Is there limit on how small fusion/fission reactor can be??

[DerekL1963]

19 hours ago, Gaarst said:

No. The fuel of fission reactors is not nearly refined enough to be able to explode. If it was, half of the world would have nukes, and we'd probably all be dead.

When it's in a commercial reactor?  No.  There's no compression and virtually no confinement and the fuel isn't sufficiently enriched.

The fuel itself?  That depends on the reactor...  commercial power reactors, certainly not, as they only use lightly enriched fuel (if it's enriched at all).  Naval reactors OTOH use very highly enriched fuel, some sources think possibly weapons grade.   Certainly enough to generate a very nasty 'fizzle' (fizzles can still yield from tens to a couple of hundred tons as well considerable fallout and contamination) if nothing else.

But note the large number of states that have both a native atomic power industry and atomic weapons.  This is no accident, as the same basic technology underlies both.  It's also why we keep an eye on countries with enrichment programs (such as North Korea, Iran, and Iraq)...  the same equipment and technology that can produce mildly enriched uranium for reactor fuel can produce highly enriched weapons grade material.  Commercial reactors generally produce "dirty" plutonium (that is, it has a large quantity of plutonium isotopes other than 239, making it useless for nuclear weapons).  But they can be designed (as in the UK), or operated (as is generally believed to have been done in North Korea) to produce "cleaner" plutonium with fewer objectionable isotopes and more Pu-239 making it useful for weapons manufacture.

[KSK]

22 minutes ago, kerbiloid said:

RTG can neither former, nor latter.
Also, it is not chain.

 

19 minutes ago, cantab said:

An RTG might meet the "fission" part (depending on the isotope used) but it doesn't meet the "reactor" part. In nuclear physics the term "reaction" is restricted to processes where two or more particles interact; radioactive decay or spontaneous fission where one particle decays into several do not count. (This contrasts to the use of the term "reaction" in chemistry, which includes one reagent turning into multiple products).

Ahh, OK. I stand corrected.

[PB666]

On 11/27/2016 at 8:25 AM, raxo2222 said:

Well (space-worthy) molten salt reactor from Interstellar has minimum size of 0.625m - this means diameter of 1.25m and similar height.

Thermal generators/engines adds to height too.

The concept here is that if you are going to go interstellar and you are using fusion energy, then your accelerations are going to be limited to the 0.001 to 0.1 a range. Your interstellar travel times are going to be years, you need a big ship.

Consequently you need alot of reactors or bigger reactors. 0.6M form factor could maybe run non propulsion systems, but not propulsion systems.

 

[raxo2222]

1 minute ago, PB666 said:

The concept here is that if you are going to go interstellar and you are using fusion energy, then your accelerations are going to be limited to the 0.001 to 0.1 a range. Your interstellar travel times are going to be years, you need a big ship.

Consequently you need alot of reactors or bigger reactors. 0.6M form factor could maybe run non propulsion systems, but not propulsion systems.

 

Well tiny reactors generating electricity are useful on probes and as plasma maintainers for bigger fusion reactors used solely for propulsion.

[Bill Phil]

https://en.wikipedia.org/wiki/Subcritical_reactor

Criticality need not apply...

[kerbiloid]

2 hours ago, Bill Phil said:

https://en.wikipedia.org/wiki/Subcritical_reactor

Quote

A subcritical reactor is a nuclear fission reactor concept that produces fission without achieving criticality. Instead of a sustaining chain reaction, a subcritical reactor uses additional neutrons from an outside source. There are two general classes of such devices. One uses neutrons provided by a nuclear fusion machine, a concept known as a fusion-fission hybrid. The other uses neutrons created through spallation of heavy nuclei by charged particles such as protons accelerated by a particle accelerator, a concept known as an accelerator-driven system (ADS) or accelerator-driven sub-critical reactor.

This just parasitizes on an external criticality. So, still needs.

