XYLENE POWER LTD.

FNR COVER GAS

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

A FNR uses argon as a cover gas to prevent the liquid sodium chemically combining with atmospheric gases.
 

THERMAL WALL PROTECTION:
A FNR contains five isolated gas spaces:
a) The space over the sodium pool which contains argon and sodium vapor;
b) The space between the hot wall and the cool wall which contains argon at a pressure 4 inchs WC above the pressure over the sodium pool;
c) The service access space which normally contains both air at atmospheric pressure and slightly above normal room temperature;
d) The rubber bladders in the argon silos which contain argon at close to outside air pressure and temperature;
5) Th argon over the dump tanks whcich contain argon and varing amountsw of NaK vapor t pressures typically 1 to 2 bar. .

The gas space over the sodium pool undergoes wide temperature variations ranging from 20 degrees C to 500 degrees C.

The service access space normally operates at close to room ambient temperature and consists of the following connected spaces:
a) Space above the cool ceiling;
b) Space between the cool wall and the inner structural wall.

The sodium pool space is separated from the service access space by an insulated thermal wall consisting of a hot wall and a cool wall. The hot side of this thermal wall must easily flex to allow for thermal expansion and contraction while remaining gas tight under small pressure differentials. However, this flexible wall would quickly fail if it was exposed to the large pressure differentials that typically occur in tornados.

There are large rubber bladders located in the nearly constant temperature argon silos. The outside of these bladders is at atmospheric pressure. The inside volume of these bladders is connected to the pool space via a gas cooler.. The effect of these bladders is to keep the pool space at normal atmospheric pressure.

A FNR uses argon as a sodium pool cover gas to prevent the liquid sodium chemically combining with atmospheric gases. This argon thermally contracts when the liquid sodium temperature drops which lowers the pressure in the pool space. The resulting pressure differential causes argon to flow from the bladders in the argon silos into the pool space until the two pressures are again equal. Similarly, when the liquid sodium temperature rises the argon in the pool space expands causing argon to flow from the pool space into the bladders in the argon silos.

This automatic pressure balancing mechanism prevents a thermal wall structural failure due to a high pressure differential.

When the sodium temperature is rising hot argon flows from the sodium pool space toward the bladders. This hot argon must flow through a gas cooler to prevent the hot argon with entrained sodium vapor damaging the bladders. This issue imposes a limit on the argon flow rate and hence the rate of rise of liquid sodium surface temperature and rate of pressure balancing between the atmosphere and the liquid sodium pool space.
 

TORNADO PROTECTION:
The sodium pool space is surrounded by service access space. That is further surrounde by the inner structural wall and steel roof dome. This rigid wall is rated for a pressure differential of 10,000 Pa to 20,000 Pa.

In the event of a tornado there will be a rapid drop in outside air pressure followed by a return to normal. That pressure change will appear across the inner structural wall. It will also cause transient swelling of the argon bladders. If the the inner structural wall is gas tight the pressures in the service access space and over the sodium pool are unaffected.

Thus in a tornado the outside air pressure may quickly fall or rise by 10,000 Pa to 20,000 Pa but the pressure difference between the service access space and the sodium space remains very small. Hence the thermal wall between the sodium pool space and the service access space is not stressed by the rapid change in outside air pressure caused by the tornado.

The system can tolerate a small air leak in the inner structural wall provided that the leakage rate is no larger than the permitted maximum argon gas flow rate into or out of the bladders.
 

CHARGING SYSTEM WITH ARGON:
The argon is obtained by separating if from air. In normal air the concentration of argon is about 0.93%. The argon separator inputs a gas mixture into its input port, discharges pure argon out one output port and discharges other gases out the other output port.

Initially the sodium is presumed to be covered by a layer of kerosene. Prior to starting a FNR it is necessary to install the argon cover gas. This step is accopomplished by starting with the argon bladders empty and using the argon separator to fill the bladders with pure argon. The bladders are then connected to the sodium pool space.

Assume that the bladder volume is comparable to the pool space volume. Then run the argon separator with both its input and its argon discahrge port connected to the pool space. Initially the input to the argon separator is about 50% argon. As the separator runs the input fraction of argon will gradually increase and due to the rejection of air the volume of total gas in the bladders will gradually decrease. When the bladder contined argon volume is almost zero the fraction of argon in the pool space will be about 100%.

At that point the kerosene cover on the sodium can be removed and sodium heating can commence.

Note that there is a check valve between the cold trap and argon separator and the argon bladders. Note that there is another check valve between the argon storage and the sodium pool space.
 

PHYSICAL SIZE CALCULATIONS:
The volume of argon over the primary sodium pool is approximately:
Pi [(13 m)^2 X 9 m] + Pi[(10 m)^2 X 1 m]
= Pi [1521 + 100] m^3 = 5093 m^3

When the reactor space cools from 520 degrees C (793 degrees K) to 27 degrees C (300 degrees K) the argon in the reactor space shrinks to:
(300 / 793) X 5093 m^3
= 1926.55 m^2

Hence the amount of room temperature replacement argon that must be supplied by the argon bladders is:
(5093 m^3 - 1926.55 m^3 = 3167 m^3 m^3

The Argon silos need additional volume to allow for sudden expansion in a tornado and for 25% of the silos being out of service. Hence the minimumr equired argon bladder volume is:
(3167 m^3 / 0.8) X (4 / 3) = 5278 m^3.

