XYLENE POWER LTD.

FNR HEAT TRANSPORT SYSTEM

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

INTRODUCTION:
A nuclear reactor produces heat. That heat is used to produce high pressure steam. Then expansion of that steam through a turbine drives a line synchronous generator to produce 60 Hz 3 Phase electric power.

This web page describes how heat is transported from the FNR primary liquid sodium pool to the steam generators in the turbogenerator halls.

In a liquid sodium cooled Fast Neutron Reactor (FNR) power plant, due to the chemical incompatibility between sodium and water. for safety purposes heat is conveyed first by medium pressure (0.5 MPa) secondary liquid sodium to sodium-salt heat exchangers which are installed in isolated heat exchange galleries constructed around the perimeter of the FNR's primary sodium pool enclosure.

Then the heat is conveyed by molten salt from the sodium salt heat exchangers to steam generators located in lower elevation turbogenerator halls in another building.

The system design approach is to optimize the performance of a 18 MWt (5.4 MWe) heat transport circuit. Then for a particular application up to 56 such identical heat transport circuits are supplied. An initial 37.8 MWe system has seven such circuits but is capable of 8 fold expansion by addition of further heat transport circuits and further turbogenerators up to a maximum of 8 turbogenerators providing a total power of 300 MWe.

This web page provides an overview of the FNR heat transport system. Other web pages focus on the design details of the Intermediate Heat Exchangers, Induction Pumps, Sodium-Salt Heat Exchangers and Steam Generators.
 

HEAT TRANSPORT LOOP ISOLATION:
There are 56 identical independent heat transport loops connecting to the 8 heat exchange galleries located around the perimeter of the primary sodium pool. Failure of any individual heat transport circuit does not cause a failure of the whole.

Each intermediate heat exchanger supplies hot liquid sodium to a dedicated sodium-salt heat exchanger which has a dedicated secondary sodium dump tank, a dedicated electric induction pump, a dedicated argon filled cushion tank and dedicated pressure relief vents. Hence any single heat transport circuit can be shut down for service while the other heat transport circuits remain operational.

Each of eight heat exchange galleries has seven associated heat transport circuits. Each heat transport loop must have the capacity to transport:
1000 MWt / (7 X 8) heat transport loop = 17.857 MWt / heat transport loop
at a liquid temperature differential of:
120 degrees C.

Each loop transfers up to 17.857 MWt of heat which in turn can be used to provide up to:
17.857 MWt / 3.3 = 5.4 MWe
of turbo-electricity generation. Thus the maximum possible system electricity output is limited by the heat transport loops to about:
56 X 5.4 MWe = 302.4 MWe
 

SECONDARY HEAT TRANSPORT FLUID:
The FNR requires a secondary heat transport fluid that is compatible with the heat exchange tube material (Inconel 600) and will not chemically react with the hot radioactive primary liquid sodium in the event of an intermediate heat exchange bundle tube failure. The secondary heat transport fluid must to be chemically stable liquid over the temperature range 15 C to 600 C.

This same secondary heat transport fluid must react with molten salt in a predictable manner to allow safe loop shutdown in the event of a sodium-salt heat exchange tube failure.

To meet these specifications modest pressure (~ 0.5 MPa) non-radioactive sodium is used as the secondary heat transport fluid. This modest pressure permits use of gasketed flanged pipe joints in the heat transport fluid pipes between the intermediate heat exchange bundles and their companion sodium-salt heat exchangers with minimal sodium leakage problems at the flanged joints. However, the entire secondary sodium loop is rated for a 4.6 MPa working pressure transients at 800 degrees K (527 C) to allow safe management of sudden sodium-salt heat exchange tube ruptures and/or steam generator tube ruptures.

The intermediate heat exchange bundles isolate the radioactive primary liquid sodium from the non-radioactive secondary liquid sodium. Should an intermediate heat exchange bundle fail, due to its higher pressure secondary sodium will always flow into the primary sodium, not vice versa. Thus the heat exchange galleries, where the sodium-salt heat exchangers, secondary sodium induction pumps, secondary sodium dump tanks and local salt dump tanks are located, are always radiation free.

Note that if the secondary sodium level or pressure decrease below predetermined thresholds as a safety measure the nitrate salt is drained from the sodium-salt heat exchanger. This drainage is accomplished by:
a) Draining the salt in this heat exchanger into a local dump tank;
and
b) Turning off the salt circulation pump;
and
c) Draining the balance of the salt to a remote dump tank located at a lower elevation than the steam generator.

To accomplish this drainage it is necessary that the steam generator be lower in elevation than the sodium-salt heat exchanger and it is necessary that the remote salt dump tank be lower in elevation than the steam generator. The supply piping must monotonically slope downward from the sodium-salt heat exchanger discharge to the steam generator salt inlet and from the steam generator salt discharge to the remote salt dump tank. The return salt piping must slope monotonically upward from the remote salt dump tank to the local salt dump tank and then from the local salt dump tank to the sodium-salt heat exchanger inlet. If the cover air pressure is removed from the rempte salt dump tank the entire nitrate salt circuit should drain into the remote salt dump tank. If cover air pressure is removed from the local salt dump tank the salt contained in the sodium-salt heat exchanger should drain into the local salt dump tank.

To achieve the desired level of safety it is essential that, even during system transients, at the sodium-salt heat exchanger the secondary sodium pressure must always be greater than the molten salt pressure. Hence if the secondary sodium loses argon cover gas pressure the salt in the sodium-salt loop must be automatically drained to its dump tanks.

The secondary sodium has a pressure head (0.5 MPa) sufficient to ensure that even during a rupture of a sodium-salt heat exchange tube and consequential nitrogen production in the salt the secondary sodium pressure is always greater than the salt pressure in the sodium-salt heat exchanger. Then if there is a sodium-salt heat exchanger tube failure sodium always flows into the salt, not vice-versa. That sodium injection forms nitrogen gas which will escape via the local atmospheric vent. It will likely also violently expel molten salt. The volume of the secondary sodium cushion tank is minimized to minimize the maximum amount of sodium that can be injected into the salt loop.
 

