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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 provides an overview as to how heat flows from the FNR liquid sodium pool to the turbo generators in the turbogenerator halls. This heat is transported first by radially flowing NaK, then by flowing nitrate salt or hot air and finally by flowing steam.
During system startup and shutdown the nitrate salt loops provide no heat transport when the salt temperature is below about 260 degrees C. Hence, to enable heat transport during system startup and shutdown, the nitrate salt is drained to a dump tank and hot air is circulated through the same buried pipes as are used for nitrate salt heat transport. These pipes are made 16 inch diameter to provide sufficient cross section for the required heat transport via hot air.
The heat transport fluid in each heat transport loop has a dedicated dump tank to allow individual heat transport loop fire suppression and individual heat transport loop service.
An important heat transport system feature is the internal horizontal baffle arrangement inside the NaK-salt heat exchangers. These baffles force the nitrate salt (or air) to follow a zig zag path that reduces the thermal stress.
In the sodiumpool under and inside the intermediate heat exchange bundle is a perforated vertical cylinder about 17 m diameter X 9 m high known as the reactor skirt. The purposes of this perforated cylinder are to ensure good high and low temperature sodium mixing and to minimize the radial neutron flux on the sodium pool walls. Part of this cylinder thickness can be gadolinium.
Other web pages focus on the design detail of each major heat transport system component including:
Intermediate Heat Exchange Bundle;
FNR NaK Loop;
FNR Induction Pump;
NaK-Salt Heat Exchanger;
FNR Nitrate Salt Loop;
NaK-HTF Heat Exchanger;
FNR Steam Generator;
FNR Temperature Profile;
Design Formulae.
This web page also addresses various technical issues that are common to all of the heat transfer loops.
In normal full power operation the nitrate salt pumps operate at nearly full speed. The variable speed induction pump adjusts the NaK flow to maintain a constant nitrate salt temperature differential (440 C - 320 C). The thermal load is controlled by controlling the speed of the nitrate salt pumps. Note that care must be taken to prevent the return NaK temperature rising above the temperature rating of the induction pumps.
The FNR sodium pool contains pure sodium to take advantage of the superior neutronic properties of pure sodium. The FNR intermediate heat transport loop contains NaK because NaK is a liquid from room temperature (20 degrees C) to above 500 degrees C. Furthermore, a small leak of NaK into the Na for a limited time is tolerable.
In the scientific literature NaK is a eutectic consisting of about 77% K, 23% Na which has a melting point of -12 degrees C. However, a problem with using this eutectic mix is that sooner or later there will be an intermediate heat exchange tube bundle leak that leads to K leaking into the sodium pool. Purifying the sodium pool is a multi-million dollar expense. Hence, it is desirable to minimize this K pollution problem. Since the purpose of using NaK is to lower the NaK melting point down to room temperature, there is no need to use such a large fraction of K. The literature indicates that 60% Na, 40% K has a melting point of 20 degrees C, which is low enough for our purposes. Hence, on this website, unless otherwise noted, the term NaK refers to an alloy containing 60% Na and 40% K.
In a liquid sodium cooled Fast Neutron Reactor (FNR) power plant, due to the chemical incompatibility between sodium and water, for safety purposes the NaK is double isolated from the water in the steam generator and the Na, which contains radioactive Na-24, is triple isolated from water in the steam generator.
Heat flows from the atmospheric pressure (0.1 MPa absolute) sodium to higher pressure (~ 0.5 MPa gauge) NaK by thermal conduction through the intermediate heat exchange tube bundles that are immersed in the sodium pool. The liquid NaK then conveys heat by liquid convection to NaK-salt heat exchangers in heat exchange galleries located outside the upper perimeter of the FNR's sodium pool enclosure. Part of this heat transfer is via uninsulated NaK return pipes that, being cooler than the surrounding Na vapor, condense that vapor. In consequence each NaK return pipe generates a continuous stream of Na condensate that should flow back into the Na pool. The pool deck and pool walls must be appropriately sloped to manage these liquid sodium streams.
The heat flows by thermal conduction from the NaK through the NaK-salt heat exchanger to separate near atmospheric pressure (~ 0.1 MPa) molten nitrate salt loops. The heat is conveyed by low pressure pumped liquid convection through below grade nitrate salt pipes to steam generators that are located in remote turbogenerator halls on the opposite side of the laneway from the nuclear island.
