Full marks for Forsmark’s welds29 November 2000
Forsmark had been suffering from regular cracks in the nozzles to the reactor pressure vessel as a result of the use of Inconel 182 as a weld filler material.
Since the 1970s, discontinuities have been found in the Inconel welds of reactor pressure vessel nozzles of BWRs. These discontinuities consist of cracks caused by stress corrosion cracking in the areas of manually performed welds made using the weld filler metal Inconel 182.
The cracks have been found in Inconel 182 butts at the ferritic nozzles as well as in the welded joints connecting to the fully austenitic safe ends.
At Forsmark, there has been a history of outages caused by such cracks (see box).
Forsmark took the decision to repair all reactor pressure vessel nozzles to prevent more damage developing and to allow longer intervals between regular inspections. Previously, inspections were required every three years. The weld repairs should allow this to be extended to 10 years.
In June 1999, Forsmark decided to repair all the problem areas on the 10 nozzles at each of Forsmark 1 and 2, and awarded the contract for this to Siemens. The 10 nozzles consist of four core spray nozzles (system 323), four feedwater nozzles (system 415), and two shutdown cooling nozzles (system 321).
The design principle of all of these nozzle welds is the same. The ferritic nozzle is clad with austenitic material on the inside. The adjacent butting was applied manually using the weld filler material Inconel 182. The safe-end, made of Inconel 600, was welded to the nozzle using Inconel 182 as weld filler metal. The Inconel 600 weld connecting the nozzle to the austenitic piping was also made using the weld filler metal Inconel 182.
Stages of repair
The first stage of the repair process to construct a full-scale 1:1 mock-up to enable the repair process to be fully tested prior to the actual repair. This was necessary to satisfy Forsmark and the Swedish Nuclear Power Inspectorate (SKI). These mock-up tests started in November 1999, and were completed by the end of February 2000.
Repairs at Forsmark 2 started in May 2000, with a contracted time for the repairs of 35 days. The repairs were actually completed in 34 days for Forsmark 2. Repair work on Forsmark 1 began in August 2000, and was completed in 32 days.
The main difficulty in carrying out the repair work was the distance of the weld from the exterior side.
First of all, it was necessary to cut the pipe outside the biolgical shield to allow access to the welds.
When this was done, a high pressure water cutting process using a sand-water mixture at 1000 bar was carried out. The area was cleaned, and a milling manipulator used to cut away a groove to 5mm beyond the transition line. The groove was cut, and the location of the transition zone was determined by delta ferrit measurement with a Förster-probe, and if necessary, the groove was extended.
Once the groove was completed and tested by eddy-current testing and VT, a welding machine carried out overlay welding of the groove. The surface of the weld was milled, and the surface of the weld was then checked with eddy current testing, ultrasonic testing, and visual examination by TV.
Inevitably, the milling and welding is a lengthy process. Several layers of weld build-up – 3 layers in the original, which was later increased to five layers. The milling task took approximately 48 hours, with the welding task taking approximately 30-40 hours.
The feedwater system has a slope of 10°. For this reason special milling is necessary to provide the smooth milling required to reacquire the geometry.
The milling manipulator that was used in the process was described in the August 2000 issue of Nuclear Engineering International, pp22-23.
In addition, Siemens took the opportunity of the outage to redesign the pipework in order to reduce the number of welds in system 321 from 36 to 25 by the use of induction bends and monoforged penetrations with integrated bends and to replace the inner and outer isolation valves in system 321 to improve the containment isolation function. It was also possible to optimise the mixing points by numerical simulation using the Fluent computer program. This reduced the risk of thermal fatigue by reducing hot spots.
Siemens received the contract for similar work for Forsmark 3 to be carried out in 2001. This will be a harder task than Forsmark 1 or 2, as it has a higher radiation level in the relevant area than the previous two units. The repairs must be carried out within an outage time of 35 days.
|History of outages caused by weld cracks in reactor pressure vessel nozzles at Forsmark|
|1989 Repair at nozzle system at Forsmark 1. 1992 Reported indications at Forsmark 2. 1996 Repair at nozzle system at Forsmark 1 by GE. 1997 Local repairs and sample cuts at Forsmark 1. 1999 Reported indications at Forsmark 2.|
|Scope of project|
|•New fluid dynamic analyses for pipe breaks inside and outside containment. •New pipe stress analyses, CFD analyses of mixing point, pipe rupture analyses, and new fatigue analyses. •New restraint and support concept (no pipe whip restraints necessary in the area of the isolation function of system 321). •Process design (including preparation of logic diagrams) for system medium operated gate valves. •Detail design for all pipes, penetrations, supports and isometrics. •Preparation of all material specifications. •Delivery of all forgings, induction bends, penetrations, supports, restraints, small bore valves, and insulation. •Performance of a 1:1 mock up for the repair of the nozzles. •All installation works. •Commissioning of system.|
|Working sequence for nozzle repairs|
|The following steps formed the basis of the working sequence for nozzle repairs. •Cut the pipe outside the biological shield. •Decontaminate nozzle, safe-end and process pipe with high pressure water cleaning. •Carry out NDE of existing weld nozzle and safe-end. •Inspect by video. Measure transition lines with Förster probe. •Mill the repair groove at the nozzle and safe-end to a depth of about 4.5mm. •Complete NDE of groove bottom and dimensional check. •Overlay welding of groove. •Surface milling of clad weld. •NDE of cladding. •Dimensional check of cladding. •Measure transition lines at safe-end and process pipe. •Cut process pipe at existing safe-end using water jet . •Bevel new weld at existing safe-end. •Measure wall diameter and thickness of weld bevel. •Transfer bevel geometry to new process pipe. •Fit new process pipe to safe-end. •Weld new butt weld safe-end and process pipe. •Surface milling from inside of new butt weld. •Milling from outside of new butt weld. •NDE of new butt weld from inside. •Eddy current testing from outside of new butt weld. •Dimensional check of new butt weld. •Closing of system.|