During the history of operation of the Hanford reactors, several methods have been utilized to establish an outlet water temperature or ``Panellit`` limit based on the stability of water flowing through the process tubes. These methods have been known as ``Excess Header Pressure Limits,`` ``Trip Before Boiling Limits,`` and the most recent, ``Trip Before Instability Limits.`` Each new concept vas accepted only after considerable experimental data had proved the new method to be safe and operationally feasible. It is anticipated that in the future other tube outlet water temperature limits will be developed based on new technology. The purpose of this document is to present, in simple terms, an understandable explanation of previous operating limits, to discuss the status of the present Instability Limits, and to outline the direction of future work in raising the outlet water temperature limits.

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This document was prepared in response to a request from the Atomic Energy Commission for a brief review of the principal ideas and technical developments which might effect power levels over the following six years. Also requested were approximate data on any pile modifications that might be involved, including rough estimates covering possible cost and completion dates. A brief discussion of plans for increasing the power levels of the Hanford Piles is provided.