SPACE NUCLEAR PROPULSION REACTOR AFT PLENUM ASSEMBLY

An aft plenum assembly for use with a nuclear thermal reactor including a pressure vessel and a nozzle assembly having a top plenum plate disposed within the pressure vessel, the top plenum plate defining a first plurality of fuel flow apertures, a bottom plenum plate disposed within the pressure vessel, the bottom plenum plate being parallel to the top plenum plate thereby defining a plenum space therebetween, the bottom plenum plate defining a second plurality of fuel flow apertures, and a plurality of tubular connections extending between the first plurality of fuel flow apertures of the top plenum plate and the second plurality of fuel flow apertures of the bottom plenum plate, wherein the aft plenum assembly is disposed between the pressure vessel and the nozzle assembly.

TECHNICAL FIELD

The presently-disclosed invention relates generally to nuclear reactors and, more specifically, to internal support structures for supporting various internal components of nuclear reactors used in nuclear thermal propulsion.

BACKGROUND

The concept of utilizing nuclear thermal propulsion (NTP) to propel spacecraft during space travel is known. Nuclear thermal propulsion has been studied and tested in both the US and former Soviet Union (FSU). Most reactor arrangements utilize separate fuel assemblies and moderator assemblies “hung” from a support structure at the forward end of the reactor. Reactor designs based on the use of a slab moderator configuration have also been developed. Examples include the Experimental Beryillim Oxide Reactor (EBOR) which utilizes cylindrical assemblies inserted into blocks of BeO, the High Temperature Reactor Experiment (HTRE) design which utilized solid moderator blocks of zirconium hydride in a similar arrangement, and the Space Nuclear Thermal Propulsion (SNTP) reactor which utilized cylindrical assemblies inserted into hexagonal blocks of moderator. While not a solid moderator block, a similar heterogeneous fuel/moderator arrangement was used for the Gas-Cooled Reactor Experiment (GCRE).

These fore-side supported configurations place the support at the cold end of the reactor core. However, the flow channel paths are particularly complicated, there are many sub-assemblies, and it is challenging to keep the aft end of the moderator assemblies cool and limit thermal stress since they are adjacent to the fuel assemblies. Furthermore, support from only the forward end places the moderator and fuel assemblies in tension and failures of either may be particularly catastrophic. However, there is little design data on prior configurations of aft side supported configurations and no workable configuration demonstrated for a SNP/NTP engine with integral reactor concept making use of high assay low enriched uranium (HALEU).

Moderator block reactor arrangements require that the moderator be supported and flow distributed into it from the aft end of reactor. While fuel assemblies can be supported from the forward end, that configuration requires a sliding seal at the end of the fuel assembly so the fuel assemblies are also supported from the aft (nozzle) end for the moderator block arrangement. Aft-side support structures must distribute hydrogen provided to the reactor for moderator cooling to the cooling channels in the moderator while interacting with nozzle exit temperatures immediately adjacent to it. The support structure must further transmit the loads from pressure differentials and from core mass/accelerations to the reactor vessel.

There at least remains a need, therefore, for improved devices for NTP engines that can support moderator block configurations while providing adequate cooling flow and support for the fuel and moderator block assemblies.

SUMMARY OF INVENTION

One embodiment of the present disclosure includes an aft plenum assembly for use with a nuclear thermal reactor including a pressure vessel and a nozzle assembly having a top plenum plate disposed within the pressure vessel, the top plenum plate defining a first plurality of fuel flow apertures, a bottom plenum plate disposed within the pressure vessel, the bottom plenum plate being parallel to the top plenum plate thereby defining a plenum space therebetween, the bottom plenum plate defining a second plurality of fuel flow apertures, and a plurality of tubular connections extending between the first plurality of fuel flow apertures of the top plenum plate and the second plurality of fuel flow apertures of the bottom plenum plate, wherein the aft plenum assembly is disposed between the pressure vessel and the nozzle assembly.

