Hyperion Mini Nuclear Reactor Set to Transform Power Generation

Originally posted by thecinic
That is some neat technology, it can't be any unsafer then a complete nuclear facility......

Yes the nuclear solution is very much safer than the atomic solution.
The Papp engine using noble gases exploded injuring and killing an observer.
They should have been behind a brick wall before deciding the pull any wires.
Spark plugging those gases is not child's play.
Dr. Moray's generator also had an explosion of sorts perhaps why the Department
of Commerce can't let the technology go overseas or is that hearsay.

Originally posted by Authenticity
It is amazing, I feel that the future of the world is leaning on nuclear reactors that are safe and can virtually run forever as long as we keep feeding it uranium which the government makes in abundance.

Governments make uranium, do they?

Do they make gold and silver too?
Next you'll tell us the government makes latinum.

Uranium is mined. Thats why we have uranium mines.
And right now it looks like the world is heading for a shortage of uranium.

Originally posted by Kailassa


Uranium is mined. Thats why we have uranium mines.
And right now it looks like the world is heading for a shortage of uranium.

uranium 235 is in short supply, true enough, that's why it'll have to be a breeder reactor - based on Thorium 232 for several reasons,, outlined only recently on this forum.

see

www.abovetopsecret.com...

and

www.abovetopsecret.com...


or use search (both ATS and web)


i don't know why nuclear energy is suddenly en vogue on ATS, but nevertheless i believe that it's, imho, basically a certainty that it'll be developed unless civilisation fails anyway because when done right (as described in the linked threads) it's everything fusion claimed to be and capable of delivering valuable rare elements as a side effect, which wouldn't have to be mined anymore.

China is doing it, India is actively pursuing a Thorium program and outside some parts of Europe, the general climate appears to be either neutral or even favorable, so it's really only a matter of time - again, if we still have any.


That said, there are aspects of small scale nukes such as nuscale and hyperion designs which strike me as highly problematic, such as once-through use of uranium fuels (no improvement on the waste front) lack of containment, less supervision, and so on. i also question the chemical safety factor of certain fuels, if you use metal oxides, the fuel won't burn and will only melt at extremely high temperatures (to the tune of 3000K), providing inherent safety even in case of an exposed core. if you're using nitrides or worse, hydrides, the fuel will chemically burn in contact with oxygen, releasing a lot of radioactivity in the process, much like Chernobyl's graphite did.

most of all, these systems will have a hard time breeding any fuel if so converted, because neutron economies are inherently better in large reactors (volume to surface ratio...).

the most important task in nuclear power is IMHO building large scale breeders that work and reliably produce energy, to prove the concept in the eyes of the public and generate enough revenue to sustain and expand nuclear power on its own without subsidies. there are small nuke plants on submarines, so yes, miniaturisation works, but that's simply not the point.

and what about waste that comes with all fission reactors? Fission still seems quite archaic no matter how it is implemented. There are some exciting fusion projects out there, particurarly focus fusion and new insights into the phenomenon of cold fusion. I look forward to the day when fission is a thing of the past, along with flat earth theory and steam engines.




focusfusion.org...

reply to post by C0bzz


^ That's amazing..

My state[Texas] has the most wind turbines

reply to post by volafox


Thats brilliant really! Iv always wondered why gas we could use would be allowed to escape and cause problems when we could be using it instead.

There are too many problems with safe containment to have these buried for small villages.

However, much more practical is a large installation, run by a utility with 24/7/365 on-site monitoring, which has a large bank of these.

A big problem with current nuclear reactors is they are so individually expensive and take many years to construct though they produce a large amount of power. In some ways there is a diseconomy-of-scale, in the same way that a skyscraper with 100,000 square meters of floor space is more than 10x as expensive as 10 buildings of 10,000 square meters.

It is more financially practical for a utility to buy a smaller number of these smaller reactors, and add more capacity as cashflow and demand require.

If they can be managed and fueled independently, that's another advantage.

