When accounting for the total amount of debris that may be present in a pressurised water reactor (PWR) containment pool during operation of the emergency core cooling system (ECCS), it is important to include a reasonable estimate of the latent dirt and foreign material that can be found in containment in addition to the debris generated by a high-pressure pipe rupture. Past and recent testing has shown that even small volumes of fibrous debris present on an ECCS sump screen can very effectively filter particulates that are present in the sump pool, leading to significant pressure losses across the composite debris bed. Debris present during routine operations that is subjected to containment spray and pool transport may contribute a significant source of particulate and perhaps fiber material. Because the PWR industry is working to estimate the quantity of latent debris present in containment, Los Alamos National Laboratory (LANL) is working, under the direction of the United States Nuclear Regulatory Commission (USNRC), to characterise the material composition and the ...