Paper presented at 20th International Conference on Nuclear Engineering. 30 Jul - 3 Aug 2012, Anaheim, CA, USA. Paper No. ICONE20POWER2012-54190
The two main designs currently being applied worldwide for pressurized water reactors (PWR) are the EPRTM and the AP-1000®, respectively based on the Design and Construction Rules for Mechanical Components of PWR Nuclear Islands code (Règles de Conception et de Construction des Matériels Mécaniques des Ilots Nucléaires des réacteurs à eau sous pression, RCC M), the French nuclear construction code, and on the ASME Boiler and Pressure Vessel Code (referred to hereafter as 'ASME').
This paper presents an interpretive comparison between the RCC-M and ASME requirements for welding and non destructive testing (NDT), limited to components within the 'nuclear island'. Differences that might have an impact on manufacturing operations or on the long term integrity of the Class 1 welds are discussed. The results are presented in the form of text and tables. In addition, individual clauses are compared to establish if they can be considered equivalent and if not, which code provides the most stringent requirements.
This paper presents a comparative assessment of the requirements provided by the RCC-M and ASME codes for the design and construction of PWR nuclear power plants, specifically for welding and associated non destructive testing (NDT) of Class 1 production welds on nuclear island components. The study conducted by the authors involved a large number of detailed comparisons of requirements for specific aspects relating to welding and NDT of major Class 1 welds for which there is insufficient space here. The intent of this paper is to highlight the major differences between the two codes, as well as providing a useful reference for organisations that are traditionally more familiar with the ASME code.
The documents compared were the ASME 2010 edition and the RCC-M 2007 edition, including the 1st Addendum, dated December 2008, and the 2nd Addendum, dated December 2009. These two codes are inevitably complex documents and the requirements for one do not necessarily map directly on to those of the other. An example which highlights the care with which direct comparisons between the two codes must be carried out is that the RCC-M code, Section I, Sub-Section A, Clause A4242 states that: 'Welds subjected to pressure shall be assigned the same class as the parts they join. If the parts have different class designations, the weld shall be assigned the most severe class'. ASME Section III does not identify welds in relation to component class in this way, but Clause NB-1130 specifies the criteria for defining the boundary of the applicability of Sub-Section NB (Class 1 components). Consequently, a weld which may be deemed to be Class 1 by one code may not be according to the other.
|Individual or legal entity responsible for the design and construction of all or part of the nuclear island
|Prefix that denotes a European Standard
|International Organisation for Standardisation
|Individual or legal entity responsible for the design and construction of the pressure equipment. The Contractor may assume the role of manufacturer for some equipment items or some services
|Reactor pressure vessel
Scope of this study
The review focused on RCC-M Section I, Subsections A (General Requirements) and B (Class 1 components), and Section IV (Welding), which broadly correspond to Section III, Subsections III-NCA and III-NB, and Section IX of the ASME, respectively (Table 1). It should be noted that Section IV of the RCC-M deals with all aspects of welding. The scope of Section IV of RCC-M is consequently wider than the one of Section IX of ASME, covering aspects contained in ASME Sections II and III. Therefore, the comparison considered these as well. The review of non-destructive testing requirements also included Section III of RCC-M and Sections V and XI of ASME. With regard to parent materials, the review was limited to clauses applicable to MnNiMo low-alloy steels, C-Mn steels and stainless steels (RCC-M grades and equivalents are provided in Annex A).
Table 1 Alignment of the relevant sections of the two codes
|Section I - Subsection A
|Section III - Subsection NCA
|Section I - Subsection B, Class 1
|Section III - Subsection NB
Some elements of Section IV of RCC-M appear in ASME III Subsections NCA and NB
Some requirements are also included in Section XI
Welding and fabrication
The structure of this section is based on Section IV of RCC-M. Unless otherwise indicated, any mention of 'A', 'B' and 'S' clauses refers to the RCC-M code and the numbering of paragraphs reflects that of the sections and subsections of the code itself. When the ASME Code is referred to in this Section, the reference is limited to ASME Sections III-NB, III NCA and IX.
RCC-M sections I-A A3500 and I-B B4000
Clause A3500 of RCC-M concerns welding documents; hence it does not strictly affect the long-term integrity of components and welds. Both clauses A3500 and B4000 call for parts of RCC-M Section IV, which are reviewed below.
RCC-M section IV
S 1320 Preheat and interpass - required temperature. Clause S1320 provides recommendations for minimum preheating temperatures, which are defined as mandatory if required in Subsections A, B or C. The minimum preheat temperatures required by RCC-M are in line with those suggested by ASME (NB-4610). In particular, RCC-M B4440 recommends minimum 150 and 175°C for the pressure retaining components of the reactor vessel and steam generator or pressurizer, respectively. These are more stringent than the min 120°C temperature suggested by ASME for equivalent material grades and thickness ranges.
It is noted that, while preheat requirements of S1320 are mandatory for Class 1 components, non-mandatory ('suggested') in ASME III-NB (Appendix D), except for welds exempted from postweld heat treatment (PWHT) which are addressed in table NB-4622.7(b)-1. In addition, ASME NB-4611 requires that the minimum preheating requirements are specified in the welding procedure specification, according to the qualification requirements of Section IX. A summary of the preheat requirements from the two codes is provided in Annex B.
Interpass. RCC-M requires qualification (S1320) but no specific requirements on the interpass temperature are given. ASME requires qualification (ASME IX) and only states that consideration should be given to quenched and tempered materials (NB-4613).
S 1330 Postheating. Table 2 shows a comparison between postheating requirements. The ASME code does not provide requirements for postheating, except for cladding or repair to cladding exempted from PWHT which are addressed in table NB-4622.7(b)-1.
Table 2 Comparison between RCC-M and ASME requirement for postheating of pressure-retaining welds in SG, RPV and pressurizer.
|RCC-M (S1330 and B4440)
|Min temperature, °C
|Min duration, hours
*Except for cladding or repair to cladding exempted from PWHT which are addressed in table NB-4622.7(b)-1.
