Chemistry measures to control antimony activity for minimising radiation exposures during long shutdowns

Due to the presence of 124Sb, a strong gamma-emitting radioisotope, significant increase in radiation fields on Primary Heat Transport piping and equipments and consequently high man-rem consumption were observed during the Biennial Shut Down of RAPS-4 in May 2003. As this incidence was observed for the first time in any Indian PHWR, a systematic study was carried out to establish the mechanism of release of 124Sb and an innovative and simple chemistry measure was suggested to prevent its recurrence. Implementation of this measure has helped in minimising man rem consumption during subsequent long shutdowns. This paper highlights the investigations carried out at RAPS-3 and 4 for establishing the source of 124Sb in PHT system and the chemistry measures to avoid man rem consumption. (author)