A Study of Fission Product Migration and Selective Leaching by Means of a Power-Bump Test.

Abstract

ABSTRACTAs part of a programme to identify and quantify the mechanisms controlling the aqueous corrosion of spent fuel,a short fuel rod was refabricated from a full-length rod from the Swedish Ringhals 1 BWR (burnup 44-48 MWd/kgU). After a conditioning period at 25 kW/m,the rod was power-bumped in Studsvik Nuclear´s R2 test reactor up to a maximum linear heat rating of 43 kW/m,with a hold time at power of 3 hours,imposing steeper radial temperature gradients on the fuel pellets. A few hours after termination of irradiation,the rod was shipped to Studsvik Nuclear´s Hot Cell Laboratory,where the pellet inventories of gamma-emitting fission products were determined non-destructively.Two 20mm long fuel/clad sections were cut from the rod at positions corresponding to power-bump LHR values of 43 and 33 kW/m.Two similar sections were cut from the original rod as references,together with a shorter section for destructive inventory analysis.The fuel/clad sections were leached in 200ml of synthetic groundwater (l23ppm HCO3) under oxic conditions for consecutive periods of 7,7,14 and 14 days.The samples arising from the tests were analysed by conventional radiometric and radiochemical methods,and also using an ICP-MS (Inductively Coupled Plasma Mass Spectrometer) instrument commissioned for active use.Fractional release data for both radioactive and stable fission products has been obtained. Since the main aim of the test was the study of the early stages of corrosion,expected to be dominated by migrational effects in the fuel during irradiation,particular attention has been paid to Cs,I,Rb,Sr, Ba,Mo and Tc. Although the migration and selective leaching of Cs and I are well-established,it is shown that there is also appreciable leaching of Rb nuclides,whereas the results show no significant Sr migration.This represents support for the hypothesis that the apparent enhanced leach rate of Sr-90 observed experimentally can be due to migration of Rb-90 precursors.