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

The NaK induction pump must be sized to overcome the NaK flow pressure head in the intermediate heat exchange bundle, the NaK-salt heat exchanger, the NaK-HTF heat exchanger, the induction pump itself 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 a 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 distributed 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 over the temperature range 20 degrees C to 450 degrees C at a liquid sodium gauge pressure varying from 0.2 MPa to 0.8 MPa. It should be hydraulic pressure tested at 1.2 MPa.

The induction pumps is used to cause turbulent variable NaK circulation through the NaK loop.

Induction pumps for NaK are built using a stainless steel flow pipe. As compared to liquid Na 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. Due to the high torpedo operating temperature the laminations must be bound together by a glaze rather than by 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 using the induction pump to modulate the NaK flow rate through the intermediate heat exchange bundle. The steam production rate is approximately proportional to the NaK flow rate and its change in temperature.

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 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.

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 1.2 MPa transient test pressure. In this respect a major issue is hoop stress tolerance at 460 degrees C. Each induction pump must be safety tested at 1.2 MPa, 460 degrees C.

The proposed three phase annular linear induction pump design involves a 24 inch OD 0.375 inch wall stainless steel induction pump flow pipe with 24 inch to 18 inch diameter reducing nipple at the discharge end and the 24 inch branch of a 18 inch X 18 inch X 24 inch tee on the pump inlet end. One of the 18 inch tee ports connects vertically to the NaK-salt heat exchanger lower manifold via an 18 inch 90 degree elbow. The other 18 inch tee path connects to the drain down tank via a 18 inch to 6 inch reducing nipple. This fitting arrangement ensures complete drainage of liquid NaK from the induction pump.

The induction pump internal torpedo is about 18 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 24 inches. Each magnetic gap is about 3.4 inches (air gap + 0.375 inch stainless steel + 3.0 inch of NaK). Each laminated iron washer thickness is about 6 inches thick, 24.2 inch ID, 36 inch OD. Each coil length is about 6 inches. Each coil is about 24.2 inch ID, 36 inches OD.

There is a cylindrical cover about 48 inch OD X 36 inch ID X 6.0 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 (6 inch + 6 inch) = 36 inch long.
Five such pumping sections are:
(5 X 36) + 6 = 186 inches long.

Outside the 48 inch OD cover is space for 5.5 inch of thermal insulation while complying with the maximum width space allowance of Pi (32 m) / 56 = 1.795 m = 70.67 inch.

At the induction pump inlet end the 18 inch X 18 inch X 24 inch tee requires 36 inches for steel length along the induction pump axis. There is an additional length requirement of 9 inches to accomodate the flanges on the 18 X 18 X 24 inch tee. There is and additional requirement for about 16 inches to accommodate the nitrate salt/ HTF pipimg. These total about:
36 + 9 + 16 = 61 inches = 1.55 m. There is an additional requireent for insulation on the flanges suggesting a totalof about 2 m.

At the induction pump discharge end the 24 inch X 18 inch reducing fitting and the 18 inch flange must fit within the available lengthwise space. Estimate this space at 1 m.

Hence the remaining length available for the induction pump flow tube is:
8 m - 2 m - 1 m = 5 m = 195 inch


In the heat exchange gallery design the induction pumps fit below the NaK-salt heat exchanger or below the Nak-HTF heat exchanger. The induction pump positioning is shown on the following diagram.

The maximum available length for the induction pump horizantal flow tube length is 5 m.

In the available space N = 10.

Thus the induction pump which when insulated is a horizontal cylinder 1.35 m diameter with a torpedo 5.6 m long.

There is an overall diameter allowance of 1.5 m in each heat exchange gallery for the induction pump.

Each induction pump rests on two dedicated support cross I beams. These beams between the two walls bear the weight of the induction pumps and the NaK-salt heat exchangers. The dump tanks fit underneath the induction pumps. The heat exchangers and induction pumps must be removed to extract the 48 inch OD dump tanks. When insulated these dump tanks are 1.5 m diameter.

The approximate volume of iron in the afore described induction pump is:
Pi[(24 inch)^2 - (18 inch)^2] X 186 inch
+ Pi [(18 inch)^2 - (12 inch)^2) X (186 / 2) inch
+ Pi [(24 inch)^2 - (23.625 inch)^2] X 186 inch
+ Pi [(12 inch)^2 - (11.625 inch)^2] X 186 inch
+ Pi [(9 inch)^2] X 186 inch
= Pi[186 inch] [252 inch^2 + 90 inch^2 + 17.86 inch^2 + 8.86 inch^2 + 81 inch^2]
= Pi{186 inch] [449.72 inch^2]
= 262,788 inch^3
= 262,788 inch^3 X (0.0254 m / inch)^3
= 4.306 m^3

The mass of this iron is:
4.306 m^3 X 7,874 kg / m^3 = 33,907 kg
= 33.9 tonnes.

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

A non-trivial issue is the weight of the induction pumps. These pumps must be supported by dual beams 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.

Inside the induction pump flow pipe the liquid sodium linear velocity increases. There must be sufficient induction pump suction head to support this pressure increase.

The induction pump flow pipe material must be engineered to dependably withstand the 1.5 MPa hydraulic test pressure. In this respect a major issue is hoop stress tolerance at 390 degrees C. Each induction pump must be safety tested at 1.5 MPa.

The wall of the 24 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 NaK. 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 November 5, 2023

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