The light water reactor has a glass jaw: its zirconium alloy cladding. It reacts with water under loss-of-coolant conditions, generating dangerous amounts of heat and hydrogen. As regulators evaluate how well the Fukushima Daiichi nuclear station performed, we should ask why the nuclear energy industry keeps all its eggs in one basket: the LWR. By J.K. August
The Fukushima Daiichi event constitutes the most serious challenge to light water reactors (LWR) since the accident in the United States at Three Mile Island (TMI) in March 1979.
Following reactor shutdown, nuclear fuel generates decay heat. Reactors require cooling to remove the decay heat. At Fukushima, this decay heat could not be removed. Loss of offsite power combined with failing emergency diesel generators led to station blackout (SBO). This caused acute undercooling problems in the reactor cores and spent fuel pools.
At high temperatures, zirconium alloy cladding (such as Zircaloy 2/4, Zirlo, and M5) reacts with the core cooling water in LWRs and generates free hydrogen, according to the reaction Zr + H2O = ZrO + H2. In addition, this reaction is exothermic; it generates heat. Once this is started, removing heat from the core becomes even more difficult.
The immediate consequences included nuclear fuel damage and partial meltdown of the cores. Hydrogen explosions damaged structures and overhead gantries. Hydrogen explosions probably damaged the unit 2 torus from overpressure. Cracked welds leaked water from containment following re-flooding to cool the fuel. Damaged fuel released moderate amounts of radiation through containment to the air, soil and sea.
From the US experience of decommissioning Three Mile Island 2 and other sites, the cost of cleanup can be estimated at around US $30 billion: $8 billion for control and remediation of the site (four times TMI 2’s $2 billion); $15 billion to build two 1500 MW nuclear stations to generate replacement power; $2 billion to remediate surrounding areas. Factoring in inflation could raise these costs.
Metal-clad fuels have high power densities combined with low heat capacity. They lose strength at high temperatures. Combined with chemically-reactive cooling water, intrinsic cladding characteristics limit these designs in undercooling events. To mitigate and control these requires special systems with other complex considerations. They make design and operations more complex, contributing to higher LWR costs.
New reactors designs should be built right, from fundamentals. Ideally, they would be intrinsically simpler. Otherwise, costs will soar and risks remain hard to project with high levels of confidence.
We should consider other reactor designs that are more resistant to fuel overheating weaknesses. One of these is the high temperature gas-cooled reactor (HTGR). That design uses ceramic-coated fuel within a high heat-capacity matrix. The graphite matrix also retains its properties well above the temperature ranges common for metal reactors. Furthermore, helium cooling eliminates chemical coolant-fuel reactions. The result is a design with more inherently safe characteristics. If Fukushima has a positive aspect, it may be to renew interest in other types of reactor design.
The nuclear industry should re-examine nuclear plant design processes from their fundamentals. It should not promote any particular design prescription, as it has in the past. It should encourage thinking ‘outside the box’ to support innovation. Processes should provide the basis for new design standards. We need to improve transparency by providing more intuitive design processes.
We should also simplify the process of licensing designs to encourage innovative and improved technology to lower cost. New approaches to regulation should include the use of relational databases to capture critical plant design content from a design’s ubiquitous documents. Such software would help licensees complete their design bases to directly reduce their licensing burden in operations.
If nuclear energy is to become viable again, nuclear investments must be attractive. They must control investor risk. We should strive to reduce or eliminate complex LWR safety mitigation systems because of their high cost. We should also strive to reduce the many added-on costs inherently risky designs require: emergency plans, dry cask storage, security forces, and their associated management and maintenance costs.
We need a new breed of intrinsically safer reactors for financial investment reasons, as much as for protecting public health and safety. We should stop imposing LWR requirements on other innovative and safer designs. These requirements financially penalize or even block them, by increasing their costs. After all, the future of nuclear power for the US and the world is in better nuclear plants.
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