Can We "Tune" The Radiation Output Of a Nuclear Device.? After reading "Project Orion", the 1950's plan to launch a large mass spaceship using small nuclear devices, (and also from common sense),  I realise that a lot of nuclear research is classified.
The output of a typical nuclear device can roughly be split into 


*

*Blast Effects (50%)  

*Thermal Radiation (40/30%)

*Fallout (20/10%.)


I have read through Nuclear Weapon Design and, although it goes into pretty exhaustive (and, to me, depressing) detail about how good a bomb we can make, I cannot see any spectrum of the radiation output.
My question is: if we wanted to create a bomb for something like a future Project Orion, or any other specific peaceful purpose, could we, in theory, "salt" the explosive mixture with elements to produce, for example, more infrared, rather than soft X rays, (which are the main component of present day bombs.)
 A: The spectrum of emitted rays is defined by the energy output of the reactions happening in the device and its surroundings. Basically, after an exothermic nuclear reaction, the produced particles smash into surrounding material and bounce for some time until their energy is dispersed. This energy heats up the medium which begins to emit black-body radiation spectrum with high temperature.
I believe this black-body spectrum is what you refer to as "soft X rays" as typical photons created directly in a nuclear reaction will be far more energetic, in the gamma ray part of spectrum, no matter what.
Black-body spectrum is just the thermal radiation emitted by any heated body. It has continuous distribution with the peak at the energy $\sim 3 kT$. So, basically, if you want to reduce the energy of the produced radiation, you will have to reduce the temperature of plasma and hence the energy of explosion. 
Anyhow, this black-body spectrum will be present only in the atmosphere and it is anyhow a pretty bad idea to detonate near the surface.
