Study of spectral heterogeneities for the interpretation of calculational trends in predicting pin power distributions in a SVEA-96+ BWR assembly

Detailed energy spectra have been calculated with the codes CASMO-4 and BOXER for neutron reaction rates in individual regions of a Westinghouse SVEA-96+ BWR fuel assembly immersed in water at room temperature, as investigated in Core 1A of the LWR-PROTEUS Phase I experimental program. First, results in 39 energy groups from the two codes were compared for 123 regions to characterise the lattice heterogeneity and to identify spectral trends qualitatively. Second, the various regions were classified into 4-5 broad spectral categories for quantitative comparisons. Third, results were collapsed to 2 groups, and then finally to 1 group, in order to obtain the principal neutron balance components, as also to derive and compare integral parameters such as the multiplication factor and average number of neutrons emitted per fission. These detailed spectral heterogeneity studies have led to the interpretation of certain trends in the prediction of experimental pin power distribution maps, which were not recognised earlier, providing a clear physical explanation for the observed discrepancies. [All rights reserved Elsevier

Diagnostic analysis of pin-removal reactivity worth experiments in a SVEA-96+ test lattice

The present paper concerns a novel type of integral database generated in the course of the LWR-PROTEUS Phase I experiments, viz. the relative reactivity effects of removing individual fuel pins from a highly heterogeneous BWR assembly. The measurements reported were conducted in the central assembly of a 33 test-zone configuration of SVEA-96+ assemblies in PROTEUS, under full-density water moderation conditions. Calculations of the pin-removal reactivity worths have been carried out using two different LWR assembly codes, viz. CASMO-4 and BOXER. The discrepancies observed between experiment and calculation have been analysed in detail, on the basis of an extended reactivity decomposition methodology. This has allowed quantification of the different phase-space contributions (in terms of reaction rate types, energy groups and spatial regions) of a given calculated pin-removal reactivity worth, thus providing useful insights regarding the most important sources of error in each of the assembly codes investigated.[All rights reserved Elsevier]

On the accuracy of reactor physics calculations for square HPLWR fuel assemblies

Although the supercritical-pressure or high-performance light water reactor (HPLWR) concept is largely based on the well-established technological experience available with conventional light water reactors, there is still no consensus on various key design features such as an optimal layout for the fuel assembly. This results mainly from the very large density variations of supercritical-pressure water in the core, which render it difficult to ensure reliable values for parameters such as power peaking factors and reactivity worths. The present paper describes studies carried out to compare deterministic and Monte Carlo codes for analysing a representative square HPLWR lattice with uniform 5%-enriched UO2 fuel. The main purpose has been to assess the prediction accuracies achievable for integral parameters such as the multiplication factor, control absorber effectiveness, moderator/coolant density reactivity feedback and pin power distributions. The results show good agreement between the deterministic and stochastic calculations for the unperturbed lattice. However, for certain perturbed situations involving, for example, local coolant density changes in the assembly or control absorber insertion, the observed discrepancies are large enough to question the basic viability of the reactor physics design, e.g. with respect to the thermal performance. [All rights reserved Elsevier

Quantification of the transferability of reactivity effect investigations in large multiregion systems

A methodology has been developed for the accurate assessment of localised reactivity perturbations in a BWR lattice embedded in a larger multiplying system, based on a full-system, unperturbed calculation, and on perturbed calculations on reduced-geometry models with reflective boundary conditions (typically, reflected-assembly calculations). Reflective reduced-geometry calculations are to be followed by a fast transferability correction for making the results representative of what full system computations would have produced. In this way, one can avoid the problem of having insufficient accuracy in the results (in spite of extremely lengthy iterations), particularly for cases of small reactivity effects. Furthermore, the factorization of reactivity effect transferability, a key feature of the developed methodology, provides valuable insight into the different effects contributing to a particular integral transferability factor, along with a quantification of the relative importance of these effects for each individually considered case. The initial investment, needed for realizing the relatively low required computational effort involved in the postcorrection procedure, is to obtain a limited number of adjoint equation solutions defined for the reference state at full system level. Application results are reported for the numerical analysis of fuel pin removal reactivity effects in LWR-PROTEUS. The latter is a programme of integral experiments, employing essentially a central LWR test zone driven critical by surrounding driver and buffer region

Development and application of a decomposition methodology for interpretation of reactivity effect discrepancies

With reactivity being the most important integral reactor physics quantity - and simultaneously the one that can be measured with the highest accuracy there is a great interest in understanding how possible space- and energy-dependent data and/or modeling discrepancies may propagate into a calculated reactivity change, and with which magnitude this occurs. In the context of pin removal reactivity effects in a light water reactor assembly, for example, it is illustrative to carry out, for any arbitrary localized material composition perturbation, a decomposition of the total effect into individual space- and energy-dependent contributions of the different unit cells in the assembly. If this decomposition is normalized to +100% in the case of a positive reactivity effect and to -100% in the case of a negative reactivity effect, an importance map is established that indicates the relative contribution (in percent) of each individual contributing cell to the total reactivity effect caused by the localized material composition change. Such an importance map can be interpreted as a sensitivity matrix that quantifies the final discrepancy in a calculated reactivity effect, with respect to its reference value, as a weighted sum of the complete collection of cell-wise data and/or modeling discrepancies. The current paper outlines the basic theory and gives certain practical applications of the proposed decomposition methodology. Thus, it is found that the developed methodology offers in-depth, quantitative explanations for calculational discrepancies observed in the analysis of fuel pin removal experiments conducted in the framework of the LWR-PROTEUS program at the Paul Scherrer Institut

