Are MSR thorium breeder cycles a good option for our energy future?

[mesa]

I would imagine it could be difficult to make drastic changes to current nuclear technologies however with the current political and general population consensus about nuclear energy does it make sense to try a radically different approach to nuclear power production?

Thorium has come up several times on the forum (especially in regards to MSR technologies) and from what I have read some problems require worked out such as delayed neutrons for power control, corrosion and tritium production (although many scientists in ORNL thought these issue could be worked out), even power distribution in the reactor and what would happen if one or more feed lines to the reactor went offline.

I have read links provided about using thorium to displace the use of fissile materials in current nuclear power plants and it seems like a logical first step but with a wary public would it be more worthwhile investing time in MSR reactors picking up where we left off at ORNL in the 70's?

 

[nikkkom]

I would imagine it could be difficult to make drastic changes to current nuclear technologies however with the current political and general population consensus about nuclear energy does it make sense to try a radically different approach to nuclear power production?

With renewables' prices steadily falling and with the recent snafu at Fukushima making a large dent in public trust in nuclear industry's ability to not shower thousands of square kilometers with Cs-137, it's difficult to convince anyone with money that spending them on MSR and/or thorium reactors R&D is worth it.

Indians may be the most enthused to do that, since they have no large sparsely populated deserts to cover in photovoltaics, they have huge and growing population, and a lot of thorium ore.

If renewables become as cheap as nuclear, nuclear power generation risks going into survival mode as old reactors are closed down with little new construction.

 

[QuantumPion]

If renewables become as cheap as nuclear, nuclear power generation risks going into survival mode as old reactors are closed down with little new construction.

By renewables you mean gas, yes. :uhh:

It is the cheap ubiquitous supply of natural gas via fracking which is putting coal plants out of business and nuclear on hold for the time being. The only thing that really drives renewable energy sources are tax subsidies.

 

[mesa]

...with the recent snafu at Fukushima making a large dent in public trust in nuclear industry's ability to not shower thousands of square kilometers with Cs-137

Exactly my point, it may be too difficult to gain public trust for current nuclear technologies so would it be easier to convince a skeptical population of a new way of doing nuclear (albeit still has developmental issues) with passive safety, reduced waste, abundant thorium supply, etc.? It seems like an easier 'sell' to that public.

...it's difficult to convince anyone with money that spending them on MSR and/or thorium reactors R&D is worth it.

Without public consensus this is certainly true, so should we invest time in educating the public about MSR technologies in an attempt to garner more widespread support for Thorium energy or continue shutting down nuclear power plants and replace them with coal and gas?

Indians may be the most enthused to do that, since they have no large sparsely populated deserts to cover in photovoltaics, they have huge and growing population, and a lot of thorium ore.

If renewables become as cheap as nuclear, nuclear power generation risks going into survival mode as old reactors are closed down with little new construction.

Thorium is fairly common, geographic areas with higher concentrations would seem to make little difference in terms of whether or not a government/private enterprise will engage in this type of power production.

Renewables are a great option where they make sense and are close to infrastructure however I have serious doubts about their economic viability without federal funding. For example, I used to run a biodiesel feedstock supply company and even though this industry could survive without the biodiesel fuel tax subsidy production would be drastically reduced due to simple economics.

 

[nikkkom]

Exactly my point, it may be too difficult to gain public trust for current nuclear technologies so would it be easier to convince a skeptical population of a new way of doing nuclear (albeit still has developmental issues) with passive safety, reduced waste, abundant thorium supply, etc.? It seems like an easier 'sell' to that public.

Thorium reactors aren't drastically better than Uranium ones wrt safety. Passive safety could/should be implemented for any future reactors, Uranium or Thorium.

Supply of Uranium is not yed a concern, and won't be for several decades at least.

This leaves us with only "reduced waste" argument.

As to selling anything to public, nuclear industry was overdoing it already. When you hear claims that reactors are safe, and then the accident happens, how much will average citizen trust the claims that "this time, honest, these new reactors are really safe, not like last ime!" ?

 

[nikkkom]

Renewables are a great option where they make sense and are close to infrastructure however I have serious doubts about their economic viability without federal funding. For example, I used to run a biodiesel feedstock supply company and even though this industry could survive without the biodiesel fuel tax subsidy production would be drastically reduced due to simple economics.

