A Fission-Fusion Hybrid Reactor in Steady-State L-Mode Tokamak

Configuration with Natural Uranium

Mark Reed

FUNFI

Varenna, Italy

September 13th, 2011

PART I: The Issue

PART II: Fission

PART III: Fusion

PART IV: Conclusions

PART I: The Issue

Why this might be a good idea

Contention

Fission-fusion hybrids could actually be more viable than stand-alone fusion reactors and

obviate some challenges of fission.

€

Qhybrid = Q fus

1

5+

4

5Q fis

⎛

⎝ ⎜

⎞

⎠ ⎟

Constraints

• D-T tokamaks

• Fully non-inductive (steady-state)

• Low confinement mode (L-mode) operation

• Pebble bed blanket with helium coolant

• Natural or depleted uranium

• Lithium-lead eutectic layer for tritium breeding (one triton per fusion neutron)

PART II: Fission

The maximum natural uranium blanket power gain

Basic Layout

Li-Pb

natural uranium with He coolant

shield

Neutronics Methodology

• Developed a subcritical Monte Carlo code (benchmarked with MCNP)

• Treated uranium and lithium layers as elongated toroidal shells (quartic solutions for neutron path lengths)

• ENDF cross-sections and other nuclear data

Blanket Variables

• Uranium toroidal layer thickness

• Lithium toroidal layer thickness

• Relative positioning of toroidal layers

• Homogenized uranium density (different pebble designs)

• Lithium enrichment

• Major and minor tokamak radii

Layer Thickness Optimization

Subcritical Neutron Multiplication

k = 0.27

k0 = 1.19

Total Power Composition

Fusion-Born Neutron Fate

Fission Results

• Blanket power gain of 7

• Tritium breeding ratio of 1.05

• Uranium layer thickness of 18 cm

• Lithium enrichment of 90% 6Li

• Helium coolant velocity ≈ 10 m/s

PART III: Fusion

The minimum tokamak size for steady-state L-mode operation

0-D Tokamak Model

• Volume-averaged parameters

• Simply relate R, a, B, q*, Pfus, and Qfus

• Current limit and safety factor (q* > 2)

• Greenwald density limit

• Troyon no-wall pressure limit (βN < 3)

• L-mode operation (H-89 scaling)

• Fully non-inductive (fNI ≈ 1)

• Solenoid flux approximately twice plasma flux

Fusion power surface density PF/AS and fixed Bmax uniquely define each operating point

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q ~Ba2 1+ κ 2

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RIP

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PAUX + Pα = PLOSS

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0-D Tokamak Relations

Stand-Alone Fusion Reactor

Q = 40, R/a = 2.6, Bmax = 15 T, PF/AS = 5 MW/m2.

Fission-Fusion Hybrid Reactor

Q = 6.3, R/a = 3.1, Bmax = 15 T, PF/AS = 3 MW/m2.

Fusion Results

• Major radius of 5.2 m

• Aspect ratio of 2.8

• Maximum on-coil magnetic field of 15 T

• Fusion gain of 6.7

• Total fusion power of 1.7 GW

• Safety factor of 3.0

• H89 = 1.48 (L-mode)

PART IV: Conclusions

What this all means

Fission-Fusion Advantages

• Fully non-inductive L-mode operation at small scale (low capital cost relative to pure fusion devices)

• Subcritical operation (flexibility and safety)

• Control of fission blanket indirectly through control of the tokamak plasma – fission blanket gain increases with time due to plutonium breeding

• No uranium enrichment (non-proliferation)

• Enhanced transmutation of long-lived fission products through (n,2n) reactions

Conclusion

Instead of complicating the already difficult challenges of fission and fusion, fission-fusion hybrids could actually simplify many difficult

aspects of fission and fusion.

A profusion of pro-fusion sentiment?

Acknowledgements

Prof. Ron Parker (fusion)

Prof. Ben Forget (fission)

M. Reed, R. Parker, B. Forget. “A Fission-Fusion Hybrid Reactor in L-Mode Tokamak Configuration with Natural Uranium”. PSFC/RR-11-1 (2011).

MIT Plasma Science and Fusion Center (PSFC) report:

Extra Slides

L-mode and H-mode

• H-mode has rough profiles that create edge-localized modes (ELMs), the bane of current fusion research.

• L-mode does not give rise to ELMs but has lower power density.

• Some current hybrid designs are based on ITER (H-mode).

Hybrid Power

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Q fus =Pfus

Paux

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Q fis =Pfis

(4 /5)Pfus

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Qhybrid = Q fus

1

5+

4

5Q fis

⎛

⎝ ⎜

⎞

⎠ ⎟

The fission blanket augments the fusion power.

At large size, increases in temperature lead to operation at the maximum D-T rate coefficient.

• T near <σv> maximum provides inherent stability (negative reactivity coefficient)

• Absolute <σv> maximum limits feasible parameter space

66 keV

T= 10 keV 100keV

Log

(D-T

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e co

effic

ient

)