Control Rods Worth: Measuring & Interfering Effect

[Stephan_doc]

What represent reactivity excess and how is it determined
How we can evaluate control rods worth by measurements and by using computer code. It is absolout necessary to take into consideration control rods interfering efect?
All problems regarding research reactor.
Also if exist some books or articles describing these subjects.

 

[QuantumPion]

Control rod worth can be measured in a few different ways. Yes it is necessary to consider the effect of control rods on each other. The following methods generally pertain to commercial PWR's but could be applicable to other reactor types.

1) Boron swap - gradually insert control rod while diluting boron to maintaining criticality, calculate rod worth based on boron worth
2) Rod swap - swap one control rod with another while maintaining criticality, if worth of first control rod is known
3) Subcritical rod worth - estimate control rod worth based on effect on subcritical multiplication of source neutrons

 

[QuantumPion]

Oh and to answer your first question, reactivity is measured by the rate of change of neutron flux.

 

[Astronuc]

Rod worth is measured by how it changes k_eff, that is by the Δk it produces as it relates to k_eff, or in other words, how much reactivity is removed when the control rod is inserted, or how much is added with the control rod is withdrawn

Individual rods have worth, and banks (sets) of rods have worth, and that will vary according to reactor conditions (hot vs cold, zero power vs hot full power), enrichment (including fissile Pu content), and burnable absorber (either in liquid form, e.g., boric acid in a PWR), or boron in the form of pyrex in special assemblies, or burnable absorber such as gadolinia, erbia, or ZrB₂ in the fuel rods. In other words, it depends on reactivity distribution in the core.

One may refer to differential rod worth, i.e., how does the reactivity or Δk change with each step or fraction of control rod insertion/withdrawal, or integral rod worth, in which the reactivity effect is measured with a rod fully inserted or fully removed.

Rod worth is important for shutdown margin, as for evaluation of transients such as reactivity insertion accidents, RIA.

With regard to control rod interference, it relates to the rate at which a control can insert reactivity into the core. Some commercial plants, both PWR and BWR, have experienced issues of control rod/blade interference. In some PWRs, assembly (guide tube) distortion has been an issue, while in BWRs, channel bow and bulge have caused slower insertion of control blades. Fuel management is a factor, as is fuel design.

 

[QuantumPion]

Rod worth is measured by how it changes k_eff, that is by the Δk it produces as it relates to k_eff, or in other words, how much reactivity is removed when the control rod is inserted, or how much is added with the control rod is withdrawn

Individual rods have worth, and banks (sets) of rods have worth, and that will vary according to reactor conditions (hot vs cold, zero power vs hot full power), enrichment (including fissile Pu content), and burnable absorber (either in liquid form, e.g., boric acid in a PWR), or boron in the form of pyrex in special assemblies, or burnable absorber such as gadolinia, erbia, or ZrB₂ in the fuel rods. In other words, it depends on reactivity distribution in the core.

One may refer to differential rod worth, i.e., how does the reactivity or Δk change with each step or fraction of control rod insertion/withdrawal, or integral rod worth, in which the reactivity effect is measured with a rod fully inserted or fully removed.

Rod worth is imported for shutdown margin, as for evaluation of transients such as reactivity insertion accidents, RIA.

With regard to control rod interference, it relates to the rate at which a control can insert reactivity into the core. Some commercial plants, both PWR and BWR, have experienced issues of control rod/blade interference. In some PWRs, assembly (guide tube) distortion has been an issue, while in BWRs, channel bow and bulge have caused slower insertion of control blades. Fuel management is a factor, as is fuel design.

I think by control rod interference he was referring to flux shadowing, where the the reduction of flux caused by the insertion of one rod reduces the worth of inserting adjacent rods. This is the reason why control rods are arranged in banks of rods in specific positions designed to account for this effect.

 

[Stephan_doc]

Yes, i mean flux shadowing caused by the insertion of one rod reduces the worth of inserting adjacent rods. Generally speacking woth of two control rods inserted simultaneously is not equal to worth sum of two rods inserted one by one. How percent is the diference? I need some research works about this phenomena.

 

[QuantumPion]

The difference is highly dependent on the location and details of the specific core loading. We calculate this type of rod worth using core simulator software.

 

[Astronuc]

In large commercial reactors, control rods are distributed across the core.

See figure 2-7 in the pdf for a core map showing the control rod groups.
http://www4.ncsu.edu/~doster/NE405/Manuals/PWR_Manual.pdf

For large BWR (764 assemblies) see - http://www.freepatentsonline.com/5677938.pdf.
The + in the figures of core maps designates a control rod. Within the core, one control rod sits in a 'cell' surrounded by 4 assemblies. The controls rods are grouped with separation among control rods in each group. There is redundancy.

For small cores, e.g., in a research reactor, the control rod density may be higher, and each control rod may be closer to its neighbor. Usually control rod worth is worked out with experiment and simulation.