NuScale’s story of simulation

In March 2017, the US Nuclear Regulatory Commission accepted the design certification application for NuScale Power’s small modular reactor, making it the first modern SMR to begin the licensing process in the West.

NuScale submitted its application to the NRC on 31 December 2016. By accepting the application, the NRC confirmed that NuScale’s submission addresses all US regulatory requirements and contains sufficient technical information for the NRC to carry out its review. NRC is expecting to complete the certification process in 46 months, or by January 2021.

“The uncommon fact that the NRC was able to accept our application during the 60-day docketing review period is validation of all of our hard work over the past eight years,” said NuScale CEO John Hopkins, upon acceptance of the application.

Simulation played a key role in this achievement. NuScale provided technical justification from the simulator to support its design certification application – the first time this approach has been used in the licensing of a US nuclear plant.

Design development

The NuScale SMR design has been under development in the USA since 2000 as part of a US Department of Energy research project. Idaho National Environment & Engineering Laboratory (INEEL) led that work with support from Oregon State University (OSU). But the real effort began in 2007, after a technology transfer agreement from OSU granted NuScale Power the exclusive rights to the design and continued use of the test facility. Today, the design is being commercialised with financial backing from Fluor Corporation and up to $217 million in matched funding from the US DOE. This public-private partnership is looking to accelerate the design, licensing, first-of-a-kind engineering, and testing, which could enable the first reactor to start operating by 2026.

The NuScale power module comprises a 160MWt reactor core housed with other primary system components in an integral reactor pressure vessel and surrounded by a steel containment pressure vessel. The reactor vessel (17.8m tall and 3.0m in diameter) contains the core (37 fuel assemblies and 16 control rod clusters), a central riser (hot leg), two integral helical coil steam generators surrounding the central riser, and an integrated pressuriser. The reactor vessel is nested within a 23.1m tall and 4.5m diameter steel pressure vessel, which functions as the containment.

Up to 12 power modules can be co-located below grade in a large common pool of water, which provides passive containment cooling and decay heat removal. Each module has a dedicated turbine-generator to provide gross electrical power of 50MWe. A 12-module plant is expected to yield a net power of 570MWe to the grid.

The first commercial NuScale module is planned for construction at Idaho National Laboratory for Utah Associated Municipal Power Systems (UAMPS), and will be operated by Energy Northwest, as per a 2013 agreement.

Simulation first

When simulator supervisor Ross Snuggerud joined NuScale in 2008, one of his first roles was looking at how many people would be needed to staff the NuScale control room. After reviewing the regulation, Snuggerud managed to convince management that a simulator would be needed to investigate staffing requirements.

Currently, in the United States regulations require a minimum number of operators per reactor and a maximum of two reactors are operated from a single control room. Operating 12 units from a single location was a “quantum leap” from anything the NRC had approved before. However, the attributes of the NuScale design – the passive nature of the safety systems, the fact it doesn’t need safety-related AC or DC power or operator action for design basis events – mean that it is different to operate than today’s conventional reactors.

After a request for proposals, US-based GSE Systems was chosen to develop simulation modules for the NuScale plant. The simulator was to be used in the design certification process, for analysis of the Human-System Interface design, controls system strategy and to develop operating procedures. “Before, simulators were built after the plant to train and license operators,” says Snuggerud. However, NuScale is using the simulator to investigate how the human- system interface will function.

“We are using the simulator to prove to the NRC that our staffing plan is safe and acceptable for the design,” adds Tim Tovar, plant operations manager at NuScale. “We originally talked about getting an exemption. But we worked through the processes with the NRC and it made sense to develop a staffing rule for the customers.”

In January 2016, the NRC described two options that NuScale could consider for addressing the 10 CFR 50.54(m) regulatory requirements related to control room staffing in the design certification application. NuScale decided to propose, as part of the rulemaking, certification of an alternative approach to control room staffing to be used in place of 10 CFR 50.54(m). This would require NuScale to provide the technical basis for rulemaking language that addresses control room staffing and control room configuration as part of the design certification. Any future combined operating license (COL) application that incorporates the NuScale design certification would, as such, not need an exemption from 10 CFR 50.54(m).

Constant evolution

The first single-unit simulator was up and running by 2010. GSE used Idaho National Laboratory’s RELAP5-3D code in an interactive, real-time simulation environment, Studvik’s S3R reactor core model and GSE’s JADETM Simulation toolset, to create an accurate model of the new reactor design. For the balance of plant (BOP) systems, which had not been designed, GSE used its experience in modelling smaller fossil plants to provide a good representation of the balance of plant. As system design matured, so did the simulation models and accuracy of the entire simulator, according to GSE.

