Applying ALARA for a large-scale backfit at Borssele29 October 1998
An extensive backfitting programme (called the Modifications Project) was carried out at the Borssele nuclear power plant in 1997. It was clear from the start that a high collective dose had to be taken into account in planning and carrying out the 16 projects involved. Maximum attention to radiation protection in all phases of the project resulted in a final collective dose of 2505 mSv, which was a factor of 4 below the initial estimate.
Over the past decade attention to risks at older nuclear power plants has greatly increased. The necessity to minimise these risks led to the implementation internationally of periodical integral safety evaluations, in which the level of safety of the plant is tested against the standards for newly-built power plants.
In 1992 this requirement was added to the operating licence of the Borssele nuclear power plant. In particular, the ten-year evaluations now had to take account of the latest developments in nuclear safety and radiation protection. This relates not only to nuclear systems, but also to the organisation, administration and personnel facilities at the station.
The first ten-year evaluation took place during 1991 and 1992. On the basis of this the Modifications Project,1 involving 16 technical modifications, was conceived and then carried out at the Borssele nuclear power plant during 1997.
In view of the extent of the work and the dose rates at places where work was to be carried out, it was clear from the start that a relatively high collective dose had to be taken into account.
On this basis it was decided to pay special attention to radiation protection at the basic design stage of the project. This led to applying to the project a set of procedures and specific measures to keep the radiation dose as low as was reasonably achievable.
RADIOLOGICAL PROTECTION PLANNING
From this point of view, the Modifications Project can be divided into three phases, namely:
• Conceptual phase, in which the justification principle was applied.
• Engineering phase, in which the ALARA principle was carried out.
• Implementation phase, in which besides the ALARA principle attention was paid to maintaining the (internal) dose limits.
In order to ensure a maximum contribution of radiation protection when choices and decisions are made during all phases of the project and to ensure that the experience of the Radiation Protection Department of the plant was included, a procedure taking account of radiation protection was laid down in advance by the general contractor Siemens/KWU and the plant. This procedure is in diagram form on p30. As a guide, a monetary value of NLG 1000 ($540) per milliSievert saved was applied.
Throughout the project all radiation protection considerations and results were documented in so-called ALARA reports and radiation protection checklists.
The integral safety evaluation led to a safety concept report which specified what technical safety requirements Borssele would have to comply with after the modifications were made. In addition the report gave a broad outline of technical modifications necessary to comply with these requirements.
In the conceptual phase a dose justification was drawn up for the planned modifications. For this purpose a comparison was made between the potential dose to the population as a result of accidents and the anticipated collective dose to the personnel in connection with the execution of the modifications. To estimate the potential exposure of the population, the definition “risk x effect” was applied whereby risk and effect were determined using a probabilistic safety analysis (PSA) specific for Borssele.2,3
On the supposition that the power plant would be in operation for another ten years after the modifications, it was concluded that the potential decrease in dose to the population justified the anticipated collective dose to workers due to the execution of the modifications.4
The doses for the individual modifications were chiefly considered legitimate on the basis of the increase in the level of safety of the installation through prevention or control of accidents and by compliance with the licence requirement that the power plant be periodically upgraded to take advantage of the latest technical developments.
These justifications were recorded in ALARA reports.
In the conceptual phase an estimate was made of the collective dose to be expected for the entire project.
For this estimate no empirical figures were available as a guide, apart from some data relating to previous work on the pressurising system, as a backfitting programme of these proportions had never before been done and many of the anticipated operations were to be carried out for the first time.
The estimated collective dose was based on the following:
• The floor plans of the rooms where work had to be carried out, the process diagrams and the instrumentation diagrams of systems on the required work.
• The dose rates measured.
• The installation effort required (number of persons, duration).
• An extra 10% for general work (radiation checks, setting up scaffolding, decontamination, etc) based on empirical figures from previous work at the plant.
• Only operations in spaces with a dose rate > 10 µSv/h were included.
This led initially to a collective dose estimate of 10 person-Sv.
Then, assuming general radiation protection measures be applied (eg distance, time, shielding constraints) it was estimated that this dose could be reduced to approximately 7.5 person-Sv. Finally, more specific measures were considered which would be expected to ensure an even greater reduction.
The engineering phase was subdivided into a basic engineering sub-phase and a detailed engineering sub-phase.
