Nuclear power stations: are they designed to shut down automatically in the event of a disaster....?

Question

like a pandemic that wipes everyone out.

I'm doing some research for something I'm writing at the moment - a bit of post apocalyptic fiction and was just wondering if my survivors would have to worry about melt downs.

Any nuclear physicists out there?

Answer 1

Quite simply, yes. All British built Atomic power stations are fitted with a 13 amp fuse.

As to the second part of your question, roughly 2 out of 4 participants on Yahoo are nuclear physicists.

Answer 2

yes, unlike older models, the modern reactors require constant intervention to keep running.

good luck with the book :P

Answer 3

yes

nuclear power stations are the safest and cleanest type of electrical power generators

Answer 4

Yeh I'm pretty sure they do!

Answer 5

Err......if a pandemic wipes us all out who is going to know.Anyhoo I think Homer Simpson has it covered.

Answer 6

Yes, the nuclear reactors have safety systems that shut down.  However spent fuel pools located at reactor sites rely on systems to circulate cold water to keep the fresh used fuel from overheating.  These cold water circulation systems are driven by electrical pumps.  In a disaster scenario where no people are left to keep things running, the electricity will eventually fail and the pumps will stop.  The fuel will eventually boil off the water from the spent fuel pool and be exposed to air.  Once the fuel is exposed to air, the temperature of the fuel will reach thousands of degrees and ignite the zirconium fuel cladding.  The fire will destroy the surrounding building and spread radioactive contamination (iodine, xenon, strontium, cesium, etc.) with the smoke.  In the US there are about 70 locations where this will happen.

So, survivors of a disaster won't have to worry about the reactors, instead, the spent fuel is the problem.  However, the really dangerous contamination will be near the plant and will wash away and be diluted within a few months or years.  The area in the Ukraine that is contaminated from the Chernobyl accident is booming with natural wildlife.  The plants and animals there don't seem to mind a little radiation in their food.

The survivors will have to worry about poor health care, lack of clean water, and poor nutrition before they need to worry about radiation from nuclear power plants.

Answer 7

Sequence of events

What happens when reactor fuel melts depends upon reactor design, and is the subject of conjecture and some actual experience (see below).

Before the core of a nuclear reactor can melt, a number of events/failures must already have happened. Once the core melts, it will almost certainly destroy the fuel bundles and internal structures of the reactor vessel (although it may not penetrate the reactor vessel). (Note that the core at Three Mile Island did melt nearly completely but stayed within the reactor vessel.) If the melt drops into a pool of water (for example, coolant or moderator), a steam explosion called a Fuel-Coolant Interaction (FCI) is likely. If air is available, any exposed flammable substances will probably burn fiercely, but the liquid nature of the molten core poses special problems.

In the worst case scenario, the above-ground containment would fail at an early stage, (due to say an FCI within the reactor vessel, ejecting part of the vessel as a missile - this is the 'alpha-mode' failure of the 1975 Rasmussen (WASH-1400) study), or there could be a large hydrogen explosion or other over-pressure event. Such an event could scatter urania-aerosol and volatile fission-products directly into the atmosphere. However, these events are considered essentially incredible in modern 'large-dry' containments. (The WASH-1400 report has been supplanted by the 1991 NUREG-1150 study.)

It seems to be an open question to what extent a molten mass can melt through a structure. The molten reactor core could penetrate the reactor vessel and the containment structure and burn down (core-concrete interaction) to groundwater (this has not happened at any meltdown to date: see China Syndrome). It is possible that, as in the Chernobyl accident, the molten mass might mix with any material it melts, diluting itself down to a non-critical state. If hot uranium dioxide is combined with iron(II) oxide a eutectic is formed which may cause the fuel to become more mobile than it would otherwise be.[1]

Note that the molten core of Chernobyl flowed in a channel created by the structure of its reactor building, e.g., stairways and froze in place before core-concrete interaction. In the basement of the reactor at Chernobyl, a large "elephant's foot" of congealed core material was found. Furthermore, the time delay and the lack of a direct path to the atmosphere would work to significantly ameliorate the radiological release. Any steam-explosions/FCI which occurred would probably work mainly to increase cooling of the core-debris. However, the groundwater itself would likely be severely contaminated, and its flow could carry the contamination far afield.