These reactors are an inherently safe design for a nuclear reactor that is but this has never been practical from a political stand point. This device gives a new meaning to "Not in my back yard." I think they will get funding and will continue their work but will only be used in remote government run installations and military bases. It might perhaps be used for electric propulsion for spacecraft but will not be adopted for small community use as the inventors envision I think.

Focus fusion also mentioned in this thread is interesting but has not matured yet to the point of commercialization. Nearer to commercialization are a number of low energy nuclear effect based power generators. I will give some citations in another post.

Originally posted by mudskipper
and what about waste that comes with all fission reactors? Fission still seems quite archaic no matter how it is implemented. There are some exciting fusion projects out there, particurarly focus fusion and new insights into the phenomenon of cold fusion. I look forward to the day when fission is a thing of the past, along with flat earth theory and steam engines.




focusfusion.org...

Only economical commercial fusion is many decades away, and fast reactors can have most of the advantages of fusion. You can keep dreaming, I'll stick to implementing fast reactors (and fast breeders) as soon as possible, while developing fusion for the future, because contrary to common belief, we actually need energy today.

deleted,

need more data[

Originally posted by Authenticity
www.nextenergynews.com

The self sufficient nuclear generator is simply buried underground

I'm not sure how sound this technology is, but my first question is, am I the only one who thinks it's more than a little inconvenient to have to dig this thing up out of the ground every 8-10 years to re-fuel it? It actually sounds like a nightmare digging that thing up every 8 years.

Interesting, but still a little misleading. Having looked at the designs produced by the company itself, the module might be the size of a phone booth, but you still need significant infrastructure below and above ground to use it.

Don't get me wrong, this is still a great leap forward if it works, but it won't be as simple as putting something in the ground and hooking it up to a grid as that article implies. You still need all the infrastructure in place.

Still good though.

another application :

marine powerplants [ for merchant naval craft ]

if the claims of the hyperion battery are to be believed [ power 25 000 homes for 5 years ] - and if that is an ecconomically viable model

then it SHOULD be viavbe to mount banks of 2 or 3 or 4 hyperion batteries in merchant ships - and power them for 5 years


the largest merchant vessels have powerplants of 40 to 90mw - and burn upto 60 million dollars / year in fuel oil

now a 25 000 home town cannot sustain a 2 million dollars / year electric bill

so this hyperion battry should be able to replace feul oil plants in merchsant vessels

Originally posted by mudskipper

focusfusion.org...

if you favor one form of nuclear energy over another, that's fine and dandy , there's just a slight availability problem, afaics.

i've taken a look at the dfp fusion site and did some web searching and while direct conversion is very much preferable in many ways, i can't help but notice that roughly half their output is supposed to be Xrays. how are they supposed to be converted into electricity, a vacuum device or by some sort of photo cell.

btw, even if it's 99% neutron free, you'll still need a blanket for that remaining percent (using what? fertile material? déjà vu?) and you'll have activation issues that will have to be addressed. (any type of) nuclear technology isn't a toy, especially at scale.

==============================================================================

Originally posted by ignorant_ape
another application :

marine powerplants [ for merchant naval craft ]

several navies have experience with such designs and i doubt they're doing it for the cost. look up the amount of fissile material used, it's at least 4 times as much as in normal LWRs (which suck). that's a lot of enrichment and it won't scale because it's just not there unless you breed fissile in appropriate reactors - so far that part alone is problematic.

Apologies for the late reply, I've been busy.

Originally posted by Kailassa

Originally posted by Authenticity
It is amazing, I feel that the future of the world is leaning on nuclear reactors that are safe and can virtually run forever as long as we keep feeding it uranium which the government makes in abundance.

Governments make uranium, do they?

Do they make gold and silver too?
Next you'll tell us the government makes latinum.

Uranium is mined. Thats why we have uranium mines.
And right now it looks like the world is heading for a shortage of uranium.

