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PostPosted: Sep 09, 2014 8:00 pm 
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Fast solid fuels reactors like the PFBR have B4C control rods. MSFR could have the same.
Rather than the messy drain plugs or exotic shapes suggested, you could have a top with surge columns. The liquid fuel can flow back in when temperature goes down.


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PostPosted: Sep 10, 2014 1:15 pm 
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Dear Lindsay...you can of course control the reactor power with the tertiary (water) feed pumps. As long as you have more than about 20% of the nominal power it should not be an issue at all. If you go below you have to take care on the decay heat that is about 5 - 6% of the nominal power directly after shut down. You have to consider as well the 135Xe poisoning. You might balance the feed pumps with these effects and shut it down to nearly 0.

A. Frequenzy converters to control the primary and secondary feed pumps are a technical standard and not really expensive. They could be placed beyond the biological shield away fom radiation. Why not controlling as well the primary and secondary pumps?

B. Regulators all over the world request that nuclear power plants need to have at least 2 independent systems to shut down the reactor quickly. What kind of systems would you use for this purpose?

C. Reactor operators and technical staff over here require a hydraulic testing during/after maintenance outages (without full power). How could you realize it?


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PostPosted: Sep 10, 2014 2:00 pm 
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Holger, I am so pleased to see that you've made yourself at home here on the EFT Forum.

Xenon, I am not counting on Xe being very challenging at all in MSR system with modest helium sparging, but I may be underestimating that challenge.

VFD's: yes they are cheap, effective and efficient, I want to minimise their use in the nuclear circuit as one more thing to go wrong, one more thing that has to be proved to a regulator to some extremely unique and high standard. I think that VFD's would be extremely useful on salt pumps to have a soft startup, but would like to avoid any reliance on them.

Shutdown Systems: I accept that, if the regulator requires two independent shutdown systems, they shall have it, my comments were focused on the need or not for control rods for normal maneuvering; and for the fast spectrum design I believe that we can live without them, if the base design is sympathetic to that outcome.

Holger wrote:
C. Reactor operators and technical staff over here require a hydraulic testing during/after maintenance outages (without full power). How could you realize it?
I may be misunderstanding your meaning, but for 0 power just trim up the fuel salt composition and or raise the cell temperature to get the power level desired. For example REBUS-3700 has a reactivity coefficient for temperature of -6 pcm/C, so for a core designed to idle at 600C, raising the temperature to the normal outlet temperature say 700C would be enough to inject -600 pcm which should bring the reactor to 0 power.


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PostPosted: Sep 10, 2014 3:11 pm 
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Dear Lindsay,

135Xe is mainly a decay product from 135I. If the degassing can extract the iodide there is no issue with the Xe..if not it is an issue when you try to restart a moderated reactor or try to increase power from a low level back...

Another neutron poison is 149Sm. In a chloride MSR it will form SmCl3

Do you, or does anyone know if the helium bubbling (degassing) extracts as well iodide?

regards

Holger


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PostPosted: Sep 10, 2014 4:35 pm 
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ORNL tried to extract the iodine but gave up. We expect some iodine in the offgas system but only a minor fraction. As far as xenon being a problem at startup I don't see that. If you shutdown fission and stop fuel salt flow for 10 hours or so you will have too much xenon present to restart fission. But you can restart fuel salt flow. This will gradually (over a few minutes time) remove the xenon and allow the reactor to ramp up output power. I guess that does introduce a minor constraint that you can't go from a no flow shutdown to full power in less than a few minutes but I don't see that as an issue.

On the other hand, if you shutdown fission procedure maintains a modest fuel salt flow in order to remove the decay heat then you will also be removing the xenon as it gets generated.


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PostPosted: Sep 10, 2014 10:17 pm 
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If there is some external calamity/civil emergency that takes out the grid for a period of time, to my way of thinking, idling the reactor with normal pump driven circulation is probably the safest and most ideal holding pattern to adopt, assuming that you have a reliable internal power supply. No concerns over Xenon pits, no settling out of fine particulate Nobel metals to concentrate in a stagnant corner somewhere, no significant changes for the nuclear island aside from a drop in thermal power and settling out at a different temperature.


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PostPosted: Sep 11, 2014 2:55 am 
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It is generally required to be able to keep the system in Hot Zero Power (HZP), that is, for a MSR, at reactor temperature with the pumps running. Thus you will need *shutdown rods*, i.e. to keep the system subcritical but with the primary completely filled.

For reactor control, that is more debatable.


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PostPosted: Sep 11, 2014 4:23 pm 
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Der Lindsay ....if there is a break down in the grid nuclear power plants usually have a couple of diesel generators to produce the necessary internal power. As you might remember I planned to use as well the decay heat from the used fuel and the noble metal plating out to generate some MW of emergency power


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PostPosted: Sep 12, 2014 1:55 pm 
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Re HZP depending on how you look at it the reactor would be very close to that, the plant certainly would be in that hot zero power condition, meeting only it's own internal loads. The requirement that I would add is that internal power must NOT come from the emergency diesel system, that must be reserved and available for emergencies.

I know that we often think that in uncertain times the best thing is to turn things off, but often in a power station environment the safest condition is the last steady state condition. Turning things off, changing their operating state often increases the risk profile on that equipment it doesn't reduce it. When I think of a working MSR core, I think that the safest condition is continuously circulating with some power that helps to stabilize those FP's with neutron high absorption, maintaining the dispersion of particulates and maintaining through mixing and degassing of the fuel salt. You keep the entire plant hot and ready to go, ready to support the grid restoration.

