Nuclear Design Analysis of SLHC Fuel Elements

Nuclear Thermal Propulsion

• Why Nuclear Thermal Propulsion?

• Background of Nuclear Thermal Propulsion and Carbide Fuel Development

• Processing and Testing of Pseudo-Ternary Carbide Nuclear Fuels

• Square Lattice Honeycomb (SLHC) Fuel Elements

• Thermal Hydraulic Design Analysis of SLHC Fuel Elements

• Nuclear Design Analysis of SLHC Fuel Elements

Nuclear Design Analysis Of A SLHC Space Nuclear Rocket Engine

ABSTRACT

The square-lattice honeycomb reactor is designed based on a cylindrical core that is determined to have critical diameter and length of 0.50 m and 0.50 c, respectively. A 0.10-cm thick radial graphite reflector, in addition to a 0.20-m thick axial graphite reflector are used to reduce neutron leakage from the reactor. The core is fueled with solid solution of 93% enriched (U,Zr,Nb)C, which is one of several ternary uranium carbides that are considered for this concept. The fuel is to be fabricated as 2 mm grooved (U,Zr,Nb)C wafers. The fuel wafers are used to form square-lattice honeycomb fuel assemblies, 0.10 m in length with 30% cross-sectional flow area. Five fuel assemblies are stacked up axially to form the reactor core. Based on the 30% void fraction, the width of the square flow channel is about 1.3 mm. The hydrogen propellant is passed through these flow channels and removes the heat from the reactor core. To perform nuclear design analysis, a series of neutron transport and diffusion codes are used. The preliminary results are obtained using a simple four-group cross-section model. To optimize the nuclear design, the fuel densities are varied for each assembly. Tantalum, hafnium and tungsten are considered and used as a replacement for niobium in fuel material to provide water submersion sub-criticality for the reactor. Axial and radial neutron flux and power density distributions are calculated for the core. Results of the neutronic analysis indicate that the core has a relatively fast spectrum. From the results of the thermal hydraulic analyses, eight axial temperature zones are chosen for the calculation of group average cross-sections. An iterative process is conducted to couple the neutronic calculations with the thermal hydraulics calculations. Results of the nuclear design analysis indicate that a compact core can be designed based on ternary uranium carbide square-lattice honeycomb fuel. This design provides a relatively high thrust to weight ratio.

INTRODUCTION

A nuclear thermal propulsion system has some advantages over the conventional chemical system for a manned mission to a distant planet, such as Mars. A nuclear thermal rocket produces an enormous energy per unit mass of fuel, and the energy-producing medium is separate from the thrust-producing propellant. From these differences, a nuclear thermal rocket with hydrogen propellant could produce a greater specific impulse (I_sp) than chemical propulsion systems. The higher specific impulse permits the nuclear rocket to accomplish its mission in a shorter time and to carry a larger payload. The separation between the energy producing medium and the thrust-producing propellant allows the nuclear rocket to utilize propellants of low molecular weight. With the low molecular weight propellant, the rocket will also have an increased thrust to weight ratio.

In 1960, the nuclear rocket was viewed as an essential part of a manned mission to Mars due to a much higher thrust to weight ratio that could be produced in these systems. The larger thrust would reduce the excursion time to Mars. The nuclear rocket program was part of the Rover Program. The first nuclear reactor for the purpose of nuclear rocket was named Kiwi. The development of nuclear rocket was continued under the experimental nuclear rocket (NRX) series. During the NRX project, a new nuclear rocket engine, NERVA, was developed. The Phoebus was the next nuclear rocket engine in the NRX series. Following the Phoebus, new high temperature fuels were developed under the Pewee and the Nuclear Furnace projects. Finally, in 1973, the nuclear rocket engine development program was terminated. Although the nuclear rocket engine program was considered to be a technical success, changing national priorities resulted in its termination (Angelo, 1985).

The work presented in this paper, is a modification of the NERVA core by using a new nuclear fuel design. It is attempted to reduce the weight of the nuclear rocket engine and simplify the core design without scarifying the thrust level. This paper presents a preliminary neutronic analysis of the new core design.

CORE DESCRIPTION The core is designed based on using a set of five disk-shaped square-lattice honeycomb fuel assemblies, which are configured in a cylindrical core. The bare core has the approximate critical diameter and height of 0.50 m and 0.50 m, respectively. Figure 1 shows the schematics of the reactor core and the reflectors. The core is fueled with solid solution of 93% enriched (U,Zr,Nb)C, which is one of several ternary uranium carbides that are under consideration for this concept. The fuel is to be fabricated as 2 mm grooved (U,Zr,Nb)C wafers. The fuel wafers are used to form square-lattice honeycomb fuel assemblies, 0.10 m in length with 30% cross-sectional flow area, shown in Figure 2 (Furman, 1999). Table 1 presents the properties of the fuel in the core.

