A solution-based technique for growing large-volume stilbene scintillators was developed in 2013; crystals up to diameters of 10 centimeters, or larger, have been grown while preserving excellent pulse shape discrimination (PSD) properties. The goal of this study is to evaluate the PSD capabilities of 5.08 by 5.08-cm stilbene crystals grown by Lawrence Livermore National Laboratory and Inrad Optics when exposed to a 1000-to-1 gamma ray-neutron ratio and operating at a 100-kHz count rate. Results were compared to an equivalent EJ-309 liquid scintillation detector. 252 Cf neutron pulses were recorded in two experiments where 60 Co and 137 Cs sources created the high-gamma field. The high count rate created numerous double pulses that were cleaned using fractional and template approaches designed to remove double pulses while preserving neutron counts. PSD was performed at a threshold of 42 keVee (440-keV proton) for stilbene and 60 keVee (610-keV proton) for EJ-309 liquid. The lower threshold in stilbene resulted in a neutron intrinsic efficiency of approximately 14.5%, 10% higher than EJ-309 liquid, for bare 252 Cf and 13% for 252 Cf in the high-gamma field. Despite the lower threshold, the gamma misclassification rate in stilbene was approximately 3 × 10-6 , nearly a factor-of-five lower than what we found with the EJ-309 liquid.
The interactions between neutron radiation and matter are nonlinear. Specifically, certain neutron nuclear reactions (e.g., fission events) generate heat, which modifies the temperature and density of the surrounding material, which in turn modify the neutron interaction probabilities (through the macroscopic cross sections). This nonlinear process is referred to as neutron transport with thermal hydraulics feedback.Linear transport problems (without feedback) are accurately described by the Neutron Transport Equation (NTE). Discretized forms of the NTE are routinely solved using iterative transport acceleration schemes, for example, the standard Coarse Mesh Finite Difference (CMFD) method. CMFD and related acceleration schemes for linear transport problems are reliable and well-understood. However, when these same iterative methods are applied to nonlinear (i.e. multiphysics) problems, significant performance and stability issues often occur.In this thesis, we propose modifications to the CMFD procedure, so that it can more robustly and efficiently solve loosely-coupled neutron transport -thermal hydraulics multiphysics problems. We refer to the new method as Nonlinearly Implicit Low-Order CMFD (NILO-CMFD). In this method, the transport-corrected diffusion equationa foundational component of the CMFD method -is modifed to include approximate thermal hydraulics and nuclear data update physics.To begin, we provide a general derivation of the 3-D NILO-CMFD method. Then, to initially analyze and test the method, we develop an approximate 1-D multiphysics model of a 3-D nuclear reactor fuel pin. The 1-D model consists of: (i) a 1-D neutron transport equation describing neutron transport, (ii) a 1-D advection-diffusion equation describing fluid temperature variation, (iii) a fuel temperature that depends on the 1-D fission heat source, and (iv) model equations that define the neutron cross sections and related nuclear data in terms of the fluid and fuel temperatures. The implementation of this model in a 1-D test code shows that the NILO-CMFD method overcomes the instabilities that occur in the standard Relaxed-CMFD (R-CMFD) method.x To test the new NILO-CMFD method on realistic 3-D problems, we implemented an incomplete version of the method, NILO-A, in the 3-D neutron transport code MPACT. For multiphysics problems, MPACT is loosely coupled to the 3-D COBRA-TF (CTF) code, which performs subchannel thermal hydraulics calculations. In every 3-D problem that we ran, NILO-CMFD successfully reduced the number of outer iterations to the low number required by CMFD for single-physics calculations. Unfortunately, due to lack of time, we could not implement the full NILO-CMFD method, which would have significantly reduced the wall clock time of the simulations. Consequently, some of our 3-D simulations required more wall clock time to converge than R-CMFD.Nonetheless, our results show that the NILO-CMFD method is a legitimate generalization of single-physics CMFD to multiphysics problems. The number of outer iterations requi...
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