Researchers at the Idaho National Laboratory (INL) have developed a reactivity control system (RCS) for the US Department of Energy’s (DOE’s) Microreactor Applications Research Validation and Evaluation reactor (MARVEL).
Based on test reactors such as the Advanced Test Reactor, MARVEL uses two mechanical reactivity control mechanisms: a single control rod and four control drums – rotating cylinders – to help keep it small. INL researchers are assembling and testing this RCS in Idaho Falls before it is shipped to the Transient Reactor Test (TREAT) facility at INL’s Materials and Fuels Complex.
MARVEL, an 85 kW, sodium-potassium-cooled microreactor being assembled at INL, is about 15 feet tall and 4 feet wide, requiring something smaller than conventional control rods. “Light water reactors typically use control rods and have a lot more volume and area,” said Anthony Crawford, a researcher in INL’s Mechatronics group. “Microreactors, particularly MARVEL, need more compact solutions, so we use control drums.”
MARVEL’s four rotating control drums are located around the outside of the core and have both reflective and absorber material. Depending on how far each drum is rotated, neutrons are either reflected into the core or absorbed, which increases or reduces reactor power. “The control drum and control rod have the same basic functions – one translates, one rotates,” Crawford said. “But control drums are more challenging because of their inertia, bearings, penetrations and rotational dynamics. That’s why industry doesn’t always use them.”
MARVEL also uses a central insurance absorber. This is essentially a control rod that, when inserted, absorbs neutrons to control reactivity. The central insurance absorber is currently intended to be used only during shutdown and is, in essence, a redundant emergency feature. Together, the control drums and the central insurance absorber provide operational control and layered safety. “In MARVEL, the control drums are the heart of how we control the reactor during operation,” Crawford explained. “The central insurance absorber provides defence-in-depth.”
When the MARVEL project was launched in 2020, the reactivity control system was just a paper design. “We had a general idea of what we wanted to do,” Crawford said. “Then we went through the full engineering process – design, design reviews, analysis – and moved into prototyping and testing.” Some three years ago, the design passed its 90% design review, with input from DOE, the Nuclear Regulatory Commission (NRC) and other experts. However, fabrication introduced new challenges. The system requires about 25 different functions to rotate and “scram” the drums. During a scram (full shutdown), a clutch is released and springs turn the drums quickly into the shutdown position while a damper smooths the motion to reduce impact.
“Through fabrication, we encountered things you don’t necessarily see in CAD (computer-aided design) models,” Crawford said. “We had to address those real-world challenges in real time with few immediately available references. Now we’re assembling it. The mechatronics team developed detailed assembly plans to make sure functionality is maintained, and we test the critical characteristics of every component along the way.”
The reactivity control system is being built to NQA-1 standards, the nuclear industry’s quality assurance benchmark. Many of the components used in MARVEL’s RCS are not available off the shelf. Most of the RCS was fabricated to exacting standards at a special machine shop at the Materials and Fuels Complex. In some cases, the engineers adapted commercial components and subjected them to extensive testing. “That means testing at receipt, during assembly, on a test stand, during pre-operation, during operation and throughout the life cycle,” Crawford noted.
During normal operation, the system must move with extreme precision to finely adjust reactor power. During a scram, it must move very quickly and reliably. The hardware must also accommodate high temperature, high flux, vibration, swelling and deflection while executing both functions. Testing the RCS before the reactor is assembled has helped the researchers better understand the system and develop strategies in the design, analysis and testing processes.
Researchers have assembled all five actuators – four for operation and one spare. The control drum skeletons are assembled, with reflector and absorber material installation pending. They have built the central shutdown absorber and its spare. These systems have been installed onto a test stand that supports each control drum and the central insurance absorber system in their ultimate deployment configurations. With this accurate configuration, the system is undergoing initial integrated qualification testing to verify speed control, precision and scram performance.
“The RCS now becomes a tool to assess the health of reactor assembly,” Crawford said. “We’ll test it on the primary cooling system, then again during dry criticality, then again with coolant.” The system also becomes a training tool. It will help operators better understand nuances… so they’ll know exactly what’s happening and how to respond.
MARVEL’s RCS is designed to adapt. “We use adjustable hard stops to limit maximum reactivity early in life, then move them out as (fuel) burnup occurs to recover capability,” Crawford said. “We can also adjust spring preload and swap dampers to fine-tune scram behaviour. Without tunability, redesigns to accommodate reactor changes could take months or years, or capability could be reduced or lost. With tunability, we can adapt quickly – a capability that is applicable not just for MARVEL, but for other reactor designs too.”
Once MARVEL is operational, engineers will be able to adapt the RCS in line with experience gained during the reactor’s life cycle. “During startup, we operate more conservatively while we learn the reactor’s behaviour,” Crawford noted. “Once we’re confident, we give the system more latitude to operate efficiently and achieve its mission as a test reactor. The RCS parameters evolve with the reactor.”
