ESA Document - Emits - ESA

s
HMM
Assessment Study
Report: CDF-20(A)
February 2004
page 404 of 422
Furthermore, the total mass of the systems is one of the key parameters for space reactors, while
virtually non-existent for designing classical terrestrial systems (except submarine reactors). To a
smaller extent this also holds true for its shape and size.
As a result of the above mentioned parameters, space reactor designs show higher reactor core
temperatures and operate with much higher enriched 235 U fuel. This leads to lower core
dimensions and mass, reduced radiator size and mass and higher efficiencies of the conversion
system. In numbers, the enrichment increases from natural-20% for terrestrial up to 93% for
space systems, and the core temperature increases from 400-573 K up to 900-1000 K.
Technically these changes imply the abandon of the pressurised water reactor design for either
liquid metal cooled or gas cooled cores. They furthermore demand the use of materials capable
of withstanding for long times high temperatures as well as high radiation (neutron) fluxes.
6.2.3 Approach
Given the high complexity of designing space nuclear reactors and especially the highly
interrelated subsystem dependencies, it was chosen to not develop a parametrical reactor model
with completely open input parameters but to base the assessment on two recently proposed
space reactor models.
For the present study, the two designs that are taken as reference were recently proposed by
European nuclear industry within an ESA contract especially for Martian surface power
generation purposes. They are both available to a technical design level sufficient for the present
study and correspond to the power level as well as lifetime requirements.
6.3 Reactor power subsystems
Some aspects of different reactor power system components are provided. The purpose of this
small section is to provide some basic elements to understand the choices and implications of the
reactor designs proposed.
Reactor core
• Thermal n core: larger (if small: limited core life due to burn-up), moderator, good
negative power coefficient (safety), rather for larger than10 MWe reactors
• Fast n core: enriched fuel, smaller, more stable power distr., low burn-up, small negative
power coefficient (safety), favourable in 100 kWe range
Conversion system
Static
Thermoelectric: mature, space proven; about 4% (adv.: cascades about7%)
Thermionic: mature, in core vs. out core systems, life-time limiting factors,
space reactor proven
AMTEC: immature, different for ionic and electronic condition, short
lifetimes, corrosion problems, 19-25%