control rod material 1

[pagoji]

Hello experts,
i have a question regarding control rod materials. any difference between using a boron control rod or a silver/indium/cadmium control rod?

also, how frequently do these rods get replaced in BWR/PWR?

thanks and rgds

Pagoji

 

[electricpete]

I work at a pwr, although my position does not involve focus on the reactor. I have never heard of the control rods being replaced.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[vpl]

Pagoji

Here's a link that you might find helpful:

http://www.tpub.com/content/doe/h1017v2/css/h1017v2_75.htm

I'm not real familiar with what different plants use, but, besides their neutron absorption capability, issues such as neutron embrittlement and cost play a role in what is selected.

To answer your second question, yes control rods are replaced, but rarely.



Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

 

[pagoji]

hello Electripete and Patricia,

Thank you for your comments.

If control rods do not get replaced (or rarely get replaced), does the rate of reaction change over time. I am visualizing this control rod as some kind of a filter which absorbs neutrons and over the course of time, the effectiveness of each control rod reduces. Let me know if I am off!!

also from your experience how many control rods go into each reactor(PWR/BWR). Is it in the hundreds or in the thousands? and are the rod dimensions standard between the different reactors.

I greatly appreciate you taking the time to respond.

Rgds

Pagoji

 

[vpl]

pogoji -- you do understand that this is working on the atomic level, right? And that there are many different types of control rods (shutdown, control and, oh yes, burnable poison rods). And that many of the rods are not in the reactor while it is operating (i.e., they are there only to shut it down).

All of the above get replaced at a frequency determined by a utilities nuclear fuel vendor. Each refueling cycle the vendor does a detailed core analysis which takes into account such factors as nuclear burnup of the fuel and any poison rods. If a vendor makes a determination that the control rods cannot function over the next fuel cycle, then those rods get replaced. While this doesn't happen often, it does happen.

In regard to your question regarding the number of fuel rods - the generic answer is "yes". Specifically, the number of control rods is dependent on the reactor design and varies considerably. I suggest you go talk to a reactor engineer or an operator to find out what you have at your site.

You also asked if the rod dimensions are standard. The answer to that is "no". In fact, the different types of rods for one vendor have different dimensions so they don't get used in the wrong place.

In short, you cannot arbritarily decide to move a control rod from one place to another nor to replace the absorpion medium. Doing so would invalidate your fuel burnup analysis and might cause you to have to reduce power or start an outage early (both major economic considerations.)

Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

 

[pagoji]

Hello Patricia,

Thanks a lot for your detailed response. Makes it more clear and answers my question.

Regards

Pagoji

 

[electricpete]

Once again I am definitely not a reactor guy. VPL I'm sure knows a lot more.

Two more thoughts:

1 - One objective in selecting control rod material is one that has a chain of large number of isotopes, all with large cross section for absorption of neutrons... in order to prevent the exact effect you talk about... diminishing effectivenss over time.

2 - For our PWR, rods spend that vast majority of time fully withdrawn from the core (control reactivity with coolant boron concentratio). I think the objective is to get even burnout of fuel including near the top. A side effect would be that rods last longer.



=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[vpl]

You're absolutely right electric pete.

Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

 

[tristanhunt]

Are there any reactor types which call for pure (elemental B) Boron rods, as opposed to carbide or alloy rods?

 

[NuclearProfessor]

Control rods come in all shapes, sizes and flavors to fit the needs and requirements of the different reactor designs.

Cruciform control rod which looks more like a blade, and is made by (GE), who is the supplier of Boiling water reactors, these blades are inserted from the bottom of the reactor. There is one blade between each four fuel assemblies.
Control rods for PWR are made in a cluster assembly, the cluster have about 15 rods in each fuel assembly. The control rods are inserted (or dropped for reactor shutdown) from the top. PWR are made by Westinghouse.

The material selection for control rods depends on several decisive factor like
High neutron absorption cross section, Mechanical strength, Chemical and structural stability, Corrosion resistance, Fabrication availability and reasonable cost .
Usually, cadmium, boron, carbon, cobalt, silver, hafnium, and gadolinium, and Europium are common elements used in control rods.

In BWR B4C Clad in stainless steel is commonly used.
In PWR both B4C and Ag-In-Cd (80% Ag-15%In+5%Cd) also clad with SS and other materials ( CW 304 SS/Inconel 627) are typically used. PWR reactors also use boron as soluble poison in the form of boric acid (H3BO3) dissolved in the coolant (referred to as borated water)

Ned Xoubi
Nuclear Engineer

 

[olezh]

Hello, all.

I will have a task to evaluate neutronic and other properties of a neutron absorber under out-of-reactor conditions: Ni-Gd alloy that might or might not be sandwiched between Cr-Ni alloy plates of low corrosivity. It will be placed between PWR and BWR assemblies for reactivity control. Has anyone heard about behaviour and properties of such alloys out of reactor?

Thanks.
Oleg.