RCC-M (S1330) states that when preheating is required, then postheating is required at a temperature equal to or greater than the minimum preheating temperature for min 60 minutes, except if PWHT is performed straight after welding without cooling to ambient temperature. According to B4440, postheating is compulsory for welding performance on SG, RPV and pressurizer, with minimum 200°C recommended for two hours.
S 1340 PWHT. The PWHT temperature ranges in RCC-M are identical to those specified by ASME III with the exception of Carbon steels (P-No1), where the ranges overlap in the interval 595 to 625°C. In particular, RCC M specifies a significantly lower minimum temperature for carbon steels (550°C versus 595°C) complies with that specified by PD 5500 and EN ISO 13445-4, which is harmonized with the Pressure Equipment Directive 97/23/EC. These standards are well established and extensively used in Europe for the manufacture of pressure vessels. In addition, the upper limit of 625°C set by RCC M is appropriate if TMCP steels are considered. The reason for this is that many of the TMCP steels are accelerated cooled to a temperature of around 620°C; heat treating at or close to this temperature will result in a substantial reduction in tensile strength due to over-tempering. Therefore, the temperature range required by RCC-M is not considered detrimental to the long term integrity of the vessel, provided that the appropriate temperature range is specified for the steel grade and manufacturing route considered. A summary of the PWHT requirements from the two codes is provided in Annex C.
S1900 Continued validity of qualifications. ASME IX (QW-100.3) states that PQRs made in accordance with ASME IX 1962 or any later edition may be used in any ASME construction. It also states that PQRs made to earlier editions may also be used, provided they meet all the requirements of the 1962 edition or later editions.
RCC-M allows use of existing PQRs qualified according to previous editions of RCC-M. In cases where heat input and interpass were not recorded during the qualification, the RCC M requires that the existing PQRs are integrated by preparing a test coupon under the same condition as the original welding procedure qualification. This would be used to establish the range of qualification for heat input and interpass.
S 3200 Welding procedure qualification - general. For the general case, RCC-M (S 3200) requires welding procedure qualification according to EN ISO 15614-1 and to a number of additional requirements defined in clauses S 3200 and B4231. Among these, the ones which are considered to have the greater effect on the qualification process and indirectly on the integrity of the production welds are listed below:
- The qualification is limited to the amperage range specified in the qualification test or in the qualification data sheet as described in S 5000.
- The qualification is limited to filler materials with exactly the same geometrical characteristics as the filler metal used for the qualification test piece. In this case, 'geometrical characteristics' was interpreted as referring to electrode or wire diameter.
In comparison, ASME III-NB-4330 requires that ASME IX be applied and provides additional requirements for toughness testing (impact test and DWT test to determine RTNDT).
S 3600 Weld overlay cladding with austenitic-ferritic or Ni-based alloys on carbon and low-alloy steels. The qualification practice of RCC-M includes the same destructive and non-destructive tests required by ASME (Section IX), as well as additional tests. The major differences between the two codes are listed here and tabulated in Annex D, which also includes reference to the relevant ASME clauses:
- RCC-M limits the base metal qualification to the grade used during testing, with the exception given in S 3612, whereas ASME IX qualifications are limited to base metals with the same P-No.
- RCC-M requires that the range of qualification for the weld overlay is based on the number of layers and the chemical analysis is performed at a depth of 2mm, after grinding 0.5mm from the surface of the as-welded overlay (S 3633b). On the other hand, ASME IX qualifications are based on the overlay thickness at which the required chemical analysis has been obtained.
It should be noted that the provisions of ASME IX apply to boilers and pressure vessels which are not necessarily designed for nuclear applications. Therefore, for nuclear components, the qualification tests not required within the ASME code are usually specified in the relevant equipment specifications.
S 3800 Tube to heat exchanger tubeplate (tube sheet) welds. The procedure and performance qualification requirements for tube to tubesheet welds are almost identical, with the exception of the leak test (not required by ASME) and the required throat thickness which is average 0.8 (min 0.66) times the nominal wall thickness of the tubes for RCC-M and minimum 0.66 for ASME (no requirements on average). A detailed comparison is shown in Annex E. These requirements only apply to the qualification of tube to tubesheet welds; therefore, it is not possible to compare the minimum throat thicknesses required by RCC-M and ASME for the completed components.
S 6000 Technical qualification of production workshops. Clause S 6000 describes how a Manufacturer can demonstrate the technical qualification of its workshop. It is not required that such qualification be assessed by an independent third party. However, it is recommended that an initial and/or periodic verification of the workshop qualification are carried out by the Contractor or by an independent third party, should the contractor also act as manufacturer.
Similar requirements are broadly covered by the ASME provisions for obtaining N certificates (ASME III, NCA-8100), although these are mainly related to verifying the application of the quality assurance manual and quality assurance programme, hence outside the scope of this review.
S 7000 Production welds. The RCC-M requirements for the performance of production welds are included in Clause S 7000, with sections following the fabrication sequence. With regard to the ASME code, such requirements are included in Section III-NB for Class 1 components.
S7400 Execution of production welds. The requirements for the execution of production welds, for instance tack welding, use of backing rings and attachments (see ASME III NB-4321(b), NB-4321.1, NB-4240, NB-4421, NB-3352 NB-4430), are generally similar and tend to overlap for most part.
A few differences were observed with regard acceptance criteria for visual and dimensional examination after welding. In various clauses on welding qualification and during fabrication, RCC-M permits no undercut, whereas ASME III NB-4424.1(c) allows 1/32in (0.8mm undercut). The reason for this discrepancy is that ASME does not consider undercut has been associated to any weld failures observed so far on Class 1 components . Similarly, root concavity is not permitted by RCC-M, whereas it is allowed by ASME (NB-4424.1(d)), provided the minimum required thickness is obtained.
Controlled peening to minimise distortion is allowed by ASME NB 4422, however, the same clause does not permit the use of peening on the initial layer, root of the weld metal or on the final layer, unless the weld is postweld heat treated. RCC-M does not allow peening (unless for special circumstances, with Contractor's approval).
