Water and liquid-metal heat pipes are considered for redundant and passive cooling of nuclear reactors and redundant operation of light-weight heat rejection radiators of space reactor power systems. The systems operate continuously for many years independent of the Sun, are enabling of deep space exploration and could provide 10’s of kilowatts of electrical power for lunar and Martian outposts. Space reactors typically operate at 900 K – 1400 K could be cooled using liquid-metal heat pipes with either sodium (900 –1100 K) or lithium (1100 K–1400 K) working fluids. The heat rejection temperatures dictate the working fluid of the radiator heat pipes; potassium for 600 – 800 K, rubidium for 500 – 700 K, and water for < 500 K. These working fluids would be frozen prior to starting up the power systems in orbit or at destination. The startup of water and liquid-metal heat pipes from a frozen state is complex, involving a number of non-linear mass and heat transport processes that require implementing appropriate procedures, depending on the type of working fluid. This paper reviews operation and design constraints pertinent to the uses of water and liquidmetal heat pipes in space reactor systems, and the modeling capabilities of the startup from a frozen state. In addition to models validation, results of design optimization of liquid-metal and water heat pipes in a number of space reactor power systems are presented and discussed.

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Uses of liquid-metal and water heat pipes in space reactor power systems

Integrated power electronic module (IPEM) planar interconnect technologies offer opportunities for improved thermal management by allowing thermal access to the upper side of the power devices. In this paper, the feasibility of using miniature heat pipes to achieve effective double-sided cooling is investigated by analyzing the complete thermal circuit associated with the power device. A nominal case was modeled using the ANSYS(tm) finite element software in a single-sided and double-sided configuration. The numerical model predicted that the double-sided configuration would result in a 13/spl deg/C reduction in the maximum temperature compared to the single-sided case, for the same 100 W/cm/sup 2/ power dissipation in the semiconductor die. This corresponds to a 15% decrease in the maximum temperature rise relative to ambient or a similar increase in allowable power dissipation. Twenty-eight percent of the heat was removed from the upper side of the IPEM in the double-sided case. An additional benefit associated with double-sided cooling was a significant reduction in the spatial temperature gradients along the surface of the IPEM which would translate to lower thermally induced stress and higher reliability. The sensitivity of the numerical predictions to important parameters; including the dielectric conductivity, contact conductance, and heat sink characteristics are numerically investigated. An experimental fixture was fabricated and used to measure a miniature rectangular heat pipe's performance characteristics and the solder joint resistance at its evaporator and condenser interfaces in order to validate the numerical model inputs and demonstrate the required heat pipe capacity. The tested heat pipe was limited to approximately 80 W/cm/sup 2/ heat flux in a vertical, evaporator-over-condenser orientation. This limit was not observed in a vertical, gravity-assisted orientation for applied heat flux up to 125 W/cm/sup 2/. Equivalent heat pipe resistances of approximately 0.12 and 0.08 K/W were measured in these orientations, respectively. The contact resistance of the indium solder joint was measured and found to be approximately 0.1 cm/sup 2//spl middot/K/W.

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Double-sided IPEM cooling using miniature heat pipes

Design and performance analysis of the nuclear reactor’s lithium heat pipes for a 110‐kWe Heat Pipes‐Segmented Thermoelectric Module Converters (HP‐STMCs) Space Reactor Power system (SRPS) are presented. The evaporator length of the heat pipes is the same as the active core height (0.45 m) and the C‐C finned condenser is of the same length as the STMC panels (1.5 m). The C‐C finned condenser section is radiatively coupled to the collector shoes of the STMCs placed on both sides. The lengths of the adiabatic section, the values of the power throughput and the evaporator wall temperature depend on the radial location of the heat pipe in the reactor core and the number and dimensions of the potassium heat pipes in the heat rejection radiator. The reactor heat pipes have a total length that varies from 7.57 to 7.73 m, and a 0.2 mm thick Mo‐14%Re wick with an average pore radius of 12 μm. The wick is separated from the Mo‐14%Re wall by a 0.5 mm annulus filled with liquid lithium, to raise the prevailing capillary limit. The nominal evaporator (or reactor) temperature varies from 1513 to 1591 K and the thermal power of the reactor is 1.6 MW, which averages 12.7 kW for each of the 126 reactor heat pipes. The power throughput per heat pipe increase to a nominal 15.24 kW at the location of the peak power in the core and to 20.31 kW when an adjacent heat pipe fails. The prevailing capillary limit of the reactor heat pipes is 28.3 kW, providing a design margin ⩾ 28%.

Reactor Lithium Heat Pipes for HP-STMCs Space Reactor Power System

OF DISSERTATION Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy Engineering The University of New Mexico Albuquerque, New Mexico

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Numerical and Experimental Investigations of a Rotating Heat Pipe

The heat pipe cooled thermionic reactor (HPTI) relies on in‐core sodium heat pipes to provide a redundant means of cooling the 72 thermionic fuel elements (TFEs) and 36 driver fuel pins which comprise the 40 kWe core assembly. In‐core heat pipe cooling was selected for the reactor design to meet the requirements for a system design with the potential to achieve a high survivability level against natural and man‐made threats and one that possesses no‐mission ending single point failures. A detailed study was performed to determine the potential in‐core heat pipe geometries which could be developed for an HPTI concept. Requirements and performance estimates were developed for two in‐core heat pipe geometries. Both nominal and faulted operating conditions were evaluated using a two‐dimensional thermal model of the core to assess TFE and driver fuel pin temperature profiles. A bow tie in‐core heat pipe geometry was selected as the optimum design using a HPTI honeycomb core structure.

Thermionic in-core heat pipe design and performance

A steady-state heat pipe analysis program, HTPIPE, operational on CRAY computers, is documented. HTPIPE has two input/output (I/O) options: pressure and temperature profiles along the heat pipe, and calculation of performance limits. Pressure and temperature profiles are calculated based on user-specified boundary conditions, which may be any two of the following: power throughput, evaporator exit temperature, source temperature, and sink temperature. The performance limits represent the maximum heat transport limits at a specified evaporator exit temperature. The calculated performance limits are the capillary, viscous, sonic, entrainment, and boiling limits. User input is interactive, with the available option of parametric variation during a single program execution to facilitate heat pipe design optimization. The hydrodynamic model is described. Program I/O is detailed and supplemented with examples. A program listing and a flow chart are included in this manual. HTPIPE is consistent with the theories and correlations used for heat pipe design at Los Alamos National Laboratory to date. 13 refs., 2 figs., 3 tabs.

Merrigan

Papers

HTPIPE: A steady-state heat pipe analysis program: A user's manual