Tony Williams

Neutronics investigations for the lower part of a Westinghouse SVEA-96+ assembly

Abstract Accurate critical experiments have been performed for the validation of total fission (Ftot) and 238U-capture (C8) reaction rate distributions obtained with CASMO-4, HELIOS, BOXER, and MCNP4B for the lower axial region of a real Westinghouse SVEA-96+ fuel assembly. The assembly comprised fresh fuel with an average 235U enrichment of 4.02 wt%, a maximum enrichment of 4.74 wt%, 14 burnable-absorber fuel pins, and full-density water moderation. The experimental configuration investigated was core 1A of the LWR-PROTEUS Phase I project, where 61 different fuel pins, representing ~64% of the assembly, were gamma-scanned individually. Calculated (C) and measured (E) values have been compared in terms of C/E distributions. For Ftot, the standard deviations are 1.2% for HELIOS, 0.9% for CASMO-4, 0.8% for MCNP4B, and 1.7% for BOXER. Standard deviations of 1.1% for HELIOS, CASMO-4, and MCNP4B and 1.2% for BOXER were obtained in the case of C8. Despite the high degree of accuracy observed on the average, it was found that the five burnable-absorber fuel pins investigated showed a noticeable underprediction of Ftot, quite systematically, for the deterministic codes evaluated (average C/E for the burnable-absorber fuel pins in the range 0.974 to 0.988, depending on the code).

LWR-PROTEUS verification of reaction rate distributions in modern 10 10 boiling water reactor fuel

Abstract HELIOS, CASMO-4, and MCNP4B calculations of reaction rate distributions in a modern, fresh 10 × 10 boiling water reactor fuel element have been validated using the experimental results of the LWR-PROTEUS Phase I project corresponding to full-density water moderation conditions (core 1B). The reaction rate distributions measured with a special gamma-scanning machine employing twin germanium detectors consisted of total fission Ftot and 238U-capture C8. The average statistical errors for the gamma scans were better than 0.5% for Ftot and 0.9% for C8. The rod-by-rod measurements were performed on 60 different fuel rods selected from the central part of a test zone consisting of actual, fresh SVEA-96+ fuel elements, thus gaining in realism by departing from conventional fuel rod mockups. In the case of Ftot, the root-mean-square (rms) of the rod-by-rod distribution of differences between calculational and experimental (C-E) values has been found to be 1.1% for HELIOS and for CASMO-4, and 1.3% for MCNP4B. For C8, the rms values of the (C-E) distributions are 1.0, 1.3, and 1.4% as obtained with HELIOS, CASMO-4, and MCNP4B, respectively. The effects of using different data libraries (ENDF/B-V, ENDF/B-VI, and JEF-2.2) with MCNP4B were also studied and have been found to be small.

Measurement and interpretation of delayed photoneutron effects in multizone criticals with partial D2O moderation

The effective fraction of delayed photoneutrons (ph) has been theoretically defined and experimentally determined in various different configurations of the LWR-PROTEUS critical facility. The peculiarity lies in the fact that the reactor has D2O in only one of the four fuelled zones, thus D(,n)H reactions take place mainly in this region. The work is divided into three parts. The first part is devoted to the description of the LWR-PROTEUS facility and to the measurements of ph. These experimental values are derived from standard inverse-kinetics analysis of neutron flux decay experiments for each of seven different configurations, with nine additional groups of neutron precursors to account for photoneutron effects. In the second part, the coupled neutron and gamma Boltzmann equations are reduced to exact point kinetics equations using the photon infinite-velocity approximation, and then to the point reactor model. Photoneutron-specific kinetics parameters, i.e. the photoneutron reactivity ph, the effective photoneutron delayed group fractions jph and the photoneutron effectiveness are rigorously defined and interpreted. In the third part, ph, jph and ph values have been estimated for the seven different configurations using only four MCNP4C photonic calculations with unitary photon sources in each of the four fuelled reactor regions. Comparisons have been made with the experiments, and the agreement obtained within 2 between the predicted and measured ph values is considered remarkably good in view of the simplicity of the models used, the approximations for the adjoint weighting and the complexity of the problem at hand

Validation of axial pin power distributions in a 10×10 BWR assembly across an enrichment boundary under full-density water moderation conditions

