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XYLENE POWER LTD.

FNR STEAM GENERATOR

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

This web page is concerned with detail related to the steam generator design.
 

NUCLEAR ELECTRICITY:
A nuclear reactor produces heat which is used to heat circulating nitrate salt. A steam generator uses that heat to produce high pressure steam. The steam expands through a steam turbine which drives a line synchronous generator to produce 60 Hz 3 Phase electric power. Ideally the steam generator acts as a constant temperature constant pressure steam source. The system power is changed by changing the steam mass flow.

A steam generator reference paper is Steam Generation - An Overview by Babcock and Wilcox.

Spray evaporator theory is discussed in the reference: Spray Evaporator Theory

Steam plants are discussed in the references: Steam Plant Operation and Industrial Boilers and Heat Recovery Steam Generators.
 

STEAM GENERATOR CONSTRUCTION:
Specifications:
Tube = 1/ 2 inch ID, 5 / 8 inch OD
Exposed Tube length = 5.0 m
Track width = 1.00 inch
37 tracks
1027 tubes
1.00 inch center to center spacing
Tubed diameter = 37 inch
Flange OD = 2.4 m (96 inch)
Shell OD = 1.2 m (48 inch)
Shell ID = 0.635 m (37 inch)
Tube sheet thickness = 11.753 inch = 0.298 m (0.3 m)
End cap thickness = 7.2 inch = 0.183 m ~ 0.2 m
Steam generator upper manifold inside length = 1.1 m
Steam generator lower manifold inside length = 1.0 m

Nitrate salt volume contained in tubes:
1027 tubes X Pi (1 / 4 inch)^2 X (0.0254 m / inch)^2 X 5.6 m
= 0.7285 m^2

Nitrate salt contained in manifolds:
(1.1 m + 1.0 m) X Pi(0.635 m /2)^2
= 0.6651 m^3

Total nitrate salt contained in steam generator:
= 0.7285 m^3 + 0.6651 m^3 = 1.394 m^3

Steam generator heat exchange area:
1027 tubes X Pi X 0.5 inch X 0.0254 m / inch X 5.0 m
= 204.88 m^2
 

STEAM GENERATOR SHELL SIZE CALCULATION:
Assume that the steam generator has an exposed tube bundle 5 m long consisting of 0.500 inch ID, 0.625 inch OD tubes on 1.00 inch staggered centers. The steam generator shell is 48 inch OD.

Barlow's formula gives:
Pm = [2 Sy W] / [3 D]
or
[3 Pm / 2 Sy] = [W / D]

However for this example:
D + 2 W = 48 inch
or
D = 48 inch - 2 W
or W / D = W / (48 inch - 2 W)
giving:
[3 Pm / 2 Sy] = W / (48 inch - 2 W)
or
(48 inch - 2 W)[3 Pm / 2 Sy] = W
or
W [1 + 2 (3 Pm / 2 Sy)] = 48 inch (3 Pm / 2 Sy)
or
W = 48 inch (3 Pm / 2 Sy) / [1 + 2 (3 Pm / 2 Sy)]

In the case of the steam generator where:
Pm = 12 MPa
Sy = 117 MPa
[3 Pm / 2 Sy] = [W / D]
= 36 MPa / 234 MPa
giving:
W = 48 inch (3 Pm / 2 Sy) / [1 + 2 (3 Pm / 2 Sy)]
= 48 inch (36 MPa / 234 MPa) / [1 + 72 MPa / 234 MPa]
= 48 inch (36 /234) / [306 / 234]
= 48 inch [36 / 306]
= 5.647 inch

Hence:
D = 48 inch - 2 W
= 48 inch - 11.29 inch
= 36.7 inch

Recall that as shown in FNR Heat Transport System the thickness T of a solid end cap is given by:
T = D [3 P / 8 Sy]^0.5
= 36.7 inch [36 MPa / 8 (117 MPa)]^0.5
= 36.7 inch [0.196116]
= 7.197 inch

This end cap thickness must be further increased to account for loss of end cap material near the end cap perimeter due to tube holes. Assume that the tubes are (5 / 8) inch OD and spaced 1.00 inch center to center. The effective tube sheet perimeter length is reduced to (3 / 8) of its original value.

However, the tube sheet becomes thicker. The resisting torque is proportional to the perimeter length and to T^2. To maintain the same overall strength, as the perimeter length drops by fraction (3 / 8) the Thickness T must increase by:
(8 / 3)^0.5 = 1.633

Thus with (5 / 8) inch diameter tube holes on 1.00 inch staggered centers the required end cap thickness becomes:
7.197 inch X 1.633 = 11.753 inch

Note that the required side wall thickness and end cap thickness greatly reduces the surface area available for heat exchange.
 

