Stopping Of Fission Fragments Research Articles

On the Behavior of Uranium Dioxide Toward Aqueous Corrosion: Incidence of Electronic Stopping of Fission Fragments

ABSTRACTThe scenario of spent fuel direct storage in a deep geological repository is currently under consideration in many countries. In this approach the nuclear fuel — mainly uranium dioxide —acts as a first barrier towards fission fragment and actinide release. Although the access of water in such a repository is rather unlikely, evaluating the safety of a disposal requires the knowledge of the mechanisms and kinetics of alteration and dissolution of the nuclear fuel in groundwaters. Despite the fact that the reported litterature on the topic is really extensive, it is still a challenging problem due to the complexity of the system: the used nuclear fuel cannot identify with uranium dioxide itself due to both radiation damage and modification of the matrix composition during reactor operation. Parametric studies in well-defined conditions on simple systems are therefore of prime importance in order to determine the relevant parameters involved in the structural transformations of the uranium-oxygen-water systenL The aim of the present work was to study the incidence of the electronic stopping of fission fragments on the behavior of nuclear fuel towards leaching in deionized water under oxidative conditions. Several features were of special interest: (i) quantification of the damage induced by swift-heavy ion irradiation prior to aqueous corrosion; (ii) stoichiometry and oxygen depth concentration of the alterated surface region; (iii) structural transformation of the crystalline structure induced by incorporation of additional O atoms; (iv) comparison between unirradiated and irradiated samples

Z2-oscillation systematics in the stopping of fission fragments

The initial energies associated with fission fragments relate them to the medium velocity region of the order of v 0Z 1 2 3 in which most of the fragment energy is lost by “electronic” stopping. Theoretical models for electronic stopping in this velocity region are based on a statistical treatment of the atoms. Thus effects due to the electronic shell structure of the stopping medium cannot be reproduced and the resulting stopping parameters are monotonically dependent of Z 2. Range and energy loss determinations offission fragments reveal a pronounced non-monotonic dependence on Z 2. Based on these results an attempt is made to determine the systematics of the Z 2-oscillations in the stopping process of fission fragments. Systematic analysis of a large number of experimental results pointed out the dependence of the stopping process on several parameters characterizing both the fragment and the target. A spectral analysis was performed and typical periodicities in the Z 2-oscillations determined. After performing a sensitivity analysis a semiempirical modified LSS-expression for the calculation of fission fragment ranges was developed which includes the Z 2-oscillations. This expression has been evaluated and compared to the original LSS-theory by means of the Theory of Information.

Defect-annealing in fission-fragment-damaged UO 2-SiO 2 system

A study was made of the radiation-induced defects annealing in the UO 2-SiO 2 system for 20–60 mol% UO 2. It was found that more than 30% of defects remain after athermal recombination and the remaining defects anneal by a thermally activated processes at temperatures up to about 750 °C, with an activation energy of about 3 eV. The defects formed in UO 2 crystallites and in SiO 2 containing dissolved UO 2 are most probably oxygen vacancies and molecular oxygen respectively. In SiO 2 containing dissolved UO 2, besides these defects there appear unbonded U atoms and also atoms resulting from stopping of fission fragments.

Influence of nuclear radiation on the electrical conductivity and thermo-electric power of the UO 2-SiO 2 system

A study is made of the effect of fission fragments on the electrical conductivity and thermo-emf of UO 2-SiO 2 system in the range 10–40 mol % UO 2. It is found that the conductivity decreases and that the thermo-emf increases after an irradiation of about 10 16 fiss/cm 3. After annealing, it is noticed that the values of electrical properties do not coincide with those before irradiation. The conductivity increases and the thermo-emf decreases. This is explained by assuming that the defects introduced into the UO 2-SiO 2 system are of two types: (a) Frenkel pairs and (b) uranium atoms and other atoms resulting from stopping of fission fragments in SiO 2 containing dissolved UO 2. Defects of the first type act as trapping centres for holes and the second type as acceptors.

Ionisierung innerer Elektronenschalen bei der Abbremsung von Spaltprodukten

The subject of the present experiment was to investigate the ionisation of the inner electronic shells following the stopping of fission fragments in various materials. The ionisation in the inner shells of the stopping material as well as of the fission fragments has been detected. The measured X-ray quanta have energies from 2 kev up to 11.5 kev. The data give evidence of the ionisation crossection and its dependance from the velocity of the fission fragments. The crossection varies appreciably with the ionisation energy. It has a value ranging from (10−17 to 10−23) cm2. Formvar as stopping material shows an ionisation crossection which increases with the tenth power of the relative velocity of the colliding particles.

Scattering and Stopping of Fission Fragments

Scattering and Stopping of Fission Fragments