Patent Publication Number: US-2011075786-A1

Title: Heat exchanger, methods therefor and a nuclear fission reactor system

Description:
BACKGROUND 
     This application generally relates to induced nuclear reactions, including systems, processes and elements which implement such processes, such as a reactor core, primary heat exchanger, or pump, immersed in a liquid coolant in a vessel and more particularly relates to a heat exchanger, methods therefor and a nuclear fission reactor system. 
     It is known that, in an operating nuclear fission reactor, neutrons of a known energy are absorbed by nuclides having a high atomic mass. The resulting compound nucleus separates into fission products that include two lower atomic mass fission fragments and also decay products. Nuclides known to undergo such fission by neutrons of all energies include uranium-233, uranium-235 and plutonium-239, which are fissile nuclides. For example, thermal neutrons having a kinetic energy of 0.0253 eV (electron volts) can be used to fission U-235 nuclei. Thorium-232 and uranium-238, which are fertile nuclides, will not undergo induced fission, except with fast neutrons that have a kinetic energy of at least 1 MeV (million electron volts). The total kinetic energy released from each fission event is about 200 MeV. This kinetic energy is transformed into heat. 
     In nuclear reactors, the afore-mentioned fissile and/or fertile material is typically housed in a plurality of closely packed together fuel assemblies, which define a nuclear reactor core. The fissile and/or fertile material may be a mixture of oxides of plutonium and uranium in the form of fuel pellets housed in fuel rods spaced apart by spacer or wire wound helically around each fuel rod 
     In addition, in a commercial nuclear power reactor, the fission heat is converted into electricity. In this regard, reactor primary coolant is pumped through the reactor fuel assemblies that define the reactor core and is heated by the fission process. In some reactor designs, the heated primary coolant is carried to a steam generator where the heated primary coolant surrenders its heat to a secondary coolant (i.e., water) disposed in the steam generator. The primary coolant then returns to the reactor core. A portion of the water that receives the heat of the primary coolant vaporizes to steam, which travels to a turbine-generator set to generate electricity. The steam that has passed through the turbine-generator set flows to a condenser that condenses the steam to water, which is then returned to the steam generator. 
     A type of nuclear fission reactor capable of safely generating electricity is a pool-type liquid sodium fast breeder reactor. In this regard, uranium-238 may be used as a fertile material. The uranium-238 absorbs neutrons and transmutes to fissionable plutonium-239 by means of beta decay. When plutonium-239 in turn absorbs a neutron, fission occurs to produce heat. In a fast breeder reactor, moderating materials, such as water, may not be desired as coolant. Rather, in such a pool-type liquid sodium fast breeder nuclear reactor, sodium is the coolant of choice because sodium does not significantly thermalize neutrons. Also, due to the heat transfer characteristics of sodium, the reactor core can operate at higher power densities so that size of the reactor may be reduced. In addition, sodium melts at about 100° C. (about 212° F.) and boils at about 900° C. (about 1650° F.). Thus, sodium can be used at high temperatures without boiling, thereby allowing high temperature and high pressure steam to be generated. This in turn provides increased power plant thermal efficiency. 
     However, the sodium coolant circulating through the reactor core becomes radioactive due to absorption of neutrons. Due to this radioactivity, reactor designers utilize intermediate heat exchange loops between the primary sodium coolant loop(s) and the steam generation loop. This lowers the of risk radioactive contamination of the turbine generator. In addition, steam generator pipe leaks may occur. If a leak were to occur in the piping carrying the sodium through the steam generator, the hot radioactive sodium passing through the steam generator will vigorously chemically react with the water and steam in the steam generator. This would radioactively contaminate the water and steam in the steam generator, thereby increasing risk of radioactive contamination of the surrounding biosphere. For all the reasons hereinabove, reactor designers incorporate use of an intermediate heat exchanger between the reactor core and the steam generator to avoid direct contact of the sodium in the core with the steam generator or turbine generator. 
     Thus, in the pool-type liquid sodium fast breeder nuclear reactor mentioned hereinabove, the intermediate heat exchanger forms a boundary between radioactive primary sodium in the reactor pool and nonradioactive secondary sodium in the steam generator. In other words, the intermediate heat exchanger, which is disposed in the pool of liquid sodium together with the reactor core, is typically used to remove heat from the fast breeder reactor core and transfer that heat to the external steam generator. 
     Attempts have been made to provide adequate removal of heat from a fast fission nuclear reactor core by use of intermediate heat exchangers. U.S. Pat. No. 4,294,658, issued Oct. 13, 1981 in the names of Peter Humphreys et al. and titled “Nuclear Reactors” discloses an intermediate heat exchange module comprising a tube-in-shell intermediate exchanger and an electromagnetic flow coupler disposed in the base region of the module for driving primary coolant through the heat exchanger. This patent addresses severe thermal shock occasioned to an intermediate heat exchanger when there is an interruption in the flow of coolant in the relevant secondary coolant circuit, for example, as caused by a failure of the secondary coolant pump. According to this patent, an object of the invention is to reduce the thermal shock occasioned to the intermediate heat exchanger of a liquid metal cooled nuclear reactor of the pool kind in such an emergency wherein there is an interruption in flow in the secondary coolant circuit. 
     Another attempt to provide adequate removal of heat from a fast fission nuclear reactor core by use of intermediate heat exchangers is disclosed in U.S. Pat. No. 4,324,617, issued Apr. 13, 1982 in the names of Michael G. Sowers et al. and titled “Intermediate Heat Exchanger For A Liquid Metal Cooled Nuclear Reactor And Method.” This patent discloses a heat exchanger that is used in a multi-pool, liquid metal cooled, nuclear reactor. This patent addresses accommodating differential thermal expansion between the structural components of the heat exchanger. According to this patent, the shell of the heat exchanger is heated to a temperature substantially greater than the temperature of the tubes in the heat exchanger by thermal communication with the hot pool and tensioning said tubes during operation by said heating of the shell and thereby accommodating differential thermal expansion in the heat exchanger. 
     Although the art recited hereinabove may disclose devices and methods that adequately serve their intended purposes, none of the art recited hereinabove appears to disclose a heat exchanger, methods therefor and a nuclear fission reactor system, as described and claimed herein. 
     SUMMARY 
     According to an aspect of the disclosure there is provided, for use in association with a pool-type nuclear fission reactor capable of generating heat, a heat exchanger capable of being disposed in a pool fluid residing in the pool-type nuclear fission reactor, the heat exchanger capable of being disposed in proximity to an interior periphery of a pool wall confining the pool fluid, the heat exchanger comprising: a heat exchanger body; and means integrally formed with said heat exchanger body for removal of the heat. 
     According to an additional aspect of this disclosure, there is provided, for use in association with a pool-type nuclear fission reactor capable of generating heat, a heat exchanger capable of being disposed in a pool fluid residing in the pool-type nuclear fission reactor, the heat exchanger capable of being disposed in proximity to an interior periphery of a pool wall confining the pool fluid, the heat exchanger comprising a heat exchanger body having a surface formed thereon defining a portion of a plenum volume. 
     According to a further aspect of this disclosure, there is provided, for use in association with a pool-type nuclear fission reactor capable of generating heat, a heat exchanger capable of being disposed in a pool fluid residing in the pool-type nuclear fission reactor, the heat exchanger capable of being disposed in proximity to an interior periphery of a pool wall confining the pool fluid, the heat exchanger comprising: a heat exchanger body defining a plenum volume therein shaped for a predetermined flow of a heat transfer fluid into the plenum volume, said heat exchanger body having a surface formed thereon defining a portion of the plenum volume; and a heat transfer member coupled to said heat exchanger body, said heat transfer member defining a flow channel therethrough. 
     According to an additional aspect of this disclosure, there is provided, for use in association with a pool-type nuclear fission reactor capable of generating heat, a heat exchanger capable of being disposed in a pool fluid residing in the pool-type nuclear fission reactor, the heat exchanger capable of being disposed in proximity to an interior periphery of a pool wall confining the pool fluid, the heat exchanger comprising: a heat exchanger body having a surface formed thereon defining a portion of a plenum volume shaped for a predetermined flow of a heat transfer fluid into the portion of the plenum volume; and a plurality of adjacent heat transfer members connected to said heat exchanger body and spaced apart by a predetermined distance defining a plurality of flow passages between opposing ones of said plurality of adjacent heat transfer members for distributing flow of the heat transfer fluid through the plurality of flow passages. 
     According to an aspect of this disclosure, there is provided a system for use in association with a pool-type nuclear fission reactor, comprising: a nuclear fission reactor core capable of generating heat; a heat exchanger body associated with said nuclear fission reactor core, said heat exchanger body capable of being disposed in a pool fluid and in proximity to an interior periphery of a pool wall confining the pool fluid; and means in heat transfer communication with said nuclear fission reactor core and associated with said heat exchanger body for removal of the heat. 
     According to another aspect of this disclosure, there is provided a system for use in association with a pool-type nuclear fission reactor, comprising: a vessel defining a pool wall having an interior periphery, the pool wall being configured to confine a pool fluid therein; a nuclear fission reactor core capable of being disposed in said vessel and capable of generating heat; a heat exchanger body capable of being in heat transfer communication with said nuclear fission reactor core, said heat exchanger body capable of being disposed in the pool fluid in proximity to the interior periphery of the pool wall, said heat exchanger body having a surface formed thereon defining a portion of a plenum volume shaped for achieving a predetermined flow of a heat transfer fluid into the plenum volume; and means in heat transfer communication with said nuclear fission reactor core and associated with said heat exchanger body for removal of the heat. 
