Tuesday, 19 January 2016

Radiation and Nuclear Safety

by Samuel Ha

I’ve recently noticed a lot of negative news articles about the dangers of nuclear power and I have some points I’d like to raise to counter these arguments. The advances in nuclear power plants over the decades is something that is rarely mentioned and is very relevant to us here in the UK, as it looks ever more likely that we will have newly built nuclear power plants at sites like Hinkley Point and Sizewell.

Over the years, rigorous safety systems have been added to old reactors and the designs of new reactors have included inherent safety features, but public perception of nuclear power still harks to the disasters of Fukushima and Chernobyl (plants designed in the 1960s). If new designs have inherent safety features and are still received as though they didn’t have them, then we need to address the fear of a nuclear accident.

The points I’m going to make in this post are about how the dangers of nuclear power are perceived.



Fukushima and the Consequences

On the off chance that you didn't know, Japan was struck by an earthquake in March 2011 that measured 9.0 on the Richter scale. This led to a tsunami that left only devastation in its wake.

Nearby were the Fukushima Daiichi reactors (Boiling water reactors (BWR) operated by Tokyo Electric Power Company, or Tepco, and supplied by Toshiba and Hitachi) [1]. Upon notification of the nearby earthquake, the reactors had control rods inserted into the core (they were turned off, in other words) and emergency cooling systems were initiated. These comprise four diesel generator driven pumps. Four, to make sure that the failure of an individual pump doesn’t present a danger.

When the tsunami arrived at the reactor sites, the waves breached the 12m sea walls and flooded the basement.
It is necessary, at this point, to say that the diesel generators were stored in the basement. All four.

After the basement was flooded, emergency cooling to the reactor stopped. But what's the issue? The nuclear reactions have stopped, thanks to the control rods! However, fuel pins contain a substantial amount of fission products after a sufficient amount of time in operation. The power given off in the decay of fission products can be as much as 7% of the total thermal power, and without active cooling, the reactor cores began to heat.

Once the core of the reactor heats up enough without active cooling, interesting things can happen. The ceramic fuel pins can melt and release fission products at an elevated rate, but more importantly, zirconium cladding (the kind used in many Light Water Reactors) can react with water. This reaction has two important outcomes: production of hydrogen and increased pressure in the pressure vessel.

In the case of multiple Fukushima Daiichi reactors, it was assumed that the pressure build up that was observed was from steam produced from the heated water, so they vented the gas into the containment buildings, not realising the gas was actually hydrogen. This was a catastrophic mistake that led to the damage of containment buildings, as the vented hydrogen gas exploded.

It was this breach of the confinement building that led to the spread of radioactive matter into the surrounding areas.
Caesium-137 and Iodine-131 are two fission products that pose some of the largest risks for humans. Iodine-131 has a half-life of approximately 8 days, while Caesium-137 has a half-life of about 30 years. Caesium, a toxic substance can be absorbed and can replace potassium in the body. Iodine is an element that is readily absorbed into the thyroid.
For anyone that’s interested, a very useful map has been created that shows radiation measurements from across Japan. It shows how much damaging radiation you would absorb if you were in different locations across Japan.

Another issue is the release of contaminated water into the Pacific Ocean from the reactor sites. There are reports of fish being caught in the Pacific testing positive for Caesium-137, above the allowable limit of 100Bq/kg.

You may be thinking “How dangerous is it that these fish have such high levels of radiation?” I’ll cover that in a few paragraphs.


Tritium

So what has this talk about Fukushima got to do with nuclear fusion? Well, fusion energy uses two isotopes of hydrogen: deuterium and tritium. Deuterium has one proton and one neutron, and is stable, whereas tritium has one proton and two neutrons, and is unstable.

Tritium has a half-life of 12.3 years and undergoes beta decay (the emission of an electron) with an average energy of 5.7keV. Since tritium is an isotope of hydrogen, it behaves almost identically to hydrogen and is readily absorbed into water and into organic tissue (e.g. humans). However, it is also very easily removed from the body.

