Review of Past and Current W-uo2 Cermet Fuel Fabrication Development and Testing

1. Introduction

Long elapsing spaceflight can betrayal astronauts to two major problems. These are extended periods of weightlessness and radiation exposure. Thus, it is necessary to develop alternate means of propulsion to that of chemic propulsion. A reasonable option being studied past NASA is nuclear thermal propulsion (NTP), whereby nuclear fuel elements made from metal/UO₂, metal/UN or tricarbides are being considered. These fuel elements must exist capable of operating in excess of 2700 K while existence compatible with the propellant, typically hydrogen [ane]. From previous studies information technology was shown that W/UO₂ as a fuel element can be used both in ability and propulsion at temperatures every bit high as 3000 One thousand [2, 3, 4]. 2 factors need to be considered when producing West/UO₂ fuel elements. The first is the importance of a uniform distribution of UO_ii particles in the tungsten matrix. If i has segregation of UO₂ particles it can pb to hot spots and ultimately failure of the fuel element. The 2nd factor which needs to be considered is the stoichiometry of the UO₂ particles. Maintaining stoichiometry is vital to ensure stability and proper operation of the fuel element. This affiliate will detail a brief history of NTP by NASA including fuel element work, followed past more contempo research on various fuel systems under consideration.

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2. NTP history

Every bit far back as the 1940s, information technology was recognized that energy from nuclear fission could be used to power spacecraft past heating a working fluid such as hydrogen and provide thrust to the rocket via expansion of the propellant through a rocket nozzle. A uncomplicated drawing of an NTP rocket is shown in Figure ane below.

Figure ane.

Schematic of NTP rocket.

Due to the high specific impulse, NTP is considered to exist the preferred propulsion method for future manned flights to Mars. Specific impulse (I_sp) is a method to measure and compare the efficiency of different propulsion systems. It is determined by the ratio of thrust to the propellant mass flow rate through the engine. The typical NTP engine would have and I_sp = 900 s, which is twice that of chemic propulsion systems. During a Mars manned mission the engine would be run for a total of 4 hours. One 60 minutes to accelerate from Globe to Mars, followed by a one hour deceleration burn. The same burn down cycles would occur on a return trip. Thus it is quite critical for the fuel elements to retain their stability during these burns.

The United states was involved in the production and testing of NTP engines during the period of 1955–1972. This was the Rover/NERVA plan which tested 20 prototype reactors during this menstruation. These prototypes included fuel test reactors, a safety reactor and prototype engines. Figure 2 beneath shows a test of one of the NERVA engines. This engine reached an Isp of ∼850 s during a ii hour burn. Twenty prototype reactors were ground tested. Fuel forms evolved over the duration of the program [ii, 3, 4, five, half-dozen, 7, 8, 9].

Figure 2.

Examination of NERVA engine.

The fuel elements used during testing were of varying compositions. These were coated graphite-matrix elements followed by advanced fuel elements consisting of UC-ZrC-C and all carbide elements ((U, Zr)C) [5, 6, 7, viii]. Most of the testing was performed using the coated graphite fuel elements. These elements were full length (52″) with a hexagonal cross section (0.75″ flat-to-flat) and 19 axial holes for propellant menstruum. These elements were arranged to create a cylindrical reactor core. The NERVA/Rover program proved NTP to exist a viable technology [9]. Several prototype reactors were produced which survived multiple restarts and power levels over 4000 MW, thrust levels of 250 klbf, maximum propellant outlet temperatures of 2550 K, a maximum net specific impulse of 850 s and over an hour of continuous operation [ix]. The fuel elements had to perform various tasks. The elements contained fissile material (UO₂ or UC₂) and graphite every bit a moderator. The fuel elements also functioned as structural components.

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3. Present-24-hour interval NTP fuels research

Shortly there are a number of fuels under consideration for NTP. These include graphite composites, tricarbides (U-Zr-Nb)C and CERMETS (MUO_two and MUN). At Marshall Space Flight Middle, we are concentrating on CERMETS (Westward/UO_two, Mo/UO_ii, W/UN, Mo/Un) and tricarbides. Consolidation of these CERMETS and tricarbides has been performed using RF induction furnaces, hot pressing and spark plasma sintering (SPS).

