Load following with Small Modular Reactors (SMR): A real options analysis

Highlights

• Nuclear power plants (NPP) are required to operate in load following mode.

• Small modular reactors (SMR) are NPP suitable for cogeneration purposes.

• SMR can use cogeneration options to perform the load following.

• The paper assesses two cogeneration options: microalgae and desalination.

• SMR plus Desalination is suitable from both a technical and economic perspective.

Abstract

Load following is the potential for a power plant to adjust its power output as demand and price for electricity fluctuates throughout the day. In nuclear power plants, this is done by inserting control rods into the reactor pressure vessel. This operation is very inefficient as nuclear power generation is composed almost entirely of fixed and sunk costs; therefore, lowering the power output doesn't significantly reduce generating costs and the plant is thermo-mechanical stressed. A more efficient solution is to maintain the primary circuit at full power and to use the excess power for cogeneration. This paper assesses the technical-economic feasibility of this approach when applied to Small Modular Reactors (SMR) with two cogeneration technologies: algae-biofuel and desalinisation. Multiple SMR are of particular interest due to the fractional nature of their power output. The result shows that the power required by an algae-biofuel plant is not sufficient to justify the load following approach, whereas it is in the case of desalination. The successive economic analysis, based on the real options approach, demonstrates the economic viability of the desalination in several scenarios. In conclusion, the coupling of SMR with a desalination plant is a realistic solution to perform efficient load following.

Introduction

According to analysis by the US Department of Energy, the global demand of energy will increase by 50% in the next 30 years, primarily in non-OECD countries [70]. The journey towards sustainable energy therefore faces several challenges, with a number of different technologies needed to achieve this long-term goal [71]. Renewable energy sources will play a lead role and need to be developed, deployed and managed, along with existing power and non-power technologies.

From this perspective, Nuclear Power Plants (NPP) can be deployed along with renewable fuel power plants and facilities (e.g. desalination plants) to achieve the long term perspective of sustainable development without the emission of greenhouse gasses (Ambitiously, the IPCC targets “zero carbon” emissions by 2100 [72]). Given the predominance of their fixed costs, NPP are considered as a base load power technology. However, given the relevant share of nuclear power in specific countries (e.g. 75% in France) and the introduction of intermittent sources of energy (i.e. solar, wind) in to the grid [73], flexibility and adaptability will be required for the load curve [1], [74], as stressed by the OECD/NEA in a recent report [2]:

“a unit must be capable of continuous operation between 50% and 100% of its nominal power (P_n), […]. Load scheduled variations (should be) 2 per day, 5 per week and 200 per year”.

Currently, NPP production follows the electricity demand (from now on “load following”) by modifying the reactivity within the core, e.g. by inserting control rods and neutrons absorbers into the coolant [1]. By doing so, the power is reduced, with a waste of potential energy and a thermo-mechanical stress on the plant whenever the power regime is changed. Unlike gas fuelled power plants, there is not a relevant cost saving in operating an NPP at a lower power level due to the substantially fixed nature of nuclear costs. Besides investment costs, O&M (Operation & Maintenance – mainly personal and insurances) costs are fixed and independent from the power rate. Again, in contradiction to conventional gas-fired plants (where fuel accounts for approximately 70%–80% of the generation cost) nuclear fuel accounts for only about 10% of generation costs, making it significantly less influential [3], [4]. A lower power rate does not translate into a significant fuel saving. Due to the complexity of the neutron dynamics within the core (fission, absorption by all reactor materials, reactions, leaks, poisoning etc.), the proportionality between power produced and fuel consumed is not linear [5], [6]. Consequently running a power plant at 50% of its power does not save more than 4–5% of its cost, while the loss of revenue extends the recovery of the capital investment.

An alternative is to keep the NPP primary circuit always at full power and to follow the load curve by using the power (both thermal and electric) of the secondary side to cogenerate valuable by-products. The goal of this paper is to assess the technical and economic feasibility of this concept by coupling multiple Small Modular Reactors (SMR), interesting because the power is fractioned, with algae-biofuel and desalinisation.

SMR are NPP with electric power output lower that 350 MWe and therefore suitable for an intrinsically modular power station. In the last 5–10 years, SMR have received an increased attention from the scientific community and nuclear industry, with several SMR now under development [7], [8]. In this paper the International Reactor Innovative and Secure (IRIS, a 335 MWe PWR) is assumed as representative of the SMR – PWR class. It is considered that its power size allows it to exploit both economies of scale (i.e. it is placed in the upper size of the SMR category), design innovation (e.g. integral primary loop) and economies of multiples (i.e. the unitary cost saving of deploying more than one unit) [9]. IRIS is a PWR integral design where every primary system component is integrated into the vessel (including fully internal primary pumps); this containment is designed to be thermodynamically coupled with the integrated primary system during accident conditions and the overall design is focused first and foremost on simplicity [10]. IRIS major design parameters and values are summarized in Ref. [11] Table 4, while the rationale for its design are recapped in Ref. [12]. Nevertheless, literature references, methodology and results for IRIS, are applicable to the whole light water SMR class.

A key advantage of adopting multiple SMR instead of a single Large Reactor (LR) is the intrinsic modularity of an SMR site. In particular, it is possible to operate all the primary circuits of the SMR fleet at full capacity and switch the whole thermal power of some of them or use the electricity produced for the cogeneration of suitable by-products. Therefore, the load following strategy is realized at site level, by diverting 100% of the electricity produced or 100% of the thermal power generated of some SMR units, to different cogeneration purposes and let the remaining units to produce electricity for the market. Either in the case of full electricity conversion or in the case of full cogeneration operation mode, the efficiency would be maximised by-design: SMR could run at full nominal power and maximum conversion efficiency and cogeneration plant size could be optimized against the thermal power rate.

Assuming 4 IRIS units, the power rates at site level would be approximately 0%, 25%, 50%, 75% and 100%; these steps are suitable for the general load following requirement by a base-load plant. Gas plants will provide the fine matching with the electricity market demand, as usual. By using SMR smaller than 335 MWe size, the possible power rates steps of the nuclear power station would be more gradual.

Several cogeneration plants can be coupled with a nuclear reactor using its thermal power and/or the electricity. The plants analysed in this paper are a biorefinery (algae) and a desalination plant because:

• These plants require low enthalpy thermal energy, as it is the case for the steam produced by Light Water Reactor SMR. More advanced GEN IV designs can provide fluids to higher temperature for a large range of industrial purpose (e.g. steel production [75]). However GEN IV design are not expected for commercial deployment in the near future, while Light Water (as PWR) is the technology implemented in the vast majority of NPP built in the last 10 years.

• These plants require higher input in terms of thermal energy than electric energy. This is ideal with the modular approach.

• The interest of institutions and countries for biofuels: the EU has set a goal of 10% of biofuel consumption on the total fuel for transportation by 2020 [13].

• Biofuel (including biogas) from microalgae is a promising technology still in the development phase. There are different types of technologies and biomass under consideration, some more promising that other. Tedesco et al. [76] gives an account of the biogas yields obtained from co-digestion of seaweed biomass and show that some species of microalgae are preferable to others.

• Nuclear-Desalination is a proven technology with PWR reactors [14], [15], [16], [17].