SECONDARY SODIUM LOOPS:
Each heat exchange gallery has an internal length of 12.5 m and an internal width of 8.5 m. Each heat transfer loop has one sodium-salt heat exchanger and is allocated a gallery length of 1.5 m. There are 7 sodium-salt heat exchangers per gallery.

At the end of the Heat Exchange Gallery furthest from the airlock are 2.00 m of Heat Exchange Gallery length and 4 m of heat exchange gallery width reserved for a corner stair well. Thus the overall
Heat Exchange Gallery inside length:
= 7 (1.50 m) + 2.00 m = 12.5 m

The last 2 m is occupied by a stairwell.

Intermediate heat exchange bundles are immersed around the perimeter of the primary sodium pool. Pipes connecting the sodium-salt heat exchangers to the intermediate heat exchange bundles go straight through the primary sodium pool enclosure wall before bending to reach the desired intermediate heat exchange bundle position. Each pipe carrying hot secondary sodium from an intermediate heat exchange bundle goes directly to its companion sodium-salt heat exchanger sodium inlet port. Each pipe carrying cooler secondary sodium back from a sodium-salt heat exchanger to its companion intermediate heat exchange bundle is routed through a flow meter (F) and then an induction pump which in normal operation sets the secondary sodium circulation rate and hence the thermal power converyed by the heat transport circuit.

There is a secondary sodium drain to a pressure rated sodium dump tank at a low point on the secondary sodium return pipe between the steam generator and the intermediate heat exchanger. The drain pipe is connected so as to fully drain the sodium in the induction pump.

Oil cooling in place of salt cooling is required to dump primary sodium heat to the atmosphere sufficiently to enable work inside the reactor space. One of the heat transport systems in each heat exchange gallery should be configured to allow oil cooling of the secondary sodium.
 

MOLTEN SALT HEAT TRANSPORT:
The molten salt acts as a thermal fluid and transports heat from each sodium-salt heat exchanger under a roadway to a remote turbine hall which contains a corresponding steam generator. The salt return pipe comes back under the roadway to the local salt dump tank which is at the basement level of the Heat Exchange Gallery. The local salt dump tank must be at a lower elevation than the sodium-salt heat exchanger to enable salt gravity drainage from the sodium-salt heat exchanger to the local salt dump tank.

The salt piping slopes monotonically downward toward the remote salt dump tank so that when the air pressure over the remote dump tank is released all the salt in the loop self drains into the remote dump tank.

The salt circulation pump is magnetically coupled to avoid hot salt leakage and is located close to the discharge from the remote dump tank.

In the event of a sodium-salt heat exchanger tube failure the higher pressure secondary sodium will enter the molten salt circuit where it will rapidly chemically react with the molten salt producing nitrogen. The nitrogen will displace salt in the sodium-salt heat exchanger tubes and manifolds, and will force molten nitrate salt to discharge out the adjacent salt vent. As the secondary sodium loses pressure and its level decreases the air pressure over the local salt dump tank is released and any remaing salt in the sodium-salt heat exchanger will drain to the local salt dump tank. The nitrogen producing chemical reaction will continue as long as there is liquid secondary sodium above the sodium-salt heat exchanger tube rupture. Thus the salt vent and the various pressure ratings must be sufficient to safely manage this rapid nitrogen production and discharge.

An important ongoing operating issue is maintenance of sufficient thermal flux and molten salt flow to prevent the molten salt from freezing in the external pipes and in the steam generator.

The pressure on the water side of the steam generator must be set sufficiently high to keep the steam generator water temperature and hence the molten salt return temperature well above the salt melting point. The molten salt supply temperature is limited by the secondary sodium temperature and the secondary sodium flow rate.

When the molten salt supply temperature falls below its melting point there will be no heat transport away from the sodium-salt heat exchanger. When it is necessary to cool the primary sodium below the salt melting point for service purposes heat should be removed from the secondary sodium using oil cooling instead of salt cooling. In normal operation oil is drained from the oil cooler to prevent high temperature oil breakdown.
 

BLACKSTART:
Before the system can generate any electricity the nitrate salt must be melted. That is achieved by injecting heat into the salt dump tanks via their electric heaters. Normally only one heat transport circuit is started at a time to minimize the auxiliary power required for black start.

Once the salt in the dump tanks has reached its design working temperature air pressure is applied to the salt dump tanks to fill the salt loop and the secondary sodium induction pump and the salt circulation pump are turned on, also under auxiliary power. This action provides molten salt borne reactor heat to the steam generator. Then the steam generator water injection pump is enabled, which allows steam generation and turbine startup. Once the turbine is operating electricity can be generated and the system no longer requires auxiliary power for continuation of black start. Thus there must be enough auxilliary power to fully turn on any one of the redundant heat transport circuits.

It is necessary to inject cover air pressure into the molten salt dump tanks to give the molten salt circulation pump suction head prior to turn-on of the molten salt circulation pump. Each dump tank requires an internal liquid level sensor to regulate its cover air pressure.

STEAM GENERATOR TUBE FAILURE:
A steam generator tube failure will cause injection of steam / water into the molten salt. The water will immediately form steam which will expand blowing molten salt out the steam generator remote molten salt vent to the atmosphere. A sudden molten salt pressure or level rise in the remote salt vent indicates a steam generator tube failure.
 

MAINTENANCE:
There is no radioactivity in the heat exchange galleries which allows safe service work or secondary sodium fire suppression in selected heat exchange galleries without a total reactor shutdown.
 

DIAGRAM:
A diagram showing one of 56 heat transport systems is shown below.

This diagram shows the intermediate heat exchange bundle and the secondary sodium pipes on the left hand side, the molten salt pipes on the right hand side and the secondary sodium dump tank in the lower left middle. The horizontal induction pump, the secondary sodium flow meter and the vertical sodium-salt heat exchanger are in the middle of the diagram. The local salt dump tank is on the lower right. Above the sodium-salt heat exchanger from left to right are the secondary sodium argon filled cushion tank, and the molten salt air cushion tank and the molten salt pressure relief vent.

Note that the heat exchange gallery inside width is 8.5 m.

Note the dedicated steel columns which support the various pieces of equipment.

