Physical Behavior of Thermal Barriers in High-Temperature Gas-Cooled Reactor Conditions

A series of tests on the mechanical behavior of thermal barriers used in high-temperature gas-cooled reactors were performed. Three full-scale mock-ups were built for this purpose, each representing one element of the different kinds of barrier involved. These barriers consisted roughly of sheets of fibrous insulating material (alumina and silica) compressed by metallic cover plates made of steel (XC-18) or a hightemperature alloy (Hastelloy alloy X). For these trials, the mock-ups were subjected to thermal conditions typical of

1. normal reactor functioning
2. incidents liable to occur in operation.

The observations carried out during these experiments concerned the behavior of the fibrous insulating materials (in particular, estimation of heat-induced relaxation effects) and that of the metallic containment structures (evaluation of deformations in the insulant holding plates). Analysis of the results obtained showed that 1. No total relaxation effect, i.e., loss of cover plate/fibrous material insulant contact, was produced whatever the thermal conditions inflicted on the barriers (97% relaxation for ∼1 h at 1260°C). 2. Relaxation phenomena occurred mainly in the early stages of exposure to the heat flux (48 and 52% relaxation effect after 100 and 20 000 h, respectively, at 800°C). 3. On the whole, the mechanical behavior of the containment structures was very good, and all fixtures remained intact. The amount of deformation of the cover plates depends closely on the temperature levels imposed but remains very limited, even under the harshest thermal conditions (<40 mm for the thermal stage at 1260°C). The heat treatments undergone by these barriers gave rise to structural changes in the alloys (XC-18, Hastelloy-X), reflected mainly by alterations in their mechanical properties (embrittlement).