Reinforced Concrete Design for Thermal Effects on Nuclear Power Plant Structures

Description

ACI 349, Appendix E, provides general considerations in designing reinforced concrete structures for nuclear powerplants subject to thermal effects. Thermal effects are defined to be the exposure of a structure or component thereof to varying temperature at its surface or temperature gradient through its cross section; the resulting response of the exposed structure is a function of its age and moisture content, temperature extreme(s), duration of exposure, and degree of restraint. The terms “force,” “moment,” and “stress” apply and are used in this report where a structure is restrained against thermally induced movements. Further treatment of these forces, moments, and stresses are contained in this report as a function of type of structure.

The Commentary to Appendix E, Section RE.1.2, of ACI349-06 (ACI Committee 349 2006) instructs the designer to consider the following:1. Linear thermal strain causes stress only under conditions of restraint, and a portion of such stress may be self-relieving. Mechanisms for relief are: cracking, yielding, relaxation, creep, and other time-dependent deformations; and2. Accident temperature transients may be of such shortduration that the resulting temperature distributions and corresponding stress changes are not significant.

Therefore, these temperature transients may not adversely affect the safe shutdown capacity of the plant. The Commentary to Appendix E, Section RE.3.3, ofACI 349-06 addresses three approaches that consider thermal effects in conjunction with all mechanical loads acting on the structure. One approach is to consider the structure un cracked under the mechanical loads and cracked under the thermal effects. The results of two such analyses are thencombined.

The Commentary to Appendix E also contains a method of treating temperature distributions across a cracked section. In this method, an equivalent linear temperature distribution is obtained from the temperature distribution, which can generally be nonlinear. The linear temperature distribution isthen separated into a pure gradient ΔT and into the differencebetween the mean and base (stress-free) temperatures Tm – Tb.