Current large PWRs utilize a core basket assembly to support the reactor pressure vessel internal assemblies. These assemblies typically include a cylindrical core barrel to separate and preheat the incoming cold leg reactor coolant from the fuel assemblies. A thermal shield surrounds the core barrel. The core barrel directs the cold leg coolant to the bottom of the core. The fuel assemblies are supported inside the core barrel. To adapt the square fuel assemblies to the shape of the cylindrical core barrel, a core former or baffle is utilized. This arrangement will be the same for the iPWR designs.

5.2.4 Instrumentation

The ability to monitor core conditions will be of key importance for iPWR designs and will differ in implementation than current large PWR counterparts. This will be discussed further in Chapter 6.

The main characteristic of new HSI software platforms is that it typically forms part of the plant’s distributed control system (DCS) software. The DCS is the system that is used for overall plant automation and the HSI forms part of the ‘front end’ that enables the operator to interact with the plant through a hierarchy of controls and displays. This system typically allows development of the functionality and displays of the HSI without the need for low-level programming, while allowing some end — user customisation. It also supports full object-orientation and component-based programming, which ensures consistency of functionality, layout and appearance of objects throughout the HSI. Systems like this also support standardised documentation and code handling formats like XML. In advanced applications, as discussed later, it would support advanced computational methods like neural/semantic networks, pattern recognition, as well as real-time and faster-than-real-time simulation.

As indicated in Section 7.2.6.5, the regulatory review process for SMRs will in most respects be the same as it has been for conventional NPPs and designers of new plants will be required to comply with the requirements described in NUREG-0711 (O’Hara et al., 2012). Specifically, they will be required to demonstrate that they have followed a risk-informed process to ensure that safety considerations have been adequately addressed in the HFE process (Boring, 2010; Hugo, 2012).

7.10.1 Standards and design guidance

There are far too many standards and guidance documents to review in this chapter, but the most important ones are undoubtedly those already mentioned in Section 7.2.5. The NRC is reviewing and updating many of its regulations and requirements to make provision for reviewing design submittals for a new generation of SMRs. Already we are seeing an updated version of NUREG-0711 (O’Hara et al., 2012), which addresses emergent aspects of plant modifications and important human actions.

NUREG-0700 (O’Hara et al., 2002) is also due to be updated and will contain new review guidance on digital systems and HSIs.

Some military and government publications that may serve as useful references for the designer could be added to the documents already listed:

• MIL-HDBK 46855: Human Engineering Program Processes and Procedures

• MIL-STD-1472: Human Engineering

• DoD Human-Computer Interface Style Guide

• FAA Human Factors Design Guide for Acquisition of COTS systems, Non-developmental items and developmental systems

• NASA-STD-3000: Man-Systems Integration Standards

Four steps are involved in this main activity. Steps 3 and 5 prepare for the required analysis, while the bulk of the analysis occurs under Step 6, followed by integration of results for presentation in Step 7. Details of the proposed process are summarized in GenlV International Forum (2011b).

Since most US SMR vendors will apply for a ‘reference’ plant design to be certified by the NRC with multiple modules included in the reference design, the NRC had to consider how it should issue the license(s) to the reference plant. Should the NRC issue a facility license to the multiple-modular plant, or should it issue a license for each module? The NRC’s consideration of this issue was addressed in SECY-11­0079 (12 June 2011). The NRC staff recommended that the reference plant can be reviewed and issued a design certification for all modules. NRC will permit a single design certification if the modules are a generic design that will support a single licensing review, SER and public hearing.

However, it decided that each module should be issued a separate license with attendant specific technical (but largely generic) specifications. This recommendation was mainly based on the phased deployment of modules, and the operation and maintenance of each module over its operating life — it may be deployed, operated, maintained, and decommissioned separately from each other module. International licensing of a multiple-module plant should consider and address these same licensing issues.

11.2.3.1 Manufacturing license

SMR modules will be manufactured in a factory-like setting and then the modules will be transported to the site for final fabrication and installation. Manufacturing in this manner offers advantages in quality and efficiency through replication, assembly-line construction, and the maintenance of a stable and skilled workforce. NRC requirements in 10 CFR 52.167 permit the issuance of a manufacturing license with necessary and sufficient ITAACs to ensure the reactor is manufactured and operated in accordance with its license. Although no application has been submitted for a manufacturing license under Part 52, the US nuclear industry recommends that the NRC develop further licensing guidance on these requirements and how ITAAC provisions will be applied both in the plant and at the site. NRC requirements also do not address the export of SMR modules since these reactors will be licensed by the importing or host country. The importing country and customer must meet all US law and regulatory export requirements, including NRC’s requirements in 10 CFR 110, ‘Export and Import of Nuclear Equipment and Material’, and DOE’s requirements in 10 CFR 810, ‘Assistance to Foreign Atomic Energy Activities.’ These requirements are in the process of revision and should be considered by both exporters of US SMRs and importing countries and companies.