The steam generators use the hot nitrate salt to produce steam at about 400 degrees C. The steam discharge pressure setting is 10 MPa corresponding to saturated steam in the bottom of the steam generator at about 310 degrees C. The hot steam (~ 400 deg C) flows out the top of the steam generator through a pressure regulating valve to a turbogenerator, which expands the steam to produce electricity.
If the steam expansion was adiabatic the steam turbine discharge temperature would be hotter than necessary. Hence, as the steam expands part of its contained heat is used for heating of incoming feed water to the steam generator.
The remaining expanded low pressure steam is condensed. The steam's latent heat of vaporization at the low temperature in the condenser is either rejected to a cooling tower or is used for low temperature district heating. The condensate is fed to a multi-stage feed water pump which raises the liquid condensate pressure back up to 10 MPa.
Note that the nuclear island is connected to the turbogenerator hall by nitrate salt filled heat transport loops. At reactor startup and cold shutdown these loops are air filled.
The nitrate salt physical characteristics impose temperature limits on this process. The nitrate salt has a specified low operating temperature limit of 260 degrees C and a practical low temperature limit of about 280 degrees C. At lower temperatures the nitrate salt is drained to a dump tank and heat transport is via hot air.
The nitrate salt loop has a design supply temperature setpoint of 440 degrees C and a return temperature of about 320 C. The nitrate salt flow rate is variable and determines the load on the FNR. At sodium temperatures below 300 degrees C the nitrate salt heat transfer may not be functioning so the thermal load is limited to about 160 MWt. At sodium temperatures in the range 300 C to 335 C the system can operate at 1000 MWt.
The reactor power is controlled by varying the nitrate salt flow rate. The differential temperature across the nitrate salt loop is nearly constant at about 120 degrees C. Hence there is a big temperature difference between the NaK return temperature and the sodium pool temperature. The heat exchangers must be designed to accommodate temperature difference.
The NaK induction pumps and the nitrate salt pumps are normally operated to maintain a constant system temperature profile over the range 10% to 100 % of rated thermal power. The differential temperature in the NaK measured at the NaK-salt heat exchanger is typically 120 degrees C at full load. The NaK supply temperature varies from about 450 degrees C at full load to ~470 degrees C at 10% load.
At full load the maximum NaK temperature is 450 C and is limited by the NaK pipe flange gasket material.
At full load the minimum NaK temperature meassured at the NaK-salt heat exchanger is 330 degrees C as required to prevent NaOH precipitation on the intermediate heat exchange bundles. Due the shunt the corresponding NaK return temperature at the intermediate heat exchanger inlet is about 390 degrees C.
The internal baffle arrangement in the NaK-salt heat exchanger ensures that the Na inlet temperature to the FNR fuel assembly remains above 400 degrees C.
HEAT TRANSPORT LOOP DESIGN:
The design approach is to optimize the performance of a single 21 MWt (7 MWe) heat transport circuit. Then for a particular FNR up to 48 identical independent heat transport circuits are used to transfer heat from the Na pool to turbogenerators that collectively output up to 300 MWe of electricity plus up to 700 MWt of low grade heat.
The reactor Na surface temperature is chosen to be 460 C. This choice serves the dual purpose of preventing fuel center line melting and enabling use of teflon based gaskets for flanged pipe connections in the NaK, nitrate salt and HTF heat transport loops.
The NaK supply temperature is nearly constant at 450 deg C to 459 C. The return NaK temperature varies from about 438 deg C at 10% load to about 400 deg C at full load. Hence the effective Na discharge temperature from the intermediate heat exchange bundles varies from about 448 deg C at 10% load to about 400 degrees C at full load.
The return nitrate salt temperature is in effect pinned by the steam generator water temperature and steam generator PRV. This water temperature is about 310 degrees C. The minimum nitrate temperature is one heat exchange drop or 10 degrees C higher at 320 C. Hence the effective full load Na discharge temperature from the intermediate heat exchange bundles is about 330 degrees C.
At full load the intermediate heat exchanger has a NaK inlet temperature of ~ 340 C and a NaK discharge temperature of 450 C.