S7600 Repair by welding. Both codes allow weld repairs and require qualification of welders, welding operators and welding procedures as per production welds (see ASME III NB-4453.2). ASME gives specific examination requirements for repair welds (NB-4453.4).
RCC-M restricts the number of repairs in the same location to maximum two, with further repairs subject to analysis of the cause of the issue and to Contractor's approval. The ASME code does not provide any specific restrictions on the number of repairs. This will depend on the required PWHT holding time and on the total PWHT length covered by the corresponding procedure qualification.
S7620 repair without post weld heat treatment (temper bead). Both codes permit weld repair without post weld heat treatment, provided the temper bead technique is applied. Whilst RCC-M only allows temper bead repairs by MMA welding, ASME III-NB, allows the following welding processes: SMAW, GMAW and FCAW (GTAW is allowed for repair welds to cladding). As summarised in Annex F, the ASME code provides more specific and stringent requirements for the performance and qualification of temper bead welding.
S7800 Production test coupons. This section of RCC-M requires that for main joints of class 1, 2 and 3 components, one production coupon representative of production welds is made per welding procedure qualification, per workshop and per pressure retaining component. Special cases (S 7822) are provided, eg tube-to-heat exchanger tubeplate welds, class 1, 2 and 3 pipe welds and others.
There is no mention of production test coupons in ASME Section III. Usually, for nuclear components manufactured to ASME, provisions for production test coupons are provided in the technical specifications for the particular component.
This part of the study focused on the examination and NDT requirements for production welding for the principal weld types in the proposed EPR reactor pressure vessel and steam generator. The assessment has been carried out with reference to the manufacturing examinations required by both codes and the associated acceptance criteria based on workmanship standards. Assessments of pre- and in-service examinations and examinations carried out as part of a determination of fitness-for-service are not included.
As might be anticipated, the general requirements for the examination of the various types of full penetration production welds for Class 1 nuclear components are very similar for the two codes. For example, such welds are all required to undergo a 100% examination by both surface and volumetric methods. The emphasis of this study was to highlight where the requirements differ markedly. This exercise is not straightforward, as the effectiveness of a particular non-destructive examination to achieve a combination of detection and adequate evaluation of discontinuities in welded joints is built up from requirements for:
- Access to the joint for examination (e.g. surfaces available for scanning, joint geometry, etc.),
- Weld surface condition (surface finish, removal of weld cap etc),
- Examination methods specified,
- Area or volume to be examined (100% for welds such as these),
- Extent of coverage (e.g. directions of magnetisation for MPT; beam angles, directions and scanning surfaces for UT; orientation of radiation beam for RT, etc.),
- Sensitivity settings or minimum levels achieved,
- Reporting levels,
- Procedures for evaluation of indications reported,
- Acceptance levels.
There is no direct mapping of each of these parameters between the two codes in most instances. Some combinations of parameters may yield similar overall levels of effectiveness; others may result in widely differing outcomes of the examination. Therefore a rigorous assessment of the minimum weld quality levels achieved when applying one code in comparison to the other was beyond the scope of this present study. The approach adopted has been to identify any clear differences in requirements between the two codes and to assess the effect of each of these where possible.
One significant difference between the two codes is that the ASME requirements for volumetric examination are based on the use of radiography, with ultrasonic testing being used as a supplementary examination. On the other hand, the RCC-M code specifies ultrasonic testing (alongside radiography) as an integral part of the examination requirements. However, a code case approved by the ASME standards committee in June 2008 (Case N-659-2) permits the use of ultrasonic testing in lieu of radiography, provided certain provisions are met.
Examination prior to and during welding
ASME requires the edge preparation surfaces for category A, B, C and D welds, 50mm or more in thickness, to be examined by magnetic particle or liquid penetrant examination. Visual inspection is not mentioned. RCC-M Clause S7360 states that a visual examination of 100% of the surfaces to be welded '...and adjacent areas' is required. The acceptance criteria are: 'The surface finish tolerances specified in the drawings and the surfaces to be welded shall have no defect liable to affect adversely the quality of the weld.' Magnetic particle examination is required by RCC-M for class 1 welds in carbon and low alloy steels and liquid penetrant examination for austenitic stainless steels and nickel based alloys. 100% of the surface is required to be examined in each case.
As far as acceptance criteria for liquid penetrant or magnetic particle examination for imperfections on weld bevels are concerned, RCC-M is more stringent. The threshold for evaluation is >1mm (1.5mm for ASME). No linear indications are permitted (laminar indications up to 25mm and linear indications up to 5mm long are permitted by ASME). The maximum size of permissible rounded indication is 2mm (5mm for ASME). Both codes specify limits for aligned multiple indications. ASME states a limit of 4 (up to 5mm, as above) separated by less than 1.5mm. RCC-M states a limit of 3 (up to 2mm) separated by less than 3mm. RCC-M includes an additional area-based criterion which is not present in ASME.
For intermediate examination of weld passes using Magnetic Particle (MPT) or Liquid Penetrant Testing (LPT). RCC-M includes the requirements for surface preparation prior to examination. Few specific requirements are given in either case.
Examination of completed welds
Extent of examination. ASME states that, for surface examination of butt welds (Category A, B, C and D joints), the external and accessible internal weld surfaces plus ½inch (13mm) of base material on either side shall be examined. A similar requirement exists for RCC-M, the only difference being that 15mm of base material on either side of the weld is to be examined.
For fully penetrated welds both codes require the completed weld to be examined by a surface method and a volumetric method. For volumetric examination, RCC M states that the examination must cover the base material for a distance of 5mm beyond the original preparation on each side for thicknesses up to 30mm and for 10mm beyond the original preparation for thicknesses of 30mm and greater. ASME does not clarify this.
It is worthy of note that RCC-M states that the entire length of the weld is to be examined. In ASME this is implied rather than explicitly stated, for example in Clause NB-5210, where it is stated that the joint concerned 'shall be examined...'.
