The Effects Of Transmutation Of Tc-99 On The PWR Fuel Cycle

It is urgent to dispose of long-lived high radioactive waste in spent fuel properly. Tc-99 is important long-lived radioactive products in the spent fuel which poses a threat to human ecological environment. Most of the commercial reactors around the world are pressurized water reactors, making Tc-99 transmute in the thermal reactor and studying its transmutation characteristics in pressurized water reactor is a significant issue, which has a great value to research.In this paper, we study the effect of transmutation of Tc-99 in PWR to the reactor core’s life. Firstly, we use the MCNP program to simulate neutron transport and build the model of pressurized water reactor. We observe the change of keff after adding Tc-99 to the fuel, on the basis of that, we study two different conditions that Tc-99 was added to the burnable poison in two different form. One is to mix Tc-99 to the burnable poison homogeneously. Another way is to plate Tc-99 film to the burnable poison rods. We study the impacts of Tc-99’s mass ratio to the keff.Then we use DRAGON+ORIGENS burn-up calculation program to calculate the transmutation rate of Tc-99 in two conditions and the change of reactivity over the core lifetime after 365 days’irradiation.According to the results of simulation, compared with keff of the normal operation of PWR, keff of core of reactor decreases slowly when Tc-99 is added to the fuel, and it increases slowly when Tc-99 is added to the burnable poison. If Tc-99 is coated on the burnable poison rods, with the quantity increased, the keff of the core decreases slowly. By analysis and calculation results, the evolution of Tc-99 in burnable poison of PWR after 365 day, as the increase of the initial loading quantity of the Tc-99, the absolute amount of transmutation of Tc-99 increases at the same time, however, the efficiency decreases. As the mass ratio of the Tc-99 of the burnable poison from 1% to 60%, the anniversary evolution rate decreases from 8.99% to 6.40%. When Tc-99 is mixed uniformly in burnable poison, and the mass ratio is 55%, the mass of Tc-99 is 332.8 kg, the evolution quantity is 21.78 kg in a years; When thickness of the coating layer of Tc-99 is 0.0442 cm on burnable poison rods, the assembly quantity of Tc-99 is 408.43 kg, and the evolution quantity of Tc-99 is 19.17 kg in a year, which approximate equals the amount of output by the core. If Tc-99 is mixed with fuel and coated to burnable poison rods, with the increase of initial load, the excess reactivity reduces, while the initial excess reactivity increased as the increase of Tc-99 loaded in reactor, which means longer life of the core. making fuel element burning more time, helps to improve the economics of the reactor.