In June 2007, Greenpeace released a report about the hazards of tritium, highlighting the seemingly alarming levels of tritium released into the Canadian environment.
And the numbers do not seem to bode well for the Canadians:

[4]

Just look at all of those enormous numbers! MILLIONS OF TERABECQUERELS!
But what’s a terabecquerel? Or even a Becquerel?
Tera is the prefix for 1012. So in Canada, over 20,000,000,000,000,000,000 Becquerels of tritium were released in one year over 6 sites. That does sound a lot, right…? The reader is definitely inclined to believe that this much radiation being released is unacceptable, and quotes a rule of thumb that anything ‘greater than 100,000 Bqs would trigger the need for some kind of action or investigation’
Well here is where it’s important to know what a Becquerel, as a unit of measurement, is. One Becquerel is equal to one radioactive decay process in one second, and is commonly referred to as a unit of radioactivity or ‘activity’ and attached to some quantity (such as 100Bq/kg, the activity level for Caesium-137 in fish).

However, before we can know what this means, and  how much danger our beloved Canucks are in, we need some more information. We know how much tritium was released, but we don’t know how much has reached any people. Luckily these numbers have been evaluated for anybody waiting with their trusty calculator, and are shown below. The following table shows the content of radioactive tritium in water and organic molecules, at different distances from a Canadian CANDU  nuclear power plant. The numbers are taken from a Greenpeace report [4] and reproduced here. For the keen beans out there, HTO is water in tritium (Hydrogen-Tritium-Oxygen) and OBT is tritium in organic matter (Organically Bound Tritium)


[4]
So now we have all the information that is needed to see what the resulting risk is of tritium actually is, which is also provided in the Greenpeace report: someone who lives within 2km of a nuclear power plant and only eats food grown in their garden will receive 20 micro Sieverts (╬╝Sv) per year.
Wait (I hear you say), now we’re using Sieverts? I thought that Becquerels were how we were doing this? What is this new unit that has been thrown at me? What does THIS MEAN?!

The Sievert is a unit used to describe radiation exposure to people and is the real decider for how dangerous exposure to radioactivity is. Not Becquerels, not Grays, not Curies and not Roentgen.

The international rules on radiation exposure for radiation workers (IRR99) states that no qualified radiation worker shall receive more than 20 milli Sieverts (mSv) in one year (that is equal to 20,000 microSieverts – 1,000 times more than is quoted above for Canada). Members of the public should receive no more than 1mSv (1000 microSieverts) from any nuclear activities or events in the UK (if a nuclear power plant were to release something radioactive to the environment, for instance).

To put this into perspective, a resident of the UK will receive an average of 2.2mSv per year, almost all coming from from natural sources, although this varies based on where you live.
Here’s a table of doses I’ve compiled for your reading pleasure!

Notably, the World Health Organisation announced that they that no deaths or cancer cases were directly attributable to Fukushima [6], including the member of staff who received an enormous dose of 678mSv. Since that report was published, the Japanese government has said that one case of cancer may be attributed to the disaster, and has received compensation. This worker received a dose of 19.8 milliSieverts over a year – which is still below the international limit. To put this into perspective, Greenpeace are concerned that a member of the public living next to a nuclear plant may receive 0.02mSv.
Notice the entry about Pacific Tuna? A radioactivity of 200Bq/kg puts these fish at twice the legal limit for radioactive caesium content. But how dangerous is it to eat this fish?


If you were to take a flight from London to Tokyo and eat five 120g tins of this tuna every day for a week and fly home, your dose from flying would be at least 300% higher than what you’d get from eating the highly radioactive tuna. On top of that, the dose you absorb from the tuna would be spread out over years. [10]

Ask yourselves: Would you be scared of eating radioactive fish from the coast of Japan?
How about the radiation dose you receive from flying? You shouldn’t be scared of either.


Better Safe than Sorry?

