Analysis of neutron diagnostic systems in large tokamaks

Štancar, Žiga (author), Snoj, Luka (mentor)

Modern fusion neutronics studies play a crucial role in the support of the development of fusion devices. Their contribution varies from the design of plasma diagnostics systems, fusion power measurements, tritium breeding studies, evaluation of radiation induced structural embrittlement to radiation protection of personnel. Present-day neutron calculations are almost in their entirety based on advanced stochastic neutron transport codes. One of the foundations of these programs is knowledge about the neutron source, in our case a hot plasma. Since uncertainties in basic simulation parameters are being propagated through the system together with the neutrons, there is an ongoing effort of trying to identify and study major uncertainty sources and improving existing physics models for describing the generation of neutrons in a tokamak. The dissertation focuses on the description of the plasma as a neutron source and begins with a study of the state-of-the-art modelling capabilities of neutron emission in tokamak plasmas. The core of the thesis is the description and application of a novel methodology for generation of realistic plasma neutron sources, called PLANET. The methodology is based on calculations of plasma transport with the TRANSP code and neutron spectra with the DRESS code, coupled to the MCNP stochastic neutron transport code. Diagnostic data and modelling results of two representative JET deuterium fuel discharges are used for neutron generation in computational JET models, analysing basic source parameters - emissivity profile, spectra shape, source anisotropy and synthetic detector response. By comparing the realistic source results with a thermal plasma, it is shown that discrepancies of integral neutron detector response, from which the total neutron rate and hence the fusion power is calculated, of up to several percent are computed, exhibiting relatively low sensitivity to changes in the neutron source. The analysis of neutron spectra shows distinct structural characteristics which arise due to the fact that plasma heating and fusion reaction anisotropy are modelled. Material activation studies show that certain threshold reactions yield results of orders of magnitude difference for different neutron source models. The developed plasma neutron source is shown to be applicable to detailed tokamak neutron source effect studies.

• Language: English
• Keywords: plasma transport, neutron spectrum, neutron transport, tokamak, JET, Monte Carlo method, neutron source, TRANSP, DRESS, MCNP, PLANET
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2019
• PID: 20.500.12556/RUL-108043
• COBISS.SI-ID: 3318372
• Publication date in RUL: 13.06.2019

Secondary language

• Language: Slovenian
• Title: Analiza nevtronskih diagnostičnih sistemov v velikih tokamakih

Abstract

Razvoj modernih fuzijskih naprav v veliki meri temelji na nevtronskih raziskavah. Slednje se uporabljajo za razvoj diagnostičnih sistemov, meritve fuzijske moči, napovedi oplojevanja tritija, ocene sevalnih poškodb materialov in obratovanja v skladu z načeli varstva pred sevanji. Novodobni nevtronski preračuni temeljijo predvsem na programih za stohastični transport nevtronov. Eden izmed temeljev stohastičnih kod je opis nevtronskega izvora, v našem primeru vroče plazme. Ker se negotovosti v izvornih simulacijskih parametrih preko transporta nevtronov propagirajo v izračunane fizikalne količine, je del nevtronskih raziskav namenjen identifikaciji in preučevanju večjih izvorov negotovosti in izboljšavi trenutnih fizikalnih modelov, ki opisujejo nastajanje nevtronov v tokamakih. V disertaciji opisujemo plazemske izvore nevtronov, začenši s študijo najmodernejših zmožnosti modeliranja emisije nevtronov iz plazme tokamaka. Glavni del naloge predstavljata opis in uporaba razvite izvirne metodologije PLANET za ustvarjanje realističnih plazemskih izvorov nevtronov. Metodologija temelji na izračunih plazemskega transporta s programom TRANSP in nevtronskih spektrov s programom DRESS, ki so sklopljeni s kodo za stohastični transport nevtronov MCNP. Diagnostične podatke in rezultate modeliranja dveh reprezentativnih devterijevih plazemskih strelov tokamaka JET uporabimo kot osnovo za postopek žreba izvornih nevtronov v Monte Carlo računskih modelih tokamaka JET, nakar preučimo osnovne izvorne parametre - profil izseva, obliko spektrov, anizotropijo izvora in sintetični odziv diagnostike. Primerjave med realističnimi in poenostavljenimi izvori pokažejo nekaj procentne razlike v integralnem odzivu detektorjev in splošno nizko občutljivost na spremembe v izvoru. Zaradi upoštevanja učinkov gretja plazme in anizotropije fuzijskih reakcij v obliki nevtronskih spektrov opazimo značilne fizikalne strukture. Pri študiji aktivacije vzorcev zabeležimo odstopanja izračunanih reakcijskih hitrosti za več redov velikosti, pri uporabi materialov s pragovnimi reakcijami. V disertaciji pokažemo, da so razviti plazemski izvori nevtronov primerni za podrobne analize učinka izvora na odziv detektorjev v tokamakih.

• Keywords: plazemski transport, nevtronski spekter, transport nevtronov, tokamak, JET, metoda Monte Carlo, nevtronski izvor, TRANSP, DRESS, MCNP, PLANET