Control Rod Lifetimes 1

[tjc4pv]

Hello All!

This is my first time here and I think this forum can be very informative!

I've been involved with control rod design criteria and one issue that has come up is the design lifetime. I have seen some plants have had problems (major & minor...) with unexpected events. How do you estimate/calculate how long a control rod (RCCAs, CEAs, etc) will last before failure? Is there an acceptable amount of damage before the control rod must be replaced (e.g., a 1/2 inch crack vs. 3 inch crack)?

Any info would be much appreciated!

 

[vpl]

tjc4pv

I'm not sure there is a simplistic answer to your questions. That is partially due to the fact that they are very general. Also, I am not familiar with any control rod failures, although there have been failures of the control rod nozzle in the vacinity of where the control rods join the reactor vessel. As you know as a designer, the control rod nozzle is the penetration connected to the reactor vessel which allows the control rod drive mechanism to drive the actual control rods in and out.

Basically, the control rods are supposed to last "for the lifetime of the plant." Estimations are based on the materials used, the weld processes involved, the nuclear design and a number of other factors, some of which are proprietary to the vendor.

Also there is not a single criteria which says what size crack is acceptable. Factors that must be considered, besides the length, are the direction of the crack, the depth, the crack growth rate, the materials involved and much more.

You can find some additional information on the Nuclear Regulatory Commission's website (

http://www.nrc.gov).

However, if you are really interested in crack phenomenon, you need to find a very patient metallurgist who can explain it to you.

Patricia Lougheed

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

 

[damienk]

another key point for RCCA is wear !

 

[electricpete]

I'm pretty sure that some plants have experienced a stuck rod due to warping.

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[vpl]

electric pete:

You're correct that there have been cases where control rods have "stuck" or been slow to respond due to warping; however, the warping is generally in the fuel, not the control rods. Certain fuels are more likely to have this problem occur than others.

For all you non-nukes reading this, utilities have to monitor control rod response times and take corrective actions if they are too slow in responding. It's not something that is "lived with."

Patricia Lougheed

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