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PostPosted: Nov 10, 2011 4:31 am 
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I think that most people would agree that by making small 'shim' additions of fissile or fertile can modulate the core reactivity quite well without using control rods. But can we routinely stop and start a MSR core by adding or removing material, can we effect enough of a shift in reactivity by those means sufficient for a number of reliable and repeatable startup/shutdown sequences in a year?


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PostPosted: Nov 10, 2011 9:34 am 
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Why is there a need for this? In case of an emergency shutdown or scheduled maintenance the core salt needs to be dumped into the dump tank. If the reactor has a negative reactivity coefficient, automatic load following should be possible. Why do you think that more has to be done than simply let the reactor heat up to the point, where it shuts down itself?


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PostPosted: Nov 10, 2011 12:56 pm 
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It will depend on the accident scenario AND the regulator attitudes.

For example, stopping the pumping means all the delayed neutrons are released in the core and none are lost to the out of core salt. This is a pretty large reactivity insertion and uses up much of our temperature margin. If you have graphite in the core then as it heats up it will also increase the reactivity. I personally suspect we will be are OK without control rods but it isn't obviously the case.

Draining to the tanks may be too slow.

Regulators are used to seeing control rods and it would take a lot of convincing to not have them. So, I suspect we will have control rods. I'm thinking it would be best if they are not used for trim but only to achieve rock solid shutdown. I'm thinking it would be a redundant system - something that if it doesn't work we still shutdown.


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PostPosted: Nov 10, 2011 1:29 pm 
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Lars wrote:
Draining to the tanks may be too slow.

Regulators are used to seeing control rods and it would take a lot of convincing to not have them.
I don't think this has much to do with speed.
Rather, it has to do with the regulators requiring a certain level of subcriticality (i.e. multiplication factor "k") defined as a "guaranteed shutdown" state.

Drain tanks will certainly accomplish that.

IMO, the only thing you have to demostrate is that any transients prior to "guaranteed shutdown" do not cause any damage.


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PostPosted: Nov 11, 2011 12:02 pm 
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It's both speed and amount of sub-criticality.

I agree with Lars, regulators are going to almost certainly require some kind of automatic, fail safe, control rod system for rapid (1 to 3 seconds) insertion of shutdown reactivity.

And also to provide the ability to show you are greater than 5.15% (usual number in the regs) shutdown at all times except reactor startup. That means when you are NOT in the dump tanks and are just about ready go critical, you have to show you are sufficiently shutdown even then.


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PostPosted: Nov 11, 2011 12:19 pm 
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There are some advantages to not having dump tanks - doing the cooling in the reactor.

There's also a thermal stress/economic risk issue. You probably don't want to rely on a large temp increase for normal shutdown, there's always a risk of something wearing out faster and that is not nice for reliability and maintenance cost. Control rods can help there, they shut down reactivity so fast there won't be much chance for fast reactivity heat up. If the control rods don't work we have a transient without scram and need to show this won't cause failures, but we can do that (based on the AHTR work so far, and we'll do better with more negative coefficients) and so we can have a very robust safety case for unplausible scenarios (complete control rod failure)while limiting economic risk for temperature rises in more plausible scenarios (station blackout).

Likewise, to limit temperature rise from decay heat buildup, even if dump tanks are used, you'd want to have another way to normally pump or mechanically dump the fuel into the dump tanks, using the freeze plug only as last line of defence.

One thing that we can do with the control rod is make it bouyancy activated. When the pumps stop (station blackout) the flow stops and this flow is what keeps the control rods up, so the control rods sink into the core, shutting it down quickly. The nice thing about this is that you have no signal of intelligence so no software or electronics signal failures. More conservative would be to have such a passive control rod and also use a normal signal driven control rod for more operator options and diversity. I don't know if these control rods add much to the cost of the system.

Some reactors are different and don't have these issues, for example Jaro's HW-MSR. This is a fail safe system that needs no control rods.

For a no-dump tanks design, we can also use temperature fusible cans of soluble neutron poison, to counter prolonged heat up effects from Pa233 decay into U233. Put these cans in the upper plenum, if temp goes up too much the cans melt and will put the neutron poison in the fuel.

I think it is well worth it to make the entire reactor system walk away safe. It will really convince people. With this ambitious goal, we need to ask ourself the question: what happens if all operators walk away from the reactor after a station blackout and multiple scram and other equipment failure. And then we go back in a few months to see if the reactor is in safe shutdown. If we can do this we've got the best safety case.


