CRITICALITY SAFETY OF TRANSURANIC STORAGE ARRAYS AT THE WASTE ISOLATION PILOT-PLANT

The Waste Isolation Pilot Plant (WIPP) facility is designed to store t ransuranic waste that will consist mainly of surface contaminate artic les and sludge. The fissile material in the waste is predominately Pu- 239. The waste is grouped into two categories: contact-handled waste, which will be stored in 55-gal steel drums or in steel boxes, and remo te-handled waste, which will be stored in specially designed cylindric al steel canisters. To show that criticality safety will be acceptable , criticality analyses were performed to demonstrate that a large numb er of containers with limiting loadings of fissile material could be s tored at the site and meet a k(eff) limit of 0.95. Criticality analyse s based on the classic worst-case moderated plutonium sphere approach would severely limit the capacity for storage of waste at the facility . Therefore, these analyses use realistic or credible worst-case assum ptions to better represent the actual storage situation without compro mising the margin of safety. Numerous sensitivity studies were perform ed to determine the importance of various parameters on the criticalit y of the configuration. It was determined that the plutonium loading h as the dominant effect on the system reactivity. Nearly all other reac tivity variations from the sensitivity studies were found to be relati vely small. The analysis shows that criticality of the contact-handled waste storage drums and boxes and the remote-handled canisters is pre vented by restrictions on maximum fissile loading per container and on the size of handling/storage areas. Analysis has shown that the k(eff ) limit of 0. 95 is met with the storage of large loadings of fissile material at the WIPP facility, while credible or realistic assumptions still pro vide a large degree of conservatism.