Pathway analysis of nuclear reaction in pulsed neutron activation

When being irradiated by neutrons, materials will produce the radioactive nuclides. It is significant to analyze the nuclear reaction pathways of activated nuclides for effective radiation protection and source term management. Currently, most fusion devices or reactors operate under pulsed conditions, which makes the nuclear reaction pathway analysis of radionuclides complex. This paper, after in-depth study, proposes an exact analysis approach for nuclear reaction pathways under pulsed conditions: pathway analysis of the target nuclides will be performed with a reverse order of phases. Then serving the encountered intermediate nuclides as “minor” target nuclides to search the pathways further, aiming to achieve the exact source tracing of the target nuclides. Based on the neutron activation calculation program ABURN, the corresponding functions are implemented and some typical tests are conducted. The trial results manifest the conclusion: not only can this methodology list the detailed pathway information of each phase under pulsed conditions, but also it gives the cumulative data regarding target nuclides in total irradiation time and total cooling time from a macro perspective. In order to enhance the efficiency of nuclear reaction pathway analysis under complex pulsed conditions, three approaches, steady-state (SS), equivalent steady-state (ESS), and continuous-pulsed (CP), are introduced to approximate the entire pulses. The computation results demonstrate that SS and ESS methods have issues overestimating or underestimating the inventory of medium-lived isotopes. While the CP method, by incorporating exact simulation for terminal pulses, mitigates inventory errors caused by pulsed approximation. In terms of nuclear reaction pathway analysis, due to the approximate treatment of pulses, not only the detailed pathways in each phase cannot be given, but also the issues of missing pathways and calculation errors of contributions may occur; even the CP method cannot perfectly solve the latter. Finally, leveraging the periodicity characteristic inherent to pulsed activation condition, a pathway analysis approach based on the last highest-level pulse is proposed, which reduces the pathway analysis of entire phases to only carry out within the last highest-level pulse. The nuclear reaction pathways of the target nuclides can be accurately given through this approach and the effectiveness of the approach has been analyzed via typical examples. For the calculation examples adopted in the paper, the refined methodology can not only grasp the detailed information of nuclides within each phase, but also substantially reduce redundant computational overhead and memory allocation for repetitive information, thereby markedly enhancing source term analysis efficiency of complex pulsed conditions in fusion reactor.