Subsequent studies have drawn heavily on the experience from these projects. Sample return mission payloads including a lander and re-ascent vehicle are likely to be several tons in mass. A 6 year round trip to Mars or a 10 year round trip to a Jovian moon, with a year’s stay time in each case, needs tens or hundreds of kWe depending on specific impulse (Isp) and propellant mass used.
The studies have indicated that for higher power levels closed cycle Brayton thermal to electrical power conversion is significantly more efficient. Although, new materials may help raise thermo-electric energy conversion from 5 to say 10%, the 17 to 20% efficiency claimed for the Brayton cycle still brings significant specific mass benefit. The relative simplicity of gas cooled reactors is an advantage in terms of long life, but experience to date has been with liquid metal cooling. Liquid metal cooling is more complex (needs additional pumps and heat exchanger), and requires significantly more energy to heat the coolant and reactor to an operating temperature for commissioning or for ‘cold’ re-starts.
Fixed, body mounted metallic radiators have high mass and area unless the operating cycle temperature can be raised significantly (radiator size varies with temperature to a fourth power law). Deployable radiators are lower mass but larger area and need an additional heat exchanger. There is the added complexity of packaging a large structure for launch and deploying it safely. Lighter materials such as carbon-carbon composites offer new options for fixed radiators.
A review of technical options for a 30 kWe and a 200 kWe nuclear power generator revealed a fair degree of commonality between the two findings.
Design constraints (identified in HiPER ) are:
o Compatibility with an Ariane 5 ECA launch: >800km in-orbit commissioning altitude, radiator compatible with the Ariane 5 fairing, Launch safety criteria (e g water immersion).
o Ten years of operation within a 15 years’ lifetime,
o Specific mass of 25 kg/kWe for a 200 kWe generated power or better,
o High temperature reactor (fast indirect or epi-thermal direct) and conversion system (Brayton cycle),
o Resilience to sudden load fluctuations,
The preferred options were pin-fuel fast reactors for indirect inter-cooled and recuperated (ICR) Brayton because of compact, low mass features or particle-based fuel reactors for Direct ICR Brayton cycle.
The operating principle is ‘load following’ through negative thermal control, accepting a degree of ‘thermal lag’, and containment with beryllium reflectors. Control rods give a more compact, low mass core but control drums require fewer shield penetrations and are simpler to rotate reliably. Both electrical and pneumatic drive should be compatible with the temperature field and high radiation environment (dry lubrication may be required).
A layered shadow shield design (Beryllium, Lithium Hydride, Tungsten or with Beryllium Oxide to overcome Lithium hydride thermal expansion sensitivity) was adopted for both 30kWe and 200kWe; Shadow angles were up to 28° and a 22.5m separation boom was planned.
Both 30kWe and 200 kWe considered turbine rotation of ~ 45Krpm but for 200kWe turbine blade creep life above 1100 K will require new materials. Indirect ICR CBC is an alternative for both 30kWe and 200kWe (more compact reactor but added complexity of liquid metal pumping, and melting liquid metal for commissioning, cold starts, etc).
Both fixed and deployable radiators are options for the 200 kW power level. At very high temperature operation, fixed radiators become compact and mass and area competitive. Also, it should be mentioned that Russia is developing a droplet radiator which allows lower system temperatures but requires a large area when deployed.
The turbo alternator output may be alternate current (AC) or direct current (DC) if rectified. AC may be used to reduce harness mass but there are potential complications in interfacing with EP or a high power instrument and the DC battery.
Power Management and Distribution (PMAD)
Turbo-alternator output could be tailored to EP or radar tube operating voltage, resulting in a reduced PMAD mass.
Battery size, coupling (DC/AC converter) and functions (commissioning, load ballast, etc) shall be studied.
The selection of CBC Brayton power conversion for both 30kWe and 200 kWe allows a high degree of focus in the technical options. It is also helpful because of the inherent ‘scalability’ of the technology. The main issues to be resolved are the trade-off between liquid metal and gas cooled reactors and the operating temperatures. Materials which allow higher temperature operation for 10 year lifetimes will tend to make the relative simplicity of gas cooled systems more attractive.
Public Acceptance And Dissemination
The importance of preparing public outreach study/material for nuclear space technology to be developed and proposed to EC / Europe was recognised. A similar approach had been used for the Prometheus programme (using the Keystone Centre in Colorado). The recent launch of RTGs and RHUs in the US still attracted small protest groups. It was essential to assemble a team that both understood the technical issues and the public concerns. This included both traditional concern about nuclear dangers and also whether it was a good way to spend government money (the case for private investment did not look strong). The US experience was that the management of public acceptance could be a relatively small part of the budget if tackled early and effectively.
Uranium enrichment was considered necessary to design a sufficiently compact reactor for space. This is one factor why a Public Acceptance assessment study is an early priority task before to take into account the suited recommendations. The fact that at launch the uranium is "new" and so weakly radio-active as shown in fig. 3 with the handling operations of the fuel is a major fact for enabling a positive Public Acceptance of fission reactor for space applications.
Thus important public considerations are safety and that government will spend tax money wisely. The following need to be established:
o Definition of the economics of the technology,
o The need of sustainability: long term output of the technology.
o Good communication: avoid news like “millions burn down on launch pad crash”.
The safety guidelines for nuclear power sources in space are provided in numerous documents .
