Martin Hertzberg: Thermodynamics rules: not business or politics |

Martin Hertzberg: Thermodynamics rules: not business or politics

While investigating the causes of the Three Mile Island accident and advising the EPRI (the Electric Power Research Institute) on their research program for the safety issues involved, it became quite clear to me that the standard design of nuclear reactors involving the interfaces: enriched uranium/zirconium alloy/liquid water is thermodynamically unstable with respect to high temperature excursions. Under normal operation, the heat generated by the uranium fission chain reaction within the zirconium tubes is conducted through those tubes to circulating liquid water coolant, which then generates steam for the power turbines. If the coolant flow is interrupted and the zirconium interface is exposed to gaseous steam, then heat transfer is diminished, its temperature rises, and the reaction between zirconium and steam generates hydrogen, which, when mixed with air to concentrations above 8 percent is highly explosive. In addition, that reaction is highly energetic which heats the zirconium further accelerating the reaction, which increases the temperature further, etc. until the zirconium cladding is destroyed and radioactive fission products contaminate the entire reactor. Firefighters know that they should never to try to put out a magnesium fire with water because of a similar reaction.

At Three Mile Island, a stuck-open pressure relief valve causes a loss of coolant, the hydrogen generated by the high temperature excursion flowed out from the reactor and was ignited generating an explosion (a pressure of 18 psi). Fortunately, the outer containment structure was able to contain that pressure. For the reactors in Japan, the loss of coolant caused by the tsunami gave the same result: The thermodynamically unstable steam/zirconium interface generated hydrogen, but in those instances the ensuing explosion destroyed the outer containment structures. When I discussed that thermodynamic instability at a National Academy conference, representatives of the nuclear power industry didn’t want to hear about it: They had already committed themselves to that design. So they settled for Band-Aids: pilot flames or catalytic devices to oxidize the hydrogen before it could reach an 8 percent concentration.

The lesson? Ignore thermodynamics at your peril!

The final outcome of the nuclear reactor disaster in Japan remains uncertain, but there is one lesson to be learned. Whether one is dealing with nuclear reactor safety or the theory of human-caused global warming, you ignore the laws of thermodynamics at your peril.

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