With respect to CERMET processing a number of approaches have been utilized in order to obtain a compatible distribution of the fissile fabric within the metal matrix. These include traditional powder processing techniques and coating the fissile material with metal using chemic vapor deposition (CVD) [x]. Recently, a fluidized bed reactor to coat UO_two with tungsten was tested [11]. CVD shows swell promise, however, there are technical issues due to the complexity of the experimental CVD apparatus and the CVD process. These issues lead to expense and long reaction times to employ the advisable thickness coatings. Thus, a technique for obtaining a uniformly coated spherical UO_ii particles was adult [12]. In this technique a small amount of high density polyethylene binder (0.25 w/o) is added to a mixture of tungsten and UO_two particles. The powders are then mixed thoroughly using a turbula, then heated and mixed on a magnetic stir plate.

Traditional sintering methods tin can exist used to densify W/UO₂. Both hot pressing and hot isostatic pressing have been used. There are drawbacks to these two methods including incomplete sintering and dissociation of UO_ii at loftier temperatures, pressures and long sintering times. Another consequence is a problem of exaggerated grain growth which tin can occur under these processing conditions.

A sintering method which has been shown to be a reasonable alternative to these traditional method is Spark Plasma Sintering (SPS) [13, 14, fifteen]. SPS leads to higher densities at lower temperatures and processing times while minimizing grain growth. Grain growth is detrimental to densification during the sintering process. In one study, UO_two was produced past hot pressing, however it was found that a large number of pores were present on the grain faces which led to a smaller grain purlieus contact area [16]. In this same study it was observed that grains which had not undergone exaggerated grain growth had pores at the grain corners. It has been observed that pores located on grain faces have greater mobility than those at grain corners and ultimately end up inside the grains [17, 18]. Joule heating is utilized in SPS which results in passing a current through the powder during sintering [19]. A pulsed electric current is utilized in SPS which leads to two different operating temperatures: the average temperature and the maximum temperature. The average temperature is lower than the melting bespeak of the materials. During current belch, material is transported past a plasma across pores of the matrix. While the pulse is off, the matrix cools quickly, and this lead to condensation of the fabric vapor within regions where there is mechanical contact betwixt grains. This mechanism leads to necking betwixt grains. There have been a number of studies accept using SPS to consolidate tungsten and a surrogate or tungsten and UO₂. In one study W/CeO_two was sintered using SPS [19]. In a 2nd report W/UO₂ was densified using SPS [xx]. A shortcoming in both of these studies was the segregation of the tungsten and the oxides. In the W/UO₂ study, the materials were mixed in a turbula for 1 hour and then hot isostatic pressed [21]. This result was a segregated CERMET due to the differences in pulverization sizes (Westward-15 μm, UO₂-200 μm) and density differences where size differences made the largest difference. In Figure 3 on tin can plainly see segregation in the sintered CERMET.

Effigy iii.

Scanning electron micrographs of hot isostatic pressed West/UO₂. Dark surface area on left is UO₂ while on right shows higher magnification image of UO_ii.

Studies [22, 23] were undertaken to eliminate this segregation using an inexpensive, simple technique. Depleted UO₂ particles were obtained from Oak Ridge National Laboratory. These particles had an average size of 200 μm. Tungsten powder with a particle size of 5/15 μm was purchased and used as the matrix fabric. In club to coat the UO_two particles with tungsten, a pulverisation processing technique was developed. In this technique, loftier molecular weight polyethylene pulverisation was milled to approximately ane μm in diameter. Next a mixture of 50 chiliad of threescore vol% UO₂, forty vol% W and 0.25 wt% polyethylene pulverization were thoroughly mixed for 45 minutes in a turbula. This pulverisation was placed in a 400 ml Pyrex beaker and then stirred on a hot plate for 10 minutes above the drib indicate of the polyethylene (101°C). This procedure was repeated until 500 g was produced. The pulverization was shipped to the Middle for Space Nuclear Research in Idaho Falls, Idaho for sintering in the SPS. Thirty ane grams was placed in a graphene die for sintering. Samples were densified at 1600, 1700, 1750, 1800 and 1850°C. Samples were heated at a rate of 100°C/minute to the sintering temperature. The pressure level was increased by 10 Mpa/minute to 50 Mpa. After soaking at the maximum temperature for 20 minutes, the pressure was decreased by 10 Mpa/minute to 5 Mpa and the temperature was decreased by xx°C/minute to room temperature.

