Viewgraphs and explanations on laser diagnostics for nuclear thermal propulsion (NTP) fuel corrosion studies are presented. Topics covered include: NTP fuels; U-Zr-C system corrosion products; planar laser-induced fluorescence (PLIF); utilization of PLIF for corrosion product characterization of nuclear thermal rocket fuel elements under test; ZrC emission spectrum; and PLIF imaging of ZrC plume

Butt, D. P.

Sappey, A. D.

Wantuck, Paul J.

English

NASA Technical Reports Server

r 
Laser Diagnostics for NTP Fuel 
Corrosion Studies 
Paul J .  Wantuck 
Chemical and Laser Sciences Division 
Los Alamos, NM 87545 
Contributors: D.P. Buff 
A.D. Sappey 
Nuclear Propulsion Technical Interchange Meeting 
NASA-Lewis Research Center, Plum Brook Station 
October 20023,1992 
I NP-TlM-92 NIP. Technology 52 1 
PREBeDiNG PAGE BLANK NOT FILMED 
https://ntrs.nasa.gov/search.jsp?R=19930017747 2020-03-17T05:26:16+00:00Z
Issues Associated with NTP Fuels 
Reactor operates at temperatures approaching 
3000 K 
Fuel is susceptible to corrosion 
Need to quantify corrosion products / rates to 
assess fuel performance 
Severe environment during test: limits methods 
of measurement 
Laser-based diagnostics offer means of probing 
environment about fuel 
Advantages: non-intrusive 
unaffected by harsh oca1 conditions 
CLS-92-1582 
In order to generate the necessary propulsive performance, the nuclear reactor will operate at 
temperatures approaching 3OOO K. Such temperatures, in combination with the high-pressure 
hydrogen propellant flowing through the core, provide a very hostile operating environment for 
the fuel material, particularly with respect to hydrogen-induced corrosion. Identifying the 
corrosion products as well as experimentally quantifying the corrosion rates of these fuels under 
a variety of conditions is critical to assess the expected lifetime of the reactor. The severe 
environment surrounding a fuel material under test is not conducive to probing by standard 
instrumentation. Thus alternative diagnostic techniques are required to provide real-time 
monitoring of corrosion products. Laser diagnostics offer a means of non-intrusively probing 
the high-temperature environment above the surface of the fuel to identify and establish spatial 
distributions and local concentrations of many of the anticipated corrosion species. 
' 
NIT: Tecbnology 522 NP-TIM-92 
NP-TIM-92 
U-Zr-C system corrosion products 
(Ux Zrl-x) Cy solid solution or (Ux Zrl-x) Cy + C 
composite fuel material serious candidate for “I’P 
reactor fuels 
- High melting temperature 
- Good resistance to % corrosion 
Calculations indicate that for H, temperatures 
>2800 K, acetylene, U(g), and Zr(g) are primary 
vapor constituents 
Zr selected as species to probc 
- Major corrosion product 
- Corrosion of (U, Zr,-,) Cy may be rate limited by 
transport of Zr away from fuel surfaces 
- Obviates need for testing in H2 environments or with 
uranium-containing samples 
CLS-92- 1957 
Solid solution refractory carbide fuel materials, such as U,Zr,&,, are being seriously 
considered as candidate NTP reactor fuels because of their high melting point and resistance to 
corrosion by hydrogen.’ Butt* has calculated the equilibrium partial pressures above the surface 
of Uo.05Zro.95Cl.o, during exposure to 1 atm of hydrogen over a temperature range of 2000 to 
3200 K. The results show that in high temperature hydrogen, U(g), Zr(g), and various 
hydrocarbon species dominate the gas-phase products. Above approximately 2800K, acetylene 
and gas phase uranium and zirconium are predicted to be the primary vapor constituents. 
Confirmation of such predictions through experimental measurements is critical to the devel- 
opment of accurate corrosion kinetics models. 
In the current study, Zr atoms were selected as the species to probe using laser techniques. This 
selection was influenced by severdl factors. First, the zirconium atom is predicted to be a major 
corrosion product. Second, the corrosion of UXZr1-,C in hydrogen may be rate limited by the 
transport of gas phase Zr away form the surface2 Third: testing a laser diagnostic for zirconium 
obviates the need to perform experiments in a hydrogen environment or with uranium- 
