Intensive radiative heat transfer in very hot gas ( >5000K, gas core nuclear reactor related ) The question: 
At temperatures above ~5000K are not stable any solid or liquid materials or even more complex molecules (such as fullerenes and Polycyclic aromatic hydrocarbons) which emit/absorb wide-spectral black-body radiation effectively. 
Simple molecules and atoms which survive such temperatures have very narrow absorption/emission lines and small absorption/emission cross-section. => They are transparent for thermal radiation.
Is there anything which would emit/absorb broadband thermal radiation at temperatures >10,000 K ?
What about plasma? I guess plasma must be both highly ionized (~hot) and at the same time dense in order to have high opacity and absorption for thermal light (Am I right?). This is a bit contradictory. Density is inversely proportional to temperature if we are limited by pressure let's say 100MPa, while ionization is increasing with temperature. Is it possible to increase opacity/absorption of gas/plasma at ~10,000-100,000 K by seeding with alkali metal which release electron very easily ? 
background:
Gas core nuclear reactor, and derived gas core nuclear rocket is a fission nuclear reactor theoretically able to achieve temperatures above 5000K (some proposals talk about 40,000-100,000 Kelvin ) which is important for high energy efficiency of electricity production (using MHD generators ) and high specific impulse of nuclear rocket.
There is a concept of space propulsion called nuclear lightbulb rocket which should achieve high specific impulse exhausting moleculer/atomic hydrogen propellant at velocity up to 20-40 km/s which means temperature ~25,000-90,000 Kelvin.
For effective function of such engine it is necessary that all the heat from gaseous nuclear core is transferred to propellant by thermal radiation. However, hydrogen propellant itself is almost transparent for thermal radiation. This would cause that the heat is transfered to the walls of rocket nozzle instead,  which would melt the walls and destroy the reactor. In order to make propellant opaque it was proposed that tiny dust particles of tungsten or hafnium-tantalum carbide particles would be dispersed in propellant gas. These particles would however evaporate above ~5000K making propellant gas transparent again. 
 A: It might be helpful to first understand why gases have such narrow absorption lines. 
Atoms in a gas are pretty far apart, sufficiently far that we can ignore their interactions for the purpose of analyzing their orbitals. That means the energy of each orbital is defined just by its orbital numbers. 1s electrons in helium gas all have the same ionization energy (Obviously this doesn't apply to double-ionizing helium)
Since electrons are fermions, the Fermi exclusion principle plays up when two atoms are close enough for electron orbitals to interact. With two hydrogen atoms, this can be resolved by having opposite spin, which explains H2. But in a solid, you have many more atoms close together, and there are only two spin directions. The result is that you get energy shifts, which widen the energy bands.
Metals in particular have wide energy bands that are only partially filled with electrons. This means that there are a lot of energy transitions available to these electrons, which means photons of many different energies can be absorbed.
Plasma's are pretty much the opposite of solids. A plasma is basically a ionized gas. As the free electrons aren't bound to specific atoms anymore, they too are capable of absorbing a wide variety of photons. But your typical plasma indeed isn't very dense. How would you exert a force to keep it together? Stars use gravity, but that's not going to work for you. Electro-magnetic forces don't work well because of the charge mix - positive nuclei and negative electrons. Weak and strong nuclear forces are too short-range.
The idea of injecting tungsten particles into the plasma sidesteps the idea of absorbing energy at 10000 K. Those particles are much cooler when injected. That means you're no longer talking about a steady-state situation, which does complicate the analysis a whole lot. These particles will rapidly evaporate and then ionize, i.e. increase in temperature, but you never attain an equilibrium since this is a rocket exhaust. You keep injecting new particles as the ions are carried away.
