The I&C challenges for small modular reactors4 November 2020
Dr Li Li examines the instrumentation and control requirements and challenges for a new generation of small modular reactors
Above: The simulator control room at NuScale Power’s small modular reactor design facility in Oregon (Photo courtesy of NuScale)
REACTOR DEVELOPERS ARE DEVELOPING A wide variety of small modular reactor (SMRs), some of which also open up new opportunities for remote deployment, co-generation, hydrogen production and other applications. Many use Generation III+ pressurised water reactor (PWR) technology similar to that used in current gigawatt-scale reactors, while others are based on advanced Generation IV technologies ranging from high temperature gas-cooled reactors to molten-salt reactors.
Each design will have its own requirements for instrumentation and control (I&C) systems for the operation, monitoring and control of the reactor, turbine island and balance of plant. Even among PWRs, the design principles and I&C architectures are very different and cannot be easily replicated from one design to another.
To understand the challenges, let’s look at three competing designs: the UK SMR being developed by a consortium of companies including Rolls-Royce, Jacobs, Assystem and Atkins with support from research institutions including Nuclear AMRC; the NuScale Power Module backed by Fluor Corporation with funding from the US Department of Energy; and the Westinghouse SMR, which adapts technologies from the established AP1000 design. Table 1 lists the key technical design parameters for these three SMRs.
Table 1: Key SMR technical design parameters
The UK SMR is a tri-axial symmetric three-loop close-coupled PWR with coolant flows driven by three reactor coolant pumps to three corresponding vertical u-tube steam generators. The cooling system is mainly forced circulating flow, but is also configured to provide natural circulation flow for passive decay heat removal. I&C system requirements include provision of defence in depth, diversity and substantial redundancy, and optimised operator interface.
Following best practice from larger plant, the UK SMR’s I&C architecture includes a reactor protection system, a hard-wired ‘diverse protection system’ (HDPS), reactor plant control system, fuel handling protection system, fuel route and hazardous material monitoring system, and severe accident management systems.
The reactor protection system provides safe shutdown in response to a fault signal arbitrated by a priority logic system, based on Hardline technology. It uses mixed analogue and non-programmable digital sensors and transducers communicating on hardwired multi-channel digital electrical networks, which benefit from the diversity of the HDPS.
The reactor plant control system will deploy an industrial programmable logic controller or distributed control system. The UK SMR design team is investigating opportunities to use smart instruments and devices to modernise the platform and improve operation productivity and efficiency.
As well as a main control room with displays, alarms and manual controls, the UK SMR I&C platform has a supplementary control room with basic station control, HDPS, safety controls and post-accident and severe-accident management systems as back-up for operating the plant in the aftermath of an accident. Non safety-related I&C systems include distributed control, networked communications and sensors, transducers and instruments up to the automation system.
The UK SMR is at a relatively early stage, with I&C design development to date focusing on reactor and safety-related systems. Rolls-Royce’s modular Spinline technology is likely to be used as a basis for the safety I&C system.
NuScale Power Module
A NuScale Power Module has two internal helical coil steam generators, in an underground high pressure containment vessel which is submerged in a stainless steel lined, water-filled concrete pool. It operates using the principles of buoyancy-driven natural circulation, even during transient or accident conditions.
Each power module operates independently in a separate compartment within a multi-module configuration, with up to 12 modules in the same water pool. The design also has two redundant passive safety systems (the decay heat removal system and the containment heat removal system) to allow decay heat to reach the containment pool.
NuScale is the most advanced SMR developer securing licensing for its design. The company submitted its design certification application to the US Nuclear Regulatory Commission (NRC) in late 2016, and received design approval in August 2020.
The power module I&C system includes a fully digital control system based on field programmable gate array (FPGA) technology, adhering to the principles of independence, redundancy, predictability and repeatability, diversity and defence in depth. NuScale has been granted around 14 patents in I&C related areas including in-core monitoring, remote monitoring, and control rod drive mechanisms.
The I&C architecture includes a module protection system and neutron monitoring system which are classified as safety-related, along with plant protection system, safety display and indication system, module control system, plant control system, in-core instrumentation system, health physics network and fixed area radiation monitoring.
The module protection system uses the highly-integrated protection system (HIPS) platform which is already approved by the NRC. The FPGA-based HIPS platform comprises programmable modules that can be interconnected in multiple configurations to support the various types of reactor safety system.
NuScale’s control room design offers some unique features. The room layout and control panels were designed using a state-of-the-art simulator as part of a comprehensive human factors engineering and human system interface approach. NuScale’s design team has produced a coherent and consistent screen-based design for the operators, who will monitor and control the multi-reactor power plant from a single control room.
