David D. Dixon

Design of a 25‐kWe Surface Reactor System Based on SNAP Reactor Technologies

A Hastelloy‐X clad, sodium‐potassium (NaK‐78) cooled, moderated spectrum reactor using uranium zirconium hydride (UZrH) fuel based on the SNAP program reactors is a promising design for use in surface power systems. This paper presents a 98 kWth reactor for a power system the uses multiple Stirling engines to produce 25 kWe‐net for 5 years. The design utilizes a pin type geometry containing UZrHx fuel clad with Hastelloy‐X and NaK‐78 flowing around the pins as coolant. A compelling feature of this design is its use of 49.9% enriched U, allowing it to be classified as a category III‐D attractiveness and reducing facility costs relative to highly‐enriched space reactor concepts. Presented below are both the design and an analysis of this reactor’s criticality under various safety and operations scenarios.

Design of a Low Power, Fast-Spectrum, Liquid-Metal Cooled Surface Reactor System

In the current 2005 US budget environment, competition for fiscal resources make funding for comprehensive space reactor development programs difficult to justify and accommodate. Simultaneously, the need to develop these systems to provide planetary and deep space‐enabling power systems is increasing. Given that environment, designs intended to satisfy reasonable near‐term surface missions, using affordable technology‐ready materials and processes warrant serious consideration. An initial lunar application design incorporating a stainless structure, 880 K pumped NaK coolant system and a stainless/UO2 fuel system can be designed, fabricated and tested for a fraction of the cost of recent high‐profile reactor programs (JIMO, SP‐100). Along with the cost reductions associated with the use of qualified materials and processes, this design offers a low‐risk, high‐reliability implementation associated with mission specific low temperature, low burnup, five year operating lifetime requirements.

Experimental Demonstration of a Heat Pipe–Stirling Engine Nuclear Reactor

Abstract Los Alamos National Laboratory and Glenn Research Center with the help of National Security Technologies demonstrated the use of a nuclear fission system as a power source that transferred heat via a water-based heat pipe to a small Stirling engine–based power converter to produce electricity. This experimental setup demonstrated that a small reactor based on heat pipes and Stirling engines is possible and produces a system with well-characterized nuclear feedback between the reactor and the power conversion system. This paper describes the experimental setup, modeling of the system, and results that confirm the basic physics of the experiment.

FRINK — A Code to Evaluate Space Reactor Transients

One of the biggest needs for space reactor design and development is detailed system modeling. Most proposed space fission systems are very different from previously operated fission power systems, and extensive testing and modeling will be required to demonstrate integrated system performance. There are also some aspects of space reactors that make them unique from most terrestrial application, and require different modeling approaches. The Fission Reactor Integrated Nuclear Kinetics (FRINK) code was developed to evaluate simplified space reactor transients (note: the term “space reactor” inherently includes planetary and lunar surface reactors). FRINK is an integrated point kinetic/thermal‐hydraulic transient analysis FORTRAN code — “integrated” refers to the simultaneous solution of the thermal and neutronic equations. In its current state FRINK is a very simple system model, perhaps better referred to as a reactor model. The “system” only extends to the primary loop power removal boundary condition; h...

Development of High Fidelity, Fuel-Like Thermal Simulators for Non-Nuclear Testing

Non‐nuclear testing can be a valuable tool in the development of a space nuclear power or propulsion system. In a non‐nuclear test bed, electric heaters are used to simulate the heat from nuclear fuel. Work at the NASA Marshall Space Flight Center seeks to develop high fidelity thermal simulators that not only match the static power profile that would be observed in an operating, fueled nuclear reactor, but also match the dynamic fuel pin performance during feasible transients. Comparison between the fuel pins and thermal simulators is made at the outer fuel clad surface, which corresponds to the outer sheath surface in the thermal simulator. The thermal simulators that are currently being developed are designed to meet the geometric and power requirements of a proposed surface power reactor design, accommodate testing of various axial power profiles, and incorporate imbedded instrumentation. Static and dynamic fuel pin performances for a proposed reactor design have been determined using SINDA/FLUINT the...

PID Control Effectiveness for Surface Reactor Concepts

Control of space and surface fission reactors should be kept as simple as possible, because of the need for high reliability and the difficulty to diagnose and adapt to control system failures. Fortunately, compact, fast‐spectrum, externally controlled reactors are very simple in operation. In fact, for some applications it may be possible to design low‐power surface reactors without the need for any reactor control after startup; however, a simple proportional, integral, derivative (PID) controller can allow a higher performance concept and add more flexibility to system operation. This paper investigates the effectiveness of a PID control scheme for several anticipated transients that a surface reactor might experience. To perform these analyses, the surface reactor transient code FRINK was modified to simulate control drum movements based on bulk coolant temperature.

DYNAMIC CHARACTERISTICS OF COMPACT FAST-SPECTRUM REACTORS

Abstract This paper focuses on some of the unique dynamic characteristics of compact fast-spectrum reactors. The study is limited to the characteristics that are relatively independent of how the reactor is integrated into a complete power system. Some of the well-established characteristics of compact fast-spectrum reactors are that point kinetics is generally very accurate for these systems and that temperature and burnup reactivity feedback mechanisms are relatively small and simple. Beyond this, there are two unique aspects of highly reflected fast reactors (e.g., space reactors) that do not occur in more traditional reactors. First, the neutron reflector has a very important impact on dynamic performance, and in some cases the temperature coefficient of the radial reflector is higher than that of the fuel. The thermal time constant of the reflector is much longer than that of any component in the core, which requires all reflector temperature and expansion effects to be modeled individually. Second, reflected neutrons have a much longer fission life span than in-core neutrons. In effect, this creates additional delayed neutron groups, referred to as geometric delayed neutron groups. These groups can have life spans orders of magnitude longer than neutrons that do not leave the core, and have much higher worth due to moderation. For compact beryllium reflected reactors there is also a measurable delayed group of photo-induced neutrons that result from delayed gammas. Another characteristic of compact fast-spectrum reactors is simplified control and the ability to passively handle a wide range of transients without control. Various transient analyses are presented that were performed by the Fission Reactor Integrated Nuclear Kinetics (FRINK) code, which facilitates near-term compact reactor design and development by providing a transient analysis tool. In its current state FRINK is a very simple system model, and the “system” only extends to the primary loop power removal boundary condition; however, this allows the simulation of simplified transients (e.g., loss of primary heat sink, loss of flow, etc.).