Technical feasibility is, in the most basic sense, defined by system operability, measured in terms of on-line operating capacity factor and thermal efficiency (as a percent of the energy resources converted to a more usable energy currency or product). Power plants typically aim to achieve at least an 85 percent capacity factor, in consideration of grid demand cycles and seasonal plant outages for equipment maintenance. The capacity factor for the current fleet of LWRs in the United States has ranged from approximately 86 to 91 percent (over the entire US nuclear fleet) from 2006 to 2012 [5]. The chemical industry strives to establish the highest capacity factor possible, sometimes reaching 98-99 percent of nameplate capacity for annual operation of the plant. Current subsidized mandates (viz., Product Tax Credits) to build wind power has decreased the capacity factor of many baseload power plants, including some nuclear plants, to around 50 percent.

Some key technical parameters that affect the feasibility of a given combination of subsystems in a hybridized plant include (from the perspective of the reactor): reactor outlet temperature, reactor inlet temperature, heat flux, heat capacity, and peak operating temperature (and, in some cases, minimum operating temperature) of either the thermal hydraulic system materials or the heat transport fluid. These characteristics dictate materials of construction, determine the optimum power generation cycle, and dictate what process applications may best couple with reactor — produced thermal energy.

When the operating schedules of two or more subsystems are co-dependent, then plant design and operating schedules must account for startup and synchronization of all subsystems. Energy storage subsystems — on both the thermal and electrical branches of the system — may be necessary buffers to ensure smooth shutdown of one or the other facility in the event of an off-normal interruption of operations or for planned maintenance. The feasibility of cycling unit operations also needs to be considered, taking into account thermal energy production ramp rates, lag in energy delivery systems, the effects of mechanical and thermal stresses, and electrical hysteresis effects on battery storage units.

Technical feasibility is also impacted by the ability to modularize unit operations. As discussed in Section 13.1.2, SMRs are ideally suited (and designed for) modularization. Multiple modules in a single plant adds complexity to the physical integration, system operations, and control. However, modular implementations also offer the opportunity to increase capacity more easily with increasing demand, to attain higher integrated system capacity factors while allowing individual subsystems to be taken offline as needed for maintenance or refueling, and provide additional operational flexibility based on changing market trends.

Given the aforementioned design differences from large LWRS not only for SMRs in general but for A-SMRs in particular, it is important to evaluate and then determine if existing assessment methods for certain key areas as developed for large LWRs are applicable or require modifications for use for A-SMRs or if new methods are required. Presently, the three areas where assessment methods are being evaluated and/or developed for A-SMRs in the ART program include the following:

• probabilistic risk assessment (PRA);

• safeguards and security; and

• economics

The overall objectives for examining and developing PRA methods at an early stage for applying them to A-SMRs is to (1) aid in early conceptual design efforts for evaluating design options from a risk and reliability standpoint and (2) formulate an A-SMR PRA framework to provide resources for evaluating future designs and assisting in licensing activities by developing quantitative methods and tools for analyzing A-SMR risks [7].

In looking at future A-SMR designs, the opportunity exists at the outset for integrating safeguards and physical security protection early on in the design process. In NRC’s SECY-10-0034, Potential Policy, Licensing, and Key Technical Issues for Small Modular Nuclear Reactor Designs [8], NRC notes that

Insulated duct (3 inches thick) Down to test section

Roof of Bldg. 308

test section

Pit

ft deep)

Figure 14.4 Severe accident testing facility for passive safety system (3 inches = 75 mm, 24 inches = 600 mm, 20 ft = 6 m).

‘Because many SMRs are still in early developmental stages and the designs are not yet fixed, the designers have a unique opportunity to determine the appropriate design basis threat; develop emergency preparedness; and integrate physical security protection, cyber security protection, and material accountability and control (MC&A) measures with the design and operational requirements during the design process and during the development of a license applicant’s physical security and MC&A programs and systems. Therefore, SMR designers are expected to integrate security into the design and will need to conduct a security assessment to evaluate the level of protection provided, including safeguards aspects of SMR-related fuel cycle and transportation activities.’

The R&D underway is focused on developing an understanding, from an economics standpoint, of some of the key differences between small modular reactors and large LWRs in terms of design (smaller size with passive versus active systems), construction (factory fabrication of modules versus stick built on site), financing costs (lower capital outlay for initial module(s) and deployment of additional modules over time
to meet increasing demand for power), learning rates for factory fabrication going from first-of-a-kind (FOAK) to-nth-of-a-kind (NOAK) production for standardized modules, and potential for reduced staffing levels. The intent is to examine and evaluate these factors for SMRs in general to establish the foundational capability to evaluate costs for SMRs in general and then modify or adapt the models and tools for A-SMRs as these designs evolve and mature in the future.

