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

Elsewhere on this website Fast Neutron Reactors (FNRs) have been identified as the primary source of energy for meeting mankind's future energy needs. This web page focuses on the design of an FNR's fire suppression system.

Assuming that an FNR is installed at a site where flooding by water is not a credible risk the worst case risk appears to be a primary sodium pool enclosure failure followed by a long sustained sodium fire that burns off the top 4 metres of sodium in the primary sodium pool, thus leading to a situation where the products of combustion could start including irradiated FNR blanket fuel. Stopping such a fire involves:
a) Temporary exclusion of air;
b) Cooling the remaining liquid sodium;
c) Flooding the remaining liquid sodium with kerosene to exclude air.

Reliable temporary exclusion of air requires a sufficient supply of argon and requires that the liquid sodium surface be sufficiently below the top of the primary sodium pool to temporarily hold the argon in place. Exclusion of air is helped if the primary liquid sodium surface is covered with non-combustable steel or ceramic floats.

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 number one issue is to reliably exclude O2 and H2O.

Water exclusion is best done by designing the site so that water and sodium cannot mix, except in the steam side of the steam generator if there is a steam generator tube failure.

Provided that surfaces are over 100 degrees C hot liquid Na reliably runs downhill like water into an Ar covered sump tank. The problem is not the Na. 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.

Consider a pressurized NaK pipe rupture into air. 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 secondary 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 fire energy release is the available supply of O2, not the available supply of NaK. Detection of a fire should trigger automatic drain down of NaK into sump tanks within a few seconds. Then use Na2CO3 to extinguish the much smaller fire related to the NaK splashes that leaked but did not run down into the sump.

The likely potential NaK leak spots are the welds around the intermediate heat exchanger, the NaK/salt heat exchanger and around the induction pumps.

Protection of the primary 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.

This author has a concern that a high altitude military attack with a 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 FNRs. This military threat issue is common to other energy sources such as other 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 reactor enclosure ceiling and a large amount of liquid sodium to drain out of the primary sodium pool and into the surrounding air filled portion of the reactor enclosure the potential damage could be very great. A ground penetrating bomb dropped from a high altitude might rupture all three steel walls containing the primary liquid sodium pool so that the primary 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 those circumstances of continuing sodium vapor production.

The only way to reliably resist such a military attack is to locate a FNR sufficiently far underground that it cannot be reached by a ground penetrating bomb.

This web page last updated August 13, 2022.

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