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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.

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.

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.

This web page last updated July 29, 2021

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