ISOE studies show local effect on dose management29 October 1999
Occupational exposure data in the ISOE database makes it possible to study doses related to tasks in the plants. In recent studies, doses related to insulation jobs and to refuelling have been examined, and show that practices are influenced by other reactors in the country, as well as other reactors of the same type. by Stefan Mundigl & Ted Lazo
The database of the Information System on Occupational Exposure (ISOE) allows dose and exposure data to be examined and analysed across the commercial nuclear industry. It has been used, for example, to analyse the average collective dose per reactor for operating reactors included in the database indicating a clear downward trend over the last decade (from 1987 to 1998).
Removing, maintaining and replacing insulation is a constant need in reactor operation and maintenance, and a continuing cause of exposure to workers. A recent ISOE study considered the evolution of doses incurred by insulation workers at 84 European PWRs in the period 1990-1997.
The 84 units selected were those providing more than three years of data for insulation job doses. The calculation was made on a three-year rolling average for each unit. The mean per country, or per sister-unit group, was then calculated using the three-year average dose of each unit belonging to the group under consideration.
In all the countries the collective dose due to insulation jobs represents 5 - 7% of the annual collective dose. Although this is relatively stable over the period under consideration, a decreasing trend in the three-year rolling average of the collective dose due to insulation jobs can be seen in most countries. Two groups can be identified: • In Switzerland, Sweden, Germany and Belgium, doses for insulation jobs were 20 -100 person mSv in the period 1990-2, and 20 - 45 person mSv in the period 1995 - 7. The greatest decrease was in Switzerland, from 100 person mSv per year in 1990-2 to 18 person mSv during 1995-7. In Sweden and Germany, after an increase between 1990-2 and 1992-4, the collective dose for insulation jobs decreased. In Germany, however, the dose during 1995-7 was 40 person mSv – still higher than during 1990-2 (which was around 25 person mSv). In Sweden, the latest doses of 25 person mSv correspond to a decrease of 45% since 1990-2 (45 person mSv).
• In Spain, France and Hungary, the level of collective doses due to insulation jobs is higher than in the first group: 120 - 140 person mSv during 1990-2, and 80 - 125 person mSv during 1995-7. A major decrease was seen in France and Spain, where the mean collective dose was nearly halved, from 140 person mSv in 1990-2 to 80 person mSv in 1995-7. In Hungary, the collective dose due to insulation jobs was quite stable between 1991-3 and 1994-6 (around 118 person mSv per year). In 1995-7, however, the collective dose of insulation jobs came below 100 person mSv. Note that in Hungary, due to the 6-loop design of Paks, the authority requires more inspection work on insulation. The collective dose of insulation jobs during a long outage could therefore be 3-4 times higher than the above mentioned 5-7% of the annual collective dose.
Exposures by sister-unit group
In order to take into account the differences in the design of the plants, the collective dose due to insulation jobs was analysed for different sister-unit groups. The collective dose within sister-unit groups was then compared with the evolution of generations.
• The level of collective dose due to insulation jobs at first generation, 3-loop, Framatome designs, has now reached around 70 person mSv. This is similar to the dose at second generation, 4-loop, units. Even when reduced, the plants of the second generation, 3-loop design, with 100 person mSv, still show doses slightly above the other generations.
• The four types of Siemens plants show greater differences. The collective dose in the pre-Konvoi and Konvoi plants is around 10 person mSv, about a factor of 14 lower than the corresponding number for the first 2-loop generation (which was around 140 person mSv in 1995-7). For this first 2-loop generation, the dose for insulation jobs in fact increased between 1990-2 and 1992-4, reaching 160 person mSv. However, it seems that the tendency is now slightly falling. The level of the collective dose for the 3-loop second generation has also decreased, to 50 person mSv.
Even though there are still significant differences between countries, or between the type and design of units, this study confirmed the global trend of decreasing collective dose for insulation jobs in nearly all plants. This trend has to be seen in the light of decreasing total collective dose which can be observed in most countries, due to the sharing of good practice between plants.