Note the space below the sodium-salt heat exchangers is kept open for moving the vertically upright mobile air locks from their truck load/unload points outside one end of the heat exchange gallery to the airlock ports underneath the pool deck corners.
 

HEAT EXCHANGE GALLERY LOWER LEVELS:
Each heat exchange gallery has lower levels where the induction pumps, induction pump power supples and the dump tanks are located. Personnel access to the various heat exchange gallery levels is via a stairwell adjacent to the long outside wall and at the gallery end far from the airlock truck load/unload point. Equipment in the heat exchange galleries is installed and removed from above using a mobile crane parked at the airlock truck load/unload point.

There must be a large air vents in heat exchange galleries for air cooling.

An important issue in the heat exchange gallery basement is isolation of potential sodium drips. Yjr likely source of these drips are the secondary sodium pipe flange connections. Drip pans should be provided to isolate dripped sodium. The pumped water sump should be located near the outside wall.

The heat exchange gallery basement water sump pump may need to discharge into a near grade level storm sewer. It would be better if it drained into the facility bottom drain located about 18 m below grade.
 

SECONDARY SODIUM PIPING:
The intermediate heat exchanger is realized with a single pass vertical tube bundle.

The intermediate heat exchange bundle tube side top connects to the sodium-salt heat exchanger shell side top via a 12.75 inch OD thick wall pipe. The intermediate heat exchanger tube side bottom connects to the sodium-salt heat exchanger shell side bottom via a 12.75 inch OD thick wall pipe. This pipe contains an induction pump that circulates secondary liquid sodium from the bottom of the sodium-salt heat exchanger to the bottom of the intermediate heat exchanger. This configuration provides heat exchange counterflow and permits removal and replacement of individual sodium-salt heat exchangers and induction pumps via an overhead crane lift. There is a gravity drain to a secondary sodium dump tank that also acts as a cushion tank.
 

PIPE WALL SEALING AND SUPPORT:
Flexible air and argon bellow wall seals are required at locations where the secondary sodium pipes pass through the inner reactor enclosure wall to accommodate thermal expansion/contraction. Under ordinary operation the reactor power is modulated by controlling the secondary sodium circulation rate via the induction pumps. This control methodology causes significant pipe thermal expansion/contraction.

The secondary sodium service pipes and the induction pump must be separately supported with threaded hanger hardware so that these pipes remain in correct position when an intermediate heat exchanger is disconnected.
 

INDUCTION PUMP:
Induction pumps are used to circulate the secondary sodium. The induction pumps must be sized to overcome the flow pressure head in the secondary sodium loops. Note that these pumps must be located on the low temperature return pipes near the primary sodium pool deck level to ensure both cool operation and sufficient positive pump suction head.

The induction pump operates by inducing a circular current in the liquid sodium. This current crosses a radial magnetic field component and hence experiences an axial force. External 3 phase coils, analogous to the stator coils of a 3 phase AC motor, create a suitable time varying magnetic field.

In normal operation the pumped liquid sodium temperature is about 330 degrees C and the induction pumps are oil cooled to protect the electrical insulation from heat damage.

When the steam generator upper manifold steam vent is opened the water temperature in the steam generator falls to 100 degrees C and the liquid sodium temperature at the induction pump falls to about 110 C, at which point the induction pump does not need oil cooling.

However, under circumstances when the steam generator contains no water either natural or pumped circulation of the secondary sodium can cause the secondary sodium temperature at the induction pump to rise to about 460 degrees C. Under these circumstances the induction pump can easily be damaged if there is insufficient pumped oil cooling. Generally, before the steam generator is drained of water the induction pump should be stopped and the secondary sodium should be gravity drained into its dump tank. Likewise, the steam generator should be charged with water before the secondary sodium is transferred from its dump tank back to the upper portion of the heat transfer loop.

The induction pumps have 16.000 inch OD schedule 80 (0.843 inch wall) 316 SS input pipes and 12.75 inch OD schedule 80 (0.687 inch wall) 316 SS discharge pipes. The 16.000 inch OD pipe runs through the induction pump but is partially obstructed by the induction pump's internal magnetic flux torpedo.

The ID of the 16 inch pipe is:
16.000 inch - 2 (0.843 inch) = 14.314 inch

Pm (14.314 inch) = 2 x 0.843 inch X 39 MPa
or
Pm = 2 x 0.843 inch X 39 MPa / 14.314 inch
= 4.593 MPa

Thus the secondary sodium loop has a maximum safe peak working pressure of 4.6 MPa at 800 degrees K. The induction pumps should be hydraulic pressure safety tested at 300 degrees K at:
4.593 MPa X (208 / 117) X 1.5 = 12.25 MPa_______.

Induction pump details are set out at FNR Induction Pump.
 

GASKET CONSTRAINT:
A major constraint on the FNR design is gasket properties. This FNR operates at too high a (temperature X pressure) product for use of elastomeric gaskets. Soft metal or compressed carbon gaskets must be used. Such gaskets do not tolerate pipe misalignment, manifold distortion or high pressures. Hence gasketed mechanical joints need near optical precision fabrication. All the intermediate heat exchange bundle manifolds bottom and top halves are sealed with such gaskets.The flange bolts must have a smaller TCE than the flange material.
 

THERMAL EXPANSION:
The sodium-salt heat exchangers are in fixed positions with respect to the concrete structure. The intermediate heat exchange bundles move due to connecting pipe thermal expansion and contration. Also the primary sodium pool inside wall moves due to thermal expansion. When the system is cold the intermediate heat excnage bundle supply pipes should be almost touching the primary sodium pool inside wall. When the system is hot there is about a 0.2 m gap between the cool sodium pipe insulation and the primary sodium inside tank wall.
 

SECONDARY SODIUM TEMPERATURE CONSTRAINTS:
Inside the tubes of the intermediate heat exchange bundle is non-radioactive secondary sodium normally at a pressure of about 0.5 MPa. The low temperature limit on the circulated intermediate sodium at full load is 320 degrees C to prevent NaOH precipitation on the outside of the tubes within the sodium-salt heat exchanger shell. The high temperature limit on the circulated secondary sodium is ~ 460 degrees C. We are assuming a full load 10 degree C temperature difference across the intermediate heat exchange tube bundle wall. At full load the dry steam in the steam generator will reach about:
(460 deg C - 10 deg C -10 C - 40 deg C) = 400 deg C.