Renewables did not have yet their share of research and development. Thermal power stations benefit from 100+ years of R&D, nuclear ones from 60+ years. What progress will be achieved after a few more decades of R&D? I don't know, but both wind power and PV show nice steady progress.

 

[mesa]

Thorium reactors aren't drastically better than Uranium ones wrt safety. Passive safety could/should be implemented for any future reactors, Uranium or Thorium.

You sure about that? According to the PSR that is the case but I have written Dr Karamakos only to find out they were ill informed as PSR was confusing thorium as a replacement fuel in current nuclear technologies as opposed to that in MSR. This has led to widespread misinformation on the net (although I would still like to do the calculations before claiming passive safety is a 'guarantee' of any type).

Supply of Uranium is not yed a concern, and won't be for several decades at least.

I will take chemical separation over isotopic any day, in this regard thorium is better hands down.

This leaves us with only "reduced waste" argument.

Is that not a major concern? Waste from thorium MSR will be drastically reduced since it does not come out with the unspent fuel.

As to selling anything to public, nuclear industry was overdoing it already. When you hear claims that reactors are safe, and then the accident happens, how much will average citizen trust the claims that "this time, honest, these new reactors are really safe, not like last ime!" ?

What do thorium MSR's have to do with empty promises made by the nuclear industry? Historically there has only been opposition by the industry against these technologies which led to the eventual shutdown of the only U233 MSR reactor built during the 60's.

Renewables did not have yet their share of research and development. Thermal power stations benefit from 100+ years of R&D, nuclear ones from 60+ years. What progress will be achieved after a few more decades of R&D? I don't know...

I am all for research and development of photovoltaics since current technologies are not economically viable without government subsidies but should we also invest in thorium based MSR technologies at the same time?

If we do not come up with viable alternative solutions then we will just have to get used to burning stuff that is ripped from the planet to produce our energy.

 

[Astronuc]

I will take chemical separation over isotopic any day, in this regard thorium is better hands down.

One still needs an initial fissile fuel feed in an MSR, whether it's U-233 or U-235. The equilibrium MSR would in theory use U-233 produced from the transmutation of Th-232.

Is that not a major concern? Waste from thorium MSR will be drastically reduced since it does not come out with the unspent fuel.

Fission of U-233 produces much the same fission product 'waste' as fission of U-235, with a slight shift. There is less waste in the sense that a Th-232-based cycle produces much less TU nuclides (Np, Pu, Am, Cm, . . . ) than a U-based cycle in which the bulk of the fuel is U-238 that becomes transmuted to TU nuclides through successive neutron capture and decay.

What do thorium MSR's have to do with empty promises made by the nuclear industry? Historically there has only been opposition by the industry against these technologies which led to the eventual shutdown of the only U233 MSR reactor built during the 60's.

The MSR was only partially demonstrated in the laboratory on a small scale. It was not amenable to commercial nuclear plants, which use light water as a moderator, coolant and working fluid via the Rankine cycle.

Adding a power cycle (steam generator) to a fluorine-based chemical process is challenging and potentially inviting a significant accident. The commercial side of the nuclear industry cannot afford the risk.

 

[mesa]

One still needs an initial fissile fuel feed in an MSR, whether it's U-233 or U-235. The equilibrium MSR would in theory use U-233 produced from the transmutation of Th-232.

Yes, and we have a large stockpile of initial fissile feed-stocks from the decommissioning of nuclear weapons that would do the job nicely of starting the breeder cycle until it becomes self sustaining. If MSR's are built this provides a viable pathway for disposal of these materials which would make for a good argument for building thorium based MSR's to a skeptical public.

Fission of U-233 produces much the same fission product 'waste' as fission of U-235, with a slight shift. There is less waste in the sense that a Th-232-based cycle produces much less TU nuclides (Np, Pu, Am, Cm, . . . ) than a U-based cycle in which the bulk of the fuel is U-238 that becomes transmuted to TU nuclides through successive neutron capture and decay.

Yes, fission products are similar but the claimed advantage with MSR by ORNL, FLIBe energy, etc. would be the removal of fission byproducts while in operation as opposed to disposal of the entire fuel assembly; although as we both know this has never been demonstrated and still requires a great deal of engineering.

