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XYLENE POWER LTD.

FNR SODIUM-SALT HEAT EXCHANGERS

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

HEAT TRANSPORT OVERVIEW:
In a liquid sodium cooled Fast Neutron Reactor (FNR) for safety purposes there are isolated secondary liquid sodium and nitrate salt heat transport loops between each immersed intermediate heat exchange bundle and the corresponding steam generator. There is an induction type circulation pump in the secondary sodium return pipe from the sodium-salt heat exchanger to the intermediate heat exchange bundle. There is a molten salt pump in the salt return pipe from the steam generator to the sodium-salt heatr exchanger.

The electricity output from a FNR is controlled by modulating the flow rates of the secondary liquid sodium induction pumps. The steam production rate is approximately proportional to the secondary sodium flow rate.

Each steam generator has a local control loop that maintains the desired water level in the steam generator by controlling the rate of high pressure condensate water injection.

In the steam generator the mass flow of molten nitrate salt is normally much larger than the mass flow of water. Hence from a safety perspective it is easier to control the water than the flow of molten salt.

This web page is concerned with design detail related to the sodium-salt heat exchanger.
 

FNR SODIUM-SALT HEAT EXCHANGER DESCRIPTION:
The purpose of the FNR sodium-salt heat exchanger is to transfer heat from the secondary sodium to the nitrate salt.

A FNR sodium-salt heat exchanger is in essence a vertical tube in shell heat exchanger with nitrate salt flowing upward inside the tubes and liquid molten secondary sodium flowing downward outside the tubes but inside the shell. A FNR sodium-salt heat exchanger and its support equipment must be designed so that a tube rupture anywhere in the heat transport system does not create a dangerous situation due salt or brine entering the secondary sodium circut, mixing with the secondary sodium and instantaneously producing large amounts of hydrogen and/or nitrogen gas.

Due to:
a) The higher heat capacity of salt as compared to sodium;
b) The changes in temperature of the sodium and the salt in normal operation;
the downward sodium volumetric flow rate in the sodium-salt heat exchanger is much higher than the upward volumetric salt flow rate. Hence the molten salt flows inside narrow tubes and the liquid sodium occupies the space around the tubes but inside the shell.

The secondary sodium is normally at a significantly higher pressure than the molten salt, which at the sodium-salt heat exchanger is only slightly above atmospheric pressure. The secondarey sodium pressure head is maintained by a argon filled cushion tank which contains some reserve sodium.

The liquid sodium is hotest on top of the sodium-salt heat exchanger and flows almost straight downwards. The molten salt is coolest on the bottom of the sodium-salt heat exchanger and flows almost straight upwards through the heat exchange tubes. These tubes may contain turbulators to improve the heat transfer. The high thermal conductivity of the liquid sodium minimizes heat transfer limitations due to laminar secondary sodium flow downwards outside the tubes.

Outside the sodium-salt heat exchanger the salt inlet and discharge pipes both go straight down to below the bottom of the sodium-salt heat exchanger before heading toward the steam generator. Below the bottom of the sodium-salt heat exchanger the salt discharge pipe has a tee going to a tall vertical pipe that is open to the atmosphere. This pipe allows emergency molten salt discharge

In the event of a sodium-salt heat exchanger tube failure the higher pressure sodium flows into the nitrate salt where it immediately generates nitrogen gas. This gas is at a lower pressure than the secondary sodium, so there is no danger of salt flowing through the tube rupture and into the secondary sodium circuit.

The salt circuit has a small cushion tank immediately above the sodium-salt heat exchanger. This cushion tank rapidly fills with nitrogen gas which then fills the tube side of the sodium-salt heat exchanger, forcing salt out the aforementioned vent to the atmosphere.

Eventually the secondary sodium circuit loses its pressure by discharge of secondary sodium and/or argon through the sodium-salt heat exchanger tube rupture. However, by then the salt side of the sodium-salt heat exchanger is filled with nitrogen gas, not salt. Hence there is no danger of salt back flowing into the secondary sodium circuit.

In the event of an intermediate heat exchange tube failure the argon pressure in the cushion tank over the secondary sodium circuit will cause the secondary sodium to flow through the rupture in the intermediate heat exchange bundle and into the primary sodium pool.

As soon as a sensor which monitors the secondary sodium level in the secondary sodium cushion tank detects a drop in the secondary sodium level a valve opens which allows compressed nitrogne gas into the top of the sodium-salt heat exchanger tube side. This nitrogen gas blows the salt out of the sodium-salt heat exchanger tube side before the argon pressure on the shell side has had a chance to decay. Hence, even if there is a simultaneous failure of tubes in both the intermediate heat exchanger and the sodium-salt heat exchanger there is still no danger of salt back flowing from the salt circuit into the secondary sodium circuit.

Connected to the salt cushion tank of the sodium-salt heat exchanger is a rupture disk connected to a large diameter vent to the atmosphere. Thus if the salt circuit vent to the atmosphere is plugged by frozen salt, the rupture disk will openventing nitrogen to the atmosphere. Thus there are two redundant means of protecting the sodium-salt heat exchanger from failure due to over pressure on the tube side.

