Be prepared28 February 1998
Significant cracking in vessel internals is an issue of growing importance for BWR operators. Experience shows that a proactive programme of analysis, mitigation, repair/replacement and inspection/monitoring is an effective way of dealing with the issue. Without an orderly plan for managing internals there can be major impacts on plant availability.
Detection of intergranular stress corrosion cracking (IGSCC) in boiling water reactor (BWR) internals has increased significantly in recent years. It is likely that much of this cracking has been found because inspections are being performed more often, in more locations, and with better techniques. The increased crack findings have caused utilities to focus on management of internals. Continued cracking and disposition of cracking through repair/replacement can be a significant financial burden to utilities, and jeopardise continued operation.
Much of the detected cracking has probably been present for some time. Most BWR plants are currently operating with very good water chemistry, making the likelihood of significant new crack initiation low. Current typical water chemistry values are at levels sufficient to reduce crack growth rates significantly compared with those that would be expected using the early water chemistry regimes. So the internals cracking encountered in recent years is most likely not an indication of new cracking, but of cracking that has been present for many years and propagating at slow rates.
Much of the recently observed cracking is significant in terms of size, extent and number of occurrences. For example, crack lengths in BWR core shrouds have been observed to be of the order of meters. While to date there have been no instances of through-wall cracking, crack depths in the core shroud typically are found to be 25% to 50% of wall, with deeper cracking in some instances.
Such significant cracking emphasises the need for a proactive approach, so that a plan of action and countermeasures are fully developed in the event that cracking is detected. This can reduce the risk of delayed start-up and lessen the impact on plant availability.
In several instances, utilities have had to rush into repair activities during an outage without adequate planning. These rushed repairs are very expensive because of the need for a rapid major design and fabrication effort and because of their impact on critical path downtime. In some cases, the installed repair is considered temporary, whereas with better preplanning and preparation, the repair could have been implemented as a permanent fix – and at lower overall cost.
To minimise the impact of any degradation, an integrated programme is needed which addresses the overall degradation issue. The components of this integrated programme should include analysis, mitigation, repair/replacement, and inspection/monitoring, as shown below. The effective use of this integrated programme can help to maintain plant availability at high levels.
Unfortunately, many plants enter outages and start inspections without having made any preparations for dealing with cracks should indications be detected. This has caused delays in plant restart while evaluations are performed to demonstrate that structural integrity is maintained for at least one more operating cycle.
By postponing the need for an immediate repair, there is time to implement an orderly repair/replacement or mitigation programme. This additional time can be useful to obtain optimum repair/replacement designs, perform testing, select appropriate materials, and pursue mitigation options. In addition, the time can also be used to thoroughly evaluate the consequences of cracking. A possible result of these activities could be the demonstration that repairs may be postponed or may not be necessary.
Proactive analysis of indications and dealing with them (disposition) has become common in the USA, as well as in other countries. Demonstration of structural integrity, as well as continued safe operation, has been intensively pursued in the USA through the BWR Vessel & Internals Project (BWRVIP), for which EPRI is programme manager.
OUTLINE OF A PROACTIVE PROGRAMME
Compared with repair/replacement and mitigation actions, proactive analysis is a cost-effective way of addressing cracking. As a first step in properly preparing for an inspection, the following studies can be used as an initial effort in a proactive programme:
STEP 1: Component study
Determine component design and material features. For example, determine the presence of crevices, and types of base and weld materials.
STEP 2: Stress and load study
Determine the stress in the components. This includes determination of stresses from all potential loads. The applicable load combinations must also be determined.
STEP 3: Environment study
Determine the environment that is in contact with component surfaces and the nature of radiation levels.
STEP 4: Degradation study
Given the results of the component and environment studies, determine the susceptibility of the component to various degradation mechanisms. In addition, this study can also determine the location and orientation of potential cracking. The results can be used to prioritise the components and to provide recommendations to mitigate, repair, replace, inspect, monitor, and evaluate cracking. The above diagram shows the relationship between high stress, aggressive environment, and susceptible material. If all three of these are present, then IGSCC may occur.
STEP 5: Consequences of degradation study
Determine the consequences of degradation in the components if cracking were to occur and go undetected. The consequences should consider impact on plant safety, plant availability and financial impact.
STEP 6: Allowable flaw size evaluation study
Given the orientation, material, geometry, stress and environment information, the allowable flaw size evaluation can be performed. This evaluation would require some assumptions about crack configuration (eg assume that cracks are through-wall in a core shroud weld). These conservative assumptions may be refined if necessary in case the flaws cannot be dispositioned (ie a safety case made) under the initial allowable flaw size criteria. Of course, this refined analysis cannot be performed until the indications have been characterised. Refined analysis would include consideration for remaining wall ligament and spacing of multiple cracks.
STEP 7: Pre-outage planning study
Pre-outage planning studies are also extremely important in preparing for potential degradation. These studies should focus on the following items:
• Conclusions of proactive studies discussed above.
• Identification of regulatory requirements regarding internals components.
• Understanding the various options for dispositioning/mitigating cracking.