The MSR was only partially demonstrated in the laboratory on a small scale. It was not amenable to commercial nuclear plants, which use light water as a moderator, coolant and working fluid via the Rankine cycle.

Agreed, a >8MW reactor with air cooled heat exchangers that runs several hundred C0 higher reactor temperatures has little in common with a modern commercially based nuclear power generating facility but this is where decades of work by the scientists at ORNL ended. Kirk Sorenson has suggested the use of a Brayton cycle to take advantage of these higher temperatures.

Either way there are still engineering hurdles to overcome although ORNL did not think these issues would be unreasonable to solve but nothing will happen without a program to build MSR's again.

Adding a power cycle (steam generator) to a fluorine-based chemical process is challenging and potentially inviting a significant accident. The commercial side of the nuclear industry cannot afford the risk.

No doubt about the dangers of working with elements like fluorine and I would imagine dealing with processing on site for removal of wastes, collection of U233 for reintroduction, etc. will pose additional engineering challenges as well.

"The commercial side of the nuclear industry cannot afford the risk", when did chemical reactions become more dangerous to handle than nuclear reactions? Let's keep this in perspective.

Looking at the hurdles vs. possible benefits do you feel it is a good idea to pursue MSR technologies given what we know about it today?

 

[nikkkom]

Yes, fission products are similar but the claimed advantage with MSR by ORNL, FLIBe energy, etc. would be the removal of fission byproducts while in operation as opposed to disposal of the entire fuel assembly; although as we both know this has never been demonstrated and still requires a great deal of engineering.

Online removal of fission products is a double-edged sword.
While it may allow to eliminate refueling stops and it reduces decay heating after scram, it is also a quite complicated process.

Essentially, you need a small reprocessing plant (!) on site. Building and operating a reprocessing plain is neither fast, nor cheap, and runs additional safety risks - ask British, US and Japanese, they have some bad experiences with it.

 

[mesa]

Online removal of fission products is a double-edged sword.
While it may allow to eliminate refueling stops and it reduces decay heating after scram, it is also a quite complicated process.

Essentially, you need a small reprocessing plant (!) on site. Building and operating a reprocessing plain is neither fast, nor cheap, and runs additional safety risks - ask British, US and Japanese, they have some bad experiences with it.

Agreed, and I would like to look at more detailed plan, perhaps FLiBe Energy has done some work in this area. I will put in a request as they have been very helpful in the past.

 

[Astronuc]

"The commercial side of the nuclear industry cannot afford the risk", when did chemical reactions become more dangerous to handle than nuclear reactions? Let's keep this in perspective.

When chemical and nuclear are combined in the same process/system - especially when the chemical processing is done remotely because of radiation. The fission products must be collected, then converted into a stable waste form. One has to deal with the volatiles (e.g., I) and gases (Xe, Kr radioisoptes), most of which are normally contained in the fuel matrix. I have the perspective of someone working in the nuclear industry.

Looking at the hurdles vs. possible benefits do you feel it is a good idea to pursue MSR technologies given what we know about it today?

A thorium-based cycle may make sense if it can be perfected and it is economical and safe. I don't believe MSR is there.

The benefit of Th-based fuel is the lack of TU (Pu, Am, Cm, . . . ) elements, which would be a few percent of an LWR fuel bundle at discharge. The U-238 would be recycled - assuming recycling/reprocessing becomes acceptable.

 

[mesa]

When chemical and nuclear are combined in the same process/system - especially when the chemical processing is done remotely because of radiation. The fission products must be collected, then converted into a stable waste form. One has to deal with the volatiles (e.g., I) and gases (Xe, Kr radioisoptes), most of which are normally contained in the fuel matrix.

No disagreement in this regard but is it really any more difficult than the way we currently do nuclear? The concepts for the reactor with molten salt are simpler in design than current technologies but there is the addition of a chemical processing plant (however it would seem fuel reprocessing is a good idea regardless of the type of nuclear reactor we utilize).

It would be nice to look at what types of solutions have been proposed (if any) for chemical processing and see if they are viable or perhaps try to sort through it independently, there is impressive talent right here on PF.

I have the perspective of someone working in the nuclear industry.

Which is why I value your input, I almost sent a PM until I saw your response today.

