Nuclear Engineering ETDs

Publication Date

9-1-2015

Abstract

Future human exploration of the solar system is likely to include establishing permanent outposts on the surface of the Moon. These outposts will require reliable sources of electrical power in the range of 10's to 100's of kWe to support exploration and resource utilization activities. This need is best met using nuclear reactor power systems which can operate steadily throughout the long ~27.3 day lunar rotational period, irrespective of location. Nuclear power systems can potentially open up the entire lunar surface for future exploration and development. Desirable features of nuclear power systems for the lunar surface include passive operation, the avoidance of single point failures in reactor cooling and the integrated power system, moderate operating temperatures to enable the use of conventional materials with proven irradiation experience, utilization of the lunar regolith for radiation shielding and as a supplemental neutron reflector, and safe post-operation decay heat removal and storage for potential retrieval. In addition, it is desirable for the reactor to have a long operational life. Only a limited number of space nuclear reactor concepts have previously been developed for the lunar environment, and these designs possess only a few of these desirable design and operation features. The objective of this research is therefore to perform design and analyses of long operational life lunar reactors and power systems which incorporate the desirable features listed above. A long reactor operational life could be achieved either by increasing the amount of highly enriched uranium (HEU) fuel in the core or by improving the neutron economy in the reactor through reducing neutron leakage and parasitic absorption. The amount of fuel in surface power reactors is constrained by the launch safety requirements. These include ensuring that the bare reactor core remains safely subcritical when submerged in water or wet sand and flooded with seawater in the unlikely event of a launch abort accident. Increasing the amount of fuel in the reactor core, and hence its operational life, would be possible by launching the reactor unfueled and fueling it on the Moon. Such a reactor would, thus, not be subject to launch criticality safety requirements. However, loading the reactor with fuel on the Moon presents a challenge, requiring special designs of the core and the fuel elements, which lend themselves to fueling on the lunar surface. This research investigates examples of both a solid core reactor that would be fueled at launch as well as an advanced concept which could be fueled on the Moon. Increasing the operational life of a reactor fueled at launch is exercised for the NaK-78 cooled Sectored Compact Reactor (SCoRe). A multi-physics design and analyses methodology is developed which iteratively couples together detailed Monte Carlo neutronics simulations with 3-D Computational Fluid Dynamics (CFD) and thermal-hydraulics analyses. Using this methodology the operational life of this compact, fast spectrum reactor is increased by reconfiguring the core geometry to reduce neutron leakage and parasitic absorption, for the same amount of HEU in the core, and meeting launch safety requirements. The multi-physics analyses determine the impacts of the various design changes on the reactor's neutronics and thermal-hydraulics performance. The option of increasing the operational life of a reactor by loading it on the Moon is exercised for the Pellet Bed Reactor (PeBR). The PeBR uses spherical fuel pellets and is cooled by He-Xe gas, allowing the reactor core to be loaded with fuel pellets and charged with working fluid on the lunar surface. The performed neutronics analyses ensure the PeBR design achieves a long operational life, and develops safe launch canister designs to transport the spherical fuel pellets to the lunar surface. The research also investigates loading the PeBR core with fuel pellets on the Moon using a transient Discrete Element Method (DEM) analysis in lunar gravity. In addition, this research addresses the post-operation storage of the SCoRe and PeBR concepts, below the lunar surface, to determine the time required for the radioactivity in the used fuel to decrease to a low level to allow for its safe recovery. The SCoRe and PeBR concepts are designed to operate at coolant temperatures ≤ 900 K and use conventional stainless steels and superalloys for the structure in the reactor core and power system. They are emplaced below grade on the Moon to take advantage of the regolith as a supplemental neutron reflector and as shielding of the lunar outpost from the reactors' neutron and gamma radiation. The SCoRe and PeBR concepts are designed with cores divided into six and three sectors, respectively. The sectors are thermally and neutronically coupled but hydraulically decoupled. Each sector is served by a separate cooling loop(s), with independent energy conversion and heat rejection radiator panels. This combination of a sectored core and power system integration with multiple loops avoids single point failures in reactor cooling, energy conversion, and heat rejection. These unique attributes would allow the reactor power system to continue operation, but at lower power, in the event one of the sectors experiences a Loss of Coolant (LOC) or a Loss of Cooling (LOCo). The power system could thus maintain a vital supply of electrical power to the lunar outpost for crew life support. The performed multi-physics design and performance analyses of the SCoRe show that increasing the diameter of the UN fuel rods increases the core excess reactivity. The larger diameter rods, however, increase the NaK-78 coolant flow bypass near the walls of the core sectors. Scalloping the dividing walls of the core sectors produces a more even flow distribution. The use of 151Eu spectral neutron poison additive to the UN fuel ensures subcriticality during a water submersion accident, for the compact SCoRe core, with the highest excess reactivity and lowest mass. The redesigned Solid-Core Sectored Compact Reactor (SC-SCoRe) with a monolithic solid core of Oxide Dispersion Strengthened Molybdenum (ODS-Mo) achieves a long operational life of 21 years at a nominal power of 1,000 kWth. The high thermal conductivity ODS-Mo core structure allows the reactor to continue safe operation in the event that one of the core sectors experiences a LOC of LOCo. The ODS-Mo solid core readily conducts heat generated in that sector to the adjacent core sectors, to be removed by the flowing liquid metal coolant. The SCoRe power system with SiGe energy conversion is fully passive and load following. In addition, the decay heat is removed safely and passively following shutdown of the reactor at end of life. Neutronics and analyses show that the PeBR can achieve a very operational life of 66 years at a nominal thermal power of 471 kWth by fueling the reactor core on the Moon. This full power operational life is beyond what is possible with a reactor fueled at launch like the SC-SCoRe. Neutronics safety analysis of the fuel pellets transport canisters for the PeBR shows that they are made highly subcritical in a water submersion accident. This is achieved using a combination of favorable geometry and neutron absorbers. The DEM fuel loading simulation of the PeBR on the lunar surface demonstrates that the PeBR core sectors can be successfully fueled in lunar gravity. Post-operation storage analyses of the SC-SCoRe and PeBR concepts show that the radioactivity in the fuel decays away to a sufficiently low level within 300 years. The radiation field around the post-operational reactor at that time is low enough to allow humans to safely handle and retrieve the cores, for potential recovery of the valuable quantities of 235U remaining in the fuel.

Keywords

Space nuclear power, lunar surface power, lunar outpost, fast reactors, NaK

Sponsors

University of New Mexicos Institute for Space and Nuclear Power Studies, United States Nuclear Regulatory Commission'

Document Type

Dissertation

Language

English

Degree Name

Nuclear Engineering

Level of Degree

Doctoral

Department Name

Nuclear Engineering

First Committee Member (Chair)

De Oliveira, Cassiano

Second Committee Member

Hall, Chris

Third Committee Member

Cooper, Gary

Fourth Committee Member

Rodriguez, Salvador

Download

Share

COinS