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Response Matrix Reloaded: for Monte Carlo Simulations in Reactor Physics
KTH, School of Engineering Sciences (SCI), Physics, Nuclear Engineering.ORCID iD: 0000-0003-4878-6711
2019 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis investigates Monte Carlo methods applied to criticality and time-dependent problems in reactor physics. Due to their accuracy and flexibility, Monte Carlo methods are considered as a “gold standard” in reactor physics calculations. However, the benefits come at a significant computing cost. Despite the continuous rise in easily accessible computing power, a brute-force Monte Carlo calculation of some problems is still beyond the reach of routine reactor physics analyses. The two papers on which this thesis is based try to address the computing cost issue, by proposing methods for performing Monte Carlo reactor physics calculations more efficiently. The first method addresses the efficiency of the widely-used k-eigenvalue Monte Carlo criticality calculations. It suggests, that the calculation efficiency can be increased through a gradual increase of the neutron population size simulated during each criticality cycle, and proposes a way to determine the optimal neutron population size. The second method addresses the application of Monte Carlo calculations to reactor transient problems. While reactor transient calculations can, in principle, be performed using only Monte Carlo methods, such calculations take multiple thousands of CPU hours for calculating several seconds of a transient. The proposed method offers a middle-ground approach, using a hybrid stochastic-deterministic scheme based on the response matrix formalism. Previously, the response matrix formalism was mainly considered for steady-state problems, with limited application to time-dependent problems. This thesis proposes a novel way of using information from Monte Carlo criticality calculations for solving time-dependent problems via the response matrix.

 

Abstract [sv]

Denna avhandling undersöker Monte Carlo-metoder som används för kritikalitets- och tidsberoende problem i reaktorfysik. På grund av deras noggrannhet och flexibilitet betraktas Monte Carlo-metoder som en ‘gyllene standard’ i reaktorfysikberäkningar. Fördelarna kommer dock till priset av betydande datorkostnad. Trots den kontinuerliga ökningen av lättillgänglig datorkraft är en råstyrka Monte Carlo-beräkningar av vissa problem fortfarande utanför räckvidden för reaktorfysikaliska rutinanalyser. De två artiklarna som denna avhandling bygger på försöker ta itu med beräkningskostnadsproblemet genom att föreslå metoder för att utföra Monte Carlo-reaktorfysikberäkningar mer effektivt. Den första metoden behandlar effektiviteten för de vitt använda beräkningarna av k-egenvärdet med Monte Carlo. Den antyder att beräkningseffektiviteten kan ökas genom en gradvis ökning av neutronpopulationens storlek som simuleras under varje kritikalitetscykel, och föreslår ett sätt att bestämma den optimala neutronpopulationens storlek. Den andra metoden behandlar tillämpningen av Monte Carlo-beräkningar för reaktortransienter. Medan beräkningar av reaktortransienter i princip kan utföras uteslutande med Monte Carlo-metoder, tar sådana beräkningar flera tusentals CPU-timmar för att beräkna flera sekunder av en transient. Den föreslagna metoden erbjuder en medelväg, med användning av ett stokastiskt-deterministiskt hybridschema baserat på responsmatrisformalismen. Tidigare har responsmatrisformalismen huvudsakligen beaktats för tidsoberoende problem, med begränsad tillämpning på tidsberoende problem. Denna avhandling föreslår ett nytt sätt att använda information från Monte Carlo-kritikalitetsberäkningar för att lösa tidsberoende problem via responsmatrisen.

 

 

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2019. , p. 45
Series
TRITA-SCI-FOU ; 54
National Category
Energy Engineering
Research subject
Physics, Nuclear Engineering
Identifiers
URN: urn:nbn:se:kth:diva-263412ISBN: 978-91-7873-384-2 (print)OAI: oai:DiVA.org:kth-263412DiVA, id: diva2:1368225
Presentation
2019-12-10, FB54, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 14:00 (English)
Opponent
Supervisors
Note

Examinator: Professor Pär Olsson

Available from: 2019-11-06 Created: 2019-11-06 Last updated: 2019-11-07Bibliographically approved
List of papers
1. Optimal neutron population growth in accelerated Monte Carlo criticality calculations
Open this publication in new window or tab >>Optimal neutron population growth in accelerated Monte Carlo criticality calculations
2018 (English)In: Annals of Nuclear Energy, ISSN 0306-4549, E-ISSN 1873-2100, Vol. 117, p. 297-304Article in journal (Refereed) Published
Abstract [en]

We present a source convergence acceleration method for Monte Carlo criticality calculations. The method gradually increases the neutron population size over the successive inactive as well as active criticality cycles. This helps to iterate the fission source faster at the beginning of the simulation where the source may contain large errors coming from the initial cycle; and, as the neutron population size grows over the cycles, the bias in the source gets reduced. Unlike previously suggested acceleration methods that aim at optimisation of the neutron population size, the new method does not have any significant computing overhead, and moreover it can be easily implemented into existing Monte Carlo criticality codes. The effectiveness of the method is demonstrated on a number of PWR full-core criticality calculations using a modified SERPENT 2 code.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Monte Carlo criticality; Fission source; Convergence; Bias; Efficiency
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-225829 (URN)10.1016/j.anucene.2018.03.046 (DOI)000431469900029 ()2-s2.0-85044790843 (Scopus ID)
Note

QC 20180412

Available from: 2018-04-09 Created: 2018-04-09 Last updated: 2019-11-06Bibliographically approved
2. Stochastic-deterministic response matrix method for reactor transients
Open this publication in new window or tab >>Stochastic-deterministic response matrix method for reactor transients
2019 (English)In: Annals of Nuclear Energy, ISSN 0306-4549, E-ISSN 1873-2100, article id 107103Article in journal (Refereed) In press
Abstract [en]

Presented is a stochastic-deterministic, response matrix method for transient analyses of nuclear systems. The method is based on the response matrix formalism, which describes a system by a set of response functions. We propose an approach in which these response functions are computed during a set of Monte Carlo criticality calculations and are later used to formulate a deterministic set of equations for solving a space-time dependent problem. Application of the response matrix formalism results in a set of loosely connected equations, which leads to a favourable linear scaling of the problem. The method offers a simplified approach compared to previously proposed response matrix methods by avoiding phase-space expansions in sets of basis functions. We describe the method starting with the fundamental neutron transport considerations, provide a demonstration on two absorber movement transients in a 3 × 3 assembly PWR mini-core geometry, and compare the solutions against time-dependent Monte Carlo simulations.

Place, publisher, year, edition, pages
Elsevier, 2019
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-263309 (URN)10.1016/j.anucene.2019.107103 (DOI)2-s2.0-85073812774 (Scopus ID)
Note

QC 20191108

Available from: 2019-11-05 Created: 2019-11-05 Last updated: 2019-11-08Bibliographically approved

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