Fukushima two years later: a scientific report (2) - Gamma spectroscopy

This post appeared in Italian on wired.it. This is the secon part of a (poor) English translation. First part is here

Not only  the food is continuously monitored: also construction wood from the forests of the region is controlled. In this case too the safety limits are much more severe than that of  the volcanic pozzolans often used in the construction of houses and Italian schools.

Monitoring of radiation in a supermarket 10 km
from the coast of Fukushima

Comparison between radiation Italy (Rome) and Japan (Tokyo and Saitama). It is possible to see how radioactive background is higher in Rome than Tokyo. In  Rome the radiation environment is dominated by the peaks of Radon 222. The arrow indicates  where   the Cesium 137 (660 keV) peak should be located.

An estimation of  the amount of Cesium in food, wood or forage requires a spectrometer capable of determining the energy of each gamma ray. Since each isotope emits gamma rays of specific energies, it is possible to determine the quantities of the various isotopes present.

Among the recent devices there is a portable detector consisting of a crystal to stop the gamma  (CsI) and a Silicon Photomultiplier (MMPC or as they call them) to detect the energy measuring the light emitted in the crystal. The simplicity of this type of tools is that they do not require high voltages, are   small as a pack of cigarettes and is used as a USB device. The cost, however, is about 20 times that of a Geiger counter.

A spectrometer can  count, for each decay, the energy of the rays that strike it. In about an hour and then it is possible to obtain a spectrum which describes the type and amount of environmental radiation. To improve the statistics and better highlight the peaks is, however, advisable to wait a while longer. The picture above shows the value measured at Rome in an apartment on the fourth floor: it is 0.25microSv/hour  (with peaks of 0.35).

In the figure above it is possible to see how the spectrum in Rome (and in much of Italy) is dominated by radon 222, a noble gas source to the high amount of environmental radiation. Usually the radon comes from the soil and tuff, but in this case, since it is an apartment on the fourth floor, is more likely to come from pozzolana used in the construction materials . In figure are compared the spectra taken in Rome with those  acquired in Japan. The value of Rome is  higher (0.25microSv / h), followed by the basement workshops of Tor Vergata (0.10 microSv / h, where, however, there is much radon), Kokubunji (0.05), and the fourth floor in Wako (0035 microSv / h). Note the almost total absence of radon in Japan. It is worth mentioning that the environmental radiation at the onsen baths is higher due to the volcanic nature of the sulphurous waters.

Monitoring radiation in wood to be used for construction

In the samples collected in the hot spots, Cesium-137 is present in large quantities: this isotope decays into an excited state of barium (emitting an electron and an antineutrino). The de-excitation of barium emits a signal characteristic of this element: a gamma ray energy of 660 keV. The process is similar to that of fluorescence, only that in this case atomic electrons are excited. The return to the ground state emits light (between 2 and 3 eV), e.g. electromagnetic radiation. The energy levels in the nucleus are thousands of times more intense and therefore the electromagnetic radiation emitted has an frequency and associated energy thousands of times greater.

To the left of the peak  there is the so-called 'Compton edge', produced by gamma rays hitting an atomic electron of the Cesium crystal in the detector and accelerating it with a slightly lower energy (depending on the angle with which it is emitted). The spectroscopic analysis of a particularly contaminated sample, taken on  the side of a mountain road between the city of Fukushima and the coast. This sample shows the presence of the isotope cesium-134, which decays into barium with several peaks at 600, 790, 1400 and 1600 keV (the latter is out of range of the detector).

The cesium-134 has a decay time of two years, therefore the presence of this isotope represents the "signature" of the origin of the  Fukushima power plant. In other cases, the absence of cesium-134 was used to show how well mushrooms that had radioactivity above the threshold of 100 Bq / kg were not contaminated by the panel, but presumably from nuclear tests in the ‘60s.

