Nuclear waste management represents the main problem related to nuclear power generation. The Swedish project for an ultimate nuclear waste repos- itory is in a very advanced state and cast iron-copper canisters have been designed with the aim of isolating the spent fuel from the environment for at least 105 years. Neutrons and gamma radiation emitted by the fuel may induce unexpected embrittlement and impair the container integrity through formation of point defect clusters and consequent copper precip- itation. Gamma radiation induces displacements by interacting with the container’s metal lattice and creating high energy electrons. To study this container embrittlement, irradiation experiments that make use of high en- ergy electron beams are required. In order to reproduce the experimental arrangement, a Monte Carlo code named ElectronDamage was developed. The code computes the damage rate induced by a collimated high-energy electron beam on one-dimensional metallic samples and also provides for the damage distribution as function of depth. The particle energy loss along its trajectory is computed by means of the Bethe-Bloch formula. The scattering cross section and the stopping power are assumed to be constant along the free path. Collisions against the lattice nuclei are modelled by randomly generating the scattering angle, according to the differential scattering cross section. A cutoff angle has to be chosen in order to make the cross section function limited and allow for the construction of a PDF. At every collision the exchanged energy is computed. A sharp displacement energy threshold is assumed and the Kinchin-Pease formula is used for the secondary displacement computation. The energy threshold to produce displacements by electrons is 630 keV in iron. Test simulations were performed in order to validate the code. Sim- ulated electron ranges and backscattering coefficients were computed for several electron energies and compared with theoretical predictions or semi- experimental values. The comparison was successful, although it showed that the code is not suitable for beams below 500 keV. Subsequently, a few relevant energies were selected with the aim of reproducing the gamma spectrum emitted by real spent fuel samples. The total damage rates were computed (in displacements/electron unit) and converted into dpa/year; considerable contributions are given by gamma fluxes whose energy is lower than 2 MeV. The damage rates provided by the literature were satisfactorily close to the computed ones. Displacement distributions presented a peak, which is located near the irradiated surface and moves inwards as the elec- tron energy increases. Finally, a parametric study of the cutoff value showed that its choice does not induce any effect on the total damage rate, whereas it causes a shrinkage of the distribution toward the irradiated surface and a slight increase of the peak value.
| Identifer | oai:union.ndltd.org:UPSALLA1/oai:DiVA.org:kth-34288 |
| Date | January 2010 |
| Creators | Messina, Luca |
| Publisher | KTH, Reaktorfysik |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Student thesis, info:eu-repo/semantics/bachelorThesis, text |
| Format | application/pdf |
| Rights | info:eu-repo/semantics/openAccess |
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