Current assessment methods related research in these three areas is listed in Table 14.4 including a brief summary of the scope for each research area.

A steel-lined concrete reactor containment building (RCB) accommodates all primary reactor systems, including the reactor assembly and associated valves and piping. A compound building (CPB), and an auxiliary building (AB) surround the RCB. A single base-mat accommodates the RPB, CPB and AB. The plant building layout is designed to reduce the surface silhouette and direct access to the RCB. Similar to the operating PWRs in the Republic of Korea, the RCB contains the radioactive fission products protects against primary coolant leakage to the environment. The RCB protects the entire reactor systems from any external air collisions.

In order to avoid damage to either the fuel or the core components, the designer has to be cognizant of the maximum reactivity insertion rate, due to control rod withdrawal (during normal or accident conditions), or by dilution of the boron in the coolant (if present). Limits on the maximum linear heat rate and margins to departure from nucleate boiling have to be demonstrated. By analyzing the reactivity worth and the potential allowed speed of movement of the control rods, the designer can limit the reactivity insertion rate.

Traditional devices for measuring reactor flux and power are grouped into two categories: excore instrumentation and incore instrumentation. There are two popular radiation measuring devices: fission detectors and ion chambers. These devices have measured reactor flux and power reliably over the last 40 years.

Ion chambers, located outside the core (excore), can detect thermal neutron flux that is directly proportional to the fission rate and reactor power. Fission chamber flux detection devices, on the other hand, have a wider range than ion chambers and are more sensitive to neutrons. Both devices are typically used in traditional large PWRs. The same technology is being considered for iPWRs.10

iPWRs, however, may be able to take advantage of recent size reductions in fission chambers, and recent accuracy improvements. For example, micro-pocket fission detectors have been fabricated and tested as incore flux monitors in the 250 KW TRIGA reactor at Kansas State University. These micro-fission detectors have shown high performance with radiation hardness to neutrons, gamma rays, and charged radiation products.11

Additionally, another emerging technology in the nuclear power monitoring field is gamma thermometers. Although approved for US nuclear use as a local range power monitor since 1982, they have not been deployed widely in US PWRs. Based on the principle of temperature difference in a thermocouple type junction which is proportional to incident gamma flux, gamma thermometers may provide an alternative option for power monitoring in the future, offering possible improvements over traditional methods in incore time response, calibration, and size.12

Although iPWRs provide the opportunity for the use of new technologies for power and flux measurement, the incorporation of new technologies will require increased development time to design and qualify new devices for this important measurement.

Unlike large nuclear power plants that typically have one control room for each unit, compact plant designs like SMRs are more likely to operate multiple modules from a single central control room. Plants that employ multi-module control rooms will inherit a number of characteristics from NPP control rooms as we know them today. As in the past, the primary purpose of the control room and the HSIs within it will still be to enable the operator to control the plant safely and effectively. They are also used to monitor and direct complex operational activities, such as optimising the combined output of the modules or units.

The most likely characteristics of multi-module control rooms would include the following:

• The need for a high level of automation, integration and synchronisation of systems, and optimisation of output. This suggests a single control room from where a minimum crew can manage the entire plant, while still being able to control and monitor the operation of individual modules and systems.

• The use of advanced HSIs to simplify the display of complex system functioning and to minimise the safety-critical, potentially high consequence nature of the control task.

• The change in the central role of the control room operator to system supervisor.

• The existence of new regulatory measures to govern the control room procedures and interface technologies.

7.7.12 LCSs

NUREG-0700 (2007) defines an LCS as ‘A place outside of the main control room where operators interact with the plant. LCSs may include multifunction workstations and panels, as well as operator interfaces, such as controls (e. g. valves, switches, and breakers) and displays (e. g. meters and VDUs)’. NUREG/CR-6146 (Brown et al., 1994) identifies multifunction and single-function LCSs:

• A multifunction LCS is any operator interface used for process control not located inside the control room and not consisting solely of manually operated valves or circuit breakers.