A further aspect of the extent of examination relates to the requirements in the manufacturing specification for examinations which also partially fulfill the requirements for pre-service examination or which are carried out under Code Case N-659-2. For the ASME code, this invokes the need for additional ultrasonic testing (NB-5111 and Section XI, Appendix I) and the requirements for performance demonstration for the procedures and personnel carrying out the examinations (Section XI Appendix VIII) in some cases.
Time of examination. The requirements of the two codes are quite similar. The main requirement is to ensure that the examinations are performed after at least an intermediate heat treatment or after a final heat treatment. Obvious exceptions are when examinations are performed at various stages during welding operations.
ASME states that if a radiographic examination is performed before an intermediate or final heat treatment, then an ultrasonic examination is required after heat treatment. A similar provision exists in RCC-M, where an additional ultrasonic examination is required if the volumetric examination is carried out before heat treatment, even if it is by UT. Further, if both radiography and ultrasonic testing are required, the final examination shall be the ultrasonic test. ASME Clause NB-5120 (c) states that all dissimilar metal weld joints shall be examined after final post weld heat treatment.
Methods to be applied. ASME Clause NB-5200 provides the 'Required Examination of Welds for Fabrication and Preservice Baseline'. The requirements for weld examination during fabrication in RCC-M are given in Section IV, Clause S7710.
Surface methods. ASME Clause NB-5200 states that surface examination shall be by either liquid penetrant or magnetic particle methods. Liquid penetrant is specified for weld metal cladding. RCC-M states that magnetic particle examination shall be used for carbon and low alloy steels. Liquid penetrant examination shall be used for austenitic stainless steels and nickel based alloys.
- Liquid Penetrant Examination. ASME Section V contains much more detailed information than the RCC-M code, which references NF-EN 571.1 (AFNOR, 1997) and AF-EN-ISO 3452-2 (AFNOR 2006) for further details. RCC-M specifies surface finish requirements (6.3µm Ra for machined surfaces and 12.5 µm Ra for castings) whereas ASME does not. On the other hand, ASME specifies cleaning requirements in great detail. Both codes specify similar normal temperature ranges for penetrant examination (5 to 52°C for ASME and 10 to 50°C for RCC-M). Both include requirements for penetrant testing at elevated temperatures.
- Magnetic Particle Examination. Both codes provide for a variety of magnetisation methods and require similar magnetisation levels.
Volumetric methods. Both codes require a full volumetric examination of full penetration welds. The RCC-M code places a greater emphasis on ultrasonic testing than ASME. For example, for full penetration butt and fillet welds in the RPV and dissimilar metal 'safe end' welds RCC-M requires both ultrasonic testing and radiography (Table S7710.1 of the code). For major category A, B, C and D welds ASME (clause NB5200) places a much greater emphasis on radiography. It is stated in a footnote that 'A radiographic examination [NB-5111 (a)] is required; A preservice examination [NB-5111 (b)] may or may not be required for compliance to the Design Specification [NCA-3252 (c)].' However, provisions exist in the ASME code for the use of ultrasonic testing where radiography is impractical (Clause NB-5279). The use of ultrasonic examination in lieu of radiography is further strengthened via the Code Case N-659-2, which was approved in June 2008, and where pre-service examinations are required.
Sensitivity and reporting requirements
Liquid penetrant examination. ASME Section V, Article 6 sets out to ensure performance is maintained by concentrating on control of essential parameters of the test. There are no specific requirements for demonstrating capability. Dwell times before cleaning are stated in the related Article 24 'Standard test method for liquid penetrant examination', as these have a major influence on sensitivity. Requirements are given for viewing illumination, 1000 lux for coloured penetrants and 1000µW/cm2 for fluorescent.
RCC-M, Section III, Clause MC4200 states that the penetrant method must be capable of detecting 100% of flaws 20µm in length and 75% of flaws 10µm long. There is no stated requirement for viewing conditions in Clause MC4000, but as the code heavily references EN 571-1, the requirements of this are implied. This standard requires 500 lux for coloured penetrants, which is one half that for ASME, and 10W/m2
for fluorescent (which is identical to 1000µW/cm2
, as required by ASME). These values are also stated in BS EN ISO 3059 (BSI 2001), reflecting general good practice for illumination conditions for both liquid penetrant and magnetic particle inspections.
Magnetic particle examination. Both codes require the use of indicators to assess the level of magnetisation. ASME Section V, Article 7 includes pie (Berthold) gauges, shims and Hall Effect tangential field probes. RCC-M also references the use of the ASME field indicators. Where field measurements are possible, ASME requires 2400 to 4800 A/m, whereas RCC-M quotes 2400 to 4000 A/m. The viewing conditions quoted in ASME are 1000 lux for coloured inks and 1000µW/cm2 for fluorescent. RCC-M requires 500 lux for coloured inks and 10W/m2 for fluorescent (=1000µW/cm2) which, again, is as stated in BS EN ISO 3059.
Radiography. Whilst ASME Section V, article 2 is referenced for the method to be applied for radiography, Article NB-5111 of Section III places some restrictions on parameters to be used for Class 1 nuclear constructions. Fluorescent screens are not permitted and alternative Image Quality Indicators (IQIs) are to be used, as stipulated in Table NB-5111-1. The radiographic method is given in RCC-M Section III MC3000. Table MC3162.1 gives the hole and wire IQI sensitivity requirements for welds.
There are some differences between the two codes. For hole type penetrameters the requirements are generally comparable, although the step-wise increase in hole diameter with material thickness means that ASME requires a lower sensitivity (i.e. requires a larger hole diameter to be visible) at 100, 150 and 400mm thicknesses. ASME specifies a higher sensitivity at 250mm thickness. For wire type IQIs the RCC-M code requires a higher sensitivity to be achieved, by requiring smaller wires to be detected, except for 50 and 250mm thicknesses where they are comparable. The comparison is given in Table 3, below for a range of thicknesses.
Table 3 Minimum penetrameter hole and wire IQI sizes to be detected to meet code requirements.
As far as film density is concerned, ASME requires a minimum density of 1.8 for X-Ray exposures and 2.0 for gamma ray. The maximum acceptable density for any film is 4.0. RCC-M requires a minimum density of 2.0 regardless of source and a maximum of 4.5.
