J. Nucl. Eng., Volume 3, Issue 3 (September 2022) – 4 articles

The nations of the world plan to stop burning carbon fuels but have not fixed on any replacement. For social and economic confidence, they need to share a proper picture of the options. The science is simply explained and not in doubt, though widely misunderstood. Energy sources belong to three distinct groups: renewable, chemical and nuclear. Since human life began, it has adopted each of these in turn. In the past, initial disruptions have been more than off-set by the rise in human values that followed. Now, to complete the final step, we confront those who would look backward to the age of renewables. Instead, the world should look forward to a heavy dependence on nuclear energy with a confidence informed by natural science and openly shared in society. Full article

The Nagoya University Accelerator-driven Neutron Source (NUANS) is an accelerator-based neutron source by ⁷Li(p,n)⁷Be reaction with a 2.8 MeV proton beam up to 15 mA. The fast neutrons are moderated and shaped to beam with a Beam Shaping Assembly (BSA). NUANS is aiming at the basic study of the Boron Neutron Capture Therapy (BNCT) such as an in vitro cell-based irradiation experiment using a water phantom. Moreover, the BSA is developed as a prototype of one for human treatment. We have evaluated the radiation field of NUANS by a Monte Carlo code PHITS. It is confirmed that the radiation characteristics at the BNCT outlet meet the requirement of IAEA TECDOC-1223. Additionally, the radiation field in the water phantom located just in front of the BSA outlet is calculated. In the in vitro irradiation experiment, the boron dose of 30 Gy-eq, which is the dose to kill tumor cells, is expected for 20 min of irradiation at the beam current of 15 mA. Full article

The application of the recently developed “nth-order comprehensive sensitivity analysis methodology for nonlinear systems” (abbreviated as “nth-CASAM-N”) has been previously illustrated on paradigm nonlinear space-dependent problems. To complement these illustrative applications, this work illustrates the application of the nth-CASAM-N to a paradigm nonlinear time-dependent model chosen from the field of reactor dynamics/safety, namely the well-known Nordheim–Fuchs model. This phenomenological model describes a short-time self-limiting power transient in a nuclear reactor system having a negative temperature coefficient in which a large amount of reactivity is suddenly inserted, either intentionally or by accident. This model is sufficiently complex to demonstrate all the important features of applying the nth-CASAM-N methodology yet admits exact closed-form solutions for the energy released in the transient, which is the most important system response. All of the expressions of the first- and second-level adjoint functions and, subsequently, the first- and second-order sensitivities of the released energy to the model’s parameters are obtained analytically in closed form. The principles underlying the application of the 3rd-CASAM-N methodology for the computation of the third-order sensitivities are demonstrated for both mixed and unmixed second-order sensitivities. For the Nordheim–Fuchs model, a single adjoint computation suffices to obtain the six 1st-order sensitivities, while two adjoint computations suffice to obtain all of the 36 second-order sensitivities (of which 21 are distinct). This illustrative example demonstrates that the number of (large-scale) adjoint computations increases at most linearly within the nth-CASAM-N methodology, as opposed to the exponential increase in the parameter-dimensional space which occurs when applying conventional statistical and/or finite difference schemes to compute higher-order sensitivities. For very large and complex models, the nth-CASAM-N is the only practical methodology for computing response sensitivities comprehensively and accurately, overcoming the curse of dimensionality in sensitivity analysis of nonlinear systems. Full article

This work presents the n^th-Order Comprehensive Adjoint Sensitivity Analysis Methodology for Nonlinear Systems (n^th-CASAM-N), which enables the most efficient computation of exactly determined expressions of arbitrarily high-order sensitivities of generic nonlinear system responses with respect to model parameters, uncertain boundaries, and internal interfaces in the model’s phase space. The mathematical framework underlying the n^th-CASAM-N is proven to be correct by using mathematical induction. The n^th-CASAM-N is formulated in linearly increasing higher-dimensional Hilbert spaces—as opposed to exponentially increasing parameter-dimensional spaces—thus overcoming the curse of dimensionality in sensitivity analysis of nonlinear systems. Full article