Om Parkash Joneja

Absolute depth-dose-rate measurements for an 192Ir HDR brachytherapy source in water using MOSFET detectors.

Reported MOSFET measurements concern mostly external radiotherapy and in vivo dosimetry. In this paper, we apply the technique for absolute dosimetry in the context of HDR brachytherapy using an 192Ir source. Measured radial dose rate distributions in water for different planes perpendicular to the source axis are presented and special attention is paid to the calibration of the R and K type detectors, and to the determination of appropriate correction factors for the sensitivity variation with the increase of the threshold voltage and the energy dependence. The experimental results are compared with Monte Carlo simulated dose rate distributions. The experimental results show a good agreement with the Monte Carlo simulations: the discrepancy between experimental and Monte Carlo results being within 5% for 82% of the points and within 10% for 95% of the points. Moreover, all points except two are found to lie within the experimental uncertainties, confirming thereby the quality of the results obtained.

Validation of an MCNP4B whole-reactor model for LWR-PROTEUS using ENDF/B-V, ENDF/B-VI, and JEF-2.2 cross-section libraries

A detailed three-dimensional, continuous-energy MCNP4B model of the LWR-PROTEUS critical facility has been developed for the analysis of whole-reactor characteristics using ENDF/B-V, ENDF/B-VI and JEF-2.2 cross-section sets. The model has been applied to the determination of the critical loading, as well as the evaluation of reactivity worths for safety/shutdown rods, control rods, and individual driver-region fuel rods. The initially obtained results for the first configuration investigated (Core 1B) indicated that, for the same geometrical and materials specifications, the ENDF/B-V data library yields the closest critical prediction (discrepancy of 64040 pcm), followed by ENDF/B-VI (98040 pcm) and JEF-2.2 (134040 pcm). The differences in results between the three data libraries were studied by considering the contributions of individual materials to the neutron balance. 235U and 238Pu cross-sections from JEF-2.2, for example, explain an effect of xa0400 pcm. Refinement of the materials specifications in the MCNP4B whole-reactor model, in particular the impurities assumed for the graphite driver of the driver and reflector regions, has been shown to reduce the final discrepancy of the ENDF/B-V based keff result to xa00.2%. The MCNP4B results for relative reactivity effects, such as control rod worths, are found to agree within experimental errors with the measured values

Radial and azimuthal 235U fission and 238U capture distributions in BWR UO2 pins : CASMO-4 and MCNP4C versus activation foil measurements

Abstract In the context of the LWR-PROTEUS program, radial and azimuthal 235U fission (F5) and 238U capture (C8) rate distributions have been calculated for zero-burnup pins of a Westinghouse SVEA-96 Optima2 boiling water reactor fuel assembly using the stochastic MCNP4C and the deterministic CASMO-4 codes. The within-pin F5 distributions predicted by the two codes are in very good agreement; the C8 distributions are more pronounced, and there are significant discrepancies between the codes, both azimuthally and radially. The calculations have been compared with experimental results obtained from activation foil measurements in two pins of the assembly irradiated in the center of the PROTEUS test zone. The measurements confirm that the two codes can accurately predict the radial and azimuthal F5 distributions but that MCNP4C within-pin C8 distributions are much more accurate than those of CASMO-4.

Comparison of Monte Carlo Simulations of Photon/Electron Dosimetry in Microscale Applications

It is important to establish reliable calculational tools to plan and analyse representative microdosimetry experiments in the context of microbeam radiation therapy development. In this paper, an attempt has been made to investigate the suitability of the MCNP4C Monte Carlo code to adequately model photon/electron transport over micron distances. The case of a single cylindrical microbeam of 25-micron diameter incident on a water phantom has been simulated in detail with both MCNP4C and the code PSI-GEANT, for different incident photon energies, to get absorbed dose distributions at various depths, with and without electron transport being considered. In addition, dose distributions calculated for a single microbeam with a photon spectrum representative of the European Synchrotron Radiation Facility (ESRF) have been compared. Finally, a large number of cylindrical microbeams ( a total of 2601 beams, placed on a 200-micron square pitch, covering an area of 1cm2) incident on a water phantom have been considered to study cumulative radial dose distributions at different depths. From these distributions, ratios of peak (within the microbeam) to valley (mid-point along the diagonal connecting two microbeams) dose values have been determined. The various comparisons with PSI-GEANT results have shown that MCNP4C, with its high flexibility in terms of its numerous source and geometry description options, variance reduction methods, detailed error analysis, statistical checks and different tally types, can be a valuable tool for the analysis of microbeam experiments.

