The potential importance of a mechanism of steam generator tube cracking occurring in polluted steam stems from three primary issues: (1) the secondary water chemistry would probably require optimising with respect to hydrazine, oxugen and polluting species; (2) the possibility that remedial measures to combat polluted steam attack could be at variance with present EPRI secondary water chemistry guidelines; and (3) the ranking of different candidate tube materials could be different in polluted steam as compared to caustic or acid sulphate liquids that have often been cited hitherto as the cause of IGA / IGSCC.

The main research objectives of the experimental work were to:

• help optimise secondary water chemistry guidelines with respect to hydrazine and oxygen concentrations in
  the feed water and in the steam generators,

• determine which pollutants in dry superheated steam lead to IGA / IGSCC of alloy 600 and improve
  understanding of this degradation process,

• test the sensitivity of other steam generator tube materials such as alloy 690 and 800 in the same
  environments,

• identify the detailed morphology of such attack so that it may be readily identified or eliminated from
  consideration during examinations of pulled tubes.
 
Three experimental tasks were proposed as follows:

• Task 1: Doped steam tests on alloys 600, 690 and 800 at elevated temperatures to isolate the role of
  pollutants and to determine the influence of temperature, total pressure and hydrogen partial pressure,

• Task 2: Two zone capsule tests incorporating a superheated, polluted steam bubble using protypic mixtures
  of realistic pollutants at normal steam generator temperatures and pressures in order to identify the
  conditions of prtective film breakdown and eraly intergranular corrosion,

• Task 3: High resolution, analytical transmission electron microscopy (ATEM) characterisation of grain
  boundary attack and crack tip morphologies to aid identification of material damage mechanisms and to
  compare with the results of pulled tube examinations from other programmes. This is the only known
  method for distinguishing different forms of grain boundary attacks in pulled tubes and establishing the
  pratical relevance of the laboratory simulation experiments in tasks 1 and 2.
 
This final report describes the results of the two year experimental programme conducted within FROG between March 2000 and March 2002. The experimental techniques and results are described for each of the three tasks. A general discussion follows in which the extent to which current results support the original hypothesis is examined, followed by the conclusions and recommendations arising from this work regarding the future management of secondary water chemistry PWR steam generators with the objective of minimising damages to steam generator tubes.