At full load the secondary sodium temperature differential can drop to 450 -330 = 120 deg C without threat of NaOH precipitation. There is feed water temperature mixing in the lower manifold of the steam generator to minimize thermal stress on the lower ends of the steam generator tubes. The injection water temperature rise from 25 deg C to 320 C is realized by feed water recuperator heat recovery from the steam immediately upstream from the turbine condenser followed by water mixing in the lower manifold of the steam generator.
 

TEMPERATURE CONSTRAINT:
At low steam loads the seconday sodium flow will decrease and the primary sodium discharge temperature from the FNR will settle at about 453 degrees C. As the steam load increases the secondary sodium flow will increase and the secondary sodium discharge temperature from the intermediate heat exchange bundle will decrease to about 450 degrees C.
 

FULL LOAD SYSTEM TEMPERATURE PROFILE (deg C):_______

	FUEL CENTER	Primary   Na HIGH	Primary   Na LOW	Secondary   Na HIGH	Secondary   Na LOW	STEAM   HIGH	STEAM   LOW	WATER   HIGH	WATER   LOW
10%	550	544	322	543	321	503	320	320	____
100%	550	440	340	429	329	440	320	320	____

 

SECONDARY SODIUM FLOW:
Some of the FNR secondary sodium flow issues are discussed in the paper titled Improving SFR Economics Through Innovation.

The FNR secondary sodium loops take advantage of the relatively large thermal coefficient of expansion of liquid sodium to promote natural circulation of the secondary liquid sodium. Even with no pumping the position of the sodiun-salt heat exchanger above the intermediate heat exchange bundles causes the secondary liquid sodium to naturally circulate.

There is an induction pump for larger controlled circulation of secondary sodium.

The secondary sodium pipe is chosen to be 12.75 inch OD, schedule 80 (0.687 inch wall). Then the pipe ID is:
12.75 inch - 2 (0.687 inch) = 11.376 inch

Then the pipe cross sectional area A is:
A = Pi [(11.376 inch / 2) (.0254 m / inch)]^2
= .06557 m^2

Define:
Rho = 0.927 gm /cm^3 = density of liquid sodium;
V = average sodium linear velocity in 12.75 inch OD pipe;
Cp = 1.23 J / gm-deg C = heat capacity of liquid sodium;

Loop Thermal power = Rho A V Cp (150 deg C) 17.857 MWt X 10^6 Wt = 0.927 gm /cm^3 X 10^6 cm^3 / m^3 X 0.06557 m^2 X V X 1.23 J / gm deg C X 100 deg C
= 7.476 X 10^6 V J / m

Hence:
V = 17.857 Wt / 7.476 J/ m
= 2.3885 m / s
which is an acceptable liquid flow velocity.

Hence the maximum secondary liquid sodium flow rate in each heat transport loop is:
A V = .06557 m^2 X 2.3885 m / s
= 0.156619 m^3 / s

Hence the secondary sodium flow rate for the entire reactor is:
0.156619 m^3 / s X 56 = 8.77066 m^3 / s
which must be matched by the primary sodium flow rate. If the available primary sodium flow rate is less than 8.77066 m^3 / s it will constrain the reactor thermal output power.

In normal operation hot (460 degree C) primary sodium flows into the intermediate heat exchange bundles just below the pool surface, flows down outside the intermediate heat exchange tubes and is discharged about 6 m below the primary sodium pool surface at a temperature of about 340 degrees C. Simultaneously secondary sodium flows up inside the intermediate heat exchange bundle tubes, entering at about 330 degrees C and leaving at about 450 degrees C.

The intermediate heat exchange bundles are single pass to realize counter current operation and to minimize material thermal stresses.

In the sodium-salt heat exchanger secondary sodium flows downwards outside the tubes while molten salt flows upwards inside the tubes.

If there is a secondary sodium leak from an intermediate heat exchange tube or manifold secondary sodium will leak slowly into the primary sodium pool.

Potential fire problems are secondary sodium leaks from the heat transport piping, sodium-salt heat excahnger or induction pump. The secondary sodium pressure driving such leaks is low. In every case the fire can be extinguished by releasing the air pressure in the molten salt dump tank to lower the molten salt level below the bottom of the sodium-salt heat excahanger and then releasing the argon pressure in the sodium dump tank which causes gravity drain down of the secondary liquid sodium into the below grade sodium dump tank.
 

MAXIMUM SAFE SECONDARY SODIUM WORKING PRESSURE:
For the secondary sodium 12.75 inch OD schedule 80 pipe the ID is:
11.376 inch

The secondary sodium pipe wall thickness is:
0.687 inch

Assume that the secondary sodium pipes are made of 316 SS pipe. A table of 316 SS yield stress versus temperature is as follows:

	TEMPERATURE (degrees K)	YIELD STESS (MPa)
300	208
400	167
500	144
600	129
700	122
800	117
900	114
1000	104

The highest anticipated secondary sodium working temperature is:
520 C + 273 = 793 K
which from the above table indicates a 316 SS yield stress of 117 MPa.

The corresponding maximum 316 SS working stress is:
117 MPa / 3 = 39 MPa.

Let Pm = maximum allowable liquid sodium pressure. Then:
Pm (11.376 inch) = 2 (0.687 inch)(39 MPa)
or
Pm = 2 (0.687 inch)(39 MPa) / 11.376 inch
= 1207.806 psi
= 1207.806 psi X .101 MPa / 14.7 psi
= 4.71 MPa
which is the maximum safe transient secondary sodium pressure due to formation of hydrogen.
 

INTERMEDIATE HEAT EXCHANGE BUNDLES:
The intermediate heat exchange bundles isolate the radioactive primary liquid sodium from the secondary non-radioactive liquid sodium. They also serve as a barrier to other radioactive species in the event of a fuel tube leak.

