A 100 t ship, TWR = 1, ISP*g = 100 km/s.
Jet power = 100 000 * 10 * 100 000 = 1011 W.
Let the waste energy (in form of X-rays) be 50% of total.
1011 W of waste power.
Let the chamber be a sphere 10 m in radius.
Equilibrium temperature ~= (1011 / (5.67*10-8 * 4 * pi * 102) )0.25 ~= 6 000 K
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A 10 000 t ship, TWR = 1, ISP*g = 100 km/s.
Jet power = 107 * 10 * 100 000 = 1013 W.
Let the waste energy (in form of X-rays) be 50% of total.
1013 W of waste power.
Let the chamber be a sphere 50 m in radius (proportionally).
Equilibrium temperature ~= (1013 / (5.67*10-8 * 4 * pi * 502) )0.25 ~= 8 600 K
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So, the temperature of the chamber would be several times greater than a solid material can survive without active cooling.
And the terawatts active cooling of tens-meters nozzle would mass so much that the ship would consist of it totally.
But even worse, it should find a way to quickly emit terawatts into space and, and additionally - not heating its own hull.
On ground you can use massive external coolers and the infinite heat sink of the ocean. But not in space.
So, this limits the encased reaction designs with low waste power, so low thrust.