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XYLENE POWER LTD.

FNR FIRE SUPPRESSION

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

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 drain down. The remaining or leaked NaK is extinguished with argon or Na2CO3./MgCO3 or possibly as a last resort, NaCl.
 

PRESSURIZED NaK PIPE RUPTURE:
Consider a pressurized NaK pipe rupture into air. Exposed NaK pressure pipes only exist in the heat exchange galleries. It is necessary to ensure that no humans are present in a heat exchange gallery when its NaK pipes are pressurized and that the maximum possible NaK mass leakage into air is manageable. Hot liquid NaK has a low viscosity and will immediately naturally drain into an Ar covered below grade sump. The limiting factor as to the fire energy release is the available supply of air, not the available supply of NaK. Detection of a fire 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 leaked but did not run down into the NaK sump.

The likely potential NaK leak spots are at gaskets, at the welds around the intermediate heat exchanger, at the NaK/salt heat exchanger, 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 in adverse circumstances such as following an attack on a FNR by a jihadist with a large aircraft or following a military attack involving precision ground penetrating bombs dropped from a high altitude.
 

Protection of the sodium pool and the NaK dump tanks is a matter of sufficient tonnes of protective 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:
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 not nice if it occurs in either your lungs or eyes.

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:
A sodium pool fire can only occur as a result of a breach of the FNR structural enclosure or its interior wall.

The sodium pool enclosure is robust and is able to withstand a major tornado, hurricane or earthquake. However, a meteorite or a malcontent could still cause a potentially serious sodium fire by breaching the sodium pool structural enclosure with an armour piercing missile. The resulting hole would likely vent argon and admit air and precipitation into the pool space. Likewise a very violent earthquake might crack the structural enclosure with the same overall effect. In either case it is essential that the resulting sodium fire be extinguished. Letting it continue burning would eventually lead to airborne radio isotopes.

In normal operation the sodium pool space contains hot sodium vapor that will self ignite on exposure to air. This web page assumes that the sodium pool enclosure has been breached and focuses on the design and operation of the FNR's sodium fire suppression system.
 

MINOR 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. The Na fire suppression sequence is:
1) Attempt to asphixiate fire with argon;
2) Attempt to asphixiate the fire with 300 m^3 of buoyant 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 liberating CO2 between 350 C and 900 C. 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 an enclosure failure involving loss of the stored argon because this fire suppression method may damage the fuel bundles, the stainless steel indicator tubes, the stainless steel pool liner, the intermediate heat exchange bundles and related pipe components.
 

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.

Minor fires can be extinguished with argon.

The major fire extinguishing strategy advocated herein is to asphixiate fire with a NaCl crust supported by the buoyant steel spheres. The first step is to cover the liquid sodium surface with several layers of buoyant steel spheres, each about 37 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 important to understand that the it is sodium vapor, not liquid sodium, that burns. When the liquid sodium surface is covered by multiple layers of buoyant steel spheres sodium vapor diffuses through the layers of steel spheres. If there is any oxygen in the sodium pool cover gas that vapor will spontaneously burn, raising the temperature of the upper layers of buoyant steel spheres. Then if NaCl is sprimkled onto the balls it melts forming a crusty sealing layer. This crusty layer will prevent further contact between the sodium vapor and the 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 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 reignition it is is necessary to remove sufficient heat from the liquid sodium pool to lower its temperature first to about 300 degrees C using nitrate salt heat transfer and then to under 200 degrees C using HTF heat transfer, so that the sodium vapor pressure falls below the point of spontaneous ignition in air.

As the liquid sodium cools its vapor pressure will decrease, which will reduce the vapor's propensity to burn in air.

When the liquid sodium temperature is below 200 degrees C its vapor pressure is low enough that the sodium will no longer self ignite in air.

Since structural enclosure repair will be required it is prudent to further lower the sodium temperature down to about 100 degrees C and then 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 sodium pool enclosure which initiates a sodium fire. Extinguishing such a fire requires:
i) Reduce FNR reactivity by activating both emergency shutdown systems to stop the fission reaction;
ii) Covering the sodium surface with several layers of buoyant steel argon filled spheres, each about 37 cm in diameter;
iii) Sprinkling the buoyant steel spheres with NaCl to form an air excluding crust;
iv) Use nitrate salt heat transfer to extract sufficient heat from the liquid sodium pool to lower its temperature from 460 deg C to 300 degrees C;
v) Charge the synthetic heat transfer fluid (HTF) isolation loops with NaK;
vi) Use HTF heat transfer to extract sufficient heat from the liquid sodium pool to lower its temperature from 300 degrees C to 120 degrees C;
vii) Continue using HTF heat transfer to extract fission product decay heat from the liquid sodium;
viii) Flood the sodium surface with kerosene to exclude air.

ix) Application of a temporary roof (eg tarps) over the enclosure opening to exclude precipitation;
x)Fixing the sodium pool enclosure and restoring argon cover gas over the sodium pool.
xi) Filtering solids out of the sodium pool.

