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PostPosted: May 04, 2013 6:40 pm 
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Cyril R wrote:
Long term reactivity changes would be compensated by daily top-up. There would never be more than a week of excess reactivity.

Well that just changes from one failure mechanism to another. Now you have to prove the system is safe if a mischievous human or errant computer dumps too much fuel in for the daily top-up (like what happens if the entire inventory of fuel is dumped in?). I do see your point about the cost and complexity of control rods.

Also, with LEU-only fuel, does reactivity drop monotonically with burnup? In the DMSR, the reactivity increases the first year, since the bred U233 is more reactive than the consumed U235.

It might be acceptable if the operators had to dilute the fuel periodically until the new batch reaches peak reactivity. As long there is space for the extra fluid.

Lars wrote:
--- An MSR regulates fine on its own.

I've seen this claim for two fluid machines (very strong negative temp coef). Are we talking about a single-fluid LEU-only machine?


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PostPosted: May 05, 2013 5:09 am 
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The entire inventory of fuel can't be just dumped in because of it's bulk - this is low enriched fuel with even greater quantities of LiF dilutant. Too much fuel addition simply won't fit in the system, so it goes by overflow line to a skimmer surge tank, which would be the spent fuel storage tank in the design I'm thinking off, so would be used normally like this. Add fresh fuel, some old fuel can't fit and will overflow.

Reactivity tends to increase a bit early on as Pu builds up, but then declines more or less linearly as Pu gets to equilibrium concentration quickly due to its high cross section, and FPs burn in linearly with burnup.


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PostPosted: May 05, 2013 8:51 am 
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Flow based control rods are seductive,
but you have to awfully sure of yr flow pattern.
In our design the average full power upward
velocity in the core is only about 0.7 m/s.
A plug somewhere could increase the
velocity at the control rod just when you need more rod.
Combining flow and control rods this way
could lead to some weird oscillations.
Pump overspeed, possibly malicious,
pushing the rods out while cooling the salt (for a while)
need to be examined.

Separating flow control and rod control
gives you a lot more ways to handle off-design scenarios,
(and a lot more ways to screw up).


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PostPosted: May 05, 2013 12:09 pm 
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The flow pattern and flow induced force are very simple things even I can calculate them, I have friction factors for various materials, I know the specific gravity, etc.

The flow rate in the core is a given, based on the orificing that you have in that axial location. It is not necessary to tweak it. Just tweak the specific gravity of the control rod, eg vary the amount of Gd metal to B4C (or just add graphite to Gd metal) so that the control rod drops in at the right reduction in flow rate deemed necessary for rod insertion.


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PostPosted: May 05, 2013 5:47 pm 
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Remember as well that the control rods are the backup control. The primary control comes from thermal expansion of the fuel salt. We need the control rods to compensate for the increase is in fissile as the Pa decays (27 day 1/2 life) into 233U. It would be nice if the control rods could also help out for the delay neutron increase (tens of seconds) as this would reduce the peak temperature a bit (tens of degrees C) but it isn't crucial.


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PostPosted: May 05, 2013 9:33 pm 
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Looking into details of control rods usage makes one appreciate how great the fluid fuel is indeed. A major issue in designing control rod patterns over a burn campaign is how to burn the (solid) fuel evenly. That is to maximize the burnup over a fuel cycle while limiting power peaking of (or hot spots in) any fuel assembly, since that makes a fuel cladding failure more likely. Achieving an even burn over the course of a fuel cycle is rather difficult, almost a matter of art, and different vendors have developed different strategies of control rod shuffling. Given the amount of fuel assemblies and possible control rod patters in a large reactor core, and the time it takes to calculate the fuel depletion per time step, it is not realistic to optimize the process in a deterministic way. Add to this the degrees of freedom in fuel element design - different fuel pins may (and often do) have different enrichment levels, different concentrations of various burnable absorbers, and these generally vary in axial zones.

Improvements in supporting simulations, in stochastic methods, combined with the experience of the power plant operators and the fuel vendors improved the even fuel burn, thus have reduced the fuel cladding failure rate, and led to the large improvement in the capacity factor during the 1990s: from 60% to 90%. Not a small feat.

When a fluid fuel is used, in particular in the form of molten salts given their large liquid temperature range, all this is immensely simplified, since the fuel is rapidly homogenized.


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PostPosted: May 05, 2013 11:35 pm 
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in case of any fluid fuel, some gas contained above the fuel and a surge tank could make an effective control system. As the temperature increases, the pressure in the gas builds up and pushes up the fuel to surge shaft/tank. Things return to original state on cooling. Overriding manual control can be exercised through the pressure of the gas, if required.


