The proposed cycle is the Th-U one. The main objectives are

1. to incinerate transuranics

2. to transmute a number of fission products.

The very high proposed thermal neutron flux reaches 1016/cm2/s. Neutron multiplication is obtained either by 233U fission, or by fission of the actinides one wants to incinerate: plutonium, americium or curium. 233U is obtained via neutron irradiation of a 232Th blanket, followed by online extraction of

ПЛІ O-J-J

Pa which is allowed to decay into U away from the neutron flux. This is made possible by the use of a molten salt (a mix of fluorides) fuel, similar to that which was used in the Oak Ridge pilot reactor. The liquid fuel circulates continuously through the protactinium extraction facility. In order to limit the number of neutron captures in protactinium, the thorium blanket is placed in a neutron flux limited to a few 1014neutrons/cm2/s. The region of maximum thermal flux is where the actinides are incinerated. Indeed, very high fluxes have the following advantages.

• Reduced lifetime of the actinides in the reactor. The lifetime of 239Pu in a thermal-neutron flux of 1016 neutrons/cm2/s is less than two days.

• Improved neutron balance of the incineration process.

• Small inventory of fissile matter in the system. The quantity of plutonium necessary to produce 3 GW in a flux of 1016 neutrons/cm2/s is as small as 8 kg, with a daily burn-out of 3.5 kg.

Fission product transmutation would be optimal in the epithermal flux region since it is in the resonances that the absorption cross-sections are

maximum. Fission products with a capture cross-section of 1 barn would live 3 years in a 1016 neutrons/cm2/s flux. In order to prevent stable fission products becoming radioactive by neutron capture, an online separation of fission products to be transmuted is necessary.

The advantages mentioned are, of course, counter-balanced by the great complexity of the system:

• An accelerator able to accelerate protons to at least 1 GeV, with intensities larger than 100 mA.

• A subcritical assembly using molten salt fuel. Although tested on a small scale at Oak Ridge, this technique has to demonstrate its resistance to very high fluxes. Corrosion problems may be serious, even if the use of hastalloy (a special nickel alloy) seemed to be satisfactory in the Oak Ridge conditions. One should also note that since the fuel itself circulates in the primary heat exchangers, any intervention on these, often delicate, components would be very difficult if not impossible.

• A complex online chemistry for separation of protactinium and fission products and continuous injection of the fuel.

Figures 12.4 and 12.5 show one of the designs which have been pro­posed, together with a diagram of the chemical processing. They show the complexity of the system which could only be implemented in countries with a very advanced nuclear technology.