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PostPosted: Jun 07, 2013 12:56 pm 
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Ed P wrote:
Cyril R wrote:
Ed P wrote:
I have a question on reactivity. If you shut off all fuel salt pumps, whether intentionally or due to power failure, would net reactivity go down due to lack the introduction of cold/denser fuel or go up due to retention of delayed neutrons in the core?


The extra delayed neutrons will increase reactivity, but it's very small. It's maybe 200-300 pcm, and the amount outside the core is probably only 25% or so, even less with compact heat exchangers. So you get maybe a 50-100 pcm increase. Maybe 10-20 degree Celsius increase in temperature, then that's it. Heatup simply due to loss of pumps will be much bigger than that (dwell time in the core increases suddenly and the heat exchanger heat removal decreases over time).


So, the core gets hotter initially and power goes down, and the heat exchanger cools down initially decreasing delta T across to the secondary coolant and decreasing heat removal as you said, yet the secondary loop cools down because (assuming a steam turbine) steam is still being drawn to the turbine. This tends to keep the heat transfer through the primary to secondary salt from decreasing too much, and primary salt really cools down. With the hotter core, and colder Hx, natural circulation will be established at some level, assuming Hx higher than the core. Wouldn't the core just go back to power with a much larger core delta T, and an average temperature roughly 10-20 degrees C higher?

Is this an ok result, or is operator intervention needed. I assumed the turbine did not trip due to low pressure.


It is not an ok result, because of risk of freezing and plant damage. If the primary pumps stop pumping all other pumps plus steam turbine feedwater pump have to stop pumping as well. The steam system will probably trip on some process parameter going out of allowable range, but a fast acting safety trip logic is probably needed as investment protection.


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PostPosted: Jun 08, 2013 7:24 am 
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Cyril R wrote:
Ed P wrote:
Cyril R wrote:
It is not an ok result, because of risk of freezing and plant damage. If the primary pumps stop pumping all other pumps plus steam turbine feedwater pump have to stop pumping as well. The steam system will probably trip on some process parameter going out of allowable range, but a fast acting safety trip logic is probably needed as investment protection.

So, sounds like tripping other loops off line is needed to protect from freezing. What about the core? Scramming control rods sounds like it would increase the likelihood of freezing, by reducing heat input to the Hx, right? So loss of fuel salt pumps does not justify the need for CDMs.

Since the freeze seal is usually located in the cold fuel salt, the plug would not melt automatically as cold salt is getting colder. So, a freeze seal dump has to be manually initiated. Independent of securing secondary and turbine systems, would a manual freeze seal dump be fast enough to protect against freezing the Hx for a loss of fuel salt pumps casualty?


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PostPosted: Jun 09, 2013 6:57 am 
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Here are some casualty transient calculations, that supposedly used MSRE for code validation.

http://moltensaltindia.org/wp-content/u ... Krepel.pdf


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PostPosted: Jun 09, 2013 7:18 am 
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I'm trying to understand the differences in casualties between LFTRs and LWRs, with the very high temperature capability of salt compared to water, but MSRs have a freezing concern that LWRs do not. For example, use of control rods tends to result in primary coolant cooling, which in MSRs could lead to freezing. How would you need to design the control rod and/or the use logic differently for an MSR?


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PostPosted: Jun 09, 2013 4:30 pm 
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Ed P wrote:
Cyril R wrote:
Ed P wrote:
I have a question on reactivity. If you shut off all fuel salt pumps, whether intentionally or due to power failure, would net reactivity go down due to lack the introduction of cold/denser fuel or go up due to retention of delayed neutrons in the core?


The extra delayed neutrons will increase reactivity, but it's very small. It's maybe 200-300 pcm, and the amount outside the core is probably only 25% or so, even less with compact heat exchangers. So you get maybe a 50-100 pcm increase. Maybe 10-20 degree Celsius increase in temperature, then that's it. Heatup simply due to loss of pumps will be much bigger than that (dwell time in the core increases suddenly and the heat exchanger heat removal decreases over time).


