Subcritical system (target-blanket) for transmutation of the actinides

Abstract

The results published in [1] attest to the fact that the transmutation of Np, Am, and Cm -the most dangerous long-lived a-emitters -can be accomplished quite effectively in a specialized system with a high thermal-neutron flux. The initial data for calculating the capacity of such a system are the amount and composition of the nuclides. These data are given in [2] for different U-Pu fuel cycles.In the present paper we report the results of investigations whose purpose was to determe the basic parameters of a transmutation system for Np, Am, and Cm. The system contains a proton accelerator, a target where high-energy protons and neutrons are converted, a subcritical blanket where multiplication of neutrons occurs by means of fission and transmutation of Np, Am, and Cm, and a system for removing the energy released in the blanket and the target.Such systems must meet requirements which limit the possible design solutions: As a result of the high cost of the accelerator, Np, Am, and Cm, which are produced by several tens of power reactors (BBER-1000), must be transmuted in the same system. For this reason, the thermal power of the blanket should equal several thousands of MW.The high capacity of th e system and the fact that efficient conversion of protons into neutrons must occur in a comparatively small volume mean that a heavy liquid metal or a eutectic alloy of such metals must be used as the target.The irrevocable losses of long-lived radionuclides in the transmutation process must not exceed 0.1% of the radiotoxicity of the actinides which are used to replenish the system. By using liquid fuel these losses can be reduced to a minimum and, which is less important, a high specific load on the fuel, necessary to ensuring low subcriticality, can be achieved.Nuclear plants, specifically, transmutation plants are an inevitable and at the same time the most dangerous location for storing radionuclides. For this reason, for safety purposes, efforts must be made to increase the average neutron flux in the blanket. With the power maintained constant, this will make it possible to decrease the amount of long-lived radionuclides which are constantly present in it. Close to optimal conditions for transmutation of the actinides are achieved with an average thermal neutron flux density in the blanket r > 10 t5 sec-hcm-2. For lower values of r at the start of the transmutation process, the long-lived radiotoxicity continues to increase for a long time ( _> 100 years) up to a comparatively high equilibrium level [1]. Such fluxes can be achieved only by minimizing the amount of construction materials and fission products in the blanket as well as the time during which the liquid fuel is kept outside the irradiation zone [3].In the case of a closed fuel cycle, the amount of plutonium used for transmutation of Np, Am, and Cm should not exceed several percent of the amount generated in power reactors. Otherwise, the cost of transmutation could be excessive, and the number of serviced power reactors with fixed capacity will be sm...