Space and nuclear safety experts from “big ESA member states” are drafting a technically sound European framework that:
o Provides a predictable, efficient, "workable" process for ESA missions,
o Addresses the main concerns of participating member states,
o Takes advantage of the existing European nuclear safety expertise and experience gained on the subject in US and Russia,
o Provides a technically sound basis for an early decision on processes, roles and responsibilities.
Europe is unlikely to fund enabling research for a space nuclear fission programme until an application (or range of applications) has been identified which is justified in terms of benefit, credibility and cost. It is difficult to determine benefit, credibility and cost until the enabling research has helped to quantify the performance which may be achieved. In this sense, a dual-mode program, to build compact reactors capable of both industrial power generation and space propulsion and power, could free this impasse. A way to start the iterative process would be a workshop  for the space science and exploration, space mission and spacecraft design and nuclear communities with a view to:
o Define specific research and development projects, including cost and schedule, to deliver the performance required for the identified science and exploration objectives based on the initial assessments made in the DiPoP project:
- High temperature reactor (including controls) and fuel materials research (potentially in collaboration with Generation IV civil nuclear power development),
- High temperature turbo-alternator materials research to overcome creep life limitations,
- Low mass, high temperature radiator materials (not-porous to helium) research,
- Low mass shielding configurations compatible with high temperature operation and efficient spacecraft architectures
- Mass efficient power management and distribution and associated safety features,
- In-orbit commissioning and end-of-life disposal,
o Identify trade-offs between objectives, performance, technical development, schedule and cost.
o Propose one or more candidate mission analysis to provide a baseline for evolution of the Technical Roadmap (in practice a family of mission analyses would be a sensible investment to establish a range of potential applications and give confidence of a multi-application program).
o Propose a program to achieve public awareness and secure public acceptance for a European space nuclear fission program.
Either ESA or the EC could sponsor a workshop (EC sponsorship is understood to be proposed). The output of the workshop and mission analysis can then provide a basis to determine specific enabling research projects in the EC Horizon 2020 programme and further mission analysis could be sponsored by ESA as part of the General Studies programme.
Past experience indicates that fission nuclear power generation is technically feasible. Subsequent studies indicate the need for significant technical development in Europe to realise the performance identified in the range of proposed applications.
From the range of applications for which space fission nuclear power is potentially necessary initial candidate selections of European missions are:
o Generating electrical services for a remote planetary outpost and high power instrument was selected for 30kWe.
o Earth threatening NEO deflection or outer planet orbital surveying mission for 200kWe fission nuclear electric propulsion. The performance needed for these applications would also support other identified applications.
Closed Brayton Cycle power conversion with either an indirect liquid metal cooled or direct gas cooled fast reactor is selected for both power levels.
Materials research into the high temperature operation (but with non oxidising gas) needed to achieve optimal mass efficiency for space reactors, Brayton turbo-alternators and radiators. Research is also needed into very high power electrical equipment and switching.
A representative review of the capabilities of European government organisations research centres, industry and universities indicated potential expertise and infrastructure for all aspects of a European space nuclear fission programme.
Generation IV civil terrestrial reactor research includes high temperature liquid metal and gas cooled projects ; there are many useful synergies, particularly in associated materials research..
Potential interest in a European space nuclear fission programme was expressed by many of the organisations contacted in the survey and covered all aspects. Evidence of sustainability of the programme is seen as a pre-requisite for both government and industry.
The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n°284081 for the Disruptive Technologies for Power and Propulsion (DiPoP) Study.
Koppel, Valentian, Blott, Jansen, Ferrari, Bruno, Herdrich, Gabrielli
[ 1] The Global Exploration Strategy: the Framework for Coordination, May 2007.
[ 2] HiP-AST-D-2.7-i1r1 HiPER Consolidated Mission Analysis 8th December 2011.
[ 3] IAC-10-A3.1.1 Assessing Space Exploration Technology Requirements as A First Step To-Wards Ensuring Technology Readiness For International Cooperation In Space Exploration by CSA, NASA, ESA and JAXA October 2010.
[ 4] IAA Commission III SG2 Nuclear Space Power and Propulsion Autumn 2007.
[ 5] HiPER Nuclear Power Generation Concept Design HiP-SEP-D-3.9-i1r0 dated 31st May 2011.
[ 6] HiPER Nuclear Power Generator Roadmap HiP-SEP-D3.8-ill0 dated 6th May 2011.
[ 7] Project of Creation of Heavy Spaceship with Megawatt-class NPPS. A. S. Koroteev, V. N. Akimov, C. A. Popov, 2010.
[ 8] European Working Group on Nuclear Power Sources for Space Report, March 2005. Section 6.2.1
[ 9] European Working Group on Nuclear Power Sources for Space Report, March 2005. Section 6.2.3.
[ 10] Principles Relevant to the Use of Nuclear Power Sources In Outer Space, 1992”, Principle 4 ammended by "Safety Framework for Nuclear Power Source Applications in Outer Space", Jointly published by the United Nations Committee on the Peaceful Uses of Outer Space Scientific and Technical Subcommittee and the International Atomic Energy Agency, Authors: JEG (Joint Expert Group of STSC and IAEA), A/AC.105/934, Printed by the IAEA in Austria 2009.
[ 11] Nuclear Power Sources Final Report, DiP-Sep-RP-002 D30.3 Nuclear Power Sources Final report, 2012, available soon on www.DiPoP.eu.
[ 12] International Atomic Energy Agency (IAEA), The Role of Nuclear Power and Nuclear Propulsion in the Peaceful Exploration of Space, Vienna, 2005.
[ 13] MEGAHIT@esf.org