Control Rod Homogenization in Heterogeneous Sodium-Cooled Fast Reactors

• The sodium-cooled fast reactor is one of the candidates for a sustainable nuclearreactor system. In particular, the French ASTRID project employs an axiallyheterogeneous design, proposed in the so-called CFV (low sodium effect) core, toenhance the inherent safety features of the reactor.This thesis focuses on the accurate modeling of the control rods, through thehomogenization method. The control rods in a sodium-cooled fast reactor are usedfor reactivity compensation during the cycle, power shaping, and to shutdownthe reactor. In previous control rod homogenization procedures, only a radialdescription of the geometry was implemented, hence the axially heterogeneousfeatures of the CFV core could not be taken into account. This thesis investigatesthe different axial variations the control rod experiences in a CFV core, to determinethe impact that these axial environments have on the control rod modeling.The methodology used in this work is based on previous homogenizationprocedures, the so-called equivalence procedure. The procedure was newly implementedin the PARIS code system in order to be able to use 3D geometries, andthereby be take axial effects into account.The thesis is divided into three parts. The first part investigates the impactof different neutron spectra on the homogeneous control-rod cross sections. Thesecond part investigates the cases where the traditional radial control-rod homogenizationprocedure is no longer applicable in the CFV core, which was found tobe 5-10 cm away from any material interface.In the third part, based on the results from the second part, a 3D model of thecontrol rod is used to calculate homogenized control-rod cross sections. In a fullcore model, a study is made to investigate the impact these axial effects have oncontrol rod-related core parameters, such as the control rod worth, the capturerates in the control rod, and the power in the adjacent fuel assemblies. All resultswere compared to a Monte Carlo-based model which served as the reference.It was demonstrated that the treatment of the radial environment, surroundingthe control rod, has a small impact on the core parameters, and the traditionalradial homogenization procedure yields reliable results. For axial interfaces withinthe control rod itself, the traditional radial homogenization procedure could nolonger capture the large environmental impact, hence 3D modeling is recommended.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Annan teknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Other Engineering and Technologies (hsv//eng)

Nyckelord

Homogenization

Monte Carlo

Fast Reactors

Reactivity Equivalence

Control rod

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