Figure 13.1 provides an overview of a generic hybrid energy system (HES) as a basis for discussion. Note that a simplified version of the illustrated HES could incorporate just a single input with multiple outputs (co-generation), or could simply integrate two input sources for a single output stream (e. g. electricity). The latter single output scenario would require load-following operation of at least one of the production sources. In this scenario the thermal energy produced by the baseload source is not optimally employed, nor is the full benefit of the financial investment realized. The latter scenario is not considered a desirable configuration, but co-generation may

offer a desirable early implementation of a tightly coupled energy system without the added complexity of multiple energy sources, one of which is highly variable.

The current energy grid employs diverse energy resources to meet demands for thermal energy and electricity. However, these resources are only very loosely coupled in most of the present implementations; each production source connects to the grid individually, and their relative input is managed on the large-scale grid as a whole. In a tightly coupled hybrid configuration these subsystems would be coupled behind the electrical grid both to meet external grid demands and to meet thermal and electrical demands internal to the integrated energy system. This definition of a hybrid system, sometimes referred to as an integrated energy park, is assumed for the remainder of this chapter.

DOE-NE’s overall objective for SMRs in general is to conduct R&D and activities that support the accelerated deployment of SMRs near-term and development of advanced SMR technologies employing innovative designs that offer options for enhanced economic performance, safety, and security as well as for other applications such as process heat and hybrid energy in addition to producing electric power. Obviously, the time horizons for ultimate deployment for these advanced SMR technologies are beyond the early to mid-2020 time frames currently projected for the nearer-term LW-SMRs.

DOE-NE’s program for supporting the near term deployment of the LW-SMRs is the ‘SMR Licensing Technical Support’ (LTS) program. The LTS program is described in Section 14.2.1 while R&D for A-SMRs as conducted under DOE-NE’s Advanced Reactor Technology (ART) program follows in section 14.2.2.

The SMART desalination system consists of four units of MED combined with a thermal vapor compressor (MED-TVC). The distillation unit operates at a maximum brine temperature of 65 °C and a supplied seawater temperature of 33 °C. The MED process coupled with SMART incorporates a falling film, a multi-effect evaporation with horizontal tubes and a steam jet ejector. One significant advantage of the MED — TVC is its ability to use the energy pressure in steam. Thermal vapor compression is very effective when the steam is available at higher temperature and pressure conditions than required in the evaporator. The thermal vapor compressor enables the low-pressure waste steam to be boosted to a higher pressure, effectively reclaiming its available energy. Compression of the steam flow can be achieved with no moving parts using the ejector. SMART and MED-TVC units are connected through the steam transformer. The steam transformer produces the motive steam by using the extracted steam from a turbine and supplies the process steam to the desalination plant. It also prevents a contamination of the produced water by hydrazine and the radioactive material of the primary steam. The steam is extracted from a turbine by using an automatic (controlled) extraction method. The extracted steam control valves vary the flow-passing capacity of the stages downstream of the extraction point. This type of control is usually used when the process steam exceeds 15% of the down-stream flow of the extraction point. The primary steam flow is condensed inside the tubes at its saturation temperature. The brine feed is sprayed outside the tube bundles by a recycling pump. Part of the sprayed water is evaporated and the produced steam is used as the motive steam for the thermo-compressor of the evaporator. Part of the condensate in the first cell of the evaporator is used as a make­up for the steam transformer, and this make-up water is preheated by the condensate of the primary steam before being fed into the steam transformer. When SMART is used for a co-generation purpose, i. e., electricity generation and district heating, it is estimated that ~80 MW of electricity and ~150 Gcal/h of heat can be delivered to the grids.

Oak Ridge National Laboratory, Oak Ridge, TN, USA

Notice: This manuscript has been authored by UT-Battelle, LLC, under Contract No. DE-AC05-00OR22725 with the U. S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes.

