Analysis of neutron induced water activation

Kotnik, Domen (author), Lengar, Igor (mentor), Snoj, Luka (comentor)

Water as a primary coolant will play an important role in the performance of fusion reactors, as after being irradiated and activated, it causes an ionising radiation field throughout the facility. Therefore, additional protection and shielding for the instruments and personnel must be adequately considered. As part of this dissertation and in direct support of ITER, the KATANA irradiation facility, which utilises a closed-water activation loop, was designed, constructed, experimentally evaluated and studied, and successfully commissioned in 2024 at the TRIGA Mark II research reactor at the Jožef Stefan Institute in Slovenia. In this dissertation, a comprehensive overview of the KATANA, which serves as a well-defined and stable high-energy (6 MeV - 7 MeV) gamma and ∼1 MeV neutron source, is given, including the design optimisation analysis, final design, experimental set-up, detector systems. The physical characteristics of KATANA were assessed by analyses of neutron transport simulations leading to activity calculations in connection with a conventional analytical approach, without using CFD calculations. To complement the computational analyses, a series of first experiments was performed to determine the operational characteristics of the KATANA, i.e. characterisation of the neutron flux within the irradiation component, dose rate measurements, spectrum characterisation of the activated water, and response to a change in reactor power and flow rate. Since the performance of the pump and thus the flow rate was gradually improved several times during the commissioning phase, not all experiments were carried out under the same conditions. KATANA facility demonstrated the desired operational characteristics in terms of high and stable water flow rates and high activity values of the observed isotopes 16N, 17N and 19O, which is essential for minimising the experimental uncertainties. The calculations and first experiments performed at KATANA provided in-depth knowledge into the operation and capabilities of the facility and essential data to serve as a basis for further, more detailed experiments. The ultimate goal of KATANA is to perform benchmark-quality experiments, e.g. validation of fluid activation codes, and to establish itself as a reference facility for the calibration of high-energy gamma-ray detectors, which will significantly support the operation of ITER and other future water-cooled fusion reactors. The work has also been done within the framework of EUROfusion as part of the work package Preparation of ITER Operation.

• Language: English
• Keywords: water activation, 16N, 17N, 19O, KATANA facility, TRIGA reactor, water activation experiments, MCNP, Fusion, ITER, particle transport
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2025
• PID: 20.500.12556/RUL-169033
• COBISS.SI-ID: 236161027
• Publication date in RUL: 08.05.2025

Secondary language

• Language: Slovenian
• Title: Analiza aktivacije vode po obsevanju z nevtroni

Abstract

Voda kot primarno hladilno sredstvo bo imela pomembno vlogo pri delovanju fuzijskih reaktorjev, imenovanih tokamakih, saj se po obsevanju z nevtroni aktivira, tj. postane radioaktivna, pri čemer se izsevajo žarki gama visokih energij in nevtroni. Tako povzroča ionizirajoče sevanje okoli vodnega hladilnega sistema. To lahko segreje superprevodne tuljave, poškoduje elektroniko ali izpostavi ljudi sevanju. To sevanje je eden glavnih virov ionizirajočega sevanja izven biološkega ščita med delovanjem tokamaka, zato je ključnega pomena dobro poznavanje tega pojava, pri čemer je omenjena orodja za simulacijo in izračun ravni sevanja okoli aktivirane vode nujno treba validirati (preveriti) z eksperimenti. V okviru doktorske disertacije in neposredne podpore fuzijskemu reaktorju ITER je bila na reaktorju TRIGA na Inštitutu Jožef Stefan zasnovana, proučena, zgrajena, eksperimentalno preverjena in in proti koncu leta 2023 uspešno zagnana nova naprava, imenovana KATANA, z zaprto vodno zanko za aktivacijo vode. Obsevalna naprava KATANA služi kot dobro definiran in stabilen vir žarkov gama visokih energij (6 MeV – 7 MeV) ter nevtronov in omogoča izvajanje številnih eksperimentov, povezanih z aktivirano vodo. V disertaciji je podan celovit pregled obsevalne naprave KATANA, ki vključuje predanalize za optimizacijo zasnove, podroben opis končne zasnove, eksperimentalno postavitev ter sisteme detektorjev. Fizikalne lastnosti KATANE so bile ocenjene z analizami simulacij transporta nevtronov, ki so vodile do izračunov aktivnosti. V izračunih je bil uporabljen konvencionalni analitični pristop brez upoštevanjem izračunov dinamike tekočin (CFD). Kot podpora računskim analizam in za določitev operativnih značilnosti KATANE je bila izvedena serija prvih eksperimentov, tj. karakterizacija nevtronskega fluksa znotraj notranjega obsevalnega Polža, meritve hitrosti doznih polj v okolici naprave, karakterizacija spektra aktivirane vode in odziv KATANE na spremembo moči reaktorja ter odziv na spremembo pretoka. Ker se je zmogljivost črpalke in s tem maksimalni dosežen pretok med fazo zagona postopoma izboljševal, vsi eksperimenti niso bili izvedeni pod enakimi pogoji. KATANA je pokazala želene operativne značilnosti v smislu visokih in stabilnih pretokov vode ter visokih aktivnosti opazovanih izotopov 16N, 17N in 19O, kar je bistveno za zmanjšanje eksperimentalnih negotovosti. Izračuni in prvi eksperimenti, izvedeni na KATANI, so zagotovili poglobljeno znanje o delovanju in zmogljivostih obsevalne naprave ter bistvene podatke, ki služijo kot osnova za nadaljnje, podrobnejše eksperimente. Končni cilj KATANE je izvajanje referenčnih eksperimentov, tj. eksperimentalna validacija računskih orodij za izračun sevalnega polja v okolici hladilnega sistema, in uveljavitev kot referenčni objekt za kalibracijo detektorjev žarkov gama pri visokih energijah, kar bo znatno podprlo obratovanje ITER in DEMO ter drugih prihodnjih fuzijskih reaktorjev, hlajenih z vodo.

• Keywords: aktivacija vode, 16N, 17N, 19O, KATANA, TRIGA, eksperimenti z aktivirano vodo, MCNP, fuzija, ITER, transport delcev