Edited November 30, 2016 by kerbiloid

[Bill Phil]

30 minutes ago, kerbiloid said:

This just parasitizes on an external criticality. So, still needs.

Quote

Most current ADS designs propose a high-intensity proton accelerator with an energy of about 1 GeV, directed towards a spallation target or spallation neutron source. The source located in the heart of the reactor core contains liquid metal which is impacted by the beam, thus releasing neutrons and is cooled by circulating the liquid metal such as lead-bismuth towards a heat exchanger. The nuclear reactor core surrounding the spallation neutron source contains the fuel rods, the fuel being preferably Thorium. Thereby, for each proton intersecting the spallation target, an average of 20 neutrons is released which fission the surrounding fissile part of the fuel and enrich the fertile part. The neutron balance can be regulated or indeed shut off by adjusting the accelerator power so that the reactor would be below criticality. The additional neutrons provided by the spallation neutron source provide the degree of control as do the delayed neutrons in a conventional nuclear reactor, the difference being that spallation neutron source-driven neutrons are easily controlled by the accelerator. The main advantage is inherent safety. A conventional nuclear reactor's nuclear fuel possesses self-regulating properties such as the Doppler effect or void effect, which make these nuclear reactors safe. In addition to these physical properties of conventional reactors, in the subcritical reactor, whenever the neutron source is turned off, the fission reaction ceases and only the decay heat remains.

Doesn't sound like external criticality. There's no chain reaction at all.

From https://en.wikipedia.org/wiki/Nuclear_reactor_physics#Criticality

Quote

When a reactor’s neutron population remains steady from one generation to the next (creating as many new neutrons as are lost), the fission chain reaction is self-sustaining and the reactor's condition is referred to as "critical".

Non-critical reactors don't have a steady neutron population from one generation to the next. It's not self-sustaining. The core isn't critical. Even if external parts are (which they don't have to be, in some cases), the core itself isn't critical.

[kerbiloid]

1 hour ago, Bill Phil said:

a high-intensity proton accelerator with an energy of about 1 GeV, directed towards a spallation target or spallation neutron source.

Trying to realize an alternative energy source for a reactor-scaled particle flux except the same or another nuke.
Unlikely they use solar panels or coal.

(I mean: what powers the accelerator?)

Edited November 30, 2016 by kerbiloid

[magnemoe]

33 minutes ago, kerbiloid said:

Trying to realize an alternative energy source for a reactor-scaled particle flux except the same or another nuke.
Unlikely they use solar panels or coal.

(I mean: what powers the accelerator?)

You would need power to start it, same as you need for an car engine or most power plants outside of solar panels. You use line power or an backup generator for this power. 
Starting large machinery is often an pretty complex operation even if you don't do the maintenance and security checks. 
Benefit of an sub critical reactor is that if you cut power to the neutron generator the reaction will stop, you con't need the redundancy you would need using control rods making it easier to make an small reactor. Might be nice for space use, less moving parts 

[kerbiloid]

33 minutes ago, magnemoe said:

neutron generator

And where the neutron generator takes neutrons, otherwise than from D+T collision or a chain fission reaction?

(As I can understand, you can't just take neutrons from the active zone, as they are not charged. You can just reflect them back in random directions.)

Don't forget, they speak not about urchin or so, but about that 1 g of neutrons per 238 grams of  U.

Edited November 30, 2016 by kerbiloid

[peadar1987]

On 29/11/2016 at 3:11 PM, RCgothic said:

Graphite-moderated water-cooled was an extremely bad design choice from a stability point of view, but it wasn't the reason the reactor went prompt critical. It just made things worse once it did.

The RMBK design was chosen because the graphite moderator absorbs fewer neutrons than water as it moderates, which allows for the use of unenriched natural uranium oxide fuel (cheap). The water coolant allows for higher power density than the other main graphite-moderated reactor type, which is gas-cooled, because of the higher heat transport capability.  High power density and large size makes the RMBK design staggeringly powerful.