containing fuel samples. 
1. S. K. Bhattacharyya. R. H. Cooper, R. B. Matthews, C. S. Olsen, R. H. Titran, and C. E. 
Walter, “Fuels and Materials for Space Nuclear Propulsion.” AIP Proceedings of the 9th 
Symposium on Space Nuclear Power Systems, Albuquerque, New Mexico, Vol. 2. pp 681- 
691 (1992). 
D. P. Butt, “Corrosion of UxZrl~,Cy Nuclear Fuel Materials in Hydrogen Gas at High 
Pressures and Temperatures,” IAF Paper No. 92-0570.43rd Congress of the International 
Astronautical Federation, Washington DC (1 992). 
2. 
523 NIP ‘l‘ecbnology 
1 Planar Laser-Induced 
Fluorescence (PLIF) 
Highly sensitive technique 
Absorption of laser light by atomic or 
molecular species followed after some finite 
time by emission from the excited state 
Intensity of emission can be related to 
concentration, temperature, velocity, etc. 
PLIF represents extension from point or 
line diagnostic to 2D or field measurement 
technique 
CLS-92-1583 
Laser-induced fluorescence (LIF) offers a highly sensitive technique for monitoring many of the 
NTP fuel corrosion products (including 23) as well as for determining properties of the NTP 
exhaust. Quite simply, LIFcan be viewed as an absorption of laser light, at P specific frequency, 
by an atomic or molecular species followed after some finite time by an emission from the 
excited state. This emission or fluorescence is, in general, at a different (typically longer) 
wavelength than the exciting laser light’s wavelength. By viewing this off-resonance fluores- 
cence, it is possible to avoid interference from scattered laser light. 
Planar LIF or PLTF represents an extension of the LIF technique from a point or line diagnostic 
to a 2D or field measurement method. In general, the species-exciting laser beam is transformed 
intoa thin sheet by the placement of cylindrical lenses into the optical train. A camenis typically 
used to collect the resulting fluorescence emission, perpendicular to the species-exciting laser 
sheet. The local intensity of the collected light can then be related to the concentration, 
temperature, or velocity of the target species. The non-intrusive natureof this technique, as well 
as its good spatial and temporal resolution, make it particularly well suited for application as a 
diagnostic in the high temperature NTP operating environment. 
NTP: Technology 524 NP-TIM-92 
f 
Utilization of PLIF for Corrosion Product 
Characterization of Nuclear Thermal Rocket (NTR) 
Fuel Elements Under Test 
- 
H, (- 3000k) 
Fluorescence Emission 
From Target 
Corrosion Product 
Corrosion Products 
(U, C, Zr, C2H,, etc.) 
- 
H2 
___t 
CLS-92- 
A PLIF scheme for measurement of corrosion species evolving during the test of an NTP fuel 
sample/element could take the above configuration. The laser sheet is passed through the high 
temperature hydrogen stream, which contains the various corrosion products, and contacts the 
surface of the fuel. Fluorescence emission from the chosen target species is then imaged 
producing a two-dimensional dismbution map. 
NP-TIM-92 525 N7Y: Technology 
Laser ablation / PLIF apparatus 
Baraaon Pressure Gauzes 
Gas Flow (Mass Flow Controlled) 
c d 
CLS-92-829 
Production of zirconium vapor for subsequent illumination by a PLIF diagnostic, is accom- 
plished by focusing a pulsed laser onto a ZrC target. This technique, known as laser ablation, 
represents a relatively simple method for producing gas phase samples of refractory materials. 
The apparatus utilized for these experiments is displayed above. The cubical (-30cm) ablation 
chamber contains five, window ports which allows optical access to its interior. The chamber 
is evacuated by a standard mechanical pump through both a liquid nitrogen and alumina-filled 
trap. The chamber can be backfilled with a variety of gases through a separate, flow regulated 
feed line. During each experiment, a slow flow of argon is maintained through this line and the 
chamber to minimize the buildup of particulate. A series of capacitance type manometers and 
thermocouple gauges are available to monitor chamber pressure. The base pressure for the 
chamber is 20 mtorr. Typical operating pressures are between 7 to 10 torr. The ZrC target 