NUMBER OF TUBES:
The calculated inside diameter D allows 37 tracks consisting of:
1 + 6 (1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 +10 + 11 + 12 + 13 + 14 +15 +16 +17 +18) tubes
= 1 + (6)(9) (19) tubes
= 1027 tubes

The steam generator heat exchange area is:
1027 tubes X 5 m X Pi X (.0254 m / 2) = 204.9 m^2

This is a contemplated average thermal flux through the FNR steam generator tubes of:
21 MWt / 204.9 m^2 = 0.103 MWt / m^2

By comparison, in a CANDU reactor the thermal flux to liquid wqter is: 2100 MWt /[380 fuel tubes X 12 bundles / tube X 0.5 m X 37 rods/bundle X Pi X 0.5 inch X .0254 m / inch]
= 2100 MWt / [151442.7 m inch X .0254 m / inch]
= 2100 MWt / 3364 m^2
= 0.624 MWt / m^2

Note that in a CANDU reactor the fuel is entirely surrounded by liquid water whereas in the contemplated steam generator only the bottom portion of the tubes is immersed in water so that at the tube bottom the heat flux per unit area is likely comparable to the heat flux at the CANDU fuel rod surface.
 

In steam generator design thermal stress relief is a major issue.
 

STEAM GENERATOR SHELL WEIGHT:
The approximate Steam generator shell weight is given by: Rho Pi [(1.2 m / 2)^2 - (0.9 m / 2)^2] (5 m)
+ Rho Pi [(2.4 m / 2)2](0.3 m) (2)
= Rho Pi [0.36 m^2 - 0.2025 m^2](5 m)
+ Rho Pi [1.44 m^2](0.6 m)
= Rho Pi [0.7875 m^3 + 0.864 m^3]
= (8.03 X10^3 kg / m^3) Pi [1.6515 m^3]
= 41.66 tonnes

TRANSPORTATION DELAY COMPENSATION:
The electricity output from a FNR is normally controlled by modulating the flow rate of the secondary sodium through the intermediate heat exchange bundles.

There is a significant transportation time delay between a change in secondary sodium flow rate through the intermediate heat exchange bundle and the corresponding change in rate of steam production.

Hence the steam generator has a steam bypass valve (BP) to the condenser to fine adjust the turbogenerator speed and generator phase with no load before connecting the electricity generator to the grid.

Each steam generator has a local control loop that adjusts the steam pressure regulating valve (PRV) to attempt to maintain a steam pressure of 11.25 MPa in the steam generator which has the effect of controlling the steam generator water temperature at 320 degrees C.

The steam bypass valve must rapidly open to prevent dangerous turbine over speed if there is a sudden loss of electrical load on the generator. Even if the secondary sodium flow rate through the intermediate heat exchanger is immediately reduced on loss of system electrical load there is a significant amount of thermal energy contained in circulating secondary sodium and nitrate salt that must be dissipated to prevent turbine over speed. That energy dissipation is achieved by bypassing steam to the condenser.

Each steam generator is fitted with a rupture disc that will vent steam to the atmosphere if the internal steam pressure exceeds the maximum safe working pressure for the steam generator shell.

Each steam generator has a local control loop which maintains the desired water level in the steam generator by controlling the rate of high pressure condensate water injection. In the event of a sudden pressure rise in the nitrate salt, indicating a steam generator tube leak, the injection water flow is cut off and the steam generator drain valve is openned.

Up to six steam generators have their steam discharge ports feeding a common turbine and their steam bypass ports feeding a common condenser for a shared large steam turbo-generator. Ideally the turbogenerator should be rated for producing about 38 MWe.

DIAGRAM:
A diagram showing the connection configuration of one of six identical steam generators in each turbine hall is shown below. One FNR can support up to eight identical turbine halls.


 

FNR STEAM GENERATOR DESCRIPTION:
A FNR steam generator is in essence a vertical tube in shell heat exchanger with water and/or steam flowing upward outside the tubes but inside the shell and molten salt flowing downward inside the tubes. A FNR steam generator and its support equipment must be designed so that when a steam generator heat exchange tube ruptures it does not create a dangerous situation. Note that the water/steam pressureis always much greater than the nitrate salt pressure. When one heat transport circuit is shut down the remaining parallel connected heat transport loops should continue safe operation.

In this design high pressure water/steam leaking through a tube rupture into low pressure nitrate salt expels that salt via an adjacent salt atmospheric vent. Another atmospheric salt vent at the Heat Exchange Gallery limits the pressure drop along the salt pipes and hence prevents salt hammer of the heat exchange tubes of the sodium-salt heat exchanger and steam generator. That hammer effect could be due to water leaking into the salt loop via a steam generator tube rupture or due to secondary sodium leaking into the salt loop via a sodium-salt heat exchanger tube rupture of by a combination or these two ruptures.

Due to:
a) The latent heat of vaporization of water;
b) The changes in temperature of the salt and the water in normal operation;
the downward molten salt mass flow rate in the steam generator is much higher than the upward water/steam mass flow rate. However, water takes the form of steam which has a relatively low density. Hence the salt flows downward inside the steam generator heat exchange tubes and the water/steam occupies the space outside the tubes but inside the steam generator shell.

