Broadway, J.

Hickman, R.

KImberlin, A.

Mireles, O.

NASA Technical Reports Server

Development Status of a CVD System 
to Deposit Tungsten onto UO2 Powder 
via the WCl6 Process
NASA Advanced Exploration System (AES) Project:
Nuclear Cryogenic Propulsion Stage
O. Mireles, A. Kimberlin, J. Broadway, R. Hickman
NASA Marshall Space Flight Center
omar.r.mireles@nasa.gov
NETS
26 February 2014
https://ntrs.nasa.gov/search.jsp?R=20140008815 2019-08-31T20:21:36+00:00Z
• NTP fuels under development
− W-60vol%UO2 CERMET
− Fuel loss through erosion
− Inherently stable W fuel element cladding
− Coat spherical UO2 fuel kernels in 40 vol% W
• Performance Advantages
− Prevent H2 propellant at 2850 K from reducing 
UO2 fuel kernels
− Minimize erosion and fuel loss
• Manufacture Advantages
− Excellent powder distribution uniformity during 
HIP can fill
− Prevents segregation during HIP can fill
− Higher green packing density
− Minimize dimensional distortion during HIP
331 and 7 channel fuel samples
• Vendor cost to coat dUO2 in W excessive
• WF6 process
− Industry standard for W deposition
− Gaseous reagent
− Excessive F contamination in UO2 substrate and W 
coating
− Residual F exacerbates fuel loss
• WCl6 process
− No UO2 chlorination with WCl6 or reaction products
− W coatings do not excessively contaminate substrate
− WCl6 preferable to WF6 for coating UO2 with W
− More complex (solid-to-vapor reagent gas)
− Not an industrially utilized process
• Develop a lab-scale prototype that utilizes the 
WCl6 process that enables cost effective 
40vol% coating of spherical dUO2 powders
SEM micrographs of spherical 
W-coated ZrO2 particles 
SEM micrograph of uncoated UO2
sol-gel particles (700x)
• Fully encapsulate UO2 substrate
• Thickness: 40 ± 1 vol%, uniform spatial distribution
• Density: pore-free, > 18.7 g/cm3 - 19.2 g/cm3
• Purity: > 99.98% W, ≤ 10 ppm impurities
• Process: must not react with UO2 substrate
• Adhesion: must not de-bond, spall, crack or blister up to 3000 K
• UO2 fuel loss: <1.9 wt% (<1 mg/cm2) when heated to 3000 K in 
flowing H2 for 2 hours
CVD System
• WCl6 process
− Temperature: 950°C (higher results 
in large columnar grains
− H2/WCl6 mole ratio: 10:1 to 30:1 
− Pressure: < 10 mm Hg (0.193 psia)
• CVD System
− Fluidized bed reactor
− Raining feed, 25 g batches
− 20 to 60 min
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• Accomplishments
− Fluidization of ZrO2
− Coating ZrO2 to 60% of target 
thickness in 20 minutes
− Demonstrated viability of the 
WCl6 CVD process
− Coating spatial uniformity 
(thickness measured through 
cross section examination)
− Path: ZrO2, HfO2, UO2
• Limitations
− Powder drop-out: difficult to 
fluidize HfO2 without high H2
flow rates & powder small 
quantities
− Complex design
− Fragile and expensive glass-
metal transition ZrO2, Dp,u = 42.649 μm, 20 min run,
t = 2.3593 μm, (59.6% of goal) 
Deposition rate = 7.078 μm/hr
ZrO2, Dp, u = 14.519 μm, 20 min run, 
t = 2.1184 μm (157.2% of goal)
Deposition rate =  6.3552 μm/hr.
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Spouted reactor design.  Fluidization pre & post deposition process
• Calculate fluidization conditions
− Estimate Reynolds number and 
terminal velocity of powders in a 
fluidized state.
• Empirical data 
− Develop correlations based on 
empirical data
− Verify correlations with observed 
fluidization behavior
• Reactor estimation
− Extrapolate calculated and empirical 
results to estimate minimum 
fluidization flow rate
− Reactor design and particle specific
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− Remf = Reynolds number for minimum 
fluidization (sphereicity > 0.93)
− Ar = Archimedes number
− g = gravity
− Dp = particle diameter
− ρp = particle density
− ρf = fluid density
− μf = fluid viscosity
− VT = particle terminal velocity
Powder
Theoretical 
Density 
(g/cm3)
Actual 
Density 
(%TD)
Particle 
Size (μm)
ZrO2 5.68 50 53 - 106
HfO2 9.68 99 100 – 200
UO2 10.97 99 50 – 150
• Fluidization Prototype
− Simplified & robust design
− Based on lessons learned
− Co-centric fluid lines
− Built and tested
• Fluidization
− HfO2 (30, 60, 100, 200 g)
− Room temperature argon
− Fluidization vs. flow rate
− Inner/outer fluidization line
− Straight vs. tapered reactor wall
− Fluidized column height behavior
− Determined minimum and optimum 
fluidization flow rates
− Data used to design inverted CVD 
reactor
Inverted Reactor   
Prototype (tapered)
Fluidization: Tapered, 103 g HfO2, 
Ar, 30 L/m outer, 1 L/m inner, flush.
Inverted Reactor   
Prototype (straight)
Co-centric tube positions
Powder loading and fluidization vs powder mass
• Reactor manifold
− Pyrex
− Co-centric reactant and fluidization lines
− Ball-socket gas connections
• Reactor Wall
− Quartz
− Tapered
− Contains powder, eliminates powder 
drop out collection hopper
• Reactor O-ring Joint
− Standard item
− Eliminates glass-to-metal transitions
− Thicker walls = robust
• Inverted Reactor
− UO2 fluidization and coating trials in 
March
Inverted Reactor Design
Ar + WCl6 vapor
H2
H2+Ar+HCl vapor
Gas line simplification, fitting/valve reduction
Data Acquisition and Control System Kalrez 4079  O-ringsReactor handling glove box.
Inverted Reactor System LayoutInverted 
reactor 
sublimer
• Demonstrated viability of the WCl6 CVD process 
to coat ZrO2 particles with W.
• Inverted reactor designs are far more forgiving 
and robust than spouted designs.
• Corrosive nature of WCl6 vapor limits reactor 
material to Inconel, pyrex, quartz.
• Transition from surrogate to dUO2 powder as 
quickly as possible in order to address changes 
in process variables specific to dUO2.
• Optimize process variables
− WCl6 powder, H2, Ar impurity content
− Reactor temperature
− Reactor heat/cool rates
− H2/WCl6 mole ratio
− Flow rates as a function of coating thickness
− Deposition rate as a function of particle size
• Coating characterization
− Thickness
− Spatial uniformity
− Impurity content
− Adhesion
− Micro-hardness
− Surface roughness
− Grain structure (epitaxial content, grain orientation)
− Grain orientation effect on coating properties (heat transfer/diffusion)
− Grain boundary population impact on fuel retention
• Potential H2 heat treatments
− Pre-deposition to clean substrate surface: effect on coating adhesion
− Post-deposition to remove impurities: effect on W grain growth
• Funding was provided by the “Advanced 
Exploration Systems – Nuclear Cryogenic 
Propulsion Stage” project.
• The authors would like to thank Roger Harper, Eric 
Stewart, Mike Houts, Jim Martin of NASA MSFC, 
and Gene Nelson of AG Scientific Glass.
• The opinions expressed in this presentation are 
those of the author and do not necessary reflect 
the views of NASA or any NASA Project.


Development Status of CVD System to Deposit Tungsten onto UO2 Powder via the WCI6 Process

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