The current electrical grid is diversified — many input sources are combined to meet the ever-varying demands for electricity. However, in this loosely coupled, large-scale energy system, grid-priority for renewables can drive baseload, dispatchable power systems to follow the remainder of the grid demand by reducing power. Although new nuclear power plants can likely operate at a power level as low as 25 percent of their rated capacity (older plants may be limited to something closer to 50 percent of rated capacity), this is not a desirable operating mode from the perspective of optimizing the use of invested capital. Baseload power plants were not designed for an operational mode that incorporates significant power cycling. Cycling of either baseload nuclear or coal plants to accommodate the variable demand that results from the introduction of highly intermittent resources on the grid results in significant wear and tear on the plant systems, increasing operations and maintenance costs, and is potentially shortening the life of the plant.

A truly ‘hybrid’ system would be tightly coupled, requiring individual subsystems to be operated in an integrated fashion to respond appropriately to grid-level transients, while optimizing the energy use and minimizing cycling within the integrated system. This generalized architecture description then begs the question of what subsystems make sense in an integrated system. The vast possibilities for hybridization must be narrowed using established performance criteria to those that are technically feasible and can efficiently and reliably meet the need for a selected region or industrial application.

The intent of the ART R&D program is to conduct R&D on non-LWR design SMRs to address their respective technology challenges leading ultimately to the design, licensing, and operation of enhanced SMR designs in the future. This class of reactors, A-SMRs, is defined within the context of the ART program as non-LW-SMR designs. The non-LWR coolants for which R&D is underway include such coolants as liquid metals (principally sodium), helium, and liquid salts. DOE-NE’s objective is to conduct impactful R&D to accelerate the development of the technologies for these innovative concepts that obviously are candidates for deployment beyond the more near term. The principal R&D areas or pathways that make up the ART program at this time are (1) development of advanced instrumentation, controls, and human-machine interface (ICHMI) that is potentially applicable to LW-SMRs as well, (2) development and testing of materials, fuels, and fabrication technologies, (3) resolution of key regulatory and safety issues, (4) development of assessment methods for evaluating A-SMR technologies and characteristics, and (5) A-SMR concept evaluations. The first four elements were initiated in fiscal year 2012 with the fifth element started in fiscal year 2014. The A-SMR concept evaluations involve early efforts at characterizing and evaluating a set of three A-SMR design concepts employing each of the three aforementioned coolants — liquid metal, helium, and liquid salts. These evaluations will be summarized in this chapter after the four principal R&D areas are discussed.

DOE-NE also has several initiatives that complement the ART program including its Nuclear Energy University Program (NEUP), Integrated Research Program (IRP), and Industry R&D Partnership program that all include R&D elements and interactions associated with A-SMRs. These various R&D programs and initiatives as related to A-SMRs are summarized later in this chapter.

Use of these non-light water coolants in A-SMR designs offers a number of potential advantages in terms of potential economic, safety, and operational benefits. However, to realize these benefits, a number of technology challenges need to be addressed, thus forming the basis for undertaking the various R&D elements as described below.

The safety approach of SMART is based on a defense-in-depth concept with extensive use of inherent safety features and passive engineered safety systems combined with proven active systems. Substantial parts of the design features of the SMART have already been proven in industry.

15.4.8.1 Application of defense-in-depth concept

In the SMART design, adopting and implementing safety features for all levels implement the defense-in-depth concept:

1st level: minimization of an abnormal operation and failures.

2nd level: control of an abnormal operation and a detection of failures.

3rd level: control of accidents within a design basis.

4th level: control of severe plant conditions, including prevention of accident progression and mitigation of the consequences of severe accidents.

Burnup of fuel is the amount of energy extracted per mass of initial fuel loaded, and the units are generally megawatt-days per metric ton of heavy metal loaded (MW d/ MTHM). For example, if a 500 megawatt (MW) thermal SMR uses four metric tons (MT) of uranium contained within the fuel and operates at full power for one year, the average burnup of the fuel will be:

Average burnup = (500*365)/4 [4.1]

which is 45 625 MW d/MTHM, or 45.6 GW d/MTHM.