From a control perspective it is easier to control the high pressure injection water than the flow of molten salt.

The injection water is normally at a high pressure (11.25 MPa) while the molten salt is normally at a very low pressure (~ 0.2 MPa).

The molten salt is hotest in the upper manifold of the steam generator and flows downwards through the heat exchange tubes. The water is coolest near the bottom of the steam generator shell and expands almost straight upwards.

During normal operation a steam pressure regulating valve (PRV) connected in line between the steam generator steam discharge pipe and the corresponding turbine keeps the steam pressure inside the shell at about 11.25 MPa. This pressure in combination with the properties of water keeps the super heated liquid water inside the steam generator shell bottom at 320 degrees C.

Also connected to the steam discharge pipe is a bypass (BP) valve to the condenser and a rupture disk vented to the atmosphere via a 12.75 inch OD schedule 160 pipe. The rupture disk is configured to rupture open at a steam pressure of about 14 MPa. The steam bypass valve also acts as a redundant over pressure safety valve. Thus there are two redundant means of protecting the steam generator from failure due to steam over pressure.

The steam generator consists of six stacked coaxial sections. Starting from the bottom:
1) A bottom manifold which discharges molten salt above its melting point. This section has schedule 160 shell and fittings. This section has a 8.625 inch OD schedule 40S molten salt discharge pipe.
2) A sleeved lower tube section where the superheated injected water is vaporized. The sleeves limit the heat flux through the tube wall near the bottom of the tubes where cool liquid water is in contact with the tube outer wall while hot molten salt is in contact with the tube inner wall.
3) A middle tube section containing steam flow baffles where the dry bulb temperature of the steam is increased;
4) An upper tube section has a _____ inch OD schedule 160 steam discharge pipe with an over pressure rupture disk vented via 12.75 inch_____ OD schedule 160 pipes to above the roof, a 12 inch _____schedule 160 electrically operated steam bypass valve to the common condenser and a steam generator pressure regulating valve (PRV) to the common turbine.

5) A top manifold which inputs molten salt from the sodium-salt heat exchanger via a 8.625 inch OD pipe and distributes it over the 721_____ steam generator tubes. The top manifold is also vented to the atmosphere via a vent pipe which is sufficiently tall to provide the gravitaional head necessary to support the required salt flow through the steam generator tubes and provide the nitrate salt pump its specified suction head.
 

FNR STEAM GENERATOR NORMAL OPERATION:
The condensate injection pump inputs condensate water from the turbogenerator condenser at about 50 degrees C and at a pressure of less than 0.02 MPa and raises its pressure to about 11.5 MPa. This pressurized water flows first through the condenser recuperator coil which raises its temperature to about 100 degrees C before the water flows to the steam generator.

Near the bottom of the heat exchange tubes there are external tube sleeves to minimize the heat flux from the secondary liquid sodium into liquid water contacting the inner tube wall. The temperature difference between the two liquids is dropped across the metal tube wall plus the sleeve wall. A thin layer of water between the tube outside surface and the sleeve inside surface ensures good thermal contact between the tube and the sleeve. The gap between the tube and the sleeve must be sufficient to allow for differential thermal expansion but not sufficient to allow significant water flow within the gap.

To minimize thermal stress in the tube wall and the sleeve wall it is essential to keep the temperature difference across the metal walls within the material rating by ensuring that the liquid water injected into the bottom of the steam generator shell is sufficiently warm.

In the bottom manifold of the steam generator there is molten salt which is normally at a temperature of about 320 degrees C.

Cool input water from the turbogenerator recuperator flows into a preheating loop which warms the input water before guiding this input water toward the steam generator tubes. The injected water mixes with the water from the steam generator bottom pool before flowing to the sleeved portion of the steam generator which operates at about 320 degrees C.

The lower portion of the tubed section of the FNR steam generator provides to the water its latent heat of vaporization. The design object is to ensure a layer of steam between the metal tube outer wall and the liquid water pool to limit the heat flux per unit area through the wall of the heat exchange tube. Thus the temperature difference across the heat exchange tube wall is minimized. The water evaporates from the bottom pool forming nearly saturated steam.

Outside the upper portion of the steam generator tubes there is no liquid water. The heat transfer is from the hot tube outer surface to water vapor. The water vapor forms a thin boundary layer adjacent to the surface of the heat exchange tubes. The effect of this boundary layer is to reduce the temperature differential across the heat exchange tube wall. Hence the temperature of the heat exchange tube metal is close to the temperature of the molten salt, so the thermal stress within the heat exchange tube metal wall is small.

The tubed section of the steam generator acts as a counter current heat exchanger which raises the temperature of the steam from its boiling point at about 320 deg C to about 400 degrees C. At light loads this steam discharge temperature rises to about 440 degrees C.

A pressure regulating steam discharge valve keeps the pressure inside the steam generator upper manifold at 10 MPa. This vapor pressure in combination with the properties of water keeps the liquid water at about 310 degrees C.

