Re-evaluation of user design requirements with a focus on

Beginning in the mid 1980s and continuing into the 1990s, user requirements documents for advanced light water reactors have been prepared. They include the EPRI ALWR Utilities Requirements and the European Utility Requirements. These documents have established sets of requirements that have been used to guide the designs of evolutionary LWRs [3]. These

35 documents have built on the accumulated experience of plant licensing, construction, operation and maintenance, and have incorporated developments in technology and safety.

They reflect lessons learned on current plants and provide the set of assumptions to be used to assess compliance with acceptance criteria.

However, user requirements can also be a cause for over-design and high capital costs. Cost benefit analysis of different design solutions has not always been carried out to determine if the requirement is in fact cost effective, or to determine whether the requirement could be met more cost effectively.

For example, user requirements documents for future plants specify plant availability factors of 87% and above, and identify design features for achieving these requirements. In fact, many well managed current plants are achieving availability factors of 87% and above. Advances which have been applied at current plants, such as high burnup fuel, better man-machine interface using computers and improved information displays, and better operator qualification and simulator training, will certainly be incorporated into new designs and will contribute to high availability figures for future plants.

Design features requested in the requirements documents for future plants for achieving the availability goals include:

• increased thermal and hydraulic design margins;

• redundancy – reducing vulnerability to single component failure, and enabling main-tenance and repair during plant operation;

• diversity – reducing the sensitivity to “common-mode” failures;

• plant layout and installations that ensure accessibility for inspection, maintenance, replacement and repair;

• design for on-line testing and maintenance; and

• design for reduced in-service inspection¹⁸.

While these features can be incorporated at the design stage to contribute to high plant availability, it is important to note that all of them, except the last two, result in added capital costs. In most designs for new nuclear power plants, margins have been introduced to enhance safety or achieve higher availability, but with added cost. The same is true for increased redundancy and diversity. A re-evaluation of requirements in these areas may well lead to major cost savings without compromising the safety or reliability of the plant.

The need for careful economic optimization can be demonstrated by considering increased design margins. Increased margins provide:

• capability to accommodate disturbances and transients without causing challenges to the plant safety, and initiation of engineered safety systems;

• an enhancement of system and component reliability, reducing the potential of exceeding specified limits which would require de-rating or shutdown;

• additional assurance that longer plant lifetime goals (e.g. 60 years) can be met.

18 An example of a design feature for reducing in-service inspection requirements and work to be done in high-radiation areas, leading to less occupational high-radiation exposure for new designs, is the enhanced utilization of forgings in the pressure retaining parts of the primary system, to reduce the number and length of welds which require inspection.

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While increased margin adds to the capital cost, substantial design margins can provide an option that may become attractive for the power generator once experience has been gained with the new plant: the operator can up-rate the plant power level. However, this potential advantage must be weighed against the possibility that the revenue from the additional generation may not offset the initial investment in substantial design margins.

Examples of user design requirements for which a re-evaluation may lead to cost savings are:

 requirements for measures to address corrosion of steam generator tubes that have been set for both low (hot leg) RCS temperature and for improved corrosion resistant steam generator materials, while the improved materials could in fact operate reliably at somewhat higher temperature;

 requirements for a very high number of cycles to be considered for load-following and frequency control which are sometimes much higher than could be realistically expected during the plant lifetime;

 requirements for large margins and specific additional design features imposed for the purpose of achieving high availability targets, considering that well managed existing plants have achieved high availability without such measures;

 requirements for increased safety margins for cases in which assumptions used to assess compliance with acceptance criteria are already very conservative (e.g. assumptions for reactor coolant pump flow, heat transfer surface areas, heat transfer coefficients, etc.);

 use of excessively conservative assumptions in assessing compliance with acceptance criteria. A typical example is the requirement to assume a very large number of steam generator tubes to be plugged (much higher than anticipated at end of plant life) in the analyses of plant accidents;

 prescriptive requirements for certain design features against severe accidents, without regard to the probability of certain sequences (e.g. the deterministic requirement to design the containment to withstand an in-vessel steam explosion);

 core management objectives which lead to the need to consider exceptionally high peaking factors in safety analyses which must assume mis-positioning of fuel assemblies;

 requirements for diversity of equipment which can have some negative consequences including added complexity of components and systems, and increased operator burden during maintenance. For example, the number of different spare parts needed is increased, which in turn leads to increased costs, and it also introduces risks that maintenance personnel will use a wrong component, make an erroneous adjustment or that operators will use a wrong manual or operating procedure. With an increasing number of somewhat similar though different components the personnel’s familiarity with the equipment and its functions will decrease. An evaluation of such user requirements for diversity should be consistent with regulatory design requirements¹⁹.

19 The IAEA document “The Safety of Nuclear Power Plants: Design”; Safety Standards Series No. NS-R-1 (1999) deals with the potential for certain common cause failures and states that the reliability of some systems can be enhanced by using the principle of diversity. It recognizes that “care should be exercised to ensure that any diversity used actually achieves the desired increase in reliability in the implemented design. For example, to reduce the potential for common cause failures the designer should examine the application of diversity for any similarity in materials, components and manufacturing processes, or subtle similarities in operating principles or common support features. If diverse components or systems are used, there should be a reasonable assurance that such additions are of overall benefit, taking into account the disadvantages such as the extra complication in operational, maintenance and test procedures or the consequent use of equipment of lower reliability”.

37 Re-evaluation and modification of user requirements could result in more cost effective designs. This could also result in relief on Technical Specifications, or other potential benefits for the plant operator.