• A single-function LCS is defined as any operator interface, excluding multifunction control panels, that is not located in the control room. This type includes all controls (valves, switches, breakers) and displays (meters, gauges, monitors) operated or consulted during normal, abnormal, or emergency operations.

We can expect that many manual actions that were common with LCSs will be replaced by automated control to eliminate many of the discrete controls mentioned above. Instead, operators will be able to use digital controls and more advanced displays to control and monitor local processes (Brown et al., 1994).

In addition to their potential safety advantages, iPWR SMRs also pose specific safety-related challenges. The main ones are briefly summarized below:

• While in principle the reliability of passive systems and components should be higher, the actual experience-based reliability data are generally scarce. Thus, a large uncertainty may need to be included in PRA/PSA.

• In particular, natural circulation plays a prominent role in the safety approach and safety systems of many iPWR SMRs, but all related relevant phenomena are not as well understood or quantified as for traditional systems.

• Smaller-power, multiple-unit sites will likely require a common control room to achieve economic competitiveness. Not all the implications for safety performance and safety analyses are well known at this time.

• Multiple SMR units at the same site will each have a smaller source term than a large unit, but a similar total source term for a similar total power. While not likely, common mode failure cannot be completely excluded. There is no clear consensus how to treat and evaluate this situation in technical or regulatory space.

• SMRs are more conducive for co-generation applications. Their market niche will likely include desalination, district heating and other industrial processes where the PWR temperature is sufficient. Conceivably, there may be coupling and feedback between the nuclear and non-nuclear portion with an impact on safety performance. However, this has not been analyzed in detail, and actual experience is lacking.

The contribution of learning (Boarin et al., 2012) applies at various levels: a better work organization on the same site, where the personnel have already had experience in the construction and assembling of previous NPP modules; a learning component in factory fabrication of the equipment; a learning component in the utilization of materials and equipment by more skilled workers, etc. A scale-up of the plant output and the attempt to introduce an original French design, i. e. the N4 reactors, towards the end of the program may only partially explain such an occurrence.

Lovins (1986) presented an interesting theoretical framework, referred to as the Bupp-Derian-Komanoff-Taylor hypothesis, that suggests that with increasing application (‘doing’), the complexity of the technology inevitably increases, leading to inherent cost escalation trends that limit or reverse ‘learning’ (cost reduction) possibilities. In other words, the technology scale-up can lead to an inevitable increase in systems complexity that translates into real-cost escalation, or ‘negative learning’. Nevertheless, learning effects have been recorded in technology-advanced industries (Frischtak, 1994); learning effect description was first published by an aeronautical engineer (Wright, 1936).

The learning effect is also visible in the Korean NPP fleet deployment costs: learning accumulation has played an undeniable effect on a progressive cost decrease (Figure 10.10). KHNP, the owner of all 21 of South Korea’s operating nuclear power reactors, has held a licensee relationship with Westinghouse since the late 1980s when the US-based company supplied the 945 MWe System 80 nuclear steam supply for Yonggwang 3 and 4. After that, KHNP was able to develop variants of System 80 for its own requirements under technology transfer terms in the license agreement. After introducing domestic innovations and updating technology over time, KHNP came up with the Korean Standard Nuclear Plant (KNSP), then the OPR-1000. The current APR-1400 technology represents a further evolution of that design. The construction and power generation costs of the APR-1400 are reported to be 10% lower than those of OPR-1000 units.

Korean NPP is the evidence that learning economy may apply to construction costs: in this case, learning effect was achieved through a concentrated construction (Figure 10.11), with the deployment of twin/multiple units on the same site and by avoiding substantial design modification in order to attain PWR plant standardization and control design complexity. (Wolsong NPP are PHWR-CANDU, provided by AECL, as the only exception to the PWR design.)

It may be argued that in principle, learning accumulation is expected to determine a construction cost and time-progressive decrease of successive NPP units, as it was in the Korean NPP fleet. Nevertheless, as far as western countries are considered, in the real world there is often no evidence of cost and time benefits in large NPP deployment programs. That is why simpler and smaller NPPs, with design modularity and high content of factory fabrication, have a higher chance of controlling complexity and exploiting standardization, enabling learning accumulation on both construction and assembling phases. SMRs are expected to benefit from anticipated learning effects, mostly arising from the construction and assembling of multiple units on the same