Density was obtained using the Archimedes method. Carbon content was analyzed using instrumental gas analysis (EAG, NY). Scanning electron microscopy with energy dispersive x-ray analysis was performed on all samples. Microstructural and chemical analyses were carried out by using transmission electron microscopy (TEM) and atom probe tomography (APT) techniques. TEM and APT specimens were prepared at phase boundaries using elevator-out methods with a focused ion beam (FIB). The size of each TEM lamella was x × 10 μm. TEM characterization was carried out using a FEI Tecnai G2 F30 STEM FEG equipped with energy dispersive x-ray spectrometry (EDS). The EDS analyses were washed in Scanning TEM (STEM) mode with a axle size of i nm. APT was carried out using a CAMECA Spring 4000X 60 minutes. APT data reconstruction was done using the CAMECA IVAS software.

Figure 4 shows a scanning electron micrograph of UO₂ particles coated with tungsten powder.

Figure 4.

Scanning electron micrograph showing UO_two particles coated with tungsten powder.

In Figure 4 one can note that the UO₂ particles are almost completely covered with the tungsten powder. The polyethylene binder is viscid to a higher place its drib signal (101°C) and coats both the UO₂ particle and tungsten particles which when subsequently mixed together results in the image in Figure 4. The mixing temperature was 140°C which led to a binder viscosity of 140 cP. As the mixture was stirred, the nearly spherical UO_ii particles rolled around the bottom of the beaker and were coated with the tungsten particles.

As can exist seen in Table 1 below, the density is relatively high at a sintering temperature of 1600°C and gradually increases upwardly to 1800°C and jumps to 99.46% of theoretical density at 1850°C.

Table 1.

Sintering temperature versus % theoretical density for SPS W/UO₂.

The lower sintering temperature densities marshal well with what was previously reported for which the density was reported as 97.9% of theoretical for Westward-Re/UO₂ at 1500°C and forty Mpa practical pressure using SPS [20]. The higher sintering temperatures and increased practical force per unit area used in this study can business relationship for an increment in density seen in Table one. It is also thought that decreasing the cooling rate to 20°C/minute could also have contributed to further densification. At lower cooling rated the insulated die volition retain estrus which will permit more densification.

Effigy 5 below is a low magnification SEM micrograph of the sample sintered at 1850°C.

Effigy 5.

SEM of SPS sintered W/UO₂ at 1850°C.

This effigy shows the distribution of UO₂ particles within the tungsten matrix. The UO₂ particles are the darker almost spherical particles in the lighter grayness tungsten matrix. As tin can be seen, the distribution of UO_ii particles is virtually compatible with the tungsten matrix. Obtaining a uniform mixture of disparate particle sizes is extremely hard using traditional powder mixing techniques without the help of a binder. At that place four backdrop which give ascent to segregation in pulverisation mixes are: particle size difference, variations in particle density, along with shape and resilience [24]. Particle size divergence has been shown to be the most of import factor [21]. Segregation of powders causes fluctuations in the size distributions of particles and this leads to variations in majority density which can impact the desired properties. There are 3 mechanisms of segregation which can occur during mixing and vibration. Vibration is oftentimes used to increase packing density in the powder which in leads to higher sintered densities. Segregation tin occur during mixing when fine particles travel further than fibroid particles during the mixing operation. If a mass of particles is disturbed so that individual particles movement, a rearrangement tin take place. This is termed percolation. Over time, gaps between particles occur, which allows particles from above to motion downward, while a particles from another location replaces them. When the powder mass contains different size particles, pocket-size particles will autumn through the large particle interstices leading to segregation. Percolation occurs when the mass of particles is disturbed due to a shear stress within the particle mass. This miracle is explained in which a large particle causes an increase in pressure in the region below information technology which compacts the cloth and stops the particle from moving downward. Upward movement allows fines to run in under the coarse particle and these in plow lock in position. If the vibration intensity is large enough the larger particles will drift to the pulverization surface. The pulverization procedure described higher up which uses polyethylene binder, overcomes this difficulty using a minimum amount of folder which burns out during the SPS process and is drawn away from the sample by the vacuum system. In all the sintering temperatures listed in Tabular array one the vacuum was ∼2 × ten⁻³ Torr. The binder acts as an adhesive for the W/UO₂ powders eliminating the disparate particle size effect. It was found that the carbon content for the mixed powders was 0.025 wt%, while the sintered samples were below the detectable limit which is in parts per million.

