As the largest heat exchange equipment in the nuclear power plant system, steam generator (SG) is mainly responsible for the heat transfer matching between reactor coolant system (RCS) and secondary system in normal operation. The steam generator incorporating U type heat transfer tube also plays an important role under accident conditions due to the considerable feedwater inventory in its secondary side for design of most nuclear power plants, such as EPR, VVER, M310, etc.

However a direct passive cooling method is adopted in the Advanced Passive 1000 (AP1000), namely the emergency core cooling system is composed of passive residual heat removal (PRHR), core makeup tank (CMT), accumulator (ACC), and (In-containment Refueling Water Storage Tank (IRWST). As a result, the effect of heat exchange of SGs are usually ignored under accident conditions especially in the loss of coolant accidents (LOCAs) for the core decay heat is mainly transferred to the cold water in IRWST and atmosphere within containment via PRHR and other passive heat exchange systems.

However, the SGs may be relatively large heat sources in some transient conditions such as station blackout (SBO) occurring at Fukushima nuclear power plant in Japan. When the level of SG secondary side is fairly high while the decay heat power is low, a reverse heat transfer phenomenon which makes the SGs become an important heat source may occur as while as the PRHR operation.

In this paper, one SBO integral effect test carried out at ACME test facility is used to investigate the reverse heat transfer phenomenon in SG U-tube under non-LOCA transient. The temperature distribution in the U-tube, the flow rate of SG, the secondary pressure and feedwater level of SGs were studied. The results show that after the PRHR system is put into operation, the average temperature of RCS obtained by cold legs and hot legs decline rapidly. The water temperatures in the hot legs and hot chamber at SG inlet are far below that in the secondary side of SGs. The reverse heat transfer then occurs in the U-shaped tube bundle, and the heat transfer function of SGs are reversed namely the SGs become heat sources from heat sink in the accident process. Thus the cooling rate of RCS caused by PRHR is slowed down.

The equivalent power of reverse heat transfer in SGs were estimated based on the energy balance method and test data. The results show that the maximum power of SG reverse heat transfer reaches about than 20% of the decaying core power at the same time. The thermal hydraulic behavior of RCS, the temperature distribution in RCS are significantly affected and therefore make an effect on accident evolution process.

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