4.1 Introduction

Small modular reactors (SMRs) share many of the same nuclear design principles as other reactor types, small and large, thermal and fast. In particular, integral pressurized — water reactors (iPWRs) build upon the extensive nuclear design experience of the hundreds of larger pressurized-water reactors (PWRs) operating around the world today. Nuclear design is the fundamental discipline that involves several major criteria/objectives for the reactor core designer/vendor, including the following:

• Safety — the fuel and the core must be designed to be able to withstand all operational demands and anticipated accidents. Closely related to this is that it must be licensable. In general, this means that constraints on fuel rod power peaking, total reactivity, reactivity coefficients, control rod worths, shutdown margin and delayed neutron fraction have to be analyzed and demonstrated to be within limits.

• Economics — the fuel and the core must be designed to be able to produce the required energy that the utility demands, over the required time period requested, whilst minimizing the fuel costs.

• Reliability — the fuel and the core must be designed to operate in a predictable and reliable manner.

• Operations — the fuel and the core must be designed for relative ease of operation, but with minimum complexity to the operator. For example, the utility may wish to increase cycle length in subsequent years.

• Strategy — the fuel and the core must be designed to be able to achieve utility strategic requirements, if and when set, such as plutonium management, load follow, fast ramp up in power.

‘Nuclear design’ is also often referred to as ‘core design’, ‘in core fuel management’ or ‘core analysis’, but regardless of the term used, there are various design features that the designer can work with to develop an effective and viable design, meeting all of the objectives listed above. For iPWRs (and large PWRs) these include, but are not limited to:

• enrichment of the fissile material;

• burnable poison (BP) types;

• BP loadings (location, number, weight percent);

• locations of fuel (fresh and irradiated) within the core;

• control rod locations and level of insertion;

• frequency and number of fuel assemblies replaced during its maintenance outage.

Therefore, the nuclear design of any reactor, including iPWRs, is more specifically focused on the core physics characteristics, the safety and operational performance of the fuel and the core, and the reactivity control systems in the fuel and the core. The designer, wherever possible, has to stay within the licensing window of experience, including fuel irradiation experience, and validated operations for the nuclear design codes, e. g., fuel types, reactor types, irradiation time and energy output. At the same time, the designer has to be looking at ways of making the nuclear design as economic as possible, whether that is by minimizing the fuel costs, or extracting the most energy from the fuel; all has to be done while staying within the safety envelope of the reactor.

It should be noted that nuclear design is not concerned with the specific design of the fuel or core components, nor the thermo-mechanical performance, or chemical aspects of the fuel, e. g., rod internal pressure, fission gas release. This is generally referred to as ‘fuel design’ and is not the subject of this chapter.

However, because of the central role the nuclear design fulfils, there are also a variety of disciplines and interfaces that the designer works with on a regular basis (see Figure 4.1), including other analysis teams (thermal hydraulics, fuel performance, mechanical design, etc.), licensing, fuel manufacturing and procurement (since the nuclear designer sets the enrichments, number of assemblies, number and type of burnable poisons, etc.), and the broader safety analysis groups (transient analysis, severe accident analysis for source terms, etc.).

For the reader less familiar with the nuclear design process, this chapter begins by introducing and summarizing the nuclear design process for PWRs, and then moves into the specifics of iPWRs; the principles and requirements are the same. The chapter describes (i) the important safety design criteria and principles in nuclear design of reactors, and in particular, iPWRs, (ii) what design features are used to achieve a viable and economic nuclear design, and (iii) how the iPWR designers and vendors have addressed the design principles and features in their respective reactors.

Safety systems are defined as the systems necessary for the safe shutdown and cooling of the reactor during accident or transient/excursion conditions. Safety instrumentation is the instrumentation necessary to monitor the selected safety parameters and send the required actuation signals to the actuation devices necessary for safe shutdown and emergency cooling. For example, the reactor trip breakers are safety equipment designed to release the control rods into the core to stop the nuclear reaction. The instruments designed to detect a parameter excursion, process the signal, and transmit a trip signal to the reactor trip breakers are also considered safety equipment. Safety signals can trip the reactor or they can actuate a system that will provide emergency cooling to the reactor. In the nuclear world, the safety related instrumentation is kept separate from the non-safety instrumentation. (Safety signals can still be used as inputs for control systems but the signal is electrically isolated first so that no control system feedback can affect the safety signal and function.) Because of this separation, safety-related instrumentation has different issues and considerations from non-safety instrumentation. The following sections discuss the techniques for safety system measurements and the unique iPWR conditions and issues involved in developing a safety I&C system for an iPWR.