Any Uranium shortage that occurs will be the result of not being able to boost production of Uranium fast enough to keep up with the growth in demand, especially as China brings 100 reactors online in the next decade. Uranium from nuclear weapons is also running out, so extra mining will be required (or a move to using Plutonium based nuclear weapons as fuel which is what the US is doing) to make up for any short-fall. With that said, it is generally acknowledged that over 100 years of Uranium exists at current consumption rates with significantly more to be discovered. Total known recoverable Uranium has increased in Australia by a factor of about 3 since 1990.

i don't know why nuclear energy is suddenly en vogue on ATS, but nevertheless i believe that it's,

Probably because I keep spamming it in chat. And this forum is made up mostly by conservative americans (in my opinion) who are pro-nuclear.

such as once-through use of uranium fuels (no improvement on the waste front)

Once through use of Thorium isn't any better unless you have some form of continuous reprocessing like in LFTR.

In any case NuScale is supposed to be an integral Pressurized Water Reactor where the reactor, coolant pumps, control rod drive mechanism, and steam generator are integrated into a single piece. It is based off PWR technology which means it should be fairly easy to license by the NRC and should leverage prior experience with PWR technology (these companies are made to make money remember). Adding reprocessing to the Hyperion is conceivable however it would not be done at the reactor site, but at the factory. I think Hyperion needs to focus on getting its design licensed and sold as fast as possible, then focus on the fuel cycle in the future.

lack of containment

Both have containment. They just might have innovative containment that is somewhat of a different design to existing technology by for example, placing the entire reactor underground. These containments might have difficulty getting licensed by the NRC, but again, they do have containment.

less supervision

Less supervision is only going to happen if it doesn't need the supervision. Lead-Bismuth batteries are far more proliferation resistant than typical LWR technology, and most SMRs have some form of passive safety. Again, there might be difficulty getting licensed this way.

if you use metal oxides, the fuel won't burn and will only melt at extremely high temperatures (to the tune of 3000K), providing inherent safety even in case of an exposed core.

You mean like at Chernobyl and Three Mile Island, both of which used Uranium Dioxide fuel? The Zircalloy fuel cladding melted and the 'metal oxide' fuel formed something similar to a molten concrete even though it technically did not melt.

if you're using nitrides or worse, hydrides, the fuel will chemically burn in contact with oxygen, releasing a lot of radioactivity in the process, much like Chernobyl's graphite did.

There's also a question of exactly how this would happen.

In a Pressurized Water Reactor coolant leaks are problematic for a number of reasons. The coolant is at an extremely high pressure therefore a significant leak will mean that it will be lost as steam, hence complicated safety systems will be required to inject water back into the core. High temperature borated water is corrosive and acidic which makes leaks more likely. And the primary loop loop leaves the reactor pressure vessel to reach the steam generators. If the cladding melts then essentially the entire core will melt down (even though the metal oxide fuel won't technically melt).

Meanwhile a Liquid Metal Reactor has the entire primary loop contained within a single vessel that operates at atmospheric pressure and the fuel will not boil under any circumstances. A containment could also be designed so that even if the reactor vessel had a leak, the coolant will always stay above the fuel (this is what PRISM does). The situation you described is therefore impossible, full stop.

the most important task in nuclear power is IMHO building large scale breeders that work and reliably produce energy, to prove the concept in the eyes of the public and generate enough revenue to sustain and expand nuclear power on its own without subsidies.

The most important issues that need to be solved are in my opinion: passive safety, capital cost, and waste. I think a smaller reactor like NuScale could help solve the first two issues, whereas a reactor like IFR/LFTR could solve all three although will probably need government support to be developed. Increased efficiency, greater proliferation resistance, and higher temperatures for synfuel production would also be nice. Breeding would only be required if there's an unprecedented scale up of nuclear power, which won't occur until those three issues are solved. Luckily, PRISM (or LFTR) can also be operated as a breeder.

