Abstract
Recent subcritical VENUS-F experiments showed that fission chambers with a threshold deposit like U-238 can essentially improve the on-line sub-criticality measurments with the beam interruption method, which is currently supposed to be the main method for the ADS MYRRHA. To suppress the uncertainty caused by fissions in the U-235 impurities, the fraction of U-235 in the U deposit should be accurately known. Three PHOTONIS CFUL01 type fission chambers with U-238 deposit were purchased for sub-critical experiments in the VENUS-F reactor. To verify the purity of their deposits, the effective U-235 masses were measured in the empty cavity of the BR1 reactor with a well-known thermal neutron spectrum. It turned out that the measured effective U-235 mass in two fission chambers is lower than the declared mass (as it should be), but this is not the case for the third fission chamber. Then, the effective U-238 mass in these FCs was measured in the well-known fast spectrum of the MARK-III convertor in the BR1 reactor. Finally, the isotopic composition was obtained and it was found that the purity of two CFUL01 FCs is in agreement with the values declared in the certificates but it is not the case for the third fission chamber. As the length of the deposit is bigger than the length of the MARK-III convertor, necessary corrections were calculated with MCNP. The developed procedure using the BR1 standard irradiation fields can be applied for calibration and impurity determination of large fission chambers.
Highlights
THE FREYA FP7 project results show that the choice of the detector deposit and position in the core or reflector chosen for the reactor instrumentation can make reactivity monitoring easier or more difficult
The effective masses of U-235 (Meff235) of CFUL01 FCs were measured in the thermal flux in the empty cavity while the effective masses of U-238 (Meff238) were measured in the fast flux of the MARK-III convertor in the BR1 reactor
Three PHOTONIS FCs of CFUL01 type with U-238 deposit, which were used in VENUS-F sub-critical experiments for beam interruption method validation, were calibrated in the BR1 reactor because the method was found to be sensitive to impurities in the deposits
Summary
THE FREYA FP7 project results show that the choice of the detector deposit and position in the core or reflector chosen for the reactor instrumentation can make reactivity monitoring easier or more difficult. Three PHOTONIS fission chambers of CFUL01 type with U-238 deposit were used for the sub-criticality measurements in a VENUS-F sub-critical core coupled with the GENEPI-III accelerator to simulate the ADS principle. To verify the purity (U-235 content) in the deposits of these FCs, the effective U-235 and U-238 masses were measured in the standard thermal and fast neutron fields of the BR1 reactor [2, 3]. These fields are normally used to calibrate miniature fission chambers (typically 2-4 mm in diameter and 10-25 mm in length). Calibration of large fission chambers with 10× bigger dimensions represents a challenge
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