Very basically, there are two components of creep in graphite, thermal and irradiation-induced with the latter dominating as carbon atoms are constantly displaced under a barrage of high-energy neutrons. This leads to ‘damage’ of the structure at the atomic level and this is manifest in large components by dimensional change, bowing, changes in strength parameters such as Young’s modulus, in thermal properties, and in potential CRACKING

Creep under continuing irradiation serves to alleviate the stresses and to delay the onset of damaging cracks... Indeed, the UK AGRs continue in operation only because of the presence of creep... and HTR reflectors and fuel components are also subject to these phenomena, whose precise behaviours clearly need to be a part of operational safety cases

Historically, there are diverse models over limited ranges of temperature and fluence, and often for graphite types which are no longer used, but no ‘unified theory’ or fully satisfactory mechanistic explanation of the phenomenon

Consequently, regulatory bodies are very keen that creep studies and analysis proceed in order to support the safety arguments

United States of America (for NGNP and irradiation services for US and UK)

United Kingdom (for AGRs)

The Netherlands (irradiation services for UK)

China (for HTR-PM safety case)

Japan (pre-Fukushima, for HTTR and future development [hydrogen production and process heat])

South Korea (for HTR for hydrogen production)

Ukraine (contributing knowledge of special graphite behaviour)

…and, formerly, South Africa

Initial progress was hampered by the loss of

South Africa (PBMR Co Pty, University of

Pretoria) from the CRP

As a consequence we have had to refocus

the CRP and reaffirm the CRP’s goals

Major reviews of irradiation-damage

mechanisms at all scales (molecular

through to whole reactor components)

undertaken, driven largely by USA and UK

New data on effects of neutron irradiation on crystal structure revealed by X ray diffraction studies of irradiated graphite (Netherlands)

Two HFIR irradiation creep rabbits built and H-451 graphite specimens subjected to detailed Pre Irradiation Examination in preparation for irradiation (USA)

Existing creep models reviewed (UK)

Application of models to core structures (China)

Experiments at INL have got under way,

and initial results are now being analysed;

This will greatly influence the thinking for

low-fluence irradiation behaviour, but high-

fluence data cannot be obtained in the

timescale even of an extended CRP;

Difficulties encountered ‘along the road’

are being fed back into the irradiation

programme

To be physically based (not just a fit to data);

To be a predictive model for creep strain as a function of neutron dose (which can be extrapolated with confidence) and used for reactor operation and design purposes (e.g., load fluctuations);

To describe accurately the experimental creep-strain data over a temp range from 300 to 1000°C;

To predict the effects of stress, temperature, and oxidation on creep strain;

To account for other physical property changes.

The objective of a ‘universal’ model is

unrealistic – too many variables in the

available (and future) datasets,

especially graphite type and fluence

Creation of a matrix of “Quality v

Relevance” to assist

in sorting data to be

it for purpose...

The objective of a ‘universal’ model is

unrealistic – too many variables in the

available (and future) datasets,

especially graphite type and fluence

To improve the understanding of the underlying mechanisms of irradiation creep through the following:

1. To collect and evaluate historical graphite creep data;

2. To develop an improved understanding of the structure (all relevant features over all relevant length scales) of graphite and its role in the creep process;

3. To elucidate the effects of non-regular loading on creep behavior within the limits of available data:

a. load-unload cycles to observe the creep stain relaxation effect on

structure;

b. subject to stress reversal to observe the effects of creep strain reversal on structure;

c. irradiate stressed pre-irradiated graphite (unstressed) to provide insights into the behavior of irradiated graphite structures to the imposition of external load.

What we’ve got:

a current evaluation of the existing models (and from that, confirmation that they are not consistent and that they do not overlap sufficiently to provide the predictive confidence needed either by the UK for AGR life extension nor for the US for NGNP (and nor, it is presumed, for the Chinese or Toyo Tanso to provide what is needed by the Chinese regulator))

a comprehensive database (albeit with some minor omissions yet to be filled) enabling independent Member States to utilise it for their own programmes

thanks to delays, an opportunity to incorporate at least some low-fluence results from current experiments

a demonstrably useful matrix of data quality versus relevance to enable appropriate datasets to be selected objectively by individual users for their purposes

developing analyses of the structural behaviour of graphite under irradiation, taken from the atomic level [Heggie et al], and the micro-meso level [Rand team]; reports due April

a useful range of analytical techniques (awaiting Nassia’ Tzelepi’s list of lab caabilities and also subject to issues of sample transport on a useful timescale - needs to include a survey of creep-test techniques)

an improved chance to influence some later parts of the experimental programmes based upon better information than we expected

support to the IAEA international collaboration from the UK TSB intervention

What we haven’t got: adequate post-dimensional-change turnaround data on any relevant

graphite an understanding of the structural behaviour of full-size component

sufficiently to make credible predictions

a newly developed model (empirical or otherwise) which fully satisfies the conditions of interest of anyone

any useful contributions from Japan or Korea access to the independent (?) thinking from the former PBMR people an understanding of CTE behaviour (in particular, its implications for the

dimensional change and creep mechanisms) an understanding of how different graphite structures affect their creep

behaviour a clear understanding of whether creep effects influence the turn-

around dose an understanding of whether, if you stress a crystallite, you change its

response to irradiation a clear view on tertiary creep

4. To reveal details of pore structure changes as a function of neutron dose as the graphite undergoes pore closure followed by new pore generation associated with volume “turn-around”;

5. To clarify the current understanding of the effect of creep on CTE and to propose further experimental and theoretical analysis to develop this understanding;

6. To coordinate and reach consensus on the design of new creep experiments and the analysis of experimental creep-strain data from them;

7. This knowledge will be made available to be applied to new reactor design and to help in the design and operational assessment of graphite components;

8. To agree common terminology and definitions (in the field of irradiation-creep damage and materials structure).

Model which is technically sound, covering the ranges of temperature/fluence relevant to UK AGR with gilsocarbon graphite, adequate to support case for continued operation and reactor life extension – Likely to be based on extant “Bradford-Davies” model with enhanced underpinning from structural review:

UK has commissioned additional experiments in HFIR (ORNL) and at Petten to support the EdF-Energy requirement

Model which is technically sound, covering

the ranges of temperature/fluence relevant

to NGNP: now that AREVA prismatic design

is likely, this will probably be based on

extant “Kelly-Burchell” model , again with

enhanced underpinning from structural

review and from experimental data,

particular HFIR creep

rabbit (right) and INL

irradiations

China needs a model to support the

conditions and life expectations of HTR-

PM – INET is being invited to play a larger

role in this CRP

...contributions from other Asian

colleagues have been disappointingly

small

Request extension to allow better utilisation of current experimental data and their analysis to assist in model refinement alongside the completed mechanistic reviews (justification partly based on loss of critical partner at early stage);

Produce TECDOC in autumn 2013 covering the extensive review of data and mechanisms

Produce second TECDOC after two more years in which the interpretation of the new experimental data will underpin the model proposals