     According to an additional aspect of this disclosure, there is provided a system for use in association with a pool-type nuclear fission reactor, comprising: a pressure vessel defining a pool wall having an interior periphery, the pool wall being configured to confine a pool fluid therein; a nuclear fission reactor core disposed in said pressure vessel and capable of generating heat; a heat exchanger body capable of being in heat transfer communication with said nuclear fission reactor core, said heat exchanger body capable of being disposed in the pool fluid in proximity to the interior periphery of the pool wall, said heat exchanger body having a surface formed thereon defining a portion of a plenum volume therein shaped for predetermined flow of a heat transfer fluid into the plenum volume; and a plurality of adjacent heat transfer members coupled to said heat exchanger body and spaced apart by a predetermined distance for defining a plurality of flow passages between opposing ones of said plurality of adjacent heat transfer members for distributing flow of a heat transfer fluid through the plurality of flow passages. 
     According to a further aspect of this disclosure, there is provided, for use in association with a pool-type nuclear fission reactor capable of generating heat, a method of assembling a heat exchanger capable of being disposed in a pool fluid residing in the pool-type nuclear fission reactor, the heat exchanger capable of being disposed in proximity to an interior periphery of a pool wall confining the pool fluid, the method comprising: receiving a heat exchanger body; and coupling means to the heat exchanger body for removal of the heat. 
     According to an aspect of this disclosure, there is provided, for use in association with a pool-type nuclear fission reactor, a method of assembling a heat exchanger capable of being disposed in a pool fluid residing in the pool-type nuclear fission reactor, the heat exchanger capable of being disposed in proximity to an interior periphery of a pool wall confining the pool fluid, the method comprising receiving a heat exchanger body having a surface formed thereon defining a portion of a plenum volume. 
     According to an aspect of this disclosure, there is provided, for use in association with a pool-type nuclear fission reactor capable of generating heat, a method of assembling a heat exchanger capable of being disposed in a pool fluid residing in the pool-type nuclear fission reactor, the heat exchanger capable of being disposed in proximity to an interior periphery of a pool wall confining the pool fluid, the method comprising: receiving a heat exchanger body defining a plenum volume therein shaped for a predetermined flow of a heat transfer fluid into the plenum volume, the heat exchanger body having a surface formed thereon defining a portion of the plenum volume; and coupling a heat transfer member to the heat exchanger body, the heat transfer member defining a flow channel therethrough. 
     According to another aspect of this disclosure, there is provided, for use in association with a pool-type nuclear fission reactor capable of generating heat, a method of assembling a heat exchanger capable of being disposed in a pool fluid residing in the pool-type nuclear fission reactor, the heat exchanger capable of being disposed in proximity to an interior periphery of a pool wall confining the pool fluid, the method comprising: receiving a heat exchanger body having a surface formed thereon defining a portion of a plenum volume shaped for a predetermined flow of a heat transfer fluid into the plenum volume; and connecting a plurality of adjacent heat transfer members to the heat exchanger body, the plurality of adjacent heat transfer members being spaced apart by a predetermined distance for defining a plurality of flow passages between opposing ones of the plurality of adjacent heat transfer members for distributing flow of the heat transfer fluid through the plurality of flow passages. 
     A feature of the present disclosure is the provision of a heat exchanger body defining a chamber therein shaped for uniform flow of a heat transfer fluid through the chamber. 
     Another feature of the present disclosure is the provision of a plurality of adjacent heat transfer members connected to the heat exchanger body and spaced apart by a predetermined distance for defining a plurality of flow passages between respective ones of the plurality of adjacent heat transfer members in order to evenly distribute flow of a heat transfer fluid through the plurality of flow passages. 
     In addition to the foregoing, various other method and/or device aspects are set forth and described in the teachings such as text (e.g., claims and/or detailed description) and/or drawings of the present disclosure. 
     The foregoing is a summary and thus may contain simplifications, generalizations, inclusions, and/or omissions of detail; consequently, those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter of the present disclosure, it is believed the disclosure will be better understood from the following detailed description when taken in conjunction with the accompanying drawings. In addition, the use of the same symbols in different drawings will typically indicate similar or identical items. 
         FIG. 1  is a schematic representation of a nuclear fission reactor system; 
         FIG. 2  is a view in horizontal section of an hexagonally-shaped nuclear fission reactor core containing a plurality of nuclear fission reactor modules and breeder fuel modules; 
         FIG. 3  is view in horizontal section of one of the plurality of nuclear fission reactor modules and a plurality of control rods therein; 
         FIG. 4  is an isometric view of a nuclear fuel rod, with parts removed for clarity; 
         FIG. 5  is a view in horizontal section of a parallelepiped-shaped nuclear fission reactor core containing a plurality of the nuclear fission reactor modules and breeder fuel modules; 
         FIG. 6  is a view in vertical section of three exemplary nuclear reactor fission modules with parts removed for clarity; 
         FIG. 7  is an isometric view of a heat exchanger; 
         FIG. 8  is an isometric view of a heat exchanger in section and with parts shown in phantom; 
         FIG. 8A  is an isometric view of a heat exchanger in section and showing a guide structure; 
         FIG. 9  is a view in vertical section of the heat exchanger, this view showing cross-flow of a primary heat transfer fluid and a secondary heat transfer fluid; 
         FIG. 9A  is a view in vertical section of the heat exchanger, this view showing counter-flow of a primary heat transfer fluid and a secondary heat transfer fluid; 
         FIG. 9B  is an exploded isometric illustration of the heat exchanger shown in  FIG. 9A  with parts removed for clarity, this view showing the counter-flow of a primary heat transfer fluid and a secondary heat transfer fluid; 
         FIG. 9C  is a view in vertical section of the heat exchanger, this view showing parallel-flow of a primary heat transfer fluid and a secondary heat transfer fluid; 
         FIG. 9D  is an exploded isometric illustration of the heat exchanger shown in  FIG. 9C  with parts removed for clarity, this view showing the parallel-flow of a primary heat transfer fluid and a secondary heat transfer fluid; 
         FIG. 10  is an isometric view of a heat transfer member having a plurality of fins on an exterior surface thereof; 
         FIG. 11  is an isometric view of a heat transfer member having a plurality of nodules on an exterior surface thereof; 
         FIG. 12  is an isometric view of a heat transfer member having a plurality of fins on an interior surface thereof; 
         FIG. 13  is a view in an isometric view of a heat transfer member defining a flow channel therethrough and a plurality of conduits disposed along the flow channel; 
         FIG. 13A  is an isometric view of a heat transfer member having wedge-shaped fins on an exterior surface thereof; 
         FIG. 13B  is an isometric view of a heat transfer member having nodules of increasing density on an exterior surface thereof; 
         FIG. 14  is a schematic illustration of a plurality of heat exchangers disposed in a pressure vessel; 
         FIG. 15  is a view taken along section line  15 - 15  of  FIG. 14 ; 
         FIG. 16  is a view in horizontal section of a pressure vessel belonging to the nuclear fission reactor system, this view showing a plurality of contiguous heat exchangers disposed in the pressure vessel; and 
         FIGS. 17-47  are flowcharts of illustrative methods, for use in association with a nuclear fission reactor, of assembling a heat exchanger. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented herein. 
     In addition, the present application uses formal outline headings for clarity of presentation. However, it is to be understood that the outline headings are for presentation purposes, and that different types of subject matter may be discussed throughout the application (e.g., device(s)/structure(s) may be described under process(es)/operations heading(s) and/or process(es)/operations may be discussed under structure(s)/process(es) headings; and/or descriptions of single topics may span two or more topic headings). Hence, the use of the formal outline headings is not intended to be in any way limiting. 
     Moreover, the herein described subject matter sometimes illustrates different components contained within, or connected with, different other components. It is to be understood that such depicted architectures are merely exemplary, and that in fact many other architectures may be implemented which achieve the same functionality. In a conceptual sense, any arrangement of components to achieve the same functionality is effectively “associated” such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality can be seen as “associated with” each other such that the desired functionality is achieved, irrespective of architectures or intermedial components. Likewise, any two components so associated can also be viewed as being “operably connected”, or “operably coupled,” to each other to achieve the desired functionality, and any two components capable of being so associated can also be viewed as being “operably couplable,” to each other to achieve the desired functionality. Specific examples of operably couplable include but are not limited to physically mateable and/or physically interacting components, and/or wirelessly interactable, and/or wirelessly interacting components, and/or logically interacting, and/or logically interactable components. 
     In some instances, one or more components may be referred to herein as “configured to,” “configurable to,” “operable/operative to,” “adapted/adaptable,” “able to,” “conformable/conformed to,” etc. Those skilled in the art will recognize that “configured to” can generally encompass active-state components and/or inactive-state components and/or standby-state components, unless context requires otherwise. 
     Therefore, referring to  FIG. 1 , by way of example only and not by way of limitation, there is shown a pool-type fast neutron nuclear fission reactor and system, generally referred to as  10 . As described more fully hereinbelow, nuclear fission reactor system  10  may be a “traveling wave” nuclear fission reactor system. Nuclear fission reactor system  10  generates electricity that is transmitted over a plurality of transmission lines (not shown) to users of the electricity. Nuclear fission reactor system  10  alternatively may be used to conduct tests, such as tests to determine effects of temperature on reactor materials. 
