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

[manaiaK]

3 hours ago, todofwar said:

So is fission/fusion just a way to get heat energy? No more efficient ways to harness it? 

Here and now, yes (nearly completely). Pretty steampunk, huh? Make a fire, boil water, use steam to power things. PV is shiny new quantum physics, but has its own problems outside Duna... er, Mars.

Relatedly, starting-2018-china-will-begin-turning-coal-plants-nuclear-reactors .

[Steel]

28 minutes ago, manaiaK said:

PV is shiny new quantum physics, but has its own problems outside Duna... er, Mars.

Well Juno went out to Jupiter on solar panels so its not that bad, and a lot of quantum physics is surprisingly well understood these days, it's just tricky is you want a lot of power

[kunok]

1 hour ago, KSK said:

Bimodal nuclear thermal engines (can be run as engines or generators) can be quite small. According to the introduction to this paper, the enhanced SNRE design (which I believe stands for Small Nuclear Rocket Engine) is  59cm in diameter by 132cm in length and can generate 25kWe through a Brayton cycle turbine.

So a 0.625m fission reactor is entirely realistic. Any size of rocket sized fusion  reactor is currently entirely fictional.  

You forget to put the paper

[Aghanim]

Fission reactor: Size is limited by how small can the neutrons interacts with the fission fuel to produce net power. Most if not all reactors today have a critical mass where the neutrons produced is sufficient to sustain further fission, but you can cheat by using neutron reflectors or neutron generators

Fusion reactor: Size is limited by how efficient can the ignition system can ignite the fuel plasma that produce net power, which with current reactor tech, is very big, and likely will get bigger because we don't have net power fusion reactor yet

My question: Is there any fission or fusion reactor tech that can explode in a nuclear explosion like in all of those science fiction stories? As far as I know fission reactors explodes with steam or hydrogen explosion, which is very bad but not catastrophic, and fusion reactors stops working when you look at it at the wrong way, with no explosions or something like that

[kerbiloid]

28 minutes ago, Aghanim said:

Is there any fission or fusion reactor tech that can explode in a nuclear explosion like in all of those science fiction stories?

A breeder reactor (i.e. on fast neutrons) theoretically can.
Also, their usage is the only way to utilize huge amounts of depleted uranium and natural thorium and build a capable thermonuclear energetics. I.e the only way at all.
(As you know, most of so-called thermonuke energetics is indeed a tricky combination of several types of fission/fusion types at once.)

Edited November 28, 2016 by kerbiloid

[KSK]

9 hours ago, kunok said:

You forget to put the paper

Sigh. I did go back and put the link in but it seems that this 'mobile friendly' forum software has a problem saving post edits when made from a tablet.

Anyway - link is here. Sorry about that and hopefully it's not paywalled for anyone.

Apparently the SNRE design was studied during Project Rover although I haven't been able to find anything definitive to say whether a working model was ever built. Incidentally, here's a nice scale drawing of the engines that were built during Rover - judged purely by eye Pewee doesn't seem to be much above a metre in diameter and Pewee is pretty old technology. I'd be surprised if SNRE and it's descendants (SNRE crops up quite a lot as the reference design for later NTR studies) weren't practical designs even if they haven't been built. 

Edited November 28, 2016 by KSK

[cantab]

9 hours ago, Aghanim said:

My question: Is there any fission or fusion reactor tech that can explode in a nuclear explosion like in all of those science fiction stories? As far as I know fission reactors explodes with steam or hydrogen explosion, which is very bad but not catastrophic, and fusion reactors stops working when you look at it at the wrong way, with no explosions or something like that

Yes and no, I think. If a reactor goes prompt-critical the activity will rise sharply and some sort of nuclear explosion may be possible. However, at most it will be equivalent to a 'fizzle' in nuclear bombs, where the explosion of a small fraction of the nuclear fuel blows the bomb/reactor apart preventing further reaction. The other likely outcome is that the extreme heat changes the reactor so the nuclear reactions are no longer prompt-critical and activity stabilises, which I believe is more-or-less what happens in a meltdown.

