IT IS BECOMING WIDELY RECOGNISED that nuclear power must be a major component of any strategy to combat climate change, because it offers the greatest potential for reduced carbon emissions in the electricity sector. Both the International Panel on Climate Change (IPCC) and the International Energy Agency (IEA) propose a significant increase in nuclear power to achieve carbon emission reduction on a global scale.
Many policy-makers view nuclear power as a mitigation for climate change and propose that the decision to use nuclear power for climate mitigation should be based on more than its carbon-free power attributes. However, others question whether nuclear power can adapt to climate change in a manner that is cost effective, ensures safety and provides reliable power, without a significant negative impact to the environment. They assert that in addition to mitigating climate change, the next generation of nuclear plants must be capable of adapting to changes in climate, such as an increased frequency in extreme weather events. The current generation of SMR designs seek to accomplish this.
In particular, the NuScale SMR (left) has significant resilience features, enabling the plant to adapt to climate change and support grid recovery. This design is in the final phases of design certification review by the US Nuclear Regulatory Commission (US NRC).
With regard to natural external events, nuclear plants already have significant resilience to high velocity winds and seismic loads. Nuclear plant containments and reactor buildings are typically designed for sustained hurricane winds and tornados, with peak velocities in excess of 460 km/h — including debris generated by such winds. Safety related buildings are designed to very stringent seismic standards that require comprehensive site-specific seismic analyses and rigorous construction and inspection practices.
Nuclear plants normally have two independent connections to the grid to assure AC power for their safety systems. They also have highly reliable diverse, redundant and independent safety systems that are available to respond to a loss of connection to the main grid. This includes redundant AC diesel backup power, DC battery banks and dedicated residual heat removal cooling systems. Many include some passive safety systems such as isolation condensers and pressurised accumulators that do not require power to perform their safety function. Some advanced plants, such as the AP1000, use passive safety systems for shutdown and residual heat removal. Furthermore all have on-site and off-site emergency preparedness plans.
Because nuclear plants can operate continuously for 18 to 24 months without refuelling, nuclear power has always provided reliable electricity to the grid — an advantage when cold weather conditions prevent fuel deliveries to other types of baseload power plants. A Nuclear Energy Institute review of responses to unusual weather events, including floods, hurricanes and polar vortexes, showed that most nuclear plants in a geographical region remained at full power, while the remainder were in safe shutdown conditions and restarted normally afterwards.
Nuclear power also has to adapt to climate change. For example, the NRC has established an inspection procedure for adverse weather protection. This requires seasonal inspections of weather-related risks (eg high winds, hurricanes, torrential rain, electrical storms, tornadoes, extreme high or low temperatures), conditions adversely affecting the ultimate heat sink (eg debris, ice blockages, frazil ice, sea grass, fish), offsite power systems, alternate AC power sources and external flooding mitigation measures. From a regulatory and design basis perspective, therefore, existing plants are better positioned than most power generating systems to adapt to extreme weather events caused by climate change.
New SMR technology will have more resilience. NuScale plant safety in response to extreme events was discussed in terms of an ability to withstand Fukushima type events. Subsequently, NuScale Power completed a research initiative, with multiple studies, to assess and enhance the resilience of the NuScale Power SMR plant design and to assess how to assure continued reliable and safe power production in the face of climate changes. Resilience features identified in those studies that would help adapt future nuclear plants to climate change include: minimal water consumption; no need for AC or DC power to ensure safety; island mode, black-start and off-grid operation capabilities; a site-boundary emergency planning zone; and highly reliable long-term power for mission-critical facilities.
In many parts of the world considering the use of nuclear power, water is at a premium. NuScale has, therefore, developed options for an air-cooled design that drastically reduces water consumption. By using air-cooling for its steam condensers and all of its balance of plant heat loads, NuScale reduces water consumption to pool evaporation, potable water use and minor losses. Water consumption, at nominal conditions, could fall to as little as 5 litres/MWh.