Note that the intermediate heat exchange bundle is single pass, bottom fed and only supported from the bottom to allow for thermal expansion and contraction. It is surrounded by a flow guide to minimize thermal stress. Similarly the NaK pipe connections above the intermediate heat exchange bundles have long arms and flexible sections with double elbows and wall clearance to further accommodate thermal expansion and contraction.
Note that the cool sodium discharged by the intermediate heat exchanger produces at full load a fuel bundle sodium discharge temperature of about 400 deg C. THIS SODIUM MUST BE WELL MIXED.
The NaK-salt heat exchanger has a full load NaK inlet temperature of 450 C and a NaK discharge temperature of 340 C, a corresponding nitrate salt inlet temperature of 320 C and a nitrate salt discharge temperature of 440 C.
At full load the steam generator has a nitrate salt inlet temperature of 440 C, a nitrate salt discharge temperature of 320 C, a feed water inlet temperature of ~ 280 C and a steam discharge temperature of about 400 C.
FNR Site Plan
The 700 MWt of low grade heat is delivered to four district heating pipe loops. Each district heating loop has 4 X 24 inch diameter buried pipes entering the FNR Power Plant site and 4 X 24 inch diameter pipes leaving the FNR Power Plant Site. Heat that is not used in district heating system is dissipated via 16 geographically distributed cooling towers, four of which are located at the corners of the FNR Power Plant site.
The use of 48 identical independent heat transfer circuits enables equipment repair and maintenance while continuing to provide uninterrupted supply of electricity and heat. This feature has been shown to be important in other sodium cooled nuclear power plants where, due to use of larger common equipment components, every minor heat exchange tube leak resulted in a prolonged system shutdown.
HEAT TRANSPORT OVERVIEW:
In a liquid sodium cooled Fast Neutron Reactor (FNR) for safety purposes the systems that transport heat out of the sodium pool use first an isolated NaK circuit and then isolated nitrate salt/air circuits between each immersed intermediate heat exchange bundle and the corresponding steam generator. There is an induction type NaK circulation pump in the NaK return pipe from the NaK-HTF heat exchanger to the intermediate heat exchange bundle. There are variable speed magnetically coupled pumps in the nitrate salt return pipes from the remote steam generators. The nitrate salt dump tanks have electric heaters to remelt the nitrate salt after a nitrate salt circuit shutdown.
The liquid level in the NaK dump tanks is controlled via the argon pressure over the NaK dump tanks. This gauge pressure varies from zero when the dump tanks are full to 0.5 MPa when the NaK dump tanks are almost empty.
The liquid level in the nitrate salt dump tanks is controlled via the air pressures over the nitrate salt in the respective dump tanks. This gauge air pressure varies from zero when the dump tanks are full to about 0.2 MPa absolute at room temperature when these dump tanks are almost empty. Under normal operation at high temperature the compressed air must be vented to maintain the nitrate salt levels. When the apparatus is cooled air must be re-injected for nitrate salt level maintenance.
The nitrate salt circuit is vented to the atmosphere via ball checks at both circuit ends to ensure that the nitrate salt head pressure is always lower than the NaK pressue and the water/steam pressure in the steam generator. The NaK pressure is maintained by compressed argon pressure in the NaK loop. The NaK level is maintained by pumping argon from the top of the system to the dump tank, or vice versa.
In the event that the NaK level or NaK pressure in any heat transport circuit decreases the air pressure over the corresponding nitrate salt dump tanks is released which causes the molten nitrate salt to drain out of the NaK-salt heat exchanger and the extended nitrate salt pipes and flow into the nitrate salt dump tanks. This arrangement prevents nitrate salt entering the NaK loop via a NaK-salt heat exchanger tube rupture.
HEAT TRANSPORT LOOP ISOLATION:
There are 48 identical independent heat transport circuits, 6 associated with each of 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 other heat transport circuits. Likewise, in principle any turbogenerator can be operated at part power using only a fraction of its 6 allocated heat transport circuits.
Each intermediate heat exchange bundle supplies hot NaK to a dedicated NaK-nitrate salt heat exchanger. Each NaK loop has three parallel connected dedicated NaK dump tanks, a dedicated electric induction pump, a dedicated argon filled cushion tank and dedicated pressure relief vents. Any heat transport circuit can be shut down for service while the other heat transport circuits remain in operation.