That’s been the gist of radiation protection since the risks were understood. Be cautious, not optimistic. Be careful and you’ll save more lives.

A piece in World Nuclear News evaluated the evacuation of Fukushima (standard procedure after any nuclear incident) and argued that the obsession with reducing the risk of radiation exposure has missed the forest for the trees; that by displacing so many people with evacuations we endanger those we wish to protect. The stress and panic that is caused by evacuation causes far more deaths than the radiation dose would.
A summary given by the World Nuclear Association [9] puts it very well:
Many evacuated people remain unable to fully return home due to government-mandated restrictions based on conservative radiation exposure criteria. However, over 1000 premature deaths have been caused by maintaining the evacuation beyond a prudent week or so. Decontamination work is proceeding while radiation levels decline naturally. The October 2013 IAEA report makes it clear that many evacuees should be allowed to return home.”

Don’t Panic

If there’s one point I have in this entire rant, it would be Do Not Panic when media outlets raise their collective voices about radiation. There is a plethora of information that describes harmless radiation, which can be used in a way to make it seem no less dangerous than the grim reaper himself, and how to work out how dangerous radiation is, is not easy.

It’s hard not to believe everything that is written in the media, and to know what counts as a reliable source. However, as soon as you hear the term Sieverts, make sure you compare the numbers with the table above, to see how much that really is. The misinformation spread about the harm that radiation, and in turn nuclear energy, does to us isn’t helped by the approach of many governments, to try to eliminate exposure to radiation at the expense of absolutely everything else, even reliable energy sources. There will be a ‘sweet spot’ for how much we should limit radiation, at the expense of useful technologies. I can’t say where that is, but I can say that it’s treated far too dangerously by the media and should be re-evaluated by governments, because losing over a thousand people through the after effects of evacuation shouldn’t be seen as the better alternative.

References:

[1] TEPCO report on core damage of units 1, 2 and 3 –http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_111130_04-e.pdf (Referenced on 12/04/15).
[2] Radiation map of Japan -
http://jciv.iidj.net/map/ (Referenced on 12/04/15).
[3] Greenpeace report on the hazards of tritium, prepared by Dr. Ian Fairlie -
http://www.greenpeace.org/canada/Global/canada/report/2007/6/tritium-report-canadian-facilities.pdf
[4] Review of Dr. Ian Fairlie’s report by R. V. Osborne - http://www.nuclearfaq.ca/ReviewofGreenpeacereport_Final.pdf
[5] IRR99 - www.sor.org/printpdf/book/export/html/6674
[6] World Health Organisation report on the Health Risk Assessment carried out after the nuclear accident at Fukushima - http://apps.who.int/iris/bitstream/10665/78218/1/9789241505130_eng.pdf?ua=1
[7] UK Radiation exposure - https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/82780/ntvfactsheet4.pdf
[8] Malcolm Grimston, “What was deadly at Fukushima?” - http://www.world-nuclear-news.org/E-What-was-deadly-at-Fukushima-2608141.html
 [9] World Nuclear Association quote - http://www.world-nuclear.org/info/Safety-and-Security/Safety-of-Plants/Fukushima-Accident/
[10] CDC Toxicity Profiles: Caesium-137 - http://www.atsdr.cdc.gov/toxprofiles/tp157-c3.pdf

4 comments:

  1. Superb read. The media and the general public have a warped view of nuclear energy due to years of misinformation.

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  2. Thanks. All respect. You might look into the difference of alpha and beta radiation with respect to ingestion and skin exposure and the consequences of the absorbed dose.

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    1. Hi there, this is Sam (author). Radiation damage in Sieverts (or milli/micro Sieverts) considers the biological dose from receiving different types of radiation (i.e. alpha is more damaging than beta), rather than just energy absorbed as radiation. This can also be explained by using a term called the 'Relative Biological Effectiveness'. One of the supporting bits of information provides the total Sv/Bq ingested, so considers the dose from Cs-137 as an internal source.

      I hope this helps!
      Sam

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