To attain the desired temperature distribution in the intermediate heat exchanger bundles the secondary sodium mass flow rate must be the same as the primary sodium mass flow rate. However, the primary sodium flow cross sectional area is larger so the primary sodium linear descent velocity is smaller.

There is a ring of 56 intermediate heat exchange bundles that are immersed in the primary sodium pool in a 19 m OD, 17 m ID ring. The heat exchange tubes are located between the pool surface and 6 m below the pool surface.

The weight of the intermediate heat exchangers is supported by a ring shaped table located in the primary sodium pool about 8 m above primary sodium pool bottom and about 9 m radially from the pool center. This table is supported by a ring of steel columns and the table is horizontally stabilized by interleg connections and by the adjacent innermost steel cup primary sodium pool liner. The top of this table is smooth to allow the supported intermediate heat exchange bundles to move back and forth radially to accommodate thermal expansion/contraction of the primary sodium pool and the radial secondary sodium piping. Apart from thermal expansion the radial position of the intermediate heat exchangers is fixed by the radial secondary pipes which in turn are fixed to the steam generators and induction pumps, which are in turn are fixed to the heat exchange gallery concrete structure. Thermal expansion, at points where these pipes pass through gas tight walls, is accommodated by use of metal bellows fittings.

The maximum outside diameter of the intermediate heat exchange manifolds is limited to:
38 inches
so that 56 of them will fit in the available space on an 18 m diameter circle in the primary sodium pool.
 

SECONDARY SODIUM CIRCULATION PUMPING POWER:
The secondary liquid sodium mechanical pumping power is:
(force) X (velocity)
= (pressure head) X (area) X (velocity)
= (pressure head) X (volumetric flow rate)

Recall that for each heat transport loop:
(volumetric flow rate) = 0.10440 m^3 / s
 

SODIUM FIRES:
Other parties using liquid sodium for heat transport have experienced repeated sodium fires. The problem is that if hot high pressure secondary liquid sodium is inside a pipe the secondary sodium tends to leak out via any mechanical joint that is less than perfect. The secondary liquid sodium operating temperature is too high for use of elastomeric gaskets. Once outside the pipe the sodium spontaneously ignites in air. That situation is unacceptable.

The solution to this problem is to operate the secondary sodium at the lowest practical pressure and to fit every secondary sodium loop with a dump tank and every sodium pipe connection with a leak detector. At the first hint of a fire release of argon pressure in the dump tank causes the sodium in the affected loop to drain into its sodium dump tank. Simultaneously the level of the molten salt must be dropped.
 

DUMP TANKS:
At the low points in the heat transport pipe loops are dump tanks with sufficient volume to accommodate all the fluid in that heat transport loop. These dump tanks have electric heaters for fluid melting and temperature maintenance. Note that the sodium dump tanks must be rated as pressure vessels and must be fitted with high pressure argon relief valves vented to the argon atmosphere over the primary sodium pool. Note that changing the temperature of the argon in the dump tank affects the argon pressure and hence the secondary sodium level. Hence each secondary sodium loop needs its own argon pressure and liquid level regulation.

The liquid sodium injection/removal port to the dump tank relies on argon pressure which is controlled by external low temperature argon valves. To charge the loop with liquid sodium it is injected into the dump tank while the contained argon is vented to an isolated atmospheric pressure argon sink.

This arrangement requires a reliable valves on small argon pipes connected to the dump tank that either vent the dump tank to the argon atmosphere or connect pressurized argon to drive liquid sodium out of the sodium dump tank.
 

DETECTION OF SECONDARY SODIUM LEAKS:
Any leak in a secondary sodium circuit will result in loss of sodium height in the secondary sodium system. Should such a level drop occur that entire heat transport loop should be shut down and the molten salt drained to its dump tank and the secondary sodium drained to its the dump tank.
 

NATURAL CIRCULATION REQUIREMENT:
There must be enough natural circulation of secondary sodium to remove fission product decay heat. This decay heat is quantified in the paper: Decay Heat In Fast Reactors.
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AUXILIARY POWER FOR DECAY HEAT REMOVAL:
There must be enough reliable molten salt circulation and enoough water available at the steam generators to remove the fission product decay heat by evaporation of that water.
 

SECONDARY SODIUM PRESSURE CHANGE DRIVING NATURAL CIRCULATION:
In normal full load reactor operation the reactor produces 1000 MWt of heat. When the chain reaction is off the reactor may still produce as much as:
0.08 X 1000 MWt = 80 MWt
of fission product decay heat.

Hence natural circulation of the secondary sodium with the steam generators at atmospheric pressure should run at over 8% of the pumped circulation rate.

The natural circulation rate will be primarily limited by the temperature difference between the secondary sodium rising leg and the secondary sodium falling leg and by the viscous flow pressure drop across the intermediate heat exchange bundle. Thus these pressure drops need to be quantified.

The volumetric TCE of liquid sodium is 240 ppm / deg C. Hence if there is a 100 degree C temperature difference between the rising and falling legs the change in sodium density is:
.967 kg / lit X 240 X 10^-6 / deg C X 1000 lit / m^3 X 100 deg C = 0.967 X 24 kg / m^3

Assume an elevation difference of 10 m. Then the corresponding differential pressure is:
0.967 X 24 kg / m^3 X 10 m X 9.8 m / s^2 = 2274 Pa

The molten salt will not transport heat until it is above its melting point of about 300 degrees C.

We need to compare these pressure drops to the viscous pressure drop across the intermediate heat exchange bundle at 1 / 10 of normal flow.
 

CALCULATE SECONDARY SODIUM VISCOUS PRESSURE DROP AT SUFFICIENT NATURAL CIRCULATION TO REMOVE FISSION PRODUCT DECAY HEAT:
An important issue with the intermediate heat exchange bundles is their ability to remove fission product decay heat by natural circulation. In natural circulation the liquid sodium flow rate is low and laminar, so the heat transfer characteristics are different from when the secondary loop is pumped. It is necessary to have a sufficient number of intermediate heat exchange tubes to allow the required natural circulation and laminar flow limited heat transfer. The viscosity of the sodium must be taken into account.
 