Each of the eight heat exchange galleries has six associated NaK heat transport circuits. Each NaK heat transport circuit has a full load capacity to transport:
1000 MWt / (6 X 8) heat transport circuits = 20.833 MWt.
Each heat transport circuit is used by the steam turbogenerator to provide:
20.833 MWt X 0.300 = 6.25 MWe
of electricity. Thus the maximum possible FNR electricity output is limited by the heat transport circuits to about:
48 X 6.25 MWe = 300 MWe
Each NaK heat transport loop uses 18 inch outside diameter (OD) schedule 40S stainless steel pipe. This pipe has an OD of 18.0 inch, a wall thickness of 0.375 inch and a linear weight of 49.56 lb / ft.__________
Each nitrate salt loop uses 16 inch schedule 40S stainless steel pipe, 16 inch OD ______wih a wall thickness of 0.322_____ inch and the pipe mass per unit length is 42.5 kg / m.______
The corresponding pipe flanges are 32 inch diameter.
The intermediate heat exchange bundle has at least 7 m below the pool deck + 2 manifold heights + bottom feed.
Thus the airlock needs an inside length of at least 10.0 m.
At full load the NaK differential temperature drop across each the intermediate heat exchange bundle is about:
450 - 340 = 110 degrees C, and the differential temperature drop across the nitrate salt pipe loop is about:
440 C - 320 C = 120 degrees C.
At part load the differential temperature across the nitrate salt loop remains nearly constant due to the use of a variable speed nitrate salt pump.
The thermal conductivities of the Na and NaK are relatively high, so the NaK discharge temperature closely follows the Na surface temperature, limited only by the thermal conductivity of the intermediate heat exchange bundle tubes.
At full load the nitrate salt flow through the steam generator tubes are turbulent, which enhances heat transfer. The thermal stresses in the steam generator are complex. The low end nitrate salt temperature is pinned at about 320 C by the water temperature at the bottom of the steam generator which is a function of the steam pressure setpoint. Along the steam generator tubes the tube temperature above the steam generator water level is set by the contained molten nitrate salt temperature whereas below the water level the steam generator tube temperature is close to the liquid water temperature in the bottom of the steam generator.
NITRATE SALT LOOP TEMPERATURE MAINTENANCE:
During normal reactor operation the nitrate salt return temperature should both be maintained above 310 degrees C to prevent NaOH depositing on the inside surface of the NaK-nitrate salt heat exchanger tubes. Hence, during normal reactor operation the NaK temperature in the NaK-salt heat exchanger is always kept above 335 degrees C. Similarly the NaK temperature in the NaK-HTF heat exchanger is always kept above 320 C. This temperature prevents deposion of NaOH (and some other metal hydroxides) on cooler heat exchange surfaces.
Below a nitrate salt temperature of about 260 degrees C the nitrate salt will freeze and no longer transport heat. In order to cool the primary sodium pool down to 120 degrees C, as is required for Na fire suppression, reactor fuel changes and intermediate heat exchange bundle service, heat supply/removal from the NaK is done by using hot air.
For heat removal the steam bypass valve from the steam generator to the turbogenerator condenser is openned. Then water in the steam generator evaporates at 100 degrees C and cools the air and hence the NaK and hence the Na down to about 120 degrees C.
THERMAL STRESS:
The thermal conductivity of the molten nitrate salt is lower than the thermal conductivity of NaK and in the NaK-salt heat exchanger the nitrate salt flow may be laminar, so there is a significant temperature drop between the NaK loop and the nitrate salt loop. This temperature difference also causes significant compressive stress in the NaK-nitrate salt heat exchange tubes leading to tensile stress in the NaK-nitrate salt heat exchanger shell. It may be necessary to allow the tube sheets to flex to compensate for this issue. It may be necessary to have an untubed perimeter region around the tubed region to permit this flexing. This matter is governed by ASME Fired Pressure Vessel code.
Thermal stress calculations are very important but are beyond the scope of this document.
