Year
2004
Abstract
The FRM-II reactor is a high-flux research reactor operated by the Technische Universitaet Munchen (TUM) at the Garching Research Centre (Germany). The reactor core consists of a single cylindrical fuel element, containing U3Si2-Al-dispersion fuel in combination with high-enriched uranium (HEU, with about 93% 235U). The fuel contains about 8 kg of uranium and is placed in 113 involutely curved fuel plates. The verification of the 235U content in this element can be based on active neutron interrogation assay using a coincidence collar. The main difficulty in implementing neutron measurements on the FRM-II elements comes from the unavailability of certified materials identical or even similar to the real items for the calibration of the instrument. To overcome this problem we tested a calibration procedure based on the Monte Carlo simulation of the Reals count rates with the MCNP-PTA code developed at the JRC-Ispra. The procedure is based on the following steps - measurement in the PERLA laboratory of the available HEU standards (MTR type fuel elements) - comparison between MCNP-PTA calculations and the experimental results in order to validate the computational model of the detector - generation with MCNP-PTA calculations of the response function of the detector with single FRM-II fuel element - on-site verification of real FRM-II elements Using this method the 235U content in the fuel element was verified with an uncertainty better than 2%. The results proved that computational calibration is now a mature technique for application to NDA measurements and can really help in reducing experimental effort and requirements of calibration standards.