ASME Section III, Div. 5 HAB, HHA

Graphite Code-재료 및 설계특성

지 세 환

2016 KEPIC week, August 30-Sept. 2, 2016, Jeju.

한국원자력연구원 VHTR 기술개발부

(042-868-2385, shchi@kaeri.re.kr)

목 차

1. 서 론

2. 흑연 재료 특성

3. ASME Section III, Div. 5 (High

Temperature Reactor), HAB, HHA

설계특성

4. 향후 전망

1. 서 론 (background)

2. 서 - 2008 년 PBMR Project (South Africa), NGNP (USA)

Projects. ASME determined to support these projects

by providing construction rules (design, licensing).

- BPV III established WG-HTGR (2008), WG-LMR

(2009), SG-HTR (2009) to develop Division 5.

- As a separate but supporting role, ASME ST LLC,

funded by NRC, developed a roadmap document

(whitepaper) how to develop ASME rules to support

both the HTGR and LMR.

-These planning activities recommended adaption of

existing ASME Code rules for near term projects,

such as the PBMR and the NGNP. In the long term, a

complete rewrite of construction rules was

recommended.

- Div. 5 = (NB, NC, NF, NH + Code Cases for

elevated temperature components + Graphite Code)

New rules

ASME Section III Division 5: HAB, HHA

Very High Temerpature Reactor

• Coated Fuel

• Graphite Moderator

• Helium Coolant

Benefits • High Temperature

- high efficiency

- hydrogen production by

water splitting

• Inherent Safety

- natural decay heat

removal

Hydrogen Production VHTR

Heat Application VHTR

Electricity Generation VHTR

2. 흑연 재료 특성

10

Steel Nuclear Graphite (Ceramic)

Region of linear elastic behavior Always non-linear behavior

Yield stress can be defined Yield stress is not definable (mechanical failure

occurs at a distribution of stress levels)

Small scatter of the strength data Larger scatter of the strength data (Weibull

distribution for tensile, compressive, and

flexural strength)

High tensile strength, fracture strain, and

fracture toughness

Low tensile strength, fracture strain and

fracture toughness

Section III no cracking permitted, Section XI

permits limited cracking

Graphite assembly is design to be crack

tolerant

Relief of peak stresses due to plasticity Relief of peak stresses by micro-cracking thus

incurring damage – this is the nonlinear

response in the material

Properties & behaviour of graphite are

fundamentally different from steel

Properties & behaviour of graphite are

fundamentally different from steel

11

Steel Nuclear Graphite (Ceramic)

Fast neutron fluence influences the material

properties (increases the NDT)

Fast neutron flunce changes all material

properties, and induces dimensional

change and irradiation creep

Local peak stresses are non-critical Local peak stresses (irradiation induced

mechanical property changes) can cause

damage

Crack initiation depends on the primary

stress

Crack initiation depends on the total stress:

(membrane, bending, and peak stresses-

no primary or secondary stress)

Cracked component replaced or repaired Cracked component is not in imminent

danger of failing- graphite components

must be design to be crack tolerant and

maintain functionality and code safety

requirements

Strength decreases with increasing

temperature

Strength increases with increasing

temperature

Properties & behaviour of graphite are

fundamentally different from steel

12

Steel Nuclear Graphite (Ceramic)

Metallic strength properties in Section II Propose nonirradiated strength distribution

properties in Section II

No irradiated properties in the code.

Irradiated properties are considered

environmental effects

Propose irradiated properties be in Section

III

Irradiated properties not included in

Section III design rules

Irradiated properties are included in

Section III design rules

Minimum of three heats are tested for

qualification and acceptance

The number of billets and batches needed

for sampling and acceptance is

statistically based

NDE used effectively in Section XI to assess

condition of component

Currently, there is no Section XI for

graphite; rely on visual inspection

Properties & behaviour of graphite are

fundamentally different from steel

13

Steel Nuclear Graphite (Ceramic)

ASTM standards define the properties of

metallic materials

ASTM graphite standard is a minimum

limit – all manufactures produce

graphite with different properties that

exceed the minimum. The variance

depends on the three manufacturing

methods

Design stress is limited proportionately to

Sm

Design stress limited by a probability of

failure from the strength distribution

Probability of failure is accomplished

through distribution of different loads

for constant material strength

Probability of failure is accomplished

through distribution of material

strengths for a deterministic load

3. ASME Section III, Div. 5, HAB, HHA 설계특성

15

Main Processes in Design – Static

Assessment

• Characterization of material strength

distribution

• Prediction of stress state in parts for

load cases

• Application of the probabilistic failure

criterion

• Comparison of probability of failure

(POF) to required reliability.

Characterize

Stress

Analysis

Failure

Assessment

Comparison

to Limits

16

Material Characterization

• The stochastic strength of the

material must be adequately

characterised.

• A two parameter Weibull

distribution is selected and

conservatism is ensured by

means of a 95% confidence

interval.

Characterize

Stress

Analysis

Failure

Assessment

Comparison

to Limits

17

Typical Tensile Strength

Characterisation

• A typical material strength distribution is shown.

• Details: – 16 samples

– Mean = 18.9 MPa (2.7 ksi)

– Std Dev = 2.4 MPa (0.35 ksi)

– Min Measured = 14.5 MPa (2.1 ksi)

• Weibull Parameters (95% confidence) – Sc = 18.3 MPa (2.7 ksi)

– m = 4.7

18

Stress Analysis

• Stress analysis requirements :-

– The entire stress state in the part to be

known for full failure assessment

– The stress state shall include all stresses

(both primary and secondary)

– Stress raisers cannot be ignored in the

stress analysis

– Appropriate treatment of irradiation and

oxidation effects to be incorporate later.

Characterize

Stress

Analysis

Failure

Assessment

Comparison

to Limits

19

Failure Assessment

• Simplified vs. Full Failure

assessment

• The simplified method trades

increased conservatism against

simplicity of application.

Characterize

Stress

Analysis

Failure

Assessment

Comparison

to Limits

20

Simplified Failure Assessment

• Determine stress allowable based on

the distribution and the required POF.

Compare to the peak stress in

analysis

Characterize

Stress

Analysis

Failure

Assessment

Comparison

to Limits

1

mallowS Sc ln 1 POF

Note: The Allowable stress is now a function of material quality.

Structural Reliability Classes (SRCs) and

Design Allowable POF

22

Full failure Assessment

• This is completed by a volume or surface integral throughout the part to yield a probability of failure directly.

• Several failure theories exist, an example of such an assessment is the classical Weibull approach.

Characterize

Stress

Analysis

Failure

Assessment

Comparison

to Limits d

),,( exp 1

V

m

ofV V

zyxP

23

Comparison to Limits

• The code defines Structural Reliability Classes (SRC’s) that the designer can design to.

• For example:- – SRC 1 → POF ≤ 10-4

– SRC 2 → POF ≤ 10-3

• The designer shall make the appropriate selection for each component and provide justification.

Characterize

Stress

Analysis

Failure

Assessment

Comparison

to Limits

4. 요 약/결론

• 초고온가스로(VHTR) 개발을 위한 ASME Sec. III, Div. 5 Graphite code 를 소개하였다.

• 화석연료고갈 및 지구온난화 문제를 해결할 수 있는 수소생산원자로 기술개발에 필요한 흑연재료기술과 설계기술개발이 요청된다.