     Referring to  FIGS. 1 ,  2  and  3 , nuclear fission reactor system  10  comprises a nuclear fission reactor core, generally referred to as  20 , that includes a plurality of nuclear fission fuel assemblies or, as also referred to herein, nuclear fission modules  30 . Nuclear fission reactor core  20  is sealingly housed within a reactor core enclosure  40 . By way of example only and not by way of limitation, each nuclear fission module  30  may form a hexagonally-shaped structure in transverse cross-section, as shown, so that more nuclear fission modules  30  may be closely packed together within reactor core  20 , as compared to other shapes for nuclear fission module  30 , such as cylindrical or spherical shapes. Each nuclear fission module  30  comprises a plurality of fuel rods  50  for generating heat due to the aforementioned nuclear fission chain reaction process. The plurality of fuel rods  50  may be surrounded by a fuel rod canister  60 , if desired, for adding structural rigidity to nuclear fission modules  30  and for segregating nuclear fission modules  30  one from another when nuclear fission modules  30  are disposed in nuclear fission reactor core  20 . Segregating nuclear fission modules  30  one from another avoids transverse coolant cross flow between fuel rods  50 . Avoiding transverse coolant cross flow prevents transverse vibration of fuel rods  50 . Such transverse vibration might otherwise increase risk of damage to fuel rods  50 . In addition, segregating nuclear fission modules  30  one from another can allow control of coolant flow on an individual module-by-module basis. Controlling coolant flow to individual nuclear fission modules  30  efficiently manages coolant flow within reactor core  20 , such as by directing coolant flow substantially according to the nonuniform temperature distribution in reactor core  20 . In other words, more coolant may be directed to those nuclear fission modules  30  having higher temperature in order to provide a substantially uniform temperature distribution across reactor core  20 . The coolant may have an average nominal volumetric flow rate of approximately 5.5 m 3 /sec (i.e., approximately 194 cubic ft 3 /sec) and an average nominal velocity of approximately 2.3 m/sec (i.e., approximately 7.55 ft/sec) in the case of an exemplary sodium cooled reactor during normal operation. Fuel rods  50  are adjacent one to another and define a fuel rod coolant flow channel  80  (see  FIG. 6 ) therebetween for allowing flow of coolant along the exterior of fuel rods  50 . Canister  60  may include means (not shown) for supporting and for tying fuel rods  50  together. Thus, fuel rods  50  are bundled together within canister  60  so as to form the previously mentioned hexagonal nuclear fission module  30 . Although fuel rods  50  are adjacent to each other, fuel rods  50  are nonetheless maintained in a spaced-apart relationship by a wire wrapper  90  (see  FIG. 6 ) that surrounds and extends spirally along the length of each fuel rod  50  in a serpentine fashion, as well known by persons of ordinary skill in the art of nuclear power reactor design. 
     Referring to  FIG. 3 , a plurality of spaced-apart, longitudinally extending and longitudinally movable control rods  95  (only some of which are shown) are each disposed within a control rod guide tube or cladding (not shown). Control rods  95  are symmetrically disposed within selected nuclear fission modules  30  and extend the length of a predetermined number of nuclear fission modules  30 . Control rods  95 , which are shown disposed in a predetermined number of the hexagonally-shaped nuclear fission modules  30 , control the neutron fission reaction occurring in nuclear fission modules  30 . In other words, control rods  95  comprise a suitable neutron absorber material having an acceptably high neutron absorption cross-section. In this regard, the absorber material may be a metal or metalloid selected from the group consisting essentially of lithium, silver, indium, cadmium, boron, cobalt, hafnium, dysprosium, gadolinium, samarium, erbium, europium and mixtures thereof. Alternatively, the absorber material may be a compound or alloy selected from the group consisting essentially of silver-indium-cadmium, boron carbide, zirconium diboride, titanium diboride, hafnium diboride, gadolinium titanate, dysprosium titanate and mixtures thereof. Control rods  95  will controllably supply negative reactivity to reactor core  20 . Thus, control rods  95  provide a reactivity management capability to reactor core  20 . In other words, control rods  95  are capable of controlling the neutron flux profile across nuclear fission reactor core  20  and thus influence the temperature within nuclear fission reactor core  20 . 
     With particular reference to  FIGS. 2 ,  3  and  4 , each fuel rod  50  has a plurality of nuclear fuel pellets  100  stacked end-to-end therein, which nuclear fuel pellets  100  are sealingly surrounded by a fuel rod cladding material  110 . Nuclear fuel pellets  100  comprise the afore-mentioned fissile nuclide, such as uranium-235, uranium-233 or plutonium-239. Alternatively, nuclear fuel pellets  100  may comprise a fertile nuclide, such as thorium-232 and/or uranium-238 which may be transmuted via neutron capture during the fission process into the fissile nuclides mentioned immediately hereinabove. Such fertile nuclide material may be housed in breeder rods disposed in specially designated breeder fuel modules  115 . Such breeder fuel modules  115  may be arranged as a “breeding blanket” around the interior periphery of nuclear fission reactor core  20  for breeding nuclear fuel, as well known in the art of fast neutron breeder reactor design. A further alternative is that nuclear fuel pellets  100  may comprise a predetermined mixture of fissile and fertile nuclides. 
     Referring to  FIG. 4 , by way of example only and not by way of limitation, nuclear fuel pellets  100  may be made from an oxide selected from the group consisting essentially of uranium monoxide (UO), uranium dioxide (UO 2 ), thorium dioxide (ThO x ) (also referred to as thorium oxide), uranium trioxide (UO 3 ), uranium oxide-plutonium oxide (UO-PuO), triuranium octoxide (U 3 O 8 ) and mixtures thereof. Alternatively, nuclear fuel pellets  100  may substantially comprise uranium either alloyed or unalloyed with other metals, such as, but not limited to, zirconium or thorium metal. As yet another alternative, nuclear fuel pellets  100  may substantially comprise a carbide of uranium (UC x ) or a carbide of thorium (ThC x ). For example, nuclear fuel pellets  100  may be made from a carbide selected from the group consisting essentially of uranium monocarbide (UC), uranium dicarbide (UC 2 ), uranium sesquicarbide (U 2 C 3 ), thorium dicarbide (ThC 2 ), thorium carbide (ThC) and mixtures thereof. As another non-limiting example, nuclear fuel pellets  100  may be made from a nitride selected from the group consisting essentially of uranium nitride (U 3 N 2 ), uranium nitride-zirconium nitride (U 3 N 2 Zr 3 N 4 ), uranium-plutonium nitride ((U—Pu)N), thorium nitride (ThN) and mixtures thereof. Fuel rod cladding material  110 , which sealingly surrounds the stack of nuclear fuel pellets  100 , may be a suitable zirconium alloy, such as ZIRCOLOY™ (trademark of the Westinghouse Electric Corporation), which has known resistance to corrosion and cracking. Cladding material  110  may be made from other materials, as well, such as ferritic martensitic steels. 
     Returning to  FIG. 1 , nuclear fission reactor core  20  is disposed within a vault or reactor pressure vessel  120  for preventing leakage of radioactive materials, gasses or liquids from reactor core  20  to the surrounding biosphere. For reasons provided hereinbelow, pressure vessel  120 , which has an interior wall surface  122 , is substantially filled with a pool of fluid or coolant  125 , such as liquid sodium, to the extent nuclear fission reactor core  20  is submerged in the pool of coolant. Pressure vessel  120  may be steel, concrete or other material of suitable size and thickness to reduce risk of such radiation leakage and to support required pressure loads. In addition, there may be a containment vessel (not shown) sealingly surrounding parts of nuclear fission reactor system  10  for added assurance that leakage of radioactive particles, gasses or liquids from reactor core  20  to the surrounding biosphere is prevented. 
     Referring again to  FIG. 1 , a primary loop coolant pipe  130  is coupled to nuclear fission reactor core  20  for allowing a suitable coolant to flow through reactor core  20  along directional arrow  135  in order to cool nuclear fission reactor core  20 . Primary loop coolant pipe  130  may be made from any suitable material, such as stainless steel. It may be appreciated that, if desired, primary loop coolant pipe  130  may be made not only from ferrous alloys, but also from non-ferrous alloys, zirconium-based alloys or other suitable structural materials or composites. The coolant carried by primary loop coolant pipe  130  may be a liquid metal selected from the group consisting essentially of sodium, potassium, lithium, lead and mixtures thereof. On the other hand, the coolant may be a metal alloy, such as lead-bismuth (Pb—Bi). Alternatively, in the exemplary embodiment contemplated herein, the coolant is a liquid sodium (Na) metal or sodium metal mixture, such as sodium-potassium (Na—K). Depending on the particular reactor core design and operating history, normal operating temperature of a sodium-cooled reactor core may be relatively high. For example, in the case of a 500 to 1,500 MWe sodium-cooled reactor with mixed uranium-plutonium oxide fuel, the reactor core outlet temperature during normal operation may range from approximately 510° Celsius (i.e., 950° Fahrenheit) to approximately 550° Celsius (i.e., 1,020° Fahrenheit). On the other hand, during a LOCA (Loss Of Coolant Accident) or LOFTA (Loss of Flow Transient Accident) peak fuel cladding temperatures may reach about 600° Celsius (i.e. 1,110° Fahrenheit) or more, depending on reactor core design and operating history. Moreover, decay heat build-up during post-LOCA or post-LOFTA scenarios and also during suspension of reactor operations may produce unacceptable heat accumulation. In some cases, therefore, it is appropriate to remove heat produced by nuclear fission reactor core  20  during both normal operation and post accident scenarios. 
     Still referring to  FIG. 1 , the heat-bearing coolant generated by nuclear fission reactor core  20  flows along a coolant flow stream or flow path  140  to an intermediate heat exchanger  150  that is also submerged in coolant pool  125 . Intermediate heat exchanger  150  may be made from any convenient material resistant to the heat and corrosive effects of the sodium coolant in coolant pool  125 , such as a suitable stainless steel. The coolant flowing along coolant flow path  140  flows through intermediate heat exchanger  150 , as described more fully hereinbelow, and continues through primary loop coolant pipe  130 . It may be appreciated that the coolant leaving intermediate heat exchanger  150  has been cooled due to the heat transfer occurring in intermediate heat exchanger  150 , as disclosed more fully hereinbelow. A first pump  170 , which may be an electro-mechanical pump, is coupled to primary loop pipe  130 , and is in fluid communication with the reactor coolant carried by primary loop coolant pipe  130 , for pumping the reactor coolant through primary loop pipe  130 , through reactor core  20 , along coolant flow path  140  and into intermediate heat exchanger  150 . 
