We will be posting responses to frequently asked questions on this page.
**Updated April 7, 5:20 P.M. EDT**
Q: Why are some of the radiation readings that are reported below the values that are reported for background?
A: There are a few reasons that this can occur. The first is that the readings may be being reported in terms of level above background in which case background dose rate for the area has already been subtracted from the values. Another reason is that the background rates in a specific area may be below the global average of background radiation. Background levels can easily vary by over 10x across the globe.
Q: There is a lot of discussion about radiation levels, but what about contamination, isn’t that a bigger issue?
A: Contamination refers to having radioactive nuclei on a surface or absorbed into some kind of material (such as into the food chain). In an event with large radioactive material release, the primary concern is with high doses (especially near the source of release) that can cause prompt heath effects. Contamination is associated with long-term health concerns. For example, I-131 fallout can contaminate food and water supplies. If people consume a large quantity of these contaminated foods over their lifetime, they can accumulate I-131 in their bodies. The accumulation of eating contaminated foods over a period of weeks to years can increase the chance of getting cancer. The exact time required to accumulate enough radionuclides to significantly affect human health depends on the concentration of radionuclides in the food supply. The land and food sources surrounding the Fukushima reactors will be monitored for long periods of time to determine contamination levels, and to prevent significant amounts from entering the food chain.
Q: Why is/was it so challenging to get water into the spent fuel pools?
A: Spent fuel pools, in the Fukushima reactors, are elevated (at the level of the top of the reactor vessel) in the reactor building. In this design, a piping system and a large pump are needed to overcome gravity to pump water into the spent fuel pool. Furthermore, high radiation levels from the venting of the containment makes it difficult to access the spent fuel pools through the building. Also, the lack of electricity made it not possible to use the regular pumping systems to fill the pools and remove the heat.
Q: What are the differences in repercussions of old fuel (many months old) melting in the spent fuel pools compared to that that is freshly removed from the reactor?
A: Older fuel in spent fuel pools has less decay heat (per unit mass of fuel) compared to fuel that is freshly removed from the reactor. This means less cooling is needed for the old spent fuel. If fuel has been in the spent fuel pool for a long time, all the short-live radioactive nuclei will have decayed away. This includes isotopes such as I-131 (half-life of ~8days), which is a primary health concern since it is volatile and can be transported to the environment easily. Thus, fuel that is several months old will essentially have no I-131 remaining. The total amount of fuel in the pool, and its age, determines how many radioactive nuclei are present and could possibly be released.
Q: Why are reactors built in earthquake prone areas such as Japan? What type of risk assessment allows this to occur?
A: The Design Basis Earthquakes (DBE) is the maximum earthquake intensity that a reactor is expected to experience in its lifetime. Reactors in areas of higher seismic activity and severity must design for a stronger DBE than reactors in less seismically intense regions. The same basic strategy is employed for general building construction. Skyscrapers are built in both Kansas City and Tokyo, but a skyscraper in Kansas City is not designed to withstand the same earthquakes as one in Tokyo.
Three key points should be noted:
- The DBE is determined through a historical database of earthquakes in the region and expert judgment. There is no way to determine if more severe earthquakes can occur than the historical database indicates. Reliable empirical data for earthquake intensity, location, and frequency has only been reliably collected for approximately the last 100 years, and prior to that the only source is historical accounts. There is no way to be 100% certain that a larger earthquake than designed for will hit the region in the lifetime of the plant
- Rather than licensing using a seismic risk assessment, risk-information is used to define the DBE that the reactor is designed to survive.
- When designing for the DBE, the failure criterion, which the reactor is designed to avoid during the accident are conservative. Failure in seismic engineering analysis is typically defined as the structural supports reaching their point of plastic deformation. Typically, metals can remain intact after this point. This conservatism can help explain why the reactors survived an earthquake larger than the DBE.
Q: Why do reactors only shut down to 7% power and not 0%?
A: Even after all of the control rods are inserted and fission power production is halted, there are still radioactive decay products in the fuel that continue to produce heat. This decay heat amounts to about 7% power 1 second after shutdown, but quickly drops off to less than 2% in 15 minutes. However, this decay heat still must be removed to prevent the fuel from heating up and being damaged. There is a more detailed discussion of decay heat at this post.
Q: If the reactor can only be shut down to 7% power is it really shutdown?
A: A reactor is considered shutdown when the core is subcritical by some specified minimum value of reactivity. This is a technical phrase that means that the neutron population is small and decreasing rapidly, all significant heat production from fission has stopped, and the reactor will not regain criticality (start back up) due to minor changes in plant conditions (temperature, pressure, coolant chemistry, etc.). A full insertion of the control rods is more than sufficient to achieve this condition; even with one rod fully withdrawn.
Q: How does one assemble the fuel rods in the first place if they are at 7% power and presumably “too hot to handle?”
A: When fresh uranium fuel is manufactured, it is composed of enriched uranium in its oxide form. Uranium is naturally radioactive, but only very slightly, and does not produce significant heat (it doesn’t require active cooling) or enough radiation to be dangerous when handled correctly. The 7% is referring to is the decay heat of a reactor about 1 second after a reactor shutdown. This heat comes from the radioactive products that are produced from fission called decay heat and is discussed in detail in another post.
Q: Can decay heat alone, if not removed by cooling water, build to the point that the core completely melts?
A: Although a complete core melt has never happened in commercial reactor, and extremely unlikely, there is sufficient decay heat such that if it was not removed from the core over an extended time, and the correct conditions existed in the pressure vessel, the core could completely melt.
Q: I see the decay heat plot and that that Fukushima units 2 and 3 are producing about 11.5MW of thermal power as of 5/17/2011, what does it take to remove that much heat production?
A: The plant conditions are constantly changing, but if we assume that the plant has been depressurized to atmospheric pressure, and water is being used to cool the reactor by boiling away, it would require about 80-100gpm of water to replenish what is being vaporized. Normally, this minimum flow would be provided by one of the various coolant systems and then subsequently recondensed and pumped back through the reactor. However, in emergencies such as at Fukushima, this flow rate is well within the flow rates available from standard firefighting systems and the water can simply be allowed to boil away.
Q: What is MOX Fuel?
A: Uranium occurs naturally on earth. We mine it, refine it, and use it as fuel in nuclear reactors. All other elements that can sustain the nuclear fission chain reaction, including plutonium, do not occur naturally on earth*. The only way to obtain plutonium is to produce it from other elements through neutron radiation. Neutrons in a nuclear reactor produce power through the fission of uranium, but they also convert some of the uranium into plutonium. Thus, all uranium-fueled reactors contain at least some plutonium.
After spent fuel is removed from a reactor, the Japanese nuclear industry sometimes reprocesses it by separating the plutonium and uranium from the fission products, which are (useless) light elements that arise when uranium splits in the fission reaction. The separated plutonium and uranium are often mixed to make new “fresh” fuel that has a higher relative plutonium concentration than the original spent fuel. This is called mixed oxide (MOX) fuel, as the uranium and plutonium are ceramics in oxide chemical form (UO2 and PuO2). Uranium-fueled reactors also typically use the oxide form of uranium (UO2). It is crucial to understand that uranium oxide fuel contains plutonium just as MOX fuel does. The only difference is the relative concentrations of the two elements. MOX fuel is one way in which the nuclear industry can “recycle” nuclear fuel in order to use the earth’s uranium resources more efficiently and responsibly.
*As a side note, the element thorium occurs naturally on earth and can be used to produce uranium, although it cannot sustain the fission chain reaction itself.