Incorporating a fission-based power source in a multi-output system (electricity and process heat) can offer significant advantages over carbon-based production sources, such as coal or natural gas, including reduced atmospheric waste streams (e. g. carbon or other gaseous emissions) and reduced impact on environmental resources (land usage or modification, permanent withdrawals of fresh water, thermal emissions, etc.). In an integrated multi-output system, thermal energy from the nuclear subsystem can be diverted to industrial applications in times of low electricity demand from the grid or during times of high renewable energy input to electricity generation. High-temperature, high-quality heat from advanced reactor concepts might be used for high-temperature industrial processes, such as hydrogen production or synthetic fuels production. Low-temperature heat from either advanced or light-water reactor systems could be applied to district heating, desalination processes, or low-temperature biomass pretreatment and ethanol production. Subcritical steam can also be superheated through process heat recuperation, chemical heat pumps, or topping heat prior to being directed toward a given high-temperature heat application.

For the current discussion, a dynamic NHES describes an integrated energy complex composed of one or more nuclear reactors coupled to renewable power generation sources (wind, solar, geothermal, etc.), and possibly linked to the production of one or more chemicals, fuels, or commodity manufacturing plants. Various exchanges between thermal, electrical, mechanical, and chemical energy could make it possible to produce, store, and deliver the highest value products to the market at the right time. Hydrogen, for example, can be generated by intermittent thermal/electrical output of a power plant instead of reducing boiler output during periods of reduced power demand on the grid. This hydrogen could supply either a captive or merchant market, depending on geographic location and market factors. Alternatively, hydrogen could be stored and dispensed to an emerging fleet of hydrogen fuel-cell vehicles, to generate power for a micro grid using a set of solid oxide fuel cells, or to firm up the variable power output from the wind or solar power plant. Other process-oriented heat applications for synthetic fuels and chemical production have been developed and evaluated by the Idaho National Laboratory (INL) in support of the Industrial Alliance for the Next Generation Nuclear Plant (NGNP) [3, 4].

A multi-input, multi-output system is the ultimate goal for a nuclear hybrid energy park architecture. These systems would be composed of two or more energy conversion subsystems that are traditionally separate, but would be physically coupled in a hybrid system to produce outputs by dynamically integrating energy and material flows among energy production and delivery systems. These couplings would occur behind the electrical transmission bus, such that all subsystems share interconnections, and the system would be operated via a unified control system resulting in a single, highly dynamic and responsive system that interacts with the electrical grid.

14.3.1 Development of advanced ICHMI

ICHMIs represent enabling technologies that can strongly impact reactor operations, performance, and safety. The A-SMRs as well as the LW-SMRs of the future will employ digital ICHMI systems. This fact coupled with a new paradigm of operating multi-modular units that may be used to produce electric power and process heat present new challenges. The ICHMI area potentially offers opportunities to offset the reduction in ‘economy-of-scale’ savings that come with the current large LWR central station plants where construction and operational costs are distributed over a larger number of megawatts of power in terms of design electrical rating (construction) as well for power production (operational). The following discussion on ICMHI R&D efforts under the ART program is drawn heavily from Wood [2, 3].

The ART ICHMI research area is defined by and structured based upon three principal drivers:

• unique operation and process characteristics;

• affordability — lower capital costs; and

• enhanced functionality.

Figure 14.1 illustrates and summarizes key technology issues that need to be examined and understood for each of these three drivers.

• Unique operation and process characteristics: SMRs in general have different process measurement needs from the current fleet of large LWRs. For A-SMRs using coolants (e. g., gas, liquid salt, liquid metals) other than water that will operate at higher temperatures, the process measurement instrumentation needs to be both chemically compatible with the coolant as well as tolerant of the higher temperature. Correspondingly, diagnostic measurements will differ for these A-SMRs. The unique, operational characteristics of most SMR designs are derived from the dynamic behavior of each general reactor class, e. g., gas-cooled, liquid metal-cooled, and differences in plant configurations. All SMR concepts will likely involve passive process systems to enhance safety. Thus, the impact of these passive systems on operability and plant performance needs to be evaluated to ensure proper consideration in control and safety requirements. Plant configurations that involve shared plant systems or resources among units, or integrated, reconfigurable balance-of-plant systems for multiple co-generation products require examination. Some SMR concepts involve sharing resources and systems among units to further reduce the up-front costs. This degree of sharing among units or modules for the purposes of reducing cost can range from minor support or auxiliary systems (emergency coolant tanks, control stations, backup electrical power, etc.) to major primary or secondary systems (e. g., turbine-generators coupled with two or more units). There may be significant dynamic coupling effects that must be taken into account within the operational controls for the plant depending on the extent of the sharing.