In May 2012, NuScale commissioned the world’s first small modular reactor control room simulator. The simulator comprised 12 independent work stations each dedicated to simulating the operation of a NuScale SMR module and a turbine generator. It served as a model control room, allowing NuScale to evaluate different approaches to the design and operation of a power plant using its technology.

“We used it to demonstrate the NuScale plant and safety systems to stakeholders, potential investors, potential customers, to help educate the regulator, staff and the engineering group,” says Snuggerud.

Tim Tovar, plant operations manager at NuScale, says the main challenge was determining how to manage resources effectively. After evaluating various scenarios, NuScale plans to have six licensed operators in the control room for a 12-unit station. One person would be responsible for monitoring all 12 units during normal operation. There would be two operators designated to support and take part in removal from service, maintenance and other tasks, while the final three operators would provide oversight and offer support for the crew.

Tovar explained how NuScale used the simulator for its staffing plan validation. This involved looking at all of the tasks an operator had to do, and in particular the time- critical activities. The information was pulled together in a database and used to generate three complicated scenarios, which included the highest workload items and safety significant items e.g. design basis events and beyond design basis events on multiple modules. Two trial runs were made on each of the scenarios in August 2016, and they were audited by the NRC.

The next stage of work is the integrated system validation. This is similar to the staffing validation, only four times larger. NuScale will go through more extensive simulator scenarios, including normal evolutions, design basis and beyond design basis events. Data will be collected and submitted to the NRC for review, a process that will take around 13 months in all. The aim is to make sure the hardware, software, training and procedures work together, according to Snuggerud.

“We are in the process of hiring the operators now,” says Tovar. In particular, NuScale is looking for three crews of six operators that have no experience of the NuScale plant. Roughly half of this crew will have been licensed to operate conventional nuclear plants, with the remainder being engineers from the Navy or other industries. The crews will be trained on the design (seven months) and then spend at least two months working on the simulator during the testing phase.

“There are things that people designing the interface think you will understand,” says Tovar, but that may not always be the case. “That is part of the reason we are working hard with independent operators. We want a fresh perspective.”

Testing on the simulator has already highlighted areas for improvement, including alarm prioritisation and changes to the status board. Snuggerud explains how, at one point, the status board created an optical illusion for some of the operators. In response, NuScale changed the way the system functioned to overcome that issue. But he stressed that highlighting such issues “is all part of the process” and how it works.

Summer successes

In May 2017, NuScale commissioned a second SMR control room simulator in its Richland, Washington office. The simulator will be used to develop plant operating procedures and training material, and also to train future plant operators.

A couple of months later, NuScale announced that the NRC had found the Highly Integrated Protection System (HIPS) Platform is acceptable for use in plant safety- related instrumentation and control systems, which it described as a “major step” toward completion of design certification.

The HIPS Platform, which uses field programmable gate array (FPGA) technology, is a protection system that was jointly developed by NuScale and Kansas-based Rock Creek Innovations over a six-year period. The platform is comprised of four modules that can be interconnected in multiple configurations to support different reactor safety systems. The system offers independence, redundancy, predictability, reliability, defence in depth and is not vulnerable to internet cyberattacks.

Earlier this year a working prototype of the HIPS platform (manufactured by UK-based Ultra Electronics) was installed and tested in the NuScale simulator in Corvallis, Oregon. While the simulator wasn’t used to justify
the HIPS design “it helped regulators to understand the safety systems,” NuScale said.

Virtual commissioning

In the nuclear industry today, many modifications are implemented in the simulators before they are carried out on an operating plant. And with the trend towards replacing analogue I&C systems with digital, simulators are often used to validate the control systems. But NuScale’s approach to simulation in the design phase is cutting edge. “We have been blazing new ground and setting the standard for new designs,” says Tim Tovar, plant operations manager at NuScale. “I hope that for advanced designs such as high-temperature gas reactors (HTGRs) and molten salt reactors (MSRs) we can utilise the lessons learned to make some of those designs feasible and efficient in getting through the regulatory process.”

Sean Fuller, senior vice president at GSE Systems, also stresses the capabilities of simulation to support design and operational testing of power plant applications, such as NuScale, as a method to validate system design, operational scenarios and control. “This is a cost-effective approach to design, currently implemented in the transportation industry, and allows engineers to gain insight through cost effective testing due to the accuracy of the modelling and the speed of dynamic simulation.”

GSE was one of the first to develop the nuclear power plant simulator to support NRC License Operator training and has
been virtually commissioning fossil power plants for 15 years. Fuller feels the concept of commissioning a plant before a shovel has been put in the ground, or simulating a potential design change for an existing plant before licensing can significantly add benefit to the nuclear sector.

“Simulation is such an effective communication tool,” says Snuggerud. “I don’t think we would have come up with the concept of operations without it, and even if we did we wouldn’t have been able to convince the regulator that it is safe. Simulation is the only way.”