Another embodiment of the present disclosure provides a nuclear thermal reactor having a reactor pressure vessel defining an interior volume, a reactor core including a plurality of fuel assemblies and moderator assemblies, the reactor core being disposed within the interior volume of the reactor pressure vessel, and an aft plenum assembly including a top plenum plate disposed within the pressure vessel, the top plenum plate defining a first plurality of fuel flow apertures, a bottom plenum plate disposed within the pressure vessel, the bottom plenum plate being parallel to the top plenum plate thereby defining a plenum space therebetween, the bottom plenum plate defining a second plurality of fuel flow apertures, and a plurality of tubular connections extending between the first plurality of fuel flow apertures of the top plenum plate and the second plurality of fuel flow apertures of the bottom plenum plate, wherein the aft plenum assembly is disposed between the pressure vessel and the nozzle assembly, and the reactor core is supported on the aft plenum assembly.

DETAILED DESCRIPTION

As used herein, terms referring to a direction or a position relative to the orientation of the fuel-fired heating appliance, such as but not limited to “vertical,” “horizontal,” “upper,” “lower,” “above,” or “below,” refer to directions and relative positions with respect to the appliance's orientation in its normal intended operation, as indicated in the Figures herein. Thus, for instance, the terms “vertical” and “upper” refer to the vertical direction and relative upper position in the perspectives of the Figures and should be understood in that context, even with respect to an appliance that may be disposed in a different orientation.

Further, the term “or” as used in this disclosure and the appended claims is intended to mean an inclusive “or” rather than an exclusive “or.” That is, unless specified otherwise, or clear from the context, the phrase “X employs A or B” is intended to mean any of the natural inclusive permutations. That is, the phrase “X employs A or B” is satisfied by any of the following instances: X employs A; X employs B; or X employs both A and B. In addition, the articles “a” and “an” as used in this application and the appended claims should generally be construed to mean “one or more” unless specified otherwise or clear from the context to be directed to a singular form. Throughout the specification and claims, the following terms take at least the meanings explicitly associated herein, unless the context dictates otherwise. The meanings identified below do not necessarily limit the terms, but merely provided illustrative examples for the terms. The meaning of “a,” “an,” and “the” may include plural references, and the meaning of “in” may include “in” and “on.” The phrase “in one embodiment,” as used herein does not necessarily refer to the same embodiment, although it may.

Referring now toFIGS. 1A and 1B, hydrogen gas is used to cool the Nuclear Thermal Propulsion (NTP) reactor components as well as provide propellant for the thrust produced by the NTP rocket engine200. In order for the NTP reactor202, which in the present example utilizes high assay low enriched uranium (HALEU), to go critical and generate heat in the reactor fuel, enriched uranium nuclear reactors rely on neutron moderating materials to thermalize, or slow, neutrons released in the fission process. The moderation of neutrons in a nuclear reactor's core is required to sustain the nuclear chain reaction in the core, thereby producing heat. The moderating material must be cooled in order to prevent melting. The same hydrogen gas that is utilized in cooling the moderator material is also routed through other areas of the reactor as coolant. Ultimately, the hydrogen gas exits the reactor by passing through, and being heated within, the fuel elements, thereby producing thrust as it exits the nozzle assembly204. As discussed in greater detail below, the moderator assemblies102and fuel assemblies104of the reactor core of the NTP reactor202are supported by an aft plenum assembly100in accordance with the present invention.

The purpose of the aft plenum assembly100, the position of which within the NTP reactor202is shown as region (A) inFIG. 1A, is to support the moderator assemblies102and fuel assemblies104of the moderator block reactor of the NTP/SNP rocket engine and to distribute the hydrogen coolant uniformly to the cooling channels in the moderator assembly102. Referring additionally toFIGS. 2A and 2B, the aft plenum assembly100preferably includes a top plenum plate106, a bottom plenum plate108, and a plurality of tubular connections110extending therebetween. Each tubular connection110connects fuel flow apertures103and105defined in the top and bottom plenum plates106and108, respectively. As well, each tubular connection110is positioned and configured to allow a portion of a corresponding fuel assembly104to protrude therethrough, as best seen inFIG. 3. Shear webs112connect the top and bottom plenum plates106and108in the manner of an I-beam configuration for an improved strength to weight ratio. The space between the top and bottom plenum plates106and108forms a plenum114that feeds the moderator cooling channels via coolant flow holes107in the top plenum plate106. In the preferred embodiment shown, the top and bottom plenum plates106and108are preferably constructed of Ti8Al1Mo1V, although alternate materials may be used.