But yes, in graphite-moderated gas-cooled designs you can't really get a loss of coolant from hotspot accident because it doesn't vaporize to form voids (it's already one big void), and the reactivity of the hotspot will reduce with negative temperature coefficient. In water-moderated designs, if you lose the water you lose the moderator, and unmoderated neutrons are less reactive, thus reducing power. Negative void coefficient.

In the RMBK, if the water vaporizes, steam is less efficient at conducting heat away than water, but the graphite still moderates. Additionally, the lack of water means fewer neutrons absorbed and mute neutrons total. Positive Void coefficient. Steam explosion.

But that that was just the endgame for Chernobyl. The prompt critical condition should not have been achievable in normal use, but it was being dicked about with, basically. 

As a fix they modified all the remaining RMBKs with neutron absorbers, and started using slightly enriched fuel.

The graphite tips on the control rods were intentional, by the way, and sat in the middle of the reactor in the retracted position. They were there to boost the reactor power as they were being withdrawn. It wasn't realised that as the tips didn't fill the entire reactor they'd locally boost power at the bottom as they were being inserted...

Yup, I worked on the UK's AGR fleet for a while.

We used to jokingly say it was a pity there hadn't been more serious nuclear accidents, as all of our training was Chernobyl, Chernobyl, Chernobyl, with a small amount of Windscale, Three Mile Island and Davis Besse thrown in. Maybe a little bit of Tokaimura if we were specifically talking about criticality incidents.

[KerikBalm]

technically speaking, bombs are reactors... so they can be quite small.

Chain fission reactions are limited by critical mass, which for certain isotopes /levels of enrichment is quite small

Fusion reactions are mainly limited by energy densities, and the machinery required to make those energy densities. This is why H bombs work... the fission reaction component generates extreme energy densities.

One could go even smaller with antimatter triggered fusion... but that requires substantial production and storage of antimatter... and if you can do that, it may not be long before you just go with pure matter-antimatter reactions and don't bother much with fusion.

[wumpus]

2 hours ago, KerikBalm said:

technically speaking, bombs are reactors... so they can be quite small.

That's pretty much the catch.  Reactors are small, containment is bigger, shielding bigger still and finally cooling is massive.

And even at 100% efficiency (Carnot says "Hi!  Nice heatsink you have.  Especially all that nice vacuum.") means that you need a cooling system (again, radiators in vacuum if you are in space) that radiates a Watt for each Watt of power you produce (and multiply that by the inefficiency when you take that into account).  Earth cooling systems are bad enough (and typically only built with massive water supplies nearby), but space makes reactors questionable (for areas where PV solar is possible.  And if you can build gold-foil cooling radiators you can build gold-foil mirrors to collect the sunlight).

[Nuke]

for magnetic confinement there are physics reasons that set a minimum cross section of your reactor. its something like 1.5 meters. something about the way ions interact with magnetic fields. the distance an ion will jump from a failed collision (no fusion) is a known quantity. you dont want those ions hitting the wall of your reactor because that wastes energy, cooling the plasma. so you make your reactor larger than that you keep those ions and can redirect them back to the center. this number applies to tokomaks and its derivatives and even polywells. polywell is much smaller because its a spherical topology, in toroidal reactors its the cross section of the tube and combined with the huge coils and massive structural requirements to support said coils you end up with a much bigger machine.

[Bill Phil]

On ‎11‎/‎29‎/‎2016 at 11:05 PM, kerbiloid said:

Trying to realize an alternative energy source for a reactor-scaled particle flux except the same or another nuke.
Unlikely they use solar panels or coal.

(I mean: what powers the accelerator?)

The reactor would likely be designed to have a positive power output even with the loss in powering the accelerator (which only needs to be on for a tiny moment per cycle). Otherwise there'd be no point, since there would just be loss.

In short: The reactor would power it.