The following equations derived on the web page titled FNR PRIMARY SODIUM FLOW can be used to find the natural circulation volumetric fluid flow Fv per round coolant flow channel.
Fv = {Pi Pg Ro^4 / [Muv Zo (N + 2)(N + 4)]}BR> where:
Pi = 3.14159
Pg ~ 1000 Pa
Ro = (0.37 inch / 2) X (0.0254 m / inch) = 0.004699 m
Muv = 3 X 10^-4 N-s / m^2
(N + 2) = Ro [(Rhos Pg) / 2]^0.25 [1 / [Muv Zo]^0.5]
where:
Zo = 6.0 m
Rhos = 849.4 kg / m^3

Numerical substitution gives:
(N + 2) = Ro [(Rhos Pg) / 2]^0.25 [1 / [Muv Zo]^0.5]
= 4.699 X 10^-3 m [(849.2 kg / m^3) (1000 kg m /s^2 m^2) / 2]^0.25 [1 / [(3 X 10^-4 N^-s / m^2)(6 m)]^0.5]
= 4.699 X 10^-3 m [25.5267 kg^0.5 / s^0.5 m] [ 1 / [4.24264 X 10^-2 (kg m s /s^2 m)^0.5]]
= 2.8272482 kg^0.5 s-0.5 kg^-0.5 s^0.5
= 2.8272482

Hence the secondary sodium natural circulation flow Fv through each tube is given by:
Fv = {Pi Pg Ro^4 / [Muv Zo (N + 2)(N + 4)]} = {3.14159 (1000 N / m^2)(0.004699 m)^4 / [(3 X 10^-4 N - s / m^2) (6 m) (2.8272)(4.8272)]}
= {3.14159 (1000)(487.55294 X 10^-12 m^2 / [(245.654277 X 10^-4 s / m)]}
= 6.23515 X 10^-5 m^3 / s

With 800 tubes intermediate heat exchange / bundle the secondary sodium natural circulation flow rate is:
800 tubes/bundle X 6.23515 X 10^-5 m^3 / s-tube = 0.06378 m^3 / s which is faster _________than the minimum required natural circulation rate.
 

Note that there is enough heat stored in the liquid sodium pool to sustain electricity production for several minutes after the reactor chain reaction is shut down. The situation being addressed here is one of induction pump off and chain reaction shutdown but continued reactor heat production due to fission product decay.
 

CORROSION:
We need to be concerned about possible long term corrosion of the enclosing materials caused by impurity NaOH in the secondary sodium. A relevant reference is corrosion by caustic soda.
 

PIPE FEEDTHROUGHS AND HANGERS:
The hot secondary liquid sodium is directly connected to the adjacent sodium-salt heat exchanger. Flexible air and argon bellow wall seals are required at locations where the secondary sodium pipes pass through the inner reactor enclosure wall to accommodate thermal expansion/contraction. Under ordinary operation the reactor power is modulated by controlling the secondary sodium circulation rate via the induction pumps. This control methodology causes significant pipe thermal expansion/contraction.

The secondary sodium service pipes and the induction pump must be separately supported with threaded hardware so that these pipes remain in correct position when an intermediate heat exchanger is disconnected.
 

PROVISIONS FOR SYSTEM MAINTENANCE:
To enable heat transport system service the secondary sodium is transferred into its dump tank.

Application of argon pressure over the dump tank and relief of argon pressure from the top of the secondary sodium loop transfers secondary liquid sodium from the dump tank into the secondary loop. Reversing this procedure allows secondary sodium to gravity flow back into the dump tank.

A liquid sodium level detector should be provided at the top of the system to indicate when to close the system fill valve. The supply of argon for presurizing the dump tank and the argon sink from the system top can both be from the primary sodium pool enclosure. Similarly when drain down to the dump tank is required the argon in the dump tank can be vented to the primary sodium pool enclosure and after the argon pressure has sufficiently dropped the argon top vent valve can be openned. Note that for safety this argon top vent valve must be vented over the primary liquid sodium pool.

To expel secondary sodium from the intermediate heat exchanger tubes the secondary sodium level is first drained down to the level of the bottom of the return pipe to the intermediate heat exchanger. Then a blanking disk is installed between the flanges of the intermediate heat exchange bundle secondary sodium discharge pipe. Then argon pressure is applied to the intermediate heat exchange discharge which will drive most of the contained sodium from the intermediate heat exchange bundle into the dump tank.

In reality, even after this procedure is complete a small amount of secondary sodium will remain in the bottom manifold of the intermediate heat exchange bundle. This small sodium accumulation presents a potential risk to maintenance personnel and a potential fire risk if at some later time oxygen is admitted into the secondary sodium piping. This risk can be minimized by use of a small tube that connects onto the bottom of the intermediate heat exchange bundle lower manifold and removes the remanent secondary sodium by vacuum suction. In general for safety the flange connections of the intermediate heat exchange bundle and the radial piping should be closed with blanking plates while the intermediate heat exchange bundle is still in the argon atmosphere over the primary sodium pool.
 

PROTECTION OF THE INTERMEDIATE HEAT EXCHANGE BUNDLE FROM HIGH TRANSIENT PRESSURES:
In the event of a sodium-salt heat exchanger tube rupture there is potential for a violent chemical reaction between the secondary sodium and the salt. This chemical reaction will cause a rapid transient rise in the secondary sodium loop pressure which if not properly managed could cause a liquid sodium pressure pulse analogous to water hammer, possibly rupturing the intermediate heat exchange bundle and possibly leading to major facility damage. Hence the secondary heat transport system, including the intermediate heat exchange bundles, must be designed to safely withstand large transient pressures and the secondary liquid sodium should normally operate at a pressure of about 0.5 MPa. Sustained high pressure is prevented by venting the molten salt to the atmosphere and by dropping the molten salt level at the first hint of trouble.

In order for the safety mechanisms to reliably there must also be a rupture disk That can provide instantaneous secondary sodium pressure relief. The aforementioned rupture disk must be able to discharge nitrogen gas from the secondary sodium circuit at a rate equal to the maximum possible rate of high pressure nitrogen formation in that circuit. That rate is limited by the maximum molten salt flow through the sodium-salt heat exchange tube rupture which is a function of the heat exchange tube inside diameter and the transient differential pressure between the molten salt and the secondary sodium.

