Identification and review of proposed modifications

As the project is restarted, its technical status has to be accurately assessed and confronted with the list of proposed modifications.

Agreement should be reached with the regulatory body about the scope of the review, research findings, technological innovations and requirements, standards and feedback from similar NPPs in operation as far as they relate to safety aspects.

The process of identifying and recommending modifications should include:

• Assessment of modifications against the plant status, to determine what can be implemented

• Demonstration of the safety enhancement achieved, resulting in their acceptance by the regulatory body

• Assessment of the modifications to be implemented with respect to the criteria listed in Section 5.3.3.

• Addressing unresolved licensing issues.

The following should be reviewed:

• Existing design and construction documents and data for completeness and acceptability.

• Present condition of the plant structures, systems and equipment and their capability of performing the desired functions.

• Present market status of installed systems and equipment, and whether these will continue to be supported with spare parts, for duration of operations.

• Site characteristics, whether any pre-existing or new features, including external hazards were not adequately taken into account in the design.

For the above assessment, the utility may employ special equipment and analysis tools, and specialist manpower.

In particular, the following systems require review, with ongoing inputs of stakeholders such as designers, vendors, project management, operations, procurement and regulatory body.

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Safety systems and engineered safety features

The review should identify the deviations from the current requirements in basic design principles and engineered design solutions. The following should be examined:

• The design, as compared to the current analytical methods and practices.

• Impact of all identified discrepancies between design and requirements, and their combined effects.

• Completeness of the earlier safety reports and documents compared to current analytical methods and licensing requirements, in particular:

- Environmental impact report and emergency planning, to understand how the existing design solutions meet the current safety criteria;

- Protective parameter analysis, emergency core cooling, etc.

• Analytical methods, computer codes, studies for completeness and relevance.

• Postulated initiating events (PIEs) and the basis on which they have been postulated, e.g.

deterministically or by probabilistic safety assessment (PSA) and/or experience form the reference plant and other similar NPPs.

• The ultimate heat sink. Careful study should also be made of the site specific changes that may have occurred to affect the ultimate heat sink, example being building of dams upstream, diversion of rivers. Another example is that revised seismic data about the site may show some essential structures like cooling towers need to be reinforced.

• Design bases for the containment and containment systems should be reassessed taking into account identified PIEs and provision of features for mitigation of the consequences of severe accidents.

Technology upgrades

The utility should conduct a comprehensive and detailed review to facilitate its own work as well as that of review by the regulatory body. The scope of the review should include, but not be limited to the following:

• Review research findings, engineering and technology innovations and upgrades, in the intervening period since halting of the project.

• Review of current standards (i.e. upgrades/modifications in standards since project suspension).

Environmental qualifications

A detailed review of the environmental qualification of the specified and procured equipment need to be undertaken, in view of:

• Technological advances in packaging, materials and systems may permit equipment to be installed safely in hostile environments.

• Safety analysis may indicate that the environment which the equipment is likely to face may be more hostile, during an accident, than originally conceived.

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Radiological protection requirements for operating personnel and public

• International practices and the current national regulations concerning exposure to radiation workers and public should be examined concerning exposure limits.

• Modern techniques, materials and equipment may be instrumental in reducing exposures (examples are use of modular design, improved handling facilities and remote handling tools for maintenance, better shielding).

Emergency preparedness and accident management

Changes in the site and its environment should be studied and their impact on emergency preparedness and accident management should be assessed. Typically these are:

• Revised site specific data with respect to the frequency of internal and external man induced events and phenomena.

• New hydrological data e.g. change in river base and water table.

• Changes in traffic, particularly traffic patterns including air traffic.

• Significant changes in population distribution, land use, expansion of existing facilities and human activities and construction of high risk installations in the region.

• New seismo-tectonic data, if applicable.

Maintainability

The results of maintainability review may be of benefit in reducing construction, commissioning and down-times and could lead to improved availability, reduced radiation exposure and reduction in staff requirements. Examples are:

• Appropriate handling facilities for maintenance may be added at resumption of project,

• Improved diagnostic sensors for monitoring equipment condition.

Process system technology upgrades

Process systems technology is likely to undergo technological improvements. These have to be assessed, because:

• Changes in major equipment and systems are likely to have major cost and time penalties.

• Lead time for changes are large.

• Depending on the status of the plant equipment, backfitting problems will need careful planning. Examples are:

- improvement in the materials of tubes in Steam generator, other heat exchangers and condensers

- changes in some technical codes, which may require changes in process equipment (valves, wiring).

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Nuclear and process instrumentation and control systems technology upgrades

This is the area, where the most rapid advances are taking place, and where the rate of obsolescence is the highest. Rapid obsolescence increases the pressure for change and updating.

Because of rapid obsolescence, the timing of implementing changes is particularly crucial.

Fortunately, the lead time for procurement for such systems is not as high as that for many other systems of the project.

Some costfaenefits that may accrue are:

• Better control of plant parameters may be achieved leading to better plant performance.

• Upgraded operator support systems, leading to better appreciation of the plant conditions, better operations and more effective incident handling.

• Upgraded in-core instruments may lead to better assessment of core physics for initial start up and a more efficient re-fueling scheme.

• Advanced nuclear and process instrumentation are likely to be computer based and therefore rules for new software validation must be clearly established.

Assistance from designers would be helpful for review of the original design documents for completeness and the ability of the design to meet current licensing requirement. The original codes and analytical techniques would also need to be reviewed for adequacy.

In relation to the vendors, the review of modifications will offer an opportunity for them to comment on choices such as updating or replacement, status, obsolescence and availability of spares and alternate sourcing of equipment and systems.