How Safe Are Nuclear Power Plants?

Editor’s note: Josep is a 17-year-old high school student in Spain. He is a nuclear energy enthusiast keenly interested in promoting both nuclear energy and renewables in his home country and abroad, for the betterment of man’s energy future.

By Josep Rey Cases

Generating energy by nuclear fission has proven to be one of the safest industrial pursuits in the world. Nuclear accidents such as Chernobyl or Fukushima happened in large part because of failings in systems—systems of people, procedure, and political interference in day-to-day and moment-to-moment actions at nuclear plants—and so I won’t have them in mind when writing the next lines.

When we talk about nuclear safety, or the construction of a nuclear power plant, and the company tells us, “This reactor has lots of safety systems and so you will never have to worry about an accident here,” our first question would be: “If nuclear plants are so safe, why do they need so many safety systems?” The answer is much simpler than a nuclear reactor: Decay heat.

chart

Table representing the decay heat of a 410 MWe (1350 MWth) reactor (approximately)

After an operating reactor has been shut down, the decay of radionuclides in the fuel continues to generate heat (this is a main disadvantage of nuclear power, and sometimes the cause of nuclear accidents). This heat represents seven percent of the maximum thermal power output one second after the reactor is shut down. After 24 hours, it represents 0.5 percent of the nominal thermal power. This may not seem like of heat, but it is. The average reactor of a 1000MW power electricity output has about 3000MW of thermal power. One day after shutdown it will still produce 15 MW of heat inside the reactor pressure vessel—enough to boil a thousand liters of water from 20 ºC to boiling point in slightly less than 24 hours.

As a result, if cooling is stopped by 1 to 2 years from shutdown, soon or later the fuel could become uncovered by water, assuming no one does anything to monitor and correct water level in the reactor pressure vessel. The same happens with the spent fuel pool at nuclear power stations.

Decay heat is the principal reason of safety concern in light water reactors, and by this reason nuclear power stations must have several safety and safeguard systems to make sure that even in the worst case scenario the fuel does not become uncovered ( or uncooled by water).

Nuclear reactors always have safety systems subdivided into other safety systems. There are also systems that can work off of steam pressure in the reactor/steam generators, even if the most powerful power sources are lost. Of course, those systems are operated through various sources of power such as batteries, diesel generators, backup generators, and the external power line.

If one system or component fails, there are additional systems (some of them are passive systems) that prevent a nuclear reactor from overheating. That is how safe a nuclear reactor actually is. In addition, new reactors such as the VVER-1200 and the AP-1000 can exist, some days, without any kind of external power (neither AC nor DC).

Another thing that may concern people about a nuclear reactor is the massive amount of power that it can produce. It is obvious that an uncontrolled chain reaction will increase the reactor’s power production by a factor of hundreds, melting it within seconds. To prevent this from happening, there are two things methods of control: control rods and nuclear poison.

Control rods are present in all kind of reactors and are designed to shut down the reactor within two seconds. (The RBMK design, however, was decreased from 18 to 12 seconds after the RBMK–design Chernobyl accident).

The second method of control is nuclear poison (boron), which is only present in pressurized water reactors. (This system and the system used to control the power of the reactor aren’t the same thing).

Both of them are made of elements (or an element)  able to absorb neutrons: Boron and cadmium in the first system and solely boron in the second one. Both of them are designed to terminate the reaction within a couple of seconds.

To sum up, we should rethink and educate the public on exactly how dangerous nuclear power plants are, because these plants are highly prepared even for the most extreme crisis.


https://twitter.com/peptry

Josep Rey Cases of Spain

Josep Rey Cases is 17 years old and is living in Catalonia, Spain. He has been interested in energy sources and nuclear power since mid-2013. He is preparing to start his undergraduate degree studying electrical engineering. He is a native Spanish-speaker learning English as a second language. Josep can be found on Twitter @peptry.

6 thoughts on “How Safe Are Nuclear Power Plants?

  1. Josep Rey

    Answering to Mitch:

    My classmates just don’t care even knowing that there are 2 nuclear reactors nearby home.