specimen is positioned on a rotating table which is externally driven by a variable speed DC 
motor. Rotation of the target prevents the formation of a pit in the ZrC disk by action of the 
ablation laser. 
A Lumonics model TE-860-4 excimer laser operating at 248 nm is used to produce the abation 
pulse. Beam energies are on the order of 100 mJ/pulse and the laser is operated with a 5-10 Hz 
repetition frequency. The beam is brought to the ablation chamber by several high reflectance 
mirrors and focused at normal incidence onto the target using a 18.3-cm focal length quartz lens. 
The ablation spot size is approximately 1 mm2 and the corresponding laser fluence at the target 
is -670 MW/cm2. The PLIF sheet is passed, at right angles, through the plume. 
NIP: 'Iechnalogy 526 NP-'IUII -92 
r . 
ZrC Plume Emission 
Plume emission images were recorded at different delay times. One representative ZrC plume 
emission image is displayed above, The ability to acquire such images represents a necessary 
step in the application of the PLIF technique. An intensified, gated uv camera (Xybion model 
ISG-250-U) with a 105 mm. f/4.5 quartz focusing lens was interfaced with an EPIX Silicon 
MUX RGB frame gabber board with a programmable bigger option. The timing of the camera 
intensifier, frame grabber board, and excimer laser was controlled using a Stanford Rcscarch 
System model DG535 programmable delay/pulse generator. The above image was captured 
50 ps after the excimer laser pulse. The ablation chamber's argon background pressure was held 
constant at 7.5 torr and the camera gate width set at 20 ps. in these expanding plasmas, atom 
velocities can exceed IO6 c d s  at low background pressures (tens of mtorr) with neutral gas 
temperatures near the target surface approaching 15,000 K. 
NP-TI&-92 527 NTP 'I'cchnology 
ZrC Emission Spectrum 
I I I I I I I 
h 
c fn 
E 
% 
.- 
a 
E! 
e 
L 
c .- 
Q 
% 
Y 
Y, 
E 
0) 
c 
I 
c - 
I I I I I I I 
2000 2500 3000 3500 4000 4500 5000 
Wavelength (A) 
L / 
CLS-92-794 
Temporally resolved ZrC plume emission spectra were recorded for regions near the target 
surface. The chamber pressure was maintained at 8 tom. A representative ZrC plume emission 
spectrum, recorded 10 ps after theexcirner laser pulse, is shown above. This emission spectrum, 
which covers a wavelength range from 200.0 to 500.0 nm, is dominated by the presence of 
zirconium atom emission (several of the many Zr(1) lines are identified in the figure). No 
emission lines from other species, such as carbon atoms are identified in the emission traces. 
"IT: Technology 528 NP-TIM-92 
100 ps 
200 ps 
300 ps 
500 ps 
1 ms 
I I I I I 1 
Zirconium carbide emission 
Argon chamber pressure = 8 Torr 
2000 2500 3000 3500 4000 4500 5000 
Wavelength (A) 
d 
CLS-92- 1590 
By adjusting the delay time, it is possible to establish emission spectra at specific times during 
the plume expansion event. For example, ZrC emission spectra recorded for increasing delay 
times indicate that the plume emission intensity has reduced to essentially undetectable levels 
at approximately 1 ms (fora background pressure of 8 torr). Suchareduction isdue toexpansion 
cooling and quenching through radiative and collisional processes. Quantifying the temporal 
behavior of plume emission intensity is important for establishing the proper delay times to 
acquire PLlF images of the expanding zirconium vapor as it reduces interference from 
background emission. 
Nt'-'llM-92 529 NTP: Technology 
PLIF Imaging of ZrC Plume 
Zr(1) ground state to excited state transition 
at 35515 cm-' pumped using Nd:YAG 
laser-pumped dye laser (R590 dye) 
Fluorescence emission monitored at 
418.76 nm using filtered, gated, uv-intensified 
CCD camera coupled to frame-grabber board 
Image capture at different times after 
excimer pulse 
PLIF sheet 30.0 mm x < O S  mm 
CLS-92- 1584 
PLIF images of the spatial distribution of the zirconium atom in the ZrC plume have been 
acquired. To capture these images, the Zr(I) ground state to the excited state (t3P2. J=2) 
transition at 35,515.4 cm-I (281.5 nm) was pumped using the frequency doubled output of a 
NdYAG laser-pumped dye laser operating on Rhodamine 590 dye. Fluorescence emission at 