Note that this thermal stress matter is more complex than it appears on the surface. In a conventional fire tube boiler the tube walls largely operate at the temperature of the surrounding water, not at the contained hot gas temperature. Since the water temperature also sets the shell temperature this issue provides a high degree of stress relief that will not occur in a liquid to liquid heat exchanger such as the NaK-salt and the NaK-HTF heat exchangers and in the cooler portion of the steam generator. In a liquid to liquid heat exchanger another thermal stress relief method, such as coiled tubes, may be required. The tubes will operte at the NaK temperature. There will be a significant temperature drop across the nitrate salt or HTF boundary layer.
HEAT TRANSPORT AWAY FROM THE NUCLEAR ISLAND:
The molten nitrate salt acts as a thermal fluid that transports heat from each heat exchange gallery, under the adjacent laneway, to a turbine hall which contains the corresponding steam generators. The return pipes come back under the laneway to the heat exchange gallery. The nitrate salt pipes should monotonically slope to corresponding dump tanks that are located at the pipe low points in the turbine hall. The dump tanks must be at a lower elevation than the NaK-salt heat exchanger and the steam generator to enable gravity drainage of nitrate salt from the heat exchange gallery and from the steam generator to the dump tanks.
The two end high points in each nitrate salt circuit are vented to the atmosphere via 16 inch diameter vents with top ball checks which are sufficiently tall to allow for the change in fluid level due to the circulation pump pressure differential.
When the air pressure over the nitrate salt dump tanks is released, provided that the nitrate salt is above 280 degrees C, all the nitrate salt in the nitrate salt loop will drain into the nitrate salt dump tanks.
The nitrate salt circulation pump is magnetically coupled to avoid hot salt leakage at the pump mechanical seal. To give these pumps positive suction head the pumps are located at a pipe loop low points close to the discharges from the dump tanks. However, there is a requirement for controlled air pressure over the dump tanks to provide for fluid level control.
TUBE FAILURES:
In the event of a NaK-salt heat exchanger tube failure the higher pressure NaK will enter the molten nitrate salt circuit where it will rapidly chemically react with the molten salt producing nitrogen gas. The nitrogen gas will displace salt in the NaK-salt heat exchanger shell, and may force some molten nitrate salt to discharge out the adjacent nitrate salt vent.
As the NaK loses pressure and its level decreases the air pressure over the corresponding nitrate salt dump tank is released so that the remaining salt in the NaK-salt heat exchanger will drain to the nitrate salt dump tank. The nitrogen producing chemical reaction will continue as long as there is any NaK above the NaK-salt heat exchanger tube rupture. Thus the salt vent diameter and the various pressure ratings must be sufficient to safely manage this rapid nitrogen production and discharge.
As soon as the nitrate salt level drops below the bottom of the NaK-salt heat exchanger the remaining NaK should be dumped into its corresponding NaK dump tank.
MINIMUM POWER:
An important ongoing operating issue is maintenance of sufficient thermal flux and coolant flow to prevent the molten salt from freezing in the extended external pipes.
The steam pressure in the steam generator shell 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 (260 degrees C). The molten salt supply upper temperature is limited by the NaK supply temperature. The molten salt return temperature must kept above 280 degrees C to prevent the salt freezing in the pipe and above 320 degrees C to prevent NaOH in the NaK depositing on heat exchange surfaces.
COOLING FOR MAINTENANCE:
If the molten salt temperature falls below its melting point there will be no heat transport away from the NaK-salt heat exchanger. When it is necessary to cool the primary sodium below 280 degrees C for service or other purposes, heat can only be removed from the NaK using HTF cooling.
The HTF is circulated using a dedicated HTF pump. The hot HTF is cooled by evaporation of water in the steam generator. The steam generator valve to the condenser is openned allowing the pressure in the steam generator to fall to atmospheric pressure. Then evaporation of water in the steam generator will extract heat from the HTF and hence the NaK and hence the sodium pool. The steam generator water level control system will automatically add additional water as the water in the steam generator evaporates.
PINHOLES:
Comment by Harry V. Winsor
Essentially, pinholes are partly an alloy composition problem, partly a fabrication QC problem, and partly a use environment problem.
Comment by: Charles Rhodes
There is a practical angle to the heat exchange bundle pinhole problem. Consider an automotive radiator. It may be designed to transfer 200 kWt of heat. Now consider a 1 GWt nuclear reactor. If it has only one stage of isolation it needs the equivalent of:
1 GWt / (200 kWt / radiator) = 5000 automotive radiators
If there are 2 stages of isolation that becomes 10,000 radiators.
