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XYLENE POWER LTD.

FNR HEAT EXCHANGE TUBES

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

INTRODUCTION:
This web page deals with FNR intermediate heat exchanger tubes and FNR Steam Generator Tubes. The physical properties of these tubes dictate many aspects of FNR design. A major constraining issue is the tolerable level of combined thermal stress and internal pressure stress in the heat exchange tubes which normally operate in the temperature range 290 degrees C to 500 degrees C.

It is shown that due to best performance under severe thermal stress the best heat exchange tube material for both the intermediate heat exchangers and the steam generators is likely Inconel 600.
 

MATERIAL PROPERTIES:
Define:
TC = thermal conductivity
TCE = thermal coefficient of expansion
DeltaT = temperature drop across steel tube wall
Y = (stress / strain) = Young's modulus
Sy = yield stress

Key material properties are set out in the following table:
PROPERTY316LHT-9D915/15TiINCONEL
Density7966 kg / m^38200 kg / m^38430 kg / m^3
TC @ 500 C15 W / m-K26.2 W / m-K20.2 W / m-K20.9 W / m-K
TCE @ 500 C18 X 10^-6 / K15 X 10^-6 / K13 X 10^-6 / K15.1 X 10^-6 / K
Y @ 25 C202 GPa---207 GPa
Y @ 250 C, no rad.-2000 GPa----
Y @ 250 C, with rad.2000 GPa----
Y @ 350 C, no rad860 GPa
Y @ 350 C, with rad1200 GPa
Bulk Y @ 500 C120 Gpa135 GPa--
Sy @ 25 C, no rad.291.3 MPa---630 MPa550 MPa
Sy @ 250 C, no rad.600 MPa-570 MPa-
Sy @ 250 C, rad900 MPa--
Sy @ 350 C, no rad.420 MPa560 MPa-
Sy @ 400 C, rad600 MPa to 900 MPa--
Sy @ 465 C, no rad725 MPa-530 MPa-
Sy @ 460 C, with rad520 MPa--
Sy @ 500 C, no rad167 MPa400 MPa to 550 MPa510 MPa579 MPa
Sy @ 500 C, with rad450 MPa to 600 MPa--
-------

INTERMEDIATE HEAT EXCHANGE TUBES:
The optimum choice of heat exchange tube material for an FNR is a complex property tradeoff. With respect to the FNR design developed on this web site natural circulation of the primary liquid sodium is used to achieve mechanical simplicity. However, with natural circulation of the primary liquid sodium the liquid sodium at the bottom of the primary liquid sodium pool operates at about 320 degrees C and the liquid sodium at the top of the liquid sodium pool operates at about 480 degrees C. Various parts of a heat exchange tube normally operate in the temperature range 310 C to 488 C. The heat exchange tubes must safely accommodate initial fuel bundle insertion in the FNR when the sodium inside the fuel tube is initially solid.

Another practical consideration in choosing the heat exchange tube material is its workability. Each FNR has 36 X 1664 intermediate heat exchange tubes and _____ steam generator tubes that must be automatically fabricated, assembled and tested.

The heat exchange tube alloy must be chemically compatible with Na, H2O, UO2, U, Pu, Zr, fission products, transuranium actinides from 20 degrees C to 488 degrees C.
 

PRESSURE AND THERMAL STRESSES:
Due to the internal pressure the inside of an intermediate heat exchange tube wall is under tension. The outside of an intermediate heat exchange tube wall is under compression. These material stresses are partially balanced by the radial heat flux which places the outside of the tube wall under compression and the inside of the tube wall under tension. Net stress will over time cause intermediate heat exchange tube material creep and hence heat exchange tube diameter increase.
 

FOR 316L STAINLESS STEEL HEAT EXCHANGE TUBES:
(DeltaT)
= (Sy)(2) / [(TCE) Y]
= [24,400 psi(2) X (101,000 Pa / 14.7 psi)] / [ (17.5 X 10^-6 / deg C) X (202 X 10^9 PA)]
= [48.8 X 101 X 10^12 deg C] / [14.7 X 17.5 X 2.02 X 10^11]
= 94.80 deg C

For a conservative safe design the maximum stress and hence the maximum operating temperature differential should be reduced by a factor of three to: 31.60 deg C

However, there is also differential pressure stress. If the stresses are to be equally divided between differential temperature and differential pressure the maximum differential temperature across the tube wall further decreases to 15.8 C.

Thus the maximum operating heat flux through the 316L stainless steel tubes is:
15.8 deg C X 15 W / m-deg C / (.065 inch X .0254 m / inch) = 143,549.4 W / m^2

The intermediate heat exchange tube area is:
Pi X (.500 inch) X (.0254 m / inch) X 6.1 m / tube X 36 bundles X 1664 tubes / bundle = 14,579 m^2

Hence the corresponding maximum possible reactor thermal power is:
143,549 W / m^2 X 14,579 m^2 = 2092,854,595 Wt
= 2092.85 MWt

In reality the maximum reactor power will be limited by the liquid sodium flow between the reactor core fuel tubes.