• Affordability — lower capital costs: ICHMI costs typically do not scale with the size of the reactor thus making them a larger cost element for SMRs in general than for large light — water reactors (LWRs). Consequently, using advanced technology effectively can help

reduce initial costs for example by minimizing cable runs and consolidating functions in highly reliable systems. Certain innovative technologies may also provide some benefit in reducing the fabrication, installation and inspection costs, financing costs, and operations and maintenance (O&M) costs. The most significant controllable contributor to day-to-day costs arises from O&M activities, which are heavily dependent on staffing size and plant availability.

• Enhanced functionality: Through application of advanced control schemes and predictive maintenance capabilities, one can minimize impacts in terms of reliability where multiple units or modules are employed. Advanced techniques for condition monitoring and reduction in reactor/system challenges via control system architectures can provide additional benefits. For those A-SMRs being used for both electric power production as well as process heat for hydrogen production or petrochemical processing where the demand for electric power and the process heat may vary, an advanced control system and integrated process diagnostics will be required. Multi-unit control with significant system integration and reconfigurable product streams has not been undertaken before. Obviously, these potentially new applications have significant implications for A-SMR system designs, construction, licensing, regulation, and operation.

Current ICHMI research areas are listed in Table 14.1, including a brief summary

of the scope for each research area.

Safety features incorporated into the SMART design enhance the accident resistance of SMART. The integral arrangement of the primary system removes large pipe connections between major components and, thus, fundamentally eliminates the possibility of LB LOCA. The canned motor RCP eliminates the need for an RCP seal, and basically eliminates the potential for a SB LOCA associated with the seal failure. The modular-type once-through steam generators are located relatively high above the core to provide a driving force for natural circulation flow. This design feature along with low flow resistance enables the capability of the system to have residual heat removal with natural circulation when normal means to transfer residual heat from the core are not available. A large volume of in-vessel gas PZR can accommodate a wide range of pressure transients during system transients and accidents. Low core power density lowers the fuel element temperature rise under accident conditions and increases the thermal margin. Negative fuel and moderator temperature coefficients yield beneficial effects on a core self-stabilization, and limit the reactor power during accidents.

15.4.8.2 Safety systems

Besides the inherent safety characteristics of SMART, further enhanced safety is accomplished with highly reliable engineered safety systems. The engineered safety systems designed to function automatically on-demand consist of a passive residual heat removal system, a shutdown cooling system and a containment spray system. Additional engineered safety systems include a reactor overpressure protection system and a severe accident mitigation system.

PRHRS

The PRHRS passively removes the core decay heat and sensible heat by a natural circulation in the case of an emergency such as an unavailability of feedwater supply or a station black-out. Besides, the PRHRS may also be used in the case of a long­term cooling for a repair or refueling. The PRHRS consists of four independent trains with a 50% capacity each. Two trains are sufficient to remove the decay heat. Each train is composed of an emergency cooldown tank, a heat exchanger and a makeup tank. The system is cooled by cooldown water or by air when the cooldown tank is dried out. In the case of a normal shutdown of SMART, the residual heat is removed through the steam generators to the condenser with a turbine bypass system.

Changes in the temperature of the fuel or core which happen from a change in power can cause changes in reactivity, which in turn result in effects on power. These feedback effects have an important consequence on the safety of reactors. For example, if an increase in power and therefore temperature leads to an increase in reactivity, this will result in a further increase in power (and temperature), and if this is not controlled, the unstable condition could lead to an accident; this is known as a positive reactivity feedback effect. However, if an increase in power and temperature leads to a decrease in reactivity, then the initial power level will be reduced, along with the temperature, and the core will be stable; this is known as a negative reactivity feedback effect. Clearly, the latter condition is how the specific iPWR core should be designed.

The term ‘temperature coefficient’ is used to express the effects of changes in temperature on reactivity, and is defined as the change in reactivity per degree change in temperature. The two major reactivity temperature coefficients of interest in nuclear design of PWRs and iPWRs are fuel temperature and moderator temperature coefficients. They are considered separately as they are caused by different conditions that have to be analyzed, but also occur at different rates.