This refers to the actual conditions under which a given HSI is used in a normal day-to-day working situation. More specifically, it refers to the specific operational scenarios where the HSI is likely to be used. This includes the roles and tasks of users — operators, technicians, engineers and managers. The attributes of tasks that will influence technology choices include experience, knowledge, skills and abilities. The usefulness and applicability of technology will then be a combination of its perceived usefulness, ease of use, and applicability to the intended environment and users. It is therefore important to carry out usability tests, prototyping sessions, meetings and user studies in the particular context of use to ensure a valid basis for technology selection.

While the topic of PRA or probabilistic safety assessment (PSA) is very broad and complex, in this section we only aim to addresses some of the SMR-specific issues.

We start with some general comments to provide a common ground and avoid misunderstandings stemming from the somewhat different licensing regulations and terminology used in different countries.

The US NRC virtual reading room (www. nrc. gov/reading-rm) and IAEA (2012b) provide useful general information. As defined at www. nrc. gov/reading-rm/basic-ref/ glossary/design-basis-accident. html, a design-basis accident (DBA) is a ‘postulated accident that a nuclear facility must be designed and built to withstand without loss to the systems, structures, and components necessary to ensure public health and safety’. Simplistically, these accidents are postulated assuming a single failure of a safety system, or a single ‘credible’ event, and need to be deterministically ‘dealt with’ with the remaining (diverse and redundant) safety-grade systems only, without the need and thus without taking credit for any action of non-safety grade systems (which presumably could be less reliable). On the other hand, there is always a probability of multiple failures and events, termed beyond design basis (BDB) events. Identifying such BDB event trees and evaluating probabilities that ultimately they may lead to core damage, or even to radioactivity release, and estimating the consequences to the public, is performed by the PRA/PSA. For further discussion on Level I, Level II and Level III PRA/PSA, the reader is directed to any PRA/ PSA textbook. We note that in the PRA/PSA space, the action of non-safety grade systems may be taken into account (credited) with appropriate probabilities.

Some of the topics and features specific to iPWR SMRs that have impact on PRA/PSA are discussed below. Most of them are relevant to safety in general and not only to PRA/PSA and may therefore be discussed also in other sections of this chapter.

The investment decision in an industrial activity largely depends on the capability of the project to adequately recover and remunerate the initial capital expenditure. The uncertainty of the capital cost estimation, i. e. the initial investment, as mentioned in Section 10.1.1, affects the ability to make a reliable estimation of the investment profitability. The uncertainty affects the scenario conditions, the project realization and operation; as a result, the stream of income generated by the project is also affected by uncertainty. Therefore, expected profitability has a degree of risk embedded and a series of different possible outcomes, depending on the realization of stochastic variables. Investment in liberalized electricity markets, as in most of the European and

North American countries, compels investors to include uncertainty in their business plan analysis and to give risk as much relevance as profitability into their decision making. The key variables to the financial performance of the investment project are ‘forecast’ in order to get a reasonable estimation of the project profitability and economic soundness. All this considered, NPPs represent a long-term investment with deferred payouts. Moreover, the nuclear industry is very capital-intensive. This means that a high up-front capital investment is needed to set up the project and a long payback period is needed to recover the capital expenditure.

The longer this period, the higher is the probability that the scenario conditions may evolve in a different, unfavourable way, as compared to the forecasts. As an example, market price of electricity might be driven downwards by unexpected market dynamics; unexpected operating or design drawbacks might also undermine plant availability. A capital-intensive investment requires the full exploitation of its operating capability and an income stream as stable as possible. On a long-term horizon, a low volatility in a variable trend might translate into a widespread range of realizations of the variable value. This condition is common to every capital-intensive industry. Nevertheless, some risk factors are specific or particularly sensitive to the nuclear industry: typically, the public acceptance, the political support in the long­term energy strategy, the activity of safety and regulatory agencies.

For these reasons nuclear investment is usually perceived as the riskier investment option among the power generation technologies (Figure 10.2). Clearly, risk is not the only or the most relevant criterion in the selection of a power generation technology. Besides risk and cost, other strategic and economic issues are included in a technology investment evaluation, such as the power generation independence, the power density (as compared to the land occupation), the power supply stability (baseload), the electricity price stability, etc. The key risk factors affecting a nuclear investment project are tentatively listed and classified in Table 10.3.