Gamma spectrum from a pure Cs 137 source

Spectrum of a sample of soil containing cesium 134 and 137 of Fukushima region

The measures in the region of Fukushima were extremely interesting, but equally important on a personal level was the contact with the local population. Far from being disheartened, the people in Tohokoku did not give up and have rebuilt many of the structures destroyed by the tsunami. Although the plant has not resulted in deaths due to radiation (morbidly sought by national and international journalists), many deaths are due to poor management of the emergency in the first frantic days after the earthquake. Others are due to suicides after resettlement. The inhabitants of the regions closer to the center have been forcefully moved away and are now rebuilding the social fabric elsewhere, so sometimes it is difficult to return to their town of origin, even if it were decontaminated. The most relevant problem is economic: the damage to the primary sector and tourism are visible to all and will require years to get back to normal.

2. end  First part is here

Fukushima two years later: a scientific report (1)

The coast of Minamisoma devastated by the march 2011 tsunami

This post appeared in Italian on wired.it. This is the first part of a (poor) english translation.

March 11, 2013  marked the second anniversary of the earthquake in Tohoku. The event, Mw 9 scale, triggered a huge tsunami on the coast of Northeast Japan. The waves killed more than 18 thousand people, destroyed 50 thousand buildings damaging three times more. The overall damage amounted to tens of billions of euro. In many places the reconstruction was completed but in the worst affected areas - such as the coast of Minamisoma - the few survivors were relocated elsewhere.

The accident of Fukushima-1 nuclear power plant added man-made disaster to nature's destruction. With the first ground vibrations, automatic failsafe systems stopped  the nuclear chain reactions  inserting the control rods in the reactor. However this also stopped the production of energy from the power plant. When - after about an hour - emergency generators were destroyed by the second wave of the tsunami, the cooling water pumps stopped . The residual heat of the fission products, which takes weeks to cool, then  caused the thermal fusion of the fuel rods. This has nothing to do with nuclear fusion but - like many sticks of chocolate left in the sun - has melted the fuel, the control and  the moderator in a single mass, making it extremely difficult, if not impossible, the removal. Without water, the molten core temperature in the reactors had  risen by thousands of degrees reacting with the  inner  zirconium hull, liberating hydrogen, a highly explosive gas that accumulated in the buildings of reactors until it caused the explosion.

Measurements and sample-gathering in the mountains of Fukushima-ken

The main cause of the accident was due to very poor (and criminal) construction choices to save money (sea barriers too low, emergency generators installed in the basement of the buildings and on the sea side, lack of training of employees who were killed by the waves). This procedures were sanctioned and certified by the collusion between the private sector (TEPCO) and the governmental agencies which should have enforced more adequate safeties.

The explosion of the buildings of the reactors of Fukushima plant released large quantities of radioactive material into the environment, mainly Cesium and Iodine. Most has been dispersed in the sea, where the damage is attenuated by the density of the water (a thousand times higher than the air) and the vastness of the Pacific Ocean. Part of it, however -  carried by the winds - has spread on Japan.

Iodine decayed in a few weeks, leaving the cesium-134 and 137 as contaminants of the environment and food. To measure the amount of environmental radiation a Geiger counter is sufficient. This  detector counts all gamma ray striking  it regardless of the gamma energy. The ionization caused by radioactive particles can be traced back to the ambient dose equivalent, measured in mSv / h.

Comparison of different radiation detectors in a hot spot next to a gutter in Fukushima-ken

I recently had the opportunity to go in the region of Fukushima to take measurements of radioactive contamination and collect soil samples and materials. As expected, the average radiation of the region is less than or slightly greater than that of Rome (0.3microSv/h): in the city of Fukushima values ​​are equal to 0.1microSv/h.

