How does the chain reaction start in a BWR reactor?

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Discussion Overview

The discussion revolves around the initiation of the chain reaction in a Boiling Water Reactor (BWR) after the fuel rods are loaded into the core. Participants explore how the reaction begins, the role of neutron sources, and the conditions necessary for achieving criticality.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants suggest that initial neutrons are provided by neutron sources distributed in the core, which lead to a low level of fission detectable by various detectors.
  • It is proposed that the chain reaction can start with a small number of neutrons, emphasizing the importance of the k-factor being slightly greater than 1 for exponential growth of neutron population.
  • One participant notes that the fuel rods are relatively transparent to neutrons, allowing them to penetrate the rod walls made of zirconium alloys.
  • Questions are raised about the possibility of spontaneous fission occurring during the loading of uranium pellets, with some participants asserting that it is not significant due to the absence of a moderator during fabrication and storage.
  • There is a discussion about the types of neutron sources used, including alpha-emitter and beryllium mixtures, and the potential use of spontaneous fission sources like Cf-252 for neutron emission.
  • Concerns are expressed regarding the nature of in-core detectors, with some participants questioning their presence and functionality in BWRs compared to out-of-core detectors.
  • One participant mentions that cosmic rays can provide a small neutron background that could theoretically initiate a critical mass, although this is not a controlled method.

Areas of Agreement / Disagreement

Participants express varying views on the mechanisms of neutron initiation and the role of different neutron sources. There is no consensus on the specifics of in-core detector types or the significance of spontaneous fission during fuel loading.

Contextual Notes

Some statements rely on assumptions about the presence of neutron sources and the conditions during fuel loading, which may not be universally applicable. The discussion includes technical details that may depend on specific reactor designs and operational protocols.

Who May Find This Useful

This discussion may be of interest to individuals studying nuclear engineering, reactor physics, or those involved in the operation and safety of nuclear reactors.

Trinity10
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Greetings,

As someone who is interested in the history of the nuclear age, I have been unable to find an answer for this:

Once the fuel rods are loaded into the fuel assembly and the assembly is loaded into the core, how does the chain reaction start which creates the heat? Do they bombard the U235 pellets with neutrons? But, all the pictures I have seen of the assemblies show the fuel rods closed off, so how do the neutrons make their way into the rods?

Any clarity would be greatly appreciated! And if any of the above is incorrect, please, by all means, do correct.

Thanks in advance.
 
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A core of fresh fuel would have neutrons sources distributed in the core so that there are some initial neutrons that cause a low level of fission which is detected by in-core and ex-core detectors.

In BWR, groups of rods are removed from the core until a couple of groups remain. The last two sets are then adjusted (one set deep, and one set shallow) to achieve criticality, and then power ascension begins.
 
Trinity10 said:
Once the fuel rods are loaded into the fuel assembly and the assembly is loaded into the core, how does the chain reaction start which creates the heat? Do they bombard the U235 pellets with neutrons?

Essentially, yes. But you don't need very many neutrons to start a chain reaction: it is sufficient that your "loop gain" is slightly larger than 1 (the famous k-factor), and from 100 neutrons, you get 101 neutrons, and then 102 neutrons, and ... about every 20 microseconds or so. This rises exponentially (hence, keep the k-factor not TOO far from 1 :-) ) until the desired neutron population is present.

The first few neutrons are usually delivered by a neutron source (mixture of an alpha-emitter and beryllium for instance).

But, all the pictures I have seen of the assemblies show the fuel rods closed off, so how do the neutrons make their way into the rods?

The rods are pretty transparent to neutrons, actually. Neutrons are beasts that can get through quite some material. We have made "neutrography" movies of a running diesel engine made of stainless steel.
But moreover the rod walls are made of for instance zirconium alloys, which are *particularly* transparant to neutrons.
 
Thank you both VERY MUCH! This helps immensely. Is it possible for spontaneous fission to occur while they load the fuel rods with the uranium pellets?

Cheers.
 
Trinity10 said:
Is it possible for spontaneous fission to occur while they load the fuel rods with the uranium pellets?

Cheers.

