1. EXPLORATORY DYNAMIC MODELING OF A NUCLEAR POWERED BRAYTON
CYCLE FOR NUCLEAR ELECTRIC PROPULSION
August 6, 2004
Robert M. Edwards
Professor of Nuclear Engineering
The Pennsylvania State University
228 Reber Building
University Park, PA 16802
(814) 865-0037
rmenuc@engr.psu.edu
NASA Glenn Research Center
Instrumentation and Controls Division
ABSTRACT
The Jupiter Icy Moons Orbiter (JIMO) is envisioned to be a nuclear powered electric ion-
propulsion system. A dynamic model of such a system will be needed to study the dynamic
interactions of components and to verify that control structures and algorithms are adequate to
ensure accomplishment of mission objectives. However, the design of the specific JIMO system
has not yet begun that can provide data to initiate legitimate system integration studies. This
research describes a possible approach to simplified modeling a closed Brayton cycle power
conversion unit and its coupling to a nuclear reactor model. Data for this exploratory modeling
was gleaned from a series of reports that were prepared by, or at the direction of, NASA in the
early 1990’s.
SUMMARY
System integration is often performed after components are far along in their design or even
construction. Required component changes that are identified late in the design and construction
process can result in costly modifications and inefficient or ineffective system performance. The
effort of this research is thus meant to provide an early indication of the data needed and
techniques employed in system integration. Figure 1 is used to describe the characteristics of the
system integration or control engineering effort for a central station nuclear power plant. Around
the periphery of the diagram are the development of detailed models of system components, such
as the reactor core with its neutronics and thermal hydraulics, the steam generator and electrical,
mechanical components like pumps, valves and turbines, and finally pressure vessel and piping.
The analysis performed in the component areas invariably involves the development of large
computer design codes with special emphasis on steady state full power conditions. Each
component code invariably grows to use all the available computer horsepower and uses an
analysis platform that is incompatible with other component codes. It is then the job of system
integration to work with these large component “round pegs” and fit them into the smaller “square
holes” of system integration. System integration therefore invariably works with simplified
models of all components in a platform that lends itself to the use of specialized system integration

2. software tools, again invariably incompatible with the component design codes. The job of system
integration is a challenging and diplomatic one because it interacts with different organizations and
philosophies. Component specialists may be reluctant to reveal data, thinking that they are the
only ones that can deal with problems that come up in their component.
The following publications were used to conduct the exploratory modeling:
1. Scoping Calculations of Power Sources for Nuclear Electric Propulsion, ORNL CR-
191133, 1994
2. Brayton Power Conversion System Parametric Design Modeling for NEP, NASA
contractor report CR-191135, 1993
3. Modular Modeling System (MMS): A Code for the Dynamic Simulation of Fossil and
Nuclear Power Plants: Overview and General Theory, EPRI CS/NP-2989, 1983
4. Preliminary Results of a Dynamic System Model for a Closed-Loop Brayton Cycle
Coupled to a Nuclear Reactor, Steven Wright, Sandia National Lab.
5. “Dynamic Analysis and Control System Design for an Advanced Nuclear Gas Turbine
Power Plant”, a dissertation in Mechanical Engineering, MIT 1990.
The first two references present steady state design data for a reactor component and Brayton
power conversion unit, respectively. The codes are capable of designing the reactor and Brayton
unit to specific power levels and compatible steady state operating conditions; however, executable
versions of these old codes were not readily available. The reports do provide full power steady
state example output data for a 50 MW reactor and 500 kWe Brayton PCU, respectively.
A dynamic model of a representative fuel pin of the 50 MW reactor was created to interface with a
model of the 500 kWe Brayton unit. Modeling of the Brayton unit was conducted using a lumped
parameter equation set presented in the 1983 Modular Modeling System Report. The paper from
Wright and the MIT dissertation suggested a way to model the compressor and turbine of the
Brayton Unit.
Figure 2 presents the top level diagram of the nuclear powered Brayton Unit in the Mathwork’s
SIMULINK dynamic modeling system. The Brayton unit simulation model was organized around
the turbine/intermediate heat exchanger (ihx) and the compressor/recuperator components. Within
the turbine/ihx block, a model of the 50 MW fuel pin is interfaced to the tube side of the ihx to
provide the heat source for the Brayton Unit. A simplified shaft dynamics model is represented at
the bottom of the diagram and simulation studies of the open loop response were conducted for
step reductions in power removed from the shaft (load). As expected, the shaft rotational speed
increases. But a counter intuitive reactor response shows an increase in power when the load is
decreased. These results are consistent with those presented in the paper by Wright (reference 5)
for another nuclear powered Brayton unit. Wright concludes that “the reactor control system will
have to be used to reduce the reactor power when load decreases”.
Much work lies ahead for system integration of the components of JIMO, and an early initiation of
system integration capabilities will make the project progress more smoothly.

3. Figure 1: System Integration
turbine
exhaust
high
pressure
recuperator
exhaust
Pi
Ti
md0
N
mdRx
rho
Po
To
mdi
J
nr
turbine_ihx
rho
mdot
Pi
Ti
mdo
N
Po
To
mdi
J
compressor-recuperatorScope9
Scope8
Scope7
Scope6Scope5Scope4
Scope3
Scope2
Scope10
Scope1
Scope
Pout
Output Point1
Output Point
1
s
N^2
Input Point2
Input Point1
sqrt(u(1))
Fcn1
2*u(1)*(60/(2*3.14159))^2/3.195
2*(Pin-Pout)*(60/2pi)^2/J
Figure 2: SIMULINK model of a Nuclear Powered Brayton Cycle
Steam
Generator &
Electrical:
Pressure
Vessel &
Piping
Core Design:
neutronics
thermal
hydraulics
Mechanical
Pumps, valves
turbines
System
Integration
(Control
Engineering)
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