Nuclear Reactors: how you start the reaction?

[Dorbs345]

I understand how nuclear reactors work, and how to stop them. But I am not understanding how you start the reaction? The fuel rods are placed in, and then what? What actually gets the process going?

 

[Drakkith]

The fuel rods are placed into a specific pattern or whatever, and once you start to take out the control rods, the neutrons from the radioactive material gets through and sets off more reactions. This starts a controlled Chain Reaction.

 

[Astronuc]

I understand how nuclear reactors work, and how to stop them. But I am not understanding how you start the reaction? The fuel rods are placed in, and then what? What actually gets the process going?

For a fresh core, special neutron sources are placed in an assembly. In BWR, the are in special containers on the core shroud (IIRC), whereas in a PWR, the neutron source assembly (NSA) looks like a control rod - the hub and fingers below the drive rod and coupling at the top. The NSAs sit in assemblies toward the periphery of the core near ex-core neutron detectors. Typically the count rate of the detectors must be 2 cps.

The primary neutron source could by PoBe, PuBe, but more common now is a small Cf²⁵²-Be source. In these sources, an alpha particle hits the Be nucleus and causes an exothermic reaction (alpha,n) producing a neutron and carbon nuclide.

In a new reactor with new fuel a neutron source is needed to get the reaction going. Usually this is beryllium mixed with polonium, radium or other alpha-emitter. Alpha particles from the decay cause a release of neutrons from the beryllium as it turns to carbon-12. Restarting a reactor with some used fuel may not require this, as there may be enough neutrons to achieve criticality when control rods are removed.
Ref: http://www.world-nuclear.org/info/inf32.html

That enough neutrons to startup a reactor. There is also a secondary neutron source Sb¹²⁴-Be. Sb absorbs a neutron and becomes radioactive. It's decay produces a strong gamma > 1.6 MeV. The strong gamma produces a photoneutron reaction in which the gamma excites a Be nucleus, which then emits a gamma ray. That source then provides neutrons for the second and subsequent startups until it is used up.

If the cycle design produces fuel with the right burnup, one can do a sourceless startup based on the spontaneous fissions from transuranic radionuclides that come from successive neutron capture by U-238 and isotopes of Pu.

 

[Drakkith]

And Astronuc to the rescue! Saving this thread from my horrible explanations!

 

[Phrak]

Yeah, thanks Astronuc. Very informative together with a good internet source. Before this, I didn't know the difference between control rods and moderators.

 

[minerva]

more common now is a small Cf²⁵²-Be source.

Correct me if I'm wrong, but I thought ²⁵²Cf neutron sources were just californium, not Cf mixed with Be, and they produce their neutrons from spontaneous fission, not from the ⁹Be(a,n) reaction?

 

[joewein]

Correct me if I'm wrong, but I thought ²⁵²Cf neutron sources were just californium, not Cf mixed with Be, and they produce their neutrons from spontaneous fission, not from the ⁹Be(a,n) reaction?

http://en.wikipedia.org/wiki/Neutron_source" seems to agree with you.

 

[Calvadosser]

I understand how nuclear reactors work, and how to stop them. But I am not understanding how you start the reaction? The fuel rods are placed in, and then what? What actually gets the process going?

In principle, if the assembly is critical (capable of sustaining a chain reaction), one stray neutron arriving from anywhere may start the reaction. If that one gets lost, another one would eventually get it started.

I remember a retired friend, who had been the engineer in charge of a UK nps telling me that a virgin reactor has less reactivity than one that has been running for a time, due to there being no plutonium in it.

I think I remember him saying that the fuel rods in a virgin reactor were therefore made with a slightly higher level of enrichment than was used in refuelling the reactor after if had been running for a while and had some self-generated plutonium to help things along.

 

[Astronuc]

Correct me if I'm wrong, but I thought ²⁵²Cf neutron sources were just californium, not Cf mixed with Be, and they produce their neutrons from spontaneous fission, not from the ⁹Be(a,n) reaction?