Typical automotive radiators leak after about 20 years, Hence a power reactor heat exchanger made to the same standards can expect
10,000 radiators X 1 failure / 20 radiator-year = 500 failures / year.
Even with the best quality control techniques it is difficult to get that number under 50 failures / year. Hence it is really important to design the facility so that individual heat exchange bundle pinhole leaks can be tolerated, identified, fixed, replaced or bypassed without a reactor shutdown.
LWRs have a hidden advantage that they can tolerate a few pinholes because the fluids on both sides of the heat exchangers are primarily water.
Harry V. Winsor Comments
Na is a "free-electron metal" (ditto the other alkali metals): Structural metals must have partially-covalent bonds (beyond the free electron metals crystal structures) to prevent dissolution in them. This should allow the lattices to resist Na attack, e.g., as shown in Na-( FCC or BCC) metals. The problem comes when impurities (aka alloying elements) must be introduced for reducing structural metal grain sizes to prevent pileup of dislocations that can nucleate cracks or soluble inclusions. Thus, a first heat exchanger may need to be a Reactor-Na--Intermediate-NaK heat exchanger (minimum solubility of Na in the metal as shown by its phase diagram with Na). This should allow the best long term performance close to the reactor, since the exchanger materials can be selected to be the best at resisting Na-Attack from both sides in both Na environments. This heat exchanger could be operated to minimize pressure stresses that might allow migration of impurities in the exchanger metal chosen.
This means that the next heat exchanger is at the reactor electrical potential (probably grounded) on one side, and it needs to separate dissimilar fluid flows to be compatible with the high temperature needed for best generation efficiency. This likely increases its size (transfer area) and allows for a thicker metal separation between flows. It also allows for replacing heat exchangers if a pinhole does develop beyond operating tolerances. Since steam generation is needed for existing turbo-generation units, it is necessary to add another intermediate stage, which will lower the temperature, affecting the thermodynamic efficiency.
The water-side of this second heat exchanger would probably need to be coated with a second metal with higher water or corrosion resistance, and the chemical nature of the (nearly pure) water would need to be maintained at optimum for resisting water attack and frequent pressure cycling. This might include sacrificial anodes and other water-to-metal potential controls.
Comments by:John Rudesill:
Heat exchanger leaks almost always occur in or adjacent to welds. The metallurgy of the metal is disturbed and generally is not the same as the bulk material and corrodes faster. These local differences promote galvanic activity. I submit that the design needs to factor in the total length of weldments in each proposed heat exchanger configuration. Multiplying by the width of each weldment will give a total weld disrupted area and adding a thickness factor will give a volume of disrupted metal that can be related to the total metal mass. The more weldment volume in the total exchanger complement the higher the probability of
Thank you both for your theoretical comments relating to heat exchanger construction. Let me add another not so theoretical comment and that is thermal stress. These heat exchangers are all of the single pass counter current type with a large temperature differential between the input and discharge of each pass.
FLUID SUMMARY:
The various fluids are:
Na@ 100 C to 500 C @ 0. 1 MPa (atmospheric)
NaK@ 20 C to 490 C @ 0.5 MPa (Pressure maintained by a head of compressed argon)
Nitrate salt @ 10C less than the NaK temperature @ 0.1 MPa (atmospheric)
Synthetic heat transport fluid(HTF) @ 10 C less than the NaK temperature @ 0.1 MPa (atmospheric)
Steam at 40 C less than the nitrate salt temperature @ 10 MPa
Water at 10 C less than the synthetic heat transport fluid(HTF) temperature @ 10 MPa
In normal operation the Na surface temperature is 460 C and the steam generator bottom water temperature is 310 C. Thus the nitrate salt discharge temperature from the steam generator is of the order of 320 C.
RELIABILITY:
The general approach is to use a large number of identical heat transport systems operating in parallel so that the consequences of taking a few heat transport systems out of service for repair and maintenance are minimal.
We must do all necessary to prevent products of corrosion accumulating in the NaK, nitrate salt and synthetic heat transport fluid.
Normally only the Na is radioactive. If an intermediate heat exchange bundle leaks that bundle is taken out of service until the next reactor shutdown, All the other heat exchange bundles can be repaired or replaced while continuing to generate electricity.