The upper portion of the steam generator must supply the sensible heat required to raise the steam temperature from 310 C to the steam generator discharge temperature of about 400 C. The upper portion of the tube bundle may also have to supply heat to vaporize small liquid water droplets that the flowing steam carries with it. It is important to vaporize these water droplets to prevent them eroding the down stream steam turbine.

At low thermal loads the NaK flow rate through the intermediate heat exchange bundle will be low. The secondary sodium return to the intermediate heat exchanger will be at a temperature close to 320 C and kept there by the pressure - temperature relationship maintained by the steam generator presssure regulating valve.

At high thermal loads the NaK flow rate thjrough the intermediate heat exchange bundle is much higher. The NaK delivers more heat to the nitrate salt and hence to the water filled portion of the steam generator. The injection water pump is forced to speed up to maintain the steam generator water level. However, the temperature difference across the steam generator heat exchange tube wall at the water level is never more than about 25 C.
 

The steam generator has a local control loop that adjusts the steam pressure regulating valve (PRV) to attempt to maintain a steam pressure of 10 MPa inside the steam generator which has the effect of setting the steam generator water temperature at 310 degrees C.

If there is a sudden loss of electrical load on the turbogenerator a steam turbine bypass valve must rapidly open to prevent dangerous turbine over speed. Even if the secondary sodium flow rate is immediately reduced on loss of system electrical load there is a significant amount of thermal energy contained in circulating NaK and hot nitrate salt that must be safely dissipated to prevent turbine damage due to over speed. That dissipation is achieved by bypassing steam to the condenser.

Each steam generator is fitted with a rupture disc that will vent steam to the atmosphere if the internal steam pressure exceeds the maximum safe working pressure of 12 MPa for the steam generator shell.

Each steam generator has a local control loop which maintains the desired water level in the steam generator by controlling the rate of high pressure condensate water injection. In the event of a sudden pressure rise in the nitrate salt, indicating a steam generator tube leak, the injection water flow is cut off and the steam generator drain valve is openned.

Up to six steam generators can be connected in parallel to serve a common turbogenerator.
 

CONDENSATE AND EMERGENCY WATER INJECTION:
It is necessary to control each injection water pump to control the water level in the corresponding steam generator. Note that the time constant of this control loop should be small compared to the rate of change of the NaK through the intermediate heat exchanger which controls the reactor power output.

A key safety issue after loss of station power is to continue removal of fission product decay heat. That requires maintaining condensate injection water flow into some of the steam generators and maintaining the circulated nitrate salt temperature to prevent the salt freezing.

If we allow the steam generator pressure to drop the water temperature in the steam generator will drop which might cause the nitrate salt to freeze. Under these circumstances the nitrate salt my need to be replaced with oil.

On loss of station power the oil circulation must be maintained to to remove fission product decay heat. When city water is available the reserve water tanks should be automatically refilled.
 

THERMAL PERFORMANCE:
At full thermal load the resulting temperature distribution is as follows:
Steam generator steam discharge temperature = 400 C
Steam generator water temperature = 310 C
Steam generator salt inlet temperature = 440 C
Steam generator salt discharge temperature = 320 C
Secondary sodium inlet temperature = 450 C
Secondary sodium discharge temperature = 330 C
Primary sodim discharge temperature = 460 C Primary sodium return temperature = 340 C  

FNR STEAM GENERATOR PRESSURE ISSUES:
In normal operation the steam generator shell side contains high pressure (10 MPa) water/steam. However, the shell should be hydraulic pressure tested to at least 18.0 MPa to safely manage 12 MPa maximum possible steam pressure transients that might occur.

The steam generator tubes contain molten salt normally at a pressure of about 0.2 MPa.

There are molten salt pressure and temperature sensors. An unanticipated transient increase in molten salt pressure or level indicates either a steam generator tube rupture or possibly a sodium-salt heat exchanger tube rupture.

The salt circuit, if working pressure rated for 6 MPa, can safely manage a high pressure water/steam tube rupture by expelling salt out the remote salt vent.

On the occurrance of a steam generator tube heat exchange rupture high pressure water and steam jet through the rupture into the tube side of the steam generator where they cause a rapid rise in pressure in the circulated nitrate salt. The rate of the pressure rise is mitigated by:
a) Use of narrow heat exchange pressure tubes which limit the incoming steam / water flow rate;
b) Use of large salt loop vents with top ball checks that are open to the atmosphere.

c) In the event of a tube rupture in either the NaK-salt heat exchanger or in the steam generator these vents will violently discharge hot liquid salt to the atmosphere. These vents need protective discharge shields to prevent potential injury to people or damage toequipment caused by this flying molten salt/oil.

d) Stopping of the injection water pump. There are two series connected pump control relays activated by independent high pressure sensors for certainty in stopping high pressure water injection into the steam generator;

e) Openning the steam generator drain valve;
f) Opening of the steam vent valve bypasses steam to the condenser.

g) Stopping the NaK induction pump and draining both the salt and the NaK to their respective dump tanks prevents further heating of the steam generator by the reactor and thus limits the total amount of transient heat that must be dissipated.

h) In the event of a single heat transport circuit failure complete closing of the steam pressure regulating valve (PRV) prevents reverse steam flow through the pressure regulating valve.
 

