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PostPosted: Nov 21, 2012 3:56 pm 
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A lot of discussions about reactors are about steady state operation, or power swings within certain limits. I understand how controllable excess reactivity is needed to manage the power output of a reactor. I also understand that in the steady state, the reactor runs using both prompt and delayed neutrons.

My question is about starting from cold. I could easily be wrong here, but I assume that in modern reactors there is some material that provides neutrons upon which the chain reaction can build, using only prompt neutrons, and taking advantage of excess reactivity. In the case of a shutdown, I also assume that after the reactor runs a while, there is enough material generated to provide delayed neutrons to restart, whether immediately for the Navy's reactors that have lots of excess reactivity potential, or after enough of the neutron poisons like xenon (don't know the isotope number) have decayed that reactivity on prompt neutrons and residual delayed neutrons is positive again (for civilian power reactors).

But let's go back to the Chicago Pile reactor, and the reactors used during WWII for plutonium production. I assume there was no material to provide neutrons to start the process (I could well be wrong again!). Did they just pull the control rods all the way out and wait for a stray neutron to naturally wander by, or a spontaneous fission producing a neutron, to get things started?


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PostPosted: Nov 21, 2012 6:21 pm 
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Yup. The material is the fissile fuel, all of which have a spontaneous fission rate high enough to kick things off.

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PostPosted: Nov 22, 2012 3:45 am 
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Gregorian Chanter wrote:
A lot of discussions about reactors are about steady state operation, or power swings within certain limits. I understand how controllable excess reactivity is needed to manage the power output of a reactor. I also understand that in the steady state, the reactor runs using both prompt and delayed neutrons.

My question is about starting from cold. I could easily be wrong here, but I assume that in modern reactors there is some material that provides neutrons upon which the chain reaction can build, using only prompt neutrons, and taking advantage of excess reactivity. In the case of a shutdown, I also assume that after the reactor runs a while, there is enough material generated to provide delayed neutrons to restart, whether immediately for the Navy's reactors that have lots of excess reactivity potential, or after enough of the neutron poisons like xenon (don't know the isotope number) have decayed that reactivity on prompt neutrons and residual delayed neutrons is positive again (for civilian power reactors).


You have it right. It is more convenient to start a large power reactor up with a neutron source. Obviously faster and more responsive to lower some neutron source into the reactor, and less chance of weird oscillations or the chain reaction petering out on you (because of xenon or something else).

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But let's go back to the Chicago Pile reactor, and the reactors used during WWII for plutonium production. I assume there was no material to provide neutrons to start the process (I could well be wrong again!). Did they just pull the control rods all the way out and wait for a stray neutron to naturally wander by, or a spontaneous fission producing a neutron, to get things started?


I doubt that the Chicago Pile was started up with a neutron source. You need a reactor to produce a neutron source. Chicago pile was the first reactor, so they couldn't have produced significant neutron source yet! Usually it is a really heavy actinide with fast alpha decay that you want. Then mix something light in, that gives off neutrons upon being struck by the alpha particles. Pu238-beryllium is one of the best and most commonly used materials today. It's also possible to use a special fuel rod (as a control rod) that has a higher enrichment.

Chicago pile was a very low power density thermal reactor (no way to cool the thing decently). Starting it up without a neutron source and fresh uranium must have taken just a while longer.


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PostPosted: Nov 22, 2012 6:22 am 
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You're right, in fact IIRC bars of californium (a relevant neutron emitter) are usually used to start up nuclear reactors


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PostPosted: Nov 22, 2012 2:47 pm 
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As I understand it a neutron source ensures good control during a startup with 100% virgin fuel and very helpful to avoid accidental supercriticality, but that it is not strictly necessary. Does anyone out there have hands on experience to share on this?

For MSR without control rods, I imagine that you would preheat the vessel, pipework and fuel to idling temperature plus say 50C to suppress criticality. Load fuel into the core and start circulating and then through natural heat loss allow the core temperature to drop low enough to become critical. If required then add shim material, fertile or fissile to fine tune the idling temperature before taking any thermal power out of the core. Alternatively one could load subcritical fuel salt and gradually add positive shim material (fissile) until criticality is achieved.

I'd like to think that plant instrumentation in the form of neutron detectors would provide valuable guidance on the k value of the core during the entire startup period, but I've never started a reactor, so this is mostly conjecture on my part.


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PostPosted: Nov 23, 2012 9:35 pm 
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Startup sources are typically used for the very first core starup and for any subsequent startup on a reactor that has been shut down a along time (2+ years) for some other reason.