A thorium-based cycle may make sense if it can be perfected and it is economical and safe. I don't believe MSR is there.

And it can not get there without an established MSR program, the question is whether or not it is worth the investment when considering the possible benefits and the ability to sell a skeptical public on a new way of producing nuclear energy. I have doubts about garnering public support for expanded nuclear energy when all I hear is 'Remember Fukushima!?'.

The benefit of Th-based fuel is the lack of TU (Pu, Am, Cm, . . . ) elements, which would be a few percent of an LWR fuel bundle at discharge. The U-238 would be recycled - assuming recycling/reprocessing becomes acceptable.

Which is still a reprocessing step. For thorium breeder MSR's it is a needed part of the plant and seems one of the more technically challenging aspects of such a reactor. With claims of passive safety, less waste, more abundant fuel source, etc. it seems like it would be easier to garner acceptance by an otherwise skeptical public.

There is still much engineering to be done but without support from the nuclear community it stands about as much chance for R&D as we have convincing the general public of going to a nuclear economy with similar technology to what we use today.

 

[nikkkom]

I have doubts about garnering public support for expanded nuclear energy when all I hear is 'Remember Fukushima!?'.

I can't blame them. Fukushima shouldn't have happened. Since it did happen, it empirically proves than nuclear industry is not up to the job of being safe enough.

Which is still a reprocessing step.

To have one reprocessing plant per fifty reactors, and reprocess an least four year cooled old fuel - the French model - is easier/cheaper than to have one reprocessing plant *per reactor* and to have to separate *fresh*, meaning very "hot", fission fragments.

 

[mesa]

I can't blame them. Fukushima shouldn't have happened. Since it did happen, it empirically proves than nuclear industry is not up to the job of being safe enough.

I agree, which makes MSR attractive with passive safety (although I reiterate I would like to run the calculations to see how 'safe' an option MSR's actually are). With growing demand I do not see how future energy needs can be met without a nuclear option included and fusion is a long way off.

To have one reprocessing plant per fifty reactors, and reprocess an least four year cooled old fuel - the French model - is easier/cheaper than to have one reprocessing plant *per reactor* and to have to separate *fresh*, meaning very "hot", fission fragments.

Agreed again, although the scientists from ORNL did not think this would be an unreasonable engineering issue and unless someone else would like to chime in I believe it is safe to say they are still the worlds leading experts in MSR nuclear technologies.

 

[nikkkom]

I agree, which makes MSR attractive with passive safety

Again. ***Passive safety is not in any way exclusive to MSR***.

It is a matter of designing a large enough heatsink which can be activated by station personnel even with no power available.
I hope nuclear industry have seen the light and *all* future reactors will have something like that.

Even Fukishima Unit 1 had something like that - Isolation Condenser. Yes, it wasn't designed to be a fully passive system, and consequently operators weren't trained to use it right in the severe accident they ended up in. It's too small (8 hours of cooling), and the steam valves couldn't be opened without power. But those design elements can be fixed.

 

[mesa]

Again. ***Passive safety is not in any way exclusive to MSR***.

It is a matter of designing a large enough heatsink which can be activated by station personnel even with no power available.
I hope nuclear industry have seen the light and *all* future reactors will have something like that.

That would be some heat sink.

Even Fukishima Unit 1 had something like that - Isolation Condenser. Yes, it wasn't designed to be a fully passive system, and consequently operators weren't trained to use it right in the severe accident they ended up in. It's too small (8 hours of cooling), and the steam valves couldn't be opened without power. But those design elements can be fixed.

The use of cooling pumps is hardly 'passive' ;)

So you feel we should continue with nuclear as it sits with the exception of adding even more safety features? Properly designed MSR's should do this passively. The engineering challenges fall on the side of chemical processing however we are all in agreement this should be done regardless of the type of nuclear power plant we utilize in the future.

If an MSR somehow overheated there is also the 'salt plug' which melts and the reactor drains into subcritical passively cooled storage tanks shutting the system down. I don't think there is any argument that this is an impressively simple yet effective design feature. If I am missing something on this point then someone please chime in.