Not to any meaningful extent - there is no moderator present during fabrication/shipping/storage of the assemblies.
 
Trinity10 said:
Thank you both VERY MUCH! This helps immensely. Is it possible for spontaneous fission to occur while they load the fuel rods with the uranium pellets?

Cheers.
Not with fresh fuel, hence the (PoBe or RaBe) sources which use energetic alphas to produce an (alpha, n) reaction which provide neutrons when the reactor is subcritical. Spontaneanous fission sources (e.g., Cf-252) are used instead of (PoBe/RaBe/PuBe) neutron sources. The Cf-252 source could be augmented with Be to achieve a higher neutron emission rate.

The radioisotope 252Cf is an intense neutron emitter that is routinely encapsulated in compact, cylindrical source capsules. Decay by alpha emission (96.91% probability) and spontaneous fission (3.09% probability) results in an overall half-life of 2.645 years and neutron emission of 2.314 x 106 s-1 ug-1, with a specific activity of 0.536 mCi/pg. The neutron energy spectrum is similar to a fission reactor, with most probable energy of 0.7 MeV and an average energy of 2.1 MeV.
Ref: http://www.ornl.gov/~webworks/cpr/pres/102606.pdf

Secondary sources typically used in PWRs are (Sb-Be) which uses activation of Sb by neutron absorption and the subsequent decay for a photoneutron source.

With exposure (burnup), reactor operators can do sourceless startup by taking advantage of the production of transuranic isotopes in moderate to high burnup fuel.
 
Last edited by a moderator:
Did they use startup neutrons for CP-1? If so, where were they placed at?

Thanks.
 
Trinity10 said:
Did they use startup neutrons for CP-1? If so, where were they placed at?

Thanks.

Actually, I don't know what they did for CP1. However, in principle, you don't even need any neutron source, because there is a tiny neutron background present due to cosmic rays. Point is, that is a really small background, and you need quite some "exponential growing" before you get to a reasonable neutron flux, which is, moreover, less well controlled in the beginning due to important statistical fluctuations. But in principle, the neutron background from cosmic radiation can start any critical mass.
 
Hi there,

Astronuc said:
A core of fresh fuel would have neutrons sources distributed in the core so that there are some initial neutrons that cause a low level of fission which is detected by in-core and ex-core detectors.

Just a quick question: What kind of "in-core" detectors are you talking about. From my understanding of BWR, neutrons and gamma detectors are all out-of-core, measuring, therefore, an average distribution.

Cheers
 
  • #10
Please ignore my response in post #5 above. Total brain fade, missed the point of the question.
 
  • #11
fatra2 said:
Hi there,

Just a quick question: What kind of "in-core" detectors are you talking about. From my understanding of BWR, neutrons and gamma detectors are all out-of-core, measuring, therefore, an average distribution.

Cheers
Neutron sources must be in the core adjacent to fuel (in a BWR) or in within neutron source assemblies (in a PWR), which look like control rod or burnable poison assemblies, and which reside in the guidetube positions in an unrodded (uncontrolled) assembly usually toward the core periphery.

BWRs have local power range monitors (LPRMs) which are located in-core between cells, and actually at the common corner of 4 cells, where a cell corresponds to four assemblies which surround a common control blade (control rod). Ex-core fission chambers are also an option.

http://www.ge-mcs.com/en/nuclear-reactor-instrumentation.html
http://www.ge-mcs.com/en/nuclear-reactor-instrumentation/neutron-monitoring-instrumentation/power-range-detectors.html
http://www.ge-mcs.com/en/nuclear-reactor-instrumentation/neutron-monitoring-instrumentation/dry-tube-assemblies.html

http://www.ge-mcs.com/en/nuclear-reactor-instrumentation/neutron-monitoring-instrumentation/startupwide-range-detectors.html

http://www.ge-mcs.com/en/nuclear-reactor-instrumentation/neutron-monitoring-instrumentation/ex-core-fission.html

See also Figure 4.1-1 Core Configuration with Location of Instrumentation
- http://www.nrc.gov/reactors/new-reactors/design-cert/abwr/dcd/tier-2/ch-4.pdf
 
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