The region of Fukushima (and not only) presents however a series of hot spots in which the environmental radioactivity is particularly high: areas with as little as a few tens of centimeters diameter, especially in the vicinity of gutters, under trees, in general where  water or snow, carrying radioactive elements, flow. In the picture is shown a point where the detectors show more than 30 microSv / hour. It is also possible to see that there are considerable differences in the measurements between the detectors: the Russian (blue on the left) and the 'pregnancy-test-like' stick (second from left) were out of scale  due to the high count rates. The discrepancies between the values ​​of the instruments are due to the different type of detector:  gas (Geiger) detect only gamma rays, while the scintillator (usually Cesium Iodide) can also reveal beta and alpha (if they have a appropriately thin window). With 30 microSv / hour, the dose in one year was slightly less than 300 mSv. According to specifications, the average environmental radiation is measured at one meter from the ground (the average height of a man) and is therefore lower (but it should be measured closer to the ground in schools). Even in the unlikely event of living  within 10 cm of the gutter, the values are still lower than those faced by astronauts on the International Space Station.

A large part of the population  has had to  learn to use the counters with competence to compensate  lack of action and moslty lack of confidence in the government. Measurements with a Geiger counter are in fact very simple to perform and characterized the first phase after the accident, where citizens' maps helped in understanding the the amount of radiation and the related environmental risks.

Daily gamma-ray spectroscopy on food samples  at a pre-school.
In blue in the logbook the Cesium region (660 keV)

People are more informed and aware of the risks and effects of radiation than -  for instance - in the area of ​​Tokyo. There is no panic or despair described in some apocalyptic  reports, although it is noticeable a latent concern for the economic and social future of the region. In the absence of adequate support from the central government (the previous majority party was heavily penalized during  last election), many public and private structures took independent actions. Many schools and kindergartens measure the daily amount of radiation in the food served to students. One of kindergartens we visited  had a gamma spectrometer operated with painstaking  care by one of the school cantine. This old Japanese measures the radiation present in the food of the children annotating quantities and characteristics. Over the months he had gained a lot of experience, identifying foods that contain higher amounts of cesium-137, such as mushrooms and renkon, lotus roots. Although below the very strict threshold 100Bq/kq (bananas have 125 Bq / kg of potassium 40), these products were then removed from the school menu.

Even though the direct danger of the radiation is then under control, the economic damage that the incident has caused is considerable: it is difficult to believe that a return to normal will occur in the next future.  The food on sale is below the (stricter than Europe) limit of  100Bq/kg, and therefore perfectly safe and below the doses present in the aforementioned bananas, Brazilian nuts...

Even if food comes  from a non-contaminated area of Fukushima, most of the people and almost all the restaurant owners prefer buying food from other regions. Some promising examples are  measurement performed directly at supermarkets, but it is difficult to estimate how these actually help the overall economy of the region. The signs of economic recovery are evident, but the production is much lower than before the accident. The small financial support has  not prevented numerous suicides among farmers who have lost their jobs.

How long does Cesium 137 from Fukushima plant stay in human body?

The maximum amount of Cesium allowed in food produced in Japan  has recently been lowered from 500 Bq/kg to 100 Bq/kg. This very conservative level is causing several problems to vegetable and meat producers, that have to throw away larger quantities of food. 

Even though the value of bananas is 125 Bq/kg of potassium 40 and up to 260 Bq/kg of Radium in brazilian nuts is higher than the current limit, it is sometimes argued that the values are unrelated. 

A conservative (and rough) estimate would give a factor 4 to "convert" from Cesium to Potassium (1Bq of) Cs = 4 Bq od K40. 

In other words  eating  100 g of Shitake with 50 Bq of Cesium   is equivalent to eating a couple (200g) of bananas. This takes into account decay modes, physiology and time to expel it from the body.  Calculations are below. Some points worth remembering are: 

Decay scheme of Cs137:
either one electron (beta-)
or one electron and one gamma.

But radioactivity from bananas is "natural" and good! There is no "natural" (good) and "artificial" (bad) radioactivity. All radioactive elements decay in the same ways and are potentially dangerous. Ceisum and Potassium decay in the same way (electron or gamma) and there is no difference among the two. 