The fuel temperature coefficient is particularly important as it acts with little or no delay in a power rise, and as such it is important that is negative. The use of slightly enriched uranium fuels (as currently used in all light-water reactors (LWRs) and planned for iPWRs) ensures that the fuel temperature coefficient is negative due to ‘Doppler broadening’. As the fuel temperature increases, the thermal vibration of the atoms in the fuel also increases, which results in a wider range of neutron energies, and the resonance peaks in the U-238 absorption cross-section are broadened, increasing the probability of a neutron capture in the U-238, and not producing a fission; this is ‘Doppler broadening’. In general, the fuel temperature coefficient only becomes of concern for those fuels with lower U-238 content that seen in traditional fuels, e. g., by increasing the proportion of U-235, or adding another material such as plutonium. Therefore, there is relatively little nuclear design in the control of this reactivity coefficient for PWRs, including iPWRs.

The moderator temperature will rise more slowly in the event of a power rise in a PWR core due to the time to transfer the heat from the fuel to the water moderator/ coolant. In PWRs (including iPWRs) the increase in temperature results in a reduction in the moderator density, and also reduces the moderator to fuel ratio. These effects generally reduce the reactivity of the core and therefore result in a negative moderator temperature coefficient.

However, for all PWRs (including iPWRs) that have boron present in the coolant, the moderator temperature coefficient becomes less negative (or more positive) if the boron concentration in the moderator is increased. The reduction in the moderator density caused by the temperature rise reduces the density of the soluble boron that is present, therefore reducing the absorption in the boron, and this effect is clearly increased for higher boron concentrations. Therefore, the nuclear design of the fuel and the core addresses this by using burnable poison (BP) rods (see Section 4.3.2 for further details) to limit the amount of soluble boron needed in the coolant to control the excess reactivity, and ensure a negative moderator temperature coefficient in the core throughout the cycle of operation.

A level transmitter is a device that translates the sensed level of a vessel or tank into an electronic signal. Direct, visual, or float level detection methods are usually not possible, nor practical, for safety system measurements, so most safety system fluid level detection is accomplished with a differential pressure device. This is a device that compares the constant pressure of the reference leg to the variable pressure of the hydrostatic volume in the vessel that represents the level of the fluid. This comparison, called differential pressure (DP), is correlated and calibrated to actual fluid level in the tank or vessel.

The safety system level measurements of pressurizer level, steam generator level, safety accumulator level, and refueling water storage tank level are performed with the differential pressure method in most PWRs today. This method is desired for its ability to accommodate level measurements where the vapor/water interface between steam and liquid is not distinct.

One of the key factors affecting the use of DP methods in level detection is availability of room or space for the sensing lines. Another issue is the practical challenge of maintaining a constant reference leg pressure. In a traditional containment, there is room for the sensing lines necessary for DP-type level transmitters and the reference leg level is maintained due to milder environmental conditions. The ‘room and environment’ may not be available in some of the iPWR designs. For these cases, new technology may provide the much needed solutions.

Acoustic or ultrasonic signal measurement devices have been commercially developed that shoot a sound wave frequency signal from the top of the tank directly to a fluid level below. These devices use the time of the signal reflection to determine level distance. These devices could be used in tanks where there is no blocking hardware between the signal source and the fluid level and in an environment where there is a clear distinction between the liquid and gas interface. It is mechanically feasible to use this type of methodology for a refueling water storage tank for example, where the gas to liquid level is distinct, but steam generator and pressurizer level might provide a challenge for this type of technology with a non-distinct water to vapor surface and a high degree of water vapor in the air/vapor space. The acoustic or ultrasonic devices for most safety purposes would need to be temperature compensated as the ultrasonic wave speed is affected by the temperature of the medium it penetrates.23

Another new technology which may be developed for level measurements is fiber optic sensing. A fiber optic sensor uses light (optics) to carry information about a process. In the distributed fiber sensing methodology, the optical signal transforms in an area of changing stress, strain, or temperature. These transformations are detected by the optical signal and calibrated to detect temperature differences or strain differences. Developing fiber for level measurement is in the early stages of development.1,4-6

Much the same as the distributed fiber measurement discussed above, level detection can also be accomplished with strands of traditional temperature devices, such as resistance temperature devices (RTDs) or thermocouples. For this method, one solution is to distribute two strands of closely located discrete heated and unheated junction thermocouples along the length of the vessel. Level is sensed by comparing the temperature difference between the heated and unheated thermocouples, on the principle that heat transfer properties between air and water will result in varying temperature differences between the heated and unheated thermocouples, thus establishing the air (steam) to water interface. This method is only as accurate as the vertical distance between the temperature sensors.