Question

7.3 Steam Generator Tube Break

(c) What particular precautions would need to be followed while shutting down such a unit?

Answer 1

Precautions
1. Many factors can affect the error associated with the steam generator blowdown method such as steam carry over of the radionuclide, partitioning effects, hideout/return, chemical
reactions, leak location, etc. For example, iodine readily reacts with sludge metals and is subject to hideout. However, it may be possible in some circumstances to obtain leak rate
which correlates with the more reliable methods, taking these factors into consideration and making the applicable corrections.
2. The steam generator blowdown sample itself may not truly reflect its own generator bulk water, due to deposition of solids within the sample line, downcomer/feedwater mixing, etc.
Each plant should evaluate the specific design for transport time and determination of a representative sample of the steam generator bulk water.,
3. A steady-state condition of the secondary system is essential to avoid gross errors in the estimate of leak rate. A change in steam generator blowdown rate, for example, can introduce error into the result until a new steady-state is achieved.
4. Inaccuracies in the measurement of the necessary flow rates can introduce errors.
5. Inaccuracies in the steam generator liquid volume also introduce error. This error is minimized by maximizing steam generator blowdown rate.

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