A COMPARATIVE REVIEW OF RUSSIAN AND WESTERN DESIGN CODE RULES FOR RBMK MAIN COOLANT CIRCUIT COMPONENTS

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1 LT International Conference STRENGTH, DURABILITY ND STABILITY OF MATERIALS AND STRUCTURES SDSMS'99 _Pariev&2ys, Lithuania September 1999 A COMPARATIVE REVIEW OF RUSSIAN AND WESTERN DESIGN CODE RULES FOR RBMK MAIN COOLANT CIRCUIT COMPONENTS N G Smith, J K Sharples AEA Technolo,_UpIc., Risley, Warrington, Cheshire, WA3 6AT United Kingdom Abstract, The Russian design code (PNAEG ) has been reviewed and compared against equivalent ASIME and Western European procedures. Areas where there are commonaltics between the Russian nd Western procedures and where there are differences have both been identified. The desi-n code rview ad comparison has included: general principles, allowable stress intensity, load ases and allowable stress criteria; and fatigue aalysis criteria. Keywords: RBMK, pressure components, esign codes 1. The Essence of the Paper This paper is concerned with reviewing the design criteria and methods in the Russian design code, PNAEG , in comparison with western methods in France, RCC-M 2 Germany, KTA 3 and UK requirements based on ASME III 4 The scope of the review is restricted to the requirements for the components of the RBMK main coolant circuit and the pressurised water reactor (PWR pimary circuit. 2. General Principles The basic approach of calculating Tresca stress intensities and comparing ' these equivalent stresses with allowable stress limits, depending on the category of stress and the severity of operational loading conditions, is the same in the Russian code a ASME 11. However, the latter code, additionally, permits the use of elastic-plastic, and limit load calculations to 44

2 satisfy respective criteria, compared to only linear elastic behaviour in the Russian code, The scope of the Russian design code specifies a maximum temperature limit of 600'C which is higher than that allowed in the ASME code, i.e. 3700C (ferritics) and 427'C (austenitics). This requires consideration of timedependent creep effects but for the temperature range of the ASME code, the failure modes are the same. There are various volumes of the ASME code and Section III NB-3000, applicable to Class I main primary circuit components, is equivalent to the Russian design code. Note that the ASME code also covers Class 2 3 components and, additionally, the material properties and allowable stress values are specified in a separate Section 11. However, the material properties required to derive allowable stress limits is an integral part of the Russian document. 3. Allowable Stress Intensity A comparison of the design factors applied to the 02 proof stress, Rp 2 and the ultimate tensile strength, R,,, between the Russian code and ASME 111, are shown in Table 6 below. Firstly, for components and pipework, the immediate difference is that the Russian code does not differentiate between the margins applied for ferritic and austenitic steels. In the ASW code, the reduced design margin on the 02% proof stress a temperature R 2, (i.e. no.2 PO II) is to take benefit from the greater work hardening characteristics of austenitic steels, compared with ferrific steels. The allowance appears not to be present in the Russian code, but the reason may be due to the presence of residual strains resulting from manufacturing processes 6 The factors applied to the ultimate tensile strength at temperature, R T are lower in the Russian code (n,,=2.5) compared to ASME III (n,=3.0). Note, also, that the allowable stresses in the Russian code are based on properties at the design temperature, regardless of the operating temperature, whilst in the ASME code the operating temperature and 20'C wchever gives the lower value) is used. This latter difference also applies to bolts and pins (studs) with, additionally, the design margin in the Russian code (no.22) being smaller than in ASME m (no.2=3) In general, therefore, the design margins applied to RpG.2 are greater in the Russian code and greater relative to R T,, in the ASME code. The net effect 45

3 on the allowable stress values will depend on the type of steel and the temperature dependence of tensile properties. Table 1. Factors Applied to Str ngth Properties to Derive Allowable Stress Intensity 61 Component Material Allowable Stresses Russian Regulations ASME Code Equipment Ferritic [cj=rnin (RTIII I S.= min (R", ; and pipelines 11.5; R 2.6) RT 21.5; R Om/ 3 RT M/3) Austenitic S.= min R2",o 211.5; RT,, -/ 1 I 1: R",, 3 RT13) I Bolts and pins Any [crj= R4, 2 1 S=min(R'0,,213;R"1Q213;) 4. Load Cases and Allowable Stress Criteria Both the Russian code and ASNIE III require design calculations to establish the basic dimensions and thickness of components under pressure loading, which is then followed by 'check' calculations to confirm that the component is able to withstand the non-nal operation and entremes of loadings expected during the design service life. Essentially, the 'calculation cases' (or loading level categories) and the allowable stress criteria are similar in oth codes. The Calculation Case No I corresponds to Design (NB ) and Levels A/B (NB ) conditions in the ASME code. An important variation in the Russian code is that for Calculation Case No 2 dynamic effects (e.g. seismic) are considered explicitly with the Non-nal Operating Conditions, with specific primary stress criteria for design-basis (DBE) or safe-shutdown (SSE) earthquakes; in the ASME code the same primary pus secondary stress criteria with or without consideration of earthquake loadings are applied. The Calculation Case No 3 refers to normal operating conditions with infringements (INOC) and corresponds to the ASME Level C - Faulted Condition (NB ) in which mechanical damage may need to be repaired before return to service. For Calculational Case No 4 this corresponds to ASME Level D - Emergency Condition (NB-3225, Appendix F) in which prevention of radioactive release, rather than continued operation of the plant, is the main criteria. 46