HEAT EXCHANGE TUBE WALL STRESS REDUCTION:
In the steam generator the tubes run consistently warmer than the shell. This issue leads to compresive tube stress and tensile shell stress.

High temperature differentials across heat exchange tube walls can potentially cause thermal stress in excess of the material yield stress. Part of the steam generator design focuses on reducing the maximum material thermal stress.

There are two general approaches to heat exchange tube wall thermal stress reduction. At the bottom of the steam generator shell , where the heat exchange tubes are surrounded by liquid water, external sleeves around the tubes are used to reduce the heat flux per unit area and hence the maximum temperature drop across the tube wall.

Higher up the tubes but still below the water level there is a layer of steam adjacent to the outside surface of the heat exchange tube which reduces the heat flux per unit area through the tube wall. Hence in this region the tube wall temperature is close to the downward flowing molten salt temperature. The heat flux through the tube wall is limited by the rate of heat transfer from the tube wall to steam, which is much less than the rate of heat transfer from the tube wall to liquid water.

The height of the water inside the steam generator is indirectly controlled by controlling the height of a pure liquid water column in an external tube connected between the top and bottom of the shell. This external tube has no radial heat flow. That external water column height is sensed and is controlled by controlling the flow rate of high pressure water injection into the steam generator spray injection nozzles.

Due to steam bubble formation the height of the water inside the shell is several times the height of the sensed water column in the external water level sensing tube. However, the pressure at the bottom of the two water columns is identical.

Below the top of the interior water level heat transferred from the tube outside wall to the steam is almost immediately absorbed by latent heat of evaporation of water that is transitioning from water to steam. Thus below the top of the water level the liquid temperature is nearly constant at about 310 degrees C at a pressure of 10 MPa, corresponding to saturated steam. The head pressure is maintained by the steam pressure regulating valve.

Above the top of the liquid water the steam becomes drier and rises to a top temperature in the range 400 degrees C to 440 degrees C depending on the system thermal load.
 

LOWER MANIFOLD THERMAL STRESS MITIGATION:
There is feed water temperature mixing in the bottom of the steam generator shell to minimize thermal stress on the steam generator heat exchange tube portions that pass through water. The feed water temperature rise from 25 deg C to 320 C is partially realized by feed water heat recouperation from the turbine discharge steam immediately upstream from the turbine condenser followed by water mixing at injection into the steam generator.
 

TUBE FAILURE PROTECTION AND MITIGATION:
In a continuously operating energy generation system sooner or later there will be a steam generator heat exchange tube failure. Such a failure will potentially allow high pressure water to flow through the rupture and into the molten salt.

There must be molten salt pressure relief vents at the steam generator and at the NaK-salt heat exchanger of sufficient size to limit the consequent rapid rise in pressure in the molten salt circuit sufficiently to prevent equipment damage.

An important issue in FNR steam generator design is preventing a water leak from a steam generator tube rupture from causing a serious accident. When high pressure water and steam jets into the low pressure hot molten salt the molten salt is propelled at a high speed toward the atmospheric vents.

SHUTDOWN SEQUENCE:
The system must be designed to:
1) Remain structurally safe in the presence of this salt hammer;
2) Prevent a dangerous pressure transient reaching the connected sodium-salt heat exchanger;
3) In the event of tube damage to the sodium-salt heat exchanger the system must rapidly and safely vent the nitrogen gas (and possibly hydrogen if there is any water in the salt loop) to prevent salt or water entering the secondary sodium circuit and causing yet more damage.
4) Open a steam generator drain valve and use the residual steam pressure to rapidly expel liquid water from the steam generator shell into the drain;
5) Turn off the injection water pump to stop the injection water flow;
6) Release high pressure steam from the steam generator shell to the condenser to stop water / steam jetting into the salt from both sides of the tube rupture;
7) Force the steam pressure regulating valve fully closed to prevent steam from other steam generators back flowing backwards through the steam pressure regulating valve;
8) Turn off the secondary sodium induction pump to prevent further heat input from the reactor;
9) Drain the salt to its dump tanks;
10) Safely isolate components for cleanup and repair.
 

RESTART SEQUENCE:
11) Confirm correct secondary sodium level and presence of argon cover gas at 0.5 MPa;
before restarting the secondary sodium induction pump at a low flow.
12) Use electric heaters in dump tanks to bring the salt up to temperature
13) Start the secondary sodium induction pump at low flow;
14) Apply air pressure over the salt dump tanks to refill the salt loop;
15) Start the salt circulation pump;
16) Close the steam generator drain valve;
17) Enable the steam generator injection water pump;
18) Use the steam bypass valve to synch the generator to the line;
19) Close the generator main contactor;
20) Increase the secondary sodium flow to achieve the desired generator power.

