Nuclear fuel performance is generally derived from large-scale rod bundle tests, which mainly include rod bundle critical heat flux (CHF) tests and rod bundle mixing tests under conditions of high pressure, high mass flux, and high power. Though few differences exist in the test section configuration and operation conditions, CHF tests and mixing tests generally share the same test loop, the same pressure-bearing housing, and similar flow channel designs. In rod bundle tests, the test section consists of an inner flow path with an outer surrounding chamber (outer chamber) filled with stagnant coolant. Depending on the operating conditions, the massive coolant in the outer chamber may become a significant source of heat loss/heat gain resulting from natural circulation in the outer chamber. The complex structure of the test section and variable conditions appear to be obstacles to heat loss/gain estimation.

In the past, most of the calculations and discussions for heat loss/gain correction were based on a steady-state assumption without exploring the impacts of true heat loss/gain resulting from potential transient operation. That is, in rod bundle tests, in order to determine the heat loss characteristics, only single-phase heat loss tests were conducted under different temperature steady-state conditions. With the limited parameter measured, the transient behaviors of the outer chamber are not reflected. In addition, potential heat gain conditions are neglected in single-phase heat loss tests. Based on heat loss test data, a correlation between heat loss and temperature difference of the test section inlet and exit is often derived. Given this situation, in this paper, lumped-parameter analyses are conducted to evaluate the heat loss/gain trend. A system code is applied to simulate the test section and surrounding systems under typical CHF test conditions to explore heat loss/gain transient behaviors, respectively. Methods to minimize the heat loss/gain effect through the design and operating scheme are discussed at the end.