A thermal shield116is located on the nozzle side of the bottom plenum plate108, which is shown in greater detail inFIG. 3. Note,FIG. 3also includes a partial view of some components of the fuel assemblies104therein. Although the preferred materials of the components of the aft plenum assembly100are capable of withstanding temperatures expected in the nozzle assembly204, the thermal shield116reduces material temperatures allowing for thinner, lighter components to meet the desired stress limits. Furthermore, the thermal shield116reduces heat transfer from the nozzle assembly204to the hydrogen gas located in the plenum region114. The reduced heat transfer from the nozzle assembly204to the hydrogen gas in the plenum region114results in higher engine specific impulse (Isp) and more uniform distribution of the hydrogen coolant to the moderator cooling channels. As shown, the thermal shield116includes a shield plate117that is preferably constructed of Tungsten and spaced from the bottom surface of the bottom plenum plate108of the aft plenum assembly100, thereby defining a gap119therebetween. The gap119of the thermal shield116collects hydrogen gas therein that cools due to becoming stagnant, thereby forming an insulative layer between the nozzle assembly204and the bottom plenum plate108of the aft plenum assembly100. Note also, an annular gap121exists between each tubular connection110of the aft plenum assembly100and the fuel assembly104components disposed therein. The annular gaps121have a similar insulative effect as the gap119of the thermal shield116as they are filled with hydrogen.

As shown inFIGS. 2A and 2B, the aft plenum assembly100also includes a flow distribution ring118. The flow distribution ring118includes a cylindrical side wall that extends between the top and bottom plenum plates106and108, thereby encircling the plurality of tubular connection110through which portions of the flow assemblies104pass. The cylindrical side wall of the flow distribution ring118is perforated so that the hydrogen coolant which enters the plenum114from three aft moderator inlet nozzles120is spread around the perimeter of the plenum114before flowing radially inward to both cool the structure and feed the moderator cooling channels. Still referring again toFIGS. 2A and 2B, toward the periphery of the aft plenum assembly100, a plurality of flow tubes122allows flow from the regeneratively cooled nozzle assembly204to feed into the reflector region124of the rocket engine200where the reactor control drums126are located. As shown inFIG. 1A, the reactor control drums126are each selectively driven by a corresponding control drum drive assembly131that is disposed above the reactor head133. The reactor head133and reactor vessel130support a fore support plate137that includes flow inlets139for the fuel assemblies104.

As noted, hydrogen coolant enters the outer region of the aft plenum assembly100from three discrete aft moderator inlet nozzles120defined within the flange128of the reactor vessel130, as best seen on the right side ofFIG. 2B. Hydrogen coolant flow distributes around the perimeter of the aft plenum assembly100due to the flow distribution ring118, after which it feeds radially inward. As the coolant flows inward, it not only cools the structure of the aft plenum assembly100, but also feeds the moderator cooling channels via holes107in the top plenum plate106. Simultaneously, the structure is withstanding a differential pressure between the plenum region/moderator region and the nozzle region which may be on the order of 6 to 10 MPa, although various pressure ranges are possible.