There must be fully redundant pressure relief safety devices (rupture disks) to ensure steam generator pressure safety because failure of the motorized valves to work as contemplated could otherwise have serious consequences. In addition to steam pressure sensors there must be water level sensor and a molten salt level sensor at the steam generator to to control the injection water pump and to sense a transient increase in molten salt pressure caused by a steam generator heat exchange tube leak.

HEAT EXCHANGE TUBE RUPTURES:
The worst FNR nightmare is an event which causes water from the steam circuit to flow backwards through the salt circuit, through the secondary sodium circuit and into the primary sodium pool. Such an event could only occur if both salt vents were blocked such that the steam generator pressure could cause water to rupture a steam generator tube, propagate backwards through the salt pipes, rupture a sodium-salt heat exchanger tube, propagate backwards through the secondary sodium piping, and rupture an intermediate heat exchange tube. This accident condition requires three simultaneous heat exchange tube failures and two atmospheric vent blockages in the same heat transport circuit.

The purpose of the system safety design is to prevent such an accident ever happening. Even if the tubes in both the steam generator and the sodium-salt heat exchanger simulataniously rupture the higher pressure in the secondary sodium circuit and the secondary sodium circuit pressure relief should prevent such an accident occuring.

The salt circuit pressure relief pipes must be large enough (12 inch diameter) to ensure that nitrogen (and perhaps hydrogen) can be vented as fast as they can be generated, which in the case of effects of a sodium-salt heat exchanger tube rupture and/or a steam generator tube rupture are limited by the maximum possible water and sodium flow rates through the throats of the ruptured heat exchange tubes.

The worst case consequence of any potential steam generator or sodium-salt heat exchanger tube rupture are physically limited by the use of 56 independent heat transfer circuits.

Note that the liquid water flow through a steam generator heat exchange tube rupture and into the salt will stop when the steam generator is partially drained. However, the steam flow through both sides of the tube rupture will continue until all the steam in the steam generator shell is vented. The over pressure in the salt should cause the steam bypass balve to the condenser to discharge steam from the steam generator shell as fast as possible so that steam does not continue to flow into the salt.

The steam pressure regulating valve (PRV) needs to be fully closed to prevent steam back flow from other steam generators.
 

STEAM GENERATOR SHUTDOWN:
1) The shell side of the steam generator contains super heated liquid water on the bottom and steam on top.

2) The molten salt feeds into the top manifold and then into the tubes. The molten salt flows downwards through the tubes before exiting via the lower manifold. This molten salt is hot on top and cooler ner the bottom. The molten salt flow inside the tubes is turbulent for good heat transfer.

3) Near the bottom of the steam generator shell the vertical tubes have external metal sleeves. The maximum thickness of these sleeves is limited by the distance between adjacent heat exchange tubes. These sleeves extend above the level in the heat exchange tubes where liquid water is in contact with the tube's outer wall. These sleeves locally reduce the temperature drop across the tube wall. In the event of a tube crack in the sleeved portion of the fuel tube the sleeve limits the rate of flow of high pressure water through the crack and into the lower pressure salt.

4) One external tube connected between the shell top and shell bottom contains a water level sensor which is used to control the injection water flow rate.

5) The top and bottom manifolds are approximately twice the diameter of the body of the steam generator. The manifold top and bottom covers are removable and are held in place by a large number of perimeter bolts. These manifold covers need special gaskets.

6) The bottom of the steam generator shell contains a pond of superheated water. The injection water inlet and shell drain ports are on the side of the shell, as are the water level sensing tube ports.

7) The top of the shell has a dedicated pressure regulating steam discharge valve, which attempts to maintain 11.25 MPa steam pressure in the shell.

8) There is a rupture disk that will vent steam from the shell to the atmosphere if the steam pressure regulating valve fails to operate properly.

9) There is a normally open steam pressure release valve which will vent steam to the atmosphere when there is loss of control power and water must be evaporated to remove fission product decay heat. During normal reactor operation this valve is kept closed by a solenoid.

10) If a steam generator tube rupture occurs the high pressure water leaks into the salt and immediately forms steam. The steam drives salt out the salt vents to the atmosphere.

11) On the occurance of a rise in pressure in the tube side of the steam generator electric power to the water injection pump is cut off and the steam generator shell drain valves that are held closed by solenoids open under the water/steam pressure.

12) The steam pressure in the steam generator shell drives liquid water in the bottom of the steam generator shell to the drain.

13) Liquid water will continue to jet through the heat exchange tube rupture and into the molten salt until the water level in the shell falls below the bottom of the tube rupture. Hence it is essential to drain the shell as fast as possible.

