Transmutation of minor actinides for restriction of radiotoxicity accumulation in long-term storage

Results of calculation study of the transmutation mode in power reactors (PWR, PHWR (CANDU), and Super-Phenix) are presented. The concept of preliminary transmutation of minor actinides before placement to the long-term storage is considered. The purpose of such preliminary transmutation before ultimate storage is to incinerate a part of actinides and to transform another part into new actinides providing low level of radiotoxicity accumulated in the storage. We show that the rate of reactions in PHWR type reactor is higher than in reactor with fast neutron spectrum. This fact is in agreement with common results obtained for continuous transmutation. In T 10 years, the radiotoxicity of actinides irradiated in VVER and PHWR type reactors is close to initial radiotoxicity, and the mass of actinides decreases by 3-4 times. In reactor with fast neutron spectrum, the similar effect can be achieved only in about 20 years. The radiotoxicity of actinides after 10-year transmutation in PHWR type reactor is determined by 244Cm. At irradiation of actinides in VVER and Super-Phenix type reactors, the radiotoxicity is determined by two isotopes: 244Cm and 238Pu. The importance of isotopes depends on neutron flux and spectrum. Most preferable seems to be the transmutation in PHWR type reactor with thermal neutron spectrum and rather high neutron flux. These results demonstrate that with preliminary transmutation in PHWR type reactor, the time required to reach an equilibrium in storage is about 50 years. Analogous time with preliminary transmutation in Superphenix type reactor makes about 500 years since the main nuclide in this case is 238Pu with half-life 87 years. Thus, by the process of preliminary transmutation during 10 years in PHWR type reactor, we replace the main isotope 241Am (half-life 432 years) in actinides transferred to the storage with main isotope 244Cm (18 years). We reduce by 24 times the half-life of the main isotope without increasing radiotoxicity. We thereby achieve the main purpose of transmutation - to obtain acceptable time in which equilibrium radiotoxicity in storage is reached, that is when the growth of radiotoxicity ceases