There are also some differences in geometric unsharpness values, Ug, permitted. ASME does not differentiate between source types but quotes thickness bands, Table 4:
Table 4 Geometric unsharpness values (ASME).
On the other hand, RCC-M quotes different Ug levels for different sources, but does not take thickness into account. The values for weld examination are given in Table 5:
Table 5 Geometric unsharpness values (RCC-M).
|X-Rays >400kV and Ir 192
|Linear accelerator and Betatron
As sources such as a linear accelerator or betatron will only be used at large thicknesses, it may be seen that the RCC-M requirements for unsharpness are more stringent than required by ASME. Taken together with the sensitivity requirements given in Table 3, the performance requirements for radiography in the RCC-M code are more stringent than those required by the ASME code for most material thicknesses.
Ultrasonic examination. Two factors are considered here, the coverage required and the sensitivity level. The RCC M code states that 14 test directions are required for weld examination, two with longitudinal waves and 12 with shear. The tests are also required to examine for imperfections parallel and transverse to the welding direction. Moreover, Figure MC2634.1.a shows that these scans have to be carried out from a combination of both top and bottom surfaces of the weld.
The ASME code, in Section V, Article 4, states that four scan directions are required (two parallel and two transverse to the weld axis). Clause T-472.1 states that a 45 degree beam (or angle appropriate for the configuration being examined) is required. This suggests that coverage is much less thorough than that specified by RCC-M. However, this clause also references the non-mandatory Appendix I to Article 4, which states that 45°, 60°, and 70° (or other suitable beam angles) are required (I-471). If these three angles and the four scanning directions are considered, this gives 12 angle beam scans, the same as for the RCC-M requirements. Add to this the requirement for straight beam scanning and the coverage requirements for the two codes are similar. It should, however, be noted that the ASME requirements are less specific about the need to test from both surfaces and that the circumstances when the additional beam angles identified in Appendix I are required to be included are far from clear.
The code case N-659-2, which permits ultrasonic examination in lieu of radiography, again states that four angle beam examinations are required. However, it also states (in paragraph (c)) that the performance of the procedure shall be demonstrated on a qualification block, described in paragraph (d), which is required to include planar flaws, at least one of which must be parallel to the fusion line. NB. This is not the same as the performance demonstration required for PSI and ISI examinations, in ASME Section XI Appendix VIII. It should also be noted that the use of this additional test to demonstrate the capability of the ultrasonic examination procedure does not apply to ultrasonic tests carried out under the ASME code in general, but only to cases where the Code Case is invoked and ultrasonic testing is used in lieu of the radiography specified in Clause NB-5200.
For test sensitivity, both codes use a DAC-based reference level. The side drilled (SDH) hole targets used to set the DAC level for weld examination are smaller for RCC-M and do not increase in diameter with increasing thickness, so the base reference level is of higher sensitivity. Ermolov showed that the response amplitude from a side drilled hole is proportional to the square root of its diameter, so that the change in sensitivity may be calculated. The differences are shown in Table 6 below.
Table 6 Calibration side drilled hole diameters for setting ultrasonic test sensitivity.
in far field
|25 - 50
|50 - 100
|100 - 150
|150 - 200
* These figures represent the amount by which the RCC M code reference level is more sensitive than for the ASME procedures.
This indicates that the reference sensitivity specified by RCC-M may be up to 6dB more sensitive than that for ASME. However, both codes include a threshold for amplitude of response for ultrasonic indications, above which they are required to be evaluated and below which they may be accepted without further investigation.
Clause T-482.1 of ASME states that all indications greater in amplitude than 20% of (i.e. 14dB lower in amplitude than) the reference level shall be investigated. For RCC-M, Section IV, S7714.4 states that indications exceeding 50% of the reference level (i.e. -6dB) are to be evaluated. Therefore, the RCC-M threshold relative to the reference (or DAC) level for investigating signals is 8dB less sensitive than for ASME. If the higher base sensitivity for the reference level in RCC-M (given in Table 6) is taken in to account, it does not compensate for the difference in reporting threshold between the two codes, so that the amplitude threshold to trigger the evaluation of a reflector according to RCC-M requirements is marginally less sensitive than for the standard ASME procedure. This is discussed further below.
As far as scanning sensitivity is concerned, ASME requires scanning at a level at least 6dB above the reference sensitivity, plus there are recommendations for scanning to be up to 14dB higher than the reference sensitivity level. RCC-M requires as high a scanning sensitivity as practical, without the trace being swamped with background noise. The ability of the two procedures to identify indications may therefore be similar, but the recommendation in RCC-M to scan at as high a sensitivity as reasonably practical suggests that this approach produces the highest achievable search sensitivity.
Surface examination. The acceptance criteria for the two codes are similar. Both use essentially the same criteria for both liquid penetrant and magnetic particle examination. The threshold size for evaluation is slightly larger for RCC-M, but the acceptance levels for non-linear indications are more stringent for RCC-M.
Radiography. The acceptance criteria for both codes are broadly the same. Neither allows any indication which is interpreted as a planar flaw. The length limits for elongated indications for thicknesses above 19mm are approximately the same. RCC-M is more stringent at small thicknesses, but this is not relevant to the main pressure boundary in the RPV and SG. The criteria for aligned indications are the same.
Ultrasonic testing. The acceptance criteria for ultrasonic examination are constructed differently for the two codes, so that a simple tabular comparison is difficult. Some important factors are:
- The requirement to investigate reflectors exceeding 50% of the reference level for RCC-M, compared with 20% for ASME, as discussed above.
- The RCC-M code requires the 'Cascade' procedure  to be used to determine the volumetric/non-volumetric nature of indications. This clause also states that, as an alternative, the procedure in NF-EN 1713:1998 may be used.*
- The acceptability of reflectors according to the RCC-M code depends on both their length and signal amplitude relative to the reference level.
* The 'Cascade' procedure was incorporated into EN 1713, where it is termed the 'flowchart' procedure. EN 1713 is now superseded by EN ISO 23279:2010. The provisions remain essentially the same.
