NEW INSIGHTS INTO ACTIVITY TRANSPORT WITHIN THE PRIMARY HEAT TRANSPORT SYSTEMS OF CANDU REACTORS

摘要

Deposition of activation and fission products on reactor out-core surfaces leads to the development of gamma radiation fields. The activation radionuclides, namely, Co-60, Zr/Nb-95, Sb-124 and Fe-59 are key contributors to the observed radiation fields around the Primary Heat Transport System (PHTS) of CANDU reactors. Outage Activity Transport Monitoring surveys are routinely conducted at various stations to monitor radiation fields. In support of the development of an improved understanding of radiation field trends, a Research & Development program under the sponsorship of CANDU Owners Group (COG) has been underway at Kinectrics since about 2007. This paper presents the key highlights of work undertaken recently which led to new insights into the activity transport of Sb-124 and Zr/Nb-95. Until the present study, understanding of radioantimony transport was largely based on the premise that antimony exists in the PHTS coolant principally as a dissolved species, both during reactor operation and shutdown. In the present work, it is shown that during reactor operation, antimony is also transported in particulate form as a result of wear from pressure tube and fuel bundle bearing pad surfaces. Wear generates ZrO_2 particulate containing both Zr/Nb-95 and Sb-124 activation products. Zr/Nb-95 also originates from fission in the form of fuel fragments; the latter, however, are released into the coolant only when relatively large fuel defects exist. Sb-124 arises from the transport of corrosion based antimony into the reactor core where it becomes activated; some of it deposits on the zirconium surfaces, while the balance remains in the coolant in dissolved form. It is shown that the wear products from pressure tube and fuel bundle surfaces can be differentiated, in the absence of fuel defects, based on the value of the Nb-95/Zr-95 activity ratio. Wear products from pressure tube surfaces are characterized by a Nb-95/Zr-95 activity ratio ≥ 2.2, while wear products from fuel bundle surfaces are characterized by a ratio ≤ 2.2. Assessment of radiochemical data for coolant, PHTS deposits, spent filters and spent ion exchange resins led to an overall understanding of the integrated transport of Sb-124 and Zr/Nb-95.