VTT TECHNICAL RESEARCH CENTRE OF FINLAND LTD

PALAMA

SAFIR2014 Final seminar

19th – 20th March 2015

Ville Tulkki

2 2 17/04/2015

Polttoaineen laaja-alainen mallinnus

PALAMA

Project focused on analysis on

nuclear fuel behaviour

4 years, 12.1 person-years, 14

researchers, 1.45 M€

Deliverables include 36

technical reports, 15

conference papers, 7 journal

papers, 2 Master’s theses and

1 Licentiate thesis

Asko Arkoma, Santtu

Huotilainen, Silja Häkkinen,

Anitta Hämäläinen, Timo

Ikonen, Seppo Kelppe,

Joonas Kättö, Henri

Loukusa, Anna Nieminen,

Jan-Olof Stengård, Elina

Syrjälahti, Ville Tulkki, Ville

Valtavirta, Tuomas Viitanen

3 17/04/2015 3

PALAMA Highlights

Completion of methodology for statistical assesment of the number of

failed rods during large break LOCA

Methodology development initiated in SAFIR2010 POKEVA

Development of FINIX fuel module

Improves fuel description in wide variety of host codes

New methodology to describe cladding response to stresses

To replace widely used strain hardening rule

Statistical evaluation of

failed rods during large

break LOCA

5 17/04/2015 5

Statistical methodology

Holistic approach that provides a fraction of failed rods

95% confidence that in 95% of LB LOCAs the actual number will be the

same or less

59 global APROS LB LOCA simulations, 1000 local fuel performance

simulations per global case

Also power histories from reactor physics codes required

First application: EPR analysis performed for STUK

STUK agreed to allow the publication of the results

A. Arkoma et al., Nuclear Engineering and Design Volume 285, 15 April

2015, Pages 1–14

6 17/04/2015 6

Calculation system

7 17/04/2015 7

Result

At 95/95 confidence and probability, at most 1.2% of all the rods in the

reactor fail

10% maximum allowed for this kind of scenario

FINIX fuel module

9 17/04/2015 9

FINIX – fuel behavior model and interface for

multiphysix applications

Reactor

physics

Thermal

hydraulics coolant conditions

clad temperature

Fuel

behavior

10 17/04/2015 10

FINIX – fuel behavior model and interface for

multiphysix applications

Reactor

physics

Thermal

hydraulics coolant conditions

clad temperature

FINIX

11 17/04/2015 11

FINIX models

Coupled thermomechanical

models

Solves the time-dependent radial

heat equation, in several

independent axial nodes

Rigid pellet (no elastic

deformations)

Cladding modeled as a thin-

walled tube

Thermal and mechanical solutions

coupled through gap conductance

and gap pressure

Burnup effects can be taken into

account in the initial state if given

by, e.g., FRAPCON

12 17/04/2015 12

FINIX coupling

Currently coupled to

Serpent 2

TRAB-1D/3D and Hextran

Provides realistic description of

fuel behaviour

Fast transient with Serpent 2

TMI-1 power peak with TRAB-1D

13 17/04/2015 13

FINIX

More information on FINIX in PALAMA special

article in the SAFIR2014 proceedings

Also: ”Module for thermomechanical modeling

of LWR fuel in multiphysics simulations”, T.

Ikonen et al. Annals of Nuclear Energy (2014),

accepted manuscript, in press.

FINIX currently licensed to Aalto, KIT

(Germany), Necsa (S. Africa) and CV Rez

(Czech Republic)

FINIX development continues in SAFIR2018

PANCHO project

Cladding viscoelastic

response

15 17/04/2015 15

Cladding creep response to changing

conditions

Cladding protects the fuel pellets

from the environment and contains

radioactive nuclides

Creep deformation at high

temperature and pressure

Commonly assumed that changes in

conditions can be accounted by

strain hardening rule

Accumulated strain invariant during

changes

Experiments show this is not true

16 17/04/2015 16

Viscoelastic model

Metal anelastic behaviour

Slow reversible deformation

Usually assumed insignificant

Irradiation and fuel behaviour

make it relevant in cladding

Can be illustrated with mechanical

analogs

Springs and dashpots parallel

and in series

Anelastic + permanent strain =

viscoelasticity

17 17/04/2015 17

Cladding creep

response

Viscoelastic model enables

reproduction of cladding response

to transient stresses

V. Tulkki, T. Ikonen Journal of Nuclear Materials,

457 (2015) , pp. 324-329

V. Tulkki, T. Ikonen, Journal of Nuclear Materials,

445 (2014) pp. 98-103.

18 17/04/2015 18

Stress relaxation behaviour

Viscoelastic model also

capable of simulating stress

relaxation behaviour

Figure displays model

performance in stress

relaxation with coefficients

fitted to creep experiment

Doctoral thesis ”Modelling fuel

behaviour and cladding

viscoelastic response” due this

summer V. Tulkki, T. Ikonen, “Modelling anelastic contribution to

nuclear fuel cladding creep and stress relaxation”,

submitted to Journal of Nuclear Materials (2015)

19 17/04/2015 19

Summary

PALAMA was 4 year project focused on fuel

performance issues

Wide variety of case studies, model

development, code development and analysis

Several highlights presented here

Completion of methodology for statistical

assesment of the number of failed rods during

large break LOCA

Development of FINIX fuel module

New methodology to describe cladding response

to stresses

More can be found from the SAFIR2014 Final

seminar proceedings

TECHNOLOGY FOR BUSINESS