Radiation protection at Dukovany

Dukovany is near Trebic in the South Moravia area of the Czech Republic. It has four PWR units of VVER 440/213 type, an interim spent fuel storage facility and an LLW depository. The plant produces 1760MWh a year, which is 20% of the republic's total generation. Until the very recent start-up of Temelin, Dukovany was the only nuclear plant in the country.

To bring nuclear safety procedures in the Czech Republic in line with EU guidelines, the country has an independent state regulator: the State Office for Nuclear Safety (SUJB). The office is equipped with significant legislative powers, and its six staff provide constant safety supervision at Dukovany.

In addition to this independent support, the plant has its own Radiation Protection (RP) department, which is charged with implementing the SUJB's requirements and developing them into local procedures.

RP staff is rigorously trained and has to sit an examination in radiation safety set by the SUJB. Its members use a vast monitoring system in the plant's central radiation control room to observe and analyse radiation data from its vicinity in great detail.

Significant principles for radiation protection

Dukovany follows three principles to ensure radiation protection. Firstly, every activity that could lead to radiation exposure at the plant has to be thoroughly examined and cleared by the SUJB. Secondly, the plant has to comply with exposure limits, which are in line with the Directive 29/96/EURATOM, and which prevent the development of deterministic exposure effects and restrict stochastic effects to within an internationally acceptable level. The third main principle is that of optimisation. This requires restricting exposure to the lowest achievable level, considering all social and economical aspects, in other words to use the ALARA principle.

Dukovany's implementation of the ALARA principle is demonstrated by:

•Its evaluation of all operations on the basis of long-term collective and individual dose rate. These activities are divided firstly by type of work and secondly by the type of equipment they involve.

•Using all recorded data for RP protective measures planning, dose planning and for the development of an effective personal dosimetry.

•Planning, monitoring and analysing collective and individual effective dose in the environment with ionising radiation. Doses at Dukovany are carefully planned for one fiscal year ahead, and they are monitored and analysed daily. The same process of planning, monitoring and analysing is also applied for all special works that would make a significant contribution to collective or individual doses. An advanced radiation monitoring and information system which collects and analyses data from several hundreds of sensors placed all over the station provides detailed information for various purposes (such as dose planning). Individual and collective effective doses monitoring is made easier by using an on-line system of electronic personal dosimetry.

•Developing special procedures to ensure radiation protection for selected work activities during maintenance, repairs and equipment modifications.

As an example of ALARA principle application, there was once a case in which a portable hand pump, which was often used to suction impurities and small particles from the bottom of the reactor vessel during outages, was replaced with another type of pump with a screen. The previous pump had no screen and was gradually contaminated during pumping which caused higher doses for manipulating personnel (about 6 mSv per outage). The new pump with its removable screen was able to catch most of the radioactive particles and thus reduce significantly the contamination of internal parts. The old pump would have caused about 360 mSv after 15 years of operation (four units), but the new one will only cause about 3 mSv for the same period. Using the internationally-accepted rate of $12,500/Sv to calculate the cost of this dose in financial terms, the new pump came out at $3750 cheaper.

Dose planning is a significant tool for radiation protection that uses the ALARA principle at Dukovany. The plant has a plan for every fiscal year that shows the anticipated spending of collective effective doses to evaluate continuous RP efficiency, and amendment measures if trends are negative. Dose planning involves collating data on the volume of planned work, the radiation status of affected rooms, the number of employees necessary for the work, and historical data on the radiation exposures of similar activities.

The correct evaluation of work activities, particularly during outages, is vital for an effective yearly plan.

A dose plan needs to be assessed and approved by an ALARA committee, headed by the technical director of the plant, and throughout the year it is used as a basis for making decisions on protective measures in radiation protection areas.

During 15 years of operation, the value of Dukovany ‘s collective effective dose (CED) has reached a very low level. The volume of performed tasks during outages can explain small variations. This level of radiation protection is down to a number of factors, such as:

•Very good plant design, which ensures a favourable radiation situation in all working rooms (particularly the layout of technological systems and civil structures, which prevent personnel exposure from other systems then these which are actually operated, good air-conditioning prevents the spread of radionuclides in the RCA, low content of Cobalt in the material of the primary circuit which is affected by neutron flux and thus there is a low content of hygienically significant Co-60).

•A reliable radiation protection system which consistently applies all the principles and relevant requirements of radiation protection including the ALARA principles.

•A vast monitoring and information system of radiation control, which provides detailed information about the radiation situation on the site and in its vicinity, and information about the personal effective doses of Dukovany workers.

•Strict, rational and professional state supervision.

•The implementation of safety culture principles all over the site.

•The application of quality assurance requirements (QA tools).

Dukovany has achieved a very high level of radiation protection but its staff feel that the results could be even better. The station sees that the basis for another improvement could be found by sharing RP operating experience worldwide