     Referring again to  FIG. 1 , a secondary loop pipe  180  is provided for removing heat from intermediate heat exchanger  150 . Secondary loop pipe  180  comprises a secondary “hot” leg pipe segment  190  and a secondary “cold” leg pipe segment  200 . Secondary hot leg pipe segment  190  and secondary cold leg pipe segment  200  are integrally connected to intermediate heat exchanger  150 . Secondary loop pipe  180 , which includes hot leg pipe segment  190  and cold leg pipe segment  200 , contains a fluid, such as a liquid metal selected from the group consisting essentially of sodium, potassium, lithium, lead and mixtures thereof. On the other hand, the fluid may be a metal alloy, such as lead-bismuth (Pb—Bi). Alternatively, in the exemplary embodiment contemplated herein, the fluid may suitably be a liquid sodium (Na) metal or sodium metal mixture, such as sodium-potassium (Na—K). Secondary hot leg pipe segment  190  extends from intermediate heat exchanger  150  to a steam generator and superheater combination  210  (hereinafter referred to as “steam generator  210 ”), for reasons described momentarily. In this regard, after passing through steam generator  210 , the coolant flowing through secondary loop pipe  180  and exiting steam generator  210  is at a lower temperature and enthalpy than before entering steam generator  210  due to the heat transfer occurring within steam generator  210 . After passing through steam generator  210 , the coolant is pumped, such as by means of a second pump  220 , which may be an electro-mechanical pump, along “cold” leg pipe segment  200 , which extends into intermediate heat exchanger  150  for providing the previously mentioned heat transfer. The manner in which steam generator  210  generates steam is generally described immediately hereinbelow. 
     Referring yet again to  FIG. 1 , disposed in steam generator  210  is a body of water  230  having a predetermined temperature and pressure. The fluid flowing through secondary hot leg pipe segment  190  will transfer its heat by means of conduction to body of water  230 , which is at a lower temperature than the fluid flowing through secondary hot leg pipe segment  190 . As the fluid flowing through secondary hot leg pipe segment  190  transfers its heat to body of water  230 , a portion of body of water  230  will vaporize to steam  240  according to the predetermined temperature and pressure within steam generator  210 . Steam  240  will then travel through a steam line  250  which has one end thereof in vapor communication with steam  240  and another end thereof in liquid communication with body of water  230 . A rotatable turbine  260  is coupled to steam line  250 , such that turbine  260  rotates as steam  240  passes therethrough. An electrical generator  270 , which is coupled to turbine  260 , such as by a rotatable turbine shaft  280 , generates electricity as turbine  260  rotates. In addition, a condenser  290  is coupled to steam line  250  and receives the steam passing through turbine  260 . Condenser  290  condenses the steam to liquid water and passes any waste heat to a heat sink, such as a cooling tower  300 , which is associated with condenser  290 . The liquid water condensed by condenser  290  is pumped along steam line  250  from condenser  290  to steam generator  210  by means of a third pump  310 , which may be an electro-mechanical pump, interposed between condenser  290  and steam generator  210 . 
     As best seen in  FIG. 5 , nuclear fission modules  30  may be arranged to define a parallelepiped-shaped nuclear fission reactor core configuration, generally referred to as  222  rather than the previously mentioned hexagonally-shaped configuration. In this regard, reactor core enclosure  40  of nuclear fission reactor core  222  defines a first end  330  and a second end  340 , for reasons provided hereinbelow. 
     Referring again to  FIG. 5 , regardless of the configuration selected for the nuclear fission reactor core, the nuclear fission reactor core  20  or  222  may be configured as a traveling wave nuclear fission reactor core. In this regard, a comparatively small and removable nuclear fission igniter  350 , which may include isotopic enrichment of nuclear fissionable material, such as, without limitation, U-233, U-235 or Pu-239, is suitably located in reactor core  222 . By way of example only and not by way of limitation, igniter  350  may be located near first end  330  that is opposite second end  340  of reactor core  340 . Neutrons are released by igniter  350 . The neutrons that are released by igniter  350  are captured by fissile and/or fertile material within nuclear fission modules  30  to initiate the fission chain reaction. Igniter  350  may be removed once the fission chain reaction becomes self-sustaining, if desired. 
     Still referring to  FIG. 5 , igniter  350  initiates a three-dimensional, traveling deflagration wave or “burn wave”  360 . When igniter  350  releases its neutrons to cause “ignition”, burn wave  360  travels outwardly from igniter  350  that is near first end  330  and toward second end  340  of reactor core  222 , so as to form the traveling or propagating burn wave  360 . In other words, each nuclear fission module  30  is capable of accepting at least a portion of traveling burn wave  360  as burn wave  360  propagates through reactor core  222 . Speed of the traveling burn wave  360  may be constant or non-constant. Thus, the speed at which burn wave  360  propagates can be controlled. For example, longitudinal movement of the previously mentioned control rods  95  (see  FIG. 3 ) in a predetermined or programmed manner can drive down or lower neutronic reactivity of fuel rods  50  that are disposed in nuclear fission modules  30 . In this manner, neutronic reactivity of fuel rods  50  that are presently being burned at the location of burn wave  360  is driven down or lowered relative to neutronic reactivity of “unburned” fuel rods  50  ahead of burn wave  360 . This result gives the burn wave propagation direction indicated by directional arrow  365 . Controlling reactivity in this manner maximizes the propagation rate of burn wave  360  subject to operating constraints for reactor core  220 . For example, maximizing the propagation rate of burn wave  360  provides means to control burn-up above a minimum value needed for propagation and a maximum value set, in part, by neutron fluence limitations of reactor core structural materials. 
     The basic principles of such a traveling wave nuclear fission reactor are disclosed in more detail in co-pending U.S. patent application Ser. No. 11/605,943 filed Nov. 28, 2006 in the names of Roderick A. Hyde, et al. and titled “Automated Nuclear Power Reactor For Long-Term Operation”, which application is assigned to the assignee of the present application, the entire disclosure of which is hereby incorporated by reference. 
     Referring to  FIG. 6 , there are shown upright, adjacent hexagonally-shaped nuclear fission modules  30 . Only three adjacent nuclear fission modules  30  are shown, it being understood that a greater number of nuclear fission modules  30  are present in reactor core  20 . Each nuclear fission module  30  is mounted on a horizontally extending reactor core lower support plate  370 . Reactor core lower support plate  370  suitably extends across a bottom end portion of all nuclear fission modules  30 . Reactor core lower support plate  370  has a counter bore  380  therethrough for reasons provided hereinbelow. Counter bore  380  has an open end  390  for allowing flow of coolant thereinto. Horizontally extending across a top end portion or exit portion of all nuclear fission modules  30  and removably connected to nuclear fission modules  30  is a reactor core upper support plate  400  that caps all nuclear fission modules  30 . Reactor core upper support plate  400  also defines a plurality of flow slots  410  for allowing flow of coolant therethrough. Primary loop pipe  130  and first pump  170  (see  FIG. 1 ) deliver reactor coolant to nuclear fission modules  30  along a coolant flow path or fluid stream indicated by directional arrows  140 . The primary coolant then continues along coolant flow path  140  and through open end  390  that is formed in lower support plate  370 . 
     As previously mentioned, it is important to remove the heat produced by nuclear fission reactor core  20  and the nuclear fission modules  30  therein, regardless of the configuration selected for nuclear fission reactor core  20 . Proper heat removal is important for several reasons. For example, heat damage may occur to reactor core structural materials if the peak temperature exceeds material limits. Such peak temperatures may undesirably reduce the operational life of structures subjected to peak temperatures by altering the mechanical properties of the structures, particularly those properties relating to thermal creep. Also, reactor power density is limited by the ability of core structural materials to withstand high peak temperatures without damage. In addition, nuclear fission reactor system  10  alternatively may be used to conduct tests, such as tests to determine effects of temperature on reactor materials. Controlling reactor core temperature by properly removing the heat from the reactor core is important for successfully conducting such tests. 
     Moreover, it may be desirable to achieve uniform flow rate of the heat transfer fluid through intermediate heat exchanger  150 . Such uniform flow rate may otherwise avoid uneven coolant flow to the nuclear reactor core and resulting core reactivity perturbations. Further, it may be desirable to provide uniform distribution of coolant flow through the heat exchanger in order to avoid preferential flow of the coolant through the heat exchanger. Avoidance of preferential flow of the coolant can mitigate development of localized temperature “hot spots” in the heat exchanger. Such localized temperature “hot spots” might otherwise decrease the operational life of the heat exchanger. Uniform flow also acts to enhance heat exchange evenly across the heat transfer surfaces of the heat exchanger, enhancing heat exchange for a given heat exchange area. The structure and operation of intermediate heat exchanger  150  addresses these concerns. 