However, the bladder volume is less than the silo volume. Design for a silo volume of at least 6000 m^3.

The base area of the silos is about:
8 silos X Pi (6 m)^2 / silo = 905 m^2

Hence the minimum required argon silo height is:
6000 m^3 / 905 m^2 = 6.63 m

There should be a sufficiently large supply of stored argon kept on-site to prevent sodium combustion while the liquid sodium temperature is being reduced following a sudden major roof failure or other unplanned emergency event.

The gas connection between the liquid sodium pool space and the argon bladders is fitted with a cold trap. During reactor warmup when the argon temperature is rising this trap protects the bladder material from high temperature argon and also condenses any radio cesium vapor that might be present in the argon due to a failed reactor fuel tube. This gas cooling system can potentially also be used to condense krypton for selective removal of Kr-85.
 

ARGON CONTAMINATION:
A practical FNR has several hundred thousand vertical fuel tubes with tops located about 6 m below the surface of the liquid sodium pool. Over time each such fuel tube builds up an internal pressure due to formation of inert fission product gases, some of which have radioactive isotopes such as Kr-85. Ideally the fuel tubes are all well sealed so that these fission product gases remain contained in the fuel tubes.

We need to keep the radioactive gas concentration in the sodium pool cover gas sufficiently low to permit suitably suited workers into the cover gas space for the purpose of eventual changing of heat exchange bundles and servicing the gantry crane. The flange connections to such heat exchange bundles are difficult to execute via robotics. The alternative is to flood the sodium surface with kerosene, vent all the argon and then replace the argon. That is a lot of argon to be discarded and still needs kerosene recovery.

Over time the cover gas will likely gradually become polluted with radioactive inert gas fission products. Hence from time to time each argon bladder must be vented to the atmosphere. Hence there is enough spare argon storage to allow bleeding off of one bladder full of contaminated argon at a time.

The space between the rubber bldders and the argon silo inner walls should be vented via a damper. In the event that the gas in a rubber blader becomes seriously radioactive that damper can be closed to prevent radioisotope leakage to the atmosphere.

Suppose that a fuel tube has a defective top plug. Once the gas pressure inside the fuel tube exceeds the liquid sodium head pressure at the top of the fuel tube the fission product gases will slowly leak out and will bubble to the surface of the liquid sodium. The liquid sodium is covered by argon gas. The fission product radioisotope gas Kr-85 will mix with that argon. Its presence can be detected with a radiation detector which monitors the argon. However, at that point we need to figure out which fuel tube in which fuel bundle is leaking and replace it or its fuel bundle to stop further Kr-85 accumulation in the argon cover gas.

Suppose that we lower the liquid sodium surface temperature to about 120 degrees C. Suppose that we then flood the liquid sodium surface with a thin layer of kerosene or something similar. This kerosene should contain an additive which promotes the formation of foam bubbles. Hence any Kr gas bubbling up from below should form visible bubbles, similar to soap bubbles that are used to detect leaks in natural gas pipes or automobile tires.

The issue is: What kerosene like liquid and what bubble promoting additive should be used? These must operate to form visible bubbles at around 120 degrees C. As the temperature of the liquid sodium is raised both the kerosene like liquid and the additive should fully evaporate and then condense on a cool surface. These materials need to be totally extracted from the reactor space as they will likely decompose at normal FNR operating temperatures of 460 to 500 degress C. An additional complication is that the argon pressure is maintained at one atmosphere by large ambient temperature argon filled bladders. We do not want the kerosene plus additive to attack the bladder material.

Argon flowing from the reactor space into a bladder is first cooled to near ambient temperature, so the vapor pressure of the kerosene plus additive inside the bladder should be very small at 20 degrees C. A device, analogous to a dehumidifier, should be used to capture most of the kerosene before the sodium temperature is raised to its normal operating temperature.

While doing this kerosene extraction we should also try to simultaneiously concentrate and extract the krypton. The krypton can then be either vented or cryogenically condensed. Are there any suitable separation membranes? An alternative is a high speed gas centrifuge to concentrate the krypton. It will also catch xenon and radon.

The object is to reject the high atomic weight inert gas fission products while rejecting minimal argon.
 

ARGON PLANT:
Before the sodium is loaded the air in the sodium pool space must be replaced by argon. Then after the sodium is loaded the argon cover gas will gradually become polluted with leakage air. The gas mixture above the sodium pool must be continuously recycled to remove and reject all molecule types except argon.

A crucial part of the FNR is its supporting argon plant. The argon plant draws in both air and a argon-air mixture via a three way valve, sends the argon molecules one way and discharges all other gas molecules to the outside. The rate of argon discharge will depend on the argon concentration in the space over the sodium pool. Hence the three way valve needs to be adjusted so that the gas volume input from the sodium pool space equals the gas volume discharged to the sodium pool space.

The argon plant must be efficient because it runs continuously. We need to focus on the detail of the argon plant. This plant must recycle the gas over the sodium many times to clean it up.

Argon is industrially extracted from liquid air in a cryogenic air separation unit by means of fractional distillation.
 

GAS	B.P. degrees C	% In Air
N2	-196	78
O2	-183	21
Ar	-186	0.93
CO2	-78.5	0.042

 

This web page last updated November 25, 2025.