Review of concrete performance at elevated temperature and hot sodium exposure applications in nuclear industry

In the normal operating conditions of fast breeder reactors [FBRs], liquid sodium is used as heat transfer medium at the maximum temperature of 550 °C. Sodium leakage accidents pose various safety issues related to loss in structural integrity and performance characteristics of concrete structures. Thermo-chemical interactions of liquid sodium with concrete at 550 °C and above result in degradation of concrete in several ways. Concrete is a composite material and exhibits instability due to thermal and chemical loading of hot sodium. The increase in concrete temperature leads to a sequence of events – dehydration of cement paste above 100 °C, crack formation in the range of 400–1000 °C, decomposition of hydrated cement products like calcium hydroxide [Ca(OH)2] at 450 °C, mineralogical changes in aggregates above 500 °C, disintegration of calcium silicate hydrates [CSHs] above 600 °C and formation of glassy powder around 1200 °C. Thermo-chemical interaction of concrete with sodium consists of three phases, namely, initiation, propagation and termination. Initiation phase is accompanied with liberation of moisture from the interior of concrete and its subsequent reactions with sodium resulting in the formation of sodium hydroxide [NaOH], hydrogen gas [H2], sodium oxide [Na2O] and sodium hydride [NaH]. Propagation and termination phase are governed by total sodium leaked, temperature at the reaction interface and types of aggregate used. To protect the structural concrete from deterioration, generally a sacrificial layer of cement composite is employed. Damage behavior of this sacrificial layer depends on total amount, exposure time and impact velocity of leaked sodium, along with composition, water to cement ratios and age of sacrificial concrete used in the floor and wall of the reactor building. This paper presents a compilation of various studies conducted by researchers over the years on performance of concrete at elevated temperature and in the presence of sodium.

► Hot liquid sodium and concrete interactions in fast breeder reactors are reviewed.
► Thermal and sodium effects are discussed separately in this paper.
► Concrete experiences chemical and thermal instability due to hot sodium.
► Uncertainties still persist in assessing the basic phenomena during the exposure.
► Microstructural study is required to understand fundamental degradation mechanisms.