I have a preference for simple safety and that is what the general public will demand if we wish to continue a nuclear program. The other option (as you suggest) is adding another level of complexity to an already Goldbergian arrangement, I have serious doubts that will win much public support. Further you even posted at an earlier time,

...as to selling anything to public, nuclear industry was overdoing it already. When you hear claims that reactors are safe, and then the accident happens, how much will average citizen trust the claims that "this time, honest, these new reactors are really safe, not like last ime!" ?

You can't have it both ways and I feel your argument is more about trying to bash nuclear as opposed to coming up with real solutions.

I am a proponent of renewables (like yourself) but we also have to be realistic about economics. I have experience in this industry and the entire thing is kept afloat by government subsidies. Without them this is not an economically feasible option at this time and requires more R&D just like MSR.

Nuclear is in the lead in development as far as economics are concerned which as power demand continues to increase will always decide the technologies we utilize first.

 

[nikkkom]

>>It is a matter of designing a large enough heatsink which can be activated by station personnel even with no power available.

That would be some heat sink.

What's the problem? For example, Fukushima site had a lot of area up in the hills - now occupied by dozens of huge tanks - which could easily accommodate ponds/tanks with many tens of thousand tons of water. Add a flexible hose and you have a gravity-fed system to refill e.g. Unit 1 Isolation Condenser.

>> Even Fukishima Unit 1 had something like that - Isolation Condenser.

The use of cooling pumps is hardly 'passive' ;)

Isolation Condenser has no pumps.
As long as all necessary valves are open, it cools the reactor.

So you feel we should continue with nuclear as it sits with the exception of adding even more safety features?

It's up to industry to perform a cost analysis and decide whether financing MSR R&D makes sense. There are other things to finance, you know. Fast reactors. Supercritical water reactors. Reduced moderation reactors. Powdered fuel. Solid thorium oxide fuel. Replacement of Zirconium cladding with SiC. etc etc etc

Properly designed MSR's should do this passively.

Properly designed _ANY_ reactor should do it passively.
MSR proponents make it look like they have some unique advantage there. It is not true. That is my point.

If an MSR somehow overheated there is also the 'salt plug' which melts and the reactor drains into subcritical passively cooled storage tanks shutting the system down.

Why do you think this passively cooled storage is easy and cool in MSR, but adding *basically the same* passively cooling to existing reactors is "another level of complexity to an already Goldbergian arrangement" in your view?
You think MSR will be significantly simpler than current reactor? On what grounds? Do you still remember that you need to have a *reprocessing plant*?

 

[mesa]

What's the problem? For example, Fukushima site had a lot of area up in the hills - now occupied by dozens of huge tanks - which could easily accommodate ponds/tanks with many tens of thousand tons of water. Add a flexible hose and you have a gravity-fed system to refill e.g. Unit 1 Isolation Condenser.

Once again adding more complexity to an already complicated system.

Isolation Condenser has no pumps.
As long as all necessary valves are open, it cools the reactor.

You are correct I had assumed you were also referring to the pumps that went offline. Either way the last time I checked the IC's did not prevent a disaster, so much for their 'passive safety'.

Properly designed _ANY_ reactor should do it passively.
MSR proponents make it look like they have some unique advantage there. It is not true. That is my point.

Their argument is pretty solid from what I have seen and I have yet to see another design that is simpler with the same theoretical level of safety all while using a fuel more common than tin. Which of the reactors you suggest would be on par with this?

Why do you think this passively cooled storage is easy and cool in MSR, but adding *basically the same* passively cooling to existing reactors is "another level of complexity to an already Goldbergian arrangement" in your view?

Because the fission products are removed, as the salt gets hotter it expands reducing criticality, and if there is still a criticality issue the MSR's freeze plug system employs subcriticality within the storage tanks. How do the reactors you propose stand up to this benchmark?

You think MSR will be significantly simpler than current reactor? On what grounds? Do you still remember that you need to have a *reprocessing plant*?

Yes I do although I have also brought up there will be engineering hurdles to reprocessing several times, if this is the only argument you have against MSR then we have a viable option here.

As far as reprocessing is concerned we are both in agreement this is a necessary step regardless of the type of nuclear we utilize. Large reprocessing facilities will be more efficient but onsite chemical processing eliminates shipment of hazardous nuclear wastes to offsite facilities.

 

[nikkkom]

Once again adding more complexity to an already complicated system.

