Ten years on: ISOE considers expansion3 April 2002
For ten years the NEA's Information System on Occupational Exposure has recorded falling exposures in nuclear plants. Now the ISOE is considering how to prepare for its next ten years.
Occupational doses in nuclear power plants have been declining since the early days of nuclear power. Since the Information System on Occupational Exposure (ISOE) was set up at the Nuclear Energy Agency (NEA) at the start of the 1990s that trend has been "confirmed and consolidated", ISOE says in its recently published report, "Ten years of experience".
Few would argue with the report's claim that ISOE has contributed to strengthening that downward trend in dose, by "improved communication and experience exchange between radiation protection managers of nuclear power plants worldwide, provided by the ISOE network, as well as the growing use of improved work management procedures developed and published through ISOE."
The ISOE databases have a number of uses for radiation protection managers. First, they provide benchmarking data that allows every plant to measure its progress against its peers. Data can be provided for similar plants - "sister" groups with the same supplier, design, and generation - or for plants in the same country, so that managers can develop precise targets for annual cumulative exposures. The databases can also be "mined" for information on dose accumulated during specific work procedures or maintenance tasks.
For example, the report investigates dose incurred during 58 steam generator replacements between 1979 and 2000. During that period collective doses per steam generator replaced showed a marked reduction - from more than 6man.Sv at the start to an average of 0.5man.Sv during the late 1990s. The database allows this information to be investigated in more depth since "that average masks quite large discrepancies," the report says. "The best results correspond to three steam generator replacements performed in 1996 and 1998 in Belgium and France, with only 0.21man.Sv per steam generator replaced." That information is used, for example, by the Krsko plant, which told ISOE: "In the bidding documentation for the steam generator replacement we have specified an acceptable range of collective dose based on the available benchmarking data from ISOE."
The ISOE's Future
The tenth anniversary report identifies challenges ahead both in improving the current system and preparing for changes in the industry.
The database is the "backbone" of the ISOE programme, the report says, and it is essential that it is "as comprehensive and updated as possible," but it notes: "This completeness can only be achieved if and when all participants are motivated to input data that is as detailed as possible, and to update their contributions regularly." The challenge, the report says is to achieve a system which is "easy to use and not time-consuming". But in the end, the report says, the continued success of the database will depend on those reporting to it. "It is the commitment of participants to report on new experiences and to share them with other radiation protection experts that determines the usefulness and success of the system."
Management of the databases is made more complicated because the ISOE maintains a two-tier access system. This provides some information to regulators who belong to the ISOE, but must ensure that additional information shared by the operators continues to be restricted. The challenge here is to ensure that regulators have "a fair share of information" while utilities can preserve "confidential channels for the direct exchange of detailed operational information."
In the future ISOE may expand to maximise its usefulness to new communities developing within the nuclear industry. Plant life extension is one developing area where the system offers a well-established community to discuss occupational exposure management issues, the report says. Meanwhile, the ISOE should prepare to offer information for plants that have been finally shut down. "As decommissioning and dismantling of nuclear power plants become more widespread, ISOE can play an important role," the report says, adding: "The use of analytical tools developed within ISOE will help achieve a higher level of protection for workers involved in these activities."
At the other end of the plant lifetime, ISOE's databases also contain important information that can be used in reactor design, to ensure that for new reactor types "an appropriate level of occupational dose management is built into their conception."
Data from 2000
In its most recent (2001) annual report, the ISOE database included occupational exposure information for 452 reactors, both operating and in cold shutdown or some stage of decommissioning, operated by 72 utilities from 28 countries. National regulatory authorities from 25 countries also participate in the ISOE programme.
One of the most important aspects of the ISOE programme is the tracking of annual occupational exposure trends. Although there are some annual fluctuations, the annual average dose per unit has generally decreased over the time period covered in the database. For PWRs it has dropped from 1.00man.Sv in 1999 to 0.96man.Sv in 2000; for BWRs, from 1.77man.Sv in 1999 to 1.62man.Sv in 2000; for Candu reactors, the dose increased slightly from 0.85man.Sv to 0.92man.Sv; for LWGRs (RBMK), from 8.09man.Sv to 5.94man.Sv. (The database contains information on only three RBMK units.)
Deviations from the downward trend are due to variations in outage scheduling, changes of cycle length, amount of work and backfitting in the plants.
The Table below summarises the average annual exposure trends for participating countries over the last three years.
For those reactors that are shutdown or undergoing decommissioning, it is very difficult to identify clear dose trends due to the reactors being of different type and size, as well as being in different phases of their decommissioning programmes.
Outage duration and doses
Annual outage duration and doses in European reactors have been analysed based on three year rolling averages between 1993 and 1999 (see Table).
For PWRs, the outage dose shows a clear decreasing trend from 1993 to 1999. During the same period, the average outage duration remained within 5% of 53 days. The dose decrease might be explained by the application of work management approaches that allowed the reduction in number of workers and - for high dose areas - the workload.
As mentioned earlier, the ISOE also analyses the data by splitting it into "sister" groups. Figures for outage dose in the second-generation Framatome plants, with three loops (F32), and the second generation Westinghouse plants, also with three loops (W32), show similar reductions between 1993 and 1999. The F32 average dose dropped from around 1931 to 1299man.mSv over the period, while the W32 average dropped from 1381 to 766man.mSv. Over the same period the minimum recorded dose remained more or less consistent within each "sister" group, dropping slightly at the end of the decade. This suggests that it is a real limit in outage dose reduction, and that the improvements in average dose arise as good practice is shared and the worst-performing plants reduce their dose.
For BWRs, the outage dose appears to be mainly influenced by the extensive maintenance programme performed in Sweden. The ISOE points out that, as the Swedish modernisation programme reached its end at the end of the 1990s, the effects began to be apparent as dose rates began to decrease. The real benefits of the modernisation programme should be gained during this decade in the form of consistent low doses.
Deregulation and dose
The widespread deregulation of electricity markets from the 1990s and continuing to the present day have affected dose management programmes as much as they have other aspects of plant operation. The ISOE addressed this question directly in a meeting in 2000 which considered "Deregulation of the electricity market and its implications on radiation protection in nuclear installations." The question arises, ISOE said, "whether an increase in efficiency and reduction in price will be followed by a declining radiological performance". The ISOE notes that so far all radiation protection indicators are still improving, and that observers have cited this as evidence that deregulation has not had a detrimental effect. However, ISOE said: "This improvement in radiation protection indicators is due to investments which were introduced 5-10 years ago. Thus the effect of deregulation on radiation protection might become apparent in a couple of years." Instead of relying on current or past dose rates, the ISOE said predictive indicators should be developed to try to judge the effect of deregulation.
The ISOE said there was still sufficient qualified personnel available in the radiation protection sector. The ratio of new personnel to personnel with plant experience had, however, changed from 15% up to 60%. Some members had reported that "self-protection" was becoming more common in power plants, requiring workers to take more responsibility for their own dose. Meanwhile, ISOE said regulators would have to adapt their procedures to account for increasingly important commercial issues."