IEC TR 63335 pdf – Nuclear power plants – Instrumentation and control systems, control rooms and electrical power systems – Specific features of small modular reactors and needs regarding standards
5 SMR specific features5.1 General
The 2018 edition of IAEA report: Advances in Small Modular Reactor TechnologyDevelopments identifies more than 50 ongoing SMR design projects worldwide. The target ofthis document is those design projects that aim at power or industrial production,thoughmany of the features listed and many of the suggestions proposed are applicable, or could be
applied，to those with different aims (e.g.,research). Also，as there is a wide variety of SMRdesigns, this document aims to be generic by focussing on commonly found SMR features.
Two key objectives of production SMRs are:
. To be at least as safe and secure as large nuclear reactors.
To be economically competitive with respect to large nuclear reactors, and also to other,
non-nuclear sources of energy.
Concerningeconomic competitiveness,SMRs cannot rely on the scale effect like largenuclear reactors,and need to take advantage of different features such as:
Modular design (whereby a significant part of a plant construction consists in thetransportationand assembly on site of fully operational modules built and pre-tested indedicated factories) in order to lower construction costs and shorten ‘constructiondurations.
Series effect(whereby multiple units are based on a standardised design，possibly withminor adjustments to take account of site-specific constraints， and of country-specificrequirements in case of worldwide deployment)to lower component costs and build-upconstruction experience.
Simplified design，whereby advantage is taken of smaller size and lower power levels forsimpler，more integrated and more passive designs. Simplified designs also tend to havepositive effects on the safety and security of NPPs.
5.2 Passive design features and systems
Many SMR designs include so-called passive design features and passive systems where the
performance of particular functions (in particular， safety functions) requires little or noexternal power and human control.For example, a passive residual heat removal system does
not require the activity of powered pumps, but relies solely on natural convection，possiblyafter the opening of a few valves. Such features are not specific to SMRs, but their small size
and low power levels facilitate their introduction.
As such features place less demand on non-passive support systems,they significantlycontribute to design simplification and to cost reduction and hopefully reliability. However, the
extensive use of such design features/systems is relatively new in nuclear power plants: there
is limited experience in operation, and thus few lessons learned regarding design,construction, ‘maintenance in operating condition, surveillance and periodictesting.Regulatory experience and international consensus on licensing approaches are also limited.
Also, the categorisation of functions important to safety could benefit from the incorporation ofpassive safety features.
5.3Mutualised operation
As a production SMR generates much less power (with sometimes a factor of up to 10 or 20)
and thus much less revenue than a large nuclear reactor,economic competitiveness needsnot only to be addressed in design and construction，but also in operation. This sometimesleads to the notion of mutualised operation, wherebymultiple SMR units at the same site areoperated by a single team (including control room personnel, field operators and maintenance
staff), approximatively the size of the team for a large reactor, and from a single main controlroom.This practice is commonly seen in non-nuclear plants (whether for power generation or
not),but in a nuclear context where safety is of primary importance， this raises a number ofissues such as:
. How to determine an adequate staffing for the different teams involved.
The operation of multiple units from the same main control room，where the handling ofoutage，incidental or accidental conditions in one or more units should not disturb~theoperation of units in normal conditions.
The role and number of supplementary control rooms. For large reactors,thesupplementary control room is essentially”a backup to be used whenthe main controlroom is not available. In the case of mutualised operation，to avoid disturbing theoperation of units in normal conditions,one might consider using one or moresupplementary control rooms for units in conditionsrequiring the intervention of largenumbers of persons，with the need of communication ‘and coordination between thedifferent rooms.
How to ensure that each operator action，whether from the control room or in the field，isperformed on the right unit.
5.4Optimised maintenance
Maintenance represents an important part of the operation and personnel costs,and SMRprojects often look for ways to optimise it, e.g, by using on-line monitoring to promotecondition-based maintenance,or off-site e-monitoring to assist local operators in prognosticsand diagnostics. Longer operation cycles (as is the case for some SMRs) and reducedinventory of active components (valves，pumps,etc.) may also need to be considered whenaddressing on-line and off-site monitoring.
5.5 Mutualised plant systems
Though there could be the occasional case of plants with a single SMR unit,many SMRdesign projects have plans for multi-unit plants，where units are in a large part independent,but where some plant systems are shared by all or some units of the plant.This naturally has
impacts on l&C architectures，control rooms,control room systems,and electrical powersystems.