Opinion: What happened at the Fukushima reactor?
Events in Japan confirm the robustness of modern nuclear technology — not a failure

By Kevin Yost
STAFF COLUMNIST – The Tech – MIT’s Online Newpaper
March 15, 2011
Link: http://tech.mit.edu/V131/N13/yost.html


As a nuclear engineer, it is depressing to read the recent reports on the Fukushima nuclear incident — not because of the incident itself (at this point I strongly believe that we will remember Fukushima as evidence of how safe nuclear power is when done right) — but because the media coverage of the event has been rife with errors so glaring that I have to wonder if anyone in the world of journalism has ever taken a physics class. My favorite: in one article, boric acid was described as a “nutrient absorber” instead of a “neutron absorber.” How many editors signed off on that line without asking, “Why would a nuclear reactor need to absorb nutrients?”
Whether it is confusion of radiation with radioactive material, flailing comparisons to past accidents, or hopeless misuse of terminology, reporting on Fukushima has been a mix of hype and speculation entirely devoid of useful information. Let’s set the record straight: the situation is under control, it is unlikely that the nuclear fuel has melted, the risk to the public is effectively zero, and, depending on whether facts on the ground have been reported correctly, it is possible that the reactors will remain capable of producing power in the future.


The Nuclear Basics
A nuclear reactor is effectively a big device for boiling water. Instead of using the combustion of fossil fuel as its heat source, a nuclear power plant uses atomic fission, mostly of uranium. This method presents two major risks. The first — which occurred at Chernobyl but is virtually impossible in a responsible reactor — is a criticality accident, in which the nuclear chain reaction becomes uncontrolled. The second, which we are dealing with today, is an overheating of the reactor core. Unlike coal, which quits generating heat as soon as combustion ceases, nuclear fuel does not stop generating heat when you stop splitting atoms.

There are several layers of protection that keep nuclear fuel contained within a nuclear plant. The first barrier is what is called the cladding — a zirconium alloy sheath that surrounds the fuel, keeps it in a geometry that is conducive to reactor management and cooling, and contains any gaseous fission products.
The second layer of protection is the reactor vessel, a steel container that houses the reactor and its coolant and makes up part of the coolant loop. Damage to the reactor vessel would mean a loss of coolant and make it difficult to keep the nuclear fuel cool.

A third layer of protection is the containment building. This is a thick, steel-reinforced concrete structure built to withstand very high heat and pressure. If the reactor vessel is breached, the job of the containment building is to withstand incredible force and contain the nuclear fuel.

Finally, in the case of Fukushima, there is a fourth layer of protection, which is essentially a dry-wall building surrounding the containment building. This building is not designed to withstand force or heat, and is basically just meant to protect workers from the weather as they work around the containment building.

Unlike most of the world’s nuclear power plants, which are pressurized water reactors (PWRs), Fukushima uses boiling water reactors (BWRs). In a BWR, the game plan is simple: just keep pouring water on, and if pressure gets too high, vent steam into the containment building. Fukushima’s engineers likely had a very clear strategy for accident mitigation in the aftermath of the earthquake. Moreover, BWRs often have large chimneys (empty volume above the fuel rods within the reactor vessel). During regular operation, this chimney would be filled with a liquid/steam bubble mixture from the boiling water — in an emergency, this volume can be packed with surplus coolant, effectively raising the thermal capacitance of the reactor vessel. Given the reactor type and the engineering rigor of the Japanese, I think we have good reason to be optimistic.


What happened?


The earthquake struck at Friday, 14:46 local time, at which point the reactor automatically inserted its control rods (neutron absorbers) into the core and ceased the fission of the nuclear fuel. At this point, reactor power was at 6.5 percent, and full cooling was in effect — a combination that should reduce the temperature of the reactor from its normal operating temperature. At 15:41, the tsunami hit and destroyed the on-site generators that were powering the coolant pumps. Once the generators were destroyed, the pumps switched to battery power. Here, the timeline gets murky — either coolant flow continued until roughly 19:46, at which point a pump failure caused flow to stop or be reduced, or it continued until roughly 23:41, at which point the battery life ran out. In either case, problems with mobile generators that had been brought in to replace the batteries prevented cooling from being immediately re-established. During this time the reactor’s power output continued to fall — at 5–9 hours after shutdown, power should have been 0.8 percent of normal. Coolant flow was re-established on Saturday, around 01:30.

It is also likely that there were small coolant leaks due to the earthquake breaking seals in the coolant system, which might have further reduced water levels in the core, but not by much — I would think only 1 percent of core volume could have been lost through small seal breaks, and no larger leaks were reported.
This is the window of time in which core damage, if it occurred, had to occur. My back-of-the-envelope calculation looks like this: The Fukushima 1 Nuclear Power Plant produces about 1350 MW of thermal power during normal operation and has a core volume of roughly 300 cubic meters, made up of about three-quarters water and one-quarter uranium dioxide. Coolant flow was interrupted for a period of 2–6 hours, during which time the core’s power output was roughly 1 percent of normal, or 13.5 MW. This means that 100–300 GJ were dumped into the core without active cooling to remove the heat.

Assuming the core began at 250°C, two hours without forced cooling would be insufficient to cause any damage, while six hours brushes against an uncertain region in which cladding melt might be possible, depending on the heat distribution within the core and the assumed heat removal rate from the primary loop without forced coolant flow.

Without exact knowledge of how long the core went without pumped coolant flow, it is difficult to determine the degree of damage the reactor might have sustained. We have two pieces of information to use, neither of which is conclusive.

The first piece of information is an explosion on Saturday at 15:30, which destroyed the outer containment building (the drywall “fourth” layer). The explosion itself was not a serious risk — the building was never meant to be a serious form of containment, but it suggests that the vapor vented out of the reactor vessel and inner, “real” containment building included some amount of hydrogen. Hydrogen can be formed from a number of pathways, including the oxidation of Zircaloy, which would suggest that somewhere in the core, the temperature had risen past 2200°C.

The second piece of information is the detection of cesium and iodine in the vented steam. The presence of these isotopes suggest, at minimum, a degradation of the fuel clad. Whether this degradation was merely an existing point defect in the cladding (not an uncommon occurrence during normal operation) or from a melting of the clad is difficult to determine without knowing how much fission product was detected. It also raises the possibility of a partial fuel meltdown.

It has been widely reported that engineers are pumping seawater into the reactor vessel to keep the fuel cool. If this is true, the reactors are effectively scrap — pumping seawater into the core would introduce too many contaminants for the reactor to remain viable. However, I think that the reporters have misunderstood. Yes, Japanese officials say that they are pumping seawater into the reactor containment. But this is likely a confusion of terms: the officials have been referring to the outer building — the one that exploded — as the “containment building” and calling the containment building the “reactor containment.” If they have been consistent in their terms, then actually, the volume between the containment building (the third layer) and the reactor vessel (the second layer) is being filled with seawater to aid in cooling.

What is the take-away?

From the information we have, we can draw a conclusion anywhere between “the reactor is undamaged and being cooled” to “the reactor cladding and/or fuel has been partially damaged, but the damage is contained and the reactor is being cooled.” The question that should be asked now is whether the reactor has any future value as an electricity-producing asset. The widely-hyped possibility of some Chernobyl-like event is inconceivable without a new, catastrophic disaster. Coolant flow has been re-established and the public is in no danger. Given the magnitude of the precipitating event — a 9.0 earthquake — and the vast property damage it caused, the events at Fukushima are not a serious reason to re-evaluate our own nuclear policy in the United States.