     The structure of intermediate heat exchanger  150  will now be described. Referring to  FIGS. 1 ,  7 ,  8 ,  8 A and  9 , intermediate heat exchanger  150  comprises a heat exchanger body  420  affixed to interior wall surface  122  of pressure vessel  120 , so that intermediate heat exchanger  150  is supported within pressure vessel  120 . As an alternative, interior wall surface  122 , with confines pool  125 , may form a rear wall of intermediate heat exchanger  150 . Heat exchanger body  420  comprises an upright generally L-shaped (in transverse cross section) rear portion  425  that defines a primary fluid exit plenum volume or exit plenum chamber  430  therein. Thus, primary fluid exit plenum chamber  430  is a part of heat exchanger body  420 . Primary fluid exit plenum chamber  430  is shaped to provide uniform flow of a first heat transfer fluid (i.e., the primary heat transfer fluid) through primary fluid exit plenum chamber  430 , as described in more detail hereinbelow. Formed through rear portion  425  of heat exchanger body  420 , but within primary fluid exit plenum chamber  430 , is a primary fluid exit port  435  that opens into primary loop coolant pipe  130 . Connected to rear portion  425  is a bottom portion  440  of heat exchanger body  420  defining a bottom plenum  450  for hot secondary sodium. Bottom plenum  450 , which has a bottom plenum exit side or port  455 , forms a box-like structure having a top surface  460  thereon to which a plurality of upright plate-type heat transfer members  470  are integrally attached, such as by welding. Each heat transfer member  470  defines a flow channel  480  therethrough that has an inlet  490  and an outlet  500  at respective ends of flow channel  460 . Inlet  490  is in fluid communication with heat transfer fluid flowing through cold leg pipe segment  200 . Outlet  500  is in fluid communication with heat transfer fluid in bottom plenum  450 . Moreover, it may be appreciated that the primary fluid is supplied to heat exchanger body  420  without use of a conduit or manifold. In other words, the primary fluid is supplied to heat exchanger body  420  conduit-free or manifold-free. It may be appreciated that pool  125  is also manifold-free. In addition, it may be appreciated that the inlet side of intermediate exchanger  150  may be manifold-free and the outlet side of intermediate exchanger  150  may be manifold-free, as well. This may decrease capital cost of constructing reactor  10  and/or fabrication cost of heat exchanger  150  because such a conduit or manifold is not required. 
     Referring to  FIGS. 8 ,  8 A and  9 , intermediate heat exchanger  150  comprises a plurality of adjacent heat transfer members  470 . The plurality of adjacent heat transfer members  470  are spaced-apart by a relatively small predetermined distance “d” for defining a plurality of flow passages  510  between the adjacent heat transfer members  470 . The distance “d” is that distance necessary for achieving even flow distribution among flow passages  510 . In other words, heat transfer members  470  are spaced-apart by distance “d” in order to evenly distribute flow of the primary heat transfer fluid through a plurality of flow passages  510 . The distance “d” between adjacent heat transfer members  470  may be designed to have different values for different reactor core configurations, as required, in order to achieve the even distribution of flow of the primary heat transfer fluid through the plurality of flow passages. This is so because a particular reactor core configuration may have in-core structure that alters or interferes with the free flow of the primary heat transfer fluid as the heat transfer fluid travels toward heat exchanger  150 . The distance “d” may be designed to have different values in order to compensate for this effect. In another embodiment, heat exchanger body  420  may comprise a guide structure  515  for guiding flow of the heat transfer fluid into heat exchanger  150 . Guide structure  515  suitably spans heat transfer members  470  and is associated with flow passages  510  such that the heat transfer fluid is guided into flow passages  510 . Heat exchanger body  420  further comprises a top portion  520  sealingly mounted on or connected to an upper portion of rear portion  425  and an upper portion of the plurality of heat transfer members  470 . Top portion  520  defines a top plenum  530  therein for receiving cooled secondary sodium flowing along flow path  532  from steam generator  210 . The cooled secondary sodium flowing along flow path  532  and the primary heat transfer fluid flowing along flow path  140  define a cross-flow configuration. In this cross-flow configuration, flow path  532  is substantially perpendicular (i.e., plus or minus) 45° to flow path  140  in intermediate heat exchanger  150 . Top plenum  530  is in communication with inlet  490  for allowing the cooled secondary sodium to flow through inlet  490 , into flow channel  470 , through outlet  500  and into bottom plenum  450 . 
     Referring to  FIGS. 9A and 9B , an alternative embodiment intermediate heat exchanger  150  comprises cold leg pipe segment  200  through which the cooled secondary heat transfer fluid flows along flow path  532 . In this regard, cooled secondary heat transfer fluid enters a plate member  534  through an opening  536   a  and exits an opening  536   b  that are formed in plate member  534 . The secondary heat transfer fluid continues along flow path  532  and enters return pipe segment  538  for returning the secondary heat transfer fluid to steam generator  210 . The cooled secondary sodium flowing along flow path  532  and the primary heat transfer fluid flowing along flow path  140  define a counter-flow configuration. In this counter-flow configuration, flow path  532  is parallel, but opposite, to flow path  140  in intermediate heat exchanger  150 . 
     Referring to  FIGS. 9C and 9D , an alternative embodiment intermediate heat exchanger  150  comprises cold leg pipe segment  200  through which the cooled secondary heat transfer fluid flows along flow path  532 . In this regard, cooled secondary heat transfer fluid enters plate member  534  through an opening  536   a  and exits an opening  536   b  that are formed in plate member  534 . The secondary heat transfer fluid continues along flow path  532  and enters a return pipe segment  538  for returning the secondary heat transfer fluid to steam generator  210 . The cooled secondary heat transfer fluid flowing along flow path  532  and the primary heat transfer fluid flowing along flow path  140  define a parallel-flow configuration. In this parallel-flow configuration, flow path  532  is parallel and in the same direction to flow path  140  in intermediate heat exchanger  150 . 
     Referring to  FIGS. 10 ,  11 ,  12 , and  13 , there are shown alternative embodiments for heat transfer member  470 . In this regard, at least one of plurality of heat transfer members  470  comprises a wall  540  defining an enhanced heat transfer surface  550  that accommodates flow of the primary heat transfer fluid along enhanced heat transfer surface  550 . In this regard, wall  540  separates hot primary sodium (i.e., a first heat transfer fluid) from cool secondary sodium (i.e., a second heat transfer fluid). At least one of plurality of heat transfer members  470  comprises at least one integrally connected external fin or external flange  560  outwardly extending from wall  540  for forming enhanced heat transfer surface  550 . External flange  560  enhances heat transfer by increasing the surface area for increased heat transfer. Alternatively, at least one of plurality of heat transfer members  470  comprises at least one nodule  570  outwardly projecting from wall  540  for forming enhanced heat transfer surface  550 . Nodule  570  enhances heat transfer by increasing the surface area for increased heat transfer. As another alternative, at least one of plurality of heat transfer members  470  comprises at least one integrally connected internal fin or internal flange  580  inwardly extending from wall  540  for purposes of enhanced heat transfer. Internal flange  580  enhances heat transfer by increasing the surface area for increased heat transfer. As yet another alternative, at least one of plurality of heat transfer members  470  comprises at least one conduit  590  extending along flow channel  490  for accommodating flow of cooled heat transfer fluid through conduit  590 . 
       FIGS. 13A and 13B  present further embodiments that include enhanced heat transfer surface  550 . In this regard, external flange  560  may have increasing heat transfer surface area as flange  560  extends from a forward portion  592  of wall  540  to a rearward portion  594  of wall  540 . As may be appreciated by a person of ordinary skill in the art of thermodynamics, a greater portion of heat transfer will occur nearer forward portion  592  of wall  540  than nearer rearward portion  594  of wall  540  because the primary heat transfer fluid flows from forward portion  592  of wall  540  to rearward portion  594  of wall  540 . Thus, more heat transfer will occur nearer forward portion  592  of wall  540  and a reduced amount of heat transfer will occur nearer rearward portion  594  of wall  540 . In order to compensate for the reduced heat transfer near rearward portion  594  of wall  540 , the heat transfer surface area of flange  560  increases as flange  560  extends from forward portion  592  of flange  560  to rearward portion  594  of flange  560 . For example, flange  560  may be wedge-shaped with a smaller end portion thereof near forward portion  592  and a wider end portion thereof near rearward portion  594 . As another alternative, density of nodules  570  (i.e., number of nodules  570  per unit area) that outwardly project from wall  540  may increase from forward portion  592  to rearward portion  594  for increasing heat transfer surface area from forward portion  592  of wall  540  to rearward portion  594  of wall  540 . This configuration of nodules  570  compensates for the reduced heat transfer occurring near rearward portion  594  of wall  540 . 
     Turning now to  FIGS. 14 and 15 , there is shown an alternative embodiment of nuclear fission reactor system  10 , wherein there are a plurality of heat exchangers, such as a first heat exchanger  600  and a second heat exchanger  610 . Each of first heat exchanger  600  and second heat exchanger  610  is coupled to steam generator  210  by a first cold leg pipe segment  620   a  and a second cold leg pipe segment  620   b , respectively, that supply cooled heat transfer fluid to heat exchangers  600 / 610 . In addition, each of first heat exchanger  600  and second heat exchanger  610  is coupled to steam generator  210  by a first hot leg pipe segment  630   a  and a second hot leg pipe segment  630   b , respectively, that allow extraction of heated heat transfer fluid from heat exchangers  600 / 610 . Moreover, if desired, there may be a first shut-off valve  640   a  installed in first cold leg pipe segment  620   a  and a second shut-off valve  640   b  installed in second cold leg pipe segment  620   b  for reasons described presently. In addition, there may be a third shut-off valve  650   a  installed in first hot leg pipe segment  630   a  and a fourth shut-off valve  650   b  installed in hot leg pipe segment  630   b  for reasons described presently. In this regard, if desired, shut-off valves  640   a / 650   a  can be closed to cease coolant flow to and from first heat exchanger  600  and thereby isolate first heat exchanger  600 . Also, if desired, shut-off valves  640   b / 650   b  can be closed to cease coolant flow to and from second heat exchanger  610  and thereby isolate second heat exchanger  610 . It may be desirable to isolate either first heat exchanger  600  or second heat exchanger  610  if a leak occurs in wall  540  of any of heat transfer members  470 . In addition, a plurality of pumps, such as pumps  660   a  and  660   b , are coupled to respective ones of plurality of heat exchangers  600  and  610  for pumping cooled heat transfer fluid from heat exchangers  600  and  610  to nuclear fission reactor core  20 . 