The corresponding allowable differential pressure P is given by:
P (.37 inch) = (Syp / 6) 2 (.065 inch)
or
P = (Syp / 6)(0.13 inch / 0.37 inch)
= 30,000 psi (.05855)
= 1756.7 psi
= 119.5 bar
= 12.07 MPa
 

OTHER TUBE ALLOYS CONSIDERED:
316L is a high performance austenitic stainless steel tube alloy that has been ASME approved for use in fired pressure vessels for over 30 years. 316L features good weldability. According to the Euporean Stainless Steel Development Association the term 316L refers to steels that comply with:
<0.030% C + <1.00% Si + <2.00% Mn + <0.045% P + <0.015% S + <0.11% N
+ {16.5% Cr to 18.5% Cr + 2.00% Mo to 2.500% Mo + 10% Ni to 13% Ni + Fe}
or + {17.0% Cr to 19.0% Cr + 2.50% Mo to 3.00% Mo + 12.5% Ni to 15% Ni + Fe}
or
+ {16.5% Cr to 18.5% Cr + 2.50% Mo to 3.00% Mo + 10.50% Ni to 13.00% Ni + Fe}

According to Gimondo 316 consists of:
{Fe + 0.05% C + 17% Cr + 2.0% Mo + 0.6% Si + 1.8% Mn + 13% Ni + 20 ppm B}
 

316 Ti is an austenitic stainless steel alloy described by Gimondo as consisting of:
{Fe + 16% Cr + 2.5% Mo + 14% Ni + 0.6% Si +1.7% Mn + 0.05% C + 0.4% Ti +0.03% P}
 

D9 is a titanium stabilised austenitic stainless steel Indian alloy described by Leibowitz and Blomquist as consisting of the weight percentages:
{65.96% Fe + 13.5% Cr + 2.0% Mo + 15.5% Ni + .04% C + 2.0% Mn + 0.75% Si + 0.25% Ti}
and described by Banerjee et al as:
{Fe + 14.7% Cr + 2.2% Mo + 14.9% Ni + .05% C + 1.3% Mn + 0.65% Si + 0.18% Ti
+ <.05% Cu + <.07% Nb + .045% V + .03% Co + <.034% Al + <.004% Sn + .005% W + <.04% N + .008% P + .005% S + <.006% As}
and is described by Karthik et al as:
{Fe + 13.5% to 14.5% Cr + 2% Mo + 14.5% to 15.5% Ni + .035% to .05% C + 1.65% to 2.35% Mn + 0.5 to 0.75% Si + 0.2% Ti}
and is described by Gimondo as consisting of:
{Fe + 13.5% Cr + 2.0% Mo + 15.5% Ni + .04% C + 2.0% Mn + 0.75% Si + 0.25% Ti}
 

15/15 Ti (12R72) is an austenitic stainless steel European alloy described by Gimondo as consisting of the weight percentages:
{Fe + 15% Cr + 1.2% Mo + 15% Ni + 0.10% C + 1.5% Mn + 0.6% Si + 0.4% Ti + 0.03% P + 50 ppm B}

15/15 Ti (12R72) has an approximate fast neutron dose limit of 120 dpa. It has a Larson Miller parameter of 23.8 at 100 MPa.
 

OTHER ALLOY PROPERTIES:
9Cr - 1 Mo steel has a well documented creep rupture life.

T91 is a ferritic-martensitic steel with Larsen Miller parameter 21.5 at 100 MPa.

A major issue with Austenitic stainless steel such as 316 used at 420 C is that under prolonged fast neutron exposure it swells as much as 25% whereas under the same neutron exposure ferritic steels expand < 1%. This swelling will reduce the flow of cooling liquid sodium through the reactor core.

The alloy D9 features a higher creep rupture strength, a lower creep rate and a lower rupture ductility than 316L.
 

HEAT EXCHANGE SYSTEM:
Heat is removed from the FNR via heat exchange tube bundles immersed in the ends of the pool. These heat exchange tubes normally operate at a temperature of up to 488 degrees C, have low pressure radioactive liquid sodium on the outside and high pressure non-radioactive sodium on the inside. Thus if there is a heat exchanger tube wall failure a limited volume of nonradioactive sodium flows into radioactive sodium, which is not a serious problem. If there is a steam generator tube failure non-radioactive sodium flows slowly into the water side of the steam generator due to the non-radioactive sodium being kept at a slightly higher pressure than the steam.

