Development of a small modular boiling water reactor combined with external superheaters

Highlights

• Conceptual design of a hybrid Small Modular Boiling Water Reactor is proposed.

• Multi-batch fuel arrangements can reduce initial excess reactivity and core F_ΔH.

• A strategy for reactivity control by varying the feedwater temperature is proposed.

• A size comparison between the SMBWR and some selected SMRs is presented.

• The emission rate of the SMBWR is compared to other fossil and renewable generation.

Abstract

The balance between sustainability, energy security and affordability are important trade-offs to consider in decarbonizing the current energy system. The two practical alternatives for moving forward seem to be either reducing the energy storage costs to enable deployment the intermittent renewables on a large scale or developing an affordable and more flexible nuclear power. A small modular boiling water reactor combined with external superheaters offers a significant improvement to the conventional nuclear system. The potential benefits include improvement in cycle thermal efficiency, reduction in the size of the vessel, and the capability to adjust load while maintaining the reactor operation at 100% of its full power.

In this paper, the conceptual design of a Small Modular Boiling Water Reactor (SMBWR) combined with external superheaters is presented along with investigation into some of its core design performance characteristics. It is found that the 4-batch in-core fuel management scheme offers a more favorable performance compared to the 3-batch scheme as it has lower power peaking, less excess reactivity, and more negative coolant void coefficient (CVC). The combination of a multi-batch fuel arrangement, coolant temperature variation, and control rods are required to control the reactivity swing in the SMBWR while keeping the power peaking below the safety limit throughout the depletion cycle.

Introduction

With the increasing concern over the Climate Change, it is likely that the future electricity grid will be relying on low-carbon generators such as nuclear, wind and solar technology. In order to meet the daily and seasonal variation of electricity demand, it is important for the low-carbon system to integrate with energy storage for the intermittent renewables or find solutions for making load-following operation of Nuclear Power Plants (NPP) more economic. de Sisternes et al. (2016) investigated the integration of low-carbon technology systems into the energy mix with respect to emissions limits and average generation costs and shows that the role of nuclear energy becomes more important as the emissions limit tightens. de Sisternes et al. (2016) also found that installation of energy storage helps reduce average electricity generation costs by increasing the utilization of wind and solar. However, under a carbon emissions limit of 100 tCO₂/GWh, average system costs (including storage costs) increase in most cases. This suggests that there is a trade-off between the system costs and the flexibility of the system to meet daily and seasonal variation of demand. On one hand, by having more nuclear on the grid, the stability of electricity supply is improved. Nuclear is a base-load provider of electricity, but its electricity cost is sensitive to the load factor as nuclear is highly capital intensive and tends to have low operating costs. On the other hand, having more renewables, such as wind and solar, would reduce the average generation cost of the electricity. However, the intermittency of wind and solar would be a problem for the stability of the grid and, thus, storage systems are required, which would increase the average system costs. Thus, the options to minimize the system cost lie in how to reduce storage costs to utilize more renewables, or reduce costs for building NPPs and develop more flexible power generation for the NPPs to meet daily and seasonal demand variation, or some combination of the two.

One option to minimize the average system costs of electricity generation is by combining NPP steam cycle with external superheaters. The incentives for that are the improvement in the power cycle thermal efficiency and the possibility to follow the load to some extent by varying the heat provided to the superheater, while maintaining the nuclear reactor operating at 100% of its full rated power and thus maximizing its economic value. According to previous studies (Wibisono and Shwageraus, 2016, Darwish et al., 2010, Zaryankin et al., 2011), combining LWR with a gas fired superheater will provide additional electric power output (up to 80% of its full rated power), improve the plant thermal efficiency by 2–5%, and its operational load variation capability can be between 100% and 65% by only adjusting the heat supplied to the external superheaters. In a Boiling Water Reactor (BWR), adding external superheaters can provide additional benefits, namely, the possibility of eliminating the steam dryer, which is located above the core increasing the BWR vessel height. A steam dryer is required for a BWR to ensure high steam quality before entering the steam turbine. Since the hybrid system operates with superheated steam, the steam dryer could be removed and, thus, the vessel size could be reduced. Furthermore, by removing the steam dryer, the total recirculation loop pressure drop within the BWR vessel will also be reduced, resulting in higher steam pressure at the turbine entry, and thus, possibility of increasing the power conversion cycle efficiency. It is important to note that by having an external superheater powered by a conventional fossil fuel such as natural gas, the NPPs will not be totally carbon emission-free even though the emissions would not be as high as stand-alone gas turbines or combined cycle gas turbines (CCGT). There is also an option to rely on cleaner heat for the superheater heat source. For example, Concentrated Solar Power (CSP) technology is able to store thermal energy by using molten salt. Thus, there is a possibility that one could use the heat stored in the solar heated salt to power the superheater, which would reduce the CO₂ emissions of this hybrid energy system practically to zero.

