Effect of hydrogen distribution in UZrHx on physical parameters of JSI TRIGA reactor

Sánchez Sanz, María (avtor), Snoj, Luka (mentor), Suikkanen, Heikki (komentor)

One potential issue with uranium-zirconium hydride (U-ZrH) fuel is that the hydrogen redistributes in the hydride when thermal and concentration gradients are present. The spatial hydrogen distribution within the fuel element depends on the history of operation and temperature profile. After a long period of time there is theoretical possibility that hydrogen might diffuse out of the fuel, thus affecting neutron transport in the reactor. This masters thesis aims to gain a better understanding of how reactor physical parameters of the TRIGA (Training, Research, Isotope Production General Atomic) Mark II research reactor at the Jo\v{z}ef Stefan Institute (JSI) are affected by such hydrogen redistributions and to study the physical phenomena responsible for these changes. For this purpose two different models were created with OpenMC Monte Carlo program, one containing the whole reactor, denoted as Reactor Core model, and another containing just a fuel cell surrounded by water without leakage, denoted as Unit Cell model. In this model, the amount of water surrounding the fuel cell corresponds to the water around the fuel element if you evenly distribute all water in the core to each fuel rod. Different homogeneous and heterogeneous hydrogen distributions within the fuel were applied in those models. Effective neutron multiplication factor, neutron flux spectrum, reactivity and effective delayed neutron fraction were calculated for all the cases. Firstly, homogeneous hydrogen distributions were applied showing that an homogeneously decrease in the hydrogen content in all the fuel rods of the reactor results in a decrease in the multiplication factor, while if one considers a fuel rod surrounded by water that has no leakage, multiplication factor increases as the hydrogen content decreases. It was concluded that whereas Rector Core model system is undermoderated, Unit Cell model system is overmoderated. Secondly, different heterogeneous hydrogen distributions, axial and radial ones, were applied in the two models created. From results obtained for both models, it is observed that axially, the most sensitive zone of the fuel to hydrogen distributions is the central part of the fuel where the flux is the largest. Its hydrogen concentration affects reactor physical parameters while the concentration at the edges of the fuel practically does not affect that much reactor physical parameters. Regarding the effect of hydrogen content in kinetic parameters of the reactor, it is concluded that the effective delayed neutron fraction does not depend significantly on the hydrogen content within the fuel.

• Jezik: Angleški jezik
• Ključne besede: TRIGA Mark II, hydrogen distribution, multiplication factor, uranium, zirconium hydride, neutron flux, OpenMC, Monte Carlo method
• Vrsta gradiva: Magistrsko delo/naloga
• Tipologija: 2.09 - Magistrsko delo
• Organizacija: FMF - Fakulteta za matematiko in fiziko
• Leto izida: 2023
• PID: 20.500.12556/RUL-149104
• COBISS.SI-ID: 163201539
• Datum objave v RUL: 03.09.2023

Sekundarni jezik

• Jezik: Slovenski jezik
• Naslov: Vpliv porazdelitve vodika v UZrHx na fizikalne parametre reaktorja TRIGA na IJS

Izvleček

Ena od možnih težav z uranovo-cirkonijevim hidridnim gorivom (U-ZrH) je, da se vodik prerazporedi v hidridu, ko so prisotni termični in koncentracijski gradienti. Prostorska porazdelitev vodika v gorivnem elementu je odvisna od zgodovine delovanja in temperaturnega profila. Po daljšem časovnem obdobju obstaja teoretična možnost, da bi se vodik lahko razpršil iz goriva in tako vplival na transport nevtronov v reaktorju. Namen magistrske naloge je pridobiti boljše razumevanje, kako na fizikalne parametre reaktorja TRIGA (Training, Research, Isotope Production General Atomic) Mark II raziskovalnega reaktorja na Institutu Jožef Stefan (JSI) vplivajo tovrstne prerazporeditve vodika in preučiti fizikalne pojave, ki so odgovorni za te spremembe. V ta namen sta bila izdelana dva različna modela s programom OpenMC Monte Carlo, eden vsebuje celoten reaktor, označen kot model reaktorskega jedra, drugi pa vsebuje samo gorivno celico, obdano z vodo brez puščanja, označeno kot model z enoto celice. V tem modelu količina vode, ki obdaja gorivno celico, ustreza vodi okoli gorivnega elementa, če enakomerno porazdelite vso vodo v jedru na vsako gorivno palico. Za vse primere so bili izračunani efektivni nevtronski multiplikacijski faktor, spekter nevtronskega toka, reaktivnost in efektivna zapoznela nevtronska frakcija. Prvič, homogene porazdelitve vodika so bile uporabljene, kar kaže, da homogeno zmanjšanje vsebnosti vodika v vseh gorivnih palicah reaktorja povzroči zmanjšanje faktorja množenja, medtem ko če upoštevamo gorivno palico, obdano z vodo, ki nima puščanja, se faktor množenja poveča, ko se vsebnost vodika zmanjša. Ugotovljeno je bilo, da je, medtem ko je model sistema, enotske celice premoderiran. V obeh modelih so bile uporabljene različne heterogene porazdelitve vodika, aksialne in radialne. Iz rezultatov, pridobljenih za oba modela, je razvidno, da je osno najbolj občutljiva cona goriva na porazdelitev vodika osrednji del goriva, kjer je fluks nevtronov največji. Njegova koncentracija vodika vpliva na fizikalne parametre reaktorja, medtem ko koncentracija na robovih goriva praktično ne vpliva na te fizikalne parametre reaktorja. Glede vpliva vsebnosti vodika v kinetičnih parametrih reaktorja je bilo ugotovljeno, da efektivni delež zakanselih nevtronov ni bistveno odvisen od vsebnosti vodika v gorivu.

• Ključne besede: porazdelitev vodika, faktor množenja, uran, cirkonijev hidrid, nevtronski tok, OpenMC, metoda Monte Carlo