MSR is also complex. It may end up being *more* complex than current reactors. Claiming that a pond, a drain pipe with a few valves and a set of hoses is too complex for NPP is ridiculous.

Either way the last time I checked the IC's did not prevent a disaster, so much for their 'passive safety'.

Only because operator didn't think it needs to be ready to deal with complete station blackout, not because IC can't do that. IC can do that.

as the salt gets hotter it expands reducing criticality, and if there is still a criticality issue the MSR's freeze plug system employs subcriticality within the storage tanks.

Criticality was not a factor in TMI and Fukushima meltdowns. In water-moderated reactors, as water gets hotter it expands reducing criticality. ;)

onsite chemical processing eliminates shipment of hazardous nuclear wastes to offsite facilities.

As I already said, in exchange it adds PITA of processing of very radioactive short-lived isotopes.

 

[mesa]

MSR is also complex. It may end up being *more* complex than current reactors. Claiming that a pond, a drain pipe with a few valves and a set of hoses is too complex for NPP is ridiculous.

You are correct, and that is not what I am saying. The reactors as they sit are already incredibly complex, adding more to it only increases this characteristic. How exactly is an MSR more complex?

Only because operator didn't think it needs to be ready to deal with complete station blackout, not because IC can't do that. IC can do that.

It's a system that did not work. Make excuses all you like but as far as we all can see IC failed. It did not make the station 'passively safe' as you suggested.

Criticality was not a factor in TMI and Fukushima meltdowns. In water-moderated reactors, as water gets hotter it expands reducing criticality. ;)

I wasn't talking about Fukushima or TMI, you asked a direct question about passive safety in regards to MSR. Please stay on topic.

As I already said, in exchange it adds PITA of processing of very radioactive short-lived isotopes.

Once again we are not in disagreement here but you seem to like bringing this up. Keep in mind the scientists at ORNL did not think this will be an unreasonable engineering issue and they are the leading experts in MSR technologies. I am open to hearing arguments (preferably something stronger than, 'it's going to be hard') against their thinking from another source even if not as qualified.

 

[nikkkom]

It's a system that did not work. Make excuses all you like but as far as we all can see IC failed. It did not make the station 'passively safe' as you suggested.

You can't read.

I did not suggest that Fukushima Unit 1 was passively safe.
It is my biggest concern with nuclear industry that they *still*, even after Fukushima, think that passive safety systems (meaning: ones which can be activated with absolutely no power) are not necessary, that they are overkill.

What I did suggest is that Fukushima Unit 1, an old reactor design, already had a system which can act as a passive safety system with minimal design changes.

 

[nikkkom]

>> Criticality was not a factor in TMI and Fukushima meltdowns. In water-moderated reactors, as water gets hotter it expands reducing criticality. ;)

I wasn't talking about Fukushima or TMI, you asked a direct question about passive safety in regards to MSR. Please stay on topic.

I am on topic. Comparing MSR to existing reactors is on-topic for this thread.

You brought up yet another bogus argument - "MSR is safer than current reactors because as it heats up, reactivity goes down". The bogusness is two-fold here:
(1) current reactors ALSO become less reactive as they heat up, this is a mandatory design requirement.
(2) past accidents have shown that keeping reactors subcritical was not a problem anyway, it isn't an area which needs fixing.

 

[mesa]

I am on topic. Comparing MSR to existing reactors is on-topic for this thread.

You brought up yet another bogus argument - "MSR is safer than current reactors because as it heats up, reactivity goes down". The bogusness is two-fold here:
(1) current reactors ALSO become less reactive as they heat up, this is a mandatory design requirement.
(2) past accidents have shown that keeping reactors subcritical was not a problem anyway, it isn't an area which needs fixing.

I was not referring to staying 'on topic' to the thread but 'on topic' to the question in which you posed.

No kidding about #1.

The advantage to MSR is the removal of the fission products, but as you said this is also the issue but if we want any possibility for any type of 'melt down proof' reactor these fission products must be removed as they are created and since they need to come out of the fuel regardless perhaps it is time to seriously consider this option.

Sorry it took so long to respond, it has been a busy week.

 

[krater]

One point that seems to be forgotten here is that the reactor with the most "passive" safety systems at Fuku I failed first, as for all its supposed lack of complexity the few simple conditions required for it to operate were simply unable to be met.

