Methodology for worker neutron exposure evaluation in the PDCF facility design

Area of Science:

• Nuclear Engineering
• Radiation Shielding
• Health Physics

Background:

• The Pit Disassembly and Conversion Facility (PDCF) is designed to process excessed nuclear weapons' plutonium pits into plutonium oxide.
• Worker safety requires stringent adherence to radiation exposure limits: 5 mSv/y (whole body) and 100 mSv/y (extremity).
• Processing tons of radioactive material necessitates robust shielding solutions for facility components like glove boxes and storage vaults.

Purpose of the Study:

• To develop and validate a consistent, cost-effective, and rigorous methodology for radiation shielding calculations.
• To determine appropriate shielding thicknesses and compositions for the PDCF to meet worker dose limits.
• To address specific challenges in shielding design, including extremity dose assessment and modeling complex geometries.

Main Methods:

• Utilized ORIGEN-S and SOURCES for generating gamma and neutron source terms.
• Employed Monte Carlo N-Particle (MCNP-4C) code for radiation transport calculations.
• Developed a consistent methodology for shielding calculations, addressing dose factor selection, extremity dose handling, and non-uniform radiation fields.

Main Results:

• Calculations informed the design of shielding for glove box walls and concrete storage vaults.
• The methodology successfully addressed challenges related to extremity doses and effective dose equivalent in non-uniform fields.
• The relative importance of neutron versus gamma dose equivalents was found to vary significantly with shielding conditions.

Conclusions:

• A validated, efficient, and rigorous shielding calculation methodology was established for the PDCF.
• The developed methods ensure worker protection by meeting target radiation exposure levels.
• Lessons learned regarding neutron and gamma dose contributions inform future shielding designs for similar facilities.