Another problem faced by the nuclear industry is the financing of large new-build NPPs. Large initial capital investment is arguably one of the main reasons for relatively slow nuclear new-build in the recent years. A small reactor could be more attractive, especially for emerging economies, as the total financial commitment to build small reactors would be lower than large reactors. The fact that the size is smaller would also reduce the duration of the NPP construction, thus, reducing the financial risk of the project as well, making it more attractive for the potential investors. In addition, as mentioned in the previous section, SMRs are claimed to offer several possibilities to counter the economies of scale through their standardization, modularization and mass production in factories.

The development of a Small Modular Boiling Water Reactor (SMBWR) offers a possible solution by utilizing the superheating concept in the SMR. One of the design features for SMBWR that was adopted is to rely on natural circulation of coolant within the reactor vessel during normal operation to simplify and reduce the system cost. This will also allow passive decay heat removal under accident conditions, although safety analyses were outside the scope of this study. By adopting natural circulation, the recirculation pumps could be eliminated from the vessel, thus enabling the removal of some of the RPV penetrations below the core.

It is easier to develop natural circulation in BWR compared to PWR, because of the greater coolant density change (two-phase flow driving head). A smaller reactor would also mean a shorter core, and, thus, lower core pressure losses, resulting in a smaller chimney height required to provide the driving head to counter the pressure losses inside the loop. By having the external steam superheaters attached to the SMBWR, the power conversion cycle efficiency of SMBWR would be improved, which means more electric power could be generated, thus improving the economics of the reactor. Furthermore, it offers the possibility for the SMBWR to reduce its load only by adjusting the external heat supplied to the superheaters, while operating the reactor at full power all the time and improving its economics. For the reasons mentioned earlier, by adding superheaters, the SMBWR would no longer need steam dryers, further reducing the vessel height. The smaller vessel dimensions offer the possibility to increase the operating pressure of the SMBWR, which would further increase its thermodynamic performance.

A number of preliminary studies on SMBWR design choices have been published previously. These studies include the investigation on the effect of operating pressure of the SMBWR (Wibisono et al., 2019) and a core configuration study (Wibisono et al., 2021). The objective of the former study was to investigate whether there was sufficient incentive for the SMBWR to operate at higher pressure. Whereas, the core configuration study was done in order to investigate the effect of core length to diameter ratio on the core performance. In the operating pressure study, it was found that increasing the operating pressure from 6.5 MPa to 10 MPa would not have a significant neutronic effect. In terms of thermal-hydraulics, with the fixed core thermal power, core mass flow rate, and inlet subcooling enthalpy, the higher operating pressure and temperature would result in a higher steam flow rate to the turbine, smaller recirculation rate, smaller core pressure drop, and a slightly taller chimney will be required to develop natural circulation (Wibisono et al., 2019). By comparing the neutronics, thermal-hydraulics, and thermodynamics of power conversion, it is shown that there is a modest but non-negligible improvement in favor of high-pressure operation. However, further studies are required, such as the implications to safety margin degradation, stability and economic performance, to confirm whether the benefits of high-pressure operation could outweigh the drawbacks that arise from having a thicker pressure vessel. Therefore, the current phase of the SMBWR development is focused on the standard BWR operating pressure, which is approximately 7.17 MPa.

In the core configuration study, the main goal was to identify the effect of core dimensions (active fuel length and number fuel assemblies) on the performance of SMBWR. As the core dimensions for SMRs are smaller, the system becomes more sensitive to leakage. Therefore, the trade-offs between neutron leakage (neutronics), chimney height requirements for natural circulation (thermal-hydraulics), and the dimensions of the core and vessel which would affect the manufacturing and transportation complexity are interesting subjects to investigate. Three core configurations, with variation of length to diameter ratio, were investigated by keeping a constant thermal power, core power density, coolant mass flow rate and inlet conditions. Table 1 summarizes the design parameters of the three compared cases (Wibisono et al., 2021). After considering the neutron economy, core performance, CPR limit, and transportation challenges, it was concluded that SMBWR development should focus on the core with 256 FAs and fuel length of 2.70 m.

This paper presents additional investigations related to the SMBWR which have not been covered in the previous publications and can be divided into two main parts. The first part of the paper focuses on the strategies that can be implemented by the SMBWR to reduce initial excess reactivity and maintain core criticality throughout the fuel depletion. While the second part of this paper highlights some additional benefits of having a SMBWR combined with external superheaters.