[kerbiloid]

4 hours ago, Bill Phil said:

In short: The reactor would power it.

If talk just about electricity,yes.

Quote

And where the neutron generator takes neutrons, otherwise than from D+T collision or a chain fission reaction?

(As I can understand, you can't just take neutrons from the active zone, as they are not charged. You can just reflect them back in random directions.)

Don't forget, they speak not about urchin or so, but about that 1 g of neutrons per 238 grams of  U.

I.e. it's not enough to create an energy itself, it's necessary to deliver it, in an appropriate form.
Say, if you have a ton of wood, you can't move a car with a fire.

Edited December 2, 2016 by kerbiloid

[RCgothic]

19 hours ago, peadar1987 said:

Yup, I worked on the UK's AGR fleet for a while.

We used to jokingly say it was a pity there hadn't been more serious nuclear accidents, as all of our training was Chernobyl, Chernobyl, Chernobyl, with a small amount of Windscale, Three Mile Island and Davis Besse thrown in. Maybe a little bit of Tokaimura if we were specifically talking about criticality incidents.

High five. 

One of my favourite incidents is Hunterstone B, Christmas 98. Total loss of power in a storm similar to Fukushima because an earlier loss of power had tripped the back ups, and with most everyone on holiday there wasn't enough manpower to reset them in case of a second loss of power. 4 hours before power was restored. Reactor was totally fine, and would have been fine for 20. AGR gas-cooled is just inherently safe comparatively. Magnox had even larger design margins.

It's a shame the on-line refuelling doesn't work. 

Edited December 2, 2016 by RCgothic

[peadar1987]

1 hour ago, RCgothic said:

High five. 

One of my favourite incidents is Hunterstone B, Christmas 98. Total loss of power in a storm similar to Fukushima because an earlier loss of power had tripped the back ups, and with most everyone on holiday there wasn't enough manpower to reset them in case of a second loss of power. 4 hours before power was restored. Reactor was totally fine, and would have been fine for 20. AGR gas-cooled is just inherently safe comparatively. Magnox had even larger design margins.

It's a shame the on-line refuelling doesn't work. 

Or when they accidentally sucked a load of sea water into the reactor core through a cracked weld in the gas circ coolant loop. Hunterston is not a lucky station!

I remember telling my girlfriend, a vet, about the Windscale fire (and how after the fire broke out, they had to cut the cooling, because, y'know, blowing oxygen over a fire!). Her instant response was "well why didn't they just use CO₂ to cool the reactor?". Took her 2 seconds to work out what it took the UKAEA a major radiological release to.

[Starman4308]

On 11/29/2016 at 4:41 AM, RCgothic said:

In a prompt critical (supercritical) reaction, enough neutrons are immediately released in each fission reaction to sustain further reactions. The timescale of this process is on the order of the travel time of the neutron between reactions (milliseconds). This is the type of reaction required for a bomb, although for reasons discussed above it would still not cause a nuclear detonation in s power plant. The fuel gets (potentially extremely, damagingly) hot and the reaction slows/stops. The speed with which prompt criticality changes the power level of a reactor makes it impossible to control, and reactors are always* designed so that they cannot go prompt critical. Reactors will always absorb too many neutrons, even with all control methods withdrawn.

The other type of criticality is delayed critical. A quirk of the fission reaction is that whilst each fission event creates neutrons, so too do the fission products a couple of seconds later as they decay. (It is for this reason a prompt critical reaction cannot be simply critical - if fission neutrons are enough to be self-sustaining, the delayed neutrons will later make it supercritical). If the reactor is operated such that on fissile neutrons alone the assembly is subcritical and delayed neutrons make up the difference to critical or supercritical as required, then the exponential coefficient is on the order of seconds and minutes rather than milliseconds. In conservative designs, reactors can take hours to build up to full power, leaving plenty of time for manual and automated control systems.

