Ducrete™ Concrete:
An advanced nuclear radiation shielding material

Introduction

Uranium metal, produced by Starmet Corporation, has long been used as shielding in selected applications where it's highly effective shielding benefits outweighed the high costs of manufacturing. However, for most nuclear material storage applications, conventional shielding materials (iron, steel, lead, and concrete) are far cheaper than uranium metal.

DUCRETE™ Concrete is an engineered material that can use the high-density attribute of uranium while minimizing the cost of production. DUCRETE™ Concrete uses aggregate manufactured from uranium oxide as the large aggregate in a concrete mixture. DUCRETE™ Concrete has a density up to 6.4 g/cm3 (400 lb/ft3). The figure shows the relative effectiveness of various materials in shielding the neutron and gamma flux from a source consisting of 24 spent PWR fuel assemblies. The hydrogen in the concrete's water of hydration compliments the uranium by efficiently attenuating the neutron flux. The uranium oxide aggregate provides the bulk of the gamma shielding portion of the matrix.

Preliminary designs of spent fuel storage casks have been made to demonstrate the advantages of DUCRETE™ Concrete over conventional concrete. The DUCRETE™ cask can provide the same nuclear shielding at about 1/3 of the wall thickness. The cask weight can be reduced about 20% to 30% allowing easier handling. The overall size allows the empty cask to be shipped from a centralized fabrication location to the point of use and re-shipped again if a centralized storage facility becomes available in the United States.

DUCRETE™ Concrete can also be used for other shielding applications such as in low-level radioactive waste storage or disposal boxes, temporary shielding in reactor facilities, and for commercial food and medical irradiator applications. With a density nearly three times that of conventional concrete, the wall thickness is proportionally reduced for the same radiation attenuation.

Process Concept

The basic process concept involves the conversion of UF6 to an oxide material such as U308 or UO2. Starting with the oxide powder, a ceramic aggregate is made using a patented process that combines the uranium with other materials to increase the chemical stability in a ceramic manufacturing process. The aggregate is then used in a Portland cement based concrete mixture replacing the typical large aggregate or gravel. Depending upon the final density of the aggregate, concrete densities between 6.0 - 7.0 g/cc can be produced. This compares to 2.4 g/cc (150 lb/ft3) for conventional concrete.

DUCRETE™ Concrete Strength

Initial compression tests using surplus UO2 fuel pellets as the aggregate have been conducted. DUCRETE™ Concrete and conventional concrete samples have been made, cured for 7, 28, and 90 days and subjected to compression and tensile testing. In summary, all early samples of DUCRETE™ test specimens had strengths in excess of 3000 psi. The latest samples produced with aggregate made by Starmet have strengths exceeding 4000 psi. Using specialized concrete formulations, strengths well over 6000 psi are expected.

Cost Effectiveness

The development of this process was funded by the US Department of Energy (DOE) to address future uses of vast quantities of surplus depleted uranium materials (over 700,000 metric tons). Aggregate production costs are expected to be part of the UF6 conversion as a lower cost alternative than disposal of the depleted uranium as waste. If such a policy eventually develops, then fabrication of components using DUCRETE™ Concrete should not be significantly higher than using traditional concrete. Weight savings and transportation features should provide added value to shielding applications, thus, justifying the additional costs. Starmet personnel will be happy to discuss these engineering features with any interested party. Most products will be fabricated in a factory environment where cost efficiencies will be derived from higher volumes and where engineering measures can be implemented to control contamination.

Future Work

Starmet Corporation and the DOE are currently working together to develop the full spectrum of engineering properties for DUCRETE™ Concrete.

Benefits to The Department of Energy (DOE)

This technology was developed as a low cost alternative for using the large quantity of uranium in the depleted uranium hexafluoride inventory. Use of depleted uranium aggregate in DUCRETE™ Concrete provides DOE an alternative to direct disposal of depleted uranium as waste. Such disposal options have been shown to cost several billion dollars. This application provides an environmentally sound use of depleted uranium while deriving useful benefits.

Application Opportunities

Starmet Corporation is interested in developing applications for DUCRETE™ Concrete and will be happy to consider new applications and sublicensing of the technology.