Theoretical and Experimental Approach Towards Generation of Thermal Scattering Law for Light Water

Safety analysis and design of nuclear systems rely on accurate computer simulation tools. These simulation tools take in basic nuclear data as an input to estimate the safety parameters of the nuclear systems and ensure its normal operation. In particular, precise knowledge of light water neutron scattering cross section data is important as light water is the most widely used moderator in thermal nuclear reactors, such as pressurized water reactors (PWRs). Since PWRs operate at temperature around 550 K and pressure around 150 bar, proper understanding of light water thermal cross section data at high temperatures and pressures is necessary. In the thermal neutron energy region, the cross sections are governed by the structure and dynamics of the scattering material described by thermal scattering law (TSL). The TSL data for light water at several temperatures are available in the standard thermal scattering libraries. No information about the impact of pressure on the TSL data exists in these libraries. Until recently, the standard TSL data libraries for light water have relied mostly on physics model and experimental data measured in the 60s. One observes some discrepancies in these evaluations both at room temperature and reactor operating temperature (550 K). Experimental TSL, for high temperature and pressure, are very scarce. There is thus a need for reviewing the existing TSL evaluations and consequently performing new experiments, to develop new evaluations valid for a large range of temperature and pressure conditions of light water. The research developed during this work, deals with the measurement and evaluation of TSL data for light water at high temperatures and pressures. To generate new TSL for light water, inelastic neutron scattering measurements were carried out at two time-of-flight (TOF) spectrometers, namely the IN4c and IN6, at the Institut Laue-Langevin (ILL), Grenoble, France. A corresponding set of molecular dynamics (MD) simulations were performed to complement the experimental data using two classical interaction models for water namely, a flexible non-polarizable TIP4P/2005f and a rigid polarizable TCPE model. Frequency spectra obtained from both TOF experiment and MD simulations at different temperatures and pressures have been analyzed and new TSL evaluations for light water have been developed. A new module named as SAB module was developed in the existing IRSN nuclear data processing code, GAIA, with capabilities to generate TSL libraries for light water at the desired temperature requested by the user. The SAB module was validated on the French plutonium temperature effect experimental program, where TSL evaluations at exact temperatures of the experiment were necessary to observe a positive temperature effect. The performance of the newly developed TSL evaluations, i.e. S(α β)JEFF-3.3-MOD, S(α β)ILL and S(α β)TCPE were tested on a series of differential, double differential and total cross section measurements available in the literature. For further verification and validation of the new TSL data, critical benchmarks available in the International Handbook of Evaluated Criticality Safety Benchmark Experiments (ICSBEP Handbook), sensitive to TSL have been used. In addition, the new IRSN 16O and 235U evaluations along with 235U Prompt Fission Neutron Spectrum (PFNS) from ENDF/B-VIII.0 were tested in conjunction with the new TSL evaluations. The outcome of this study leads to a better interpretation of the impact of temperature and pressure on TSL in PWR applications. It is suggested that a similar study for nuclear reactors operating with different moderators (for instance heavy water, graphite, etc.,) should be conducted to investigate whether a similar behavior like the one observed in light water is seen.