Thanks for this explanation; I think I finally understand the difference between prompt-critical and supercritical. In normal reactor operation, you never quite get to a self-sustaining reaction using just the prompt neutrons; you use the delayed neutrons (from the decay products of the original fission event), which have slower timescales for release, to go critical/supercritical. Because of the much slower timescales there, the machinery has seconds instead of microseconds to adjust the control rods and bring the reaction either subcritical or supercritical as necessary.

Prompt-critcal is when you do have a self-sustaining reaction on just the prompt neutrons, and because those neutrons are released at the moment of fission (which is itself a very fast process), the reactor quickly goes runaway and melts down because the scale of the reaction goes from a relative handful of nuclei to "way too many" in milliseconds or less.

Bombs, of course, are when you induce a prompt-critical reaction deliberately, and so suddenly that a good fraction of the nuclei react before your fissile material goes from "bomb" to "rapidly expanding cloud of plasma". It can't really happen in a reactor because they melt down/explode long before most of the nuclei have had a chance to react.

[magnemoe]

On 30.11.2016 at 8:36 AM, kerbiloid said:

And where the neutron generator takes neutrons, otherwise than from D+T collision or a chain fission reaction?

(As I can understand, you can't just take neutrons from the active zone, as they are not charged. You can just reflect them back in random directions.)

Don't forget, they speak not about urchin or so, but about that 1 g of neutrons per 238 grams of  U.

You have neutron generators, no they are probably not practical for running an reactor. 
However as I understand they are used in nuclear bombs to get them to ramp up faster. 

[kerbiloid]

25 minutes ago, magnemoe said:

You have neutron generators, no they are probably not practical for running an reactor. 
However as I understand they are used in nuclear bombs to get them to ramp up faster. 

Just microscopic amounts of neutrons to initiate a chain reaction inside. while such subcritical reactor means that hundreds tons of inert U or Th will be forced to decay under an external neutron flux.

(Of course, not literally all fuel, but definitely its amounts comparable with a usual reactor fuel consumption.)

Edited December 2, 2016 by kerbiloid

[wumpus]

5 hours ago, peadar1987 said:

Or when they accidentally sucked a load of sea water into the reactor core through a cracked weld in the gas circ coolant loop. Hunterston is not a lucky station!

I remember telling my girlfriend, a vet, about the Windscale fire (and how after the fire broke out, they had to cut the cooling, because, y'know, blowing oxygen over a fire!). Her instant response was "well why didn't they just use CO₂ to cool the reactor?". Took her 2 seconds to work out what it took the UKAEA a major radiological release to.

I'm missing the point as well.  If they weren't afraid of a containment leak, why didn't they just use water as a coolant?  Also, wiki mentions "fearing explosion from the recombination of hydrogen and oxygen": I suppose they might recombine somewhere suboptimal, but the net energy has to be lower.  I suspect using cooler bits of the fire to "presteam" the water would keep down massive pressure changes from vaporizing water (and this also assumes that the reactor is *so* *hot* that you can afford to give up the phase change energy because the steam will absorb that much energy.  I'd be impressed if even a burning reactor could get that hot).

[RCgothic]

Because Windscale wasn't a prototype for a water cooled reactor, it was a prototype for a gas cooled reactor.

Why go gas cooled? Because water absorbs too many neutrons as it moderates, making it impossible to get a sustainable reaction out of un-enriched natural uranium and enriching uranium at the time was difficult and expensive and supply was basically dependent on the US, which was undesirable for political reasons. Also, enriched fuel is less good at generating plutonium than natural uranium, and we wanted Pu to make bombs with.

Water-cooled graphite moderated is an option for a reactor operating on natural uranium, but as previously mentioned (Chernobyl RMBK design) that has stability issues.

So that's why the UK went gas cooled. I think the logic for Windscale was that it would be cooled by an open cycle of air and thus they could skip the pressure vessel and heat exchangers and just exhaust up the chimney.