Lunar Applications for Nuclear Thermal Propulsion

1. Lunar Applications for Nuclear Thermal Propulsion  Steven D. Howe 6/16/09

2. Agenda CSNR perspective Logical path Current projects

3. CSNR Perspective Goal of space exploration is understanding our �neighborhood�, i.e. the solar system Unmanned scientific missions for science and as a precursor to human missions Ultimate goal is the expansion of human civilization throughout the solar system

4. CSNR Perspective Current propulsion technologies are insufficient for human expansion past the Moon we need the �steamship� equivalent to the sailing ships of the past what is the next propulsion technology - fission, fusion, electric propulsion, sails, beams? According to the Independent Review Panel convened in 1999 to review the propulsion technologies examined in the NASA Advanced Space Transportation Program: �The Review Team categorized fission as the only technology of those presented [45 concepts were presented] which is applicable to human exploration of the near planets in the near to mid-term time frame��

5. Recent Assessments NASA�s Mars Architecture Study (Dec 2007) concluded that the NTR was preferred for human missions to Mars National Research Council committee (S. Howe served as one of 23 members) that reviewed the NASA Exploration Technology Development Program (ETDP) reported (8/21/08) that the one technical gap in program was no funding for the NTR.

6. Benefits of the NTR have been shown for several missions Moon - Reduce costs of implementing a Lunar Outpost Mars - Faster missions for humans; reduced radiation exposure; lower costs for cargo; adaptability to hazards Good Asteroid - rendezvous Bad Asteroid/comet - rapid interception Outer solar system � time to �first science� within a decade for orbitor missions to outer planets and to Kuiper Belt fly-through

7. Therefore, if we eventually need a NTR for human Mars missions, how do we develop a system that is reliable, safe, and known operational performance? Use the NTR to support lunar outpost development and cargo supply Get more mass to the Moon per Ares V launch cost savings Fewer launches Higher mission success probability Get operational experience Reliability data Find weak links for space ops Develop man-rating for a Mars mission

8. Lunar Trajectory Objectives Minimum ?V trajectory Time-of-Flight (TOF) is not a significant concern. Insertion into either equatorial or polar LLO

9. NTR-Based ESAS Architecture

10. Enhanced mission performance(2006 CSNR Summer Fellows study)

11. Can we develop and test a NTR in the current world According to the Independent Review Panel convened in 1999 to review the propulsion technologies examined in the NASA Advanced Space Transportation Program: �Previous studies during the Space Exploration Initiative prioritized the critical issues for developing a nuclear propulsion system as 1) ground testing, 2) fuels development, and 3) enhanced performance��

12. Ground testing - Sub-surface Active Filtering of Exhaust (SAFE) Nuclear furnace proved abiltiy to scrub exhaust Scaling to full power engines implies a costly faciltiy SAFE offers one cheaper option if proven feasible If fuel doesn�t leak, then cheaper scrubber is possible

13. Testing in the Current Environment INL/CSNR completed NTP testing assessment for NASA Prometheus program office (2007) Desert Research Institute sub-contract completed Validated previous SAFE evaluation by Howe et al in 1998 Produced design of a sub-scale proof-of-concept experiment for ~$1M

14. Tungsten Cermet Fuel Hot hydrogen compatibility Better thermal conductivity Potential for long life reactors High melting point (~3700 K) Resistance to creep at high temperatures Smaller reactor core then carbide fuels Good radiation migration properties �Cladding� from same metallic material Contains fission products and uranium oxide in fuel More radiation resistant than carbon

15. Tungsten Loss Rate

16. Accident Scenarios for Homogenous Core Design

17. Tungsten NTR Fuel elements

18. Fuels Development The requirements of the NTR place rigorous constraints on the fuel While �normal� power reactor fuel can�t work in the NTR, NTR fuel could work in a power reactor Development of one fuel form to serve both power and propulsion could ultimately be a cost savings for the program The 2009 CSNR Summer Fellows are examining concepts for high temperature power conversion to utilize the NTR fuel in a lunar reactor

19. Conclusions The benefits of using a NTR for many types of missions have been shown for many years The NTR opens the solar system to rapid exploration Testing and fuel development are major issues A single solution to these issues is the fuel form Most questions about the candidate fuel forms can be addressed for modest expense using electrically heated testing Development of one fuel form for power and propulsion could provide significant program savings

20. backups

21. Why aren�t nuclear rockets in use today? Concept proven during Rover/NERVA Performance demonstrated for high-thrust, restarts, lifetime TRL-5 or 6 demonstrated by 1969 37 years after the proof, we are still using chemical rockets with 50% of the performance