Figure 6 below shows EDS maps for the sample sintered at 1850°C.

Effigy half-dozen.

SEM (left figure), EDS maps (UO_two—bluish and Westward—orange) for sample sintered at 1850°C.

In this effigy one can see the SEM in the figure on the left and the ten-ray maps on the center and right figure. The UO₂ particles are blue and the tungsten matrix is orangish. Ane can also note some fracturing of the UO₂ particles. This is virtually probable due to the pressure level during sintering but could likewise be caused during the grinding and polishing operation. This should be avoided to lessen the probability that uranium tin escape and diffuse to the tungsten matrix and to the fuel surface. One can also note some UO_two particle-particle contact which can lead to hot spots in the fuel during functioning. This in turn could lead to disruption of the fuel chemical element.

Using loftier resolution manual electron microscopy (HRTEM) information technology is possible to image the boundaries between UO₂ particles and the tungsten matrix. Figure 7 below shows a typical region in the sample sintered at 1850°C.

Figure 7.

HRTEM image of boundary of West/UO₂ for sample sintered at 1850°C: (a) is the low resolution purlieus and (b) is the high resolution image of this boundary.

This type of boundary is typical for all samples except for the one sintered at 1600°C which also showed an dissonant third stage. This is shown in Figure viii below.

Figure eight.

TEM of sample sintered at 1600°C showing stage U_0.aneWO₃.

There was an dissonant phase which was identified as U_0.oneWO_three, space grouping Pm-3m (221) which is a cubic structure. This was based on its electron diffraction blueprint and by because the diminutive ratio of U:W = 1:10 which is consistent with EDS results. Tungsten trioxide, WO_iii, has a monoclinic structure with a space group mP32. It is possible that this anomalous phase is WO₃ with uranium contamination, since U_0.1WO₃ phase has not been previously identified in the literature. Since the space group and crystal structure are different for these two phases, this could be a new phase. High resolution, high intensity ten-ray diffraction could be performed on all sintered samples to make a definite determination. It could be that the U_0.1WO₃ phase forms due to the availability of oxygen vacancies from the UO₂ reduction due to sintering in vacuum. The EDS line scan across the W/UO₂ purlieus for the sample sintered at 1850°C is shown in Figure 9a.

Figure ix.

EDS line browse across the W/UO₂ boundary for sample sintered at 1850°C in (a) and the results are in (b).

Effigy 9a shows the length of the line scan beyond the boundary. In Figure 9b it can exist seen that the uranium has diffused approximately xv nm into the tungsten matrix. The greenish line is the uranium bend and one measures where it crosses over the blue bend (tungsten). For all other sintered samples, it was seen that the uranium diffused approximately 10 nm into the tungsten matrix. The atom imaging probe analysis for the sample sintered at 1850°C is shown below in Effigy 10.

Figure 10.

3-D element maps from UO_ii particles and atomic percent from sample sintered at 1850°C.

It can be seen from Figure 10 that the uranium and oxygen were present in the form of UO, UO_ii and UO_iii. The nitrogen present is near likely from the nitrogen gas backfill used during SPS at room temperature after cooling. Silicon was observed for all samples except the one processed at 1750°C. Its origin is unknown but almost probable is an impurity picked up during grinding and polishing. The carbon present in the sample which is from the polyethylene binder used during powder processing. The above data led to a formula given as UO_i.95 which is slightly sub-stoichiometric. The samples sintered at 1600, 1650 and 1700°C were also calculated to have this same formula. The only difference was for the sample candy at 1750°C which had the formula UO₂.