The reasons for this are twofold:
1. Nuclear instrumentation uses neutrons from fission or other nuclear reactions as an extrapolation of actual core power level. During startup you want your core criticallity to occur at a high enough fission rate to be able to see it on this instrumentation. This will help prevent adding excessive positive reactivity after criticality has been achieved. Excessive positive reactivity after criticality will cause the reactor power to rise very fast. Your power increase rate can be uncontrollably high even before it will reliably register on your instrumentation. Adding a startup nuetron source provides a base power level and allows you to observe the approach to criticality on instrumentation, thus allowing you to "see" criticality and stop adding positive reactivity sooner.

2. Verifies with a real signal source that your instrumentation is actually functioning.


Reactors that have operated for even a few dozen hours at full power will have sufficient fission product decay sources of neutrons for many months after shutdown to not require a startup neutron source. You can startup any reactor without a nuetron source. You have small levels of instrinsic neutron sources with any of the fissile materials in the core due to spontaneous fissions as well as cosmic ray interactions ans some decay products. Doing this is possible but you will have to depend on subcritical neutron multiplication and that will mean the startup will take many hours, maybe as long as a full day if you do not want to go critical before you can see it on instrumentation.


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PostPosted: Nov 23, 2012 11:00 pm 
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Not all neutron sources are produced by reactors. Beryllium mixed with an alpha emitter will emit neutrons. The Chicago Pile used radium and beryllium as a neutron source.


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PostPosted: Nov 23, 2012 11:30 pm 
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So, getting down to the brass tacks, would a LFTR need a neutron source?

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PostPosted: Nov 24, 2012 4:46 am 
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Ned Speirs wrote:
Not all neutron sources are produced by reactors. Beryllium mixed with an alpha emitter will emit neutrons. The Chicago Pile used radium and beryllium as a neutron source.


Radium, that's interesting. Where did they get the radium from? From processing natural uranium like the Curies did? Perhaps such a small reactor doesn't need much neutron source to start, and processing natural uranium for its radium is practical.


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PostPosted: Nov 24, 2012 4:55 am 
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KitemanSA wrote:
So, getting down to the brass tacks, would a LFTR need a neutron source?


From reading the ORNL documents, they didn't think it was necessary. Once some of the short lived actinides are produced, that makes enough constant neutrons even after shutdown, from reactions with beryllium and fluorine. This stream of neutrons was considered enough by ORNL to startup the reactor.

For startup the first, this source doesn't exist, unless of course you startup on TRUs. I'm hoping to startup on TRUs so that solves that problem. Without TRU startup though you could just keep adding fissile till the reactor went critical. With noble gas removal and constant fuel additions startup for a LFTR is much easier than a solid fuelled reactor.

ORNL did plan on some graphite rods which they could insert for extra power. Presumably these would be used during startup and load following. I would prefer to not bother with such mechanical devices, with the associated control and electronics, inside such a difficult environment. In stead it seems that just starting up by first preheating to operating temps, then gradually adding fuel is the easiest way to go. The only mechanical thing we really need for control is the pumps. So it makes sense to me to just use the pump as the control and safety (pump trip) feature, and fuel additions for longer term reactivity control. This way the pump trip could be the scram without a signal of intelligence for any safety rods.


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PostPosted: Nov 27, 2012 2:25 pm 
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Speaking of neutrons, make your own neutron source at home!

http://www.youtube.com/watch?v=uQ8vwevCq8Q


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PostPosted: Apr 02, 2014 5:32 am 
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Does anybody know how long it took to restart the ORNL molten salt experiment reactor after it was shutdown for the week ?
I mean time until 100% power or some specific setting.

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PostPosted: Apr 06, 2014 10:10 pm 
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The radium-beryllium neutron start up source some early piles used is called a neutron howitzer, a lab source of neutrons dating back to the 1930s that was useful for many experiments in which a neutron beam was desired (lots of shielding and geometry were important for any collimation. Polonium or other alpha emitters can be used with beryllium as well.


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PostPosted: Apr 12, 2014 3:32 pm 
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From CP-1 to large LWRs, people use sub-critical multiplication, the "1/M plot" method, to start the reactors. See pages 1-9 here for a good explanation: http://www.cedengineering.com/upload/Re ... ration.pdf

The reactor is usually heated up near the operation temperature before the startup, so that it starts with all the expected thermal feedbacks.

PS: I have heard that the old wolves of Russian nuclear program would start fresh reactor cores on background neutrons, but it may be a tall tale.


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