Thorium fission fragment rocket

Fission fragment rockets have been proposed where the thrust is
provided by the very energetic fission fragments from fissioning
uranium. The extremely high speed of these fragments could yield an
ISP of up to 1,000,000 making it suitable for very deep space
missions. In such a reactor, the fissionable material is arranged so
that not only can criticality be achieved but so the fission fragments
can escape from the core to be directed via magnetic fields. To
achieve this, two types have been proposed, the disc reactor where
discs of U are rotated so that criticality occurrs on a segment of a
rotating U disc and it emits fragments and it rotates so the disc can
cool. There is also a proposed "Dusty Fission Fragment" reactor where
the fuel is composed of submicron particles of sufficient density to
achieve criticality but small enough so the fragments can escape and
the fuel particles can cool. Both designs suffer from the need to
cool the fissioning fuel while maintaning criticality.
I propose a Thorium fission fragment rocket that does not achieve
criticality but does act as an "Energy Amplifier" as in Carlo Rubia's
original idea. However, in this concept, we intend to use the fission
fragments from the U233 (produced from the Thorium) to produce
thrust. Fission is driven by means of spallation neutrons from a
proton beam onto a heavy target material or by means of a good neutron
emitter such as Cf. The "gain" from such a device would be on the
order of 60x as originally proposed by Rubia. The Th would have to be
arranged to absorb the starting neutrons and to capture the neutrons
emitted by the U233 with very high efficiency but to also allow the
emitted fission fragments to be used for thrust. Thus, fine particles
would probably be best similar to the "Dusty Fission Fragment
Rocket".
The advantages of my Thorium Fission Fragment Rocket (TFFR) would be
that the fuel particles would be cool because there is no criticality
meaning that design is much easier and there is no possibility of a
"runaway" reaction. Furthermore, the power output and hence the
thrust can be varied by varying the neutron input whereas the "Dusty
Fission Fraqment Rocket" has a minimum power level needed to maintain
criticality.