In the best case scenario, the reactor vessel would hold the molten material (as at Three Mile Island), limiting most of the damage to the reactor itself. The American Nuclear Society has said "despite melting of about one-third of the fuel, the reactor vessel itself maintained its integrity and contained the damaged fuel".[2] However the Three Mile Island example also illustrates the difficulty in predicting such behavior: the reactor vessel was not built, and not expected, to sustain the temperatures it experienced when it underwent its meltdown, but because some of the melted material collected at the bottom of the vessel and cooled early on in the accident, it created a resistant shell against further pressure and heat. Such a possibility was not predicted by the engineers who designed the reactor and would not necessarily occur under duplicate conditions, but was largely seen as instrumental in the preservation of the vessel integrity. (However, it should be noted that the reactor vessel was inside a containment building, as in all American nuclear plants, so a failure of the reactor vessel would not mean that radioactive material is released into the environment.)

The CANDU reactor is designed with at least one, and generally two, large low-temperature and low-pressure water reservoirs around its fuel/coolant channels. The first is the bulk heavy-water moderator (a separate system from the coolant), and the second is the light-water-filled shield tank. It has been shown that even under severe loss-of-coolant conditions these backup heat sinks are sufficient to prevent either the fuel meltdown in the first place (using the moderator heat sink), or the breaching of the core vessel should the moderator eventually boil off (using the shield tank heat sink). [Allen et al.]

[edit] The three final defenses against a loss of cooling

A great deal of work goes into the prevention of a serious core damage event. If such an event were to occur, three different physical processes are expected to increase the time between the start of the accident and the time when a large release of radioactivity could occur. It is also important to understand that retaining the fission products within the core for some time will reduce the size of the radioactive release. This is because the worst isotopes in a fission product mixture are short lived. For example if all the iodine in a core was released one week after criticality was terminated by a SCRAM then the thyroid dose suffered by the population would be lower than if the iodine had escaped the plant one hour after the reactor was scrammed. Even while the Chernobyl accident had dire off-site effects much of the radioactivity remained within the building, if the building was to fail and dust was to be released into the environment then the release of a given mass of fission products which have aged for twenty years would have a smaller effect than the release of the same mass of fission products (in the same chemical and physical form) which had only undergone a short cooling time (such as one hour) after the nuclear reaction has been terminated. However if a nuclear reaction was to occur again within the Chernobyl plant (for instance if rainwater was to collect and act as a moderator) then the new fission products would have a higher specific activity and thus pose a greater threat if they were released. N.B. to prevent a post accident nuclear reaction steps have been taken (such as adding neutron poisons to key parts of the basement).

These three factors would provide additional time to the plant operators in order to mitigate the result of the event:

The time required for the water to boil away (coolant, moderator). Assuming that at the moment that the accident occurs the reactor will be scrammed (immediate and full insertion of all control rods), so reducing the thermal power input and further delaying the boiling.

The time required for the fuel to melt. After the water has boiled, then the time required for the fuel to reach its melting point will be dictated by the heat input due to decay of fission products, the heat capacity of the fuel and the melting point of the fuel.

The time required for the molten fuel to breach the primary pressure boundary. The time required for the molten metal of the core to breach the primary pressure boundary (in light water reactors this is the pressure vessel; in CANDU and RBMK reactors this is the array of pressurized fuel channels) will depend on temperatures and boundary materials. Whether or not the fuel remains critical in the conditions inside the damaged core or beyond will play a significant role.

[edit] Effects

The effects of a nuclear meltdown depend on the safety features designed into a reactor. A modern reactor is designed both to make a meltdown highly unlikely, and to contain one should it occur. In the future passively safe or inherently safe designs will make the possibility exceedingly unlikely.

In a modern reactor, a nuclear meltdown, whether partial or total, should be contained inside the reactor's containment structure. Thus (assuming that no other major disasters occur) while the meltdown will severely damage the reactor itself, possibly contaminating the whole structure with highly radioactive material, a meltdown alone will generally not lead to significant radiation release or danger to the public. The effects are therefore primarily economic[3].

In practice, however, a nuclear meltdown is often part of a larger chain of disasters (although there have been so few meltdowns in the history of nuclear power that there is not a large pool of statistical information from which to draw a credible conclusion as to what "often" happens in such circumstances). For example, in the Chernobyl accident, by the time the core melted, there had already been a large steam explosion and graphite fire and major release of radioactive contamination (as with almost all Soviet reactors, there was no containment structure at Chernobyl).

Answer 8

If everyone is wiped out there will be no survivors.