[en] Since August 9, 1979, Diablo Canyon has experienced four fires in which the penetration seal material ignited and burned. On March 25, 1986, Toledo Edison reported that the silicone foam sealant in a fire barrier penetration in the main steam line room of Davis-Besse appeared to have baked and pulled away from the pipe. This penetration was filled with low-density silicone foam, which is rated for a maximum temperature of 425 F. The main steam lines normally operate near 600 F. The licensee's corrective actions were to stuff ceramic fiber (Kaowool) into the seal for increased fire resistance and to expand the existing fire watch. The silicone foam seal material in the diesel generator exhaust pipe penetrations at Diablo Canyon was used as replacement material for the combustible foam plastic-type seals, which were involved in the 1975 fire at the Browns Ferry nuclear power plant. In addition to diesel generator exhaust pipe penetrations, some main steamline penetrations may be sealed with this material. The material is apparently designed to withstand maximum ambient temperatures of 400 F. and temporary exposure to 500 F., but not long-term exposure to higher temperatures. Although the measurement at Diablo Canyon in 1982 indicated a pipe temperature of about 600 F., average pipe temperatures are probably much higher. The licensee for Diablo Canyon determined that only six penetrations (all associated with diesel generator exhaust pipes) potentially exposed the silicone foam penetration seal material to high temperatures. For these penetration openings, the licensee plans to install a penetration seal material designed to withstand long-term exposure to high temperatures