22. Tech summary Rover/NERVA demonstrated that a nuclear core at full power (keep the hot parts at 2550K and the cool parts cool) could operate for the require duration, have multiple restarts, produce high thrust, have high Isp, and operate safely Through the CY2000, some expertise remained in human resources and some parts remained in physical resources. While blueprints and documents remain regarding design, the rest is essentially gone. Thus, there is little carry over The major issues with the NERVA system were 1) mid-band corrosion (lifetime) and 2) radioactive effluent (impacts testing and space operations) Any new program will start with knowledge but no hardware and should be targeted to address the major issues

23. Issues - Emissions NERVA tests showed significant emission of radioactive gases and particulate during operation NTR performance benefit is enhanced if operations begin in LEO Emission of radioactive species into LEO may be precluded in public viewpoint Arguing relative amounts compared to galactic cosmic ray background does not erase the mental image of radioactivity raining down onto the Earth Radiation emitted by the operating NTR can impact �big observatories� indicating that a hot reactor may not be allowed to orbit but must be ejected on the first burn No periapsis pumping

24. Fractional release rate

25. Issues - proliferation Launch aborts must be considered Fast reactors offer less chance for criticality on submersion than epi-thermal systems but contain more fissile material Dispersion upon reentry is not attractive from an environmental impact perspective Even though the engine has no fission product inventory and is �cold� Engine should stay intact upon reentry Dropping a few hundred kilograms of fissile material into foreign states could be considered a high risk Could constrain launch profile Could dictate fuel form

26. Cladding Failure of Early NTR Designs

27. Lifetime of Cermet Fuels Not limited by erosion of tungsten-cermet fuels Actual limitation Quantity of nuclear material Integrity of non-nuclear rocket components Poison buildup Possible space-cold effects(ductile to brittle transition) Operation temperature(max Isp of ~950 s)

28. Design Benefits of a Fast Reactor Greater power density Lighter core design thanthermal reactors Burn-up of transuranics generated in the reactor Reflectors instead of moderating material Fast reactors can be controlled using the reflector systems with control drums

29. GE-710 HTGR PROGRAM 1962-1968 Accomplished a flexible, basic fuel rod design, assessed a fabrication process and evaluated performance objectives through both non-nuclear and in-pile testing Four different program objectives Gas cooled reactors (fast spectrum; open and closed loop operation) Gas cooled reactor for closed loop operation only Brayton cycle space power Fuel element technology development program

30. SINTERING STUDIES Consistent fuel loadings of 46wt% UO2 1-2 mm diameter W particles Crucible design to achieve desired density Sintering temperature to minimize fuel dissociation Minimization of CTE difference between fuel and cladding

31. Maintaining Thermal Subcriticality Boron-carbide control drumsabsorb excess neutrons Melting of the core wouldput it in a non-critical state Loss of the beryllium reflectorensures the reactor cannot go critical Addition of tungsten and rhenium absorb neutrons at the thermal energies 4 to 5 orders of magnitude greater than carbon

32. Thermal Poison: Rhenium-187

33. NTR Design

34. NTR-Based ESAS Architecture

35. NTR-Based ESAS Architecture

36. NTR-Based ESAS Architecture

37. Rocket Operation Parameters Single Reactor Specific Impulse = 850 s Thrust = 150 kN (34 klbf) Temperature = 2300 � 2500 K Hydrogen Flow Rate = 18.0 kg/s Thermal Power =  650 MW Cermet: W-Re(6.5 w/o)-UO2 (60 v/o, 93% HEU)

38. Fabrication of Frozen Pellet Bed samples using the SPS furnace

39. 2009 Summer Fellowship Topics Advanced Heat Exchanger Concepts � NASA is pursuing technology development of Fission Surface Power (FSP) systems for the lunar and Mars surface. A potential FSP concept uses a pumped liquid metal reactor cooling loop coupled to either Stirling or Brayton power conversion. System performance is very sensitive to this heat transfer interface. The participants will develop heat exchanger concepts that are efficient, lightweight, reliable, compatible with the working fluids, and feasible to build. FSP Shield Options � Reducing mass and complexity are important aspects of space system design. The use of water as a radiation shield has the potential to reduce the mass and complexity of fission surface power (FSP) systems. Landed mass can be further reduced if water for the shield can be obtained in-situ. Participants will investigate water shield design from both a radiation attenuation and thermal management standpoint. Potential shield canister materials that have adequate long-term compatibility with water in a moderate radiation environment will be identified. Detailed radiation transport and thermal management calculations will be performed. Variable and fixed-orientation shields will be investigated. Methods for effectively using potential in-situ sources of water will be devised. NTR Intercept of Short Period Comet � Evaluate the potential performance of a NTR for interception of a massive low-period comet inbound to Earth. The participants will design the NTR for various thrust, specific impulse, and lifetime modes. Innovative NTR designs will also be investigated. Advanced High Temperature Power Reactor design � assess feasibility of using the NTR core as a source of high temperature fluid for power conversion. Ultra-high temperature systems such as Brayton, Rankine, and MHD will be evaluated. Specific components benefiting from high temperature refractory alloys will be identified.