Evaluation of the Tritium Content in Light Water Reactor Control and Absorber Rods to Obtain Data for the Fuel Cycle Backend

Tritium inventories and tritium distribution have been determined in boron glass absorber rods discharged from a pressurized water reactor first-cycle core and in spent boron carbide (B₄C) control rods from a boiling water reactor. The total tritium inventory in the boron glass absorber rods from the Stade nuclear reactor amounts to ∼8.0×10¹⁰ Bq (2.2 Ci) per rod. Of this, 99.6% was fixed in the boron glass itself and 0.4% in the Al₂O₃ pellets. The 4×10⁻³% fractions in the tube cladding and support pipe and the 1×10⁻¹% fraction in the fill gas accounted for an insignificant part of the total tritium inventory of the rod. This experimentally determined tritium inventory was a factor of 5 larger than that suggested by the calculated estimate. The discrepancy between analyzed and calculated values can be explained by tritium formation from lithium impurities in the boron glass, where a 30-ppm lithium content would be adequate for this tritium inventory to be generated by the reaction ⁶Li(n, α)³H. Evaluation of the B₄C control rods from the Lin-gen nuclear reactor after 3 yr of operation gave a 3.2×10¹⁰ Bq (0.85-Ci) tritium inventory per B₄C rod, while the total tritium inventory for a control rod assembly containing 60 B₄C rods was ∼1.9×10¹² Bq (50 Ci). The tritium generated was essentially bound 100% in the B₄C, since the hulls contained only 6×10⁻³% and the fill gas only 2×10⁻⁴%.