The loss of uranium from the UO₂ particles and uranium migration into the tungsten matrix can be understood in terms of the generation of oxygen vacancies during sintering in a vacuum environment. An reaction for UO_two if oxygen vacancies are abundant is given past Eq. (ane).

With the loss of oxygen there are ii possible defect reactions that tin occur. The first reaction is electronic compensation leading to the creation of oxygen vacancies and electrons. This is shown in Eq. (2).

Ionic substitution can lead to the germination of oxygen vacancies and reduction of the metal oxide on their sites as shown below in Eq. (3).

The result of either of these reactions will exist a sub-stoichiometric uranium oxide and gratis uranium every bit shown in Eq. (4).

The gratuitous uranium from this reaction is and then available to diffuse into the tungsten matrix. This mechanism occurs due to Fick's law of diffusion. The importance of the presence of gratis uranium in sintered W/UO₂ samples cannot be overstated. These materials will be exposed to hydrogen gas in a thermal cycling environment during engine operation. When thermal cycling takes place in a hydrogen surroundings, hydrogen will penetrate into the tungsten matrix by both grain boundary and majority diffusion. The hydrogen can then combine with the free uranium leading to uranium hydride. Uranium hydride can also be formed past reaction with the UO_two particle. This is shown in Eqs. (5) and (vi) below.

The costless uranium has a melting signal of ∼1130°C and will rapidly lengthened along the tungsten grain boundaries and form UH₃ at ∼225°C. The formation of UH_iii leads to large increases in volume and which can event in tungsten grain separation. This grain separation creates avenues for migration of UO_two to the CERMET surface. This results in the loss of UO_two and can lead to mechanical failure. The gratis uranium non only forms UH₃, just can likewise reoxidize to form UO_ii. Both mechanisms effect in a large volume expansion and loss of mechanical integrity. At that place is besides a departure between isothermal and cyclic heating. It has been shown that cycling heating results in more than fuel loss than isothermal heating in hydrogen [25].

It has been establish that oxides such as ThO_ii, Ce₂O₃ and Y_iiO₃ reduce fuel loss when added to the CERMET powder [25]. The observation was that the oxide additives did non increment the solubility of uranium in UO₂, but stabilized UO₂ against oxygen loss. Two mechanisms were proposed to explain the stabilization confronting oxygen loss: (i) oxide additives lower the partial tooth free energy of oxygen in the UO₂. This precludes the possibility of forming gratis uranium upon cooling and (2) when metal oxide is added to the CERMET powder, uranium is transformed to a hexavalent country. This hexavalent country precludes the formation of uranium metal. The UO₂ maintains an oxygen-to-metallic ration of ii.0–2.one by forming a defect lattice structure. To maintain electrical neutrality, the uranium ions will be in the hexavalent state. U ^{iv +} cannot exist reduced to the metal in the presence of U ^{6 +}. Thus, the initial loss of oxygen from the CERMET will be accompanied past oxygen vacancies rather than the formation of free uranium. The use of hyperstoichiometric uranium oxide (UO_2 + x). UO₂ CERMETS in which the O/U ratio of the starting composition was varied between one.93 and 2.05 was studied. It was shown in this written report that at that place was minimal effect of varying this ratio. Thus the most likely candidate to stabilize UO_two during sintering and thermal cycling in hydrogen volition be a rare globe oxide add-on.

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4. Conclusions

In this chapter a brief history of the nuclear thermal propulsion programme was given. A present-twenty-four hour period research into processing and properties of nuclear fuel elements was discussed. In detail Westward/UO₂ which was spark plasma sintered was discussed. Uranium migration into the tungsten matrix was observed for all samples. The presence of uranium was explained in terms of oxygen vacancy generation due to processing in vacuum and the migration of the uranium past Fick's police of improvidence. Possible solutions to this trouble were also discussed.

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Acknowledgments

The author would like to thank the Nuclear Thermal Propulsion role at Marshall Infinite Flight Centre for funding this work. The author would also like to thank the Eye for Space Nuclear Research for performing the spark plasma experiments and the Heart for Advanced Energy Studies for performing TEM and atom probe measurements. Both institutes are located in Idaho Falls, Idaho, USA.

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Conflict of interest

There is no disharmonize of involvement represented by this work.

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Source: https://www.intechopen.com/chapters/66178

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