Steven C. van der Marck

HTR Pebble Fuel Burnup Experimental Benchmark

The HTR pebble fuel experiment HFR EU1bis was irradiated in the High Flux Reactor, Petten, The Netherlands, in 2004 and 2005. It consisted of five fuel pebbles from the German HTR program (GLE4 type, UO2 fuel, 16.75% enrichment) and six minisamples (UO2 fuel, 9.75% enrichment). Its instrumentation included three flux monitor sets. The experiment was loaded in a REFA-170 rig, surrounded by a strongly moderating filler element. The central fuel temperature was held at 1250°C during the irradiation. In the framework of the European RAPHAEL project, Post Irradiation Examination (PIE) has been done at NRG in Petten, The Netherlands and at JRC ITU in Karlsruhe, Germany. In Petten, flux monitor analysis has been done, whereas in Karlsruhe, a quantitative evaluation of γ-emitters was used to make a burn-up determination. A benchmark description based on this experiment has been written by NRG. Until now, five RAPHAEL project participants have modeled the experiment, each with their own neutronics code system. Participating codes are three versions of MONTEBURNS (MCNP with ORIGEN), MURE/MCNP and OCTOPUS (MCNP with FISPACT). The pebble burnup and isotopic inventories (Bq/gram initial HM) of selected fission products and actinides in the fuel pebble samples are both calculated and determined by gamma spectrometry, mass spectrometry and ion chromatography by JRC-ITU. Additionally, two participants calculated the flux monitor activities that were measured by NRG. A burnup measurement of 11.0 % FIMA by gamma spectrometry could be confirmed by calculation. Differences between the various modeling approaches and the experimental burn-up determination will be discussed.Copyright

Detection of Shielded Sealed Radioactive Sources in Radioactive Waste by Non-Destructive Assay Techniques

Experiences dealing are seldom reported on the detection of sealed radioactive sources (SRS) or its shielding in waste by non-destructive assay (NDA) methods in literature. Further the knowledge and experience in this field of waste characterization will vary from specialist till basic. The main aim of this paper is to give some guidance with the aid of an overview or scheme with which the possibility can be assessed of the detection and of a shielded SRS in (historic) waste packages. This aim could be reached by simulating the gamma flux emitted by a SRS at the outline of a standard 220 litres drum. The simulations have been performed with the Monte Carlo Neutron Photon transport code (MCNP). The results, visualized by means of iso-plots, are then be used for assessing the detection probability of the SRS by available NDA techniques. The following conclusions could be made: • “Heavy and dens” objects (e.g. shielded sealed radioactive source) can be detected and an indication of its location can be obtained by a sudden significant decrease of the gamma photon flux (cold-spot) at the outline of the drum. This cold-spot is caused by the “heavy or dens” object itself. A cold-spot can be detected the best by analyzing and visualizing the gamma photon flux from the backscatter area (e.g. range 100 keV up to 200 keV) and second best by analyzing the total gamma flux. • With relative simple radiological equipment (e.g. dose rate meter) the possible presence and an indication of the location of a shielded SRS can be estimated. • The detection possibility of detecting a shielded SRS depends not only on the applied NDA technique, but also on the applied effort (e.g. number of measurements, measuring time, applied measurement grid), the properties of the waste packages (e.g. density of the waste matrix and uniformity of the inside dispersed radioactivity), the location and the properties of the shielding of the SRS (e.g. geometry). • For confirmation and the determining of the exact location advanced NDA techniques (e.g. transmission computer tomography or other NDA that techniques gives three dimensional results) have to be applied.Copyright