Fission Fragment Direct Energy Conversion (FFDEC) Into Electricity Can Dramatically Improve the Specific Mass of Fission-Based Electric Propulsion Rocket

Magnetic Collector for Traveling Wave Direct Energy Conversion of Fission Reaction Fragments

A. G. Tarditi1, J. H. Scott2 1Electric Power Research Institute,, Knoxville, TN 2NASA Lyndon B. Johnson Space Center, Code EP3, Houston, TX

.Work performed under contract from NASA Johnson Space Center, Propulsion and Power , Energy Conversion branch (EP3)

. Fission fragment direct energy conversion (FFDEC) into electricity can dramatically improve the specific mass of fission-based electric propulsion rocket . This study is focused on the conversion via traveling wave DEC, that has the advantage of being able to generate high frequency power (MHz range) and does not require high voltage technology, unlike electrostatic energy conversion. . The FFDEC is considered as a best fit to an accelerator-driven fission core to improve the efficiency and the overall specific mass . Future work: – Fragment extraction – Neutralization issues

Goal: improve drastically the fission-electric rocket specific mass to reach .The problem of fragment extraction: need a thin core .Fission criticality: presence of a moderator impacts system mass .Solution: – Subcritical fission core (thin, light), neutron source driver, electrical power re-circulating from FFDEC – Interesting analogy: for a fusion reactor DEC requires aneutronic fusion, that also requires a driven system

Schematic of proposed Fission Fragment Rocket. Fissile dusty plasma fuel is confined to dust chamber, where RF induction coils heat the plasma. Fission fragments are collimated by the magnetic field either to collection electrodes for power, or exit the re-actor for thrust.

7 © 2015 Electric Power Research Institute, Inc. All rights reserved. Previous Work on Fission DEC • Fission fragments direct energy conversion has been considered in the past for increasing power plant efficiency [1-3] and for space propulsion [4-6] • These concepts are focused on the direct conversion of the charged fragments utilizing high-voltage DC electrodes. ______[1] S. A. Slutz et al. ,Phys. Plasmas 10, 2983 (2003) [2] P. V. Tsvetkov, et al., Trans. American Nucl. Soc., 91, 927 (2004) [3] http://www.ne.doe.gov: 2003 and 2004 annual reports [4] R. Clark and R. Sheldon, AIAA 2005-4460 (2005) [5] G. Chapline and Y. Matsuda, Fusion Technology 20, 719 (1991) [6] P. V. Tsvetkov, et al., AIP Conference Proceedings 813.1, 803, (2006)

Making Nuclear (Fission) Electric Rocket a More Appealing Option for Space Propulsion .Direct energy conversion instead of steam cycle – Less heat: less radiators – No pumps, pipes, generators: lowering mass

– Lowering mass + Improving efficiency

Reactor with Lower Specific Mass a (kg/kW)

Electric Power Power Conversion Power Conditioning

Exhaust

Electric Propulsion Primary Energy Thruster Source

Typical Electric Propulsion Concept: separate electric power generation and propulsion systems

.Acknowledgement: fission fragment direct energy conversion into propulsion by R.A. Clark and R. B. Sheldon – This proposed approach is complementary to the fission fragment direct utilization for propulsion since it can provide a more versatile scenario where a lower Isp is provided by plasma acceleration while at the same time some of the extremely high Isp provided by the fission fragment beam is being reduced.

. Exploring of DEC configurations that could be implemented within a nuclear fission core . Requires collecting and collimating a beam of charged fission fragments . Options: - thin solid core for optimal fragment extraction, [1] - dust core [4] - gas core (e.g. vortex confinement, [7]) ______[7] Sedwick, AIAA Journal of Propulsion and Power, Vol 23, No. 1, Jan-Feb 2007.

.Concept: a thin fissile layer is needed to extract most of the fragments

Neutrons

Fragments Fissile Layer Structural Support

• 235U => 140Xe + 94Sr + 2n • Consider a 100 MeV 140Xe fragment with a +20e charge 140 7 • Xe fragment speed vXe=1.17∙10 m/s • For a inter-electrode TWDEC distance of d=1 m the

frequency of the AC power is f0=vXe/2d=5.85 MHz

Travelling Wave Direct Energy Converter (TWDEC) [Momota, 1990, 1992] Conceptual scheme: the energy of the bunched ion beam is collected in the Decelerator Section producing AC electric output. A small residual energy in the beam is absorbed at the end.

• No high-voltage electrodes, collects the energy of the beam through a series of electrode pairs, each at a smaller alternating potential. • Electrodes capacitively coupled to a density-modulated (bunched) beam of charged particles • Beam bunches travel through of properly spaced electrodes inducing an alternating potential • Alternating current has several advantages over DC in terms power conditioning and distribution

.2D PIC code (XOOPIC) in cylindrical geometry – Beam: 100 MeV, charge 20 e, atomic mass A=100 amu. – 2 cm radius, 1 m length – electrodes 5 long, 10 cm apart,AC potential several kV range

t=t >t

t=t0 1 0 4 cm 4

1 m Particles injected Particles leave

. Charged fission fragments (positively charged, about 20 electron charges) are magnetically collected and focused . Fission fragment beam of relatively low density, to avoid significant space charge effects.

Alternating-gradient beam focusing

• Solenoidal magnetic field B0= 0.5 T: - 140Xe fragment gyroradius= 1.71 m

Collimated Fragment at reduced drift speed into TWDEC Fragment Beam • Side injection can reduce drift speed and TWDEC frequency • Bunching can provide the non-adiabatic injection required to capture the ions.

No electric field Cases with 25% Velocity Spread Region with retarding electric field

.Fragments velocity spread can be utilized for forming a bunched beam

No electric field 10% Velocity Spread Region with retarding electric field

Region with retarding electric field 25% Velocity Spread Doubled electric field

beam still made of particles spiraling with different radii around the field mv 5 r  ii0 lines, but characterized by longitudinal i drift speed. qBi vf faster particles will have a smaller 1 fraction of their original speed directed along the solenoidal magnetic field unidirectional 4 lines. beam of fragments at injection into a solenoidal magnetic different speeds field: faster particles entering at a larger angle to the field lines. velocity filter: particles with 3 2 larger speed will be deflected less

The beam is injected into a bending magnetic field (velocity filter). Particles with higher speed will be deflected less. Three cases are shown. Fast (F), medium (M) and slow (S) particles are co-injected. After the deflection the incident angles

with respect to the solenoidal magnetic field are different: aS > aM > aF.

• Fission fragments need thin fissile fuel elements • Fragments carry large positive charge (≈20 e) and can be collimated with magnetic fields • Traveling Wave DEC converts fragment energy into AC electric power • Subcritical fission driven systems combined with fission DEC may become the most efficient option

• In “real life” multiple fragment products (different masses and energies) must be considered • Multiple channels may be required for efficiency • Electron flow must be dealt with