In Westinghouse’s SMR, both the reactor vessel and the passive core cooling system are below ground in a compact, high-pressure steel containment vessel. The coolant system has eight sealless canned motor pumps, mounted horizontally to the shell of the pressure vessel below the closure flange. The coolant flows are driven by the symmetric octagonal attached canned motor pumps through the fuel assemblies.
The I&C system uses the proven Ovation digital control system platform, which is approved by the NRC and currently being deployed in AP1000 plant. Largely inherited from the AP1000, the Westinghouse SMR I&C system comprises the protection and monitoring system, diverse actuation system, plant control system, data display and processing system, operation and control centres system, radiation monitoring system, in-core instrumentation system, special monitoring system and turbine operating system.
The protection and monitoring system provides appropriate safety-related functions to maintain the plant in a safe shutdown condition, and also controls non-safety-related components which are operated from the main control room or from a remote shutdown workstation.
Other systems are not safety-related. The diverse actuation system provides an alternate means of initiating a reactor trip and actuating selected engineered safety features.
Sensors, instruments and smart devices
Since these SMR designs are based on proven PWR technology, most sensors and instruments are commercial off-the-shelf products which are already certified for the nuclear industry. It should be relatively straightforward to qualify these for SMR deployment.
Other SMR designs based on Gen IV technologies are likely to require new kinds of advanced sensor, some of which are now in development. Prototype technologies have been reported using techniques including Johnson noise thermometry, fibre optics and ultrasonic sensors to measure the higher temperatures, pressures and flow rates of different types of coolant. However, none of these will be commercialised and certified for SMR deployment within the next few years.
Above: An artist’s rendering of NuScale Power’s small modular nuclear reactor plant (Photo courtesy of NuScale)
Digitalisation provides tremendous advantages in automation and reliability over analogue systems manually controlled by human operators. Digital smart devices are increasingly used in the upgrade of operating plant as well as new build — for example, it was recently reported that the number of operational personnel at Russia’s Novovoronezh II has been reduced by 30–40% thanks to digitalisation and automation.
However, embracing the latest digital components and I&C technologies will bring new challenges to SMR designers, licensees and regulators to approve the new entrants, particularly for smart devices that require qualification.
The issue of cyber security will be critical for the certification of digital I&C systems. According to the Industrial Control Systems Cyber Emergency Response Team, part of the US Department of Homeland Security, cyber-attacks and security infringement targeting control systems have increased significantly in recent years. With more digital smart devices used in nuclear power plant of any size, it must be a priority to protect the vulnerability of smart devices and digital I&C system from cyber attack and malicious sniffing from hostile individuals or organisations.
In the UK, the I&C Nuclear Industry Forum has developed an innovative assessment tool for smart instruments and devices intended for use in nuclear safety-critical applications. The Emphasis (Evaluation of Mission Imperative High-integrity Applications of Smart Instruments for Safety) tool uses established good practice to certify smart sensors or instruments before they can be deployed in power plant. It is compliant with the IEC61508 and ISO9001 standards, has been accepted by the UK regulator Office for Nuclear Regulation (ONR), and is now being put into practice by vendors and licencees.
Codes, standards and regulations
Although the commercial concept of SMRs has existed for decades, no country yet has a licensing or certification process for I&C systems, structure and components exercised by a dedicated SMR-related standard.
The SMR industry must engage with codes and standards organisations to address the specific changes necessary, and collaborate with academics and research institutes on codes and standards development.
Existing standards can be relatively easily adapted for the PWR type of SMRs, because the industry and regulators have sufficient experience and knowledge on codes and standards for large-scale PWRs. But in most cases, new codes and standards will need to be developed for advanced reactors.
The development of new codes and standards is a very lengthy process. It will take resources and time to publish a new standard for the nuclear industry, and we might not see one ready before new SMR developers file their design certification application.
In the US, the NRC recently completed NuScale’s design certification application, and has issued an early site permit to the Tennessee Valley Authority for potential construction of two or more SMRs of unspecified design at the Clinch River site. In the UK, the ONR has established a dedicated Advanced Nuclear Technologies team to oversee guidance and processes for the regulation of SMR and advanced modular reactor technologies. The UK SMR project team is aiming to complete the ONR’s Generic Design Assessment process in time to start building the first of a kind power plant in 2025.
Many SMR designs, including the three considered above, have the potential to be used for co-generation of process heat, district heating or desalination. This will introduce additional complexity for regulators to approve the power plant I&C design, if the co-generation processes are located at same site for economic reasons. This brings new challenges because extra safety measures must be considered. For example, additional safety features of the control system and evacuation plan must be approved by an adequate jurisdictional authority for the orderly shutdown of both the nuclear plant and industrial processes in the event of an accident.
Author information: Dr Li Li, Head of the digital I&C group at the UK’s Nuclear Advanced Manufacturing Research Centre (Nuclear AMRC)