Amazon has it marked down to $140.57 right now.   It is a bit high I must admit.  I paid more a few months ago.

Am242m nuclide engine has been suggested some years back. 
One problem is it's an excessive gamma ray emitter and I'm not too sure how well walls coated with thin Am242m would hold up under the supersonic flow of H2 gas maybe some scouring effects?
I think one rule in design of reactor block cores is you're not suppose to loose fissile material out the exhaust. 

Just a little clarification: generally in a properly designed engine, you won't get supersonic flow in the reactor part of the engine. Generally, the coolant (LH2) first is used to cool the nozzle and pressure vessel that contains the reactor. Most of the coolant is gassified by this. Then it goes to the reflector where it is preheated. (The reflector almost acts like a recuperator in a steam plant.) The warm, fully gassified coolant is then sent to the core. Is it passes through the hot tubes the gas gets hotter but its speed is mosty constant. It is approximately constant speed, constant pressure: the pressure drop across the core is a small fraction of the overal stagnation pressure. The gas exits the core and enters a space just ahead of the nozzle: this is called the plenum and is roughly the equivalent of the combustion chamber on a standard rocket engine. The plenum's purpose if to dampen turbulance and equalize pressure distributions by slowing the hot, high pressure gas down to approximately stagnation which is slower than the speed of sound in that medium. Then the hot, stagnant gas exits the nozzle and expands aprroximately adiabatically--little heat is lost in the expansion so almost all thermal energy is converted to kinetic energy. As the temperature and pressure drop, so does the speed of sound in the gas, but the gas accelerates to supersonic speeds, usually exiting the nozzle at between 3.5-4 Mach (speed of sound in that gas, which is not to be confused with the speed of sound in air.) Thus supersonic flow only occurs in the expansion part of the nozzle in a properly designed engine.

Most of the fissile losses in the NERVA and ROVER engines occured from mechanical and thermal stresses caused fractures to open in the graphite matrix which exposed the fissile materials (UC mostly) to hot hydrogen which corrodes it by stealing carbon (the reduced uranium metal will then evaporate because of the high temperatures.)

The solution is to use something that does not corrode, like an alloy with tungsten.

GoogleNaut, one point I would like to make is that this Rubbia concept is not even close to the proven NERVA designs.  It may or may not be feasible but it will eject fission fragments into the propellant flow.  This direct heating is how this engine achieves its 2.5 to 3 times the specific impulse of standard solid core NTR.