For both codes no cracks or crack-like indications ('non-volumetric' for RCC-M) are permitted. The implications of this are discussed further below. For other indications, the ASME code, Clause NB-5331, has relatively straightforward criteria, based on the length of the indication, for signals exceeding the reference DAC level. Maximum permitted lengths are:
- 6mm for t ≤ 19mm
- 1/3t, for 19mm < t ≤ 57mm
- 19mm for t > 57mm.
For the RCC-M code, there is a more complicated relationship that takes material thickness, signal amplitude and length of the indication into account. This is summarised in Table 7.
Table 7 Acceptance criteria for non-planar flaws in full penetration welds from the RCC-M code S7714.4.
|t < 50mm
|The following are the maximum acceptable lengths:
- For 3/2Hr < Hd - unacceptable
- For Hr < Hd ≤ 3/2Hr - 20mm
- For ¾Hr < Hd ≤ Hr - 30mm
- For ½Hr ≤ Hd ≤ ¾Hr - 60mm
|t ≥ 50mm
|The following are the maximum acceptable lengths:
- For 2Hr < Hd - unacceptable
- For 3/2Hr < Hd ≤ 2Hr - 20mm
- For Hr < Hd ≤ 3/2Hr - 30mm
For ½Hr ≤ Hd ≤ Hr - 60mm
Hr = the amplitude from the hole in the reference block (2mm diameter), see Table 6.
Hd is the amplitude of response from the indication.
The criteria in Table 7, above, are difficult to compare directly with the ASME criteria. Therefore, to examine the overall effectiveness of the two sets of criteria, the thresholds (for non-planar flaws) have been plotted (for thicknesses up to 200mm) in Figure 1. This shows the various levels plotted against the ASME Reference Sensitivity Level, as determined by the side-drilled hole reflectors presented in Figure 1, which is set at 0dB. All other amplitude thresholds are plotted relative to this.
Figure 1. Representation of the Reference, Evaluation and acceptable amplitude levels for non-planar flaws for the RCC-M code in comparison with the ASME Reference Level (represented by 0dB) and Evaluation level, -14dB
With reference to Figure 1, first, the effect of the constant hole diameter for the RCC-M calibration block (Table 6) is evident; the RCC-M Reference Level becomes progressively more sensitive than that of the ASME code with increasing component thickness.
Second, the difference between the 20% DAC (-14dB) evaluation threshold for indications for ASME and the 50% DAC (-6dB) threshold for RCC-M is apparent, with the RCC-M threshold being 7dB less sensitive at small thicknesses, although this difference reduces to 2dB for 150 to 200mm (the maximum thickness plotted in the figure).
However, when the evaluation criteria are taken into account several other factors are observed:
- Whereas the ASME code only considers rejection of indications for which the amplitude exceeds the Reference Level (i.e. 0dB) (with some thickness-dependent length provisos, as stated above), indications may be unacceptable to RCC-M with amplitudes just exceeding the (RCC-M) evaluation level if their length exceeds 60mm.
- No indications exceeding 30mm long are permitted under RCC-M with signal amplitudes greater than the ASME Reference Level (0dB).
- The maximum amplitude permitted for any indication deemed to be non-planar in nature under RCC-M is 2.52dB above the ASME level and no indications greater than 20mm long are permitted to have response amplitudes greater than 0dB.
- The ASME code does not impose a maximum permitted amplitude for an indication. One may argue that the much shorter permitted indication lengths for a given material thickness under the ASME Code (see above) will place an effective limit on the possible amplitude from a non-planar manufacturing flaw.
The main conclusion that may be drawn from Figure 1 is that the rejection criteria for non-planar flaws detected by ultrasonic testing are more severe under the RCC-M code.
Detection of planar flaws. Whilst the criteria for evaluation of non-planar flaws are quite elaborate, as discussed above, both codes share a simple requirement that does not permit flaws determined to be planar in nature to remain in the production welds. The effectiveness of the use of ultrasonic testing to enable such potentially significant flaws to be removed from the production welds rests, firstly, on the ability of the procedures applied to detect them and, secondly, on the correct identification of an indication being from a planar flaw.
An overriding characteristic of planar flaws is that the responses are more highly directional than for more 'thread-like' imperfections of more-or-less circular cross-section (for example slag inclusions), for which the response is largely independent of the angle of incidence. To be successful in detection of such flaws there needs to be, firstly, a high scanning sensitivity, so that weak reflections from poorly oriented flaws may be identified and, secondly, a variety of appropriate angles of incidence on to potential flaw planes (for example perpendicular incidence to the fusion boundary, if this can be achieved) to allow the specular reflected responses from the plane of the flaw to be observed.
The RCC-M code applies a higher scanning sensitivity than ASME. The ASME code requires a scanning sensitivity where the gain is increased by 6dB after setting the Reference Level. RCC-M not only has a more sensitive Reference Level, see above, but requires the scanning to be at as high a sensitivity as practical without noise and grain scatter being a problem. Therefore, the requirements of RCC-M are more likely to enable weak reflections from poorly-oriented planer flaws to be observed than if the ASME procedures are followed. Further, RCC-M requires at least 12 angle beam directions, covering both longitudinal and transverse scans and using both top and bottom surfaces of the component, whereas the ASME procedures only require, as a minimum, four angle beam directions. This will reduce the likelihood that a signal of sufficient amplitude to require evaluation will be obtained from a poorly-oriented planar flaw when using the ASME procedures than will be the case for RCC-M.
On the other hand, both codes apply an evaluation threshold, above which it is necessary to investigate the characteristics of the indication. This threshold is lower for ASME than for RCC-M, so there is some uncertainty as to whether it is more effective to put more resources into increasing the chances of obtaining a high enough amplitude from potential planar flaws to require evaluation at 50% of the Reference Level, as in RCC-M, or to have a lower absolute amplitude threshold for evaluation, as in ASME. Where the ASME Code Case N 659-2 is invoked, there is a requirement to demonstrate the effectiveness of the ultrasonic test procedure to detect planar flaws in a test block, whereas RCC-M has no comparable requirement. However, this demonstration is not required for ASME unless a case is being made for the use of ultrasonic examination in lieu of radiography under the Code Case and this requirement does not generally apply.
