Neutron absorber inserts for 55-gal drums

Transport and temporary storage of more than 200 g of fissile material in 55-gal drums at the Rocky Flats Environmental Technology Site (RFETS) have received significant attention during the cleanup mission. This paper discusses successful applications and results of extensive computer studies. Interim storage and movement of fissile material in excess of standard drum limits (200 g) in a safe configuration have been accomplished using special drum inserts. Such inserts have constrained the contents of a drum to two 4-{ell} bottles. The content of the bottles was limited to 600 g Pu or U in solution or a total of 1200 g for the entire drum. The inserts were a simple design constructed of stainless steel, forming a vertical cylindrical pipe into which two bottles, one on top of the other, could be centered in the drum. The remaining drum volume was configured to preclude any additional bottle placement external to the vertical cylinder. Such inserts in drums were successfully used in moving high-concentration solution from one building to another for chemical processing. Concern about the knowledge of fissile material concentration in bottles prompted another study for drum inserts. The past practice had been to load up to fourteen 4-{ell}more » bottles into 55-gal drums, provided the fissile material concentration was <6 g fissile/{ell}, and the total drum contents of 200 g fissile was not exceeded. Only one determination of the solution concentration was needed. An extensive safety analysis concluded that a single measurement of bottle content could not ensure compliance with double-contingency-criterion requirements. A second determination of the bottle contents was required before bottles could be placed in a 55-gal drum. Al alternative to a dual-measurement protocol, which is for bolstering administrative control, was to develop an engineered safety feature that would eliminate expensive tests and administrative decisions. A drum insert design was evaluated that would ensure subcriticality in a drum even if the concentration of fissile material solution exceeded 6 g/{ell}. The challenge posed to the study was to maximize the number of bottles with a concentration up to 150 g/{ell} that could be safely placed into a drum. A concentration of 150 g of fissile material per liter is the maximum expected solution concentration in bottles at RFETS. A series of computer analyses were undertaken to design special drum inserts. A design was completed for a 55-gal drum insert to hold up to ten bottles containing 600 g fissile material in solution per bottle or 6 kg per drum. This insert would protect against accidental criticality caused by misloading high-concentration-solution bottles into a drum designated for low-concentration-bottle transport. The drum with the inserts could be used for transport or temporary storage of high-concentration solutions in 4-{ell} bottles. Conceptually, the drum with its inserts could also be employed for interim storage and transport of fissile material in forms other than solutions in 4-{ell} bottles.« less