With a comprehensive understanding of these three items, a short and long-term programme may be outlined which can identify the following:
• Which components should be inspected over the next 5 to 10 years. This includes the inspection needs at each outage, and expanded inspection scope in the event that cracking is observed.
• What mitigation actions are necessary to reduce potential for cracking, for example hydrogen addition and noble metal addition.
• What repair/replacement activities should be implemented.
With proper implementation, this proactive plan can help a utility focus resources on specific locations in vessel internals that are critical to the safe and economic operation of the plant. Once focused on the essential components and issues, inspection, monitoring and repair/replacement options may be used in the most cost-effective manner.
The importance of planning ahead cannot be over-emphasised. Not only should flaw evaluations be prepared ahead of time to use during inspection, but discussions with regulators should be carried out prior to use of the flaw evaluation methodology in order to gain acceptance of the methodology. This has been done in the USA by the BWRVIP and has allowed timely resolution of cracking issues.
The integration of flaw evaluations and analysis with planned inspections is also extremely important. It is very important that utilities have a disposition plan (ie means of dealing with indications) prior to performing an inspection in order to avoid last minute emergencies.
EXAMPLE: CORE SPRAY LINE
As mentioned earlier, disposition of indications has become relatively common. For example, cracking has been dispositioned in core shrouds, top guides, core spray lines, and jet pumps. The following is a summary of the application of the proactive evaluation approach to the core spray line. There have been several instances where this approach has been successfully used in order to disposition cracking in this component.
The function of the internal core spray line is to provide a flow path for coolant directly into the core as part of the emergency core cooling system (ECCS). Thus, it plays an important role in assuring safety in the unlikely event of a significant loss of coolant event when the core may not be fully reflooded by other means. The core spray line is located in the annulus between the reactor pressure vessel and the core shroud. It begins at the vessel core spray nozzle and penetrates the core shroud, where it is connected to the core spray sparger.
Cracking has occurred at many locations in core spray lines at several plants. Cracking has been predominately IGSCC at the circumferential welds. There has been a high incidence of cracking at the T-box junction, and recently, significant through-wall cracking at the creviced sleeve connections has been seen.
• Step 1 (component study): The core spray line is made from austenitic stainless steel, generally either Type 304, 304L or 316L. The piping diameters range from 4 inch to 6 inch. There are several circumferential welds that were not solution heat treated. This material contains carbon which, depending on amount, can be a significant contributor to IGSCC. The weld process can be flux or non-flux, which can lead to different material behaviour. In addition, the vertical portion of the piping contains a sleeve that connects two pipe ends, which creates a severe crevice.
• Step 2 (stress and load study): Since the circumferential welds are not typically solution annealed, significant weld residual stress is present at these locations. In addition, significant stress may be present due to cold-springing during construction. Applied stresses are also present in the core spray line, such as those due to deadweight, hydraulic loads, thermal expansion, and seismic loads.
• Step 3 (environment study): The region where the core spray line is located is highly oxidising, which results in a high electrochemical corrosion potential (ECP). In addition, the presence of crevices tends to concentrate ionic species that promote IGSCC. The fluence at the core spray location is not high enough to cause significant reduction in ductility or increase the potential for irradiation assisted stress corrosion cracking (IASCC).
• Step 4 (degradation study): Based on the results of the first three steps, the three required conditions exist in the core spray line which make it susceptible to IGSCC: aggressive environment; high sustained stress; and susceptible material.
• Step 5 (consequences of degradation study): The core spray line is part of the ECCS, and thus a safety related component. Plants typically have margin against leakage and this must be verified on a plant specific basis. The impact of any leakage caused by through-wall cracking on two-thirds core coverage and peak-clad-temperature (PCT) must be determined.
• Step 6 (allowable flaw size evaluation study): Allowable flaw sizes are required for all susceptible circumferential welds. Methodology has been developed by the BWRVIP and applied in several cases to disposition cracking in core spray lines. The methodology is based on a limit load approach (similar to methodology contained in the ASME Boiler and Pressure Vessel Code, Section XI, Appendix C), that recognises the ductility of austenitic stainless steel. The allowable flaw evaluation determines the maximum length of a through-wall flaw that can be present and still maintain structural integrity, including appropriate safety factors, throughout an operating period.
• Step 7 (pre-outage planning study): Based on the results of Steps 1 to 6 above and similar studies for other high priority components, a pre-outage planning study can be performed to develop the most effective outage activities. This would involve identification of inspections, mitigation activities, and repair/replacement activities needed for implementation during the upcoming outages. In addition, regulatory requirements would need to be included in this planning study. One of the possible valuable results from this study could be the identification of inspections that may not be necessary.
JET PUMP RISER CRACKING
In order to assure long-term, economic, safe operation of nuclear power plants, an orderly plan to deal with degradation issues must be in place. Proactive evaluations are a cost-effective first step on this journey.
A good illustration of the benefits of the proactive approach is recent experience with jet pump riser pipe cracking. This has suddenly emerged as an important issue in the past year and utilities have had to respond rapidly. The use of a proactive, step-by-step programme would have resulted in more orderly implementation of a management plan for this component.