Critical experiments involving fuel pin gamma-scans have been performed to characterise total fission reaction rate axial distributions in a Westinghouse SVEA-96+ assembly under full-density water moderation [these investigations are part of the co-operation between PSI and Unterausschuss Kernenergie (UAK) or association of the Swiss nuclear operators, viz. Kernkraftwerk Leibstadt AG, Kernkraftwerk Gosgen-Daniken AG, BKW FMB Energie AG and Nordostschweizerische Kraftwerke AG]. The measurements have covered an 80-cm long region of the SVEA-96+ assembly involving two important axial heterogeneities: (a) the enrichment boundary separating the lower axial region (average 235U enrichment of 4.02 wt.%, 14 burnable absorber fuel pins) from the upper axial region (average 235U enrichment of 3.70 wt.%, 16 burnable absorber fuel pins), and (b) the presence of spacers at about 12 cm below this enrichment boundary. The experimental results have been compared to HELIOS/PRESTO-2 calculated values at three different levels, by means of radial point-wise, axial point-wise and axial node-wise comparisons. The results show good agreement (typically within 2%) between HELIOS/PRESTO-2 nodal pin powers and corresponding experimental values, with the largest discrepancy of 6-9% being highly localised in a pin subject to strong flux gradients and spectral interactions. Further improvements would require explicit three-dimensional lattice calculations and the development,of three-dimensional pin power reconstruction methods

On the choice of delayed neutron parameters for the analysis of kinetics experiments in 235U systems

Abstract The six-group, delayed-neutron parameters which have been incorporated into the ENDF/B-VI and JEF-2.2 evaluations represent a significant departure from the, until recently, almost universally adopted Keepin experimental data. This note quantifies the influence of the new data on the analyses of kinetics experiments in a reflected 235U system and uses the results to make some preliminary recommendations as to the best choice of data to be used in such analyses. In particular, it is shown that the choice of parameters in the JEF-2.2 library appears to be inconsistent and leads to the prediction of non-physical behaviour in models employing this data.

Epithermal Inverse Kinetic Measurements and Their Interpretation Using a Two-Group Point-Kinetic Model

Abstract Two of the methods that can be used for the measurement of the subcriticality of a multiplying system are the inverse kinetic (IK) and the pulsed neutron source (PNS) techniques. These methods depend considerably on correction factors and/or kinetic parameters, which usually need to be calculated using the same neutronic codes as those being validated via the experiments. The use of epithermal detectors to reduce the dependence of area-ratio PNS measurements on calculated correction factors was reported previously. In the current work, for the first time, epithermal detectors have been used for IK measurements. As in the case of the PNS experiments, these were carried out in core/reflector configurations with large spatial effects, systematic comparisons with thermal measurements clearly bringing out the considerably lower sensitivity of the epithermal IK results to calculational corrections. A new two-group point-kinetic model has currently been developed as an extension of the usual theoretical basis (employing a single energy group) for analyzing kinetic experiments. This has been essential for justifying the analysis methodology employed for the epithermal IK measurements.

Subcriticality measurements using an epithermal pulsed-neutron-source technique in pebble-bed HTR configurations

The Pulsed-Neutron-Source technique, commonly used for the measurement of the subcriticality of multiplying systems, suffers from the fact that the values of reactivity obtained are dependent upon calculated correction factors and/or kinetic parameters. Calculations have been carried out to quantify the energy dependence of these correction factors in graphite moderated systems, and these indicate that epithermal neutron distributions are much less affected by kinetic distortion. Measurements subsequently made at the PROTEUS facility of the Paul Scherrer Institute in Switzerland have confirmed this effect and show that even in strongly reflected systems the reactivity deduced from epithermal measurements is, within experimental uncertainties, spatially independent

Investigation of the keff-variation upon water ingress in a pebble-bed LEU-HTR

Accidental water ingress in pebble-bed type, high temperature reactor (HTR) cores can result in large positive reactivity changes and in consequent reductions in shutdown margins, since such systems generally tend to be undermoderated. The prediction of such effects, which are particularly important in the case of low-enriched uranium (LEU) fuel is a challenging task due to the complex interaction of, on the one hand, an increase in moderation and, on the other, a reduction in neutron streaming. Experiments have been carried out at the PROTEUS facility of the Paul Scherrer Institute in Switzerland to quantify water ingress effects in LEU-HTR systems and to assess the performance of certain deterministic codes in predicting them. In order to simulate water ingress, polyethylene rods were inserted between the pebbles of various deterministic configurations and a pulsed-neutron-source technique was used to measure the variation of keff with the stepwise removal of these polyethylene rods. Generation time, as well as reactivity measurements, were compared with calculations, made with various types of streaming corrections, for three different subcritical states

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.