magistrsko delo
Domen Kotnik (Author), Luka Snoj (Mentor)

Abstract

V magistrskem delu sta predstavljeni validacija in praktična uporaba pred kratkim razvitega programa ADVANTG, ki združuje Monte Carlo transportni program MCNP z determinističnim transportnim programom Denovo. Cilj programa ADVANTG je avtomatizacija procesa generiranja parametrov redukcije variance (utežna okna) in modifikacija izvora za izračune MCNP, kar posledično pospeši simulacije v smislu računskega časa. Zanesljivost in dosledno delovanje ADVANTG-a smo testirali na dveh računsko zahtevnih referenčnih eksperimentih, izbranih iz zbirke ICSBEP: eksperiment ''Labirint'', s katerim smo analizirali sipanje nevtronov oz. vrednosti nevtronskega fluksa v 3-delnem betonskem labirintu z nevtronskim izvorom \ce{^{252}Cf}, in eksperiment ''Nebni sij'', s katerim smo analizirali sipanje nevtronov in fotonov v zraku nad odprtim delujočim reaktorjem. Učinkovitost posamezne simulacije smo določili s statističnim testom FOM. Z uporabo parametrov redukcije variance, ki smo jih generirali deterministično z ADVANTG, smo znatno pohitrili stohastične simulacije. Z uporabo programa ADVANTG smo za prvi primer eksperimenta ''Labirint'' stohastične simulacije pospešili za vrednost 1400 (nevtroni). Za primer eksperimenta ''Nebnega sija'', s katerim smo obravnavali tako nevtrone kot fotone, pa so ti faktorji pospešitve stohastičnih simulacij znašali 30000 za nevtrone in 1400 za fotone. V obeh primerih so rezultati, pridobljeni s hibridno metodo, ostali znotraj statistične negotovosti analognih simulacij. S tem smo potrdili, da program ADVANTG zgolj pospeši analogne simulacije, pri čemer ne vnese sistematskih napak (ang. bias). Pridobljeno znanje in izkušnje z uporabo ADVANTG-a smo uporabili za analizo realnega primera odlagališča za nizko- in srednjeradioaktivne odpadke (NSRAO), ki se bo gradilo v Sloveniji. Dosedanji izračuni sevalnih doznih polj okoli odlagališča služijo zgolj kot približki/ocene, saj ne upoštevajo vseh učinkov povratnega sipanja v zraku oz. vključujejo zgolj poenostavljene geometrijske modele zaradi zahtevnega izračuna transporta fotonov v betonsko-jeklenih vsebnikih. Program ADVANTG nam tako omogoča izračun brez potrebe po dodatnih poenostavitvah. V optimalnem primeru smo analogne simulacije MCNP pospešili prib. za faktor \mbox{10000}. V sklopu obravnave smo analizirali vpliv različnih konfiguracij odlagališča na vrednosti letnih fotonskih doz za različno oddaljene merilne pozicije na površju. Za vse konfiguracije odlagališča, z izjemo ko upoštevamo NSRAO iz JEK, so bile vrednosti letnih fotonskih doz na račun žarkov $\gamma$ pri vseh merilnih pozicijah manjše od predpisanih vrednosti mejnih doz (delavec: 20 mSv/leto). Za primer odpadkov iz JEK smo naredili dodatno analizo, pri čemer smo podrobneje modelirali vsebnike N2b, v katerih so shranjeni odpadki NSRAO. Z variacijo dodatne količine polnilne malte smo ugotovili potrebno debelino, ki zagotovi, da so vrednosti fotonskih doz nižje od predpisanih mejnih vrednosti. ADVANTG se je izkazal kot izredno močna in zanesljiva metoda, zato bo v kompleksnejših primerih nujno potreben za pridobitev statistično relevantnih rezultatov.

Keywords

jedrska fizika;radioaktivni razpadi;odlagališče NSRAO;transport delcev;

Data

Language: Slovenian
Year of publishing:
Typology: 2.09 - Master's Thesis
Organization: UL FMF - Faculty of Mathematics and Physics
Publisher: [D. Kotnik]
UDC: 621.039
COBISS: 3313508 Link will open in a new window
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Other data

Secondary language: English
Secondary title: Validation and practical use of the ADVANTG code for neutron and photon particle transport
Secondary abstract: In this master thesis a validation and evaluation of the recently released ADVANTG code, which combines a well-known Monte Carlo (MC) transport code MCNP with a deterministic transport code Denovo, is presented. The aim of ADVANTG is to automate the process of generating the variance reduction parameters (weight-windows) for the fixed source MCNP calculations, which consequently accelerate the simulations in terms of the required CPU time. Reliability and consistent performance of the ADVANTG code were tested on two computationally demanding benchmarks from the ICSBEP handbook: the ''Labyrinth'' experiment where neutron fluxes were measured and calculated in a three-section concrete labyrinth for a \ce{^{252}Cf} neutron source and the ''Skyshine'' experiment where neutron and photon scattering in the air above an open operating reactor are simulated. The efficiency of each simulation has been obtained by the statistical test FOM. Compared to the analog MCNP simulations, the speed-up factors or the increases in relative efficiency of up to 1400 (neutrons), for the ''Labyrinth'' case, and up to 30000 (neutrons) and 1400 (photons), for the ''Skyshine'' case, were achieved using the ADVANTG-generated variance reduction parameters. As the mean values obtained by the ADVANTG-accelerated simulation sit within the statistical uncertainties of the analog simulation for both cases, it was shown that no additional bias is introduced by the ADVANTG code. Because of its reliability and consistent performance the ADVANTG code was used to analyse $\gamma$ dose rates from the future Slovenian silo type low and intermediate level waste (LILW) repository. Using ADVANTGs variance reduction technique different detailed calculations of $\gamma$ dose rates were once again achievable in reasonable time. Contrary, all previous calculations used rough geometric models or other different kind of approximation, which did not take into account the effect of backscattering in the air properly. For the optimized ADVANTG settings the maximum speed-ups up to 10000 were achieved compared to the analog MCNP simulation. Within the framework of the study we analysed the influence of different repository configurations on the values of the annual photon dose rates for different measurement positions on the surface. For all configurations of the repository, with the exception of using LILW from NPP Krško, the annual photon dose rates, due to $\gamma$ rays, were for all measurement positions lower than the prescribed limit values (worker: 20 mSv/year). For the latter, an additional analysis was made, where the concrete N2b disposals, in which the LILW is stored, were modelled in more detailed. To ensure that the photon dose rates are lower than the prescribed limit values, the required thickness of the additional grout was determined by varying the additional quantity of the grout. The ADVANTG code has proven to be a powerful and reliable tool for generating effective variance reduction parameters and to greatly accelerate analog MCNP simulations. Therefore, in even more complex cases, it will be necessary or the only option for simulations with the statistically relevant results.
Secondary keywords: nuclear physics;radioactive decays;LILW repository;ADVANTG;
Type (COBISS): Master's thesis/paper
Study programme: 0
Embargo end date (OpenAIRE): 1970-01-01
Thesis comment: Univ. v Ljubljani, Fak. za matematiko in fiziko, Oddelek za fiziko
Pages: XVI, 134 str.
ID: 11144267