[magnemoe]

4 hours ago, cantab said:

Yes and no, I think. If a reactor goes prompt-critical the activity will rise sharply and some sort of nuclear explosion may be possible. However, at most it will be equivalent to a 'fizzle' in nuclear bombs, where the explosion of a small fraction of the nuclear fuel blows the bomb/reactor apart preventing further reaction. The other likely outcome is that the extreme heat changes the reactor so the nuclear reactions are no longer prompt-critical and activity stabilises, which I believe is more-or-less what happens in a meltdown.

Yes, however its not much hot plasma in the fusion chamber in any fusion reactor designs, the reactor chamber would be way more massive. Worst case you might get damage to the reaction chamber.
An fision reactor is hard to shut down as the core has lots of heat energy, for the large power reactors just stopping the reaction is not enough as the heat itself will damage the reactor critical, this was that happened in Fukushima, ironically this is not the case for small reactors, as the thermal energy is so much lower, Reactors in submarines will shut down at once even if ship fail catastrophically 
An meltdown is an situation there the reactor core melts, this happened in Chernobyl as the steam explosion damaged the control rods.
An fusion reactor would be like an oil burner if you see an fission reactor as an bonfire. An oil burner will stop if you stop the oil flow, it would also stop if you flood the chamber with oil. 
Sci-fi movies has reactors exploding as nuclear bombs as you have to have explosions in movies. Can not have realism come and remove the cool explosions. 

[mikegarrison]

On 11/27/2016 at 11:09 AM, todofwar said:

So is fission/fusion just a way to get heat energy? No more efficient ways to harness it? 

All devices are just a way to use energy. And all useful devices can be considered to be a way to make use of the flow of heat from a more ordered state to a less ordered state.

[cantab]

On the topic of exploding reactors, the SL-1 accident is an example. The reactor, designed for a few megawatts, went prompt-critical and its thermal output hit 20 thousand megawatts in four milliseconds, creating a steam explosion and propelling a piece of the reactor fast enough to nail a man to the ceiling.

https://en.wikipedia.org/wiki/SL-1

That's probably about as bomb-like as a fission reactor is going to get. I suppose in some reactor types it's possible that the fuel itself, rather than a moderating coolant, would vaporise and drive the explosion, but the general principle will be the same.

[Gaarst]

19 hours ago, Aghanim said:

My question: Is there any fission or fusion reactor tech that can explode in a nuclear explosion like in all of those science fiction stories? As far as I know fission reactors explodes with steam or hydrogen explosion, which is very bad but not catastrophic, and fusion reactors stops working when you look at it at the wrong way, with no explosions or something like that

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.

[todofwar]

2 hours ago, mikegarrison said:

All devices are just a way to use energy. And all useful devices can be considered to be a way to make use of the flow of heat from a more ordered state to a less ordered state.

I was talking about a heat engine specifically, which has an inherent efficiency limit. Fuel cells are not bounded by the same limit, and I don't think PVs are either. Was just wondering if nuclear really is just a fancy way to boil water when all is said and done.

[pincushionman]

1 hour ago, todofwar said:

…Was just wondering if nuclear really is just a fancy way to boil water when all is said and done.

In terms of terrestrial power production, it absolutely is, just like most coal or natural gas plants. They're all glorified steam engines,

[linkxsc]

13 minutes ago, pincushionman said:

In terms of terrestrial power production, it absolutely is, just like most coal or natural gas plants. They're all glorified steam engines,

It aint gloriius but it works.

Course I get dirty looks from people when I tell them that I went to school for 2 years to learn about hot water flowing through a pipe.

[RCgothic]

Nuclear engineer here. 

No, no fission reactor design can detonate like a nuclear explosion. When the reaction runs away out of control the fissionable material heats up, and there are several mechanisms by which that makes the fuel less reactive, bringing the reaction to a new equilibrium state. The fuel may melt, but if there's an explosion it's going to be as a result of other materials present in the core (hydrogen, steam, molten salt explosions).