Becquerels and Sieverts are two different things, to be used in different contexts. Becquerels represent the amount of radiation in an object (food, ground, trees...). Sieverts represent the equivalent dose to which the human body is exposed. Think of Becquerels as the luminosity of an UV light bulb of a tanning machine and Sieverts as the amount of tanning we might get from it.  It is not straightforward to pass from Becquerels to Sieverts, since it depends on the exposure time, if the food is ingested etc...

Decay scheme of K40,
either one gamma or one electron

Does Cesium accumulate in human body? No, it is expelled in one to four months, compared to one month of Potassium:

                               Cesium 137 and 134. The biological half-life of Cesium is between one and four months. ICRP standard  give 110 days for man and 65 days for women.  Some quotations:  (53+-12 days in children). 112 days in adults. Some differences between population may arise: biological half time is somewhat shorter among Scandinavian males (84 days), not in females (64 days).

                                Potassium 40. Our body contains about 3700 Bq of potassium 40, corresponding to about 0.32 mSv in an year. The biological half-life of potassium, that is the time it takes to expel half of it from our body is 30 days. (data from here).

So, how do I compare radiation from potassium and from Cesium? 

On average, and very roughly, 100 Bq from Cesium results in 4 times more Sieverts than 100 Bq from Potassium. This takes into account the factor 2 of the decay (Cesium decays in 82% of the time in an electron and then one gamma) and another factor 2 for the biological half-life (two months for Cesium vs one month of Potassium). In the most conservative case  you might assume a factor 8 (2 for the decay and 4 months of maximum half-life).

Nuclear Plants, Radiation and Food in Japan - updates

Currently in English, Italian translation will follow. 

Power Production

No electricity is generated from nuclear plants in Japan. All but one reactors are down, officially for maintenance. Whether they will be reactivated is still debated. Oki reactors are now less likely to be put back on line, pending the complaints by local government and some still unclear safety issues.

As of now, both Kanto and Kansai (that are on different grids) are consuming about 80% of the full power production. If this figures are correct, there is the clear possibility to have scheduled black-outs this summer, especially if it will be warmer than last year. The situation might be more critical for Kansai  which had a larger nuclear-produced energy fraction and that last summer had still the nuclear power plants online. 

Environmental radiation in case of no decontamination
in 10 and 20 years from now

Environmental radiation

Outside the exclusion zone, Japan has  an average  lower environmental radiation than Italy.  

Recent simulations show that - in absence of any decontamination - the Fukushima expelled Cesium will leave the exclusion zone unsafe for more than 10 years. This is hardly surprising given the 30 years half-life of Cesium 137. Decontamination strategies are under study, either by removing the ground, by letting radiation penetrate deepen in the soil in case of hot spots, or by physically separating the radioactive material from the rest. A truck from Toshiba has been set up to demonstrate the principle. The first method seems more expensive but is much more reasonable,  provided that all the material is stored around the site of the now destroyed power plant. The truck option would be the cleanest one, but cost of the procedure and the possibility of large scale production are not clear. 

Is there radiation in food in Japan?

A vegetable wholesaler has been found relabeling the origin of Fukushima cucumbers in other prefectures. He was just scolded, no fine or charges. The cucumbers were not dangerous in themselves, since it still is below the new limit of 100Bq/kg for Cesium, but the lack of legislation is worrying. This new lower limit (previously was 500Bq/kg) is causing a lot of trouble to fruit and vegetable companies, since the value is extremely low too meet. .  All food (and our body) contains radioactive material: as a reference it is worth remembering that bananas  have 125 Bq/kg of  Potassium 40 (even though it is expelled faster from the huma. body).

Left and right: radiation data of Coop food products

All are below the new limit of 100Bq/Kg (mentioned in the top page). The food with highest radiation content are Shiitake mushrooms. Note that Germanium detectors have an higher resolution than those of NaI  and can distinguish between the isotopes of Cs134 and Cs137. 

The total amount of radiation in Ge-measured products is the sum of the two isotopes. Cs134 has a shorter half-life, two years, compared to the 30years of Cesium 137, so it is expected that Cs134 content will be negligible by next year.

Rate of disappearance of Cesium from human body
from here