XYLENE POWER LTD.

FNR ARGON SYSTEMS

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

FNR ARGON SYSTEMS OVERVIEW:
A FNR has multiple argon systems. These include:
1) Two cyrogenic systems to separate argon from other atmospheric gases;
2) One argon cover gas system for the sodium pool including a cold trap for condensing sodium vapor and large rubber bladders for storing spare argon cover gas at atmospheric temperature and pressure;
3) Argon in the dump tanks for controlling the NaK level in each of 48 heat transfer systems;
4) Argon at about 60 psi in an expansion tank for maintaining hydraulic sodium pressure;
5) Argon for operating hydraulic sodium control valves.
6) Argon at 4 inches WC in the thermal wall to prevent sodium vapour entering the aerogel insulation between the hot and cold walls;
7) A mass spectrometer tuned to argon and sodium for detecting argon and sodium vapour leaks.
 

1) CRYOGENIC SYSTEMS:
Argon is about 0.934% of dry air. Pure argon at various pressures performs multiple essential functions related to a sodium cooled FNR. Lack of pure argon will force a FNR shutdown. Hence, for reliable FNR operation, there should be two identical cryosystems for separating argon from air.
 

2)SODIUM POOL COVER GAS:
Argon acts as a one atmosphere inert cover gas over the sodium pool. Depending on the sodium temperature the cover gas can contain up to 4 inches WC of sodium vapor. The atomic density of the cover gas varies with the sodium temperature. Hence the cover gas is connectede to large rubber bladders that are surrounded by air at one atmosphere. As the sodium warms up the cover gas will expand through a check valve and cold trap and fill the bladders. when the sodium cools the bladders will discharge their stord argon back into the pool space via another check valve.

To charge the cover gas system the bladders are first emptied and are isolated from the pool space. Then the bladders are filled with pure argon from the crysystem argon discharge. Then the input to the crysystem is connected to the pool space and the argon output from the cryosystem is connected to the bladders. The cryosystem is run which sucks a gs mixture from the sodium pool space. That gas mixture is relaced by pure argon from the bladders with a bit from argon extraction from the air over the pool space. This step is repeated until there is no more discharge of other gases from the cryosystem and the bladders are almost empty.
 

3) DUMP TANK ARGON:
Each group of dump tanks is charged with enough argon to properly set the NaK level in the group heat transfer system vacuum discharge line.
 

4) HYDRAYLIC SODIUM PRESSURE MAINTENANCE:
The movable fuel bundle actuators are raised by admission of hydraulic Sodium to the actuators. There is a shared tank of sodium at a pressure of about 60 psi (0.4 MPa). A cycling pump draws sodium from the pool to charge this tank to 60 psi. An argon bubble at the top of this tank maintains the hydraulic sodium pressure.
 

5) ACTUATOR VALVE CONTROL:
Each actuator has a control valve the selects either:
a) Fill the actuator with hydraulic sodium to insert the corresponding movable fuel bundle;
b) Do nothing;
c) Drain the actuaator to the sodium pool to withdraw the corresponding movable fuel bundle.

These valves operate at a high temperature (up to 500 deg C). These valves are thermally isolated from their corresponding electronic controls using argon and low thermal conductivity tubing.
 

6)THERMAL WALL PRESSURIZATION:
The argon pressure in the space between the hot and cold walls is maintained via a blower with a 4 inch WC discharge pressure. The input of this blower is connected to the bladders. In normal operation the blower maintains this pressure difference but there should be no net argon flow. As the temperature in the space between the hot and cold walls rises argon will leak backward through the blower and into the bladders. In the event of a leak in the hot wall argon will leak into the pool space which will prevent sodium vapour migrting into the space between the hot and cool walls.
A leak in the cool wall can be found using soap bubbles. A leak in one of the bladders can also be found using soap bubbles.  

7) ARGON LEAK DETECTION:
It is necessary to be able to detect, locate and fix argon leaks. Oncool surfaces soap bubble detecion can often be used. However, in other cases a leak detector consisting of a mass spectrometer tuned to sodium or argon is necesary.
 

This web page last updated October 5, 2026