FAQ about nuclear power in Japan

[Otaku]

http://www.nei.org/filefolder/FAQs_Japanese_Nuclear_Situation_03232011_3.pdf

Frequently Asked Questions:
Japanese Nuclear Energy Situation
Updated 3.23.2011

How serious are the releases of radiation from Fukushima Daiichi? Do they represent a threat to
human health? Will we see an increase in cancer rates in future years?

As a result of fuel damage in at least four of the Fukushima reactors, significant releases of
radioactive materials have been detected at the site. The implications of these releases on the health
and safety of the public are not yet fully understood. The Japanese government implemented
emergency planning procedures and evacuated residents within a 20km (12.5-mile) radius of the plant before
the radiation releases were detected. Authorities are also distributing potassium iodide tablets to
specifically protect against exposure from radioactive iodine that may be present in the releases and
are monitoring the evacuees for potential exposure. Any speculation about possible health effects
would be premature until more accurate and complete data becomes available.


What happens when you have a complete loss of electrical power to operate pumps in a BWR-3 or 4
reactor with Mark I containment like Fukushima Daiichi Units 1-4?

If plant operators cannot move water through the reactor core, the water in the reactor vessel begins to
boil and turn to steam, increasing pressure inside the reactor vessel. In order to keep the reactor
vessel pressure below design limits, this steam is then piped into what is called a “suppression pool”
of water or “torus” – a large doughnut-shaped tank that sits beneath the reactor vessel.
Eventually, the water in the suppression pool reaches “saturation” – i.e., it cannot absorb any
additional heat and it, too begins to boil, increasing pressure in containment. In order to stay within
design limits for the primary containment, operators reduce pressure by venting steam through filters
(to scrub out any radioactive particles) to the atmosphere through the vent stack.

If operators cannot pump additional water into the reactor vessel, the water level will begin to drop,
uncovering the fuel rods. If the fuel remains uncovered for an extended period of time, fuel damage,
possibly including melting of fuel, may occur. If there is fuel damage, and steam is being vented to
the suppression pool, then to primary containment, then to secondary containment (in order to relieve
pressure build-up on plant systems), small quantities of radioactive materials will escape to the
environment.


What would happen to the used fuel in the storage pools if cooling was lost?

We do not know the precise condition of the used fuel storage pools at Fukushima Units 1, 2, 3 and 4.
Used nuclear fuel at the Fukushima Daiichi plant is stored in seven pools (one at each of the six
reactors, plus a shared pool) and in a dry container storage facility (containing nine casks). Sixty
percent of the used fuel on site is stored in the shared pool, in a building separated from the reactor
buildings; 34 percent of the used fuel is distributed between the six reactor fuel storage pools, and the
remaining six percent is stored in the nine dry storage containers. The used fuel pools at the
Fukushima Daiichi reactors are located at the top of the reactor buildings for ease of handling during
refueling operations. There are no safety concerns regarding the used fuel in dry storage at
Fukushima Daiichi.

Used fuel pools are robust concrete and steel structures. Pools are designed with systems to maintain
the temperature and water levels sufficient to provide cooling and radiation shielding. The water
level in a used fuel pool typically is 16 feet or more above the top of the fuel assemblies. The used
fuel pools are designed so that the water in the pool cannot drain down as a result of damage to the
piping or cooling systems. The only way to rapidly drain down the pool is if there is structural
damage to the walls or the floor.

If the cooling systems are unable to function, the heat generated by the used fuel would result in a
slow increase in the temperature of the spent fuel pool water. The operating temperature of the pools
is typically around 40 degrees C or 100 degrees F (the boiling point for water is 100 C or 212 F).
This slow increase in temperature would result in an increased evaporation rate. Rapid evaporation of
the water will not occur.

Exact evaporation rates would depend on the amount of used fuel in the pool and how long it has
cooled. The rate at which the pool water level would decrease (due to evaporation or mild boiling) in
the absence of cooling system function would not be expected to lower water levels by more than a
few percent per day. Given that there is approximately 16 feet or more of water above the used fuel
assemblies, operators would have time to find another way to add water to the pools before the fuel
would become exposed.

