Some Aspects of Ultimate Disposal

Plutonium Criticality Considerations in Reprocessing Wastes and Contaminated Soils, Roger D. Carter (ARHCO-USA) 

The minimum concentration limit, of course, is not truly a concentration liniit but an H/Pu ratio limit at a maximum of about 3600. If, by some method, a normal solution with 10 g Pu/liter were to be expanded by a factor of 10, the plutonium concentration would become 1.0, but the H/Pu ratio and the k , would remain the same and a critical mass would still be possible, although much larger. Therefore, any system containing dilute plutonium below 7 g/liter cannot be dismissed arbitrarily as being safe as long as If/Pu ratios are 3600. 

There are a number of situations where the existence of dilute plutonium with low H/Pu ratios might have to be considered to effectively control criticality. Among these are the dispersion of plutonium in soil, the accumulation of plutonium in fuel processing wastes, and the accumulation in filter materials. These systems may not only have finite critical masses at plutonium concentrations 7 g/liter, but may also contain added scatterers which can reduce the critical masses of the systems to several times less than those of pure plutonium-water systems. 

The decrease in critical mass caused by poorly absorbing neutron scatterers has been shown before. These materials tend to decrease the critical mass because they scatter neutrons isotropically, thus tending to hold them within the system. The solid of the Hanford plant contain a number of these materials (oxygen, Silicon, aluminum, etc.). Calculations show that if plutonium is dispersed within the soil, the soil components cannot be Ignored. 

The more elaborate 1973 Z-9 characterization defined the three-dimensional distribution below the 9- X 18-m floor. Maximum plutonium concentration was 20 g/liter in a small, thin region, with 2 to 10 g/liter in large areas of soil down to a depth of 30 cm. The higher concentration regions were found to have water volume fractions up to 60 vol%. A detailed three-dimensional analysis was performed using the GEM4 and KENO Monte Carlo computer codes, with the koe of the trench calculated to be 0.5 (Sefs. 6 and 7). 

---