There must be a secondary sodium pipe shunt that reduces the secondary liquid sodium inlet temperature to the steam generator to limit the thermal stress on the steam . Otherwise when the inlet water temperature to the steam generator is low the differential temperature across the steam generator tube wall may be so high as to damage the steam generator through thermal stress.

There are 12.75 inch OD 10.126 inch ID pipes from each end of each heat exchange bundle manifold up to 90 degree elbows and then back towards the steam generators. Each 12.75 inch OD inlet pipe to a heat exchange manifold has an induction type circulation pump and a disconnection flange. Each 12.75 inch heat exchange manifold discharge pipe has a disconnection flange.

The heat exchange bundles are completely isolated from one another. Each bundle feeds a dedicated steam generator and has a dedicated expansion tank. Hence in the event of a problem one heat transport loop can be shut down while the other heat transport loops remain fully operational.

At the low point in the pipe mains are sealed sodium sumps with sufficient volume to accommodate all the liquid sodium in the relevant intermediate heat transport circuit. Sodium is transferred from this sump to the intermediate heat transport circuit by applying argon pressure over the sump while evacuating the loop cushion tank. These sumps require electric immersion heaters for liquid sodium melting/temperature maintenance. Note that these sumps must be rated as pressure vessels and must ve fitted with high pressure valves and pressure relief valves vented to the argon atmosphere.
 

The hot secondary liquid sodium is piped outside the reactor building to adjacent buildings that contain steam generators and associated downstream non-nuclear equipment. Under ordinary operation the reactor power is modulated by controlling the intermediate sodium circulation rate. The intermediate sodium transport pipes must have at least two 90 degree elbows with arms sized to allow for thermal expansion-contraction and possible earthquake related movement.
 

Another major constraining issue is the combined thermal stress and internal pressure stress in the tubes which form the intermediate heat exchanger. In addition to internal pressure the intermediate heat exchanger has a significant temperature differential across the tube wall. This temperature differential can potentially lead to high thermal stress at the point where the cool secondary return sodium is first heated by the primary liquid sodium. This problem is minimized by keeping the primary liquid sodium temperature stratified.

One of the issues with Inconel is long term creep. This issue is particularly important in the intermediate heat exchanger. To minimize the effect of long term creep on primary sodium flow the tubes in the intermediate heat exchanger are arranged in a square lattice rather than a staggered lattice and the tube center to center distnace is made 0.75 inch.

In the steam generator the material stress due to differential pressure across the tube wall is relatively small because the liquid sodium pressure is controlled to track the steam pressure. However, the thermal stress can be very large at the point where inlet water to the steam generator is first heated by liquid sodium that is on its way back to the intermediate heat exchanger.
 

HEAT EXCHANGE TUBES:
Inconel 600 is a high nickel alloy that maintains its yield stress rating at high temperatures and hence is widely used in high temperature heat exchangers where there may be both substantial pressure differences and high thermal stress. It is described by American Special Metals and Rolled Alloys Inc. as:
> 72% Ni (+ Co) + 14.0% to 17.0% Cr + 6.00% to 10.00% Fe + < 0.15% C + < 1.0% Mn + < 0.015% S + < 0.50% Si + < 0.50% Cu

Inconel-600 is only used in heat exchangers that are outside the neutron flux. The inconel 600 must be chemically compatible with Na and H2O at 100 to 500 degrees C.
 

FOR INCONEL 600:
(DeltaT) = (Sy)(2) / [(TCE) Y]
= [579 MPa (2)] / [ (15.1 X 10^-6 / deg C) X (207 X 10^9 Pa)]
= [1158 X 10^6 Pa deg C] / [15.1 X 207 X 10^3 Pa]
= 370.5 deg C

For a conservative safe design the maximum stress and hence the maximum operating temperature differential should be reduced by a factor of three to: 123.5 deg C

In order to allow for half the allowable stress being due to internal gas pressure further reduce the operating temperature differential by another factor of two to 61.75 degrees C.

Thus the conservative operating heat flux through the Inconel 600 tubes of the primary to secondary heat exchanger is:
61.75 deg C X 20.9 W / m-deg C / (.065 inch X .0254 m / inch) = 781,693 w / m^2

The heat exchange tube surface area is:
Pi X (.500 inch) X (.0254 m / inch) X 5.5 m / tube X 1500 tubes / bundle X 36 bundles = 11,844 m^2

The maximum allowable internal gas pressure causes a hoop stress of:
(Sy / 6) = 24,400 psi / 6
= 4067 psi.
(Max Pressure) X (.500 inch - .130 inch) X L = 4067 psi X 2 x .065 inch X L
or
Maximum pressure = 4067 psi X .130 inch / .37 inch
= 1429 psi
= 97.2 bar
 

This web page last updated April 25, 2016

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