Joint European Torus
The Joint European Torus (JET) experimental facility was used to investigate the conditions necessary for the initiation and stable operation of a nuclear fusion reactor. A programme of work was undertaken in preparation for the decommissioning of the JET facility. A key feature of this programme was the assessment of the facility's likely radiological condition during and after shutdown. Assessments of radioactive inventories and other nuclear responses were made based on neutronics modelling of JET, and confirmed through direct measurement.
Direct measurements were made of neutron fluence and energy spectrum throughout the JET Torus Hall during deuterium-tritium (D-T) operations. MCBEND Monte Carlo calculations were also carried out to determine the neutron flux spectra at the measurement locations. Finally the SENSAK spectral unfolding code was used to determine best estimate values of the neutron flux spectra which were statistically consistent with the calculations and with measurements.
These fluxes were then used to obtain improved values of the calculated inventory of radioactive isotopes at the end of the operation.
Typical Swedish BWR
Serco Assurance has recently used MCBEND in a fluence assessment for a typical Swedish BWR. The project was a joint venture between the Swedish utilities. Fluence analysis is used to establish the neutron irradiation embrittlement of the RPV and reactor internals.
The results will be used to make recommendations for in service inspections of these regions in Swedish BWRs. For this purpose fluences and neutron damage were calculated throughout the reactor. Dosimetry in a capsule just in board of the shroud, which was extracted in 1997, was used to validate the MCBEND calculations. Comparison of detector activation for a Cu63(n,a)Co60 detector just in-board of the shroud gave a C/M value of 1.03, thus demonstrating the validity of the the calculations .
Magnox Reactor
The UK's Magnox reactors are large, gas-cooled, graphite-moderated units with natural uranium fuel, and they have operated successfully for some decades. Over the last few years, most of them have had their licences to operate renewed in order to extend their lifetimes.
Part of the safety case required was to demonstrate that the pressure vessel (usually spherical and made of steel) was still resistant to cracking. This meant that detailed calculations of neutron damage were required all around the pressure vessel, and especially at welds. The illustration (provided by VISTA-RAY) shows a MCBEND model for the calculation of damage around the pressure vessel, and clearly illustrates the detail required in the modelling of the diagrid which supports the reactor and the charge tubes at the top of the reactor.
The calculations, allied to measurements at a number of locations near the pressure vessel, indicated that the pressure vessels were sound and contributed to the case for their continuing operation.
The design of flasks for the transport of irradiated fuel demands accurate calculations of the effectiveness of the shielding if maximum carrying capacity is to be achieved within the restraints of weight and dimensions whilst meeting the regulations limiting the external dose-rates. The provision of efficient shielding for both neutrons and gamma-rays which does not interfere with the many other requirements such as mechanical strength and heat-transfer, or access to items such as lifting trunnions, lid bolts, and sampling points, means that the designer has to be able to assess the performance of complicated configurations of materials.
Monte Carlo is the preferred method for performing such shielding calculations because it is able to fully represent the three-dimensional geometry of the flask. The model (provided by VISTA-RAY) of the TN12 flask which was used in calculations carried out with the MCBEND code is shown in the illustration. With MCBEND it was possible to represent the detailed features of the fuel and the flask to give the dose-rates corresponding to the actual design.
Gamma Density Logging Tool
The central feature of this project was to demonstrate the capabilities of the MCBEND Monte-Carlo code in predicting downhole performance of commercial gamma density logging tools.
Two commercial logging tool models were set up in MCBEND. The downhole perturbations of thin beds, including inclined thin beds, and elliptical borehole shapes were studied. The illustration (provided by VISTA-RAY) shows a MCBEND model of a Gamma Density Tool in a formation with two inclined thin beds.