Sorry about that - I was mixing thoughts. Traditionally, i.e., more common, Cf-252 n-sources are just Cf-252 using SF neutrons. I was thinking of an advanced design.

 

[QuantumPion]

In principle, if the assembly is critical (capable of sustaining a chain reaction), one stray neutron arriving from anywhere may start the reaction. If that one gets lost, another one would eventually get it started.

I remember a retired friend, who had been the engineer in charge of a UK nps telling me that a virgin reactor has less reactivity than one that has been running for a time, due to there being no plutonium in it.

I think I remember him saying that the fuel rods in a virgin reactor were therefore made with a slightly higher level of enrichment than was used in refuelling the reactor after if had been running for a while and had some self-generated plutonium to help things along.

That may be the case with magnox but not PWR's. Plutonium breeding does add reactivity to burned fuel, but the reactivity is almost always lower than fresh. The exception is a hot once-burned may be on par with a fresh assembly loaded with burnable poisons. Virgin PWR first cycles have to have lower enrichments and a short first cycle due to peaking factor limits.

 

[joewein]

The same is true for BWRs. The enrichment of the first fuel load of the Fukushima I reactors was lower than for subsequent loads. In unit 1 it was 2.1% vs. 3.4%. In units 2-5 it was 2.2% vs 3.6%. In unit 6 it was 2.2% vs. 3.4% (http://fukushima.grs.de/sites/default/files/Technische_Daten_Reaktoren_Fukushima_Nr_1_0.pdf" )

 

[Astronuc]

First cores also have lower batch enrichments than reloads because they have no fission product inventory, particularly no Sm, Pm and other REs. Most if not all NPPs started with annual cycles and subsequently transitioned to 18-mo or 24-mo cycle lengths, and some in between.

 

[swl]

The NSAs sit in assemblies toward the periphery of the core near ex-core neutron detectors. Typically the count rate of the detectors must be 2 cps.

Why 2 cps? What if the initial count rate was at 20 cps? How about 0.2 cps?
Would the core eventually start on it's own given enough time?

It's been fun watching you guys/gals play in the magical nuclear garden. I can see how, with just one hit off the 'nuclear crack-pipe', an engineer could be hooked for life.

 

[Drakkith]

It's been fun watching you guys/gals play in the magical nuclear garden. I can see how, with just one hit off the 'nuclear crack-pipe', an engineer could be hooked for life.

What?

 

[swl]

What?

I meant to say that it is fascinating to the point of being addictive.
I'm sorry for my failed analogy.

 

[jim hardy]

""It's been fun watching you guys/gals play in the magical nuclear garden. I can see how, with just one hit off the 'nuclear crack-pipe', an engineer could be hooked for life.""

i understood your metaphor.



to your question --

2cps gives reasonable assurance that your instruments are working so you're not attempting to start up blind. 20 or 200 would be better.
as you approach criticality by whatever means , the subcritical multiplication increases and count rate goes up. you do this gingerly and wait between changes for it to settle, likely plotting on graph paper so you can extrapolate expected critical point which you approach real cautiously.

every time you double the count rate you have moved halfway to critical. you stop often and check to make sure it's behaving as expected. when you finally get critical the counts increase on their own and if you did a good startup, slowly.

since the subject interests you , ,,
see if your library can get you Freeman Dyson's book "Disturbng the Universe".
https://www.amazon.com/dp/0465016774/?tag=pfamazon01-20
He's aboout the last living Manhattan Project scientist and the book is semi-autobiographical. you still see him on tv occasionally and I'm surprised the industry has not got him to speak on Fukushima. What a great guest for the NOVA show! maybe i'll write to them..

here's some old timers' reminiscences of the "good old days out in the desert" you can read online.
http://www.osti.gov/energycitations/purl.cover.jsp?purl=/6294929-xawLBY/

bon appetit