So, the core gets hotter initially and power goes down, and the heat exchanger cools down initially decreasing delta T across to the secondary coolant and decreasing heat removal as you said, yet the secondary loop cools down because (assuming a steam turbine) steam is still being drawn to the turbine. This tends to keep the heat transfer through the primary to secondary salt from decreasing too much, and primary salt really cools down. With the hotter core, and colder Hx, natural circulation will be established at some level, assuming Hx higher than the core. Wouldn't the core just go back to power with a much larger core delta T, and an average temperature roughly 10-20 degrees C higher?

Is this an ok result, or is operator intervention needed. I assumed the turbine did not trip due to low pressure.


The power level (including decay heat) would be dramatically lower - basically matching your ability to remove heat from the system with the passive system. The key feature required is that the passive system remove heat more quickly than the decay heat alone, otherwise the temperature will keep climbing until you remove heat at the same pace it is generated.

A second, slower thing to consider is that the 233Pa will keep decaying to 233U and this is a noticeable increase in reactivity. In a 1Gwe machine we are talking about adding 80kg of 233U to the system or 5-10% not a few pcm but 10,000 of them! Fortunately we have time since the half-life is 27 days but you must have a means of ensuring cold shutdown - either the controls rods drop in place, or the freeze valve kicks in. I don't think of this as being too terribly challenging since we have plenty of time.


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PostPosted: Jun 10, 2013 2:28 am 
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Ed P wrote:
I'm trying to understand the differences in casualties between LFTRs and LWRs, with the very high temperature capability of salt compared to water, but MSRs have a freezing concern that LWRs do not. For example, use of control rods tends to result in primary coolant cooling, which in MSRs could lead to freezing. How would you need to design the control rod and/or the use logic differently for an MSR?


The freezing concern is not a major safety concern. It means you have too much cooling. Nuclear reactors are in real trouble when there's not enough cooling, not too much. Freezing is about damage to the investment (plant).

What freezes is the salt in the primary HX. After that you can't circulate fuel anymore, flow is blocked, so subcooling stops. System heatup will then tend to thaw the salt in the heat exchanger, but only if the secondary pumps also trip.

The trip logic is very simple. If the primary pumps all fail, all the secondary pumps are tripped. In a station blackout, the most likely failure of all the primary pumps at once, that's a given. In other events the trip system must trip the secondary pumps. Simple hardwired system will do. Sensors and a wire. Trip on redundant instrument failure.


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PostPosted: Jun 10, 2013 10:08 am 
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Cyril R wrote:
Ed P wrote:
I'm trying to understand the differences in casualties between LFTRs and LWRs, with the very high temperature capability of salt compared to water, but MSRs have a freezing concern that LWRs do not. For example, use of control rods tends to result in primary coolant cooling, which in MSRs could lead to freezing. How would you need to design the control rod and/or the use logic differently for an MSR?


The freezing concern is not a major safety concern. It means you have too much cooling. Nuclear reactors are in real trouble when there's not enough cooling, not too much. Freezing is about damage to the investment (plant).

What freezes is the salt in the primary HX. After that you can't circulate fuel anymore, flow is blocked, so subcooling stops. System heatup will then tend to thaw the salt in the heat exchanger, but only if the secondary pumps also trip.

The trip logic is very simple. If the primary pumps all fail, all the secondary pumps are tripped. In a station blackout, the most likely failure of all the primary pumps at once, that's a given. In other events the trip system must trip the secondary pumps. Simple hardwired system will do. Sensors and a wire. Trip on redundant instrument failure.

Agreed. Freezing is not a safety issue. It is a design constraint - there are certain combinations we can't use because of the risk of freezing. I should mention that when there is a freeze it means the salt occupies less volume than it did as a liquid. This is not a problem so far as I know. However, when the thaw occurs one must be careful not to thaw a section of pipe while both ends are still frozen. The salt expands on melting and as an incompressible liquid will burst the pipe.

