Processing Ultra High Temperature Zirconium Carbide Microencapsulated Nuclear Fuel

Abstract

The known fully ceramic microencapsulated fuel (FCM) entrains fission products within a primary encapsulation that is the consolidated within a secondary ultra-high-temperature-ceramic of Silicon Carbide (SiC). In this way the potential for fission product release to the environment is significantly limited. In order to extend the performance of this fuel to higher temperature and more aggressive coolant environments, such as the hot-hydrogen of proposed nuclear rockets, a zirconium carbide matrix version of the FCM fuel has been invented. In addition to the novel nature to this very high temperature fuel, the ability to form these fragile TRISO microencapsulations within fully dense ZrC represent a significant achievement.

Claims

1) A nuclear fuel product resulting from the incorporation of fragile fuel microencapsulations within an ultra-high temperature ceramic matrix.

2) The product of claim 1 where the fissile-bearing constituent is a Tri Structural Isotropic fuel (TRISO) or a variation of TRISO utilizing zirconium carbide (TRIZO).

3) The product of claim 1 where the fissile-bearing constituent is a Bi Structural Isotropic fuel (BISO).

4) The product of claim 1 produced through a transient eutectic phase process employing rare earth sintering aids utilized to suppress processing temperature.

5) The product of claim 1 where the matrix is near fully dense ZrC.

6) The methodology of claim 4 where the sintering aids are Al.sub.2O.sub.3 and/or SiO.sub.2.

7) The product of claim 1 where the matrix is near fully dense ZrC is achieved through a non-stoichiometric reaction of ZrC, ZrH and C where the ZrC is in the range of 0-10 mass percent and free carbon in the range of 0-4 mass percent.

8) The product of claims 1-7 utilized as nuclear fuel for nuclear reactor or nuclear thermal propulsion.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIG. 1 is a schematic of the (known) FCM fuel geometry. The TRISO fuel form (right-most) is embedded in a matrix of fully dense Ultra High Temperature Ceramic. Form factor is arbitrary.

REFERENCED NUMERALS IN THE DRAWINGS

[0010] 1 Microencapsulated fuel [0011] 1A Fissile fuel kernel [0012] 1B Outer pyrolitic carbon layer of microencapsulated fuel [0013] 1C SiC or alternate UHTC layer of microencapsulated fuel [0014] 1D Inner pyrolitic carbon layer of microencapsulated fuel [0015] 1E Buffer graphitic layer of microencapsulated fuel [0016] 2 Ceramic fuel sleeve [0017] 3 FCM mixture to be cold pressed [0018] 3A FCM constituent mixture: Zr and C powder, microencapsulated fuel, silica, aluminum oxide, and/or neutron poison rare earth oxides. [0019] 3B FCM constituent mixture: Zr and C powder, yttrium oxide, aluminum oxide, and neutron poison rare earth oxide.

DETAILED DESCRIPTION OF THE INVENTION

[0020] FIG. 1 provides a schematic similar to the known fully ceramic microencapsulated (FCM.) Whereby the known FCM utilizes SiC powder as a major matrix constituent, FIG. 1 portrays a fuel that is comprised of the fissile-fuel containing microencapsulation, in this case depicted as a Tri-Isotropic (TRISO) particle (item 1.) The new ZrC-matrix UHTC FCM fuel is demonstrated with acceptable matrix density and encapsulation integrity, post-processing, as achieved by the following means: [0021] A) Transient Eutectic-Phase Approach: Utilizing a combination of kinetic ball milling and high-shear milling to combine a certain volume fraction of ZrC powder and small percentage of SiC and oxide (such as Al.sub.2O.sub.3 and Y.sub.2O.sub.3) powders. This combination of ZrC, SiC and oxide powders form the dense matrix of this UHTC microencapsulated fuel. The compact of FIG. 1, forming the UHTC-FCM are fabricated at pressures not exceeding 20 MPa and temperatures not exceeding 2000 C. to attain a continuous, low porosity, ZrC matrix surrounding TRISO particles which remain unbroken and intimately bonded with the matrix following processing. The amount of oxide eutectic aids in the starting powder mix for processing the UHTC FCM fuel is up to 3 weight percent. The amount of SiC in the starting powder mix is up to 30 weight percent.

[0022] Specifically, the transient eutectic phase (TEP) SiC mixture is comprised using 94% SiC, 3.9% Al2O3, 2.1% Y2O3 by mass. The density was critically sensitive to both amount and ratio of rare earth additives. The SiC powder used were either 35 nm or 85 nm nanophase powder produced by chemical vapor deposition process. The TEP SiC mixture was mixed with milling media, dried, deagglomerated and re-dispersed in atmosphere prior to use. The feedstock material consists of powders sourced from commercial vendors and processed under typical methodologies found in ceramic powder forming and sintering. The TEP SiC mixture was added to ZrC in ratios of ZrC with 10 wt % TEP SiC mixture and re-mixed as previously described. The feedstock powder is mixed with a proprietary set of dispersants, binders, flow plasticizers and release agents, which assist in rheological properties needed for forming operations.

[0023] Sintering was conducted inside graphite tooling, configured for 10 mm diameter. Green bodies are formed in-die by loading the powder mixture directly into prepared graphite tooling. A cylindrical graphite die with a cylindrical cavity was lined with graphite foil. This conducts heat into the pellet and provides a release interface for the consolidated pellet. The pellet is formed by pouring the ZrC-based powder, followed by compaction by spark plasma sintering (SPS) for 10 minutes at 20 MPa at room temperature during the vacuum cycle. Before sintering the pressure was reduced to 10 MPa. The applied pressure was limited in order to establish processing conditions compatible with TRISO particles, which fail at low pressures at room temperature. Sintering temperatures of 1875 C. for 10 minutes. After sintering, the ZrC-10% TEP SiC mixture made into solid pellets. The pellets achieved 93% theoretical density. [0024] B) Hydrogen Aided Reaction Sintering: Utilizing a non-stroichiometric mixture of ZrC powder, ZrH and carbon powder the UHTC FCM ZrC matrix takes advantage of enhanced diffusion in a sub-stoichiometric ZrC and the decomposition of ZrH at approximately 900 C. In the absence of hydride decomposition the temperature and pressures were in excess of 2000 C. and 60 MPa with <95% theoretical density. With the addition of percent levels of ZrH the compact achieves near full density at temperatures under 1800 C. Powder handling and direct current sintering is carried out in a similar fashion to the Transient Eutectic-Phase Approach. Pressures not exceeding 20 MPa and temperatures in the 1650-1800 C. range produce a dense matrix and rupture-free TRISO microencapsulations. ZrH additions up to 10 weight percent by mass are demonstrated effective with free carbon in the range of 0.1 to 4% by mass.