Originally posted by C0bzz

Once through use of Thorium isn't any better unless you have some form of continuous reprocessing like in LFTR.

not that much better, you'll still need some fissile startup charge but as long as breeding ratio is 1 or more you'll never run out of fissile, that's the main difference. the fuel elements will fail at some point, though.

lack of containment

Both have containment. They just might have innovative containment that is somewhat of a different design to existing technology by for example, placing the entire reactor underground. These containments might have difficulty getting licensed by the NRC, but again, they do have containment.

the illustrations i've seen indicate a reactor several dozen feet below the surface with a heavy lid on

www.hyperionpowergeneration.com...

if accurate, it's better than nothing of course, but it just doesn't compare to a LWR containment. underground would mean several hundred feet, preferably in solid rock, that's not the case here, so the module is both more accessible and less contained. maybe it's enough, as is, really not my call, it's just less that's all, the cost advantage has to materialize somehow, i guess.

You mean like at Chernobyl and Three Mile Island, both of which used Uranium Dioxide fuel? The Zircalloy fuel cladding melted and the 'metal oxide' fuel formed something similar to a molten concrete even though it technically did not melt.

had these been metal fueled reactors, more heat would have been released by chemical reactions and (what was left of) fuel integrity would have vanished. maybe less so for TMI, since the containment held and the fire would have run out of oxygen soon, but Chernobyl 4's core was exposed to air and the graphite was bad enough, had the rest been consumed by fire, more radioactivity would have been released.

if you're using nitrides or worse, hydrides, the fuel will chemically burn in contact with oxygen, releasing a lot of radioactivity in the process, much like Chernobyl's graphite did.

There's also a question of exactly how this would happen.

in contact with air, at high temperature hydrogen is expelled from the metal and burns, the remaining metal burns, too, creating lots of excess heat and steam, which will expel some nuclear fuel.in the case of nitrides, nitrogen will react with air, at sufficient temperatures although the reaction is endothermic (the core won't breach unless it's overheated, right? when the nitrogen is gone, metallic fuel will of course burn as merrily as before)

In a Pressurized Water Reactor coolant leaks are problematic for a number of reasons. The coolant is at an extremely high pressure therefore a significant leak will mean that it will be lost as steam, hence complicated safety systems will be required to inject water back into the core. High temperature borated water is corrosive and acidic which makes leaks more likely. And the primary loop loop leaves the reactor pressure vessel to reach the steam generators. If the cladding melts then essentially the entire core will melt down (even though the metal oxide fuel won't technically melt).

yes, PWRs have their problems, sure. steam can be recondensed within the containment, however, providing an efficient way of cooling, unless you run totally out of water, that is.

Meanwhile a Liquid Metal Reactor has the entire primary loop contained within a single vessel that operates at atmospheric pressure and the fuel will not boil under any circumstances. A containment could also be designed so that even if the reactor vessel had a leak, the coolant will always stay above the fuel (this is what PRISM does). The situation you described is therefore impossible, full stop.

what would happen to your reactor vessel when the coolant temperature exceeds the capabilities of the material?

lower pressure addresses (primary circuit only) leak issues - note that both major accidents (TMI, Chernobyl) didn't result from leaking.. overheating can happen without a coolant leak and the only way to really remedy this is a nuclear fuel that expands adequately at higher temperatures..

in contact with air, at high temperature hydrogen is expelled from the metal and burns, the remaining metal burns, too,

Hydrogen gas is only evolved at very high temperatures of around 1000 degrees C (2000F) when the Zirconium fuel cladding reacts with the steam (remember air doesn't contain hydrogen). Metal fuel and nitride fuel may be a bad idea for light water technology, but liquid metal technology is far different.

In the case of Hyperion (which is the only reactor that uses Nitride fuel that I know of) the cladding is HT-9 steel and the entire core is immersed in low pressure lead which has enormous thermal inertia and conductivity. Nitride fuel also conducts heat much better than oxide fuel. I am not familiar with how reactive steel and nitride fuels are with air or water, but since the entire core is immersed in low pressure lead, the possibility of any reaction occurring with either is practically zero.

The likely hood of a reactor vessel failure is extremely low because it's low pressure, and in any case it is possible to build a backup reactor vessel (which is what IFR / PRISM does) because they are easy to fabricate since the steel doesn't have to be as thick as a LWR.

