Philippine Journal of Science
153 (S1): 1-14, Nuclear Science and Technology
ISSN 0031 – 7683
Date Received: 30 Jun 2023
Effects of Radiogenic Helium Release on Plutonium-Beryllium Neutron Source Encapsulation Safety
Frederick C. Hila1,2*, Cheri Anne M. Dingle1, Alvie A. Astronomo1, John Jerome G. Mataac1, and Eduardo R. Magdaluyo Jr.2
1Department of Science and Technology–Philippine Nuclear Research Institute (DOST-PNRI), Commonwealth Avenue, Diliman, Quezon City 1101 Philippines 2Department of Mining, Metallurgical, and Materials Engineering, College of Engineering, University of the Philippines Diliman, Quezon City 1101 Philippines
*Corresponding author: fchila@pnri.dost.gov.ph
Hila F et al. 2024. Effects of Radiogenic Helium Release on Plutonium- Beryllium Neutron Source Encapsulation Safety. Philipp J Sci 153(S1): 1–14.
ABSTRACT
This study evaluates the accumulation of radiogenic helium in plutonium-beryllium (238PuBe and 239PuBe) sources, which are encased in tantalum and 304 stainless steel layers, and explores the potential consequences of its theoretical liberation from the PuBe crystal lattice on the integrity of the encapsulation. The primary goal is to estimate the safe encapsulation periods of three PuBe neutron sources (PH-NUMA, PH-NUMB, and PH-MONS) intended for use at the Philippine Nuclear Research Institute. A new Python code was developed to calculate the time-dependent helium buildup in Pu-bearing materials. Theoretical internal pressures from helium buildup were used to assess the safety of encapsulation using both the ASME BPVC. VIII.1-2019 standard code and finite element analysis (FEA) conducted with Ansys Mechanical software. Results indicate that the internal helium buildup in general 238PuBe sources reaches a plateau at a faster rate compared to 239PuBe sources due to the limitation of the 238Pu decay chain. In contrast, 239PuBe sources have sustained buildup regardless of the Pu grade due to α-generation based on combinations of decay series of 238Pu, 239Pu, 240Pu, and 241Pu. Analysis of theoretical internal pressures using the ASME code suggests that the Philippine PuBe sources, PH-NUMA and PH-NUMB, have safe encapsulation periods exceeding 1,000 yr and PHMONS exceeding 10,000 yr. However, FEA simulations highlight that the safety factor at the modeled weld crack becomes a determining factor in the maximum safe encapsulation period, especially under shallow weld penetration. The Ansys simulations of PH-NUMA and PH-NUMB suggest safe encapsulation periods of 400 and 200 yr, respectively, under the assumption of an undesirably shallow weld penetration of 30%. It should be noted that this study assumes the full release of helium atoms from the PuBe crystal structure, which is unlikely to occur at ambient temperatures.
Keywords: helium buildup, neutron sources, plutonium-beryllium, safe encapsulation period, safe working life, weld penetration