# Vapor Core Reactor Question

Hi I am someone researching vapor core reactors from a historical perspective. I have a technical question. In a congressional document I found it is stated that a vapor core reactor would have 35kW of power per square inch. Am I right in calculating that a cubic inch of a vapor core reactor would have 43 megawatts per cubic inch? Or would this only apply to the surface area hence a cubic inch would have 6x35kW or 210kW?

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mathman
Try nuclear engineering forum.

mfb
Mentor

You have to know the size and shape of the reactor to convert the surface power flux to the power density (it is probably not a single cubic inch large).

I have no idea how you got that 43 MW/inch3 value. And it is a strange mix of SI and US units, using SI everywhere makes it much easier.

Astronuc
Staff Emeritus
Hi I am someone researching vapor core reactors from a historical perspective. I have a technical question. In a congressional document I found it is stated that a vapor core reactor would have 35kW of power per square inch. Am I right in calculating that a cubic inch of a vapor core reactor would have 43 megawatts per cubic inch? Or would this only apply to the surface area hence a cubic inch would have 6x35kW or 210kW?
It would help if one cited the congressional document. The thermal flux could be derived from the neutron flux. Thermal flux might be meaningful in a tubular fuel rod or cooling channel, but it doesn't mean a lot in a gas or vapor core.

Power density of 43 MW/in3 doesn't look right, especially for a gas core. LWRs have power densities on the order of 50 kW/l for BWRs and 100 kW/l for PWRs.

Gas cores have very low mass density, so power density would be low, unless the vapor has very high enrichment and is driven by a high neutron flux.

Nuclear engineering does use some unusual mixed units, e.g., kW/ft, but normally we don't use kW/in2 or kW/in3. I prefer working in SI.

etudiant
Gold Member
Has there ever been a vapor core reactor built? It sounds like a candidate for spontaneous disassembly.

Astronuc
Staff Emeritus
Has there ever been a vapor core reactor built? It sounds like a candidate for spontaneous disassembly.
To my knowledge know, at least not in the US. There have been a lot of conceptual designs, basically related to nuclear space propulsion.

Terrestrially, it would have to be sealed, and that would involve complications of what to do with the fission product accumulation. With respect to that concern, it would make more sense to use a molten salt system. Also related would be extraction of the thermal energy in the 'vapor' or 'gaseous core. How much can be extracted by an MHD systems versus a more conventional thermal conduction/convection system?

As for the core design, it would require some driver assemblies, and a reflector, in which to maintain a bulk of the reactivity and delayed neutrons, in order to ensure control.

The concept of a gas core reactor has been around for about 5 or 6 decades.

Gas-Core Nuclear Rocket Design
By David Hitchcock

Here is my source. It is from NASA's semi-annual report to Congress.

etudiant
Gold Member
To my knowledge know, at least not in the US. There have been a lot of conceptual designs, basically related to nuclear space propulsion.

The concept of a gas core reactor has been around for about 5 or 6 decades.

Gas-Core Nuclear Rocket Design
By David Hitchcock
Excellent reference! Thank you, Astronuc.
It is a fun read and just inspiring to find that there was good long range nuclear applications work done in the 1970s and 1980s. A little like the NASA proceedings on Radiant Energy Conversion in Space, which also dates back to that period and which examined the engineering issues of laser power space propulsion and space based power satellite satellites. What happened to stop these developments dead? I noted the copyright had been renewed in 2009, but nothing new was added in the available excerpt.

I find it notable that the power densities of LWR's and BWR's have not increased substantially. Why haven't the advances made in solid fuels from the ROVER/NERVA programs been spun off to create much more powerful compact reactors for electricity production? The PEEWEE's had 5MW per liter power densities. When it comes to gascore there are documents from the 1970's claiming 50,000 degree fahrenheit temperature having been achieved. The capability of powering the whole nation on a handful of reactors has been reached. Why not build 50 one for each state? We could crack hydrogen or use the Haber Bosch process to make fuel for vehicles. This could potentially halt the climate change we are all experiencing.

It would help if one cited the congressional document. The thermal flux could be derived from the neutron flux. Thermal flux might be meaningful in a tubular fuel rod or cooling channel, but it doesn't mean a lot in a gas or vapor core.

Power density of 43 MW/in3 doesn't look right, especially for a gas core. LWRs have power densities on the order of 50 kW/l for BWRs and 100 kW/l for PWRs.

Gas cores have very low mass density, so power density would be low, unless the vapor has very high enrichment and is driven by a high neutron flux.

Nuclear engineering does use some unusual mixed units, e.g., kW/ft, but normally we don't use kW/in2 or kW/in3. I prefer working in SI.

Last edited:
Astronuc
Staff Emeritus
Here is some background on the nuclear propulsion programs in the US.
http://www.lanl.gov/science/NSS/issue1_2011/story4full.shtml

Why haven't the advances made in solid fuels from the ROVER/NERVA programs been spun off to create much more powerful compact reactors for electricity production? The PEEWEE's had 5MW per liter power densities.
Nuclear thermal propulsion is a very different technology than power generation. Cores for rockets would operate for short times, from minutes to days, which means a lot of power for short times. LWR power plants used to operate on annual cycles (290-330 days before refueling), but now we operate reactors for 18 (490-530 days) or 24 month (660-730 days) cycles, and the fuel will operate for two or three cycles, or perhaps four cycles. The higher the power and temperature, the shorter the time.

Note in the LASL/LANL article the time of operation for the Rover/NERVA duration (which are records):
• 850 seconds of specific impulse
• 90 minutes of burn time

A temperature of 50,000°F (27,778°C) is not practical for solid material, particularly under stress. Corrosion, creep, creep fatigue, stress/strain are all design considerations. MHD systems can operate with such high temperatures, but then one would have to capture the fission products from the gas stream, and the temperature would have to be reduced for mechanical systems.

Hydrogen generation by nuclear thermal process is yet another technology.
http://www-pub.iaea.org/MTCD/Publications/PDF/Pub1577_web.pdf

Of course, there are critics to use of nuclear energy for hydrogen production.
http://www.nirs.org/factsheets/hydrogenbynuclear.pdf

The Jason 1970
Astronuc
Staff Emeritus