In conclusion, there's no way for the Nitride fuel to react with air and burn because it will always be covered by lead which can not boil away and leaks are unlikely.

what would happen to your reactor vessel when the coolant temperature exceeds the capabilities of the material?

Short answer - it won't because the reaction should naturally stop before then, leaving only decay heat.

Lead has significant thermal inertia and conductivity and the entire core is immersed in a large pool of it. I haven't seen any significant safety information about Hyperion, but here's what GEN-IV have to say about the SSTAR, a 20mw(e) lead fast reactor with nitride fuel (which is probably similar to Hyperion).

Primary coolant circulation for direct heat removal (DHR): Natural
Direct heat removal (DHR): Reactor Vessel Air Cooling System + Multiple Direct Reactor Cooling Systems

www.gen-4.org...

In conclusion, due to the inherit properties of lead the coolant should always cover and circulate around the core, which will effectively transfer the heat from the fuel to the coolant. From there, the passive RVACS (Reactor Vessel Air Cooling System) can passively take away heat. Hence the coolant shouldn't reach the point where it begins to melt the reactor vessel.

Chernobyl wouldn't have happened because it was the result of the water boiling, reducing the amount of neutrons being absorbed leading to the power to rise extremely quickly. The Lead Fast Reactor has 'inherent negative reactivity feedback' and even if it did have a positive reactivity feedback the coolant wouldn't of flashed to steam resulting in an explosion because lead boils at 1700 degrees. Instead, the lead would slowly heat up until the reaction was stopped by operator intervention. Three Mile Island was a loss of flow incident the initiator of which wouldn't be possible in the first place, but in any case you have natural circulation in this reactor.

Here's a video for PRISM / IFR safety:



And here's what Dr. Yoon Chang has to say about IFR:

Even in worst case accident events (loss-of-flow and/or loss-of-heat-sink without scram like TMI-2 or Chernobyl initiators), the initial coolant temperature rise will cause thermal expansion of fuel assemblies which increases neutron leakages, and hence the power is brought down all by itself without operator actions or safety systems. Ironically in these events, as the inherent feedbacks try to bring down the power, the Doppler feedback actually contributes positive reactivity. (Recall that Doppler was necessary to protect against inceasing power. When power is coming down, it tries to raise the power.) This feature is unique only with the IFR. The metal fuel operates at low temperature because of a high thermal conductivity (a factor of 10 higher than oxide), so the stored reactivity, (Doppler coefficient) x (temperature difference), is too small to override the negative feedback due to coolant temperature rise. In other words, it's the temperature difference rather than Doppler coeffient itself that enables this unique inherent safety. Therefore, in IFR the Doppler feedback is adequate to deal with overpower transients, and at the same time it enables inherent safety features in the other extreme accident conditions.

www.thesciencecouncil.com...

To take away the decay heat once the reaction is stopped, PRISM / IFR has natural air circulation around the reactor that is always enabled. (This is actually same system as Lead Fast Reactor - RVACS). The reactor looses half a megawatt of heat because the safety system is always running and cannot be turned off.

Oxide fuel has very low conductivity so can be far less safe than a nitride or metal approach in a fast reactor. Metallic fuel is far superior.

Very Low Probability Events – Oxide Fuel (cont’d)

Accident termination is by an energetic disassembly of the core
• a large power excursion to mechanically disperse the core
• likely failure of the reactor vessel
• containment structure mitigates releases to the environment

Kaboom. That would be a mess.

Very Low Probability Events – Metallic Fuel (cont’d)

The result is some core damage, but no reactor damage
– Fermi-1 experienced a metallic fuel melting accident, and was
reloaded and subsequently operated before being shut down


The favorable response to even the most severe accidents is due to
the thermophysical properties of metallic fuel
– Relatively low melting point and high thermal conductivity
– Compatibility with liquid sodium coolant, even when molten
– The key is limited early fuel pin failure and fuel removal from the
core

www-pub.iaea.org...

