ABSTRACTA fuel leaching experiment has been in progress since 1977 to study the
dissolution behaviour of used CANDU fuel in aerated aqueous solution. The
experiment involves exposure of 50-mm clad segments of an outer element of a
Pickering fuel bundle (burnup 610 GJ/kg U; linear and peak power ratings 53
and 58 kW/m, respectively), to deionized distilled water (DDH2O,
∼2 mg/L carbonate) and tapwater (∼50 mg/L carbonate). In 1992, it was
observed that the fuel in at least one of the leaching solutions showed some
signs of deterioration and, therefore, in 1993, parts of the fuel samples
were sacrificed for a detailed analysis of the physical state of the fuel,
using SEM and optical microscopy. Leaching results to date show that even
after >6900 days only 5 to 7.7% of the total calculated inventory of
137Cs has leached out preferentially and that leach rates
suggest a development towards congruent dissolution. Total amounts of
137Cs and 90Sr leached are slightly larger in
tapwater than in DDH2O. SEM examinations of leached fuel surface
fragments indicate that the fuel surface exposed to DDH2O is
covered in a needle-like precipitate. The fuel surface exposed to tapwater
shows evidence of leaching but no precipitate, likely because uranium is
kept in solution by carbonate. Detailed optical and SEM microscopy
examinations on fuel cross sections suggest that grain-boundary dissolution
in DDH2O is not prevalent, and in tapwater appears to be limited
to the outer %0.5 mm (pellet/cladding) region of the fuel. Grain boundary
attack seems to be limited to microcracks at or near the surface of the
fuel. It thus appears that grain-boundary attack occurs only near the fuel
pellet surface and is prevalent only in the presence of carbonate in
solution.
Development of High Performance BWR Fuel Bundle. Thermal-hydraulic Performance of CYCLONE Spacer.
