Review of heat transfer behavior in supercritical water cooled reactor

Supercritical water cooled reactor (SCWR) is one of the six reactor technologies under the US led generation IV international forum (GIF). SCWRs are promising advanced nuclear systems because of their high thermal efficiency (i.e., about 45% as opposed to about 33% for current LWRs) and considerable plants implification with a mission to generate low cost electricity. Various types of SCWRs i.e., pressure vessel type,pressure tube type etc. are under development stage. Literature survey showed that the majority of experimental data for measuring fluid temperature and wall temperature by varying pressure, mass flux, heat flux and geometry were obtained in vertical tubes, some data in horizontal tubes and annuli, and a few in other flow geometries including scaled down fuel bundles. But, actually fuel bundles are used in SCWR core. Therefore, more experimental data are needed for fuel bundles to verify the prediction of heat transfer coefficient. Empirical generalised correlations based on experimental data are obtained for heat transfer coefficient calculations at supercritical pressures. However, there is no consensus on the general trends in the predictions of heat transfer coefficient CFD codes are also used for the prediction of heat transfer coefficients. Wide variation in the prediction of the results is also observed for various turbulence models like standard kε,kω, RNG, RSM etc. In this paper heat transfer co-efficient obtained by experimental results, empirical correlations and with the use of CFD code is discussed. In general, the experiments showed that there are three heat transfer modes of fluid at supercritical pressure: normal heat transfer, improved heat transfer and deteriorated heat transfer.Heat transfer at critical and pseudocritical pressures is influenced by the significant changes in thermo-physical properties. Mechanism for enhancement/deterioration in heat transfer and criteria for onset of deterioration has also been discussed in this paper. Two important issues are identified which needs to be resolved for thermal-hydraulics/regulatory point of view: (a) heat transfer deterioration near pseudocritical temperature which leads to increase in wall temperature similar to dryout in LWRs, (b) fuel design criteria based on the critical heat flux is not applicable for the SCWRs because of no phase change of water inside the core. (author)