418.76 nm, corresponding to the transition from the t 3 p 2  excited state to the dF2 (J=2) 
intemediate state at 11640.7 crn-', was captured with the uv-intensified CCD camera. A dye 
laser sheet approximately 30.0 mm by 0.5 mrn was formed for passage through the plume by 
a lens combination consisting of a + I  W mm (converging) lens and a -100 mm (diverging) focal 
length cylindrical lens and a 1 SO mni focal length spherical lens. 
NIY: lechnology NP-TIM-92 
(PLIF Image of Zr in Ablated Plume 
NP-I1M-92 
A PLIFimage showing the spatial distribution of ground state zirconium atoms in the ZrC plume 
is shown above. The fluorescence emission was filtered using a m 3 9 5  filter. The image was 
recorded 625 ps after the ablation laser pulse with the camera gate width set at 20 ps. The spots 
observed in this image are pieces of hot (radiating) sputtered material from the target. 
53 I NI'P: 'l'kchiiology 
Summary 
Utilized a focused excimer laser to ablate material 
Zr prevalent in plumes 
Temporally resolved CCD image of plume emission 
from ZrC targets 
generated 
Zplurne 1c ' ms ('Background = 8 torr); Text z 12,000 K 
PLIF utilized to successfully image Zr atom distribution 
.PLIF technique should be able to monitor Zr about 
in plume 
fuel element under test 
CLS-92- 1585 
We have utilized a focused excimer laser to ablate material from ZrC targets for the purpose of 
developing appropriate laser-based diagnostics for gas-phase, corrosion products from hydro- 
gen-exposed U,Zrl -& fuel elements proposed for NTP application. Temporally and spatially 
resolved emission spectra from the produced vapor plumes show the dominating presence of 
zirconium atoms. Temporally and spatially resolved images of the ZrC plume emission have 
also been recorded. The PLIF technique has been successfully used to image Zr atom 
distributions in the ablated ZrC vapor plume and thus could potentially be utilized to monitor 
Zr about fuel elements under test. 
N1P: lechnology 512 NP-TIM-92 
Future Activities 
Investigate other fluorescence excitation 
wavelengths for Zr 
Expose samples to rf-heated, hydrogen containing 
flow and probe flowfield around and downstream of 
sample with PLIF diagnostic 
Investigate other NTP fuel materials; 
(U-Nb-C) system 
Quantify Zr, Nb, etc. concentration in terms of 
fluorescence emission 
I 
CU-9'2-1586 
Potential future activities include investigating other zirconium atom excitation wavelengths to 
ascertain the optimal transition for obtaining PLIF images in the ZrC ablation plumes. 
Following such determination, ZrC samples will be exposed to radio-frequency Q heated, 
hydrogencontaining flows and the PLIF diagnostic will be used to measure Zr atom dismbu- 
tions in the region surrounding the samples or in the nozzle exhaust flow. 
For this study, a radio frequency discharge driven flow system will be used to produce a 
continuous, high temperature, chemically-clean gas stream. The unique feature of this system, 
which has been described by Wantuck,'.* is the use of an inductively coupled plasma tube as 
a high enthalpy gas source. A 50 kW rfgenerator is used to supply power to the tube where the 
the gasis heated to between 5000 to 10,000K. The systemconfiguration will allow twodifferent 
modesof ZrC sample heating, namely, placement of the sample within the plasma tube for direct 
inductive/plasma heating or positioning downstream of the nozzle exit for heating by the gas 
stream. The first configuration approximates corrosion species distribution in an NTP exhaust 
flowfield. The second configuration best simulates propellant flow over a fuel element. In both 
cases, a dye laser beam, operating at the same wavelength employed for the plume PLLF 
illumination studies, will be used to probe the nozzle exit flowfield or the region surrounding 
the gas-stream heated ZrC sample. 
1. P. Wantuck and H. Watanabe."Radio Frequency (RF) Healed Supersonic Flow Laboratory," AlAA 
Paper No. 90-2469,1990. 
2. P. J. Wanluck, R. A. TeMant, H. H. Watanabe. "Supcrsonic Combustion of a Transverse-Injected H2 
Jet in a Radio Frequency Heated Flow," AlAA Papcr No. 91-2393, 1991. 
NP-TIM-92 533 NTP: Technology 


Laser diagnostics for NTP fuel corrosion studies

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