The combined effects of the earthquake and tsunami illustrate the risks of putting all your eggs in one basket
The architecture of energy distribution in major industrial nations like Japan, the U.S., France, the U.K. , and Germany is based on very large power plants, often several at a single site, which are tied into huge transmission and distribution (T&D) grids.
The vulnerability of this model was tragically illustrated March 11, 2011, as six nuclear reactors representing about 10% of the nuclear generated electricity in Japan were permanently taken out of service by a single natural disaster.
In addition to the loss of electricity, radiation hazards from some of the crippled reactors have created significant safety issues of workers at the others.
In engineering parlance, the decision to build all six reactors at Fukushima at a site on the earthquake prone coastline created a single point of failure. TEPCO's decision not to take historical records of tsunami events into account in setting the height of the protective sea wall sealed the fate of the reactors.
When the earthquake struck, the reactors shut down as planned and emergency generators switched on as expected in such an event. However, the 15 meter high tsunami breached the 5 meter sea wall sweeping away the fuel tanks for the diesel generators and the electrical switch gear needed to deliver their power to reactor cooling system pumps.
Are resilient networks of SMRs an answer?
What if industrialized countries began to mitigate the risk of similar future events by building resilient networks of small modular reactors? What if instead of building a single 1,000 MW nuclear reactor, a utility laid plans to build six-to-eight small modular reactors (SMRs) in locations near major demand centers for electricity?
A resilient grid of SMRs built in a distributed network would be much less susceptible to damage from natural disasters or man-made disruptions. If one SMR goes out of service, it doesn't create a regional blackout for everyone else in the utility's service area.
Another issue that comes to mind is whether continued reliance on traditional light water reactor (LWR) designs is the only feasible path forward for SMRs? A lot of emphasis, perhaps too much, has been made on the production of hydrogen when fuel assemblies with their zirconium cladding are uncovered from cooling water. At Fukushima three of the six reactors suffered significant damage to their secondary containment structures from hydrogen explosions.
Design legacy of the Integral Fast Reactor lives on
I've been exchanging emails with Irfan Ali, CEO of Advanced Reactor Concepts (ARC), a Reston, VA, firm that is developing a 100 MW SMR. It is based on the design concepts of the Integral Fast Reactor which was demonstrated at the Argonne West site of the Idaho National Laboratory.
In a white paper released in April 2011 to address the issues surrounding Fukushima, Ali says the time is right for an objective assessment of alternative energy distribution architectures. The Fukushima reactors were 40 years old and built to standards that would not be accepted in today’s regulatory environment. The reactors provided a significant portion of the electricity used in Japan, 6 GWe of the 45 GWe that comes from nuclear reactors in that nation when all of its plants are online.
What about emergency shutdown?
The U.S. Nuclear Regulatory Commission is still grappling with the challenge of how to conduct a safety review of a SMR using sodium cooling systems and uranium alloy fuel. Preparing for that review, it is fair to say the NRC will find some of its wisdom about LWR pumps and cooling system is not directly relevant to some aspects of the new designs. There will be a steep learning curve for the agency,
Citing the design of the sodium cooled ARC-100 design, ARC’s Ali discusses the advantages of natural circulation pathways that carry decay heat away from the fuel rods. There are no pumps hence the lack of a need for electricity to run emergency cooling systems. Cooling loops are backed up by air circulation outside the containment structure.
Also, he explains the fuel is a uranium metal alloy rather than uranium oxide as used in LWR designs. The steel cladding doesn't present a risk of hydrogen production.
Perhaps the most novel element of the reactor is what happens if heat transfer from the reactor to the turbine is stopped by accident or other interruption. In effect, the reactor sits on its hands and does nothing.
In technical terms, the passive feedback mechanism which shuts the reactor down is based on the physics of the design. Rising coolant temperatures cause structural elements to thermally expand which allows more neutrons to leak out of the core rather than be absorbed by the fuel creating fission. As a result, high heat causes the neutron chain reaction to shut down.
A key issue at Fukushima is the widespread release of radioactive from the turbine buildings and spent fuel pools. In a sodium cooled reactor using uranium alloy metal fuel, the iodine is chemically bound inside the reactor limiting the potential for radioactive releases.
Increasing interest in mitigating risk with SMRs
The ARC-100 reactor design concepts contain intriguing safety measures which might benefit highly industrialized countries seeking a more resilient power grid. Similar benefits might come from other SMR designs including those that use conventional LWR designs. It depends in part on the pace of advancement in fuel cladding materials science.
The key idea is to find ways to avoid future consequences of having too much electrical generation capacity invested in a single site. This is especially important in areas where there is a potential for earthquakes, tsunami, and other natural disasters or man-made disruption. SMRs buried underground add the natural containment of that design paradigm to their protective envelope.
Changes will also be needed in the way rate structures are set for resilient networks of SMRs compared to rates for single large plants. Perhaps the lure of lower costs for T&D architectures will be an incentive for utilities to speed up their assessments of SMRs in the wake of Fukushima?
Prior coverage on this blog
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