Analysis of MNSR core composition changes using the codes WIMSD-4 and CITATION.

Area of Science:

• Nuclear Engineering
• Computational Physics

Background:

• Miniature Neutron Source Reactors (MNSR) are crucial for research and isotope production.
• Accurate modeling of fuel burn-up and fission product accumulation is essential for reactor safety and operational efficiency.

Purpose of the Study:

• To develop and apply a computational methodology for simulating fuel burn-up in an MNSR core.
• To identify the primary fission products affecting MNSR reactivity and core lifetime.
• To assess the impact of fission product accumulation on excess reactivity.

Main Methods:

• Utilized WIMSD/4 and BORGES codes to generate a microscopic cross-section library for MNSR core elements across six neutron energy groups.
• Employed the 3D CITATION code, with modifications to the NUCY subroutine, to calculate fuel burn-up, actinides, and fission product concentrations.
• Simulated reactor operation, including sporadic periods, using an equivalent continuous operation model.

Main Results:

• The study identified Samarium-149 (Sm-149) as the dominant fission product influencing MNSR core lifetime, followed by Gadolinium-157 (Gd-157) and Cadmium-113 (Cd-113).
• Accumulation of these fission products over 100 continuous operation days resulted in a 4.3 mk reduction in excess reactivity.
• The simulated reactivity reduction showed good agreement with empirical data (3.5 mk) for the reactor's discontinuous operation history.

Conclusions:

• The computational approach accurately predicts the impact of long-lived fission products on MNSR core lifetime and reactivity.
• The simulation methodology provides a valid approximation for assessing the long-term behavior of MNSR cores.
• While effective for long-lived products, the continuous operation approximation may overestimate short-lived radioactive product concentrations like Xenon-135 (Xe-135).