Applying ALARA for Replacements of Steam Generator Feedwater

Applying ALARA for Replacements of Steam Generator Feedwater Distribution Pipes at Paks NPP Gábor Volent István Metzger Paks NPP, Hungary

Paks NPP 1982 Unit 1 1984 Unit 2 1986 Unit 3 1987 Unit 4

Erosion and Corrosion Damages on Feedwater Distribution Pipes of Steam Generator Similarly to other VVER-440 type NPPs erosion and corrosion damages on feedwater distribution pipes of steam generator were observed at Unit 1 of Paks NPP in 2000. An extensive replacement programme was started. In the frame of this programme the replacement of feedwater distribution pipes was carried out on 14 steam generators in 2001. (Unit 1 - 3 SGs, Unit 2 - 5 SGs, Unit 3 - 6 SGs)

Steam Generators in VVER-440 type NPPs 1.

Steam Generators in VVER-440 type NPPs 2.

Dose Rate in Steam Generators at Paks m. Sv/h A significant part of the replacement work can be performed inside the steam generator. We assumed 50 hours/SG.

ALARA in Replacement of Feedwater Distributions Pipes In the view of the extent of work and the dose rate at the place of work, it was clear from the start that a relatively high individual and collective dose has to be taken into account. ALARA approach played important role in preparation as well as implementation phase.

First phase of preparation 1. The preparation started 8 months before than the first replacement was performed. Ø analyze of experience of VVER-440 type NPPs Øconsider of procedure of replacement and all aspects of work (dose rate reduction, schedule of work. . . ) Øpreliminary dose plan Ødifferential cost-benefit analyses (25000 HUF /~100 USD per man m. Sv saved) An ALARA report was completed at the end of first phase of preparation.

First phase of preparation 2. The ALARA report suggested the use the following dose rate reduction measures:

Second phase of preparation 1. Multidisciplinary team co-ordinated the preparation activity. Scheduling of works: Proper scheduling of replacement works and harmonization of dose reduction measures with conditions of other outage works. Manpower: The goal was that none of the workers reach the monthly or yearly plant level dose limit. Working environment: Light, temperature and ventilation. Optimization of work load in relatively high dose rate area. Training: The training on full size SG mock-up prepared the workers to work in more efficiently.

Full Size Mock-Up

Second phase of preparation 2. ALARA report was completed separately for each Unit one month before than the outage of Unit started: Ø results of cost-benefit analyses: the earlier planned dose rate reduction measures were not changed Ødetailed dose plan for replacement

Work implementation 1. Multidisciplinary team co-ordinated the replacement works. Role of RP personnel: Provide assistance and advice. Control of individual and collective dose: The individual and collective dose of workers were controlled continuously by operational dosimetry. The dose data were followed annualized by supervisors.

Work implementation 2.

Work implementation 3.

Work implementation 4. Feedback data and experience collection for the preparation and implementation of replacement work Øreduce the duration of the work inside the steam generator Ømodification of shielding Ømodification of access

Work implementation 5.

Dose results of replacement The highest individual dose did not reached 15 m. Sv.