Drying of Reactor Containment Walls of Concrete during Past and Future Decades

Two types of nuclear reactor containments are studied, for BWR reactors and PWR reactors in Sweden. They both have a thick concrete wall with a steel lining inside, but the BWR containment walls are enclosed in a building, protecting it from the outdoor climate. The PWR containment walls are exposed to outdoor weather conditions. The conditions inside the containment walls include high temperature, up to around +40-50°C. Such a concrete structure, with one-sided drying of a 0.8 m thick wall, will dry extremely slow, in spite of the very high temperatures in part of the structure. This was seen in an earlier study after some 30 years of drying were moisture measurements were performed. A drying model for concrete containments is presented and the various parts are described in detail and partly experimentally quantified. Especially the moisture effects on the extent of the cement reactions chemically binding water are discussed. A first prediction with a simplified model shows fairly good agreement with measured data. INTRODUCTION For most ageing processes in reinforced concrete structures the moisture conditions are important and sometimes decisive [1][2]. In nuclear reactor containments the moisture conditions in the concrete will mainly affect the shrinkage and creep of the containment walls and, as a consequence, the stress losses of the prestressing reinforcement [3]. For the possibility, and rate, of metal corrosion the moisture conditions are also decisive [2]. Nuclear reactor concrete containments change their moisture conditions extremely slow, because of the significant dimensions of the structures, in spite of their exposure to high temperatures. Parts of the structures are exposed to very high temperatures, above +50°C, for decades. Since the inner parts of the walls simultaneously are very humid, the cement reactions continue during the service period, binding water chemically to the cement. In this paper the drying processes of the concrete containment walls are further examined, following the ideas by several researchers, from Hilsdorf (1967) [4] to Nilsson (2006) [5]. BOUNDARY CONDITIONS The first series of measurements of the climatic conditions were performed on a running BWR reactor, at the outer surfaces of the containment wall [6]. The general layout of that reactor containment is shown in figure 1. Temperature and relative humidity (RH) were recorded during several months at the concrete surfaces and in the outdoor air. The measuring points are shown in figure 1. From the measurements it was clear that the BWR containment walls do not have a significant temperature gradient, not even at the top of the containment, where inside temperatures reach some +50°C. It was also clear that the concrete surfaces are very dry, below 15 % RH at the top. MEASURED DRYING PROFILES After some 30 years of service, moisture measurements were performed in a BWR reactor containment wall [7][8]. The coring points are shown in figure 1. In figure 2 the measured relative humidity (RH) profiles through the bottom of a containment wall are shown. The temperature during service was here around +25°C. From the measured moisture profiles it is obvious that the walls are still drying, after 30 years. In the lower parts of the wall RH still is above 90 % in the inner part next to the steel plate. In the upper part where the temperature has been close to +50°C during some 30 years of service, RH still is around 85 % close to the steel plate [7]. The drying-out process is obviously extremely slow, because of the one-sided drying of the very thick walls. From the measured RH-profiles it is also obvious that the warmer parts of the wall are drying in very much dryer conditions. An extrapolated RH at the concrete surface in Figure 2 fits well with the RH at the drying surface measured by [7], corresponding to 35-40 % RH. In the upper part where the temperature during service was around +50°C, a surface RH of 10 % RH was found [7], which corresponded well with the measured profiles. SMiRT 19, Toronto, August 2007 Transactions, Paper # DH01/3