    When I published my first article I became a bit “popular” around the local media, people and teachers due to my age when writing an article.

    After all, I try to give the best of me to defend nuclear power (of course considering that there are disadvantages such as the spent fuel).

  2. Dr K S Parthasarathy

    My hearty congratulations to Josep for writing this brief essay on the safety of nuclear power plants. Credit should also go to ANSNUCLEARCAFE.ORG for publishing the essay.

    Nuclear community should reach young people and make them aware of the necessity for nuclear power. Many years ago, the British Nuclear Fuel Complex (BNFL) used to publish a very informative monthly called “ATOM”, covering various aspects of nuclear power.
    ATOM used to select, if I recall correctly, one from among the many essays written by high school students every year and give a cash award of £ 100/- to the winning essayist. The magazine also used to publish the essay unedited in one of the issues of the magazine.

    Similarly, the Department of Atomic Energy in India organizes an annual essay competition on an all India basis. Students may write essays on any one of three topics in English or in any one of the 22 recognized Indian official languages. This years will be the 27th. Over 30 short listed essayists from each topic will visit Mumbai to make oral presentations. First, second and third prizes carry Rs 21,000,Rs 14,000 and Rs 7000 respectively.Those who did not get any prize will receive consolation prize of Rs 3000/-.

    All participants will visit Bhabha Atomic Research Centre, the leading research centre for nuclear science and technology and Tarapur Atomic Power Station.I described the programme in detail as I felt that all nations currently operating nuclear power plants and those who plan to construct them may start similar programmes.

    In India hundreds of participants came into contact with nuclear scientists and engineers.. There are unique instances in which these young ambassadors challenged antinuclear scare mongers when the latter start disseminating disinformation on nuclear technology

  3. Brian Mays

    I consider there to be five barriers in a typical nuclear plant that stand in the way of any kind of radioactive release.

    For a radionuclide (such as a fission product) to be released into the environment it must do the following:

    (1) Escape the fuel pellet. The hard, uranium-oxide ceramic pellet that comprises the fuel is actually pretty good at containing many of the radionuclides. The radionuclides that are gaseous (e.g., Xenon) have the easiest time here.

    (2) Pass through the cladding. This is the primary engineered barrier against release. Nevertheless, holes in the cladding due to wear, or (in the extreme case) damage to the fuel due to high temperatures can allow radionuclides to pass into the coolant.

    (3) Exit the Reactor Pressure Vessel. Once in the coolant, the material must escape from a steel pressure vessel with walls that that are usually over half a foot thick. But even if that barrier is breached, the material must …

    (4) Escape containment. This is the barrier that most people think about when they think about nuclear plants (probably because it is the most prominent). It is a reinforced concrete structure that is designed both to protect the pressure vessel from outside events (storms, tornadoes, bombs, etc.) and to prevent the release of material from inside. But even that is not the last barrier. Outside of containment the material must …

    (5) Leave the reactor building. Even outside of containment, much of the radioactive material is still held within the walls of the buildings that make up the nuclear plant. For example, during the Three Mile Island accident some of the coolant containing radioactive material was pumped from the containment building to the auxiliary building. Although this material was now outside of the reactor containment, the vast majority of this material remained there and never entered the environment.

  4. Josep

    Yes, that’s right. I know how the reactive feedback works in a nuclear reactor and how does that changes reactor power production (thermal power, ofcourse). BWR’s and PWR’s work in reverse, for instance.
    If you slightly increase power in a PWR by the same way you do it in a BWR, you end decresing it.

  5. Dennis Mosebey

    Excellent overall summary especially given the age of the writer. The only thing I would have added is a brief statement about the 3 barriers to release to the public which are the fuel cladding itself, the Reactor Coolant System and the Containment Structure. These in conjunction with the inherent negative reactivity feedback of Doppler and moderator temperature coefficient and the protection systems and redundancy ensure the public safety. Overall though for one so young a most excellent article.

  6. Mitch

    Josep, tell us how your classmates and community take to posts of this article. Articles like this here is just preaching to the choir where it doesn’t count.

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