Fibrous debris generated from insulation materials during a Loss of Coolant Accident (LOCA) might be transported to the containment sump strainer. Once the fibrous debris reaches the sump strainer, it might accumulate on the strainer causing loss of Net Positive Suction Head (NPSH), called the upstream effect, or it might penetrate through the strainer becoming a source of clogging of flow channel in the core (in-vessel effect). This is the main idea of the Generic Safety Issue (GSI) 191 initiated by the U.S. Nuclear Regulatory Commission (U.S.NRC). In this study, the authors focused on the quantity of the fibrous debris bypass through a containment sump strainer which eventually might cause the in-vessel effect. In order to produce the fibrous debris, a method suggested by the Nuclear Energy Institute (NEI) was applied to one-side-baked NUKON fiberglass insulation materials. The quantity and characteristics of debris penetrating through the strainer may be affected by the properties of water such as pH and electrical conductivity (EC). Total quantity of the debris bypass for a certain period was measured and the effects of the water chemistry were observed. At higher pH and lower EC more fibrous debris penetrated the strainer.