A pale yellow liquid was found in the vacuum scattering vessel in which the neutron detector is installed on the BL02 spectrometer in J-PARC MLF. The substance was characterized as phthalic acids, which are known plasticizers for poly(vinyl chloride) cable sheathing. The leakage mechanism of the plasticizers needs to be urgently clarified, as they contaminate the vacuum vessel and could be the cause of the background increase in the experimental neutron data. We purchased commercially available LAN-cables and evaluated the damage mechanism of the cable sheaths based on the changes in the weight and mechanical strength of the cables after gamma irradiation, neutron irradiation, and vacuum treatment. The results show that alpha, beta, or gamma irradiation caused a local disruption of the polymer chains of the cable sheaths, and depending on the extent of disruption, plasticizers or other additives in the cable sheath leaked out. Polyimide-coated LAN-cables are not currently commercially available. However, polyimide sheaths were not damaged by neutron irradiation over a short period of time (about half a day), indicating that they are a suitable candidate for cable sheaths.