Transportation of radioactive waste containing organic materials: new methodology based on oxygen consumption

The French Atomic Energy and Alternative Energies Commission carries research programs in various nuclear fields, including radioactive waste management. These activities require transport of different types of radioactive materials with a wide range of transport packages. CEA packaging are designed, constructed and operated to meet safety and regulatory requirements. Application for package approval requires a complete and thorough safety demonstration in which flammable gas generation mechanism have become a major issue in the last few years. Depending on the type of waste and conditioning, CEA has developed different approaches. One of the critical problems concerns organic materials within waste packages and conditioned in closed containers, where gas release is limited by diffusion through a barrier such as a polymer film or a cement matrix. Then, hydrogen produced by organic materials radiolysis accumulates in the interstices between the waste, occurring potential flammable conditions. Most of current safety studies focus only on the production of flammable gas, which is often too conservative: in parallel to hydrogen release, oxygen consumption leads to the depletion of the combustive. CEA laboratories have developed a methodology based on the concomitant production of hydrogen and oxygen consumption by radiolysis of organic materials. This approach was applied to transport packages used for some cemented waste produced in CEA waste treatment facilities. Using a simple mathematical model, a generic non-flammable criterion is deduced. It depends principally of the ratio of the mean hydrogen radiolytic yield to the mean radiochemical yield of oxygen consumption for waste materials. Generally this criterion is rather easy to fulfill for gamma or beta irradiation, but not always for alpha irradiation. A specific experimental program was developed to evaluate hydrogen emission and oxygen consumption for organic materials in contact with alpha particles in intermediate-level long-lived waste. Simulation of alpha particles was performed using high energy swift heavy ions. This presentation will precise situations in which this demonstration applies, present the different hypothesis and justify the choice of irradiation conditions to simulate alpha particles. Finally the effect of parameters such as temperature, dose rate and atmosphere will be discussed.