14) Even after the water level in the shell is reduced residual steam will leak into the salt from both sides of the heat exchange tube rupture as long as there is steam pressure in the shell. Hence the shell steam pressure must be rapidly reduced to zero by opening an electrically operated steam pressure relief valve.

15) There is a redundant salt pressure relief vent over the steam generator to relieve over pressure in the salt circuit, possibly due to salt freezing.

16) There are two water level sensors for controlling the steam generator water level under both normal operation and cold shutdown conditions.


 

STEAM GENERATOR INSTALLATION:
The steam generators are installed to achieve precise pipe alignment with the corresponding turbogenerator. The nitrate salt pipes are potentially more tolerant of alignment error with holes in the outside wall of the turbine hall. These holes can be larger to give more mis-alignment tolerance. Moreover, the nitrate salt pipes are long, have multiple elbows to permit thermal expansion/contraction and operate at a low pressure.
 

STEAM GENERATOR HEAT EXCHANGE AREA:
With the aforementioned steam generator design the temperature difference across a heat exchange tube metal wall and the steam boundary layer is about 40 degrees C. Where the tubes are immersed in liquid water the full load tube wall temperature drop is typically about 10 degrees C.

Assume each sodium-salt heat exchanger feeds one steam generator.

The steam generator must withstand the steam pressure. However, the steam generator has turbulent fluid flow on both sides of its tubes so it can operate with less tube area than the corresponding intermediate heat exchange bundle.

The contemplated steam generators are realized using 5 m lengths of 24 inch diameter thick wall (610 mm OD) pipe. This pipe is available in 59.54 mm wall thickness to safely contain a pressure of up to 13.7 MPa @ 427 degrees C.

The inside diameter of this pipe is:
610 mm - 2(59.54 mm) = 490.92 mm

As shown elsewhere on this web page each 5 m long X 2 foot outside diameter steam generator shell will accept 625 X 0.500 inch ID. 0.625 inch OD tubes.

Within each such steam generator bundle there is a heat exchange area of:
256 tubes X 5 m / tube X Pi X (.0254 m / 2) = 196,448 inch^2
= 126.74 m^2

The corresponding heat flow rate per bundle limited by Inconel 600 conductivity is: 20.9 Wt / m-deg K X 126.74 m^2 X (1 / .065 inch) X (1 inch / .0254 m) = 1,604,401 Wt / deg K
= 1.604 MWt / deg K

The design target for each heat to electricity conversion subsystem is:
1000 MW / 56 = 17.857 MWt. Thus this target can be met with a tube wall temperature drop of:
17.857 MWt / [(1.604 MWt / deg K)] = 11.133 deg C.
 

FNR STEAM GENERATOR CONSTRUCTION DETAIL:
Each steam generator has two end manifolds, each rated for the steam generator working pressure (11.25 MPa). The manifolds are nominally 48 inches OD and 58 inches OD when insulated. The manifold covers are removeable for interior access and are held in place by numerous bolts. Nitrate salt pipe connections are made to the manifold sides.

The steam discharge port from each steam generator is 12.75 inch OD_____ and connects to a pressure regulating valve facing the turbogenerator. There are two other 12.75 inch______ steam discharge ports, one to the condenser andd the other to a rupture disk and vent. The vents are fabricated from schedule 160 pipe. One of the design objects is to minimize the volume of steam contained in the steam generator and its connected pipes.

The water inlet to the steam generator shell bottom must be pressure rated for the steam generator working pressure. The other shell bottom ports must enable water level sensing and rapid shell drainage.

Outside the heat exchange galleries the 8.0 inch OD salt pipes angle monotoniclly downwards towards the remote salt dump tank. These pipes go under the roadway. Each salt pipe loop feeds a dedicated steam turbogenerator.
 

FNR STEAM GENERATOR TUBE CONFIGURATION:
The 5 / 8 inch diameter tube sheet holes on 1.0 inch centers reduce the effective tube sheet perimeter length by the fraction 3 /8.

The tube open area for nitrate salt flow is:
Pi (1 / 4 inch)^2 X 1027 tubes
= Pi (64.19 inch^2)
= Pi (8.01 inch)^2
which is about equivalent to a 16 inch pipe. Hence the nitrate salt flow rate in the external 8 inch diameter pipes will have 4 X the velocity of the nitrate salt in the steam generator tubes.

However, due to the improved density and heat capacity of the nitrate salt as compared to sodium 8 inch pipes for the nitrate salt are believed to be sufficient.
 

TUBE FAILURE FLOWS:
Assume a heat exchange tube size of 0.500 inch ID.