Each heat exchange bundle is rated at 21 MWt.
HEAT EXCHANGER LAYOUT:
In the heat exchange galleries the equipment relating to each of the independent heat transport loops is laid out in radial rows with a row center to row center spacing of 1.85 m at R = 18.5 m.
The minimum room inside length required for this equipment layout is:
5 (1.5 m) + 2 (1.25 m) = 7.5 m + 2.5 m = 10.0 m
In the heat exchange galleries the internal width allowance is 8.0 m. In the turbogenerator halls the width allowance for the steam generator related equipment is 8.5 m + allowance for discharge steam piping.
BLACKSTART:
Bootstrap Warmup Procedure:
1) Use electricity or a natural gas boiler to heat the HTF;
2) Use hot HTF to heat the NaK;
3) Use the NaK to melt the sodium;
4) Use reactor heat to raise the Na and NaK temperatures to 320 C;
5) Use hot NaK and hot water to melt the nitrate salt in the steam generator;
6) Circulate the nitrate salt;
7) Raise Na temperature to 460 C which raises the NaK temperature to 450 C and raises the nitrate salt temperature to 440 C;
8) Engage the turbogenerators;
9) Synchronize to grid.
On cool down we must be careful to lower the nitrate salt temperature before the nitrate salt goes to its dump tank to prevent damage to the HTF.
The nitrate salt must flow through the steam generator before flowing to the nitrate salt dump tank so that it is cool enough to not damage the heat transfer fluid.
RECOVERY FROM SERVICE:
Assume that the primary sodium pool temperature is initially above 120 degrees C.
Assume that initially the NaK, the HTF and the nitrate salt are all in their dump tanks.
The first step is to apply argon pressure over the NaK dump tank to fill the NaK loop.
Then turn on the NaK induction pump. The NaK temperature should rise to the sodium pool temperature.
Transfer HTF from its dump tank into the HTF loop.
Raise the primary sodium temperature by movable fuel bundle insertion up to about 300 degrees C. The NaK temperature will rise causing the HTF temperature to rise.
Use HTF to heat the nitrate salt in its dump tanks to about 300 degrees C.
Then apply air pressure over the nitrate salt dump tanks to transfer the nitrate salt into the nitrate salt loop. The nitrate salt tanks require a dedicated molten salt liquid level sensor to regulate their cover air pressure. Turn on the nitrate salt pump. Heat flowing from the steam generator shuld maintain the nitrate salt temperature.
Enable the steam generator water injection pump. The water level in the steam generator should rise to its set point. This water will form steam at a rate determined by the flow rate of NaK through the intermediate heat exchange bundle.
Raise the primary sodium pool temperature to 460 C. The presssure in the steam generator will rise to about 10 MPa at which point the steam PRV should open releasing steam to the turbine.
While producing steam at low power set the turbine slightly below 1800 RPM. As the generator output voltage reaches phase synchronization, close the contactor.
Now gradually increase the NaK flow rate through the intermediate heat exchanger to increase the electricity generator output.
During black start only one heat transport circuit is started at a time to minimize the auxiliary power required for electric heating during black start.
Once the turbine is operating electricity can be generated and the system no longer requires auxiliary power for continuation of further black start steps. Thus there must be enough auxilliary power to fully turn on at least one generator.
STEAM GENERATOR TUBE FAILURE:
A steam generator tube failure will cause injection of steam / water into either the molten nitrate salt or the HTF. The water will immediately form steam which will expand blowing molten salt or HTF out the steam generator molten salt or HTF vent to the atmosphere. A sudden pressure or level rise in either vent indicates a steam generator tube failure.
MAINTENANCE:
There is no radioactivity in the heat exchange galleries, which allows safe service work or NaK fire suppression in selected heat exchange galleries without causing a total reactor shutdown.
DIAGRAM:
A diagram showing an end view of a heat exchange gallery. Each gallery has 6 heat transport loops, one behind the other.
The left hand side of this diagram shows the intermediate heat exchange bundle and the NaK pipes. This diagram shows the molten salt heat transport pipes on the right hand side and the four parallel connected NaK dump tanks in the lower left middle. The horizontal induction pump, the NaK flow meter and the vertical NaK-salt heat exchangers are in the middle of the diagram. Above the NaK-salt heat exchanger from left to right are the NaK argon filled cushion tank and the molten nitrate salt pressure relief vent.