In a practical accident scenario the chemical reaction forms nitrogen which rapidly raises the secondary sodium pressure. The nitrogen is vented via the secondary sodium cushion tank rupture disk. Most of the nitrogen forms in the molten salt and is vented to the atmosphere via the molten salt vent.

Each secondary sodium heat transport loop is normally modestly above ambient pressure. The secondary sodium dump tank also acts as an expansion tank that attenuates any pressure pulse in the secondary liquid sodium.
 

CONSTRAINTS:

1) The airlock design for primary sodium pool access must be large enough to accommodate the intermediate heat exchangers and related flanged pipe stubs.

2) There must be enough secondary sodium natural circulation to dependably remove fission product decay heat so that the system remains safe on loss of electric power to the sodium induction pumps.

3) Need thick concrete walls between the primary sodium pool and the heat exchange galleries to protect workers in the heat exchange galleries from gamma radiation emitted by decay of Na-24 into Mg-24.
 

THERMAL SIPHON CALCULATIONS:
Define:
V = linear sodium velocity in pipe g = acceleration of gravity
Rho = density of liquid sodium
A = pipe cross sectional area
H =verage height of temperature difference
Cp = heat capacity of sodium ~ 0.3 cal / gm-deg C

Assume that the steam generator is mounted so that its center is 5 m above the surface of the primary sodium pool. Then the top of the steam generator tube bundle is 8 m above the top surface of the primary liquid sodium pool. This height is consistent with the sodium vapor pressure constraint. For H = 5 m:
V^2 = 1.176 m^2 / s^2 or
V = 1.0844 m / s

Thermal power = Rho A V Cp (150 deg C) = 0.927 gm /cm^3 X 10^6 cm^3 / m^3 X A X 1.0844 m / s X 1.23 J / gm deg C X 150 deg C
= 185.466 X 10^6 J A / m^2 - s
= 185.466 MWt A / m^2

Thus neglecting viscosity and assuming ideal heat exchanger and reactor performance the maximum heat transport capacity is:
185.466 MWt / m^2 X 8.705 m^2 = 1614.5 MWt.

However, there will be a reduction in intermediate heat exchanger performance because the primary side of the intermediate heat exchanger operates in the laminar flow region. The effective tube wall thickness is increased by about (1 / 8) inch of sodium. At 700 deg K the liquid sodium has a thermal conductivity of 70.53 W / m-deg K.

Hence the temperature drop delta T across the (1 / 8) inch thick liquid sodium boundary layer is given by:
875 X 10^6 Wt / 56 bundles = [(delta T) (70.53 Wt / m-deg K) X 823 tubes X 6 m X Pi X (0.5 inch) / (1 / 8)] inch
or
(delta T) = [875 X 10^6 Wt / [(56 bundles) X (70.53 Wt / m-deg K) X (823 tubes / bundle) X 6 m X Pi X (0.5 inch) / (1 inch / 8)]
= 3.57 deg K

The web page FNR HEAT EXCHANGE TUBES
indicates that at full rated power the temperature drop across the intermediate heat exchanger tube metal wall is 7.40 deg C. Hence the total temperature drop across the intermediate heat exchange bundles at full power is about:
3.57 deg C + 7.40 deg C = 10.97 deg C.

The differential pressure established by the falling sodium column is:
P = [(873.2 -849.4) / 2] kg / m^3 x 6 m x 9.8 m / s^2
= 699.72 kg m / s^2-m^2

Neglecting viscosity:
P = Rho V^2 / 2
or
V = [2 P / Rho]^0.5 = [2 (699.72 kg m / s^2-m^2) / (849.4 kg / m^3)]^0.5 = 1.2836 m / s
= maximum possible falling sodium flow velocity

The corresponding falling sodium volumetric flow rate = 1.2836 m / s X 0.3893 m^2 = 0.4996 m^3 / s

This is about twice the required flow rate. However this flow rate will be retarded by viscosity. This flow rate can be further reduced as required by adding a baffle to the primary side of the intermediate heat exchanger.

It is necessary to maintain the design temperature differential across the reactor in order to develop the required primary sodium natural circulation through the reactor.

Note that each intermediate heat exchange tube bundle contains baffles that cause zig-zag downward flow to enhance heat transfer. The baffle design detail may have to be experimentally optimized. Note that the baffle gives each tube bundle an effective open area on the primary sodium side of about 1 m^2_____.

TEMPERATURE CONSTRAINT:

As the primary liquid sodium flows through the reactor at full power its temperature increases from 340 C to 440 C. In this temperature range in a fast neutron flux the fuel tube material HT-9 undergoes goes material embrittlement.
 

The corresponding intermediate sodium flow rate in the intermediate heat exchanger tubes is:
(321.05 m^2 / hr-exhanger) / (823 tubes / exchanger) = 0.3900 m^3 / hr-tube
= (0.3900 m^3 / hr-tube) X (1 hr / 3600 s)
= 1.08333 X 10^-4 m^3 / s-tube

The corresponding intermediate sodium flow rate in the steam generator tubes is:
(321.05 m^3 / hr-exchanger) / (625 tubes / exchanger)
= 0.51368 m^3 / hr-tube
= (0.51368 m^3 / hr-tube) X (1 hr / 3600 s)
= 1.4268 X 10^-4 m^3 / s-tube

The following equations derived on the web page titled FNR PRIMARY SODIUM FLOW can be used to find the pressure drop Pd per round coolant flow tube neglecting natural circulation:
Fv = [Pi Pd Ro^4] / [Muv Zo (N + 2)(N + 4)]BR> where:
Pd = pressure drop along tube in Pa (1 Pa = 1 kg m /s^2 m^2) Fv = volumetric flow rate / tube Pi = 3.14159
Ro = (0.37 inch / 2) X (0.0254 m / inch) = 0.004699 m = tube inside radius
Muv = 3 X 10^-4 N-s / m^2 = sodium viscosity
(N + 2) = Ro [(Rhos Pd) / 2]^0.25 [1 / [Muv Zo]^0.5]
where:
Zo = 6.0 m = tube length
Rhos = 849.4 kg / m^3 = density of sodium at the reactor operating temperature

Recall that:
Fv = [Pi Pd Ro^4] / [Muv Zo (N + 2)(N + 4)]
or
Pd = [Fv / (Pi Ro^4)] [Muv Zo (N + 2)(N + 4)] Equation #1

Recall that:
(N + 2) = Ro [(Rhos Pd) / 2]^0.25 [1 / [Muv Zo]^0.5]
or
N = {Ro [(Rhos Pd) / 2]^0.25 [1 / [Muv Zo]^0.5]} - 2 Equation #2

Try and initial interim value of N = 1.0 in equation #1 and solve for an interim value of Pd.
Substitute that interim value of Pd into equation #2 and solve for a new interim value of N.

