How does the chain reaction start in a BWR reactor?

[Trinity10]

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.

 

[Astronuc]

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.

 

[vanesch]

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.

 

[Trinity10]

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.

 

[gmax137]

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.

 

[Astronuc]

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.

 

[Trinity10]

Did they use startup neutrons for CP-1? If so, where were they placed at?

Thanks.

 

[vanesch]

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.

 

[fatra2]

Hi there,

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

 

[gmax137]

Please ignore my response in post #5 above. Total brain fade, missed the point of the question.

 

[Astronuc]

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