XYLENE POWER LTD.

FNR INDUCTION PUMP

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

INDUCTION PUMP:
The NaK induction pump must be sized to overcome the flow pressure head in the intermediate heat exchange bundle, the NaK-salt heat exchanger, the three way valve and the NaK piping. Note that the induction pump must be located on the NaK low temperature return pipe and must be physically as low as possible in the NaK loop to ensure positive suction head.

An induction pump operates by inducing a circular current in the NaK. This current crosses a radial magnetic field component and hence the NaK 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.

The induction pump relies on the relatively high electrical conductivity of hot liquid NaK as compared to the electrical conductivity of stainless steel.
 

The induction pump must dependably operate at about 330 degrees C at a liquid sodium gauge pressure of 0.5 MPa. It must not be damaged by occasional brief exposure to 460 degree C NaK and should be hydraulic pressure tested at 3 MPa.
 

INDUCTION PUMP FUNCTION:
The induction pumps is used to cause a constant NaK flow through the NaK-salt heat exchanger. The induction pump also promotes turbulent fluid heat transfer in the intermediate heat exchange bundle tubes and in the NaK-salt heat exchanger tubes.

Induction pumps for NaK are built using a stainless steel flow pipe. Stainless steel has a relatively high electrical resistivity and is non-magnetic. In the middle of the induction pump flow pipe is the ferromagnetic core known as the "torpedo" made from transformer iron laminations. However, due to the high torpedo operating temperature the laminations must be bound together by a glaze rather than a varnish.

An induction pump uses external AC solenoids to induce a circular currents in liquid NaK surrounding the torpedo shaped core. These currents interact with radial magnetic field components to cause an axial force on the liquid NaK.

The FNR power is controlled by modulating the flow rate of the NaK through the intermediate heat exchange bundle using a three way diverting valve located at the induction pump discharge. The induction pump keeps the NaK flow rate through the NaK-salt heat exchanger nearly constant. The steam production rate is approximately proportional to the NaK flow rate through the intermediate heat exchange bundle. This arrangement keeps the NaK temperature at the induction pump and three way valve nearly constant at 330 to 340 C.

A major constraint on induction pump efficiency is the electrical conductivity of the surrounding stainless steel flow pipe wall. The pipe wall forms a parasitic current ring path which reduces the circulating current in the liquid sodium and hence reduces the pump efficiency.

In theory the efficiency of an induction pump could be substantially improved by using a flow pipe material with a higher electrical resistivity. However, with non-metal materials there are major problems in achieving dependable leak proof high pressure high temperature connections between the induction pump flow pipe and the connected extended pipes.

The induction pump will require custom made components. One of the fabrication issues is whether or not the torpedo can be inserted after the diameter reducing flow tube end pieces are welded in place.
 

INDUCTION PUMP DESIGN:
For this FNR application we considered three different induction pump designs.
a) A single phase design offers analytical and construction simplicity. It is similar in concept to a floating conducting ring around a long ferromagnetic core energized by a single phase solenoid. Linear non-conducting vanes on the surface of the torpedo minimize parasitic ring current through the liquid sodium. Center feeding allows pumping at both ends of the torpedo. A disadvantage of this design is a limited pump head pressure due to only one pressure increase step. Other major disadvantages are too many fittings, too much pressure loss at many 90 degree elbows and too much parasitic current loss via the fitting castings used to form the flow tube.

b) A three phase helical design is similar in concept to a three phase squirel cage electric motor. The motor action causes the liquid sodium to revolve around the torpedo. A spiral vane on the surface of the torpedo imparts an axial force on the liquid sodium. An advantage of this design is that in principle the 3 phase stator can simply be the stator from a large three phase motor. A disadvantage of this design is that it involves bearings in the hot sodium that are almost impossible to access for service. This pump design was rejected due to its potential bearing maintenance implications.

c) A three phase annular linear induction pump is similar in concept to a linear induction motor. This design is complex to analyse theoretically but it offers the benefit of ability to work against a higher pump head, since its head capacity increases with the number of stages. This general pump design has been successfully used by others in relatively low pressure sodium cooled reactor applications.

The induction pump flow pipe material must dependably withstand the 3 MPa transient test pressure. In this respect a major issue is hoop stress tolerance at 330 degrees C. Each induction pump must be safety tested at 3 MPa, 330 degrees C. Thus the induction pump flow pipe is 16 inch OD Schedule 40S stainless steel pipe.