On this basis, it can be seen that the burnup requirements are derived from the utility needs, both in terms of the energy output of the reactor, and the refueling frequency, e. g., yearly or every 18 months.

In order for the fuel to achieve its design burnup, both for cycle of operation when it is first loaded, and for its design lifetime, sufficient excess reactivity has to be present in the fuel, and the fuel has to be replenished on an appropriate frequency. For a typical iPWR, this could range anywhere between one and five years, although is more typically 12 to 24 months for modern LWRs.

Although the excess reactivity is not a design limit or requirement directly, it is key to ensuring that the PWR core maintains criticality at full power operating conditions throughout the cycles of operation, including compensation for fuel depletion, buildup of fission product poisons (such as xenon and samarium), and loss of reactivity due to changes in temperature of fuel, moderator, etc. There is no specific limit on the amount of excess reactivity allowed either, but there are other design parameters such as negative reactivity coefficients or shutdown margin (see Sections 4.2.2 and 4.2.4) that are affected by the level of excess reactivity. Since the multiplication of neutrons varies in distribution in the core from one cycle to the next, and during the cycle of operations itself, it is important that the excess in reactivity is controlled both globally within the core, and also locally within the fuel assemblies, and the designer has a number of ways in which the core global and local reactivity and hence power can be controlled; these are described in Section 4.3.2.

There are however, limits on burnups, partially due to vendor warrantees associated with the fuel, but also licensing limits for the maximum fuel rod average burnup, but this is fuel design and vendor specific in both cases. Fuel rod average burnups of 60 to 62 GW d/MTHM are typical for large PWRs and since many iPWR designs rely on the fuel experience gained in large PWRs, a similar limit can be envisaged for all iPWRs, at least within the first few cycles of operation, i. e., before greater experience of iPWR operation and fuel irradiation is gained.

A pressure transmitter is a device that translates physical force to an electrical signal. The most common type of force transducer uses a diaphragm, piston, bourdon tube, or bellows to sense the physical force, and various strain/force sensing devices to convert the deflection of the physical element to an electrical signal. Traditional strain sensing devices include: capacitive cells, piezoresisitive strain gauges, piezoelectric quartz material, and electromagnetic devices.

Companies such as Rosemount, Cameron/Barton, Foxboro, and Ultra systems have specialized in safety system pressure measurements in the United States (US). These transmitters may still function successfully on some iPWR designs, but many will have to be re-engineered for different mounting configurations, size constraints, and environments. Many iPWR designers, when faced with a modification program, may choose to go with new technologies rather than modifying the old ones. The new technologies may offer advantages in size, redundancy, accuracy, and environmental resilience. Some of these new technologies include micro-electro-mechanical systems (MEMS) sensors, fiber optic sensors, and ultrasonic sensors.

In the optical fiber category, a company called Luna Innovations has developed and successfully tested fiber optic pressure sensors, like the one shown in Figure 6.1, in a research reactor environment. These fiber optic pressure sensors have been shown to operate in radiation environments with flux levels much higher than those compatible with most electronic pressure sensors. With traditional technology it is necessary to protect traditional electronic pressure transmitters from harsh radiation conditions near the core; this requires the use of long pressure sensing lines, which limit the response time to pressure transients and increase the number of wall penetrations. Luna’s fiber optic pressure sensors are designed to operate in harsh environments. When these pressure sensors were combined with Etalon-based fiber optic temperature sensors providing temperature compensation, drift effects were minimized. The attractiveness of this technology for iPWRs is obvious, with the elimination of sensing lines, the minimization of penetrations, the small size of the sensor, the rapid response to pressure fluctuations, and the operability in high radiation fields. With these attributes, this technology bears merit for primary and secondary side pressure measurement.1

Another new technology for pressure sensing is the polymer-derived ceramic MEMS sensor. At the forefront of this technology, a company, Sporian Microsystems

has developed a pressure/temperature sensor made to survive high temperatures (Figure 6.2). This technology offers a solution for pressure sensing in iPWRs due to its hardy environment survivability and its small size. The small size allows for the installation of redundant units and the measurement of pressure at many points, possibly with fewer penetrations than traditional sensors.

These new technologies have attributes like small size, heat survivability, radiation hardness, fast response, and low maintenance. These attributes are highly valued in iPWR designs for obvious reasons.

