Study of air and steam leak rate through damaged concrete wall

Full text of publication follows: The leak rate prediction of air and steam through a cracked concrete wall is an extremely important issue in assessing the safety of nuclear reactor containment building. Furthermore the relation between air leak rate and steam leak rate on the same wall could have some interest for safety prediction. This laboratory study investigates the transfer of fluids through a wall of 1.3 m of thickness, with a focus on two cases: one on a mechanically damaged concrete by compressive stress and another one on a crossing artificial flaw in a construction joint realized in the concrete specimen (cylindrical / section 0.1925 m2 / length 1.3 m). The both specimens were made of ordinary concrete (compressive strength: 35 MPa). To initiate residual compressive cracks, the specimen (A) was loaded in compression under controlled strains until a level of 90% of the failure strain was reached. To create a crossing artificial flaw in a construction joint, the concrete was set in the mould in two times, the second time, a water saturated sand bed was placed on the surface of the hardened concrete to realize the flaw along a diameter of the specimen (B). The permeability of damaged concrete wall was studied comparatively under two conditions, but without appreciable stresses applied on. The first condition was at ambient temperature, a reference test of permeability, with dry air, gave the characteristics of permeability and the type of flow through the specimen. In this case, the used method consisted to proceed by stages. The imposed pressures on the exposed face were successively 0.1, 0.18, 0.23, 0.28, 0.34 and 0.42 MPa, the other face was at atmospheric pressure. The second condition was an accidental scenario with simultaneous effects of temperature and gas (a mix of air and steam) pressure applied on a face, the other one remaining at atmospheric pressure and temperature. During the test, the lateral face of the cylindrical specimen was thermally isolated and made leak-tightness. So, an uni-dimensional experimental analyse was performed. The accident conditions consisted of a rise from ambient conditions to a temperature of 141 deg. C and a relative pressure of 0.42 MPa (steam pressure of 0.377 and air pressure of 0.043 MPa) in 3 hours, the maximum values been kept constant for several days. Thermocouples, pressure taps and moisture gauges were implanted in the specimens at the moment of the casting, to provide local information of the inner wall under the simultaneous effects of temperature and steam pressure. At the outside of the specimen, the chamber enclosed a condenser with humidity meter and thermocouple, so it was possible to quantify separately the saturated air flow with a flow-metre and the condensed water. As indicators of the field investigated: the permeability tests at ambient temperature gave the flows of air under 0.42 MPa: 5.42 Nl/(h.m2) for A, and 938 Nl/(h.m2) for (B). These experiments offered some elements on the relations between air flow during a permeability test and gas more liquid flows during accidental test. The experimental results were compared to the leak rate evaluation formulae available in the literature. (authors)