Designing a bomb to detonate (even to get it to fizzle) is very difficult.  You need to convince most of the fuel to react before heat increase brings the reactivity down. This requires both a very dense concentration of fissile material (else neutrons won't propagate through the entire core fast enough) and a very low concentration of non-fissile nuclei (which absorb valuable neutrons). Additionally, the need to exclude non-fissile nuclei also generally excludes the use of a moderator. A moderator is a material that slows down the 'fast' neutrons emitted by fission events to a 'thermal' level which more readily react with fissile nuclei. Without a moderator the fissile material is less reactive, so yet greater density of fissile nuclei is required.

This all add up to very high enrichment, typically 95-98%. Even fast reactors don't normally get this high. Most reactor fuel is uranium oxide (UO2) enriched to about 5%, although the presence of the oxygen atoms makes the effective reactivity even lower.

Finally, if you attempt to make critical assembly casually, it will just heat up as portions go critical before the full mass. Therefore a very rapid change of geometry is required, either compression or gun type in order to set off the final detonation.

Reactors on the other hand are designed not to explode! Not only do they lack any means to effect the final geometry change, sufficient fuel enrichment, and also have far too many foreign nuclei in the way, they are carefully designed not to operate in dangerous reaction regimes. They do this by manipulating several types of criticality:

In a sub-critical assembly the reaction is not self sustaining, and if the reaction was previously critical or supercritical the reaction power will be reducing.

A critical assembly is one in which the number of neutrons released is precisely as many as is required for the reaction to be self-sustaining at its current power level.

A supercritical assembly is one in which each reaction increases the neutron flux. The reaction thus grows exponentially.

A power plant must operate in all these regimes. A plant that could not go supercritical could not start up.  By adjusting the number of neutrons absorbed in the reactor the power level is controlled.

However there are two further types of criticality which are extremely important to the design of reactors, referring more to the response time than whether the power level is changing:

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.

And all that is why the worst that can happen given total control/coolant failure is a meltdown and not a mushroom cloud.**

*The Soviet RMBK design can in certain situations, which is why Chernobyl had a prompt critical excursion when it was messed about with by people who didn't know what they were doing. The heat build up caused a steam explosion and graphite fire. Annihilation of the cooling systems caused the core to melt. 

**Ok, conventional explosions can also cause mushroom clouds, but you know what I mean.

Edited November 29, 2016 by RCgothic

[peadar1987]

19 minutes ago, RCgothic said:

 

*The Soviet RMBK design can in certain situations, which is why Chernobyl had a prompt critical excursion when it was messed about with by people who didn't know what they were doing. The heat build up caused a steam explosion and graphite fire. Annihilation of the cooling systems caused the core to melt. 

 

Positive void coefficient ftw

[cantab]

2 hours ago, RCgothic said:

Reactors on the other hand are designed not to explode!

OK, let's say that the Star Federation has ignored that bit. The reactor's been designed for maximum power, minimum mass, high operating temperatures*, and whatever other aspects of performance, and safety has been completely thrown out. The reactors on a starship aren't designed to be bombs, but they're not designed to not become bombs either. Basically if the reactor designers and operators are very foolish, what kind of boom might be possible?

(*The hotter the reactor core, the hotter its cooling radiators, and since black body radiation scales with temperature^4 that's a big advantage.)

[KSK]

33 minutes ago, cantab said:

OK, let's say that the Star Federation has ignored that bit. The reactor's been designed for maximum power, minimum mass, high operating temperatures*, and whatever other aspects of performance, and safety has been completely thrown out. The reactors on a starship aren't designed to be bombs, but they're not designed to not become bombs either. Basically if the reactor designers and operators are very foolish, what kind of boom might be possible?

(*The hotter the reactor core, the hotter its cooling radiators, and since black body radiation scales with temperature^4 that's a big advantage.)

From  @RCgothic's excellent post (which I feel smarter for having read!), I would say a Chernobyl like incident. Reactor goes prompt-critical, which is bad news for whatever spacecraft it's attached to, but the sudden temperature rise, lack of geometry change to the fissile mass, and (unless the Star Federation is willfully reckless) insufficient enrichment of said fissile mass, should stop it going nuclear-boom.

