Nuclear Engineering ETDs

Publication Date

5-2009

Abstract

This work developed designs and analyses models of centrifugal flow compressor and radial-inflow turbine of integrated Brayton Rotating Units (BRUs) for space reactor power systems. These models calculate the polytropic efficiency of the compressor and turbine, and the electric power and thermal efficiency of the BRUs and the power system for shaft rotational speed of 30 - 55 krpm, reactor power of 120 - 471 kWth, turbine inlet temperatures of 900 - 1149 K, and He-Xe working fluids of 15 and 40 g/mole. The turbine and compressor models are coupled to the developed model of the rotating shaft in the integrated model of the BRU. Two BRUs designs with induction magnet alternators, UNM-BRU-1 and -2, are developed for peak electric power of 41.9 kWe at peak thermal efficiency of 26.7%, when operating at a shaft rotational speed of 45 krpm, compressor and turbine inlet temperature of 400 K and 1149 K, and thermal power input of 157 kWth UNM-BRU-1 is designed for He-Xe working fluid of 40 g/mole, while UNM-BRU-2 is for He-Xe of 15 g/mole. The effect of replacing the induction magnet alternator in UNM-BRU-1 and -2 with state-of-the-art, more efficient Permanent Magnet Alternator (PMA) is also investigated. PMAs increased the peak electric power and peak thermal efficiency of the UNM-BRU-1 and -2 from 41.9 kWe and 26.7% to 44.2 kWe and 28.5%. A third BRU design, UNM-BRU-3 with PMA, provides a peak electrical power of 54.2 kWe at thermal efficiency of 34.5%, when operating at the same nominal conditions as UNM-BRU-1 and -2. UNM-BRU-3 uses He-Xe of 40 g/mole.

The performance space reactor power system with a Submersion-Subcritical Safe Space (S^4) reactor and three Closed Brayton Cycle (CBC) loops, each with a UNM­ BRU are calculated. These results are for shaft rotational speeds of 30 - 55 krpm, reactor thermal powers of 120 - 471 kWth (or 40 - 157 kWth for BRU) and turbine inlet temperatures of 900 - 1149 K. The integration of this power system avoids single point failures in reactor cooling and energy conversion. The reactor core is divided into three sectors that are hydraulically independent, but thermal and neutronically coupled. Each reactor sector is coupled to a separate CBC loop with a BRU, and separate heat rejection radiator panels. Thus, a loss-of-coolant (LOC) in one CBC loop, would not force the power system to shutdown. Instead, it will continue to operate but at a reduced reactor thermal power. The fission power generated in the sector experiencing LOC will be conducted to adjacent sectors and removed by the circulating gas in these sectors.

The results show that the most efficient power system is that with 3 UNM-BRU-3. For this system, when operating at shaft rotational speed of 45 krpm, decreasing the turbine inlet temperature from 1144 K to 900 K, decreases maximum thermal efficiency and net electrical power of the system from 33.8% and 159.3 kWe to 21.1% and 99.4 kWe, but increases system pressure at exit of the compressor from 1.05 to 1.3 MPa. The thermal efficiency and net electrical power of the system at a turbine inlet temperature of 900 K are attractive for space missions. Operating at such relatively low temperature (~ 900 K) makes it possible to use common steel structure, prolonging the operation life and enhancing the reliability of the power system.

Sponsors

Institute for Space and Nuclear Power Studies for funding this research

Document Type

Thesis

Language

English

Degree Name

Nuclear Engineering

Department Name

Nuclear Engineering

First Committee Member (Chair)

Mohammed El-Genk

Second Committee Member

Jean-Michel Tournier

Third Committee Member

Norman F. Roderick

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