Current LWR's contribute to a lack of resilience in most modern grids, in part due to the xenon pit associated with a high inventory of FP's in-core, but a carefully crafted design can create a very flexible MSR NPP capable of black starting and grid restoration duties, inverting the usual paradigm regarding nuclear plants during a time of natural disaster/grid emergency. And I believe that all of that can be done without control rods for some designs, the need for shutdown rods is accepted as a requirement from the regulator.


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PostPosted: Sep 12, 2014 1:57 pm 
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HolgerNarrog wrote:
Der Lindsay ....if there is a break down in the grid nuclear power plants usually have a couple of diesel generators to produce the necessary internal power. As you might remember I planned to use as well the decay heat from the used fuel and the noble metal plating out to generate some MW of emergency power
I think that we need to go one better and keep the core critical, otherwise we cannot keep the lights on for very long as the decay reducing rapidly with time.


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PostPosted: Sep 13, 2014 12:56 pm 
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Dear Lindsay.... the German nuclear power plants were prepared for a mode to produce the internally required power- about 3% of the full load. For an MSR the same is in general possible. This is a scenario that was planned in the 70ies to keep the nukes ready after a nuclear war until the infrastructure and grid are expected to become somehow repaired several month later.

An important emergency reason to shut down the reactor is the loss of external cooling. In Fukushima the Tsunami destroyed the cooling water inlet. If there is no water cooling available it is to reduce the reactor power ideally to 0 to avoid an overheating. Air cooled diesel, or air cooled auxilliary turbines are the best methods in such a scenario. The diesel fuel is sufficient for a couple of days or weeks the same is with auxilliary turbines using decay heat. I would expect that an industrial society should be able to get additional diesel fuel or repair the external cooling in a couple of days.


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PostPosted: Sep 13, 2014 8:06 pm 
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The decay heat of fuel in the reactor can be used via an RTG arrangement even when the reactor has been shut down. The used fuel also has enough heat for a long time, which is normally dissipated in pools of water.
A suitable arrangement to tap these source of power will enable the replacement of diesel generators. As no moving part systems, they could be more reliable in stressful conditions.
A container of used MS fuel could be wired up with RTG to act for decades. It could be used for operating control/shutout rods and other operating/safety loads.


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PostPosted: Sep 14, 2014 1:29 am 
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HolgerNarrog wrote:
Dear Lindsay.... the German nuclear power plants were prepared for a mode to produce the internally required power- about 3% of the full load. For an MSR the same is in general possible. This is a scenario that was planned in the 70ies to keep the nukes ready after a nuclear war until the infrastructure and grid are expected to become somehow repaired several month later.

An important emergency reason to shut down the reactor is the loss of external cooling. In Fukushima the Tsunami destroyed the cooling water inlet. If there is no water cooling available it is to reduce the reactor power ideally to 0 to avoid an overheating. Air cooled diesel, or air cooled auxilliary turbines are the best methods in such a scenario. The diesel fuel is sufficient for a couple of days or weeks the same is with auxilliary turbines using decay heat. I would expect that an industrial society should be able to get additional diesel fuel or repair the external cooling in a couple of days.
Good point Holger if the cooling water supply has been destroyed, then idling producing just enough power to meet internal load will not be possible. One could use an air cooled condenser, but any natural event sufficiently severe to destroy the main cooling system will probably warrant a full shutdown and achieving cold shut down status as soon as reasonably practical, but that will come with its own challenges if you have no reliable internal or external power. That would be an interesting scenario to design and plan for.


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PostPosted: Sep 15, 2014 1:31 am 
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Lindsay wrote:
HolgerNarrog wrote:
Dear Lindsay.... the German nuclear power plants were prepared for a mode to produce the internally required power- about 3% of the full load. For an MSR the same is in general possible. This is a scenario that was planned in the 70ies to keep the nukes ready after a nuclear war until the infrastructure and grid are expected to become somehow repaired several month later.

An important emergency reason to shut down the reactor is the loss of external cooling. In Fukushima the Tsunami destroyed the cooling water inlet. If there is no water cooling available it is to reduce the reactor power ideally to 0 to avoid an overheating. Air cooled diesel, or air cooled auxilliary turbines are the best methods in such a scenario. The diesel fuel is sufficient for a couple of days or weeks the same is with auxilliary turbines using decay heat. I would expect that an industrial society should be able to get additional diesel fuel or repair the external cooling in a couple of days.
Good point Holger if the cooling water supply has been destroyed, then idling producing just enough power to meet internal load will not be possible. One could use an air cooled condenser, but any natural event sufficiently severe to destroy the main cooling system will probably warrant a full shutdown and achieving cold shut down status as soon as reasonably practical, but that will come with its own challenges if you have no reliable internal or external power. That would be an interesting scenario to design and plan for.


Far better is to not rely on diesels and the like. The ESBWR for instance doesn't rely on diesels, it relies on alternative, bunkerized water pool heat sinks when the main heat sink is unavailable. After 7 days fire water supply to the pools is needed if power is not restored yet. Opening a manual valve in the yard and connecting a fire water truck is a lot easier than maintaining diesel generator output, fuel consumption, etc. Diesel generators don't like water, so any flooding can do major damage. The water pools don't care about flooding (that would be a good thing for them, more heat sink).

We can add diesels, or better yet gas turbines, as defence in depth & investment protection, but if we rely on them for the safety case we get these crazy quality control requirements and enless reviewing of the regulatory authorities.


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PostPosted: Sep 15, 2014 10:16 am 
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The pump required to refill an ESBWR's cooling pool is rather small really - its one of those things two people can easily carry.
And I think that it should be possible to use seawater in the IC system without ruining the core since it never coems into contact with it.
But I might be mistaken about that.

So helicopter-in a small generator and some pipe and use it to pump seawater into a plant in the aftermath of a catastrophic accident.


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