Alternate embodiments of aft plenum assemblies in accordance with the present invention are shown inFIGS. 4 through 7.FIG. 4shows alternatives to the nozzle side thermal shield116which is discussed above with regard toFIG. 3. Specifically, rather than utilizing a shield plate117to form a gap119between the shield plate117and the bottom surface of the bottom plenum plate108, as shown inFIG. 3, a flow baffle plate134is mounted to the bottom surface of the top plenum plate106. The flow baffle134preferably includes a cylindrical side wall141, and a circular bottom wall143defining a plurality of apertures145through which portions of the tubular connections110extend. As shown, whereas the majority of the apertures145do not allow flow therethrough, one or more central apertures145ahave a diameter that is greater than that of the corresponding tubular connection110that extends therethrough. As such, the moderator cooling gas flows radially-inwardly between the top surface of the bottom plenum plate108and the bottom surface of the bottom wall143of the flow baffle134before flowing through the central aperture(s)145ainto the cooling channels of the moderator assemblies102, as shown by the arrows inFIG. 4. The flow baffle134is beneficial as it improves the heat transfer coefficient on the plenum114side of the bottom plenum plate108. Note, flow baffle134may be used in addition to the nozzle-side thermal shield116.

In yet another alternate embodiment shown inFIG. 5, an internally-cooled bottom plenum plate108may include passages109formed therein to allow coolant flow. Preferably, the top surface of the bottom plenum plate108defines a plurality of blind bores111that are in fluid communication with the internally-formed passages109of the bottom plenum plate108. Moderator cooling gas flows into a plurality of the blind bores111that are disposed outside of the flow distribution ring118, radially-inwardly through the internal channels109, and out of the plurality of blind bores111that are disposed within the flow distribution ring118, thereby cooling the bottom plenum plate108.

FIGS. 6 and 7show configurations of the aft plenum assembly100that allow the internal shear webs112to be omitted. Specifically, gussets136may be attached to either the upper surface of top plenum plate106or the bottom surface of the bottom plenum plate108so that they bear against the inner surface of the core barrel138or the inner surface of the nozzle assembly204, respectively. Note, in the disclosed configurations the gussets136are not secured to either the core barrel138or the nozzle assembly204. As noted, the gussets136allow for the elimination of the shear webs112discussed with regard to the previous embodiments.

Referring now toFIGS. 8, 9A, and 9B, a flow adapter plate140for possible use with the aft plenum assembly100is described. The flow adapter plate140is received adjacent the upper surface of the top plenum plate106and includes a plurality of apertures142that correspond to the tubular connections110of the aft plenum assembly100, and a plurality of flow holes144formed therein. As shown, a bottom side of the flow adapter plate140includes machined-out regions that define a plenum space146with the top surface of the top plenum plate106. Hydrogen coolant that enters plenum space146through holes107of the top plate106then exits the flow holes144of the flow adapter plate140, which are aligned with the flow holes of the moderator assemblies102. As such, the coolant gas passes upwardly through the flow channel of the moderator assemblies102, as discussed previously. Note, in an alternate embodiment of the aft plenum assembly100in which the flow adapter plate140is utilized, the number of holes107in the top plenum plate106may be greatly reduced by the use of fewer, larger holes107, on the order of 5 to 6 mm each. The flow adapter plate140is preferably made from Ti-8Al-1Mo-1V, and may be either welded or bolted to top plenum plate106of the aft plenum assembly100, or may just be received thereon.

As shown inFIGS. 9A and 9B, an alternate embodiment of the aft plenum assembly100includes a flow adapter plate201and a flow distribution ring218that differs from that of the previously discussed embodiment. As shown, the flow adapter plate201is disposed between the top and bottom plenum plates106and108, and parallel to both. The outer perimeter203of the flow adapter plate201intersects the side wall of the flow distribution ring218so that the side wall is operated into a solid top portion218aand a perforated bottom portion218b. As such, the cooling flow of gas flows radially-inwardly between the top surface of the bottom plenum plate108and the bottom surface of the flow adapter plate201before flowing upwardly into the moderator assemblies102by way of a central opening205defined in the flow adapter plate201.

These and other modifications and variations to the invention may be practiced by those of ordinary skill in the art without departing from the spirit and scope of the invention, which is more particularly set forth in the appended claims. In addition, it should be understood that aspects of the various embodiments may be interchanged in whole or in part. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and it is not intended to limit the invention as further described in such appended claims. Therefore, the spirit and scope of the appended claims should not be limited to the exemplary description of the versions contained herein.