Tube open cross sectional area
= Pi (.25 inch X 0.0254 m / inch)^2
= 1.2667 X 10^-4 m^2

In a tube:
Flow power = force X velocity
= (pressure X area X velocity)

Flow power = kinetic energy / unit time
= (mass / sec) X (velocity)^2 / 2
= (density X area X velocity X (velocity)^2 / 2

Equating the two expressions for flow power gives:
pressure = density X velocity^2 / 2
or
Velocity = [2 X pressure / density]^0.5
or
volumetric flow rate = area X [2 X pressure / density]^0.5
or
mass flow rate = density X area X [2 X pressure / density]^0.5
= area X [2 X pressure X density]^0.5

Hence the maximum flow rate of super heated water through a single heat exchange tube in a single direction is:
= 1.2667 X 10^-4 m^2 X [ 2 X 11.25 X 10^6 Pa X 1000 kg / m^3]^0.5 = 1.2667 X 10^-4 m^2 X [2 X 11.25 X 10^6 kg m /s^2 m^2 X 1000 kg / m^3]^0.5 = 1.2667 X 10^-4 m^2 X 15 X 10^4 kg / s m^2
= 19.0005 Kg / s

In a tube failure situation the tube will leak in both directions into the salt. Thus immediately after a single tube collapse superheated water may enter the salt at:
2 X 19 kg / s = 38 kg / s

This water will immediately flash to steam.

At 0 degrees C, 1 atmosphere (.101 MPa), one mole (18 gm) of water vapor occupies 22.4 lit. At 430 deg C one mole of water vapor at one atmosphere (0.1 MPa) occupies [(430 + 273 ) / 273] X 22.4 lit = 57.68 lit. At 430 deg C, 2 MPa one mole of steam occupies:
57.68 lit / 20 = 2.884 lit

Thus if the pressure in the vent pipe is 2 MPa the volumetic flow rate will be:
38,000 g / s X 2.884 lit / 18 gm = 6088 lit / s
= 6.088 m^3 / s

Recall that:
mass flow rate = area X [2 X pressure X density]^0.5

Thus at 2 MPa the maximum water vapor mass flow venting rate is:
mass flow rate = 0.10382 m^2 X [2 X 2 X 10^6 Pa X 18 g / 2.884 lit]^0.5
= 0.10382 m^2 X [2 X 2 X 10^6 kg m / s^2 m^2 X 6.241 kg / m^3]^0.5
= 0.10382 X 4.996 X 10^3 kg / s
= 518.7 kg / s

Thus under these circumstances the maximum tolerable number of simultaneous steam generator tube failures is:
(518.7 kg / s) / 38 kg / s-tube failure) = 13.65 tube failures

When a heat exchange tube initially fails salt will be discharge from the vent like it is shot from a blowgun. There will be a slug of salt about 10 m long. An important questionis will it be accelerated fast enough to prevent nitrate salt vent failure due to over pressure.______________ This steam pressure will create a pressure pulse in the salt exhaust vent which blows salt out its top. That vent and its connected piping must be robust enough to withstand the pressure pulse.

Assume that the vent pipe is 16.000 inch OD schedule 80 (0.843 inch wall) so the ID is 14.314 inch. Then the volume of that pipe is:
Pi (14.314 inch / 2 X .0254 m / inch)^2 X L
= 0.10382 m^2 L

Hence if L = 10 m the vent pipe will reach its maximum safe pressure of 2 Pa MPa in about:
= 2.52 s__________

Thus on the occurrence of a single heat exchange tube failure the 16 inch ball check must move from full closed to full open in about _____ seconds.

On the occurance of a steam generator heat exchange tube failure the 1st object is to reduce the liquid water level in the steam generator shell as fast as possible so that only steam feeds the rupture. The second object is to reduce the steam pressure in the steam generator shell as fast as possible to limit the mass of steam ultimately injected into the nitrate salt via the tube rupture.
 

WATER MASS FLOW RATE:
During normal operation each steam generator must supply:
1000 MWt / 48 = 20.833 MWt of heat to water flowing through it.

20.833 MWt
= [(380 deg C X 1 cal / gm deg C X 4.186 J / cal) + (1200 J / gm)] [flow rate]
or
[flow rate]
= [20.833 X 10^6 J / s] / [(380 deg C X 1 cal / gm deg C X 4.186 J / cal) + (1200 J / gm)]
= [20.833 X 10^6 gms / s] / [(380 X 4.186) + (1200)]
= [20.833 X 10^6 gms / s] / [2790.68]
= 7465.21 gms / s
= 7.465 kg / s

STEAM GENERATOR THERMAL STRESS RELIEF:
The nitrate salt loop temperature differential may be 100 degrees C but the temperature differential across the steam generator tube walls below the water level should be under 20 degrees C. One way of reaching this objective is to keep the liquid water that is in contact with the heat exchange tubes at its boiling point so that the heat exchange tubes under the water surface are covered with steam bubbles. However, that means that incoming water must be preheated before it contacts the exposed heat exchange tubes. It is contemplated that the lower portions of the heat exchange tubes in the steam generators will be sleeved to reduce thermal stress.

By keeping a layer of steam between the heat exchange tubes and the water in the base of the steam generator the local heat flux through the tube wall is reduced. It is necessary to keep the temperature differential across each heat exchange tube wall under 20 degrees C to limit wall stress.
 

This web page last updated April 23, 2022

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