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.

Description

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

MICROFICHE APPENDIX

Not Applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a nuclear fuel. More specifically this invention describes a new fuel form and the method for fabrication of this inert matrix fuel whereby a fragile fuel microencapsulation is consolidated within an ultra-high-temperature-ceramic (UHTC) (zirconium carbide) that serves as a secondary and very high-temperature barrier to fission product release.

2. Description of the Related Art

There are many known types of nuclear fuel for both research, power producing nuclear reactors, and reactor for space exploration. The most common example of nuclear fuel is the ceramic uranium oxide pellet that is contained within a thin metallic cladding. That cladding both provides a rigid structure to hold the fuel and serves as the barrier to fission product release to the coolant stream. A second example of nuclear fuel is an inert matrix fuel (IMF) in which a fissile material such as (or containing) U-235 is dispersed in an inert host matrix. That inert matrix may be a refractory ceramic and is intended to provide a rigid host for the fuel as well as provide a measure of fission product retention. Yet a third example is a microencapsulated fuel, in one example known as the fully ceramic microencapsulated FCM™ fuel. This fuel type, similar to that of the IMF, has a distinct non-fuel matrix surrounding a plurality of fueled particles otherwise known as microencapsulations. In contrast with the IMF the microencapsulated fuels utilize an engineered fuel microencapsulation, such as the tri-isotropic (TRISO) or bi-isotropic (BISO) fuel forms which layers pyrolitic graphite and SiC (for the TRISO) around the fissile fuel kernels thus providing barriers to fission product retention within the fuel. The historic and most common porous host matrix is graphite. Such a fuel was developed as early nuclear thermal propulsion rockets with good success. This FCM fuel has the attribute of having both a primary fission product barrier (the TRISO fuel particle) and a secondary barrier being the SiC matrix. This combination thus provides two rugged barriers to fission product release.

The SiC matrix of the known FCM™ fuel is fabricated through a Transient Eutectic-Phase (TEP) process whereby rare earth oxides are utilized to reduce the sintering temperature and pressure required to achieve full density for the SiC matrix. As taught in U.S. Pat. No. 9,299,464 B2 and 2017/0025192 A1 this process is essential in allowing a process window for which the fragile TRISO particles will not be crushed and thereby rendered ineffective in a fuel application. However, while the standard FMC product is considered robust for application temperatures up to 1850° C., above that temperature unacceptable SiC matrix degradation occurs due to instability of TEP SiC. Moreover, for application in systems such as nuclear thermal rocket engines the reaction between SiC and hot hydrogen is unacceptable. In order to move into a higher performance regime, a higher temperature ultra-high temperature ceramic (UHTC) that replaces the SiC matrix of the microencapsulated fuel is put forward. Typical UHTC's and their maximum application temperatures are provided in the table below. Of those materials listed zirconium carbide has a number of attractive features as an ultra-high temperature fuel matrix, though has historically been very difficult to process, requiring temperature and pressure well in excess of that which would crush modern TRISO fuel microencapsulation. The present invention provides a process to fabricate a zirconium carbide matrix microencapsulated fuel at conditions favorable to the use of TRISO fuel.

				Melt or Decomp.
		Crystal	Density	Temperature
Material	Formula	structure	(g/cm3)	(° C.)
Niobium nitride	NbN	Cubic	8.470	2573
Tantalum nitride	TaN	Cubic	14.30	2700
Vanadium carbide	VC	Cubic	5.77	2810
Silicon carbide	SiC	Cubic	3.21	2820
Zirconium nitride	ZrN	FCC	7.29	2950
Titanium nitride	TiN	FCC	5.39	2950
Tantalum boride	TaB2	HCP	12.54	3040
Titanium carbide	TiC	Cubic	4.94	3100
Titanium boride	TiB2	HCP	4.52	3225
Zirconium boride	ZrB2	HCP	6.10	3245
Hafnium boride	HfB2	HCP	11.19	3380
Hafnium nitride	HfN	FCC	13.9	3385
Zirconium carbide	ZrC	FCC	6.56	3400
Niobium carbide	NbC	Cubic	7.820	3490
Tantalum carbide	TaC	Cubic	14.50	3768
Hafnium carbide	HfC	FCC	12.76	3958

BRIEF SUMMARY OF THE PRESENT INVENTION

The present invention provides the concept for a zirconium-carbide-matrix, ultra-high-temperature ceramic matrix fuel designed for advanced nuclear application. As envisioned, in comparison with standard FCM fuel which would achieve a nominal specific impulse in the range of 500-600 s, the use of ZrC-based or pure ZrC matrix UHTC FCM fuel would achieve in the range of 700-850 s. Application of this fuel includes nuclear rocket engines and systems requiring fuel of limited fission product release to operate at temperatures in excess of 2500° C. Use of ZrC matrix UHTC FCM could incorporate TRISO of standard or more rugged SiC micropressure vessels for short durations in the temperature range of 2200-2400° C., 2 hr time periods. Advanced microencapsulation whereby ZrC shells replace the SiC microencapsulation of the TRISO can be considered for longer life or higher temperature application. The TRISO containing ZrC is known to the literature (i.e. TRIZO), displaying similar crush strength to standard modern TRISO.

The critical step towards achieving fabrication of the ZrC matrix UHTC FCM is the ability to consolidate a near full density matrix of ZrC while not compromising the function of the entrained second phase fissile fuel: to not crush, deform, or substantially react layers. In the case of a SiC (or ZrC) TRISO microencapsulation this means consolidation without rupture of a significant (<10 ppm) number of the SiC (or ZrC) protective shell layers. This is accomplished through suppression of the normal pressures and temperatures required for sintering of zirconium carbide. In addition to a low failure fraction of TRISO particles a measure of success for the matrix is to achieve near full density without interconnected porosity. As described, two methods have achieved acceptable levels of success in producing ZrC matrix UHTC FCM: A) Transient Eutectic-Phase Processing, B) Hydrogen Aided Sintering.

BRIEF DESCRIPTION OF THE DRAWINGS

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

• 1 Microencapsulated fuel
  • 1A Fissile fuel kernel
  • 1B Outer pyrolitic carbon layer of microencapsulated fuel
  • 1C SiC or alternate UHTC layer of microencapsulated fuel
  • 1D Inner pyrolitic carbon layer of microencapsulated fuel
  • 1E Buffer graphitic layer of microencapsulated fuel
• 2 Ceramic fuel sleeve
• 3 FCM mixture to be cold pressed
  • 3A FCM constituent mixture: Zr and C powder, microencapsulated fuel, silica, aluminum oxide, and/or neutron poison rare earth oxides.
  • 3B FCM constituent mixture: Zr and C powder, yttrium oxide, aluminum oxide, and neutron poison rare earth oxide.

DETAILED DESCRIPTION OF THE INVENTION

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:

• 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 Al₂O₃ and Y₂O₃) 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.

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.

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.

• 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.

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 Al2O3 and/or SiO2.

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.