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PostPosted: Nov 11, 2011 3:21 pm 
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USPWR_SRO wrote:
It's both speed and amount of sub-criticality.
I agree that's true for reactors with lots of excess reactivity and a short neutron lifetime, like the current fleet of LWRs, which run for a year-and-a-half without refueling.
That's because transients in such circumstances could potentially be very damaging, and must be guarded against, at all costs -- which they are, by having those fast shutdown systems.

An MSR with no excess reactivity to begin with, is an entirely different animal.
As stated earlier, the only concern here is whatever reactivity can be added by delayed neutron precursor hold-up in the reactor (versus elsewhere in the PHT loop).
If it is demonstrated that a potential power spike from this extra reactivity is not damaging, then there is simply nothing worse that physics can throw at us in such a situation.
Ignoring the physics could just as easily result in other absurd requirements, such as having to include a High Pressure Emergency Core Cooling System (HP-ECCS) water injection system for an MSR operating at ambient pressure.
Total nonsense !


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PostPosted: Nov 11, 2011 6:23 pm 
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It's a key design advantage that a hypothetical LFTR would have minimal excess reactivity at all times. But there IS excess reactivity. How much extra depends on how often you add fuel, and that is going to be determined by how much control reactivity you have available, either through temperature rise or though moveable control systems (control rods).

In any case, there WILL be excess reactivity, no, I agree not in the same order of magnitude as a LWR that has 1.5-2 years of fuel preloaded in, but still plenty for a power excursion.

I stand by my assertion that a rapid reactivity control system of some type WILL be required by regulators. You can argue about the unfairness of it all, or you can work with it.


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PostPosted: Nov 11, 2011 8:55 pm 
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The reactivity is a strong function of temperature. So there is excess reactivity when the fuel is cold and when the fuel is hot the system is sub-critical. This should be sufficient to provide the trim control.

I sincerely hope we do not use control rods as trim because then you have additional sources for an accident (suppose the trimming control rods were set at maximum and they somehow got rapidly removed).

I do think shutdown control rods where any one is sufficient to shutdown the reactor and there are interlocks to prevent the reactor from operating while the shutdown rods are in place makes some sense as long as they do not introduce some new failure mode.


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PostPosted: Nov 12, 2011 6:28 am 
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Is it safe to remove only a small fraction of the salt from the core to make it subcritical? If we avoid dump tanks this way, how do we replace parts like pumps?


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PostPosted: Nov 12, 2011 7:14 am 
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Burghard wrote:
Is it safe to remove only a small fraction of the salt from the core to make it subcritical? If we avoid dump tanks this way, how do we replace parts like pumps?


Sure, you can have spare capacity in the HX, expansion tanks, and other out of core volumes. This could be your minor reactivity shim control.

Replacing pumps could be done online if it is a canned pump (we'll have excess pump capacity in the other pumps or can run at slightly reduced power). The internal parts (impeller) can best be replaced offline as it requires opening the primary loop. To do that, first cool down the bite out of the decay heat, then pump the fuel salt to a temporary buffer tank. Another tank holds a clean fuel salt for flushing, flush a couple times to decontaminate, then remove the flush salt back to the tank and replace the part. When it is checked for leaks, pump in the flush salt again to flow test it, if that works fine then remove the flush salt again and put in the fuel salt.

I should point out the actual HX, pump impellers etc replacement is a very infrequent business. Take a look at how long PWR steam generators last, for example. HXs with tube in shell can have remote operated plugging of leaking tubes so you can continue operating.


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PostPosted: Nov 12, 2011 1:18 pm 
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Burghard wrote:
Is it safe to remove only a small fraction of the salt from the core to make it subcritical? If we avoid dump tanks this way, how do we replace parts like pumps?

This is pretty effective in a two fluid reactor but in a 1 or 1.5 fluid reactor it is surprising how much fuel salt must be removed to bring the reactor to a solid sub-critical state.


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PostPosted: Nov 13, 2011 12:10 am 
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If you have a metallic thorium blanket that can get into core through a temperature linked mechanism, and can be pulled back, it can replace the control rods by altering the fissile-fertile ratio.


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PostPosted: Nov 14, 2011 3:55 am 
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Lars wrote:
Burghard wrote:
Is it safe to remove only a small fraction of the salt from the core to make it subcritical? If we avoid dump tanks this way, how do we replace parts like pumps?

This is pretty effective in a two fluid reactor but in a 1 or 1.5 fluid reactor it is surprising how much fuel salt must be removed to bring the reactor to a solid sub-critical state.


How much salt needs be removed in a 1 fluid reactor for reactivity <0.95?


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