Evaluation and application of the fission matrix based burnup methodology on the TRIGA reactor

Pungerčič, Anže (author), Snoj, Luka (mentor), Haghighat, Alireza (comentor)

Nuclear fuel depletion calculations for TRIGA research reactors are addressed in the thesis. Complete operational history of the TRIGA Mark II reactor at "Jožef Stefan" Institute was recorded and analyzed, enabling depletion simulations. Burnup effects of the TRIGA fuel were studied using deterministic TRIGLAV and stochastic Serpent-2 codes. Main conclusions are that because the reactor core is small and compact, 3D depletion has to be done, dividing the fuel in at-least 15 axial depletion zones. The analysis also verified an approximation of simulating operational history only after 1991 on P=250 kW. Knowing the depletion history, a novel fission matrix-based burnup methodology bRAPID, based on the RAPID Code System, was applied to the depletion model of JSI TRIGA reactor. The methodology allows for calculation of nuclear fuel depletion by combination and interpolation of RAPID's burnup dependent fission matrix (FM) coefficients to take into account core changes due to burnup. Main part of the thesis is the verification and validation of the bRAPID methodology. Verification was performed by comparison to experimentally validated Serpent-2 Monte Carlo depletion calculations. The results show that the burnup methodology is capable of accurately calculating the k_eff changes with burnup, 3D fission source re-distributions and pin-wise, axially dependent fuel burnup and nuclear fuel nuclide composition. Experimental validation was carried out on three different experiments: excess reactivity measurements, and two fuel rod reactivity worth experiments. For all measured parameters RAPID with its bRAPID methodology was within 1sigma of measured uncertainty, indicating great agreement. Finally, both bRAPID and Serpent-2 were applied for depletion calculations of three experiments at JSI TRIGA in which discrepancies were resolved by taking into account fuel depletion. Results from both codes indicated a better agreement (6 %) in absolute measured values of three dosimetry reactions, showing the importance of knowing TRIGA fuel. Main outcome of the thesis is having an experimentally validated new depletion methodology, which retains the accuracy of the Monte Carlo code with a speed-up factor of 600, if fission matrix database is pre-calculated. Such methodology is capable of spent nuclear fuel characterization on the fly while the reactor is in operation and is applicable to other research, small modular, and micro reactors.

• Language: English
• Keywords: Nuclear Fuel Depletion, Fission Matrix, bRAPID, TRIGA Reactor, Nuclide Inventory, Experimental Validation
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2024
• PID: 20.500.12556/RUL-159277
• COBISS.SI-ID: 200675075
• Publication date in RUL: 05.07.2024

Secondary language

• Language: Slovenian
• Title: Ovrednotenje in uporaba metode fisijskih matrik za izračun zgorelosti goriva na reaktorju TRIGA

Abstract

V disertaciji so naslovljeni izračuni zgorevanja jedrskega goriva v raziskovalnem reaktorju TRIGA. V ta namen je bila zabeležena in analizirana celotna obratovalna zgodovina reaktorja TRIGA Mark II na Institutu "Jožef Stefan". Raziskan je bil vpliv zgorevanja TRIGA goriva z uporabo determinističnega TRIGLAV in stohastičnega programa Serpent-2. Glavni zaključek analize je, da je zaradi kompaktnosti reaktorske sredice, potrebna simulacija v 3D z uporabo vsaj 15 aksialnih zgorevalnih con. Analiza je prav to potrdila predpostavki, da je za natančne rezultate dovolj simulacija zgodovine po letu 1991 na polni moči reaktorja P=250 kW. S pridobljenim znanjem je bila na definiran IJS TRIGA zgorevalni model, aplicirana nova metoda za izračun zgorelosti bRAPID, ki temelji na metodi Fisijskih Matrik (FM) in je del programskega paketa RAPID. Metoda omogoča izračun zgorevanja jedrskega goriva s kombinacijo in interpolacijo zgorevalno odvisnih koeficientov FM, ki opisujejo zgorevalne spremembe v sredici reaktorja. Glavni del disertacije sta bili verifikacija in validacija omenjene metode bRAPID. Verifikacija je bila narejena s primerjavo rezultatov, pridobljenih z že eksperimentalno validirano kodo Serpent-2. Primerjava je pokazala dobro ujemanje pri različnih parametrih, kot so zgorevalne spremembe k_eff, 3D-pre-porazdelitve nevtronov v sredici, 3D-zgorelost in isotopska sestava na ravni aksialno razdeljenih gorivnih elementov. Eksperimentalna validacija je bila narejena na treh različnih eksperimentih: dveh ekperimentalnih meritvah reaktivnostne vrednosti gorivnih elementov in na meritvi presežne reaktivnosti sredice. Vsi izračunani parametri s programom RAPID in z metodo bRAPID so bili znotraj 1sigma izmerjenih negotovosti, kar kaže na odlično ujemanje. Nazadnje sta bila tako bRAPID kot Serpent-2 uporabljena za zgorevalne izračune treh eksperimentov na TRIGI, pri katerih so bile opažene razlike, pojasnjene z upoštevanjem zgorevanja. Rezultati, pridobljeni iz obeh programov za reaktorske preračune, so pokazali boljše ujemanje (približno 6 %) v absolutnih vrednostih meritev iz treh dozimetričnih reakcij. Slednje dokazuje pomembnost poznavanja zgorelosti TRIGA-goriva in vključitev v obstoječe modele. Glavni izid disertacije je eksperimentalno validirana nova metodologija za izračun zgorevanja jedrskega goriva, ki ohranja natančnost Monte Carlo metode s faktorjem pohitritve približno 600, pod pogojem da je podatkovna baza fisijskih matrik že izračunana. Takšna metodologija je sposobna karakterizacije rabljenega jedrskega goriva med delovanjem reaktorja in omogoča aplikacijo na drugih raziskovalnih, malih modularnih in mikro reaktorjih.

• Keywords: zgorevanje jedrskega goriva, fisijske matrike, bRAPID, TRIGA reaktor, izotopska sestava, eksperimentalna validacija