Development of corrective actions

A multi-pronged approach was adopted to address the above areas so as to achieve the desired results.

Areas covered included administrative measures, enhancement and reinforcement of quality of procedures and practices, and incorporating several design modifications for reduction of external as well internal radiation exposures to plant personnel.

5.1. Administrative measures

1) Setting up targets to reduce collective doses to 70% in next 3 years for old plants (10%/yr).

2) Redefining targets of collective doses for new plants in line with prevailing industry standards.

3) To budget (Plan in advance) the collective dose at the beginning of every calendar year, giving details of works planned, including dose commitment.

4) Introduction of concept of collective dose constraint at 80% of above budgeted dose when plant operator are required to inform regulator.

5) Decontamination or replacement of highly contaminated equipments as a policy.

6) Introduction of penalty clause for contract workers for radiation-protection rule violations.

7) Overview the preparedness prior to commencement of high person-rem intensive jobs.

5.2. Work practices

1) Restricting entry to shutdown-accessible areas immediately after reactor shutdown.

2) Keeping coolant under pressure and under purification after shutdown.

3) On line purification of Moderator and PHT systems water even during reactor cold and shutdown state.

4) Drying/decontamination of heat exchangers before taking ISI activities.

5) Use of umbrella concept of isolation and maintenance during shutdowns. (Elimination of repeat isolation and normalization).

6) Revised procedure to keep adjusters/PSS elements inside the core while working in RCM area.

7) Extensive use of temporary shields, Lead aprons, lead fiber mats, pile up shields.

8) Discontinuing practice of slurring the PHT resins for ejection.

9) Optimizing ISI activities, heat exchangers and steam generators.

10) Avoid power ramps and follow a defined power-raise programme for improved fuel performance.

11) Enhancement of OE sharing -periodic meet of concerned departments.

12) Reinforcement in use of maintenance procedures to avoid frequent failures/repeat jobs.

13) Replacement, ejection or recharging of ion exchange columns preferably during reactor shutdown.

14) Use of mixed based resins for chemical control.

15) Chemical decontamination of primary heat transport system, essentially prior to major refurbishment job such as en-masse coolant channel replacement.

5.3. Design modifications/improvements

1) Removing shortfalls in fuel fabrication and improving the fuel bundle testing techniques.

2) Adoption of Cobalt free material for reactivity mechanism balls, pump seals.

3) Incorporation of quick disconnects type electrical connectors in MOVs. (time saving).

4) Increased use of remote operated tools for ECT of heat exchanger tubes.

5) Creep measurement with the help of the fuelling machine.

6) Incorporating changes in layout of piping or changes in access control route.

7) Elimination of potential source of light water leaks in shutdown accessible areas (additional valves installation, deletion of redundant valves, installation of caps at open ends and seal welding, use of bellow seal valves etc.)

8) Replacement of hot insulation having metal sheets with quick connectors- reduced work duration in installation /removal.

9) Modified Jigsaw panels, reduced number of panels and reduced fasteners.

10) Shielding of crane operator cabins (save even small contributors).

11) Replaced of pumps having mechanical seals with canned rotor pumps in Moderator system.

12) Replacement of Zircaloy-2 pressure tubes with Zr-Nb 2.5 pressure tubes to reduce work required for creep elongation adjustments, component replacement.

5.4. Improvements to reduce internal doses 5.4.1. Tritium source control

1) Improving leak tightness of heavy water systems — Routine watch on LC pump out (1/day), LIGs etc, use of D2O sniffer.

2) In-situ post maintenance leak tightness checking of inter space of double gasket joints, before charging D2O.

3) Installation of vent condenser in PHT LC system vent line.

4) Incorporation of PHT LC system integrity test during long shut downs.

5) Use only fresh water in ALPAS tanks (change in procedure from using system water having high 1H³).

6) Flushing of heat exchangers with fresh (Uncontaminated) D2O and drying thereafter to bring down Tritium content in air prior to ISI/maintenance.

7) Diaphragms (elastomers) replacement on routine basis as PM.

8) Adopting internal draining of D2O system and equipment.

9) Maintaining strict segregation between PHT and Moderator water during collection and processing.

10) Swapping of water- during refurbishment of plants PHT water of comparatively low Tritium is drained, up graded and used in Moderator system, thereby reducing Tritium in high pressure

& high temperature PHT system).

11) Elimination of potential source of heavy water leaks (additional valves installation, deletion of redundant valves, installation of caps at open ends and seal welding).

12) Use of chloride free insulation for D2O stainless steel tubes & pipe to avoid stress corrosion cracking such as Delayed Neutron Monitoring tubes, Instrument tubes and LC pipes.

13) Ensuring D2O draining prior to opening of equipment/system.

14) Ensuring system integrity prior to charging of D2O

15) Ensuring system configuration prior to D2O transfer/charging.

5.4.2. Tritium DAC¹ control

1) Improved availability and performance of D2O vapour recovery dryers, particularly during shutdown. Versatile mode of operation of one dryer.

2) Improving V1-V2 integrity.

3) Ventilation flow balancing.

4) Increased purge &Temporary Supply of fresh air in S/d accessible area during maintenance.

5) Dedicated DAC coordinator during shutdown.

6) Centralized vacuum mopping system to collect spilled heavy water.

5.5. Upgrades in radiological monitoring/protection

7) Introduction of computerized dose management system, dose trending.

8) Introduction of display of area radiation field and Tritium DACs on local computer network.

9) Large-scale use of alarm dosimeters.

10) On-line display of area radiation monitors in shift health physics control room.

11) Computerizations of radiation work permit control.

12) Use of light-weight mask air hoses.

5.6. Reinforcement of other areas

13) Reinforcement of training and retraining of workers, especially crew based training.

14) Introduction of practical demonstration and hands-on practice of using protective equipments.

15) Pre job briefing on radiation-protection aspects.

16) Reinforcement of review meetings to achieve ALARA doses for dose intensive activities.

1 Derived air concentration (typically of tritium).

17) Management observation programme by line managers.

18) Corporate review programme based on WANO-Peer Review methodology.

6. Conclusion

Setting clear and high expectation from company executives, using a Holistic approach to address important issues, reinforcing of training, the continuous evolution of radiation-protection practices, and a management-observation programme have yielded good results and led to a reduction in collective doses in NPPs.

DOWNSIZING OF THE PHT PURIFICATION FILTER CARTRIDGE IN WOLSONG