I don't believe that simplicity necessarily equates to reliability. In fact one could argue that simple systems are fundamentally crippled by a limited means to deal with challenges they may face by virtue of their "simple" means of operation.

As has been discussed here before one major drawback of condenser type systems involves the potential backdoor for damaged fuel materials to egress the RPV through a direct route through damaged condenser piping, defeating the robust containment structure. This backdoor is absolutely necessary in order to provide heat a route to escape the containment, however the risk of releasing radioactivity in this way is so high that IC systems at Fuku were forced offline by automatically closing valves when control power was lost. As has also been discussed here, RCIC provides a much more attractive option of retaining containment integrity albeit at a cost of greater complexity. But theoretically this system could have been augmented by "passive" supplementary water for such time when temperatures began to render RCIC cooling ineffective as the supression pool apparently began to boil and there was no place for heat left to go.

None of these systems of course can be realistically expected to operate on an indefinate time scale nor can it ever be totally ruled out that conditions of a severe accident would render them less effective.

MSRs are clearly a technology that would require significant development to make it to commercial production, but I think the question that needs to be asked is this: Is it worth the potential consequence of unforseen damage, accidents, results, ect. to develop this alternative technlolgy in the current environment of commercial nuclear power production? Would it be worth the risk of finding out that unforseen challenges of the MSR fuel cycle could lead to worse scenarios than we have already seen in 60 years of commercial uranium fission? Or would development be better directed to providing actual, demonstrable improvements to the safety and efficiency of our current, established fuel cycles and technologies?

I don't have any concrete answer to those questions except to say that when it comes to new tech, it is wise to expect the unexpected. Also, when it comes to economics it is wise to expect someone to appeal to the lowest common denominator when it comes to safety, which often opens the door to unexpected, "impossible" consequences.

 

[nikkkom]

One point that seems to be forgotten here is that the reactor with the most "passive" safety systems at Fuku I failed first, as for all its supposed lack of complexity the few simple conditions required for it to operate were simply unable to be met.

I don't believe that simplicity necessarily equates to reliability. In fact one could argue that simple systems are fundamentally crippled by a limited means to deal with challenges they may face by virtue of their "simple" means of operation.

IC of Unit1 was not operating simply because operator was never planning on making it work in complete SBO *including emergency generators and batteries*. Operator thought that some power will be available.

Station personnel had no training and no instructions what to do if all power is lost.

No thought was given on whether, for example, the valves will "fail open", or can be opened manually, when they lose power, so that steam can reach IC in this condition. Two of four valves were even totally inaccessible since they are inside PCV, so even if station personnel, without training and without instruction, under stress, would figure out that they need to open them, they can't.

As has been discussed here before one major drawback of condenser type systems involves the potential backdoor for damaged fuel materials to egress the RPV through a direct route through damaged condenser piping, defeating the robust containment structure. This backdoor is absolutely necessary in order to provide heat a route to escape the containment, however the risk of releasing radioactivity in this way is so high that IC systems at Fuku were forced offline by automatically closing valves when control power was lost.

Did it prevent release of radioactivity? (Rhetorical question)

As has also been discussed here, RCIC provides a much more attractive option of retaining containment integrity albeit at a cost of greater complexity.

Where does RCIC discharge thermal energy *to*? What if that heat sink overheats, can it be easily refilled?

But theoretically this system could have been augmented by "passive" supplementary water for such time when temperatures began to render RCIC cooling ineffective as the supression pool apparently began to boil and there was no place for heat left to go.

Exactly. RCIC doesn't cool the whole shebang. It smears temperature around PCV, it doesn't dump it to the outside.

 

[krater]

Nikkkom, your points are all very valid in terms of why IC failed to prevent what unfolded at Daiichi unit 1. Precisely because of failures in the design of this passive cooling system, there was no means left to cool the fuel when power was lost entirely. Had personnel been better instructed, maybe it would have made a difference. Maybe not, when other shortcomings of this very old system are taken into account.

One question that comes to mind for me, regarding release of radioactivity: Had the IC system failed open, or failed to close off completely after SBO, would it in your opinion have had a chance to mitigate or reduce the eventual releases that did occur? Do you think that when the conditions inside the PCV began to rise beyond design constraints, that this system which to my mind amounts to a big hole in the containment would have been able to cope? Or do you believe that had the system been left open it would have prevented things from reaching this critical stage?