4 5. Fatigue Analysis Criteria The method of analysing the fatigue life is similar in both codes and involves elastic analysis to determine the peak stress (or strain) amplitude, and a fatigue design curve to determine the number of allowable cycles. Fatigue damage due to various types of cycles assumes linear accumulation, ninf, and is acceptable providing the cumulative value is less than unity. However, within these generally similar procedures, there are some significant differences which are described, as follows. Firstly, in both cases, the peak stress distribution is calculated elastically, and then a strain enhancement factor is applied to take account of elasticplastic straining above the material yield limit. However, in the Russian code, the strain enhancement factor is based on equivalence of elastic and elasticplastic strain energy and makes use of static or cycle stress-strain diagram for the particular material. Almost since its inception, the ASME code has included a Ke factor (NB ) wich is used to correct the elastic calculation when the range of primary plus secondary stress range exceeds the 3Sm limit. However, it is recognised that the Ke factor is over conservative for lar-er strain rad-es and is under conservative at lower levels of plastic straining when the material proportional limit is just exceeded. It would appear that the method in the Russian code is more accurate and, it is interesting to note, a similar advanced method has been introduced into the latest version of RCC-M [2] in order to overcome the deficiencies of the ASME method. Further differences are that the Russian code provides a specific method for combining cycles (as in RCC-M) and takes account of enhanced damage accumulation due to superimposed high frequency cycles (e.g. vibration) upon low frequency transients. However, the most fundamental difference is in the derivation of the fatigue design curve. In the ASME code, this is based on uniaxial constant amplitude tests which are used to derive a mean failure curve, after which design margins of 2 and 20 on strain and cycles (whichever gives the lower curve) is used to derive the fatigue design curve. In the Russian code, the failure curve is empirically derived using Manson-Langer type equations making use of specified values of ductility ZT and ultimate tensile strength, R T Design margins of I and 20 are then applied to derive the fatigue assessment curve. Fatigue curves are provided for austenitic and ferritic steels for various values of R T 2 ' R T, R 'r, Z 'rand Young's Modulus, up to IO' cycles. Empirical equations are included for extrapolating the curves out to 10" cycles. Fatigue 47

5 curves are also provided for bolting materials with design margins in the Russian code of and 1.5 compared t 2 and 20 in the ASNIE code. 6. Additional Comments The Russian code is concerned with assessing ratcheting or progressive deformation and provides very detailed analytical procedures and assessment diagrams. The procedure appears to be more complex than the methods and criteria in the ASME code which specifies the 3 Sm limit (NB to ensure shakedown) and thermal stress ratchet criteria based on the Bree diagram (NB ) restricting combinations of primary and thermal stresses Concludin- Remarks 1 The development of Russian normative documents began in 1969 and, in this process, a detailed study of ASME code 1968 version) was made. Hence, there are strong similarities in the design methods and criteria although there are differences, as highlighted in the previous sections. References I Standards to the Tensile Strength of Equipment and Pipevvork at Nuclear Power Plants. PN.6X-G (In Russian) 2. RCC-M 'Design and Construction Rules for Mechanical Components of PWR Nuclear Islands - Section I - Subsection B: Class I Components, June German Nuclear Safety Standards (Kerntechischer Aussehuss - KTA). 4. ASME Boiler and Pressure Vessel Code. Rule for Construction of Nuclear Power Plant Components, Section III, Subsection NB: Clause I Components. 5. Smith N. G. A Comparative Review of International Design and Construction Codes for Advanced PWRs. BNES Seminar, Pressure Component Standards for APWRs - A Review of Structural Integrity Related Design Codes and Licensing Requirements, London, United Kingdom, February, Filatov V M and Rivkin E Yu. NPP Structures Strength: Russian Strength Regulations and ASME Code. ICONE-4, Volume 5, ASME pp