Characterization of gamma field in the JSI TRIGA reactor

Ambrožič, Klemen (author), Snoj, Luka (mentor)

The work presented in this thesis deals with the characterization of gamma field inside a nuclear reactor by experiments and computational modelling. In the first part of the thesis an outline of the nuclear with neutrons and neutron transport. A description of high energy photon and electron reactions and importance of their coupling for accurate calculations of energy deposition. Particle transport equations are presented with emphasis on deriving adjoint operators used for variance reduction of Monte Carlo particle transport codes. Characterization of gamma radiation field using Monte Carlo transport codes only takes into account prompt gamma generation from fission, inelastic scattering and prompt (n,gamma) reactions. Previous evaluations suggest a roughly 30 % underestimation compared to measurements. A JSIR2S code package for delayed radiation field calculations has been developed and validated by numerous experiments. Characterization of neutron and prompt gamma radiation field inside the JSI TRIGA reactor core irradiation facilities was performed using the kerma approximation. The computational model was later expanded and the criticality source term translated to a fixed source for calculations of variance reduction parameters. The methodology has been validated by experiments, showing good agreement for neutrons, while underestimating the gamma field due to neglecting delayed radiation field. Several experimental campaigns were performed at JSI TRIGA reactor using fission and ionization chamber and Thermoluminescent dosimeters. An experimental procedure for estimation of the delayed gamma fraction was developed. Validation of the JSIR2S was performed on the above mentioned measurements, showing agreement within the uncertainty. use case on using the JSIR2S for calibration of semiconductor detectors in the JSI TRIGA reactor after reactor shut-down is described. The JSIR2S code package is also applied to shut-down dose rate calculations in fusion problems showing good agreement with experiments and similar two-step and single-step methodology codes for delayed radiation field characterization.

• Language: English
• Keywords: Delayed radiation, Activation, MCNP, TRIGA, Monte Carlo particle transport, Fission, Fusion, R2S, D1S, Kerma, Dose, Dose equivalent, Fission chamber, Ionization chamber, TLD, Semiconductor dosimetry
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2020
• PID: 20.500.12556/RUL-116136
• COBISS.SI-ID: 16825859
• Publication date in RUL: 17.05.2020

Secondary language

• Language: Slovenian
• Title: Karakterizacija polja žarkov gamma v reaktorju IJS TRIGA

Abstract

Doktorsko delo obravnava eksperimentalno in računsko karakterizacijo polja žarkov gama v jedrskih reaktorjih. V prvem delu predstavimo jedrske reakcije z nevtroni, njihov transport in transport nastalih visoko energijskih fotonov in elektronov ter izpostavili pomembnost njihovega prelivanja za izračune deponirane energije. Predstavimo tudi splošno transportno enačbo s poudarkom na adjungiranih operatorjih, ki se uporabljajo za redukcijo variance v preračunih transporta delcev z metodo Monte Carlo. Pri uporabi metode za Monte Carlo transprot delcev se navadno upoštevajo samo promptne reakcije produkcije žarkov gama kot so fisija, neelastično sipanje in promptne reakcije (n,gamma). Meritve kažejo na 30 % podcenitev izračunov, kar pripisujemo zakasnelemu sevanju. Razvili smo programski paket JSIR2S za izračune polja zakasnelega sevanja. Paket smo validirali na vrsti eksperimentov na reaktorju IJS TRIGA ter na nekaterih fuzijskih eksperimentih za izračun hitrosti doze po zaustavitvi. Začetno karakterizacija polja nevtronov in promptnih žarkov gama v obsevalnih mestih sredice reaktorja IJS TRIGA smo izvedli v približku kerme. Razširjen računski model smo uporabili za izračune sevalnega polja zunaj biološkega ščita reaktorja. Nevtronski izvor smo iz preračunov kritičnosti prevedli na fiksni izvor, ki smo ga uporabili za določanje parametrov redukcije variance. Za polje nevtronov dobimo ujemanje znotraj negotovosti, polje žarkov gama pa podcenimo ker ne upoštevamo polja zakasnelega sevanja. Na reaktorju IJS TRIGA smo izvedli več eksperimentov z meritvami s fisijskimi in ionizacijskimi celicami ter termoluminescentnimi dozimetri. Razvili smo proceduro za eksperimentalno določanje prispevka polja zakasnelih žarkov gama. Pridobljene rezultate smo uporabili za validacijo JSIR2S, kjer dobimo ujemanje znotraj negotovosti. JSIR2S smo uporabili tudi za kalibracijo polprevodniških dozimetrov v polju žarkov gama po zaustavitvi reaktorja IJS TRIGA. JSIR2S smo aplicirali tudi na fuzijske probleme za izračune hitrosti doze po zaustavitvi reaktorja, ki kažejo na dobro ujemanje z eksperimenti oziroma z preračuni s podobnimi programskimi orodji z uporabo eno ali dvo-koračne metode.

• Keywords: Zakasnelo sevanje, Aktivacija, MCNP, TRIGA, Monte Carlo transport delcev, Fisija, Fuzija, R2S, D1S, Kerma, Doza, Dozni ekvivalent, Fisijska celica, Ionizacijska celica, TLD, Dozimetrija s polprevodniki