One factor that is not explicitly covered by either code is that the procedure for examination of a specific weld would normally be expected to be tailored to be suitable for the weld geometry concerned. Both codes allow a choice of ultrasonic beam angles and ASME does recommend a beam angle '...appropriate for the configuration being examined'. (Section V, Article 4, T 472). It would be the case for any high quality fabrication that ultrasonic test procedures would be appropriate for the weld geometry being examined.
It is the opinion of the authors that when the more extensive scanning requirements for RCC M (in terms of both number of angles and scanning sensitivity) are also taken into account the ultrasonic testing requirements for production welds are more stringent than for ASME for the detection of planar flaws because the larger number of test directions required is more likely to result in a significant response being observed from at least one of them.
Personnel qualifications. Clause NB-5500 of ASME Section III specifies the American Society for Nondestructive Testing (ASNT) recommended practice SNT-TC-1A as the basis for personnel qualification for nondestructive testing. It is identified that this is a minimum requirement for personnel training and qualification. RCC-M Section III, Clause MC8000 states that personnel shall be qualified in accordance with NF-EN 473.
The principal difference between these approaches is that the SNT-TC-1A scheme may be administered by the employer of the inspector, whereas the EN 473 scheme requires the candidate to be examined by an independent body. The latter therefore provides much greater assurance that a consistent standard of qualification, and therefore competence, is achieved for personnel performing non-destructive tests.
It should be noted that if pre-service ultrasonic examinations are required under ASME, or if ultrasonic testing is performed under Code Case N-659-2 under some circumstances, the performance demonstration requirements of Appendix VIII of Section XI must be met. This requires further assessment of the capabilities of the technician.
The conclusions below were obtained from an interpretive comparison between RCC-M and ASME requirements. The authors do not intend 'more stringent' requirements as 'favoured'.
Welding and fabrication
- The comparison between the welding requirements in RCC-M and ASME code is summarised in Annex G.
- The RCC-M requirements for Class 1 welds in the EPR are in most instances considered equivalent or more stringent than those set forth by ASME, with the exception of repair without PWHT (item 3 below).
- The ASME requirements for repair without PWHT (temper bead) are more stringent than those provided by RCC-M.
Non-Destructive Examination of welds
- In general terms the requirements for examinations of full penetration welds are similar and are thorough. However, there are differences in how the examination requirements are built up. The cumulative effect of the different measures on the ability to achieve an absolute level of quality for both of the codes examined is difficult to quantify.
- For examination of the surfaces of the prepared edges to be welded, it appears that RCC-M is more stringent than ASME.
- The requirements for methods of examination to be applied to completed welds are similar for both codes. For surface examination, RCC-M is slightly more specific than ASME, requiring magnetic particle examination for ferritic steels, whereas ASME allows either liquid penetrant or magnetic particle examination.
- The sensitivity and acceptance levels for surface methods appear to be similar for both codes.
- For volumetric methods, the RCC-M requirements for radiography, both in terms of sensitivity to be achieved during the examinations and in the acceptance levels, are more stringent than ASME.
- For ultrasonic testing, RCC-M is considered to have much better defined requirements for coverage in terms of number of beam direction and angle combinations. The reference sensitivity is also higher than for ASME. The requirement in RCC-M to scan at the highest practical sensitivity may result in a better detection performance. Furthermore, a comparison of acceptance levels in terms of the amplitudes and indication lengths indicate that the overall results for rejection of imperfections will be more stringent for the RCC-M code.
The authors would like to acknowledge the contribution of the colleagues Rita Banks, Jackie Brand, Dave Godfrey, Sayee Raghunathan and Andy Woloszyn. This paper contains UK public sector information published by the UK Health and Safety Executive and licensed under the Open Government Licence v1.0.
- Ermolov I N (1972) 'The reflection of ultrasonic waves from targets of simple geometry', Non-Destructive Testing 5, pp87-91.
- Institut de Soudure (1997) Classification of ultrasound weld indications as volumetric/non-volumetric through the application of the 'cascade' method. IS US 319-21.
- Keshab et al, 2005: ' Design verification for reactor head replacement'. 18th International Conference on Structural Mechanics in Reactor Technology (SMiRT 18) Beijing, China, August 7-12, 2005 SmiRT 18 F08 6.
COMPARISON BETWEEN SOME RCC-M AND ASME GRADES OF LOW-ALLOY STEELS, C-MN STEELS AND STAINLESS STEELS USED IN REACTOR PRESSURE VESSELS (RPV), STEAM GENERATORS (SG) AND REACTOR COOLANT PUMPS (ENTRIES COMPILED BY TWI OR QUOTED FROM LITERATURE ).