Reliable temporary exclusion of air from hot sodium by this method requires that the liquid sodium surface be about 1 m below the pool deck to temporarily hold in place a 1 m thickness of buoyant argon filled steel spheres. The buoyant argon filled steel spheres must be small enough (37 cm diameter) to fit between the indicator tubes and around the intermediate heat exchange bundles.
 

References:
Carbonation of the EBR-II Reactor

Na Fires French Report
 

Na HEAT REMOVAL:
In the event of a FNR enclosure failure which admits air it is necessary to shut down the nuclear reaction and to rapidly cool the Na pool to prevent Na vapor spontaneously burning in air. Nitrate salt is used for heat transport when the Na temperature is above 300 degrees C. However, nitrate salt is unsuitable for low temperature heat transport because the melting point of the nitrate salt is too high. There must also be Heat Transfer Fluid (HTF) loops that will continue removing heat from the NaK and hence from the Na until the Na temperature drops down to about 100 degrees C.

The heat transport system must protect the HTF from high temperature deterioration when the Na temperture is above 335 C. this objective is reachewd by draining the HTF associated NaK loops when the Na temperature is over 300 degrees C.

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 net cool the Na it is necessary to remove about 160 MWt. Thus in each heat exchange gallery there are 5 NaK-salt heat exchangers and there is one NaK-HTF heat exchanger. When the Na temperature is less than 300 degrees C heat transport is via HTF and the maximum net heat transfer rate is 160 MWt. When the Na temperature is above 335 degrees C heat transport is via nitrate salt and the maximum heat transfer rate is 840 MWt.

The full 1000 MWt of reactor capacity is only available at liquid sodium temperatures in the range 300 Cto 335 C.

This constraint will reduce the NPP electric power output to:
0.3 X 840 MWt = 252 MWe.

The nitrate salt and the HTF loops both dump heat to steam generators. When the Na and hence the NaK temperature drops to under ~ 300 degrees C due to nitate salt freezing heat transport through the nitrate salt drops almost to zero. Then the NaK loops associated with the HTF loops are refilled. Heat transport through the HTF continues but this heat is at too low a temperature for efficient electricity generation. In these circumstances, to reduce the injection pump electricity load the HTF fed steam generators have their pressure regulating valves set near 30 psia and heat transported by the HTF is dumped by evaporation of water in the reduced pressure steam generators. The resulting wet steam can be either condensed or used for comfort heating.

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 HTF related heat transport system once heat transport via nitrate salt is no longer functional.
 

The heat that must be removed from the Na pool to drop its temperature from 460 C to 300 C is about:
4000 tonnes X 1000 kg / tonne X 1000 g / kg X 160 deg C X (1 / 3) cal / g deg C X 4.2 j / cal
= 8.96 X 10^11 j

The initial net heat transport rate is:
840 MWt = 840 X 10^6 j / s

Initially there may also be 80 MWt of fission product decay heat, resulting in a cooling capacity of:
840 MWt - 80 MWt
= 760 MWt.

Thus the initial cooling time from 460 deg C to 300 deg C is:
8.96 X 10^11 j / (760 X 10^6 j / s)
= 1179 s

Below 300 degrees C the heat transport rate is limited by the HTF heat transport capacity, implying that sufficient heat removal to prevent Na spontaneous combustion in air may take hours.

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 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. Note that the domed roof slope at its top edges might be only 45 degrees. 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 between NaK pipes in the sodium pool space. Each layer of spheres in a cabinet is two spheres wide and three spheres deep. Each cabinet stores ~ 29____layers of spheres. There are 44 such cabinets mounted between NaK pipes.

Note that the power feeds for each heat exchange gallery come 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. Thus each NPP has eight independent sub units. Thus, as long as the enclosure is intact and the equipment is reasonably maintained there will always be sufficient house power.
 

NaCl RESERVOIRS:
The NaCl is stored in and dispensed from eight dome 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 distribution patterns overlap.

We might consider loading the bottom of each fire suppresent reservoir with Na2CO3 and the upper portion of each reservoir with NaCl. Hence if the problem can be solved with just Na2CO3 it is not necessary to use stainless steel damaging NaCl.
 

BUOYANT SPHERE DESIGN:
Ths spherical float design dimensions are detailed on the web page titled:FNR Indicator Tubes.

Use (1 / 16) inch thick sheet steel to make ~ 14.8 inch = 37 cm diameter hollow steel spheres. Fill the spheres with argon. Use these buoyant spheres for limitimg the rate of air access to the liquid sodium 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

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

Hence the inequality
3 P / Sy < Rhos / 6 Rhow
is easily satisfied.

Consider practical values:
t = R / 118.4
R = 7.4 inch, t= 1 / 16 inch

The required equatorial weld length per sphere is:
2 Pi R = Pi (14.8 inch) = 46.6 inches

We need to choose the sphere diameter to be 14.8 inches to provide good sodium surface coverage between the indicator tubes. Each sphere occupies the footprint of one fixed fuel bundle.
 

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 other major nuclear power plants and major hydro-electric dams.

If the military attack was executed in a manner that caused both a large hole in the FNR enclosure ceiling 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 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.

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 15, 2023.

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