Capital cost and construction lead-time have pre-eminent importance. Construction time and cost overruns are considered to be the most adverse occurrence able to undermine the nuclear power economics. Throughout the construction period, the project will be exposed to commodity price risk, vendor credit risk, engineering and construction contract performance risk, supply chain risk, sovereign risk, regulatory risk, etc. The construction phase is the most affected by the investment risk. The magnitude of a project overrun is often difficult to estimate while construction proceeds and even more difficult to rein in (Dolley, 2008). The ability to estimate construction cost in the past has proved very limited, as confirmed by US data reported in Table 10.4.

Thus, financing of nuclear power is affected by risk perception. Risk has a cost, which is transferred to the cost of capital in terms of ‘risk premium’, as a remuneration for possible negative outcomes (Damodaran, 2011). The ‘rating’ associated with an investment project represents the probability of financial default: as far as the ‘rating’ is low (i. e. the risk is high), the risk premium applied on the cost of capital (equity or debt) is high. Therefore nuclear projects usually bear high cost of money, compared with other energy sources. For this reason and for the long debt duration, IDCs represent a relevant part of the capital cost (Figure 10.3): any increase in the

cost of capital would be a significant burden on the project economics. Besides risk premium considerations, IDCs are also heavily affected by the construction period, where financial exposure is the highest and the project pays compound interests on the invested capital.

Hence nuclear business risk derives from:

• capital intensive nature, with huge sunk costs and high financial exposure during very long PBTs;

• very long-term market forecast reliability;

• unexpected external unfavourable events (such as natural events, public acceptance/political support withdrawal) or intrinsic drawbacks to the project economics (such as construction time and cost overruns, operating unavailability).

SMRs may represent a valuable option to mitigate several among the risk factors previously discussed. Due to their features, SMRs are able to reduce the severity of many risk factors in pre-construction, supply chain, construction and operating phases (Locatelli et al., 2011a). An IAEA investigation (Barkatullah, 2011) on the topic reached the conclusion that SMRs may present mitigation factors against some

Figure 10.4 Risk factors: differential impact on SMRs and large reactors. Adapted from Barkatullah (2011).

major financing challenges of nuclear power (Figure 10.4). In particular, lower up­front investment of an SMR and low construction lead-time are key features able to decrease the financial risk of the investment. These are discussed in the following sections.

12.1 Introduction

For an assessment to be made of the supply chain and manufacturing options available for small reactors, the fundamental requirements for cost-effective deployment need to be explored and understood.

12.1.1 Economic development

From the dawn of nuclear power there has been a concerted effort to increase the economic performance of each installation; that is maximising the revenue created from each plant verses capital investment incurred. Broadly, the improvement to the economic performance can be classified into three areas:

• design;

• manufacturing;

• operation.

The resulting change in economics from any of these can be observed in changes to the overnight cost of capital (expressed in $/kW), that is the capital cost of the plant expressed in $ per kW of electrical power output.

The ambition to improve the economic situation has resulted in a drive to increase the power output from individual generating units, and this is not specific to nuclear installations. However it can also be observed that for each generating option there are economic ceilings where increased capacity no longer delivers a lower cost of operation. For coal this point came in the early 1970s, as shown in Figure 12.1. Equally the relationship between power and cost is defined with a non-linear characterisation; for example the additional cost to increase the plant size from 600 to 1200 MWe is not double the original investment, as shown in Figure 12.2.

So what are the limits to going bigger and specifically does this present an opportunity for the small reactor?

System security refers to vulnerability to physical and cyber attacks, including vulnerability of both the installed facility and the supply chain (equipment feedstock). An integrated energy system based on domestically secure natural resources would be preferred over one having external dependencies due to the associated risk factors.

A secure, reliable control system must also be designed and demonstrated for these complex nuclear hybrid energy systems. A resilient control system will ensure that the integrated energy facility is safe from cyber attack. Robust control also ensures safe, predictable response of the integrated system to off-normal conditions, such as physical attack or natural phenomena including earthquake or flood.