|Example of component
|16 MND 5
|SA-533 Type B Class 1
|RPV core shells
|16 MND 5
|SA 508 Grade 3 Class 1
|RPV head flange, lower/upper head
|18 MND 5
|SA 508 Grade 3 Class 1
|SG tubesheet, nozzles, primary head
|20 MND 5
|SA 508 Grade 3 Class 2
|Alternative to 18 MND 5 for SG tubesheet
|20 MND 5
|SA-533 Type B Class 2
|Alternative to 18 MND 5 for pressurizer SG shells
|P355NH - EN10222
|SA 516 Gr 70
|SG steam outlet nozzle safe end
|Z2 CN 19.10
|SA 182 F304L
|Z2 CND 18.12
|SA 182 Type 316L
|SG and RPV safe ends
|20 NCD 14-7
|~A 508 4N Class 1
|Disk flywheel (reactor coolant pump)
RCC-M AND ASME REQUIREMENTS FOR PREHEAT TEMPERATURES FOR CLASS 1 COMPONENTS
|Carbon and carbon-manganese (C-Mn) steels
|Min 100°C recommended (mandatory for class 1 components) for steels with Rm<440Mpa in as-welded condition and E(1)>40mm
|Min 95°C suggested for:
P-No1 Gr1 (CE≤0.3 and t>38mm)
P-No1 Gr2 (CE≤0.3 and t>25mm)
Min 120°C suggested for:
P-No1 Gr1 and Gr2 (CE>0.3 and t>25mm)
Min 10°C suggested for all other P-No1 Gr1
calls for Appendix D
D 1210.1 and 3
|Min 125°C recommended (mandatory for class 1) for steels with Rm>440Mpa in as-welded condition and E(1)>20mm
|Low alloy steels (except Cr and Cr-Mo steels)
|Min 125°C recommended (mandatory for class 1) for steels with Rm>450Mpa or E(1)>15mm
|Min 120°C suggested for:
P-No1 Gr3. P-No3 Gr3 and P-No 11A (UTS>485Mpa or t>16mm)
Min 10°C suggested for all other cases in this group
|NB-4611(3) (mandatory) calls for Appendix D (non mandatory), D 1210.3
|Minimum 175°C recommended for RPV
|Minimum 150°C recommended for SG and pressurizer
|Cr and Cr-Mo steels
|Min between 150°C-300°C recommended (mandatory for class 1) for steels with Rm>400Mpa or E(1)>15mm
|Min 150°C suggested for:
P-No4 (UTS>415Mpa or t>13mm)
Min 10°C for all other P-No4
Min 205°C suggested for:
P-No5 (UTS>415Mpa or Cr>6% and t>13mm)
Min 150°C for all other P-No5
|NB-4611(3) (mandatory) calls for Appendix D (non mandatory), D 1210.4 and 5
(1) E= equivalent thickness according to S1310.
(2) Cr and Cr-Mo steels not included in the scope of work.
(3) NB-4611 requires that the minimum preheating requirements are specified in the welding procedure specification, according to the qualification requirements of Section IX.
RCC-M AND ASME REQUIREMENTS FOR PWHT
|Material grade to RCC-M (ASME)
|PWHT temperature, °C
|Minimum PWHT holding time
|Carbon and C-Mn steels
|550-625 (recommended min 575)
(min 30 min,
|Low alloy steels (excl Cr and Cr-Mo steels)
2h plus 15 min
|Cr and Cr-Mo steels up to 1.25Cr-0.5Mo
5h plus 15 min
|5h plus 15
|Cr and Cr-Mo steels up to 5Cr 0.5Mo
5h plus 15 min
|5h plus 15
RCC-M AND ASME REQUIREMENTS FOR CLADDING PROCEDURE QUALIFICATION
|Type of test
|All NDE required in production (see S 7700)
|Four side bends (two parallel and two normal to welding direction)
|Four side bends (two parallel and two normal to welding direction)
|2+0.5mm below as-welded surface of cladding(1)
|Any distance from weld interface(2)
|Determination of δ ferrite content
|Two macrographic sections (one parallel and one normal to welding direction)
|S I 500
|S I 600
(1) Qualification rage based on number of layers (S 3616).
(2) The distance from the approximate weld interface is the minimum qualified overlay thickness.
(3) May be required by the relevant equipment specifications.
RCC-M AND ASME REQUIREMENTS FOR WELDING PROCEDURE AND PERFORMANCE QUALIFICATION OF SG TUBE TO TUBESHEET WELDS
|Six tubes to be welded
|Five demonstration mock-ups
|ASME IX, QW-303.5
|EN ISO 15614-8
|ASME IX, QW-202.6, QW-193
|Number of tubes
|Liquid penetrant test
|Yes, as in production
|Yes, on 10 tubes
Mean weld throat thickness 0.8e and no individual value below 0.66e (e=nominal tube wall thickness)
|Yes, on 10 tubes
Minimum leakage path (weld throat) 2/3 specified tube wall thickness
COMPARISON BETWEEN RCC-M AND ASME REQUIREMENTS FOR TEMPER BEAD REPAIR
|RCC-M Clause S7620
|ASME Sect III NB-4622.9 and Sect IX QW-290
- Temper bead process is applied
- Approved by Contractor
- Contractor may require stress analysis
- Qualified as production welds
- Limited to MMA
- Electrodes stored between 100-150°C
- No specific reference to vacuum-packed electrodes
- Low hydrogen electrodes, stringer beads
- Minimum preheat temperature > qualification
- Surface temper beads required
- Post heating >200°C for at least 2 hours
- NDT after 48 hours
- Temper bead process is applied
- Specific requirements for qualification (QW-290) with specific essential variables
- Hardness requirements more stringent than RCC M
- Limited to SMAW, GMAW, FCAW, GTAW
- Electrodes for SMAW stored at 105-175°C
- No specific reference to vacuum-packed electrodes
- Low hydrogen electrodes
- Minimum preheat temperature > qualification
- Surface temper beads required is used in qualification
- Post heating 230-290°C for at least 2 hours (P No1) or 4 hours (P-No 3)
- NDT after 48 hours
- Provides a specific repair method.
COMPARISON OF RCC-M VS ASME REQUIREMENTS FOR WELDING AND FABRICATION
|Indicates more stringent standard or equivalence
|Reference clause in RCC-M Section IV and ASME(3)
|RCC-M vs ASME(2)
|R for some grades A for others
|Welding Procedure Qualification
|RCC-M refers to ISO standards
|Weld overlay cladding
|Qualification of tube to tubeplate welds
|The reference clauses only apply to qualification, not to completed components
|Qualification of welders and welding operators
|RCC-M refers to ISO standards
|Qualification of workshops
|Not in ASME
|Storage and use of welding consumables
|Preparation of surfaces for welding
|Execution of production welds
|Repair by welding
|Repair without post weld heat treatment (temper bead)
|Production test coupons
|Not in ASME
(1) √=topic included in the code/spec, X=topic not included in the code or specification.
(2) 'R' means RCC-M is more stringent than ASME. 'A' means RCC-M is less stringent. '=' means they are considered equivalent.
(3) ASME Section II, unless specified.