Another option is to use a combination of air-cooling for the steam condensers with water-cooling for the balance of plant heat loads. For this option water consumption, at nominal conditions, would drop to 150 litres/MWh.
This is a fraction of the water consumption of typical thermal power plants. However, the cost of reduced water consumption is an overall reduction in net power output of 5-7%. This may be a reasonable trade-off for water- starved regions.
NuScale is resilient to a loss of AC grid connection or complete loss of all AC and DC power. As part of its design certification application, a comprehensive assessment of NuScale plant safety was submitted to the NRC in January 2017 (see NRC’s ADAMS website, docket number 52-048). In December 2017, NRC released its Safety Evaluation Report (SER), approving NuScale Power’s “Safety Classification of Passive Nuclear Power Plant Electrical Systems” Licensing Topical Report, in which the company established that the design can be safe without reliance on any safety- related 1E electrical power. Passive safety systems provide significant resilience, because they do not require AC or DC power. If all power is lost, the NuScale reactors will shut down without operator or computer intervention, and remain cooled for an unlimited period without needing to add water. The fuel in the fuel pool will remain cool for five months without adding water. Extreme weather events resulting in a loss of connection to the grid do not present a safety challenge.
There are also features of a NuScale plant that would enable it to become ‘first responder power’ for grid recovery should a severe weather event cause damage to transmission lines. In the event of a loss of connection to the grid, the NuScale plant would immediately transition to ‘island mode’, in which one NuScale module would power the internal house loads while the other 11 modules remain in a hot operational mode. Because of their small size, 200MWt, each module can reject 100% of its full-power steam production directly to its dedicated condenser. By bypassing the turbine, the plant is able to remain at hot operational conditions (full or reduced thermal power output) without connection to the main grid. This type of off-grid operation is possible from a licensing standpoint because neither AC nor DC power is required for safety.
Following repair of the transmission lines, the NuScale plant could dispatch power in 60MWe increments as needed to support grid recovery. If all reactors are required to shut down following a loss of connection to the grid, a NuScale plant can start up from cold conditions without external grid connections using a small onsite back-up generator. A single power module would be started first to enter island mode operation after which full power output for the plant can be restored.
NuScale has submitted its methodology for calculating the size of a 12-module plant emergency planning zone (EPZ). The US NRC currently requires an EPZ with a 16km radius around a nuclear plant. By contrast, a NuScale EPZ could be reduced to the site boundary because of the small source term, very low core damage frequency, lack of reliance on AC or DC power, additional barriers to fission product release and enhanced passive safety features. The EPZ analysis performed in support of the Tennessee Valley Authority Clinch River early site permit, audited by NRC, indicates that for NuScale plant design basis and severe accidents, the dose at the site boundary would not exceed regulatory requirements. NRC has initiated an emergency preparedness rulemaking with regard to SMRs that would permit a site boundary EPZ if certain conditions are met. The final rule and final guidance are due to be submitted to the Commission in February 2020.
Power sources for mission-critical facilities such as hospitals, data centres, national laboratories and military bases must also adapt to extreme weather events associated with climate change. Micro-reactors and SMRs can play an important role. The NuScale 12-module plant has a redundant array of independent reactors. This configuration can simultaneously provide power to the main grid and a dedicated micro-grid. This offers high availability for power levels of 60-120MWe — sufficient to power large mission-critical facility micro-grids.
A 12-module NuScale plant could provide 120MWe to a dedicated micro-grid at 99.95% reliability for the 60-year life of the plant. At 60MWe, the reliability would be 99.98%, which corresponds to zero output for only four days over the 60-year lifetime of the plant. If a catastrophic event were to damage the transmission grid and transportation infrastructure so neither fuel nor power could be delivered to the site for a long period, the multi-module NuScale plant operating in island mode has a significant advantage. If the micro-grid remains intact or can be restored, a 12-module plant can provide 120MWe to the micro-grid of a mission-critical facility for 12 years without delivering new fuel to the site.