site. Given the power size of a nuclear site, more SMR units should be fabricated and installed than LRs, with improved chances to learn. General learning accumulation may be recorded at the engineering procurement and construction (EPC) level residing in the human resources knowledge and approach to the project management, and to the organization and procurement issues, such as supplier selection. This learning applies independently of the site location of the new NPP and is therefore indicated as ‘worldwide’ learning in Figure 10.12. In addition, site-level learning accumulation is also applicable on successive NPP units built on the same site, residing in the best, refined practices and actions by local staff. The magnitude of the two effects is comparable (Boarin and Ricotti, 2011a). The learning effect is destined to fade out over the first five to seven units (Carelli et al., 2010). For this reason, in a mature phase of the market, worldwide learning is not a differential factor for SMRs and LRs, while SMRs keep the benefit from the on-site learning accumulation, which applies in case of multiple units built on the same site.

Implicit in the flowline concept is the standardisation of product design. In a volume manufacture situation, a small reactor vendor will not be able to cope with customisation demands without destroying the economic model for the sector. The design will therefore be fed through a sealed manufacturing route. In itself this is no different from many other high-quality, assured manufacturing operations but it raises questions such as the regulator’s approach to manufacture of components that could be fitted into any one of a number of reactors going through the flowline at that point. Although this issue could be seen as implicit in the NRC’s Part 52 licensing regime, it is not obvious that implications such as this have actually been considered in the detail required.

There is little flexibility in the current electrical grids to accommodate high penetration of variable renewables. Three significant challenges currently exist:

1. The geographical location of renewable energy resources are often removed from the major population centers.

2. High variability and, hence, low reliability exist for renewable resources, such as wind and solar.

3. The dispatchability of renewable resources is limited (meaning it cannot be throttled up or down on demand).

For illustrative purposes, projected wind turbine generation rates for one week are shown in Figure 13.3 for a wind farm in Wyoming where a 40 percent annual capacity factor has been realized. There are periods of high or low output that last for days, but significant changes in the generation rate can occur over a very short time (an hour or two). There are also periods of intermediate levels of generation.

There are several strategies for operating a hybridized power generation system to smooth the variability of wind generation. In the first and conceptually simplest, the primary heat source in the hybrid system (e. g. an iPWR or advanced SMR) and the wind farm are operated together to produce a combined electrical output that is constant, essentially mimicking base load generation. For example the wind farm

shown in Figure 13.3, would correspond to making 300 MWe at all times, or as close to this objective as possible. This type of operation would allow the integrated hybrid system to replace an existing baseload 300 MWe coal-fired power plant that was shut down either by obsolescence or to avoid the cost of future CO2 capture requirements or emissions penalties.

A difficulty with this approach is that because of wind’s low availability, on the order of 30-45 percent for regions considered attractive, the primary heat source in the hybrid system must generate power for the balance of the operation, or 55-70 percent of the total energy generated. This, in turn, means that the primary heat source of the hybrid system delivers heat to the coupled process application only when the wind blows, 30-45 percent of the expected annual total, and during some of that time only reduced heat delivery is available. This level of operation would not be economical for the process plant.

An alternative operating basis is to supply a specified minimum amount of heat to the process plant so its operating rate might vary between, for example, 70 and 100 percent of its nominal capacity as needed to offset wind fluctuations. This leads to exceptionally large process plants coupled to relatively small wind farms, both outside desirable ranges. Alternatively, an auxiliary fossil-fired steam generator (a third input system) could provide heat to the process plant when nuclear heat is not available. However, it would have to run 55-70 percent of the time, largely countering the CO2 emission advantages of a nuclear-based hybrid system.

Another strategy is to address only the problematic high frequency components of wind variability. Power grids are well adapted for handling the diurnal variations in demand, so wind variability on that timescale can be handled similarly using the same equipment as long as the wind capacity is no more than approximately the amount of diurnal cycling, typically about 30 percent of the daily peak. In this operating mode, by switching heat output between the power system and the process plant, the hybrid energy system would generate electricity such that the total of the wind-derived and the nuclear-derived electricity does not vary faster than a specified rate. The ability of an SMR hybrid system to compensate for rapid changes in wind generation depends on how fast steam can be switched between the electrical generation and process plants, not on how fast the reactor itself can respond to a transient. In the case of a rapid drop in wind generation, the electric output from the nuclear plant would rise rapidly to moderate the drop in wind generation.

The following section addresses possible applications that could be coupled to a hybrid energy system to maximize use of the thermal energy generated while still meeting the electricity demand. While some the industrial processes would utilize only thermal energy, others require both thermal and electrical energy input from the integrated system.