تجزیه و تحلیل عملکرد از سوخت فشرده ذرات پلوتانیا پوششی برای واحدهای بخاری های ایزوتوپ

Coated plutonia particle fuel has been proposed recently for use in radioisotope power systems and radioisotope heater units for a variety of space missions requiring power levels from milliwatts to tens or even hundreds of watts. The 238PuO2 fuel kernels are coated with a strong layer of ZrC designed to fully retain the helium gas generated by the radioactive decay of 238Pu. A recent investigation has concluded that helium retention in large-grain (⩾200 μm) granular and polycrystalline fuel kernels is possible even at high-temperatures (>1700 K). Results of performance analysis showed that this fuel form could increase by 2.3–2.4 times the thermal power output of a light weight radioisotope heater unit. These figures are for a single-size (500 μm) particles compact, assuming 10% and 5% helium gas release respectively, and a fuel temperature of 1723 K, following 10 years of storage. A binary-size (300 and 1200 μm) particles compact increases the thermal power output of the RHU by an additional 15%.

Recently, a coated plutonia particle fuel compact was proposed for potential use in advanced radioisotope heater units (RHUs) and radioisotope power systems (RPSs) (Sholtis et al., 1999). The fuel compact consists of ZrC-coated 238PuO2 fuel kernels dispersed into a graphite matrix with a packing density for single size particles of up to 62.5% by volume. This packing density increases to ∼73% when two particle sizes with a diameter ratio of 4 are used. The 238PuO2 fuel kernels are covered with a 5-μm thick, pyrolytic graphite (PyC) inner coating for protection during the application of the outer ZrC coating by chemical vapor deposition (CVD). The CVD process is also used to apply the inner PyC coating. The ZrC, a very strong material that is ductile at temperatures in excess of 2000 K (Storms, 1962 and Ramgopal, 1974), serves as the primary containment vessel of the plutonia fuel kernel and the helium gas generated by the radioactive decay of 238Pu. The thickness of the ZrC coating depends not only on the fuel temperature, the helium gas release fraction, and the storage time, but also on the plutonia fuel microstructure. Polycrystalline plutonia kernels fabricated using sol–gel or thermal plasma processes offer great promise for almost full helium retention, even at high-temperatures in excess of 1700 K (El-Genk and Tournier, 2000). Fig. 1(a) shows a cross-sectional view of a coated plutonia fuel particle and Fig. 1(b) shows an illustration of a binary-size coated particle fuel compact (CPFC). Full-size image (24 K) Fig. 1. (a) A cross-sectional view of a coated plutonia fuel kernel. (b) Binary-size, CPFC. Figure options The graphite or carbon–carbon composite matrix of the fuel compact is designed to accommodate the thermal expansion of the fuel particles and is spongy and structurally weaker than the ZrC coating. Thus, upon impact on solid surfaces following a launch abort or re-entry accident, cracking of the CPFC is likely to occur through the graphite, leaving the coated fuel particles intact. The design concept of the CPFC is analogous to that of the “pomegranate” fruit, in which the holding structure is spongy as well as weak under applied tensile stress to protect the fruit seeds. In addition, the carbon-based matrix of the compacts is perfectly compatible with the aeroshell material of RHUs (Schock, 1980 and Schock, 1981). In addition to their structure strength, the coated plutonia fuel particles offer a promise for enhanced safety. The fuel kernels are intentionally sized (⩾300 μm) to prevent any adverse radiological effects. They are non-respirable, non-inhalable, and if ingested, would simply be excreted with no radiological effects (Hoover, 1999). In addition, this coated fuel form offers excellent design flexibility as the CPFC could be made into different shapes and sizes to provide thermal power from milliwatts to tens or even hundreds of watts. 1.1. Potential applications The CPFC can be made into heating tapes, buttons, or paints as RHUs or miniaturized RPSs for satellites, spacecraft, and planetary exploration probes. For example, a button-like CPFC heat source could be used with miniaturized thermoelectrics (TEs) to provide electrical power in the milliwatt range for more than 10 years. The heat rejected by the TE couples could be used for thermal management of the space probes, thus achieving 100% energy utilization. Larger size CPFC in the form of pellets or disks could be used in higher power RHUs as well as in RPSs to produce power in the range from one to tens or even hundreds of watts. One could envision fabricating thin CPFC wafers the size of an Oreo cookie or smaller in which a thin layer of fine-weave pierced fabric (FWPF, a carbon–carbon weave used in re-entry aeroshells) is used on both sides of the fuel compact for thermal protection. These “cookies” like CPFCs could be attached to surfaces more readily than the more bulky, present-day LWRHUs. Fig. 2(a) shows an example of a miniaturized CPFC-RHU for thermal management of electronics circuit boards. The CPFC fuel, used in conjunction with low-temperature TE couples (Caillat et al., 2000), could be used in milliwatt CPFC-RPSs. These RPSs provide both thermal and electric power for circuit boards, as shown in Fig. 2(b). Full-size image (17 K) Full-size image (17 K) Fig. 2. (a) A miniaturized CPFC-RHU for supplying thermal power to circuit boards on board of spacecraft. (b) Milliwatt CPFC-RPS and RHU for supplying both thermal and electric power to circuit boards on board of spacecraft. (c) Conceptual design of a high-efficiency (∼25%) CPFC-RPS for space exploration missions. Figure options The proposed CPFC design offers four basic