enclosure repair will be required it is prudent to further lower the sodium temperature down to about 100 degrees C and then to exclude air by flooding the sodium surface with kerosene.
AUTOMATION REQUIREMENT:
Assuming that an FNR is installed at a site where direct flooding by water is not a credible risk the worst case risk is a breach of the inner structural wall and the thermal wall which initiates a sodium fire. Extinguishing such a fire requires:
i) Reduce FNR reactivity by withdrawing movable fuel bundles to stop the fission reaction;
ii) Covering the sodium surface with several layers of hollow buoyant argon filled steel spheres, each about 20 cm in diameter;
iii) Sprinkling Na Cl onto the buoyant steel spheres with to form an air excluding crust;
iv) Use of NaK heat transfer to extract sufficient heat from the liquid sodium pool to lower its temperature from 460 deg C to 120 degrees C;
v) Flood the sodium surface with kerosene to exclude air.
Reliable temporary exclusion of air from hot sodium by this method requires that the liquid sodium surface be at least 1 m below the pool deck to temporarily hold in place a 0.7 m thickness of buoyant argon filled steel spheres. The buoyant argon filled steel spheres must be small enough (20 cm diameter) to fit between the indicator tubes and around the intermediate heat exchange bundles.
References:
Carbonation of the EBR-II Reactor
Na HEAT REMOVAL:
At the commencement of an emergency heat dump the nuclear reaction is shut down but fission product decay causes up to 80 MWt of continuing heat generation. In order to rapidly cool the Na it is necessary to remove at least 160 MWt. In each heat exchange gallery there are 6 NaK-steam generators, each rated at about 21 MWt. As a minimum two full heat exchange galleries should be operational at all times to immediately provide emergency heat removal if required.
An important issue is that to prevent Na combustion after a sudden enclosure failure there must be a sufficient emergency standby electricity capacity to power the NaK related heat transport system.
The heat that must be removed from the Na pool to drop its temperature from 460 C to 200 C is about:
4000 tonnes X 1000 kg / tonne X 1000 g / kg X 260 deg C X (1 / 3) cal / g deg C X 4.2 j / cal
= 1.4 X 10^12 j
In normal operation the initial net heat transport rate is:
1000 MWt = 1000 X 10^6 j / s
Initially there may also be 80 MWt of fission product decay heat, resulting in a cooling capacity of:
1000 MWt - 80 MWt
= 920 MWt.
Thus the initial cooling time from 460 deg C to 200 deg C is:
1.4 X 10^12 j / (920 X 10^6 j / s)
= 1522 s
= 25.36 minutes
We need to prevent Na combustion during the hours subsequent to a roof failure. We need to asphyxiate any sodium fire and we need to prevent fresh air and precipitation contacting the hot sodium vapor emitted by the sodium pool.
Consider use of a domed roof with a constant external radius of curvature to enable an external roof repair with preformed curved steel sheets. A preformed circular steel roof patch could be stored outside the dome at its top center. The patch could be slid into position while being supported by a cable fastened to the top center of the roof.
The buoyant steel spheres are stored in wall cabinets located 2 m to 5 m above the pool deck. These cabinets discharge hollow buoyant steel spheres at the airlock locations. Each layer of spheres in a cabinet is seven spheres deep. The sphere releases must be triggered by Ar pressure.
Note that the power feeds for each induction pump mustcome from the corresponding turbogenerator hall, so in essence each turbogenerator hall operates independent of the others. Two adjacent turbogenerator halls share a common on-site cooling tower, but the two cooling systems are not piped together except for the condensers which share a common district heating loop. Thus each NPP has eight independent generation units. Thus, as long as the equipment is reasonably maintained there will always be sufficient house power.
NaCl RESERVOIRS:
The NaCl is stored in and dispensed from eight roof spce mounted reservoirs, each which contains about 40 m^2 of the fire suppressent NaCl/Na2CO3/MgCO3. Thus the total storage is about:
8 resevoirs X 40 m^3 / reservoir= 320 m^3.
The sodium pool surface area is:
Pi 100 m^2
The density solid NaCl is:
2160 kg / m^3.
Hence the maximum possible mass of stored NaCl is:
320 m^3 X 2160 kg / m^3 = 691,200 kg
= 691.2 tonnes
If the fire suppressent is in granular form the fire suppressent mass might be as small as 300 tonnes.