The main actions undertaken to reduce exposures in this work are (according to a 1997 questionnaire in the ISOE network):
• Replacing normal insulation by “cassette insulation” This is easy to remove and replace, and reduces exposure time by at least a factor of 2-3.
• Improving scaffolding and using quick-assembly versions.
• Reducing the amount of insulation to be removed.
• Selecting the best work time during the outage schedule, for example when the piping is full of water.
• Introducing a specific radiation work permit.
• Improving insulation markings before removal, to facilitate the replacement.
• Improving storage, to prevent damage.
• Using team working.
• Carrying out training on a mock-up.
Another study based on ISOE data is currently analysing refuelling doses, investigating the trends as a function of reactor type and generation. The ISOE 1 database has been used to extract data on doses during refuelling for PWRs and BWRs.
A first conclusion which can be drawn is that, since 1990, refuelling in both PWRs and BWRs has been relatively stable in terms of the doses and also – not shown here – in terms of number of workers involved, and job duration. It is also clear that reporting these data is not uniform in terms of how “refuelling” is defined. Although tendencies in dose, number of workers and job duration are stable, there are considerable numerical differences.
Concerning average collective doses by country or by sister-unit group, for BWRs doses tend to be 50-100 person mSv. In PWRs, the average seems to be somewhat higher, at 50 - 150 person mSv.
Finally, it can be seen that national practices seem to have a larger influence on refuelling doses than sister-unit practices.
The average refuelling dose for the Westinghouse 3-loop, 2nd generation (W32) sister-units found in Belgium, for example, does not track particularly well with the average for all W32 units around the world. However, the average refuelling dose for W32 units in Belgium tracks well with the average for all plants in Belgium. The same tendencies are seen for W32 units in Sweden and Spain, as well as for W21 plants in Belgium and Switzerland. This tends to suggest that shared national experience (using the same language) seems to be more valuable to plants than experience from sister-unit in other countries.
These preliminary conclusions point toward additional studies. The objective of such studies is to provide some useful information to ISOE participants, but also to demonstrate the various types of information to be found in the database.
|Introducing the ISOE|
|The Information System on Occupational Exposure (ISOE) was created by the OECD Nuclear Energy Agency (NEA) in 1992, as a communications network among participating utilities and national regulatory authorities, and a programme for the collection, analysis and dissemination of occupational exposure data. The International Atomic Energy Agency (IAEA) co-sponsored the programme, for countries which are not members of the NEA. A joint NEA/IAEA secretariat has since been formed. The objectives of the ISOE programme are to: Promote and co-ordinate international co-operative undertakings in the area of protection of workers at nuclear power plants. Analyse annual occupational exposure data from commercial nuclear power plants included in the ISOE database. Provide a forum for communication between radiation protection experts. Databases administered by the ISOE include information on occupational doses for 422 reactors - operating in cold shut down or in some stage of decommissioning - operated by 77 utilities in 26 countries. National regulatory authorities from 21 countries also participate in the programme. Some 88% of the world's operating commercial nuclear reactors participate in the programme, (383 from a total of 434). The largest blocks of reactors outside the programme are in the Russian Federation (29 units) and in India (10 units). The ISOE database is divided into four parts. ISOE 1. Dosimetric information from commercial nuclear power plants in operation, including: Annual collective dose for normal operation. Maintenance/refuelling outage. Unplanned outage periods. Annual collective dose for certain tasks and worker categories. ISOE 2. Plant-specific information pertinent to dose reduction, such as materials, water chemistry, startup/shutdown procedures or cobalt reduction programmes. ISOE 3. Radiation protection related information for specific operations, jobs, procedures, equipment or tasks for: Effective dose reduction. Effective decontamination. Implementation of work management principles. ISOE D. Dosimetric information from nuclear plants which are shut down or being decommissioned. All information is supplied by participating utilities, who have full access to the data. Participating authorities have access to a reduced database, including data from utilities in their own country. Utilities and authorities both benefit from the ISOE communication network, as well as the data analysis and summary reports. The ISOE database forms the basis for various types of data analysis and studies of occupational dose trends.Information and trends can be found in the recent publication Occupational exposures at nuclear power plants: eighth annual report of the ISOE programme 1998 (OECD, 1999).|