However, here’s a couple of the technical safety considerations.
- During an unprotected loss-of-flow, the coolant temperature rises rapidly, which introduces negative reactivity feedbacks due to radial expansion of core, control rod drive-line expansion, etc. This brings the power down.
- The high thermal conductivity of metal means that the fuel has far less stored heat to be dissipated in the event of loss of coolant flow, greatly reducing the temperature swings, making passive convective cooling much more feasible. In the safety demos at EBR-II in 1986, completely passive effects brought the coolant (and core) temperature down to ~200 degrees F below the boiling point of the sodium coolant.

bravenewclimate.com...

the illustrations i've seen indicate a reactor several dozen feet below the surface with a heavy lid on

www.hyperionpowergeneration.com...

if accurate, it's better than nothing of course, but it just doesn't compare to a LWR containment.

Doesn't need a LWR containment. In a LWR the containment has to deal with high pressure steam.

yes, PWRs have their problems, sure. steam can be recondensed within the containment, however, providing an efficient way of cooling, unless you run totally out of water, that is.

I consider PWR and BWR technology extremely safe, but they don't hold a candle to liquid metal technology. If there is a large break in a PWRor BWR then it is required to insert the control rods immediately as well as inject large amounts of water into the core so the core is only momentarily (or not at all) uncovered by water.



With a SFR with metallic fuel all thats required is natural air circulation which cannot be turned off and is running all the time.

I'm going to quote wiki, whoever edits the nuclear pages on wikipedia know what he is talking about.

Hyperion Power Generation

Fuel and Coolant Selection

According to Hyperion, the uranium nitride fuel incorporated in the design is generally similar in physical characteristics and neutronics to the standard ceramic uranium oxide fuel that is used at present in modern light water nuclear reactors. However, it has certain beneficial traits - higher thermal conductivity - and thus less retained heat energy - that make it preferable over oxide fuels when used at temperature regimes that are greater than the 250 to 300 °C (482 to 572 °F) temperatures found in light water reactors[8]. By operating at higher temperatures, steam plants can operate at a higher thermal efficiency. The presentation by Hyperion at the ANS 2009 conference mentions the use of the Doppler inherent negative temperature coefficient of reactivity in this reactor as a means of control.[9] Nuclear scientist Alexander Sesonske avers that nitride fuels have both received very little development (as of 1973) and seem to have a very favorable combination of physical properties - especially in fast reactors.[10] Whether this carries over to lead-bismuth cooled reactors is a question not answered in the reviewed literature, though the Soviet Union has worked with this type of reactor before in naval service; in particular, the Alfa class submarine - well known in the West for its high speed operation - was driven by such a lead-bismuth reactor which is known to have worked very effectively.[8]

The Hyperion module has sufficient fuel for 3650 full power days at 70 MWth, is capable of load following, and is meant to be built in pairs; one module can be at power, while another can be under installation or uninstallation at the same time, ensuring reliable supply of electricity.[8]

Safety, Control, And Transport

Four mechanisms of control are used in the reactor. There are two types of control rods - rapid shutdown rods, designed to promptly absorb a large quantity of reactivity from the reactor to bring it below the shutdown margin, and fine-grained working control rods, also known as shims, which are used to compensate for the long-term decrease in reactivity (long-term decrease in Keff) that comes from the nuclear fuel being depleted and fission products being formed. The shims, in particular, have 1.5 metres (4.9 ft) of travel distance which they slowly travel over the life of the reactor. There is a secondary shutdown system consisting of neutron-absorbing boron carbide balls that can be launched into the core in the event the shutdown rods are not responsive and rapid shutdown is called for. Fourth, there is the prompt negative temperature coefficient of reactivity, which prevents the reactor from remaining critical if it should enter into an unsafe temperature range. The reactor is designed so that once shut down, it does not require external agencies aside from natural conduction and convection to surrounding natural media to remove residual heat, qualifying it as highly safe.[8]

The reactor weighs 20 tonnes (44,000 lb) fully fueled (including coolant), and it can be transported by truck or by rail to its destination. Radiation protection during transport is integral, making it nearly impossible for any transport accident to threaten the release of radiation. As the coolant is composed of lead (a strong absorber of gamma radiation), the reactor is very safe for humans to be in close proximity to while the reactor is transported; further, if the reactor is allowed sufficient time to eliminate decay heat prior to transport, the lead-bismuth coolant will be in solid phase, thus fixing the internals of the reactor in place, causing the reactor to behave as a single piece of metal if subjected to external shock.[8]

en.wikipedia.org...