Optimised non-invasive method to determine 238U-captures-to-total-fissions in reactor fuel

High-resolution gamma spectroscopy was performed on individual fuel rods of a fresh, highly heterogeneous Boiling Water Reactor (BWR) fuel assembly, after irradiation at low power in the PROTEUS reactor at PSI, to determine the ratio of neutron captures in 238U (C8) to total fissions (Ftot). The methods for correcting the measured gamma count-rates for attenuation within the fuel rods and the collimator shielding are now so advanced that one can obtain several independent estimates of Ftot for each fuel rod by using gamma lines from different fission products with a wide range of energies. The recording of a detailed irradiation history further allowed the gamma activities to be precisely corrected for radioactive decay between the time of irradiation and measurement. Due to these facts, and the good statistical quality of the present data, the main limitation on the accuracy of C8/Ftot comes from uncertainties on the basic nuclear quantities: fission yields and beta-gamma branching ratios. The various steps involved in determining C8/Ftot from recorded gamma spectra, and their contributions to the final error, are analysed. The results for 80 fuel rods of varying material composition exposed to different neutron moderation conditions in the reactor core are compared with the results of a Monte Carlo full-assembly calculation. [All rights reserved Elsevier

Void reactivity coefficient benchmark results for a 10x10 BWR assembly in the full 0-100% void fraction range

A boiling water reactor SVEA-96+ fresh fuel lattice has been used as the basis for a benchmark study of the void reactivity coefficient at assembly level in the full voidage range. Results have been obtained using the deterministic codes CASMO-4, HELIOS, PHOENIX, BOXER and the probabilistic code MCNP4C, combined for almost all cases with different cross section libraries. A statistical analysis of the results obtained showed that the void reactivity coefficient tends to become less negative beyond 80% void and that the discrepancies between codes tend to increase from less than 15% at voidages lower than 40% to more than 25% at voidages higher than 70%. The void reactivity coefficient results and the corresponding differences between codes were isotopically decomposed to interpret discrepancies. The isotopic decomposition shows that the minimum observed in the void reactivity coefficient between 80% and 90% void is largely due to the decrease in the relative importance of the 157Gd(n, ) rate with increasing voidage, and that the fundamental discrepancies between codes or libraries are mainly governed by the different predictions of the 238U(n, ) variation with voidage. [All rights reserved Elsevier]

Characterisation of radial reaction rate distributions across the 92-pin section of a SVEA-96 Optima2 assembly

Characterisation of the SVEA-96 Optima2 boiling water reactor assembly, in terms of the radial distributions of normalised total fission and 238U capture rates, is reported at its central elevation, i.e. at the 92-pin section, where the one-third part-length pins are replaced by water. Measurements performed in the PROTEUS facility are compared with MCNPX predictions. The calculation model included the measured locations of the SVEA-96 Optima2 assemblies and sub-assemblies, within the PROTEUS test zone. Predicted and experimental fission and 238U capture rates are found to agree, respectively, within 3.5% and 4% for all pins. Fission rates in the burnable-absorber UO2-Gd2O3 fuel pins have been predicted without bias using the ENDF/B-VI data library but show an average 1.4% under-prediction with the JEFF-3.1 data library. A slight overestimation of the total fission rate in the pins located at the periphery of the assemblies was observed and has been attributed to an inaccurate modelling of the pin positions. However, there was no systematic bias observed due to the absence of the one-third pins at the corners of the assembly.[All rights reserved Elsevier]

Effects of void uncertainties on the void reactivity coefficient and pin power distributions for a 1010 BWR assembly

An important source of uncertainty in boiling water reactor physics is associated with the precise characterisation of the moderation properties of the coolant and by-pass regions, with significant impact on reactor physics parameters such as the lattice neutron multiplication, the neutron migration area and the pin-by-pin power distribution. In this paper, the effects of certain relevant void-fraction uncertainties on reactor physics parameters have been studied for a BWR assembly of the type Westinghouse SVEA-96 using CASMO-4, HELIOS/PRESTO-2 and MCNP4C. The SVEA-96 geometry is characterised by the sub-division of the assembly into four different sub-bundles, by means of an inner by-pass with a cruciform shape. The study has covered: (a) the effects of different cross-section data libraries on the void coefficient of reactivity, for a wide range of void fractions; (b) the consideration of a water film inside the sub-bundle walls, and (c) the impact of partly inserted absorber blades producing very different void fractions in different sub-bundles. [All rights reserved Elsevier