The preparations of the individual modifications in both sub-phases took account of radiation protection. In determining the best radiation protection measures to use, the IAEA safety standards (relating to radiation protection for design aspects of new nuclear power plants5) were applied, insofar as they were implementable.
In this phase the emphasis was on the optimisation of measures within the framework of ALARA.
In the basic engineering phase the possible technical variants and options concerning possible set-up locations were examined, insofar as these were applicable. Considerations between the variants and the eventual choices of variants were based on a cost-benefit analysis as described in ICRP 556, a so-called ALARA procedure. This procedure is shown in the following diagram.
This analysis was carried out exclusively for modifications in which:
• The collective dose for execution of a modification is > 10 mSv. Or,
• The annual collective dose of the future operation is > 5 mSv as a result of the modification.
The results of the optimisation process were recorded in ALARA reports. In cases where no variants were considered the reasons were documented.
A cost-benefit analysis was undertaken to examine whether decontamination was an optimisation possibility for carrying out work on the pressurising and residual heat removal systems in view of the anticipated high doses. On the basis of the “cost per unit dose saved” criterion of NLG 1000 ($540) per mSv, only the decontamination of the residual heat removal system was eligible for implementation.
For this reason chemical decontamination of part of the residual heat removal system was carried out at the start of the implementation phase.
The radiation protection measures to be taken for the options chosen during the basic engineering phase are determined in the detailed engineering phase. In this respect the requirements set out in the IAEA Safety Series No 50-SG-D9 were of particular importance.
After an initial detailed engineering plan had been drawn up, it was assessed and, as far as possible, optimised from a radiation protection angle. The aim was to keep the dose for implementing the modification and for operating the modified installation as low as was reasonably achievable.
Important optimisation considerations were:
• Set-up locations.
• Accessibility for maintenance.
• Simplification of maintenance work.
• Use of tried maintenance-friendly components.
• Correct choice of material (eg Co content to be <= 2000 ppm).
• Prevention of leakage.
• Prevention of deposition (hot spots).
• Shielding against other radiating components.
• Determining radiation protection measures for the implementation of the modifications.
The results of the optimisations were documented in radiation protection checklists. These checklists were drawn up by the designer of the modification together with the radiation specialist of the supplier (a model of a checklist is shown).
During the implementation phase radiation checks were conducted by the Radiation Protection Department of the plant. This department also checked whether the radiation protection measures proposed in the engineering phase were actually taken and applied. It could demand additional radiation protection measures, if desired.
The planning of the use of personnel was based on the internal annual limit at Borselle of 10 mSv including the dose sustained previously. In exceptional cases this individual dose could be raised to 15 mSv with the permission of the Radiation Protection Department. This exception was applied only in a few isolated cases.
At the end of the project the collective dose was 2505 mSv (2.5 Sv), which is a reduction by a factor of four of the initial estimate.
Considering the extent and complexity of the operation and the prevailing radiation levels at the Borssele nuclear power plant, this is an excellent result. Our final conclusion was that the exceptional attention paid to radiation protection during all phases of the project ensured such a low collective dose.
|Borssele Modifications Project|
|The Borssele nuclear plant is owned by the N V Elektriciteits – Produktiemaatschappij Zuid-Nederland. The plant houses a 480 MWe PWR built by Siemens/KWU which went into operation in 1973. The ten-year integral safety evaluation carried out during 1991/92 led to the Modifications Project whose aim is to ensure that the plant complies with the latest internationally recognised technical standards in nuclear safety and radiation protection. The purpose of the modifications are the following: • Improve the general operation and reduce the risk of system failure. • Increase the level of redundancy for safety-related tasks. • Increase the spatial separation of subsystems which are mutually redundant. • Increase the diversity of components/systems of which the redundancy applied was considered insufficient. • Fully automate the execution of safety functions in case of failures. A Radiation Protection Co-ordinator was appointed within the project organisation at the start of the project. The co-ordinator’s main task was to ensure that the radiation protection principles were applied in all phases of the project. In 1992 the implementation of the modifications was assigned to Siemens/KWU through a turnkey contract. The total project budget was NLG 467 million (about $250 million). In 1993/1994 an extensive licensing procedure, comparable to one for the construction of a new power plant, took place. The implementation of the modifications was largely realised during an extended outage period in 1997. The entire modification and the annual refuelling took from 8th February to 14th July 1997 inclusive.|