Connected to the cushion tank of the secondary sodium is a rupture disk vented to that atmosphere which provides emergency protection against over pressure in the secondary sodium circuit.

The sodium-salt heat exchanger consists of six stacked coaxial sections. Starting from the bottom:
1) A bottom manifold which preheats the incoming salt. This section has schedule 160 shell and fittings;
2) An externally sleeved lower tube section. The sleeves limit the heat flux through the tube wall near the bottom of the tubes where on system startup there may be cold salt on the inside of the tube and hot liquid secondary sodium on the outside of the tube. This section has a shell side 12.75 inch OD schedule 80 liquid sodium discharge pipe.
3) A middle tube section where the rising salt is heated by the falling secondary sodium;
4) An upper tube where the temperature of salt is maximum. This section has a 12.75 inch OD schedule 80 sodium inlet pipe.
5) A top manifold which collects molten salt from the heat exchange tubes and outputs it via a 12.75 inch OD pipe.
6) The top manifold also has a 12 inch schedule 160 pipe vented via a rupture disk to above the roof. This pipe will collect gas emitted by the salt and may require periodic gas release via a small vent.
 

FNR SODIUM-SALT HEAT EXCHANGER PRESSURE ISSUES:
In normal operation the sodium-salt heat exchanger shell side contains liquid sodium which is atr a moderate pressure. However, the shell should be hydraulic pressure tested to at least 9.0 MPa to safely manage 6 MPa maximum possible pressure transients that might be caused by a transient accident condition.

The sodium-salt heat exchanger tubes normally contain molten salt at atmospheric pressure. After fabrication the tube side of the sodium-salt heat exchanger should be hydraulic pressure tested at 18 MPa.______

The shell side of the sodium salt heat exchanger is filled with liquid sodium.

The sodium-salt heat exchanger has both sodium and molten salt level sensors. An unanticipated decrease in liquid sodium level indicates either an intermediate heat exchange bundle leak or a sodium-salt heat exchanger leak or a secondary sodium dump tank argon charge leak.

This arrangement can safely absorb transient high pressures in either the secondary sodium circuit or the sodium-salt heat exchanger.

On the occurrance of a steam generator tube failure high pressure water and steam both jet through the rupture into the shell side of the steam generator where they contact salt and cause a rapid pressure rise in the salt on the steam generator tube side. The rate of the pressure rise is mitigated by:
a)Use of narrow heat exchange pressure tubes which limit the water flow rate into the salt;
b)Use of upward sloping salt pipes which rise to a vent open to the atmosphere.

c) A gas vent which rises straight up from the sodium-salt heat exchanger upper manifold to the atmosphere. In the event of a either a sodium-salt heat exchange tube rupture or a steam generator tube rupture this vent will violently discharge hot liquid salt. This vent needs a protective shield to stop possible damage to people or equipment caused by this flying hot molten salt.

This pressure relief arrangement protects the sodium-salt heat exchanger from large steam induced molten salt pressure pulses. Note that the 12 inch pipes conveying secondary sodium between the intermediate heat exchange bundle and the sodium-salt heat exchanger are designed so that the pressure differences between the secondary sodium and both the molten salt and the primary sodium are sufficient to prevent either molten salt or primary sodium entering the secondary sodium circuit.

THe secondary sodium induction pump is located close to the pool deck level to ensure that during heat transport loop operation it always has sufficient suction head pressure.

Below the primary sodium pool deck level is a secondary sodium dump tank for holding secondary sodium to permit heat transport loop maintenance. If the argon pressure in the secondary sodium dump tank is released the secondary sodium will drain down almost to the pool deck level. This feature can be used to extinguish a secondary sodium fire.

The secondary sodium argon cover gas is installed with the system cool at an absolute pressure of about 0.2 MPa. If the cushion tank argon pressure rises to 1.0 MPa absolute for even a few milliseconds the loop safety shutdown sequence should be initiated.

The secondary sodium heat transport system is fabricated with schedule 80 pipe to have a working pressure rating of 6.0 MPa and must be hydraulically tested at 9.0 MPa so as to ensure safety in the presence of multiple steam generator heat exchange tube ruptures.

It is difficult to realize good high temperature high pressure sodium seals at flanged mechanical joints. Hence normally the secondary sodium operates below 0.2 MPa gauge pressure. The secondary sodium system is rated for a much higher pressure simply to ensure that there is no major damage when there is a sodium-salt heat exchanger tube rupture or a steam generator tube rupture. It is possible that as a result of a major pressure pulse gasketed flange joints may leak liquid sodium, in which case the secondary sodium must be drained down into the dump tank and the flange joint sealing gaskets replaced. The implication of this is that a tube failure may result in that heat transfer loop being out of service until the next reactor shutdown. However, the large number of independent heat transfer loops minimizes the consequence of a single loop shutdown.

There must be protection against both secondary sodium and molten salt hammer in each secondary sodium circuit. This objective is achieved by prevention of salt and sodium mixing in the secondary sodium heat transport piping.

The 24 inch diameter sodium-salt heat exchanger shell should be schedule 160.
 

This web page last updated September 10, 2021

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