Uranium Nitride kills oxide for this use and doesn't have the disadvantages you talked above. It's safer because it has a higher conductivity which means less stored heat and helps with passive safety. Same is true for metallic fuels. Metallic fuels can operate on a harder neutron spectrum which means higher breeding ratios can be attained. There's no possibility of it burning either.

if you think this is something you should see the mini ones they power the triangles with

Originally posted by C0bzz

In conclusion, there's no way for the Nitride fuel to react with air and burn because it will always be covered by lead which can not boil away and leaks are unlikely.

only at 1750C. the question is whether reactivity excursions can occur fast enough to reach such temperatures.

what would happen to your reactor vessel when the coolant temperature exceeds the capabilities of the material?

Short answer - it won't because the reaction should naturally stop before then, leaving only decay heat.

yeah, like the IFR, the test, iirc, was full power followed by loss of cooling (simulated b turning the pumps off) followed by doing nothing. that's a reasonable accident scenario and the design handled it with ease. if the Hyperion type's behavior is similar, it's of course a big advantage, but that doesn't mean that all conceivable types of accidents are automatically covered, does it?

f-ex. what would happen if one tried hard (bug or user error) to trigger a criticality excursion from low operating power in an undercooled core, along the lines of Chernobyl? doppler broadening doesn't play a large role yet, fuel expansion is subject to inertia. at this point, thermal inertia is all you've got, right? the HX will of course contain some working fluid - water? so the question is whether one can presume the secondary side of the HX safe enough to allow pressure to escape.

if it really can't overheat, well then the point is moot, but i doubt such (purposefully stupid) tests have been done. of course, i agree that metallic fuel will do a better job at preventing an accident, but if one happened (not necessarily in operation, might as well occur during refuelling), there's the potential for chemical fires. a trade-off of sorts, as i see it.

Chernobyl wouldn't have happened because it was the result of the water boiling, reducing the amount of neutrons being absorbed leading to the power to rise extremely quickly. The Lead Fast Reactor has 'inherent negative reactivity feedback' and even if it did have a positive reactivity feedback the coolant wouldn't of flashed to steam resulting in an explosion because lead boils at 1700 degrees. Instead, the lead would slowly heat up until the reaction was stopped by operator intervention.

granted, the RMBKs' feedback issues drastically increased the severity of the accident, the underlying error mode could still be problematic (as outlined above already), because once you exceed nominal temps, any amount of additional heat is bad news. decay heat is a function of the last power setting before shutdown, right? if so, which effect does a rapid ramp up from a cold condition have on the cooling requirements later? lead boiling isn't the problem, it's structural failure from overheating. of course, some sort of salt or silica based phase change system could be used to greatly increase heat capacity if needed, but so far it's not included, iirc.

Even in worst case accident events (loss-of-flow and/or loss-of-heat-sink without scram

is this really the worst case scenario? isn't reactivity excursion more dangerous because it can happen much faster? regarding natural circulation, it's of course more reliable than pumps and won't suffer from power loss, although obstructions could still shut down the flow, couldn't it? in such a small unit, heat conduction might relieve many of these problems anyway, unless i'm mistaken.

i appreciate the material you gathered and will have a closer look at your sources.


PS:

The presentation by Hyperion at the ANS 2009 conference mentions the use of the Doppler inherent negative temperature coefficient of reactivity in this reactor as a means of control.[9] Nuclear scientist Alexander Sesonske avers that nitride fuels have both received very little development (as of 1973) and seem to have a very favorable combination of physical properties - especially in fast reactors.[10]

btw i can see the nitrogen producing a lot of C14, too

www.energyfromthorium.com...

science.howstuffworks.com...

there's a reason people lived with the poor conductivity of oxides i guess.