Verification and Determination of the Decay Heat in Spent PWR Fuel by Means of Gamma Scanning

Decay heat is an important design parameter at the future Swedish spent nuclear fuel repository. It will be calculated for each fuel assembly using dedicated depletion codes, based on the operator-declared irradiation history. However, experimental verification of the calculated decay heat is also anticipated. Such verification may be obtained by gamma scanning using the established correlation between the decay heat and the emitted gamma-ray intensity from ¹³⁷Cs. In this procedure, the correctness of the operator-declared fuel parameters can be verified.

Recent achievements of the gamma-scanning technique include the development of a dedicated spectroscopic data-acquisition system and the use of an advanced calorimeter for calibration. Using this system, the operator-declared burnup and cooling time of 31 pressurized water reactor fuel assemblies was verified experimentally to within 2.2% (1) and 1.9% (1), respectively. The measured decay heat agreed with calorimetric data within 2.3% (1), whereby the calculated decay heat was verified within 2.3% (1). The measuring time per fuel assembly was ~15 min.

In case reliable operator-declared data are not available, the gamma-scanning technique also provides a means to independently measure the decay heat. The results obtained in this procedure agreed with calorimetric data within 2.7% (1).