barriers to prevent potential release of the fuel and the helium gas. The first barrier is the large-grain or polycrystalline plutonia fuel matrix; the second is the multi-layer coating of the plutonia fuel particles; the third barrier is the fuel compact matrix; and the fourth barrier is the FWPF aeroshell (Fig. 1 and Fig. 2). High performance, reliable, low mass, and long operation lifetime (5–10 years) RHUs and RPSs are required for several new NASA missions planned over the next few years. Examples include the Saturn Rings Observer, Solar probe, Europa Lander, Cryobot and Hydrobot, and the Titan Explorer missions which call for electrical power requirements in the 20–200 W range and 6–10 years mission duration. CPFC-RPSs could meet these power requirements and present several advantages such as scalability, reliability, and retention of helium gas. Modular CPFC-RHUs could be developed for a variety of thermal and electrical applications up to 20 W thermal or electric, that are 30–50% lighter than current LWRHUs (Schock, 1981 and Johnson, 1997). The CPFC-RHUs could be used either by themselves, or in conjunction with low-temperature TE converters having an efficiency of 4–6%, currently under development at the Jet Propulsion Laboratory, to provide up to 150 mW at power densities >100 mW kg−1 (Fig. 2(b)). The CPFC-RHUs are easily scalable to meet mission power requirements ranging from a few watts to hundreds of watts, or even a few kilowatts. They could be coupled with advanced, 18% efficient vapor anode, multi-tube alkali-metal thermal-to-electric converter (AMTEC) cells (Fig. 2(c)). Higher efficiencies and specific powers could be obtained by cascading the AMTEC cells with segmented TEs (Caillat et al., 2000) for the bottom cycle (Fig. 2(c)). For example, a CPFC-RPS which cascades an 18% efficient AMTEC cells (El-Genk and King, 2001) operating between 1100 and 650 K, with a 9% efficient segmented TE converter (Caillat et al., 2000) operating between 650 and 350 K, would have an effective conversion efficiency of about 25%. The CPFC-RHUs could also be used in conjunction with high-temperature segmented TE (η∼15%; Caillat et al., 2000), Stirling engine units (η∼23–30%; Mason, 2000), or even a He–Xe Brayton engine (η∼23–28%; El-Genk, 2001), depending on the electric power and mass requirements for the power system. CPFC-RPSs with higher energy conversion efficiency are quite attractive because they require a smaller amount of 238PuO2 fuel. The coated plutonia particle fuel form is also ideal for use in scientific probes requiring both thermal and electric powers and in which the appropriate type and shape of the RHU can be fabricated to optimize the design, operation, and the functionality of the probe. Some of the space missions that employ planetary probes for in-situ analysis of surface materials require that the He gas be fully retained in order to avoid contaminating the environment and skewing the sensitive measurements. Such an option and design flexibility are not attainable using the current LWRHU (Schock, 1981 and Johnson, 1997) and general purpose heat source (GPHS) designs (Schock, 1980). The coated fuel particles could also be compacted into a brick form for high power CPFC-RHUs that could be lighter than current GPHSs (Schock, 1980). In current LWRHUs, the refractory cladding of the plutonia fuel pellets is kept at relatively high-temperatures (>1173 K) to ensure sufficient ductility when impacting solid surfaces. In addition to being very heavy, these claddings (platinum–30%rhodium in LWRHUs and iridium in GPHSs) must be kept well below their melting temperatures and those of any eutectics that could form in a solid propellant fire. These temperatures are maintained with the aid of multi-layered, low conductivity, PyC insulation sleeves (Fig. 3). Conversely, the CPFC has no temperature constraint: the ZrC coating forms an eutectic with carbon at the very high-temperature of 3123 K (Storms, 1962). Thus, most of the internal thermal insulation sleeves and the refractory cladding in the LWRHU (Fig. 3) could be replaced with CPFC. This would result in higher thermal power, at lower or same mass, or in smaller size and lower mass, for the same thermal power. Full-size image (21 K) Fig. 3. Current LWRHU (Johnson, 1997). Figure options 1.2. Objectives During FY 99, an exploratory effort sponsored by the Department of Energy was initiated to investigate the potential of the coated plutonia particles fuel form for RHUs and address fabrication and performance issues. The specific tasks investigated were to: 1. review the fabrication technology of coated plutonia fuel particles; 2. review the release mechanisms of helium gas in small-grain (7–40 μm) granular plutonia pellets in GPHSs and LWRHUs, and examine the applicability of these mechanisms to the He release from large-grain (⩾200 μm) and polycrystalline fuel kernels (El-Genk and Tournier, 2000); 3. review the spectrum of credible launch and re-entry accident environments that the coated particle fuel could potentially experience. Based on this review, design and functional requirements for coated particle fuel were established (Sholtis, 2001); 4. develop a design and performance model of coated fuel particles to investigate the impact of using single- and binary-size CPFC on the thermal power output of a RHU. Also quantify the effects on the RHU thermal power of the helium gas release fraction, fuel temperature, and storage time before launch; and 5. identify future research and testing needs to confirm the coated particle fuel's potential operation and safety promise. This paper provides a summary of the work done under this joint exploratory research effort and presents and discusses in details results on the performance of the CPFC-RHU.

