7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Low Power Fusion Driven -Subcritical Thorium Fission Reactor Concept
Motoyasu Sato1, 2, Naoto Kobayashi1,2, Shin Nakatani2
1Chubu University, 2Fusion Fission Powers Co. Ltd.
e-mail(speaker): satomoto@isc.chubu.ac.jp
Abstract:
The present study introduces a novel approach to the next generation of fission reactor controlled by a DD𝜇 fusion,
addressing the demand for enhanced safety, reduced size, and minimized radioactive waste.
1. Introduction
E. Fermi and others, in 1942, devised a method forcontrolling atomic reactors by mechanically withdrawing
control materials (rods) that were pre-inserted into asupercritical state reactor to maintain criticality. To halt
the reaction, the control rods would be inserted
mechanically in reverse. This method involves inserting
negative reactivity by the control rods absorbing excess
neutrons. Reactors of this design have potential risks ofexceeding criticality if control is lost. Therefore, multiple
measures are implemented to ensure the safety of thereactor. However, the Chernobyl accident is believed tohave occurred in this manner, and public opinion remains
skeptical of this approach.
In contrast, subcritical reactors only become criticalwhen additional neutrons are introduced from the outside.
If the input of external neutrons is halted, the reactions
decrease exponentially, and a runaway reaction isfundamentally impossible. This concept should beapparent to anyone with a little thought.
Why, then, since 1942, haven't reactors utilized nuclear
fusion for starting up? The reason is that nuclear fusion
requires configurations significantly different fromreactors, such as large confinement magnetic fields orlaser systems, and it demands a different topology andsize. Additionally, achieving break-even in fusion stillrequires a considerable period of research.
2. Muon Catalyzed Fusion
Muon-catalyzed fusion experiments, demonstrating thegeneration of fusion neutrons, were already conducted in
the 1980s. In recent years, precise theoretical analyses
have been published [1]. This sequence of experiments
and theories indicates that the average free path(approximately the distance traveled) of muons diffusing
is comparable to that of high-density neutral gas
molecules. From an engineering perspective, the core sizeof a muon-catalyzed fusion reactor can be determinedbased on this distance. If the convergence radius of themuon beam can be narrowed down to 25mm, the fusion
core could be about the size of a chicken egg. The
possibility exists to drive a reactor by inserting a muoncatalyzed fusion core into a subcritical post-modern
reactor and generating neutrons. 240Pu and MA to
1/100,000 and 1/1,000,000, respectively. Nuclear reactors
using such a Th series as fuel were only implemented at
the United States' Shippingport Atomic Station during the1977-82 period.
3. High-level radioactive waste
Replacing reactor fuel with 233U-232Th can reduce theprobabilities of generating This reactor, which generated
an average of 60MW of power, was the first to supply

electricity to the commercial power grid. The reactor wassafely dismantled, and its spent fuel underwent precise
analysis, meticulously investigating burn-up levels,
damages, and other factors. Detailed reports have beenpublished. There were no incidents of melting or damage,and the increase in contaminations of Pu and MA
remained below analytical limits.
4. Fusion-Driven Subcritical Fission Reactor Concept
A pioneering fission reactor design emerges from theShippingport Thorium Reactor model. It integrates a core
blanket, the "Seed," and an outer cylindrical blanket, the"Sheath." Loaded with radioactive 233U within 232Th, theseblankets maintain subcritical conditions. Fusion neutrons,
introduced externally, regulate the fission reactor,
sustaining it just below criticality.
Neutron interactions within dictate the multiplication
factor. Two fusion core arrangements are studied. The
first, placing a core at the Seed's center[fig1a], increases
neutron gradient, reducing keff. The second aligns fusion
cores at the Seed's boundary[fig.1b], enhancing keff bysaving neutron loss.
Real reactor complexity hinders analytical solutions. A
virtual reactor via supercomputer modeling, inspired by
Shippingport, captures neutron flow through diffusion
equations. Fuel distribution validity is ensured bycomparing spent fuel changes with real data. Using this,a novel Low Power Fusion-Driven Subcritical Thorium
Fission Reactor aims for completion by 2033.
References[1] T. YAMASHITA et al., “Roles of Resonant Muonic
Molecule in New Kinetics Model and Muon Catalyzed
Fusion in Compressed Gas,” Sci. Rep., 12, 6393 (2022);
https://doi.org/10.1038/s41598-022-09487-0.
[2]A, IIYOSHI, M. SATO, teal., A Muonic Fusion
Hybrid Reactor, Fusion Science and Technology (2023)
https://www.tandfonline.com/loi/ufst20
Fig. 1 A small muon fusion neutron sources arrangement
Muon fusion at the center of Seed

Muon fusion cores arranged on the boundary of Seed

Light water moderator
っっっっReflectivematerialsっっっFig.1aっFission Blanket
233
Th97％+233U3%

In the center
Fusion Core 2.45MeV
For excitation from subcritical
As an external neutron source
Using fusion neutronsFig.1bnumber of smaller neutrons on theboundary of Seed blanket equal tothe peripheral control rod pull-out
i.e., Changes the Buckling of Seed

Figure 1. muon fusion neutron sources arrangements