The fire suppressent is held in place in its reservoirs by normally closed plugs that in an emergency are lifted by strong electromagnets. For each reservoir the NaCl flows down onto a spinning head that distributes the NaCl over a 10 m diameter circle like a spinning lawn sprinkler. For coverage certainty the discharge head NaCl distribution patterns overlap.
INDICATOR TUBE SPACING:
The center to center spacing between adjacent indicator tubes is:
(2)^0.5 X 21 X (9 / 16) inch = 16.70 inch
Hence the largest acceptable floating sphere diameter is 8.00 inch
HOLLOW BUOYANT SPHERE DESIGN:
The indicator tube design dimensions are detailed on the web page titled:FNR Indicator Tubes.
Use (1 / 32) inch to (1 / 16) inch thick sheet steel to make ~ 8 inch = 20 cm diameter hollow steel spheres. Fill the spheres with argon at one atmosphere.. Use these buoyant spheres for limitimg air access to the liquid sodium surface and for supporting a fire asphyxiating NaCl layer.
Rhos = density of liquid sodium
Rhow = density of sphere wall
t = thickness of sphere wall
Sphere weight = 4 Pi R^2 t Rhow g
Sphere buoyancy = (4 / 3) Pi R^3 Rhos g
Sphere weight < Sphere buoyancy
4 Pi R^2 t Rhow g < (4 / 3) Pi R^3 Rhos g
or
t Rhow < R Rhos / 3
or
t < R Rhos / 3 Rhow
For 50% buoyancy:
t ~ R Rhos / 6 Rhow
R = 4 inch
Rhos / Rhom ~ 1 /10
t <(2 / 3)(1 / 10)inch
or
t < (1 / 15) inch
Now consider hoop stress:
P 2 R < (Sy / 3) 2 t
or
3 P R / Sy < t
Hence:
3 P R / Sy < R Rhos / 6
or
3 P / Sy < Rhos / 6 Rhow
At 300 K = 0 C, P = 101,000 Pa - 101,000 = 0
At 1200 K = 900 C, P = 404,000 Pa - 101,000 Pa = 303,000 Pa
Sy / 3 = 10,000 psi X 101,000 Pa / 14.7 psi
= 68,707,483 Pa = 68.7 MPa
3 P / Sy = 303,000 Pa / 68.7 MPa
= .303 / 68.7
= 0.00441
However typically Rhow ~ 8 Rhos
Rhos / 6 Rhow = (1 / 8)(1 / 6) = 1 / 48 = .02
Consider practical values:
t = R / 118.4
R = 4 inch, t = 1 / 32 inch
The required equatorial weld length per sphere is:
2 Pi R = Pi (8 inch) = 25 inches
The technology for sphere fabication needs further investigation. Metal spinning as in lamp reflectors?
VOLUME AVAILABLE FOR FIRE SUPPRESSION SPHERE STORAGE:
Use pool space perimeter cabinets 3 m high X 1.5 m deep X Pi (22 m)(48 / 56) m long
3 m high X 1.5 m deep X Pi (22 m)(6 / 7) long
= 85 Pi m^3
These cabinets can dump their contained spheres via the airlock trays.
Volume to be filled = 1 m X Pi (100 m^2)
= 100 Pi m^3
Hence the buoyant spheres will fill about 85% of the volume over the sodium pool.
DETERMINED MILITARY ATTACK:
This author has a concern that a determined military attack with a large precision guided ground penetrating bomb is a potential and credible threat to FNRs. If that threat is real it may be prudent to shut down the relevant FNRs. This military threat issue is common to other major energy sources such as major hydro-electric dams.
If the military attack was executed in a manner that caused both a large hole in the FNR inner structural enclosure and a large amount of liquid sodium to drain out of the sodium pool the potential damage could be very great. A large ground penetrating or armour piercing bomb dropped from a high altitude might rupture all three nested steel cups containing the liquid sodium so that the liquid sodium drains out of the sodium pool and leaves fuel tubes without cooling. These fuel tubes heated by decaying fission products could vaporize the remaining sodium causing a sodium vapor fire. The only way to stop such a fire is to exclude air, which might be very difficult to do in circumstances of continuing sodium vapor production, other than by the method described herein.
The only certain way to reliably resist such a determined military attack is to locate a FNR sufficiently far below grade that it cannot be reached by a ground penetrating missile or bomb.
This web page last updated December 10, 2025.
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