The Thorium Reactor would then be used to heat a propellant like hydrogen,

On May 14, 1961, the world's first nuclear ramjet engine, "Tory-IIA," mounted on a railroad car, roared to life for just a few seconds. Three years later, "Tory-IIC" was run for five minutes at full power, producing 513 megawatts and the equivalent of over 35,000 pounds force (156 kN) thrust.

I understand that - but if the thorium is solid, the heat has to be conducted from the thorium fuel to the hydrogen, and the system is thermally limited. Liquid fuel will creep and deform, and might lead to blocked coolant channel, which if continuing to melt results in disintegration of the core. Also, one still needs a power supply for the laser, which means some of the propellant must be bled off and passed into a power cycle.If the thorium is a vapor, intimately mixed with the hydrogen gas (propellant), that might work, but then one loses a portion (possibly a good portion) of the Th vapor with the fission products and hydrogen, which significantly increased the molecular mass of the propellant which reduces the specific impulse.I'll have to check the sources cited, but generally, I do not trust Newscientist.There have been gas core concepts, but all have been hypothetical, and based on my experience, I am skeptical about all gas core concepts.The Pluto core had to survive temperatures of ~2,500°F (1,600°C) andSeconds and minutes prove that the core configuration can go critical and hold power - but a viable system must operate for hours or days. And I'm sure this was highly enriched U, since it was a compact core.Accelerator driven systems (spallation based) are notoriously inefficient.Actually there was consideration of nuclear ramjets for propulsion in the atmospheres of Jupiter and Saturn. The craft would descend into the atmospheres, collect the gases and blast back into orbit. Well - maybe some day in the future.