A nuclear primer from BarCap
Posted by Tracy Alloway on Mar 15 09:33.
Here’s a nice piece of research from Barclays Capital.
The UK bank has enlisted the help of a former nuclear safety employee to discuss events at Fukushima Daiichi, the Japanese nuclear plant hovering on the edge of meltdown. For what it’s worth, BarCap’s energy team doesn’t think there was an operator error at the plant — the force of the earthquake combined with the effect of the tsunami “simply exceeded what the plant was designed to withstand.”
Here’s their rundown:
Never, never, never allow the water level in a nuclear reactor to fall below the level of the fuel. This is the mantra pounded into the minds of nuclear power plant operators all over the world (one of the authors of this report used to provide such training at a commercial nuclear plant in the US). No doubt, such training was on the minds of the Japanese plant operators as they struggled to respond to the effects of the earthquake and tsunami last Friday.
It is hard to overemphasize the importance of the “keep the fuel covered” training and design of these plants. Under standard operating conditions, the reactor water in a pressurized water reactor (PWR) or boiling water reactor (BWR, the type of all the units affected in Japan by this event) contains only trace amounts of radioactive materials. A spill or venting of it is not a threat to public safety. All the highly radioactive materials remain inside the nuclear fuel, which itself is encased in tubes, around which the reactor cooling water flows. The nuclear reaction is easy and fast to stop. Indeed, all indications are that the Japanese reactors were safely shut down when the earthquake struck.
But the halting of the nuclear reaction is only step one in caring for a plant. A few percent of the thermal power output of the reactor is produced by the radioactive decay of the daughter products of the fission process (each uranium or plutonium atom that undergoes fission produces two new atoms, which are radioactive and therefore give off energy). When the reaction is stopped, this radioactive decay heat remains – leading to an ongoing need to provide cooling water to the reactor (in fact, used fuel must be cooled for a few years before it no longer needs to be surrounded by water). If the cooling water flow in a reactor is not maintained, the water will boil off, the heat will eventually cause the tubes surrounding the fuel (fuel cladding) to fail, and the subsequent over-heating of the fuel itself will cause the radioactive materials in the ceramic fuel to be liberated into surrounding water or steam in the reactor. Once this occurs, all water or steam drawn from the reactor, and water or steam vented inside the containment building (which houses the reactor) will be contaminated. The reactor creates a wide range of radioactive daughter products, and the commonly produced iodine and cesium in the reactor water or in the atmosphere (as has been reported) are indicators that the fuel has over-heated and at least partially melted.
Keep the fuel covered with water, and the fuel will not overheat, keeping the radioactive materials safely encased in the fuel. This concept is so ingrained in nuclear plant designers and the regulators of the industry that there are automated systems to provide this function, and backups to the automated systems, and backups for the backups, and contingencies for when even those are lost. Large emergency diesel generators (think of locomotives without wheels) are installed to provide the electricity to feed electric-driven pumps. Steam from the heat still generated in the reactor is used to pump water. The redundancy of the backup water and power systems provides the assurance that in a worst-case event, even though a plant might have been damaged by some external or internal event, operators will ensure that they keep the reactor fuel covered.
But, as previously noted: The central problem behind almost every hurdle faced by the workers at the Fukushima nuclear power plant has been – and remains – a lack of power supply. Since electricity was knocked out by the Tsunami it has been impossible to run the pumps that cool the reactor cores and circulate water around storage pools used to keep spent fuel rods cooled.