     Referring to  FIG. 16 , there is show an embodiment, wherein a plurality of heat exchangers  670   a ,  670   b ,  670   c ,  670   d ,  670   e ,  670   f  and  670   g  are arranged side-by-side or contiguously around interior wall surface  122  of pressure vessel  120 . This embodiment provides another configuration for using intermediate heat exchanger  150 . 
     Referring to  FIGS. 1 ,  6 ,  7 ,  8 ,  8 A,  9 ,  10 ,  11 ,  12 , and  13 , operation of intermediate heat exchanger  150  will now be further described. In this regard, heat generated by fuel rods  50  in nuclear fission reactor core  20 , due to the fission process, is taken-up by the primary heat transfer fluid, also referred to herein as the first heat transfer fluid. As the heat is generated, first pump  170  is operated to suction or draw the first heat transfer fluid from heat exchanger  150  and then pump the first heat transfer fluid past fuel rods  50 , through flow slots  410  in upper core support plate  400  and into coolant pool  125 . Continued operation of first pump  170  will then draw the first heat transfer fluid through flow passages  510  and into primary fluid exit plenum chamber  430 . As the first heat transfer fluid flows through flow passages  510 , the first heat transfer fluid will come into intimate contact with enhanced heat transfer surface  550 . As the first heat transfer fluid flows in intimate contact with enhanced heat transfer surface  550 , cooler secondary heat transfer fluid flows from steam generator  210 , along cold pipe segment  200 , into top plenum  530 , through inlet  490 , through flow channel  480 , through outlet  500  and into bottom plenum  450 . Thereafter, the second heat transfer fluid exits bottom plenum  450  through exit port  455  to be received by hot leg pipe segment  190  that passes through steam generator  210 . The second heat transfer fluid that travels along the portion of hot leg pipe segment  190  and that passes through steam generator  210  transfers its heat to body of water  230  for generating steam  240 . Second pump  220  is operated to bring the cooler secondary fluid from steam generator  210  to top plenum  520 . 
     Still referring to  FIGS. 1 ,  6 ,  7 ,  8 ,  8 A,  9 ,  10 ,  11 ,  12 , and  13 , heat transfers from the first heat transfer fluid of higher temperature flowing through flow passages  510  to the second heat transfer fluid of lower temperature flowing through flow channels  480 . This heat transfer occurs by conduction through wall  540  of heat transfer member  470 . 
     Still referring to  FIGS. 1 ,  6 ,  7 ,  8 ,  8 A,  9 ,  10 ,  11 ,  12 , and  13 , the plurality of adjacent heat transfer members  470  are spaced-apart by the previously mentioned predetermined distance “d” in order to evenly distribute flow of the primary heat transfer fluid through plurality of flow passages  510 . As previously mentioned, primary fluid exit plenum chamber  430  is shaped to provide uniform flow of a first heat transfer fluid (i.e., the primary heat transfer fluid) through primary fluid exit plenum chamber  430 . In this regard, an upper portion of primary fluid exit plenum chamber  430  is disposed closer to interior wall surface  122 , so that primary fluid exit plenum chamber  430  has a smaller volume than a lower portion of primary fluid exit plenum chamber  430 . In other words, volume of primary fluid exit plenum chamber  430  becomes greater nearer exit port  435  than inlet  490 . This shape for primary fluid exit plenum  430  provides uniform flow of the first heat transfer fluid (i.e., the primary heat transfer fluid) through primary fluid exit plenum chamber  430 . 
     Illustrative Methods 
     Illustrative methods associated with exemplary embodiments of the nuclear fission reactor system and the heat exchanger will now be described. 
     Referring to  FIGS. 17-47 , illustrative methods, for use in association with a nuclear fission reactor capable of generating heat, are provided for assembling a heat exchanger. 
     Turning now to  FIG. 17 , for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  680  of assembling a heat exchanger starts at a block  690 . At a block  700 , the method comprises receiving a heat exchanger body. At a block  710 , means is coupled to the heat exchanger body for removal of the heat. The method stops at block  720 . 
     Referring to  FIG. 18 , for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  730  of assembling a heat exchanger starts at block  740 . At block  750 , the method comprises receiving a heat exchanger body. At block  760 , the method comprises coupling means to the heat exchanger body for removal of the heat. At a block  770 , the method comprises coupling a heat removal means configured to achieve a predetermined flow of a heat transfer fluid into the heat exchanger body. The method stops at block  780 . 
     Referring to  FIG. 19 , for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  790  of assembling a heat exchanger starts at a block  800 . At a block  810 , the method comprises receiving a heat exchanger body. At a block  820 , means is coupled to the heat exchanger body for removal of the heat. At a block  830 , a heat removal means is coupled that is configured to achieve a predetermined flow of a heat transfer fluid into the heat exchanger body. At a block  840 , a heat removal means is coupled that is configured to achieve a substantially uniform flow of a heat transfer fluid into the heat exchanger body. The method stops at a block  850 . 
     Referring to  FIG. 20 , for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  860  of assembling a heat exchanger starts at a block  870 . At a block  880 , the method comprises receiving a heat exchanger body. At a block  890 , means is coupled to the heat exchanger body for removal of the heat. At a block  900 , a heat removal means having an enhanced heat transfer surface is coupled. The method stops at a block  910 . 
     Referring to  FIG. 21 , for use in association with a pool-type nuclear fission reactor, an illustrative method  920  of assembling a heat exchanger starts at a block  930 . At a block  940 , means is coupled to the heat exchanger body for removal of the heat. At a block  950 , a heat exchanger body defining a plenum volume therein of predetermined shape is received for achieving a substantially uniform flow of the heat transfer fluid through the heat exchanger body. The method stops at a block  970 . 
     Referring to  21 A, for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  971 , of assembling a heat exchanger starts at a block  973 . At a block  975 , the method comprises receiving a heat exchanger body. At a block  977 , means are coupled to the heat exchanger body for removal of the heat. At a block  978 , a manifold-free heat exchanger body is received. The method stops at a block  979 . 
     Referring to  FIG. 22 , for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  980 , of assembling a heat exchanger starts at a block  990 . At a block  1000 , the method comprises receiving a heat exchanger body having a surface formed thereon defining a portion of a plenum volume. The method stops at a block  1010 . 
     Referring to  FIG. 22A , for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  1011   a , of assembling a heat exchanger starts at a block  1013   a . At a block  1015   a , the method comprises receiving a heat exchanger body having a surface formed thereon defining a portion of a plenum volume. At a block  1017   a , a guide structure for guiding flow of the pool fluid is received. The method stops at a block  1019   a.    
     Referring to  FIG. 22B , for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  1011   b , of assembling a heat exchanger starts at a block  1013   b . At a block  1015   b , the method comprises receiving a heat exchanger body having a surface formed thereon defining a portion of a plenum volume. At a block  1017   b , a guide structure for guiding flow of the pool fluid is received. At a block  1018   b , a guide structure configured for achieving substantially uniform flow of the pool fluid within at least a portion of the heat exchanger body is received. The method stops at a block  1019   b.    
     Referring to  FIG. 22C , for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  1011   c , of assembling a heat exchanger starts at a block  1013   c . At a block  1015   c , the method comprises receiving a heat exchanger body having a surface formed thereon defining a portion of a plenum volume. At a block  1017   c , a heat exchanger body having an inlet guide structure for guiding inlet flow of the pool fluid is received. The method stops at a block  1019   c.    
     Referring to  FIG. 22D , for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  1011   d , of assembling a heat exchanger starts at a block  1013   d . At a block  1015   d , the method comprises receiving a heat exchanger body having a surface formed thereon defining a portion of a plenum volume. At a block  1017   d , a heat exchanger body having an outlet guide structure for guiding outlet flow of the pool fluid is received. The method stops at a block  1019   d.    
     Referring to  FIG. 22E , for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  1011   e , of assembling a heat exchanger starts at a block  1013   e . At a block  1015   e , the method comprises receiving a heat exchanger body having a surface formed thereon defining a portion of a plenum volume. At a block  1017   e , a guide structure for preventing contact of the pool fluid with the pool wall is received, the pool fluid being disposed within at least a portion of the heat exchanger body. The method stops at a block  1019   e.    
     Referring to  FIG. 23 , for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  1020  of assembling a heat exchanger starts at a block  1030 . At a block  1040 , the method comprises receiving a heat exchanger body having a surface formed thereon defining a portion of a plenum volume. At a block  1050 , a reactor vessel defining a portion of an outlet plenum volume of non-uniform shape is received. The method stops at a block  1060 . 
     Referring to  FIG. 24 , for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  1070  of assembling a heat exchanger starts at block  1080 . At a block  1090 , the method comprises receiving a heat exchanger body having a surface formed thereon defining a portion of a plenum volume. At a block  1100 , a heat exchanger body is received that is capable of being in heat transfer communication with a nuclear fission reactor core. The method stops at a block  1110 . 
     Referring to  FIG. 25 , for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  1120  of assembling a heat exchanger starts at a block  1130 . At a block  1140 , the method comprises receiving a heat exchanger body having a surface formed thereon defining a portion of a plenum volume. At a block  1150 , the method comprises receiving a manifold-free heat exchanger body. The method stops at a block  1160 . 