Methods/Data Testing for Advanced LWR Fuel Designs and Operational Modes in the PROTEUS Facility

In the framework of the LWR-PROTEUS programme, integral experiments are being performed for the validation of reactor physics methods/data applied to the analysis of modem light water reactor (LWR) fuel assembly designs. One of the important aspects pertaining to this programme is the investigation of the sensitivity of integral quantities, such as reaction rate spatial distributions, local reaction rate ratios and reactivity effects, to different nuclear data sets, e.g. ENDF/B-V, ENDF/B-VI and JEF-2.2. Data sensitivity studies are currently reported, largely based on continuous-energy Monte Carlo calculations at PROTEUS-core, BWR fuel assembly and pin-cell levels. Data effects could thus be investigated systematically, independent of the inherent limitations of deterministic codes such as geometrical simplification and resonance shielding approximations. The results show a significant sensitivity of system reactivity to the data set employed, calculated values being larger by several hundreds of pcm with JEF-2.2 than with ENDF/B-V or B-VI. As regards detailed reaction rate distributions in BWR assemblies (investigated in LWR-PROTEUS Phase I), there appears to be only a weak dependence on the data library used. In the context of the investigations planned with highly burnt fuel rod segments in the Phase II experiments, certain studies made at pincell level have indicated that the effects of basic data differences are likely to be much lower than uncertainties in fuel composition predictions.

Definition and Analysis of an Experimental Benchmark on Shutdown Rod Worths in LEU-HTR Configurations

Abstract Although high-temperature reactors (HTRs) are endowed with a number of inherent safety features, there are still aspects of the design that need particular attention. For concepts in which shutdown rods are situated outside the core region, as is the case in contemporary modular pebble bed designs, accurate calculations are needed for the worth of these shutdown rods not only in normal operation but also under accident conditions in which significant changes occur, for instance, due to inadvertant moderation increase in the core (ingress of water or other hydrogeneous compound). Corresponding validation experiments, employing a variety of reactivity measurement techniques, were conducted in the framework of the HTR-PROTEUS program employing low-enriched uranium pebble-type fuel. Details of the experimental configurations, along with the measurement results obtained, are given for two different HTR-PROTEUS cores, in each of which four different shutdown rod combinations were investigated. Comparisons made with calculations, based on both approximative deterministic models and geometrically “near-to-exact” Monte Carlo analyses, have clearly brought out the sensitivity of the experimental results to calculational correction factors when conventional (thermal) techniques are used for reactivity measurements in such systems. Considerably greater systematic accuracies are reflected in the experimental shutdown rod values obtained using specially developed epithermal techniques, and it is these results that are recommended for benchmarking purposes.

Validation of new electron and positron data libraries

Abstract Recently, the original Lawrence Livermore National Laboratory Evaluated Electron Data Library [S.T. Perkins, D.E. Cullen, S.M. Seltzer, LLNL, CA, UCRL-50400, 31 (1991)] was extended down from 10 eV to 1 eV. Efforts have also been made to generate a similarly formatted data library for positrons – EPODL. These libraries now include data in the energy range from 1 eV to 100 GeV for electron and positron interactions with neutral atoms of the elements H through Fm. Validation of the new data has been carried out by comparison with experimental results for single ionization and differential elastic scattering cross-sections. The calculated cross-section data are generally found to be in good agreement with reported measurements.

Real-time treatment planning optimisation for brachytherapy

In this paper, we present an integrated system for real-time dose distribution calculation and treatment planning optimisation for brachytherapy of prostate cancer, with a special emphasis on the visual integration of the dosimetry and target images obtained from the open magnetic resonance system. This system involves a fast method to calculate dose distributions of multiple concurrent radioactive sources, based on the combination of elements from a database of pre-calculated dose distribution maps for single sources, combined linearly to provide the final dose distribution map. Simulated annealing, in conjunction with the inverse planning method, is used to determine the source dwell times at preselected locations in order to optimally irradiate the tumour while preserving the surrounding healthy tissues. This algorithm, implemented in FORTRAN, is integrated into a computer-assisted treatment planning tool, written in JAVA, using the runtime class and RMI API of Java. The whole system is now under clinical testing at the Geneva University Hospital.

Data Sensitivity of Design Calculations for High Conversion D2O Reactors

Light water high conversion reactor (LWHCR) experiments carried out during the late 1980’s at the PROTEUS facility in Switzerland included the investigation of a 11wt% PuO2/UO2 lattice with a volumetric moderator-to-fuel ratio of 0.95, in which the H2O was replaced by D2O. The current paper presents a systematic investigation of the effects of different data libraries on calculation/experiment (C/E) comparisons of k∞ and reaction rate ratios for this particular test lattice. Effects of library changes for the corresponding dry (fully voided) lattice have also been studied, permitting conclusions to be drawn on the data sensitivity of the void coefficient and its components. The results obtained serve to quantify, in the rather stringent testing situation provided, the data sensitivity of reactivity and reaction rate ratio predictions for a high conversion D2O-reactor under both normal and fully voided conditions. Two parallel self-consistent calculational routes have been employed. These are the Monte Carlo code, MCNP, in conjunction with pointwise cross sections based on JEF-2, ENDF/B-VI and ENDF/B-V data, and a deterministic route, MICROX-2/ONEDANT, using pointwise and 193-group libraries generated via NJOY processing of JENDL-3 and JEFF-3T, in addition to JEF-2, ENDF/B-VI and ENDF/B-V data.