Brazil: FBNR [Implementation of Proliferation Resistance and Physical Protection Measures in Innovative Small and Medium Sized Reactors]

The FBNR is a small reactor of 70 MW(e) with no requirement for on-site refuelling. It is a pressurized LWR with spherical fuel elements. The reactor is modular in design, and each module is assumed to be fuelled at the factory.The distinguishing feature of the FBNR in terms of non-proliferation is that under shutdown conditions, all fuel elements remain in the fuel chamber, where only a single flange needs to be sealed and controlled for safeguards purposes. The spent fuel of the FBNR is in a form that can be used directly as a source of radiation for irradiation purposes. The concept is based on both sealing the fuel chamber and denaturing the fuel itself. The sealing of the fuel in the fuel chamber, accompanied by surveillance, enables continuous inspection of the fuel — guaranteeing effective control. The isotopic denaturing of the fissile fuel, both in the 233U–thorium cycle as well as in the classical uranium–plutonium cycle, would further increase the proliferation resistance, as it will require isotope or chemical separation technology to produce weapons-grade materials. Adopting a thorium cycle as an intrinsic measure will hinder the possibility of misuse of nuclear materials for nuclear weapons. The mixing of thorium with LEU or plutonium results in the production of 233U, which is diluted along with 235U in 238U. The access to 233U is only possible through isotope separation techniques. In addition, the production of gamma emitting 208Tl in the thorium cycle is a hindrance to nuclear proliferation. If the uranium– plutonium cycle is applied, one can increase the 238Pu concentration by adding neptunium to the fresh fuel. Starting from a certain concentration of 238Pu (around 8%), the alpha decay heat is so strong that the metallic plutonium sphere in a nuclear device, as well as the surrounding chemical explosives, becomes plastic or even melts. This effect means that the fuel of the reactor at any time is less useful for weapons. In any event, LWRs tend to create the less desirable plutonium isotopes over time as the reactor operates normally, thus tending to denature the plutonium as it is created. Thus, the combination of sealing the reactor and the isotopic denaturing of the irradiated fuel will also increase proliferation resistance