What would be the technical considerations of a practical dense plasma pinch based neutron source for running a nuclear fission process?
I can think of a few:
1. Energy Efficiency - is this actually a problem? Would a neutron source produce enough neutrons per energy input to not harm the power plant total efficiency?
Depends on the fission reaction you are planning. Thorium could be mixed with ‘high level’ actinide waste and end up with a lump of a fuel type that is almost as fissile as the operating point of a regular uranium reactor. Not sure if anyone has tried that.
The ‘almost’ is then a clue as to how to drive and run such a reactor - ‘all’ that is then needed to drive such a reactor is an input of neutrons that will tip the neutron budget of the fuel over towards ‘energy producing’, because thorium won’t run a chain reaction of neutron production like uranium so the extra neutrons are needed to ‘make up for’ the extra neutrons that would otherwise come out of the uranium process.
Thorium itself takes quite a bit of neutron irradiation to get it reacting enough but as the reactor runs the required input of neutrons will come down as the actinides build up. This scheme has been proposed as the ‘Rubbia Energy Amplifier’, and is driven by a spallation source of neutrons (in which protons are accelerated into lead, which could then also act as the coolant once in melts.)
Whereas if a thorium reactor had a blend of actinides then the required neutron input would be low initially but go up as the actinides decay away. You could then either ramp up the neutron rate, or re-blend the fuel.
(Not sure if many folks have yet considered making thorium fuel more fissile by blending with actinide waste, which also has the added benefit that the waste gets eaten up?)
So the answer to your question rather hinges on whether you are seeking to drive a fissile process essentially exclusively by neutrons (thorium energy amplifier), partially (my scheme with blended actinides) or not at all with neutrons (regular uranium slow reactor).
2. The rate at which neutrons are produced being at a sufficient rate/power to vary the thermal output of a fission process… would be very nice for load following in a power system. Slower neutron production can gradually reach full power through the build up of fissile byproducts.
This, depends on the mix, but in theory there would be a blend where the thorium and the actinide blend would be relatively stable in its demand to be driven by a neutron source. It would also be relatively safe, because you switch off the neutrons and you switch off the reaction. Obviously, it would still have a heat half-life to consider, but at least the fuel would not go critical by itself.
3. Can something be scaled up properly and still be feasibly built? Are there practical limits?
In order to have high output and good control (and maintain a safe K_eff), the driver neutron source needs to supply what is generally considered to be a huge amount of neutrons: ~1E+15 n/s per MWth or so. When you talk about a 20-MWth ADS power plant, generally you might expect to need 2E+16 n/s. Continuously, around the clock, with high availability!
Obviously, amateur Farnsworth fusors are out of the running. IEC devices have been proposed for the application (by George Miley and others), but the technological readiness or even plausibility remains undemonstrated. If one considers only neutron source technology that is presently well-developed and is reasonably reliable, you come down to the following more-or-less distinct possibilities (with wall-plug efficiency estimates):
-Spallation (3E+09 neutrons / joule)
-Low-energy reactions (p, Li), (p, Be), etc. (1E+07 neutrons / joule)
-Photonuclear reactions (I don’t know enough about present facilities)
-Fusion (typical commercial DT generator is ~3E+07 neutrons / joule)
Spallation wins on efficiency. It also will win on reliability, since the big spallation research sources (SNS, LANSCE, etc.) have been tooling along at 80-90% availability for years. You need a half-mile-long linac to get proton energies in the GeV range, so that’s a possible downside of spallation. But at least this is a technology that’s ready to go. For your 20-MWth plant, you would want to have an accelerator like LANSCE (800 kW beam).
A spallation spectrum consists of mostly fast neutrons, typically peaking in the MeV range, but having some particles with 10- and 100-MeV range energies. “Thermal” neutrons have a Maxwellian energy spectrum at whatever ambient temperature is applicable, e.g. a mean energy of 0.025 eV near room temperature.
In an ADS, the spallation source provides a small contribution to the total neutron population. Most neutrons are the result of fission in the fuel, and will have the characteristic Watt spectrum of fission. There will be a few (n,xn) neutrons in there too, because the high-energy tail of the spallation spectrum will cause these reactions. The source may or may not need special moderation in addition to whatever moderator surrounds the fuel. It’s not obvious to me—but rather a research question—what sort of flux tailoring scheme will best serve various types of fuel and various kinds of spallation targets.
The lifetime of the target in a spallation source is typically determined by gas production and consequent embrittlement of the target if it is solid and has to retain some kind of mechanical or thermal integrity. This is because spallation produces all kinds of light nuclear byproducts such as hydrogen and helium in addition to neutrons. If the target is liquid, the gas production issue applies to the container it’s in.