[en] Embalse NGS is provided four in-line autoclaves for a routine materials surveillance program. Through a cooperative program between the utility management and the Research and Development Branch of CNEA, the routine program was expanded to include Materials Tests according to the on-going R and D programs. The availability of the autoclave facilities and the joint Embalse/R and D program proved very valuable in occasion of the resin ingress into the PHTS due to the collapse of a Johnson strainer on April 9, 1988. The details of the incident have been communicated previously, and it is not the purpose of this presentation to describe the sequence of events on April 9, 1988, and following days. Rather, we present here the results of the materials surveillance program that was adopted to follow the materials behaviour and to check the possible negative impact of the resin ingress. The mixed bed resins were subjected to hydrothermal degradation under high radiation fields (the reactor went into hot shut-down state in two hours and was further cooled in a six hours' interval). Formation of ammonia, carbon dioxide, sulfate, nitrate and chloride are all possible. The long term surveillance program was carried out during the following 400 EFPD, on corrosion coupons provided by AECL and on additional samples prepared in CNEA on original materials from Embalse's PHTS. Comparisons were made of the behaviour of AECL materials ingressed in the Autoclave System in 1983, CNEA materials inserted in 1986, and additional CNEA coupons installed in 1988, after the incident