Post Irradiation Examination of MEGAPIE – How radiochemical analytics helps looking inside a high-power liquid metal spallation target

Title page of issue 131 (2016) of the European Physical Journal Plus, showing the design of the MEGAPIE target

Matter and Material

Almost exactly 10 years ago, in August 2006, the MEGAwatt Pilot Experiment (MEGAPIE) – a key experiment aimed to demonstrate the feasibility of a high-power liquid metal target – started operating at the spallation neutron source SINQ at PSI. The target, filled with 920 kg liquid LBE, worked successfully till December 2006, delivering neutrons with an 80% higher yield. Besides the excellent performance during operation, showing the high potential of the liquid metal technology, the examination of the target after the experiment is of tremendous importance for scientists and engineers all over the world. Therefore, an extended Post Irradiation Examination Program has been launched to investigate material properties of the structure materials as well as to determine the radionuclide inventory and the physico-chemical behavior of safety-relevant isotopes in the target.

For the radioanalytical work, the colleagues of the Hotlaboratory (AHL) of PSI extracted more than 70 samples from representative positions within the target, comprising bulk LBE, samples from the interfaces between the vessel walls and the LBE as well as the cover gas and the LBE.

The radiochemists at PSI could not only identify and quantify 20 radionuclides using α- and γ- spectrometric methods at PSI and accelerator mass spectrometry (AMS) at ETH Zurich, but also investigated their spatial distribution within the target. They found that radionuclides do not necessarily remain dissolved in the irradiated LBE. Instead they tend to accumulate on surfaces, and therefore have depleted abundance in the bulk LBE. The unequal distribution appears to be more pronounced for strongly electropositive metals (133Ba, 173Lu, 172Hf/Lu, 60Co, 148Gd) and less or completely absent for nobler metals (101,102Rh and 110mAg), respectively. These findings have a high impact on the safety assessment of nuclear installations, especially concerning dose rate estimations and the evaluation of structure material damage.

Theoretical predictions of the radionuclide inventory performed in collaboration with colleagues from CEA/Saclay and ESSS Lund, based on calculation codes and models like INCL/ABLA, MCNPX or FLUKA, mostly agreed well or at least fairly with the experimental values. However, in several cases considerable discrepancies were found. The knowledge on the actual radionuclide production rates serves as benchmark and helps to improve the codes and models.