Using a pool type reactor where all such pipe and the primary HX are all under a buffer salt is attractive for increasing the thermal mass of the system. This makes it possible to have a totally passive cooling system. The AP1000 which has a passive cooling system that works for the first 3 days (IIRC) of station blackout but then needs outside help (refilling its water tank) or there is trouble. A LFTR with a pool arrangement and passive cooling equal to 0.5% of full power could have enough thermal capacity to last indefinitely. I'm thinking that the same thermal mass would ensure that freezing never occurs in a working reactor or during normal outages or during emergencies. Freezing would occur only during very long outages (for example the 18 month outage San Onofre just went through) and then only when the operator chooses to allow it. So I'm thinking with a pool type reactor there is no concern about freezing in the primary salt loop.


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PostPosted: Jun 17, 2013 10:48 am 
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I agree, the advantage of the pool type of reactor is important for preventing freezing, and also for decay heat removal if a dump tank is not used. However, we would still need to design to minimize the chance of freezing in a pool type because it is not really a uniform pool, there has to be a lot of non-fuel salt between the critical core, and the heat exchanger because they need to be neutronically separated. If graphite is used and a Hastalloy N Hx shell, for neutronic separation, would that improve or hurt the conduction heat transport from the core to the Hx, compared a pure pool of LiFBeF (assumption to get rid of the neutronic and decay heat factors)? Does someone have the thermal conductivities of graphite and LiFBeF at hand?


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PostPosted: Jun 17, 2013 10:59 am 
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What are the different casualties for which a control rod would be needed? Or is it just in there as a defense in depth/back up system in case the freeze plug failed to melt?


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PostPosted: Jun 17, 2013 11:33 am 
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Ed P wrote:
I agree, the advantage of the pool type of reactor is important for preventing freezing, and also for decay heat removal if a dump tank is not used. However, we would still need to design to minimize the chance of freezing in a pool type because it is not really a uniform pool, there has to be a lot of non-fuel salt between the critical core, and the heat exchanger because they need to be neutronically separated. If graphite is used and a Hastalloy N Hx shell, for neutronic separation, would that improve or hurt the conduction heat transport from the core to the Hx, compared a pure pool of LiFBeF (assumption to get rid of the neutronic and decay heat factors)? Does someone have the thermal conductivities of graphite and LiFBeF at hand?


I expect there will be boron carbonite absorbers on the outside of the reactor core (outer edge of the blanket for a 1.5 or 2 fluid system or just the outer edge of the reactor for single fluid. This serves two functions a) it reduces the neutron flux load seen by the structure and thus increases its strength and b) it reduces the neutron flux load outside the core where the neutrons are not useful and create pain. The blanket salt may enhance the neutron attenuation some but I expect we will first severely attenuate the neutrons in the absorber. The thought is that the store for the old fission products is at the bottom of the pool so we have a natural heater and since the salts expand with heat this will create a natural circulation.


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PostPosted: Jun 17, 2013 11:37 am 
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Ed P wrote:
What are the different casualties for which a control rod would be needed? Or is it just in there as a defense in depth/back up system in case the freeze plug failed to melt?

Backup in case freeze plug fails. Second, to allow cold shutdown without draining the fuel if that is desired (for example, to service the drain tank). The requirement is to be inserted rapidly enough to compensate for the increase in reactivity due to Pa233 decay - which has a 27 day half-life. This will add around 80kg of u233 which is a significant enough increase in reactivity that we don't want to handle this with a temperature rise.


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PostPosted: Jun 17, 2013 11:59 am 
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Freezing is not much of an issue with a buffer salt pool type design. This is how today's aluminium is produced - large bath of molten NaF-AlF3 with Al203 dissolved in it for electrolysis, the bath is allowed to freeze at the top and sides of the walls of the bath, this is your insulation and protection for the wall as well. If the buffer salt is subcooled, it will freeze where it's losing heat, which is at the top and on the walls, so you basically get a similar situation as the aluminium electrolysers. This may mean that a buffer salt with a similar freezing point as the fuel salt is actually attractive, freezing of the buffer salt is easily accomodated and will protect the fuel salt from freezing. There is even the option of allowing this freeze during normal operation, just like the aluminium smelter. This has obvious advantages, but I haven't looked at it because I can't model freezing and thawing yet. Need different software.


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