Experience with MIDUS, a type B(U) ⁹⁹molybdenum transport package

Abstract

The medical isotope ⁹⁹molybdenum (⁹⁹Mo) decays to ^99mtechnetium (^99mTc), and ^99mTc is the most widely used radioactive isotope for nuclear medicine diagnostic studies for the heart, kidneys, lungs, liver, spleen and bone, among others. A major supplier of ⁹⁹Mo replaced its fleet of ⁹⁹Mo transport casks with a new type B(U) package designed and manufactured by EnergySolutions, and certified by the US Nuclear Regulatory Commission. The packages have been in service for 3 years, and an operational database is now available. This paper discusses the particular challenges for the Medical Isotope Depleted Uranium Shielded Transport Package (MIDUS) design and licensing, and how the field performance of MIDUS has compared with design objectives. The MIDUS package was designed to have approximately twice the capacity as its predecessor, but with the same outside envelope dimensions. Depleted uranium shielding was necessary in order to meet size and capacity requirements. Bulk shielding was not the only design challenge, since the MIDUS payload was in liquid form. After a 2002 incident involving elevated radiation levels on a package containing ¹⁹²Ir, the Nuclear Regulatory Commission staff recommended special mitigation capabilities for the MIDUS package. Unlike many payloads, high specific activity liquids could penetrate into the thin gap between the cask shield plug and body, bypassing much of the package’s shielding. The paper describes the MIDUS shielding design, how the mitigation shielding was accommodated, and compares predicted and measured operational dose rates. Owing to the short (66 h) halflife of ⁹⁹Mo, the schedule for packaging and shipping MIDUS is critical. Every hour lost to delays on the front end amounts to a 1% loss of product, affecting both patients and medical staff awaiting procedures. The MIDUS package was designed for reliable, efficient operations. The paper reviews the MIDUS operational performance including typical package duty cycles, process time for package preparation, preshipment leak tests, leak test failure frequency and package unloading.