Erfahrungs- und Forschungsbericht 2012 - Ensi

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Erfahrungs- und Forschungsbericht 2012 - Ensi

Project Goals

Objective

The main areas of SCIP II are treated in four tasks.

In Task 0, existing ramp data is reviewed and evaluated.

In Task 1, the PCMI behaviour and pellet

properties are studied. The remaining two tasks

treat the cladding failure mechanisms and critical

parameters with respect to material properties and

operational parameters. There are progress reports

issued for all four tasks once a year.

Task 0: Review of existing Studsvik

ramp data

Correlations of data from historic ramps performed

at Studsvik under various programs are

made available.

Task 1: Pellet-cladding mechanical

interaction (PCMI)

PCMI is the mechanical driving force for the

three fuel failure mechanisms Delayed Hydrogen

Cracking, PCI, and Hydrogen Embrittlement.

The cladding strain is primarily controlled by the

pellet behaviour, in combination with the initial

fuel rod thermo-mechanical conditions. For

a given power increase, the cladding strain is

determined by the initial pellet and cladding

conditions as well as the pellet expansion. The

cladding stress is in turn mainly determined by

strain and stress relaxation behaviour.

Task 2: Pellet-cladding interaction (PCI)

PCI failures are caused by stress corrosion cracking,

where the stress corrosion agents are fission

products, notably iodine. The cracks are initiated

at the inner surface of the cladding and may

propagate outwards through the cladding wall.

Task 3: Hydrogen induced failures

Hydrides will form in claddings that contain

hydrogen levels that exceed the solubility limit.

Two types of hydride induced failure mechanisms

were studied within SCIP; Hydrogen embrittlement

(HE) and Delayed Hydride Cracking (DHC).

In SCIPII a study is included on measuring local

mechanical properties of hydrides and the matrix

in the vicinity of hydrides. The Task also includes

a study of the effect of hydrides and H in solid

solution on creep/relaxation behavior. This issue

is related to PCMI in transients, where fuel cladding

relaxation may be affected by cladding H

content.

The overriding objective of the continuation of the

SCIP programme (SCIP II) is to contribute to more

reliable fuel, by further deepening the understanding

of mechanisms leading to fuel failures driven

by pellet-cladding mechanical interaction (PCMI)

and pellet-cladding interaction (PCI).

SCIP II has a broader scope compared to SCIP,

including advanced modern cladding and pellet

materials.

Task 0: Review of Existing Studsvik

Ramp Data

The main objective of this task is to compile, evaluate

and investigate possible trends and correlations

in anonymous ramp test data from a large

number of ramp tests performed at the Studsvik

R2 reactor since 1970.

This task started after the SCIP II meeting in

November 2009. Since then ramp test data for

more than a 1000 ramp tests have been compiled.

The bulk of the data has been ana-lysed by making

database statistics, plots of different parameter

combinations and searches for trends and correlations

in the data. The results of this analysis have

been presented at the PRG meetings in June 2010,

November 2010 and May 2011.

The final report will be available at the end of 2012.

A generic ramp database will also be delivered by

the end of 2012.

Task 1: Pellet Cladding Mechanical

Interaction (PCMI)

In SCIP II Task 1, 12 ramp tests in total are to be

performed in Halden. Fuel rods with additives from

several suppliers are available for the ramp tests,

as well as fuel rods with gadolinia pellets and with

high density pellets. Rods with standard pellets are

also tested as references. The ramp tests and subsequent

PIE are performed within Subtask 1:2, 1:3

and 1:4. The rods are also examined within Task

2 and 3.

In addition to the 12 ramp tests and examinations,

11 old ramp tested rods are examined within Task 1

(Subtask 1:1). Among the rodlets there are pellets

with Gd, Cr-Si, Cr-Si-Al and MOX.

Old and new results from ramps and PIE of three

rodlets with large grain size have been presented

in meetings year one, 2 and 3.

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ENSI Erfahrungs- und Forschungsbericht 2012

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