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It is my understanding that solid-core nuclear thermal rocket reactors are extremely aggressive designs that operate at the limits of materials and have an incredibly high specific thermal power.
Do space nuclear reactors meant for generating large amounts of electrical power share the same properties?
A NTR (nuclear Thermal Rocket) runs at power levels from about 1.2 to many hundreds of Gigawatt.
With a reactor core mass less than 1/1000th as much as a "normal" contained ground-based nuclear power reactor of similar power.
A NTR operates at a core temperature of about 3000K
A power reactor operates at a core temperature of about 300c (570K-isk )
Yes, NTR is "Agressive" seen from this viewpoint.
Compared to space nuclear fission power systems: They simply DO NOT scale up very well without the shielding and cooling abilities of ground systems. Unless you incorporate the shielding and containment, and replace your external gravity-assisted water cooling system with a self-contained closed loop radiative cooling system. In which case the space reactor will be MORE massive than the ground power station.
At present tech, the absolute pinnacle of electric power nuclear reactor suitable for use in space is the KiloPower units. Which, as the name suggests, provide power in the single-digit kilowatt range. And it is not quite ready yet, TRL 5-6 or so. (full scale, lab tests, of prototypes, in non-field-conditions)
At 1500kg each for the 1Kw unit, the mass per power is somewhat...disappointing. A cluster matching the power of the smallest NERVA would mass 1700 metric tons.
I will argue that these reactor types are not directly comparable.
While a high "alpha" (power/mass) is desirable for both NTRs and reactors for electric power, the limiting factor that causes the most headache for NTR materials is reactor temperature, with Isp proportional to the square root of temperature (not quite proportional, due to hydrogen disassociation).
The Isp in turns ends up having an exponential impact on vehicle mass due to the tyranny of the rocket equation.
In a reactor generating electricity on the other hand, temperature has no impact on Isp when used for electric propulsion.
Temperature does have an impact on thermodynamic efficiency though, as a heat engine with a higher temperature gradient is more efficient (with diminishing returns). This is furthermore made complicated by heat engines in space being run "hot" on the cold end to cut down radiator mass.
So while system mass is a shared goal, it's calculated differently. For a NTR the mass is mostly engine + propellant, while for an electric system it's reactor + heat engine + radiators.
And while peak temperature is a shared goald, it's governed by different physical laws, gas velocity and heat engine efficiency.
Furthermore, a NTR provides high impulse over minutes, while electric systems provides a small impulse over months. Not even the mission profiles are comparable.
It's apples to oranges, so "aggressive" does not carry much meaning.