Understandably one might surmise that since the IC system had not boiled empty that the extra time might have been enough to get a handle on the situation, but it seems to me we are talking a matter of a few hours max before cooling would have failed altogether and once again we're back in the boat we ended up in, full meltdown, inability to safely vent only this time with an additional release path available to melt byproducts (open IC system). Given the state of the plant and the surrounding areas I don't know if portable pumps could have been lined up in time to continue IC operation indefinitely or not. It certainly would have given all three units a better shot at coping had things happened this way, possibly avoiding the confusion and chaos in the aftermath of unit 1 explosion.

You are correct in your criticism that not only did RCIC not save unit 2/3, but it does not provide a function which IC-type systems would in that the heat doesn't really go anywhere once it is removed from the core. As I stated, to my mind this is really the price you pay for omission of the "big hole in the containment" that an external condenser becomes should control of it be lost. Theoretically a reactor could be very effectively cooled by the environment should one simply axe the containment altogether. Place personnel far enough away during operation, cross fingers, sit back and count the millions you saved on heavy expensive construction and hope to hell nothing goes wrong cause if it does, sure the RPV will radiate heat very efficiently, and will likely begin transporting nuclides to the environment with similar efficiency when something finally gives. I understand the argument that since RCIC and its heatsink are by definition located in a place that's difficult or impossible to access in a worst case scenario it creates a vulnerability of no way to actually dump heat, at least in common implementation.

A more ideal method might call for a happy medium of a sort of hybrid IC/RCIC design that puts a layer of isolation between the internal recirculation system and the external or semi-external heatsink. This would not be unlike the primary generation system of power reactors; the catch is this would involve another layer of complexity on safety systems with the associated costs, financial and otherwise. For the frequency that beyond design basis accidents occur I doubt this would be received warmly by the industry. The public, that might be another story. Some modern designs actually seem to aim in this direction but have also drawn criticism for seemingly using the addition of "better" safety systems to cut back on costs in other areas such as overall robustness of the design.

 

[nikkkom]

One question that comes to mind for me, regarding release of radioactivity: Had the IC system failed open, or failed to close off completely after SBO, would it in your opinion have had a chance to mitigate or reduce the eventual releases that did occur? Do you think that when the conditions inside the PCV began to rise beyond design constraints, that this system which to my mind amounts to a big hole in the containment would have been able to cope? Or do you believe that had the system been left open it would have prevented things from reaching this critical stage?

Unit 1 IC was sized for 8 hours of cooling until shell-side boils dry. Reportedly, this is larger than many similar US installations!

8 hours is a significant length of time. If station personnel would have training to engage IC at once, they would have plenty of time to find a fire truck and some fresh water to refill IC. Note that since shell side of IC is at atmospheric pressure, refilling it does not require high-pressure pumps. Contrast this with the pains they had organizing water injection into overheated and overpressured RPVs and PCVs!

If IC would save Unit 1 from overheating, Unit 1 would not experience hydrogen explosion - the first explosion, which occurred on March 12.

If this explosion would be prevented, the workers might be more successful dealing with Unit 3, which in reality also finally overheated and released hydrogen, which exploded on March 14.

Understandably one might surmise that since the IC system had not boiled empty that the extra time might have been enough to get a handle on the situation, but it seems to me we are talking a matter of a few hours max before cooling would have failed altogether and once again we're back in the boat we ended up in

No, it would be different. IC is *far* easier to refill: you don't need to fight with internal pressure.

full meltdown, inability to safely vent only this time with an additional release path available to melt byproducts (open IC system).

The "system" opened up ANYWAY. It could not possibly be worse than it actually is today.
Do you have any data to back up you assumption that IC piping is more fragile than other barriers? Why do you think it would have failed first?

 

[nikkkom]

"big hole in the containment" that an external condenser becomes should control of it be lost.

One solution: IC piping can be be intentionally designed for a higher pressure than all other piping contained inside PCV. Then in a meltdown that other piping ruptures first, and released radioactive steam ends up in PCV.

Second solution: add a heat exchanger inside PCV which physically separates RPV water/steam from the steam circulating through IC.