Environmental Engineering Reference
In-Depth Information
J-weld of the 47
th
CRDM head penetration. The leak in the J-weld was found
through helium leakage testing; ECT found some indications of cracks on
the J-weld. After grinding of the J-welds, dye-penetration testing was carried
out. Cracks were observed on the portion of dye penetrant indication located
along the grain boundary. It was ascertained by additional grinding that the
long crack was connected with other cracks. The reactor vessel head (RVH)
was replaced in 2007. The new head had a penetration nozzle and J-weld made
from Alloy 690. There are 23 in-service PWR plants in Japan. At the present,
14 plants have replaced RVHs and seven additional plants will replace RVHs
with Alloy 690 TT in the near future. One power plant has had CRDM nozzle
penetrations which have been thermally treated with Alloy 690 since plant
construction. Other power facilities in Japan have solved the issues of CRDM
head penetrations by reducing the temperature at penetration.
In April 2003, a small amount of boric acid sediment was found in two
BMI penetrations (No. 1 and No. 46 of a total of 58) in South Texas Project
Unit 1 (STP Unit 1). This is the only evidence of leakage of BMI nozzle
penetration reported in facilities in the United States up to now. The BMI
penetrations of STP Unit 2 were built with drilled Alloy 600 bar, and con-
nected to the lower head of the nuclear reactor vessel by welding Alloy
82/182 J-grooves.
In January 2003, a small cracking signal was detected on the internal sur-
face area of the BMI penetration nozzle, and 50 BMI penetrations were
found in Takahama Unit 1 in Japan through ECT. This indicator was within
the permissible limit (
3 mm depth) but it was concluded that the facility
in this unit was likely to be at the beginning of PWSCC. The utility applied
water jet peening on the surface in the BMI penetration nozzle after remov-
ing the crack indication. Laser and water jet peening are used for relief in
other Japanese PWR plants. The peening method has been carried out in
welding J-grooves in this location.
In September 2003, the thirteenth regular inspection of the PZR nozzles
in Tsuruga Unit 2 in Japan was conducted. Cracks were found in the weld
zone of the pressure relief line nozzle stub. This was the fi rst case where sed-
iments of boric acid were found. In ultrasonic testing on the relief line stub,
two indicators were found located on the repair weld zone. In ultrasonic
testing of other nozzle stubs, an indicator on the safety valve was found,
but nothing was detected elsewhere. According to the observation, cracks
remained in the weld zone only and developed in a circumferential direc-
tion of the pipe. The cause was analysed as SCC created in nickel-based
alloy (600 type), also of the same type as the material welded in the PWR
fi rst coolant environment. The welding metal for the weld zone of the pipe
nozzle on the pressure relief valve, the pipe nozzle stub on the pressure
relief valve and the safe end was changed to nickel-based alloy (690 type)
which has resistance to SCC. At the end of 2008, most of the pressurizer
≤
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