Integrated Energy Systems
The small footprint, operational flexibility, and enhanced safety of SMRs make them well suited to powering integrated energy systems.
SMR-enabled integrated energy systems can significantly broaden the market for nuclear energy. Because of their high level of safety and resilience, most can be located close to commercial loads that require power and heat.
Nuclear power can play a major role in reducing carbon emissions beyond the electricity sector to include the industrial and transportation sectors. The Figure illustrates IES applications considered in a series of NuScale studies conducted with universities, industry and national laboratory collaborators. The studies include flexible power operations, hydrogen production, process heat and power for oil refineries and water desalination.
An integrated system can incorporate SMRs that provide either fixed baseload or flexible power. The operating strategy will depend on the type of SMR used.
For example, an SMR that operates solely at a fixed maximum power for baseload electricity production would likely choose to operate its fuel at a higher linear power with a tighter margin to the design limit. It would not be designed with significant steam turbine bypass capability since its primary focus is maximising electricity production. In contrast, an SMR intended for flexible power output would operate with greater margin to fuel limits to permit relatively rapid changes in power output in order to match variable loads. It would also provide a means for up to 100% of the main steam to bypass the turbine for use in process heat applications.
Efficient use will require some level of operational flexibility, particularly in remote locations where the SMR is the only source of industrial and residential power.
The multi-module design offers operational flexibility. Each module can bypass 100% of its steam output to its condenser or to an industrial process (hydrogen production or industrial heat). By adjusting the valve position on the steam turbine, the electrical power output can increase from 12MWe (20%) to 60MWe (100%) in 27 minutes or reduce power from 100% to 20% in 8 minutes. In this mode, the thermal power (200MWt) of the module remains constant. This permits a relatively seamless transition from electric power production to thermal power production for industrial processes. It also allows for rapid load following. The module has significant thermal power manoeuvrability. Thermal power can increase from 20% to 100% in 96 minutes by control rod motion. This is adequate for load following for solar farms and industrial processes that vary slowly over hours to days.
NuScale Power teamed with Fluor Corporation to conduct a preliminary technical and economic assessment to evaluate the feasibility and desirability of using NuScale power modules to support oil recovery and refining processes. An economic assessment was performed for a representative refinery case sized to process 250,000 barrels/day of crude oil. The cost differential between using nuclear-generated electricity and heat relative to the reference scenario of using natural gas was calculated for a variety of natural gas prices and potential CO2 tax penalties. Based only on operating costs, the 10-module NuScale plant is competitive with the reference case for natural gas prices as low as $5/MBtu, even with no CO2 tax. The capital investment for the NuScale plant can be recovered in 25 years if the natural gas cost exceeds $9.5/MBtu without a carbon tax, or $7.5/MBtu with a $40/MT CO2 penalty.
By providing both process steam heat and electrical power, a 10-module NuScale plant would reduce CO2 emissions from the refinery by 40% or 200Mt/hr.
While steam-methane reforming is the most common method of producing hydrogen in the USA, the resulting emission of CO2 is a concern. Electrolysis is a clean source of hydrogen and oxygen from water. High-temperature steam electrolysis is an emerging technology and is 40% more efficient than conventional water electrolysis.
NuScale teamed with researchers at the Idaho National Laboratory to study the technical and economic feasibility of producing hydrogen using the HTSE process coupled to a six-module NuScale plant. It found that a six-module NuScale plant with 50MWe modules would produce approximately 190Mt of hydrogen and 1500Mt of oxygen per day. Only 1.15MWe, or 2.4%, of the total power output was required to raise the steam outlet temperature from 300°C to 800°C at the mass flow rates required. Thus a single 60MWe module could produce enough hydrogen to power roughly 70,000 fuel cell vehicles.
As the penetration of renewables increases, the need for systems that can balance and stabilise the grid using carbon-free power will become increasingly important. An integrated energy system can assure full utilisation of the SMR power output, while providing economic carbon-free electrical and thermal power to industrial processes.
Author information: Jose Reyes is co-founder and chief technology officer of NuScale Power
