Application of energy balance and direct substitution methods for thermal margins and data evaluation

Thermal limits, such as critical hear flux (CHF), flow excursion (FE) and incipient boiling (IB), are used to determine the maximum operating power for a variety of thermal systems, most importantly nuclear reactors. In doing so, a variety of analytical tools, concepts, and techniques are involved. Concepts and definitions involved include the critical heat flux ratio (CHFR), the critical power ratio (CPR) or departure from nucleate boiling power ratio (DNBPR), the direct substitution method (DSM), the heat balance method (HEM), local correlations, system or global correlations, and upstream effects. Some of these concepts and terms cause confusion and misunderstanding in the technical community as to their real physical meaning and their proper application. Some investigators argue strongly in support of using the CPR for determining safety margins in nuclear reactors rather than the CHFR and still others argue that the use of the HEM for thermal limits data and correlation evaluation gives ''consistently better predictions than when using DSM''. Those claims are an indication of a general confusion in understanding and applying these concepts. The intent of this paper is to first define and clarify the issues involved and second to show that either concept, CHFR or CPR, can be used for defining margins. The selection depends on the interest of the investigator and is conditional on understanding what the results mean physically. However, the present author believes that the use of HEM for data evaluation can be misleading and tends to imply incorrectly a closer agreement between the data and the correlation than can be justified. Furthermore, some correlations cannot be used correctly for certain applications, in spite of the fact that they seem to be ''stastistically superior''.