FIGURE 1. The square-lattice honeycomb complete dimensions including reflectors

FIGURE 2. The square-lattice honeycomb fuel assembly

TABLE 1. The properties of the core

 

Five fuel assemblies are stacked up axially to form the reactor core. Based on the 30% void fraction, the width of the square flow channel is about 1.3 mm. The hydrogen propellant is passed through these flow channels and removes the heat from the reactor core. A 0.10-m thick radial graphite reflector in addition to a 0.20-m thick axial graphite reflector are used to reduce neutron leakage from the reactor. In addition a 0.35-m thick 30% void graphite reflector is placed at the bottom of the core, as shown in Figure 1.

 

METHODS OF CALCULATION

To perform nuclear design analysis, a series of neutron transport and diffusion codes are used. The preliminary results are obtained using a 16-group cross-section model. Table 2 shows the 16-group models used in the calculation. The codes that were used to perform the analysis are COMBINE (Grimesey, 1990) and VENTURE (Shapiro, 1990).

 TABLE 2. 16-group energy models used in COMBINE.

 

The temperatures in the core are varied from 300^oC to 2200^oC. To account for these temperature variations in the core, an average temperature is used for each fuel assembly. A separated COMBINE run was performed to obtain group constants for each sections of the core. Each fuel assembly is modeled as a unit cell. Figure 3 shows the schematics of a unit cell. The multi-group average cross-sections are used to perform a two-dimensional core nuclear design analysis, utilizing the VENTURE code. The core is divided into seven regions, five representing fuels and two representing the axial and radial reflectors. VENTURE was used to determine the axial and radial neutron fluxes, and the axial and radial power distributions in the core.

FIGURE 3. A unit cell for COMBINE cross-section calculation.

RESULTS AND DISCUSSION

Results of core design calculations are presented in Figures 4-8. Due to the lack of enough moderation, the spectral characteristics of the core are different from those of the NERVA core.

FIGURE 4. Axial flux and radial flux distribution for neutron group 1 to group 4.	Figure 4 shows the axial flux and radial flux distributions of neutrons in groups 1 through 4, and it is also shows that the majority of the neutrons are in the group 3, which correspond to the energies from 111 to 821 KeV. This indicates that the core has a relatively fast spectrum. The presence of niobium in the fuel reduces the activity of the core under excessive moderation, which may be resulted from accidental water submersion. It is also possible to use tantalum, hafnium or tungsten, instead of niobium to further reduce the possibility of water submersion criticality of the reactor.
FIGURE 5. Axial flux and radial flux distribution for neutron group 5 to group 8.	Figure 5 presents the axial flux and radial flux distributions for neutrons in-groups 5 through 8.
FIGURE 6. Axial flux and radial flux distribution for neutron group 9 to group 12.	Figure 6 presents the axial flux and radial flux distributions of neutrons in-groups 9 through 12. The effects of the radial and axial reflectors begin to appear in the group 9, shown in Figure 6.
FIGURE 7. Axial flux and radial flux distribution for neutron group 13 to group 16.	Finally, Figure 7 shows the axial flux and radial flux distributions of neutron in-groups 13 through 16. The values of fluxes presented in these figures are relative fluxes, and they are not adjusted for power of the system. According to Figure 7, the effect of a 0.20-m and a 0.35-m thick axial graphite reflector are shown by the flux spikes in the reflector regions. Figure 7 also shows the effect of a 0.10-m radial reflector on the peaks in the reflector regions.
FIGURE 8. Axial power and radial power distributions in the core.	Figure 8 shows the axial power and radial power distributions in the core. Results of these calculations indicate that the reduced core dimensions contribute to a higher value of neutron leakage from the core. From this information, an option would be to control the reactor by regulating the neutron leakage from the core. A convenient option is to use radial windows to control the radial leakage of neutrons, thereby controlling the reactor power operations. Figure 8 shows that the dominant majority of power is produced in the upper half of the core. The axial and radial power peaking factors are determined to be 2.3 and 1.7, respectively.

CONCLUSION

A detailed neutronic analysis of the square-lattice honeycomb nuclear rocket core was performed. Results indicated that the core, which utilizes non-moderated uranium ternary carbide fuels, has a relatively fast spectrum. It was also shown that it is feasible to control the reactor power by regulating the radial neutron leakage from the core. A key feature of the core design is the use of the square-lattice honeycomb fuel, which is more amenable to simple and less expensive manufacturing. Further analysis needs to be performed to establish the optimum design of the core and fuel materials.

 

REFERENCES

Angelo, Jr., J. A. and D. Buden. Space Nuclear Power, Orbit Book Company, Inc., Malabar, Florida, 1985.

Furman, E. "Thermal Hydraulic Design Analysis of Ternary Carbide Fueled Square-Lattice Honeycomb Nuclear Rocket Engine," in 16th Symposium on Space Nuclear Power and Propulsion, edited by M.S. El-Genk et al., New York, AIP Conference Proceedings, 1999.

Grimesey, R.A., Nigg, D.W., and Curtis, R.L. COMBINE/PC-A Portable ENDF/B Version 5 Neutron Spectrum and Cross-Section Generation Program, EG&G Idaho, Inc., Idaho Falls, Idaho, 1990.

Shapiro, A., Huria, H.C., and Cho, K.W. VENTURE/PC Manual A Multidimensional Multigroup Neutron Diffusion Code System Version 2, EG&G Idaho, Inc., Idaho Falls, Idaho, 1990.