The organisational requirements for technology selection are often found in policies, standards, design style guides, cost-economic considerations and vendor preferences. These requirements may also influence operational requirements. Control room staffing is one of the aspects of power plant operations that is strictly regulated by

US Code of Federal Regulations and any deviation from the regulation is subject to scrutiny and proof of concept. This has a direct influence on the way control rooms, HSIs and automation systems are designed.

As indicated before, the underlying assumption of new designs is that higher levels of automation will enable multi-module as well as single-module plants to be controlled by fewer operators than are required for conventional LWR or boiling water reactor (BWR) plants. When this is verified, it may to lead to staffing strategies where a single operator may be able to handle multiple modules or multiple processes under normal operating conditions. In addition, the physical layout of the plant, the reduced need for LCSs and manual controls in the plant, and the availability of remote surveillance equipment, may mean that fewer field operators will be required.

However, in the absence of sufficient operating experience and proven technical bases, the only reliable way to estimate the number of operators required under varying operational conditions would be to use advanced task analysis and modelling methods and tools such as computational human performance modelling. Predictive human performance information produced in this way could eventually be verified in the full scope plant simulator.

Use of inherent safety features and safety-by-design eliminates certain accident initiators. Use of passive safety systems makes their functioning more probable when something does happen, and removes dependence on an external power source.

The combined impact reflected in most SMR designs is to reduce significantly the CDF (core damage frequency, per reactor-year) and LERF (large and early release frequency, per reactor-year) probabilities. Thus, CDF values of 1.0 X 10-7 to 1.0 X 10-8 per reactor-year are typically claimed for SMRs, with LERF probabilities usually at least one order of magnitude lower than CDF. This, if proven correct, would allow operating 10 000 reactors for 100 years with only a small probability of an accident leading to core damage, and practically negligible probability of any radiation release. In contrast, the Generation-II CDF values of 1.0 X 10-4 to 1.0 X 10-5 imply that for 400 reactors operating over 50 years a CDF event should not be excluded, and they in fact did happen. While the consequences of all commercial nuclear accidents combined are orders of magnitude smaller than the health effects that would have been caused by producing the same amount of electricity by fossil fuel, they have negatively impacted public opinion, and iPWR SMRs (and advanced reactors in general) could help address that public concern.

One of the main concerns for investors is unexpected delays during construction of a NPP and the related cost escalation. Faced by the above-mentioned risks, several investors stand ‘frozen’ and wait and see the market evolution, the strategies of their competitors or wait for a more mature phase of a specific reactor plant concept to exploit cost reduction and learning accumulation.

As argued by IAEA (Barkatullah, 2011), reduced plant size and complexity and design simplifications, enabled by the SMRs, should allow:

• better control on shorter construction lead-time — leaner project management (e. g. higher factory-fabrication content, modularization of reactors);

• lower supply chain risks — increased number of suppliers and reduced need of special and ad hoc manufacturing and installations;

• better control on construction costs — if plant complexity of gigawatt electric (GWe)-scale nuclear plants has been a driver of cost escalation (Grubler, 2010), SMRs should enable economies from standardization and accelerated learning. The ability to meet cost projection should also improve.

The small reactor has two attributes that can meet this cost challenge. Firstly, the smaller physical size of the vessels permits larger assemblies to be transported to site in a complete and tested condition. The implication is that a significantly higher portion of the plant can be built and tested in a factory prior to delivery.

The second facet is that small reactors are deployed in larger volumes to meet the same capacity requirement. This larger volume justifies investment in manufacturing techniques that larger, low volume nuclear plants have been unable to justify previously. Building plants at these projected volumes present a challenge significantly different to anything the nuclear industry has encountered previously.

Potential hybrid systems could utilize proven LWR technology or proposed advanced reactor technologies that would operate at higher temperature and, hence, provide higher temperature heat for non-electrical applications. SMRs offer unique opportunities for hybrid system applications due to their small size, modular implementation, operational flexibility, and investment flexibility. Potential SMR-hybrid implementations are explored below, with a focus on system siting and nuclear-renewable integration. Detailed discussion of potential non-electrical applications that could be integrated in such a system is provided in Section 13.5.