Ultra thin elements in nuclear reactors

There is a growing interest (14) in using 242mAm as a nuclear fuel. The advantages of 242mAm as a nuclear fuel derive from the fact that 242mAm has the highest thermal fission cross section. The thermal capture cross section is relatively low and the number of neutrons per thermal fission is high. These nuclear properties make it possible to obtain nuclear criticality with ultra thin fuel elements. The possibility of having ultra thin fuel elements enables the use of these fission products directly, without the necessity of converting their energy to heat, as is done in conventional reactors. There are three options of using such highly energetic and highly ionized fission products: a. Using the fission products themselves for ionic propulsion. b. Using the fission products in an MHD generator, in order to obtain electricity directly. c. Using the fission products to heat a gas up to a high temperature for propulsion purposes. n this work we are not dealing with a specific reactor design, but only calculating the minimal fuel elements' thickness and the energy of the fission products emerging from these fuel elements. Our reactor is composed of an ultra thin fuel element and a Be O moderator on both sides, a unit cell is composed from 242mAm fuel and 20 cm of Be O as the moderator on both sides of the fuel element. The geometry is an infinite slab one. The criticality (kα = 1) calculations were performed with the one-dimensional unit cell calculation code BOXER (9). The neutron spectrum was calculated in 70 energy groups and transport calculations were performed in 36 energy groups. The results obtained are ultra thin fuel elements of 2.5μ (2.5 x 10-6 m). In a more practical design, a working volume must be introduced. As a result, we will have two layers of fuel, each about 1.25 μ (1.25 x 10-6 m). With such a thin fuel, many of these fission products can reach the working zone