Note the dedicated steel I beam equipment supports. The maximum width of each such I beam is 0.20 m to allow installation and subsequent replacement of individual NaK dump tanks that have maximum flange diameters of 1.3 m.
Note that NaK-salt and NaK-HTF heat exchanger staggering enables a NaK-salt heat exchanger shell diameter of 1.2 m (~ 48 inches) and flange diameter of 1.8 m (72 inches). Note that the NaK-salt heat exchanger centerline to end wall clearance is 1.25 m which is barely sufficient for the NaK-salt heat exchanger end manifold flanges + insulation.
The FNR site plan shows the 2.0 m wide space between adjacent heat exchange galleries that is dedicated to the airlocks that are required for moving fuel bundles and intermediate heat exchange bundles from their truck load/unload points into or out of the sodium pool space.
HEAT EXCHANGE GALLERY LOWER LEVELS:
Each heat exchange gallery has a lower level where the NaK dump tanks are located. Personnel access to the various heat exchange gallery levels is via a stairwell at the gallery end farthest from the airlock truck load/unload point.
The electric drives for the NaK induction pumps are wall mounted adjacent to the induction pumps.
Equipment in the heat exchange galleries is installed and removed from above using a mobile crane parked at the airlock truck load/unload point. The roof over the heat exchange galleries must be easily removable and replaceable.
There must be a large air vents in heat exchange galleries for outside air cooling.
An important issue in the heat exchange gallery is isolation of potential NaK drips. A likely source of these drips are the NaK pipe flange connections. Drip pans should be provided to collect dripped NaK.
In the basement under each heat exchange gallery is an isolated space that is used for an argon bladder. This space is air flow connected to the service access space under the pool deck. Any water penetrating this space from the outside must flow along the floor and into the FNR bottom drain located about 18 m below grade.
INDUCTION PUMP:
Induction pumps are used to circulate the NaK. The induction pumps must be sized to overcome the flow pressure head in the NaK loops. Note that these pumps should be located on the lower temperature NaK return pipes near the sodium pool deck level to ensure both cool operation and sufficient positive suction head.
The induction pump operates by inducing a circular current in the liquid NaK. 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 full load operation the pumped cool NaK temperature is about ~ 400 degrees C and the induction pumps are oil cooled to protect the electrical insulation from heat damage.
However, under circumstances when the NaK-HTF heat exchanger is uncooled either natural or pumped circulation of the NaK can potentially cause the NaK temperature at the induction pump to rise to about 345 degrees C. Under these circumstances the induction pump and HTF can easily be damaged if there is insufficient pumped HTF cooling. To prevent wide temperature excursions the NaK induction pump should be stopped if the HTF pump is stopped or if the corresponding steam generator contains no water.
The induction pumps have 24.000 inch OD schedule 40S (0.375 inch wall) 316 SS flow pipes and 18.00 inch OD schedule 40S (0.375 inch wall) 316 SS discharge pipes. The 24.000 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 24 inch pipe is:
24.000 inch - 2 (0.375 inch) = 23.25 inch
Induction pump details are set out at FNR Induction Pump.
GASKET CONSTRAINT:
A major constraint on the FNR design is gasket material properties. This FNR operates at too high a (temperature X pressure) product for use of normal elastomeric gaskets. Soft metal, teflon 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 NaK-salt heat exchanger manifolds bottom and top halves as well as the pipe mechanical joints are sealed with such gaskets.The flange bolts must have a smaller TCE than the flange material to prevent bolt loosening at high temperatures.
PIPE AND SODIUM POOL THERMAL EXPANSION:
The NaK-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 contraction. Also the sodium pool inside wall moves due to thermal expansion. When the system is cold the outside of the cooler intermediate heat exchange bundle NaK inlet pipes should be almost touching the sodium pool inside wall. When the system is hot there is about a 0.2 m gap between this pipe outside and the sodium pool inside wall.
WELDING:
The manifold welds must be deep penetration equal in quality to the welds used on high pressure natural gas distribution pipelines. X-ray inspection and possibly a helium leak detector should be used for confirming weld quality.
This web page last updated April 18, 2024.
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