This document consists of information about the cooling systems of Hanford Production Reactors. It is the purpose of this letter, written May 8, 1961, to update recommendations of high-tank temperatures required to insure acceptable high-tank backup support. It does not appear practicable to relate the somewhat arbitrary standards of reference for backup support to process operating limits, i.e., reactor power levels, particularly as a mandatory requirement. For this reason, we have chosen at this time not to include these limits as a process standards` requirement. Considerable emphasis should be placed on insuring that the high-tank backup support be maintained in the maximum practical manner. Tables are attached showing high-tank temperature versus reactor power level relationships to satisfy the intent of our backup criteria and, through compliance, will insure that high-tank flow rates will be adequate during a reactor emergency following the loss of electrical power and steam.