The proposed three phase annular linear induction pump design involves a 16 inch OD induction pump flow pipe with 16 inch to 12.75 inch diameter reducer on one end and the branch of a 16 inch X 16 inch X 16 inch tee on the other end. One main tee path connects vertically to the steam generator lower manifold via a 16 inch to 12.75 inch reducing nipple. The other main tee path connects to the drain down tank via a 16 inch to 6 inch reducing nipple. This fitting arrangement ensures complete drainage of liquid sodium from the induction pump and the downstream 3 way valve.

The induction pump internal torpedo is about 12 inch OD. There are a series of three phase coils along the flow tube length separated by washer like laminated iron sections with IDs of slightly over 16 inches. Each magnetic gap is about 1.4 inches (0.375 inch stainless steel + 1 inch of sodium + air gap). Each laminated iron washer thickness is about 2 inches thick, 16 inch ID, 32 inch OD. Each coil length is about 3 inches. Each coil is about 16 inch ID, 32 inches OD.

There is a cylindrical cover about 36 inch OD, 32 inch ID X 3.1 m long again made from laminated iron to complete the magnetic circuits. The coils are cooled by circulating oil.

Thus each 3 phase pumping section is 3 (2 inch + 4 inch) = 18 inch long.
 

SPACE CONSTRAINT:
In the present heat exchange gallery design the induction pump fits below the steam ganerators. The induction pump positioning is shown on the following diagram which shows one intermediate heat exchanger-NaK-salt heat exchanger pair in a heat exchange gallery.

The available lengthfor the induction pump is 4 m____ of which about 0.9 m is lost to the various fittings leaving 3.1 m for the horizontal induction pump flow tube length.

Thus the pump flow tube should be N(18 inches) + 2 inches in overall length.

In the available space N = 6.

Thus the induction pump which when insulated is a horizontal cylinder up to 36 inch OD X 110 inch long (0.9 m X 3.1 m). An eight inch thickness of external insulation may increase its OD to about 52 inches (1.3 m).

There is an overall height allowance of 3 m ___in each heat exchange gallery for the induction pumps.
 

SUPPORT COLUMNS:
Each induction pump and each steam generator rests on a dedicated supports. Cross I beams between the two walls bear the weight of the induction pumps. The dump tanks fit in the spaces left free by the induction pumps.
 

INDUCTION PUMP WEIGHT:
The approximate volume of iron in the afore described induction pump is:
Pi[(18 inch)^2 - (16 inch)^2] X 110 inch
+ 7 Pi [(16 inch)^2 - (8 inch)^2) X 2 inch
+ Pi [(8 inch)^2 - (7 inch)^2] X 110 inch
+ Pi [(6 inch)^2] X 108 inch
 
= Pi [7480 inch^3 + 2688 inch^3 + 1650 inch^3 + 3888 inch^3]
= Pi [15,706 inch^3]
= 49,342 inch^3
= 49,342 inch^3 X (0.0254 m / inch)^3
= 0.8086 m^3

The mass of this iron is:
0.8086 m^3 X 7,874 kg / m^3 = 6367 kg
= 6.367 tonnes.

Hence the induction pump requires serious design optimaization to reduce its weight.
 

INDUCTION PUMP SUPPORT:
A non-trivial issue is the weight of the induction pumps. These pumps must be supported so that the pump weight is not transferred onto the feed pipes or other equipment. Note that this support must not block the pipe to the dump tank. The support also must be consistent with the induction pump's external thermal insulation.
 

INDUCTION PUMP DETAIL:
Inside the induction pump flow pipe the liquid sodium linear velocity increases by about a factor of two. There must be sufficient induction pump suction head to support this flow rate increase.

The induction pump flow pipe material must be engineered to dependably withstand the 3 MPa hydraulic test pressure. In this respect a major issue is hoop stress tolerance at 330 degrees C. Each induction pump must be safety tested at 3 MPa.
 

INDUCTION PUMP INEFFICIENCY:
The wall of the 16 inch OD stainless steel flow pipe forms a single turn around the laminated iron torpedo which converts much of the applied electrical energy into heat. Part of the applied electrical energy forms circulating current in the liquid sodium. The interaction of that circulating current with the radial magnetic field in the magnetic gaps accelerates the liquid NaK. Most of the heat generated is absorbed by the liquid sodium. However, conducted heat and energy dissipated in the external laminated iron must be absorbed by the pumped oil coolant.

The induction pumps form a considerable parasitic electrical load on the FNR. A significant effort should be applied to maximize induction pump efficiency.
 

This web page last updated April 26, 2022