     Referring to  FIG. 26 , for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  1170  of assembling a heat exchanger starts at a block  1180 . At a block  1190 , the method comprises receiving a heat exchanger body defining a plenum volume therein shaped for a predetermined flow of a heat transfer fluid into the plenum volume, the heat exchanger body having a surface formed thereon defining a portion of the plenum volume. At a block  1200 , a heat transfer member is coupled to the heat exchanger body, the heat transfer member defining a flow channel therethrough. The method stops at a block  1210 . 
     Referring to  FIG. 27 , for use in association with a pool-type nuclear fission reactor, an illustrative method  1220  of assembling a heat exchanger starts at a block  1230 . At a block  1240 , the method comprises receiving a heat exchanger body defining a plenum volume therein shaped for a predetermined flow of a heat transfer fluid into the plenum volume, the heat exchanger body having a surface formed thereon defining a portion of the plenum volume. At a block  1250 , a heat transfer member is coupled to the heat exchanger body, the heat transfer member defining a flow channel therethrough. At a block  1260 , a heat transfer member is coupled that is configured to achieve a predetermined flow of a heat transfer fluid into the heat exchanger body. The method stops at a block  1270 . 
     Referring to  FIG. 28 , for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  1280  of assembling a heat exchanger starts at a block  1290 . At a block  1300 , the method comprises receiving a heat exchanger body defining a plenum volume therein shaped for a predetermined flow of a heat transfer fluid into the plenum volume, the heat exchanger body having a surface formed thereon defining a portion of the plenum volume. At a block  1310 , a heat transfer member is coupled to the heat exchanger body, the heat transfer member defining a flow channel therethrough. At a block  1320 , a heat transfer member is coupled that is configured to achieve a predetermined flow of a heat transfer fluid into the heat exchanger body. At a block  1330 , a heat transfer member is coupled that is configured to achieve a substantially uniform flow of a heat transfer fluid into the heat exchanger body. The method stops at a block  1340 . 
     Referring to  FIG. 29 , for use in association with a pool-type nuclear fission reactor, an illustrative method  1350  of assembling a heat exchanger starts at a block  1360 . At a block  1370 , the method comprises receiving a heat exchanger body defining a plenum volume therein shaped for a predetermined flow of a heat transfer fluid into the plenum volume, the heat exchanger body having a surface formed thereon defining a portion of the plenum volume. At a block  1380 , a heat transfer member is coupled to the heat exchanger body, the heat transfer member defining a flow channel therethrough. At a block  1390 , a heat transfer member is coupled having a conduit extending along the flow channel. The method stops at a block  1400 . 
     Referring to  FIG. 30 , for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  1410  of assembling a heat exchanger starts at a block  1420 . At a block  1430 , the method comprises receiving a heat exchanger body defining a plenum volume therein shaped for a predetermined flow of a heat transfer fluid into the plenum volume, the heat exchanger body having a surface formed thereon defining a portion of the plenum volume. At a block  1440 , a heat transfer member is coupled to the heat exchanger body, the heat transfer member defining a flow channel therethrough. At a block  1450 , a heat exchanger body is received that is capable of being in heat transfer communication with a nuclear fission reactor core. The method stops at a block  1460 . 
     Referring to  FIG. 31 , for use in association with a pool-type nuclear fission reactor, an illustrative method  1470  of assembling a heat exchanger starts at a block  1480 . At a block  1490 , the method comprises receiving a heat exchanger body defining a plenum volume therein shaped for a predetermined flow of a heat transfer fluid into the plenum volume, the heat exchanger body having a surface formed thereon defining a portion of the plenum volume. At a block  1500 , a heat transfer member is coupled to the heat exchanger body, the heat transfer member defining a flow channel therethrough. At a block  1510 , a heat exchanger body is received that is capable of being in heat transfer communication with a traveling wave nuclear fission reactor core. At a block  1515 , a heat exchanger body capable of being in heat transfer communication with a traveling wave nuclear fission reactor core is received. The method stops at a block  1520 . 
     Referring to  FIG. 32 , for use in association with a pool-type nuclear fission reactor, an illustrative method  1521  of assembling a heat exchanger starts at a block  1523 . At a block  1525 , the method comprises receiving a heat exchanger body defining a plenum volume therein shaped for a predetermined flow of a heat transfer fluid into the plenum volume, the heat exchanger body having a surface formed thereon defining a portion of the plenum volume. At a block  1527 , a heat transfer member is coupled to the heat exchanger body, the heat transfer member defining a flow channel therethrough. At a block  1528 , a manifold-free heat exchanger body is received. The method stops at a block  1529 . 
     Referring to  FIG. 33 , for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  1530  of assembling a heat exchanger starts at a block  1540 . At a block  1550 , the method comprises receiving a heat exchanger body defining a plenum volume therein shaped for a predetermined flow of a heat transfer fluid into the plenum volume, the heat exchanger body having a surface formed thereon defining a portion of the plenum volume. At a block  1560 , a heat transfer member is coupled to the heat exchanger body, the heat transfer member defining a flow channel therethrough. At a block  1570 , a heat transfer member is coupled having a wall defining an enhanced heat transfer surface thereon. The method stops at a block  1580 . 
     Referring to  FIG. 34 , for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  1650  of assembling a heat exchanger starts at a block  1660 . At a block  1670 , the method comprises receiving a heat exchanger body defining a plenum volume therein shaped for a predetermined flow of a heat transfer fluid into the plenum volume, the heat exchanger body having a surface formed thereon defining a portion of the plenum volume. At a block  1680 , a plurality of adjacent heat transfer members are connected to the heat exchanger body and spaced apart by a predetermined distance for defining a plurality of flow passages between opposing ones of the plurality of adjacent heat transfer members to distribute flow of the heat transfer fluid through the plurality of flow passages. The method stops at a block  1690 . 
     Referring to  FIG. 35 , for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  1700  of assembling a heat exchanger starts at a block  1710 . At a block  1720 , the method comprises receiving a heat exchanger body defining a plenum volume therein shaped for a predetermined flow of a heat transfer fluid into the plenum volume, the heat exchanger body having a surface formed thereon defining a portion of the plenum volume. At a block  1730 , a plurality of adjacent heat transfer members are connected to the heat exchanger body and spaced apart by a predetermined distance for defining a plurality of flow passages between opposing ones of the plurality of adjacent heat transfer members to distribute flow of the heat transfer fluid through the plurality of flow passages. At a block  1740 , a plurality of adjacent heat transfer members configured to achieve a uniform flow of the heat transfer fluid into the heat exchanger body are connected. The method stops at a block  1750 . 
     Referring to  FIG. 36 , for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  1760  of assembling a heat exchanger starts at a block  1770 . At a block  1780 , the method comprises receiving a heat exchanger body defining a plenum volume therein shaped for a predetermined flow of a heat transfer fluid into the plenum volume, the heat exchanger body having a surface formed thereon defining a portion of the plenum volume. At a block  1790 , a plurality of adjacent heat transfer members are connected to the heat exchanger body and spaced apart by a predetermined distance for defining a plurality of flow passages between opposing ones of the plurality of adjacent heat transfer members to distribute flow of the heat transfer fluid through the plurality of flow passages. At a block  1800 , a reactor vessel is received defining a portion of an outlet plenum volume of non-uniform shape. The method stops at a block  1810 . 
     Referring to  FIG. 37 , for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  1820  of assembling a heat exchanger starts at a block  1830 . At a block  1840 , the method comprises receiving a heat exchanger body defining a plenum volume therein shaped for a predetermined flow of a heat transfer fluid into the plenum volume, the heat exchanger body having a surface formed thereon defining a portion of the plenum volume. At a block  1850 , a plurality of adjacent heat transfer members are connected to the heat exchanger body and spaced apart by a predetermined distance for defining a plurality of flow passages between opposing ones of the plurality of adjacent heat transfer members to distribute flow of the heat transfer fluid through the plurality of flow passages. At a block  1860 , a heat exchanger body is received that is capable of being in heat transfer communication with a nuclear fission reactor core. The method stops at a block  1870 . 
     Referring to  FIG. 38 , for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  1880  of assembling a heat exchanger starts at a block  1890 . At a block  1900 , the method comprises receiving a heat exchanger body defining a plenum volume therein shaped for a predetermined flow of a heat transfer fluid into the plenum volume, the heat exchanger body having a surface formed thereon defining a portion of the plenum volume. At a block  1910 , a plurality of adjacent heat transfer members are connected to the heat exchanger body and spaced apart by a predetermined distance for defining a plurality of flow passages between opposing ones of the plurality of adjacent heat transfer members to distribute flow of the heat transfer fluid through the plurality of flow passages. At a block  1915 , a heat exchanger body capable of being in heat transfer communication with a nuclear fission reactor core is received. At a block  1920 , a heat exchanger body is received that is capable of being in heat transfer communication with a traveling wave nuclear fission reactor core. The method stops at a block  1930 . 
     Referring to  FIG. 39 , for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  1940  of assembling a heat exchanger starts at a block  1950 . At a block  1960 , the method comprises receiving a heat exchanger body defining a plenum volume therein shaped for a predetermined flow of a heat transfer fluid into the plenum volume, the heat exchanger body having a surface formed thereon defining a portion of the plenum volume. At a block  1970 , a plurality of adjacent heat transfer members are connected to the heat exchanger body and spaced apart by a predetermined distance for defining a plurality of flow passages between opposing ones of the plurality of adjacent heat transfer members to distribute flow of the heat transfer fluid through the plurality of flow passages. At a block  1980 , at least two heat transfer fluids having a cross-flow orientation are accommodated. The method stops at a block  1990 . 
