Codes & Rules for Gen-IV nuclear system design
Davide Bernardi
ENEA Brasimone
International Workshop on
Innovative Nuclear Reactors cooled by
Heavy liquid metals:
Status and Perspectives
Pisa, April 17th – 20th , 2012
OUTLINE
Nuclear design codes
Overview of RCC-MR design rules
Types of damages
Criteria levels
Operating conditions categories
Breakdown of stresses
Classification of stresses
Prevention of P-type damages
Prevention of S-type damages
Current upgrade of the Code for Gen IV system design
Creep-fatigue tests on P91
Summary
NUCLEAR DESIGN CODES
Main codes for the design of high T (LM) reactor components
• RCC-MR (French code) [2007]
• ASME Sect. III – Div. 1 - Subsection NH (USA code) [2010]
• RCC-Mx (French code for research reactors, e.g. JHR) [2008]
• RCC-MRx (2010 draft, currently under revision for public edition, next ref. code
for EU GEN IV prototypes, but also ESS, JHR, ITER, TBM,…) [2012]
• ISDC - IC (ITER Structural Design Criteria for In-Vessel Components) [1997]
• Domestic codes (Japan, Russia,…) some not available in European languages
In the following, reference will be made mainly to RCC-MR
TYPES OF DAMAGES
P-type damages: damages resulting from constant or monotonic loadings
Excessive deformation (immediate or time-dependent)
Plastic collapse (immediate or time-dependent)
Time-dependent fracture (creep rupture)
S-type damages: damages resulting from cyclic loadings
Progressive deformation (ratchetting)
Progressive deformation (ratchetting) + creep
Fatigue
Fatigue + creep (creep-fatigue interaction)
Buckling: damages resulting from elastic or elastoplastic instability
Load-controlled buckling
Strain-controlled bucking
Time-dependent bucking
Fast fracture: fracture occuring without appreaciable global deformations
(RCC-MR Code does not contain rules for prevention of Fast Fracture)
Ductile tearing
Brittle rupture (fragile or semi-fragile tearing)
CRITERIA LEVELS
Level A: aims at protecting the equipment against P-type and S-type damages
It garantees the highest level of safety margins against both P-type and S-type damages
throughout the entire life of the component
Level C: aims at protecting the equipment against P-type and buckling damages
Garantees a relaxed level of safety compared to Level A (no fatigue analysis)
Small overall deformations can occur if some loading (although satisfying Level C) exceeds
Level A criteria
In this case, it could be necessary to inspect the component before re-using it
Number of stress cycles limited to 10
Level D: aims at protecting the equipment against same damages of Level C but with lower safety margins
Not always possible to put again in service components subjected to loadings limited only by
Level D criteria
ASME code introduces also a Level B criteria for pressure retaining enclosures (this is not provided
in RCC-MR: in this case, regulations in force for pressure-retaining equipment must be applied)
OPERATING CONDITIONS CATEGORIES
1st Category Operating Conditions (SF1) and 2nd Category Operating Conditions (SF2)
normal operations (including normal operating incidents)
Start-up
Shut-down
Criteria level to be met: Level A
3rd Category Operating Conditions (SF3)
Emergency conditions (implying shut-down and appropriate inspection)
Criteria level to be met: at least as severe as Level C
4th Category Operating Conditions (SF4)
Highly improbable conditions but whose consequences are of safety relevance
Criteria level to be met: at least as severe as Level D
Operating conditions and associated level criteria must be specified in the Equipment
Specifications of the component
BREAKDOWN OF STRESSES
Stress tensor σij obtained by elastic analysis
Membrane stress :
+h/2
σij m (xk) = 1/h ∫-h/2 σij (xk) dxk
Bending stress :
+h/2
σij b (xk) = 12 xk /h3 ∫-h/2 σij (xk) xk dxk
Linearized stress :
σij l (xk) = σij m (xk) + σij b (xk)
Non linearized stress (peak stress) :
σij nl (xk) = σij (xk) - ( σij m (xk) + σij b (xk) )
CLASSIFICATION OF STRESSES
Primary stress (P)
Part of the total stress that does not disappear under a small deformation
Ex. : stress due to mechanical loads (e.g. weight, pressure, external loads such as distributed
or concentrated loads,…)
Warning: if risk of elastic follow-up exists, it is prudent to consider all the membrane stresses
as primary stresses
Secondary stress (Q)
Part of the total stress that disappears under a small permanent deformation (except the peak stress)
Ex: thermal stresses, strain-controlled stresses (e.g, swelling, ), support displacements,
geometrical discontinuities ,…
Peak stress (F)
Part of the total stress due to a geometrical discontinuity, or a non linear stress distribution
Very local stress, that is unable to induce a global deformation of the structure
Only influence on fatigue or creep-fatigue damage
Ex: non linear stress, stress due to a minor discontinuity, local thermal stresses
CLASSIFICATION OF STRESSES
All symbols refer to sets of 6 quantities
representing the stress tensor components
(ex: Pm ≡ σijm is the membrane part of the