At the surface of the used fuel pool, the dose rate from gamma radiation emanating off the used fuel
assemblies is typically less than 2 millirem per hour. If the water level decreases, gamma radiation
levels would increase substantially. This increase would be noticed at the radiation monitors near the
reactor buildings.


Do the events indicate that iodine tablets should be made widely available during an emergency?

The thyroid gland preferentially absorbs iodine. In doing so it does not differentiate between
radioactive and nonradioactive forms of iodine. The ingestion of nonradioactive potassium iodide
(KI), if taken within several hours of likely exposure to radioactive iodine, can protect the thyroid
gland by blocking further uptake of radioactive forms of iodine. KI does not protect any other part of
the body, nor does it protect against any other radioactive element.

Populations within the 16km (10-mile) emergency planning zone of a nuclear plant are at greatest risk of
exposure to radiation and radioactive materials including radioactive iodine. Beyond 10 miles, the
major risk of radioiodine exposure is from ingestion of contaminated foodstuffs, particularly milk
products. Both the EPA and the FDA have published guidance to protect consumers from
contaminated foods within a 50 mile radius.

What caused the explosions at Fukushima Daiichi Units 1-3?

The explosions at Units 1, 2 and 3 appear to have been caused by a build-up of hydrogen.
The uranium fuel pellets are enclosed in metal tubes made of a zirconium alloy. When exposed to
very high temperatures, the zirconium reacts with water to form zirconium oxide and hydrogen.
This appears to have happened at Fukushima Daiichi Units 1 and 3, when a portion of the uranium
fuel was uncovered. It is assumed that the hydrogen found its way into the reactor building,
accumulated there, and ignited. Although significant events, the explosions did not appear to
compromise the integrity of the primary containments or the reactor vessels at these units.

The explosion in Unit 2 appears to have happened as a result of a similar phenomenon. The hydrogen
appears to have ignited inside the reactor. Although a breach in the Unit 2 containment was
suspected shortly following the explosion, more recent reports indicate primary containment is
maintaining pressure which would indicate integrity has not been compromised.


Did the reactor cores melt at any of the Fukushima Daiichi reactors? Was there any fuel damage?

Fukushima Daiichi Units 1, 2, and 3 have experienced some fuel damage, since the fuel rods or
portions of the fuel rods were uncovered (not covered with water) for some period of time. There is
no evidence of a complete core meltdown at any unit, however. The information we have suggests
that the basic core configuration so far remains intact, so some water or steam cooling through the
core is occurring.


Are there any additional concerns associated with the mixed oxide fuel in Unit 3?

Unit 3 installed some mixed oxide (MOx) fuel assemblies during its last refueling outage in
September, 2010. Mixed oxide fuel is a combination of uranium oxide and plutonium oxide, and is
not used in the U.S. reactors, except for limited experimental testing. Failure to keep MOx fuel
assemblies covered with water – and the resulting overheating and damage to the MOx fuel
assemblies, and release of fission products – does not pose an additional threat when compared with
the traditional uranium oxide fuel assemblies. The melting point of the MOx fuel assemblies is also
similar to uranium oxide fuel assemblies, so the risk of damage due to overheating does not increase
with the use of MOX fuel.


Do the events indicate that evacuation zones around plants should be extended?

The 16km (10-mile) emergency planning zone around nuclear power plants as determined in 1978 by a multiagency
federal task force is appropriate and should not change due to the accident at Fukushima
Daiichi. In the United States, a nuclear plant’s emergency response plan must provide protective
measures, such as sheltering and evacuation of communities within a 10-mile radius of the facility.
Japan used a similar plan. During the accident there, the Japanese government has issued evacuation
orders for a 20-kilometre (12.5-mile) radius around Fukushima Daiichi, and a 3-kilometre radius
around Fukushima Daini.