     Referring to  FIG. 40 , for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  2000  of assembling a heat exchanger starts at a block  2010 . At a block  2020 , the method comprises receiving a heat exchanger body defining a plenum volume therein shaped for a predetermined flow of a heat transfer fluid into the plenum volume, the heat exchanger body having a surface formed thereon defining a portion of the plenum volume. At a block  2030 , a plurality of adjacent heat transfer members are connected to the heat exchanger body and spaced apart by a predetermined distance for defining a plurality of flow passages between opposing ones of the plurality of adjacent heat transfer members to distribute flow of the heat transfer fluid through the plurality of flow passages. At a block  2040 , at least two heat transfer fluids having a counter-flow orientation are accommodated. The method stops at a block  2050 . 
     Referring to  FIG. 41 , for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  2060  of assembling a heat exchanger starts at a block  2070 . At a block  2080 , the method comprises receiving a heat exchanger body defining a plenum volume therein shaped for a predetermined flow of a heat transfer fluid into the plenum volume, the heat exchanger body having a surface formed thereon defining a portion of the plenum volume. At a block  2090 , a plurality of adjacent heat transfer members are connected to the heat exchanger body and spaced apart by a predetermined distance for defining a plurality of flow passages between opposing ones of the plurality of adjacent heat transfer members to distribute flow of the heat transfer fluid through the plurality of flow passages. At a block  2100 , at least two heat transfer fluids having a parallel-flow orientation are accommodated. The method stops at a block  2110 . 
     Referring to  FIG. 42 , for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  2120  of assembling a heat exchanger starts at a block  2130 . At a block  2140 , the method comprises receiving a heat exchanger body defining a plenum volume therein shaped for a predetermined flow of a heat transfer fluid into the plenum volume, the heat exchanger body having a surface formed thereon defining a portion of the plenum volume. At a block  2150 , a plurality of adjacent heat transfer members are connected to the heat exchanger body and spaced apart by a predetermined distance for defining a plurality of flow passages between opposing ones of the plurality of adjacent heat transfer members to distribute flow of the heat transfer fluid through the plurality of flow passages. At a block  2160 , at least one of the plurality of adjacent heat transfer members is coupled having a wall defining an enhanced heat transfer surface thereon for increased heat transfer through the wall. The method stops at a block  2170 . 
     Referring to  FIG. 43 , for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  2180  of assembling a heat exchanger starts at a block  2190 . At a block  2200 , the method comprises receiving a heat exchanger body defining a plenum volume therein shaped for a predetermined flow of a heat transfer fluid into the plenum volume, the heat exchanger body having a surface formed thereon defining a portion of the plenum volume. At a block  2210 , a plurality of adjacent heat transfer members are connected to the heat exchanger body and spaced apart by a predetermined distance for defining a plurality of flow passages between opposing ones of the plurality of adjacent heat transfer members to distribute flow of the heat transfer fluid through the plurality of flow passages. At a block  2220 , at least one of the plurality of adjacent heat transfer members is coupled having a wall defining an enhanced heat transfer surface thereon for increased heat transfer through the wall. At a block  2230 , at least one of the plurality of adjacent heat transfer members is coupled having a flange outwardly extending from the wall for forming the enhanced heat transfer surface. The method stops at a block  2240 . 
     Referring to  FIG. 44 , for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  2250  of assembling a heat exchanger starts at a block  2260 . At a block  2270 , the method comprises receiving a heat exchanger body defining a plenum volume therein shaped for a predetermined flow of a heat transfer fluid into the plenum volume, the heat exchanger body having a surface formed thereon defining a portion of the plenum volume. At a block  2280 , a plurality of adjacent heat transfer members are connected to the heat exchanger body and spaced apart by a predetermined distance for defining a plurality of flow passages between opposing ones of the plurality of adjacent heat transfer members to distribute flow of the heat transfer fluid through the plurality of flow passages. At a block  2290 , at least one of the plurality of adjacent heat transfer members is coupled having a wall defining an enhanced heat transfer surface thereon for increased heat transfer through the wall. At a block  2300 , at least one of the plurality of adjacent heat transfer members is coupled having a flange inwardly extending from the wall for forming the enhanced heat transfer surface. The method stops at a block  2310 . 
     Referring to  FIG. 45 , for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  2320  of assembling a heat exchanger starts at a block  2330 . At a block  2340 , the method comprises receiving a heat exchanger body defining a plenum volume therein shaped for a predetermined flow of a heat transfer fluid into the plenum volume, the heat exchanger body having a surface formed thereon defining a portion of the plenum volume. At a block  2350 , a plurality of adjacent heat transfer members are connected to the heat exchanger body and spaced apart by a predetermined distance for defining a plurality of flow passages between opposing ones of the plurality of adjacent heat transfer members to distribute flow of the heat transfer fluid through the plurality of flow passages. At a block  2360 , at least one of the plurality of adjacent heat transfer members is coupled having a wall defining an enhanced heat transfer surface thereon for increased heat transfer through the wall. At a block  2370 , at least one of the plurality of adjacent heat transfer members is coupled having a nodule outwardly projecting from the wall for forming the enhanced heat transfer surface. The method stops at a block  2380 . 
     Referring to  FIG. 46 , for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  2390  of assembling a heat exchanger starts at a block  2400 . At a block  2410 , the method comprises receiving a heat exchanger body defining a plenum volume therein shaped for a predetermined flow of a heat transfer fluid into the plenum volume, the heat exchanger body having a surface formed thereon defining a portion of the plenum volume. At a block  2420 , a plurality of adjacent heat transfer members are connected to the heat exchanger body and spaced apart by a predetermined distance for defining a plurality of flow passages between opposing ones of the plurality of adjacent heat transfer members to distribute flow of the heat transfer fluid through the plurality of flow passages. At a block  2430 , a heat transfer member is coupled having a conduit extending along a flow channel for flow of the second heat transfer fluid through the conduit. The method stops at a block  2440 . 
     Referring to  FIG. 47 , for use in association with a pool-type nuclear fission reactor capable of generating heat, an illustrative method  2450  of assembling a heat exchanger starts at a block  2460 . At a block  2470 , the method comprises receiving a heat exchanger body defining a plenum volume therein shaped for a predetermined flow of a heat transfer fluid into the plenum volume, the heat exchanger body having a surface formed thereon defining a portion of the plenum volume. At a block  2480 , a plurality of adjacent heat transfer members are connected to the heat exchanger body and spaced apart by a predetermined distance for defining a plurality of flow passages between opposing ones of the plurality of adjacent heat transfer members to distribute flow of the heat transfer fluid through the plurality of flow passages. At a block  2490 , a manifold-free heat exchanger body is received. The method stops at a block  2500 . 
     One skilled in the art will recognize that the herein described components (e.g., operations), devices, objects, and the discussion accompanying them are used as examples for the sake of conceptual clarity and that various configuration modifications are contemplated. Consequently, as used herein, the specific exemplars set forth and the accompanying discussion are intended to be representative of their more general classes. In general, use of any specific exemplar is intended to be representative of its class, and the non-inclusion of specific components (e.g., operations), devices, and objects should not be taken as limiting. 
     Moreover, those skilled in the art will appreciate that the foregoing specific exemplary processes and/or devices and/or technologies are representative of more general processes and/or devices and/or technologies taught elsewhere herein, such as in the claims filed herewith and/or elsewhere in the present application. 
     While particular aspects of the present subject matter described herein have been shown and described, it will be apparent to those skilled in the art that, based upon the teachings herein, changes and modifications may be made without departing from the subject matter described herein and its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications as are within the true spirit and scope of the subject matter described herein. It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases “at least one” and “one or more” to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim recitation to claims containing only one such recitation, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and/or “an” should typically be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of “two recitations,” without other modifiers, typically means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to “at least one of A, B, and C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, and C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances where a convention analogous to “at least one of A, B, or C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, or C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that typically a disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms unless context dictates otherwise. For example, the phrase “A or B” will be typically understood to include the possibilities of “A” or “B” or “A and B.” 
     With respect to the appended claims, those skilled in the art will appreciate that recited operations therein may generally be performed in any order. Also, although various operational flows are presented in a sequence(s), it should be understood that the various operations may be performed in other orders than those which are illustrated, or may be performed concurrently. Examples of such alternate orderings may include overlapping, interleaved, interrupted, reordered, incremental, preparatory, supplemental, simultaneous, reverse, or other variant orderings, unless context dictates otherwise. Furthermore, terms like “responsive to,” “related to,” or other past-tense adjectives are generally not intended to exclude such variants, unless context dictates otherwise. 
     Therefore, what are provided are a heat exchanger, methods therefor and a nuclear fission reactor system. 
     While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. For example, with reference to  FIG. 14 , shut-off valves  640   a / 640   b / 650   a / 650   b  may each be coupled to respective ones of a plurality of thermocouples (not shown) disposed in pipes  620   a / 620   b / 630   a / 630   b . A controller could selectively and progressively open and close the shut-off valves depending on the temperature of the heat transfer fluid entering and leaving heat exchangers  600 / 610 . That is, the amount heat transfer that is desired within the heat exchangers as a function of temperature sensed by the thermocouples could be preprogrammed into and stored in the controller. The temperatures within the heat exchangers could be detected by the controller via the thermocouples and the controller could then operate the shut-off valves by progressively opening and closing the shut-off valves to bring the heat transfer occurring within the heat exchangers into substantial agreement with the preprogrammed value stored within the controller. In this manner, heat exchangers  600 / 610  could be selectively operated to provide precise amounts of heat transfer within the heat exchangers by allowing the controller to automatically adjust the valves. 
     Moreover, the various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims. In addition, the corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or acts for performing the functions in combination with other claimed elements as specifically claimed.