The potential of coated 238PuO2 fuel particles compact for future use in advanced RHUs and RPSs is investigated. A stress and design model of the coated plutonia fuel particle was developed and used to investigate the performance of both single- and binary-size CPFC-RHUs as a function of the helium gas release fraction, for a 10-years storage before launch and fuel temperature up to 1723 K. This fuel temperature corresponds to the predicted peak value during an accidental re-entry heating pulse. Results indicated that large-grain (⩾200 μm) or polycrystalline 238PuO2 fuel kernels would retain most of the helium gas generated by the radioactive decay of 238Pu. Recent estimates of the He gas release from large-grain and polycrystalline 238PuO2 fuel kernels showed that He release in large-grain (⩾200 μm) 238PuO2 fuel kernels at 1723 K could be less than 7% and even lower in polycrystalline fuel kernels. At fuel temperatures ⩽1000 K, the He release will be nil. Large-grain fuel kernels could be fabricated using binderless agglomeration or similar processes, while the polycrystalline fuel kernels could be fabricated using sol–gel or thermal plasma processes. Although these processes have successfully been used in the fabrication of UO2 and mixed-oxide fuel kernels, they have not been demonstrated for the fabrication of 238PuO2 fuel kernels. In addition, using CVD techniques to apply the PyC and ZrC coatings on plutonia kernels is yet to be demonstrated. Performance analyses of a conservatively designed CPFC-RHU indicate that its thermal power could be 2.3 and 2.4 times that of the LWRHU, at essentially the same total mass. The CPFC-RHU uses the same FWPF aeroshell, and two of the three inner PyC insulation sleeves of the LWRHU. These performance figures of the CPFC-RHU are for a single-size (500 μm) coated 238PuO2 fuel particle compact, 11% and 7% as-fabricated fuel porosity, and 10% and 5% helium gas release, respectively. In the CPFC-RHU, the fuel pellet and its refractory cladding and the inner PyC insulation sleeve in the LWRHU are replaced with CPFC. Using a binary-size (300 and 1200 μm) CPFC increases the thermal power of the CPFC-RHUs by an additional 15%. The CPFC is a promising fuel form for use in advanced RHUs and RPSs. In addition to enhancing the thermal power output, it offers enhanced safety and unique design flexibility, since it could be fabricated in different sizes and shapes. CPFC-RHUs and RPSs could meet the thermal and electric power needs for future spacecraft and planetary exploration in the range from a few milliwatts to tens and even hundreds of watts, for more than 10 years. Several issues for future research, given in order of priority and order of conduct, are recommended. 1. Investigate and demonstrate the fabrication techniques of large-grain (⩾200 μm) and polycrystalline plutonia fuel kernels and the application of the PyC and ZrC coatings using CVD processes. 2. Perform helium gas release tests from large-grain and polycrystalline fuel kernels, both coated and uncoated, to confirm the recent estimates of the He gas release (El-Genk and Tournier, 2000). 3. Fabricate CPFC-RHUs and perform fracture impact tests and analysis to provide data to benchmark models. These data and analysis results could also be used to guide the development and the selection of the most appropriate graphite matrix material for CPFCs. Ultimately, mechanical, thermal, and aeroablation testing of coated particle fuel in simulated accident environments will subsequently be needed.