During loss of coolant accidents (LOCA) the release of insulation material attached to pipes and vessels may lead to disturbances of the long-term residual heat removal from the reactor core during sump recirculation operation. Investigations on the behavior of insulation material in LOCA-conditions were particularly performed for fibrous insulation materials in the past.
Analyses and experiments were carried out to demonstrate the appropriateness of using aerogel as insulation material for the use in nuclear power plants, which is differently designed compared with fibrous insulation materials. The investigations included:
• thermal conductivity behavior of cassettes filled with aerogel of various bulk densities (purely and with additives) at a hot pipe compared to mineral wool filled cassettes
• thermography of aerogel
• fragmentation behavior of non-calcined and calcined aerogel during steam jets of 110 bar
• complex tests with integral character concerning transport, accumulation and head loss behavior in the sump and during flow-through a single FA-dummy at the test facility ‘Zittau Flow Tray’ (ZFT) under consideration of the fluid composition (boric acid, floor grating, dust, paint, rust)
• investigations on activating aerogel with a neutron source
• tests on the mechanical stability of the cassettes
• γ-irradiation testing
• determination of the halogen content (esp. chlorides) with AOX
• durability in boron during increased temperatures
• viscosity analyses of dissolved aerogel in boric acid.
Aerogel filled cassettes (bulk density: 100 kg/m3) surrounding on a horizontal pipe at 320 C showed an insulation factor of W = 1.13 and W = 1.25 (for 110 kg/m3) in comparison to standard mineral wool-filled cassettes.
Two variants of non-calcined and calcined aerogel were fragmented at the “Fragmentation facility” with steam pressures of 110 bar. After the fragmentation tests non-calcined aerogel (hydrophobic type) was observed as swimming masses with huge surface-volume-ratio. The steam fragmentation of calcined aerogel (hydrophilic type) showed as result a suspension consisting of deionized water and aerogel.
Complex experiments were carried out concerning the behaviour of steam fragmented suspensions of calcined aerogel during sump operation at ZFT. An 18×18 PWR-fuel assembly dummy was used downstream of a 3×3 mm sump strainer. Input masses of released insulation material, concentration of boron acid, the strainer surface and velocity as well as the flow through the FA-dummy were scaled accordingly based on the German PWR “Biblis B” plant conditions. The steam-fragmented calcined aerogel did not show any deposit on the sump strainer, on the debris filter at the FA-bottom nor on the spacers in a PWR-FA-dummy. Accordingly, at these positions no significant head loss build-up was detected.