Edit. I'm also guessing that using sufficiently enriched material is a bad idea even for the Star Federation because it would just make the reactor too difficult to control.

Edited November 29, 2016 by KSK

[RCgothic]

35 minutes ago, peadar1987 said:

Positive void coefficient ftw

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...

5 minutes ago, cantab said:

OK, let's say that the Star Federation has ignored that bit. The reactor's been designed for maximum power, minimum mass, high operating temperatures*, and whatever other aspects of performance, and safety has been completely thrown out. The reactors on a starship aren't designed to be bombs, but they're not designed to not become bombs either. Basically if the reactor designers and operators are very foolish, what kind of boom might be possible?

(*The hotter the reactor core, the hotter its cooling radiators, and since black body radiation scales with temperature^4 that's a big advantage.)

Hmmm. Well if I were designing a high power density reactor without shielding, water is heavy and doesn't get that hot, so I'd throw out all water-based designs.

There have been a few designs that might inform design of a spacecraft engine.

UHTREX is gas-cooled, graphite moderated, and operates without fuel cladding. Very efficient fuel burnup, and very high coolant outlet temperature (1300'C).  The drawback is a very contaminated fuel circuit, but we don't care about that. Details are scarce, but based on an original diameter of 13ft with shielding I'd estimate 3m diameter spherical pressure vessel (~11t) filled with graphite (~29t) and .5t fuel with .5t for fuelling . 41t and 3MW output, burns through 6 fuel element per day for an endurance of ~200 days at full power. Roughly 73W/kg, but also required turbines and radiators.

This is clearly dominated by the moderator, so let's throw that out too. I don't think molten salt will save any mass as a coolant when we consider the turbine loop volume, so Gas-cooled fast reactor it is. This type of reactor is very rare, with none built to my knowledge, but they're are a few later designs. Even so it's difficult to be specific.

If I were to speculate, I'd say you could probably get the same power output out of a reactor half the diameter of UHTREX with no moderator. The trade off is enriched fuel, but money no object for the star federation, right?

That would be about 1MWe/tonne, for the reactor itself. 3t for a 9MWth/3MWe reactor core.

It would need to radiate about 2MWth per MWe,  or ~70m2 to exhaust 6MWth for a 9MWth 3MWe reactor at 850'C reactor inlet temperature. I found a NSS article implying that would probably weigh around 0.7t.

Best guess for a bespoke 3MWe turbine generator plant is approx 4t.

Call that all 9t including refuelling and control. 333W/kg for the whole system.

And if one of these let's go you still won't get a nuclear detonation. I expect the pressure vessel would go off like a fragmentation grenade, spraying glowing chunks of high velocity debris.

[micr0wave]

Isn't a RTG some sort of fission reactor ?

i mean, nuclear decay is transformed into electrical energy, so i think it's the smallest we can build with current technology.

 

[kerbiloid]

No. RTG doesn't use chain reaction, its just utilizes decay energy.

[RCgothic]

RTGs have specific power of around 5W/kg for comparison.

[KSK]

3 hours ago, micr0wave said:

Isn't a RTG some sort of fission reactor ?

i mean, nuclear decay is transformed into electrical energy, so i think it's the smallest we can build with current technology.

Well who knew. According to Wikipedia, 

"...nuclear fission is either a nuclear reaction or a radioactive decay process in which the nucleus of an atom splits into smaller parts."

So I suppose by that definition an RTG is a fission reactor. I suspect most people would go with @kerbiloid's definition though - I know that was my first thought.

 

Edited November 29, 2016 by KSK

[kerbiloid]

19 minutes ago, KSK said:

According to Wikipedia, 

Quote

A nuclear reactor, formerly known as an atomic pile, is a device used to initiate and control a sustained nuclear chain reaction

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

Edited November 29, 2016 by kerbiloid

[cantab]

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).

Edited November 29, 2016 by cantab