Repeat this process itteratively until N and Pd converge.

Then the sum of the Pd values in combination with the exchanger intermediate sodium flow rate give the mechanical load for an ideal circulation pump.

This intermediate liquid sodium flow will develop a momentum change pressure at a sharp 90 degree elbow of:
(2.010 m / s)^2 X 1 m^2 X 927 kg / m^3 = P X 1 m^2
or
P = (2.010 m / s)^2 X 927 kg / m
= 3746 kg / m s^2
= 3746 Pa
= 3746 Pa X 1 bar / 101,000 Pa
= 0.03709 bar

This pressure change is one possible method of measuring the fluid velocity in the pipe.
 

This intermediate liquid sodium flow will develop a momentum change pressure at a sharp 90 degree elbow of:
(2.010 m / s)^2 X 1 m^2 X 927 kg / m^3 = P X 1 m^2
or
P = (2.010 m / s)^2 X 927 kg / m
= 3746 kg / m s^2
= 3746 Pa
= 3746 Pa X 1 bar / 101,000 Pa
= 0.03709 bar

This pressure change is one possible method of measuring the fluid velocity in the pipe.
 

SECONDARY LIQUID SODIUM FLOW PATH:
This configuration balances flows, optimizes heat transfer and minimizes thermal stresses. The standard piping connection arrangement for each high pressure secondary sodium circuit starting at the 12 inch induction pump: 1 X 12 inch straight pipe, 1 X 12 inch 90 degree elbow, the intermediate heat exchanger lower manifold, the intermediate heat exchange tubes, the intermediate heat exchanger upper manifold, 1 X 12 inch straight pipe, 1 X 12 inch 90 degree elbow, one 12 inch horizontal straight pipe section, 1 X 12 inch 90 degree elbow, 1 X 12 inch straight pipe section, the steam generator upper manifold, the steam generator tubes, the steam generator lower manifold, 1 X 12 inch straight pipe section, 1 X 12 inch 90 degree elbow, 1 X 12 inch X 16 inch adapter, 1 X 16 X 16 X 16 inch tee, 1 X 16 inch to 6 inch adapter down to a high pressure dump tank, and a 16 inch connection to the 16 inch induction pump flow tube. There is one small drain/fill valve for each heat exchange system. This arrangement permits practical and safe identification, isolation, draining, replacement and refilling of any defective heat transport loop component.The pipes must have sufficient positioning play to allow for thermal expansion-contraction and possible earthquake related movement.
 

SECONDARY SODIUM VOLUME:
The volume of each secondary sodium circuit can be estimated by assuming that everywhere along that circuit except in the steam generator the cross sectional area is approximately the same as the cross sectional area of a 12.75 inch schedule 80 pipe.

Thus the minimum pipe length equivalents are:
Intermediate heat exchanger tibes = 6 m
Steam generator tubes = 6 m
4 Manifolds = ???? Horizontal pipes = 2 X (2 m) = 4 m
Vertical pipes = 4 X 0.5 m = 2 m

Hence total equivalent pipe length = 30 m

System volume = Pi (6.4 inch)^2 X 30 m X (.0254 m / inch)^2 = 2.49 m^3

Required secondary sodium drain down tank volume = _______

Thus the total secondary sodium volume is about:
56 X 2.49 m^3______ = 79.70 m^3_______
 

INSTALLATION:
Each heat exchange gallery has an internal length of 12.5 m and an internal width of 8.5 m. Each intermediate heat exchanger-steam generator pair is allocated a gallery wall length of 1.5 m.

Equipment in the heat exchange galleries is lowered by crane from above.

An important issue in the heat exchange gallery basement is separation of water and sodium drips. Water is only likely to leak near the outside wall. Elsewhere there could be sodium drips. A cement ridge should be provided across the basement floor to separate these two accumulations. The water sump and the water sump pump should be located near the outside wall.

As a rule of thumb electrical equipment should be mounted on the inside wall which is less subject to water penetration. There will need to be a large air vent in the access stair well end wall for air cooling.

The heat exchange gallery basement water sump pump will likely need to drain into a near grade level storm sewer. It would be better if it drained into the facility bottom drain at 16 m below grade.

At the basement level the floor is divided so that any sodium drips stay near the inner wall and any water drips stay near the outer wall.
 

At 600 degrees C the yield stress of pipe steel is 193 MPa.

The yield pressure at 600 deg C is:
193 MPa X (2) (1.594 inch / 12.812 inch) = 48.0 MPa

Hence for safety the secondary heat transport system working pressure should be less than:
48.0 MPa / 3 = 16.0 MPa

The corresponding saturated steam temperature is:
664 deg F = 351 deg C
 

Assume the use of 16.0 inch OD, 12.812 inch ID pipe. The flow cross sectional area of each such pipe is:
Pi (6.406 inch)^2 X (.0254 m / inch)^2 = 0.0831746 m^2

Let V = average axial flow velocity of secondary liquid sodium in the 16 inch OD pipe. Then:
V = (0.1672 m^3 / s) / [0.0831746 m^2)]
= 2.010 m / s

WELDING:
The manifold welds must be deep penetration equal in quality to the welds used on high pressure natural gas distribution pipelines. Possibly a helium leak detector should be used for confirming weld quality.
 

This web page last updated September 21, 2021.