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PostPosted: May 06, 2013 1:17 pm 
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ondrejch wrote:
Looking into details of control rods usage makes one appreciate how great the fluid fuel is indeed. A major issue in designing control rod patterns over a burn campaign is how to burn the (solid) fuel evenly. That is to maximize the burnup over a fuel cycle while limiting power peaking of (or hot spots in) any fuel assembly, since that makes a fuel cladding failure more likely. Achieving an even burn over the course of a fuel cycle is rather difficult, almost a matter of art, and different vendors have developed different strategies of control rod shuffling. Given the amount of fuel assemblies and possible control rod patters in a large reactor core, and the time it takes to calculate the fuel depletion per time step, it is not realistic to optimize the process in a deterministic way. Add to this the degrees of freedom in fuel element design - different fuel pins may (and often do) have different enrichment levels, different concentrations of various burnable absorbers, and these generally vary in axial zones.

Improvements in supporting simulations, in stochastic methods, combined with the experience of the power plant operators and the fuel vendors improved the even fuel burn, thus have reduced the fuel cladding failure rate, and led to the large improvement in the capacity factor during the 1990s: from 60% to 90%. Not a small feat.

When a fluid fuel is used, in particular in the form of molten salts given their large liquid temperature range, all this is immensely simplified, since the fuel is rapidly homogenized.
Right !

And I strongly suspect that this is one big reason why professional reactor physicists generally show little interest in MSRs: It would pretty much put them out of business, after the initial design & trials period.

Same thing in Canada and all the countries using Candu reactors: The constant (daily, throughout the year) fuel bundle shuffling and associated fuel burn strategies keep reactor physicists busy for entire careers !


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PostPosted: May 08, 2013 2:36 am 
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It will be interesting to see how the Chinese, the developing world workshop, deal with the matter. They are developing the nuclear energy as a major weapon in economic domination of the world.


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PostPosted: Jun 06, 2013 11:13 pm 
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Lars wrote:
Control rods as a hard shutdown mechanism would be OK but I think you still end up proving you can shut down even if they fail. Using the force of the flow to hold them out of the salt seems attractive to me as a passive means of control.
UC Berkeley investigated and documented for PB-AHTR, a floating shutdown rod slightly buoyant under normal conditions, that would sink into the core if it became too hot, I like the idea, the only problem is the balance of forces is very small so a small mechanical resistance would stop it moving. I've occasionally pondered a mechanical feature that displaces fuel salt from the core in response to high temperature to create an automatic shutdown feature, not unlike a waxstat in a car engine. (this cannot be an original thought). Overall I really like the idea of not having control rods at all if possible as it introduces another contingency that one has to deal with.


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PostPosted: Jun 07, 2013 4:34 am 
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Lindsay wrote:
Lars wrote:
Control rods as a hard shutdown mechanism would be OK but I think you still end up proving you can shut down even if they fail. Using the force of the flow to hold them out of the salt seems attractive to me as a passive means of control.
UC Berkeley investigated and documented for PB-AHTR, a floating shutdown rod slightly buoyant under normal conditions, that would sink into the core if it became too hot, I like the idea, the only problem is the balance of forces is very small so a small mechanical resistance would stop it moving. I've occasionally pondered a mechanical feature that displaces fuel salt from the core in response to high temperature to create an automatic shutdown feature, not unlike a waxstat in a car engine. (this cannot be an original thought). Overall I really like the idea of not having control rods at all if possible as it introduces another contingency that one has to deal with.


The PB-AHTR has also done modelling that shows that even if the control rods fail and pumps fail as well (or just power failure) then the temperatures are quite reasonable, short term maybe a 200 degree C rise, then it drops again, fission starts up again, but at a low level and the equilibrium temperature is very much acceptable for any alloy that can normally operate at 700 degree C anyway.

This is a very different result than the helium cooled PBMR, which has massive fuel damage in case of this ATWS + SBO event. Basically, the added fission power can't be efficiently conducted away; molten salt is much better at moving out that heat to whatever passive cooling system you have.

So, it is very robust. MSRs can be more robust still, because of fuel expansion, and a lower fuel operating temperature with the fuel in the salt meaning more margin for shutdown plus no fuel cooldown adding reactivity, with the right design a more negative power coefficient than the PB-AHTR can be achieved.


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PostPosted: Jun 07, 2013 8:20 am 
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As far as thermal controlled control systems, I've thought of having sealed tubes in the core of either helium or liquid lithium going to reservoirs outside the core. The reactivity control would be by negative void coefficient or thermal poison, respectively. Not sure you need a reservoir for the gas, but i included it to prevent positive reactivity from a leak of salt into the tube, and to be able to determine if there is a leak. The tubes would expand with incresed temperature, thus increasing the void or poison drawn into the core and reducing reactivity. But why do it? All it does is enhance the existing capability of the salt and make things more complicated and expensive. Also a thermally induced control like this is not a backup, since it is part of and partially replacing normal operational characteristics. You need a system that only thermally actuates when you go above a maximum normal self control temperature. But the freeze seal drain plug already does that. Is there a thought that the freeze seal is too slow?


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PostPosted: Jun 07, 2013 8:30 am 
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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?


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PostPosted: Jun 07, 2013 9:26 am 
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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).


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PostPosted: Jun 07, 2013 10:05 am 
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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.


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