stress tensor due to primary loads)
Lm : local membrane stress due mechanical loading and associated to geometrical or load
discontinuities. Added to primary stresses for prudence (risk of elastic follow-up)
PL = Pm + Lm : local primary membrane stress
PREVENTION OF P-TYPE DAMAGE
- NEGLIGIBLE creep
creep::
The following conditions must be satisfied:
1. General primary membrane stress intensity
2. Local primary membrane stress intensity:
3. Primary membrane + bending stress intensity:
where the stress intensities are calculated from the corresponding stress tensor components using either
the Tresca method or Von Mises method and Sm (θm) is the allowable limit calculated as a function of the
mean temperature θm for different materials:
PREVENTION OF P-TYPE DAMAGE
- NOT NEGLIGIBLE creep
creep::
creep stress curves
Creep usage fraction
where Tj is the maximum allowable time
obtained from the characteristic creep
stress curves St for the time interval tj with
maximum temperature θj and max stress σj
The following conditions for the creep usage fraction must be checked for compliance with Level A and C
criteria:
1)
for the general primary membrane stress
2)
for the local primary membrane stress + primary bending stress
Coefficients Ω and Φ take into account the effect of creep on the primary stresses and are given
in the Code for specified geometries and stress ratios
PREVENTION OF S-TYPE DAMAGE:
RATCHETTING
Conditions to prevent P-type damages must be firstly satisfied, then check:
- NEGLIGIBLE creep
creep::
Efficiency diagram
For the typical operating period calculate :
(secondary stress range)
Secondary ratio for the primary membrane stress:
Secondary ratio for the sum of primary stresses:
Calculate efficiency indexes v1 and v2 from the efficiency diagram
Calculate effective primary membrane stress intensity:
Calculate effective stress intensity for the sum of primary stresses :
Verify the allowable limits (Level A criteria)
- NOT NEGLIGIBLE creep
creep::
When creep is not negligible the procedure is the same except that Pb is multiplied by the
coefficient Φ and specified allowable limits for accumulated plastic+creep strains must be checked
PREVENTION OF S-TYPE DAMAGE:
RATCHETTING
Alternative rule: 3Sm rule
For the typical operating period for which compliance with Level A is required, the following limit must be
checked at all points of the structure:
Note that since typically: Sm = 2/3 Rp0.2 →
3 Sm ≡ 2Rp0.2 (justification from Bree diagram)
ASME code considers only the 3Sm rule for prevention of progressive deformations
PREVENTION OF S-TYPE DAMAGE:
FATIGUE
- NEGLIGIBLE creep
Fatigue usage fraction must be less than 1 at all points of the structure and for all cycles j requiring
compliance with Level A criteria:
<1
where the maximum number Nj of allowable strain cycles is obtained from the fatigue curves at the
maximum temperature θj
due to strain concentration
PREVENTION OF S-TYPE DAMAGE:
FATIGUE (Creep
Creep--Fatigue)
Fatigue)
Creep-fatigue interaction diagram
- NOT NEGLIGIBLE creep
• Fatigue usage fraction (Level A compliance)
Calculated as for the case
with negligible creep
Creep strain accumulated in
one cycle calculated by creep
laws considering an effective
stress σk to be calculated from
a combination of ΔP, ΔS, Ks
(symmetrization coefficient)
Secondary stress range
Max primary stress
intensity during the
holding time
Symmetrization coefficient
(given by the Code)
• Creep rupture usage fraction (Level A compliance)
with
CREEP-FATIGUE TESTS ON P91
(ENEA Brasimone – MATTER project)
Objective: to check the extent of applicability and the degree of conservativism of the
creep-fatigue interaction diagram of RCC-MR for P91 material
Creep rupture
usage fraction (W)
Creep-fatigue specimen
Pure creep tests
NOT allowable area
Expected
experimental
points for
creep-fatigue
tests
Pure fatigue tests
Allowable area
Fatigue usage
fraction (V)
CREEP-FATIGUE TESTS ON P91
(ENEA Brasimone – MATTER project)
The creep-fatigue tests will be carried out in
agreement with standard ASTM E 2714-09
Test conditions:
material: P91
environment: air
tests under strain control
symmetric alternate stress
total deformations applied: > 1% (2 levels)
hold-time (10-30 minutes range) at tensile and compression (TBC) peaks
temperature > 500°C.
Example of creep-fatigue cycle with hold time in tensile
ε0
t
-ε0
SUMMARY
A short overview of the RCC-MR design rules has been presented,
focusing on the methodologies used to prevent different types of
damages (P-type, S-type)
Needs of updates concerning in particular creep-fatigue interaction
diagram for 9Cr-1Mo F/M steels (P91) have been described
Experimental creep-fatigue tests to be carried out in the frame of the
MATTER project and aimed at obtaining creep-fatigue data for P91
under different operating conditions are planned in order to improve
the extent of applicability and the degree of conservativism of the
existing interaction diagram given in the Code
Thank you for your attention