72 Research focus and highlights – Nuclear energy and safety
PSI Scientific Highlights 2012
Sample extraction on the MEGAPIE liquid lead-bismuth spallation neutron target Daniel Kuster, Viktor Boutellier, Sandro Hahl, Rudolf Schwarz, Andreas Spahr, Hans Leu, Herbert Schweikert, Yong Dai, Michael Wohlmuther, Didier Gavillet and Andreas Lagotzki, Hot Laboratory (AHL), PSI
MEGAPIE is an experiment aiming at demonstrating the safe operation of a liquid-metal spallation target at a beam power level of 1 MW in the SINQ target station at PSI. It is an international collaboration of ten partners and is of relevance for accelerator-driven reactor systems, in which an accelerator and target provide the neutrons necessary to maintain a fission reaction. The MEGAPIE experiment will be an important ingredient in defining and initiating the next step towards building a target for the accelerator of a dedicated accelerator-driven reactor system (ADS). These reactor systems would use nuclear waste to produce energy, at the same time reducing the overall amount of radioactive waste or transmuting long-lived radioactive nuclides into short-lived ones. Sample extraction on parts of the target, including all preparations and handling in the PSI hot cells, began in 2011 and has continued in 2012.
After the four-month period of operation of the MEGAPIE target
PbBi out of the segments using our oven, which was designed
in the PSI Swiss Spallation Neutron Source (SINQ) in 2006
in-house, and cutting and cleaning the samples from the steel
[Link 1], the target was cut into pieces at the Swiss interim
housing rings. To avoid changing the sample structure, the
storage facility for radioactive waste – the “Zwischenlager
maximum temperature in the melting process was kept below
Wuerenlingen AG” (ZWILAG) – adjacent to PSI.
200 °C. To avoid the escape of polonium and tritium, the oven was kept under low pressure during the melting process. After melting, the oven was opened and vented with argon through
Target preparation
a filter system consisting of 2 HEPA and 2 activated carbon filters, as well as 3 water columns (tritium filter). In the exhaust,
Ten cylindrical segments (maximum diameter 400 mm,
several filter papers showed no contamination of the exhaust
maximum height 300 mm), filled with PbBi, arrived at the Hot
air after flooding three times with argon.
Laboratory in the spring of 2011, when first inspection was made and PbBi samples taken [1]. After inactive test operation for melting and cutting, the active part of the procedure
Target sectioning
followed in 2012 in the hot cell, with the melting of all the The first sectioning in the hot cell was followed by numeriAir-driven cutting blade
cally controlled milling, using a machine of type “OPTIMUM BF 20 Vario”, with an air-driven rotating cutting blade. This sectioning was performed without using any cooling
Sample ring
fluid. To keep the temperature of the samples below 200 °C, control of the blade feed was very sensitive. Also, the cutting blade was changed quite frequently. To avoid high contamination in the hot cell, the milling machine was encased in an additional alpha box (see Figure 1) and an exhaust tube was connected from the machine housing directly to the hot-cell exhaust filter. Before final wire cutting, the samples were cleaned in a hot oil bath, followed by wiping off the rest of the PbBi and clean-
Figure 1: First cutting with CNC milling machine in the hot cell.
ing them finally in an HNO3 acid bath for some minutes.