OpTimization of exposure and waste management for different accidental releases
GUETAT, Philippe
C.E.A. / Direction Military Application, and Protection and Nuclear Safety Direction Fontenay-aux-roses, Bat 52 - 92265 Fontenay-aux-rose Cedex France
The radioactive waste is substances which contain radionuclides and justifies a control of radiation protection, and for which no later use is foreseen. Then, two points have to be developed: one is: what needs a control of radiation protection? And the other is: how can we apply the general principles of waste management to the particular waste resulting of an accidental release?
Reference values are given in term of doses, dose rates, surface and mass activities and concentrations [1][2]. It raises the question of how large are the margins adopted at each step of the decision, and the ones which are also incorporated in the dose assessments.
In some region the natural dose rate is up to tens to hundred times higher than the mean 2 mSv/y. And no adverse genetic, carcinogenic, or other malign effects of those higher doses have ever been observed among people, animals and plants. For acute exposure, the survivors of the atomic bombing of Hiroshima who received less than 200 mSv have not suffered induction of cancers, and it has not been observed any genetic effects at any level allowing life.
Considering radiobiology, and the effect of radiation on the DNA, it is interesting to remember the spontaneous damages to DNA.
DNA Damage / spontaneousover 1 month / Chernobyl
youngchildren / Fukushima
youngchildren / Radio-induced damages /month
to thyroid cells / Whole body
exposure / 3 Gy (2 and more) / Max 0.08 Gy / Ref. 50 mGy/y
simple strand breaks / 300 000 – 1700000 / 3000 / 80 / 4.5
Nucleobase loss / 400000 / 6000 / 160 / 9
Bases modification / 100000
Double-strand breaks / 240 / 120 (50%) / 3 (1%) / 0.2
Table 1: DNA damages for a thyroid cell per month, spontaneous natural background, and for the most exposed group of children at Chernobyl and Fukushima and for an absorbed dose of 50 mGy per year.
Because of no consumption of milk and green vegetables after Fukushima accident and no very short lived nuclides no effect will be visible in Fukushima.
The International Commission [3] considers that the maximum value for a reference level is 100 mSv incurred either acutely or in a year. Nevertheless, this is not the reality. In the same way, using 0.1 Sv only in situation as the saving of live or to prevent a disaster is clearly not properly calibrated. The question is to evaluate simultaneously the interest of evacuation to avoid short or long term exposures, and at the contrary the interest to maintain populations locally not to induce other more conventional but real dangers and let people take care of their own way of life, tools and capital.
ICRP references values for population are given between 1, 20 and 100 mSv [3]. The band, 20 mSv - 100 mSv, applies in unusual situations.
For the determination of food and feed derived concentrations, the reference level for the mean annual internal dose of its inhabitants will not exceed around 1 mSv,. In practice, corresponding calculated doses for ingestion are between 0.1 and 0.5 mSv for a year. These calculations do not take into account the natural decrease during the growth and the fact for the next crop, soil-plant transfer is much smaller than foliar deposition. This means overestimation by a factor 3 to 10. For plutonium, it is more than a factor of 10 and for tritium; it is a factor 100 at least. This has important consequences on crops and environment management in term of surface concerned.
Case : dry deposition for 1 MBq.m-2 Total depositionExternal irradiation / infinite surface / Cs134 / 5.5 µSv.h-1 / 33 mSv.an-1
Cs137 / 2. µSv.h-1 / 12 mSv.an-1
Homogeneous after ploughing
3 Bq/g / Cs134 / 0,9 µSv.h-1 / 5,3 mSv.an-1 / Fed. Guidance n° 12
Depth 30 cm
Cs137 / 0,33 µSv.h-1 / 2 mSv.an-1
inhalation / Cloud dose thyroid
Adult
Child 1year / I131 / 5 to 20 mSvth
10 to 40 mSvth / Vdep = 0.5 to 2 cm.s-1
(Bq.m-2.s-1)/ (Bq.m-3)
Vegetable / Foliar deposition / aerosols / ≈106 Bq.kg-1 / (1 to 5 kgFreshW.m-2)
Soil-plant transfer / Cs 134 137 / 30 – 300 Bq.kg-1 / (Fresh weight)
Cow Milk / feed contamination / Cs 134 137 / 1.105 Bq.L-1 max / (Dynamic model)
Cow Milk / Feed contamination / I131 / 2 105 Bq.L-1 max
Cow meat / soil-> grass->meat / Cs137 / 50– 500 Bq.kg-1 / Long term
Milk ingestion / Child 1year thyroid / I131 / 1 to 4 Svth / Vdep = 0.5 to 2 cm.s-1
Exemption levels / BSS - Transport / Cs 134 137 / 10 Bq.g-1
Table 2 : Relations between different criteria
for the population, for the most severe reactor accident leading to some areas at the level of some MBq/m2 of each cesium, external irradiation dose reference can be between 1 and 300 mSv.y-1. Real exposure can be slightly above 100 mSv.y-1. And the choice should be done in comparison with other disaster as earthquake, tsunami, flood or fires. In fact thereis a physical border between cleaning and plowing. In the first case, this needs a special organization, a lot of people, delivers dose to workers at relatively short term and produce waste. In the second case, a large part can be done by farmers in the following months. It reduces by a factor of 4 the dose rate, and do not produce waste. 134Cs decrease reduces long term level of 2/3 after few years, and the following crops should not be above the commercial levels for most of the plants. The level of 2 or 3 MBq.m-2 for 137Cs is probably the physical good level, external doses starting at the level of 90-130 mSv.y-1, but falling during the first year to 40-60 mSv.y-1 because plowing and cleaning, and falling again in the next 4 years around 10-15 mSv.y-1.then after it becomes stable.Evacuation brings a lot of disorganization in the local life, anxiety and stress[4] and should be as limited as possible in space and time. There is a single good raison for early evacuation, which is the exposure of children to iodine by inhalation, for some weeks [5]. Distribution of stable iodine for children may be useful if evacuation is not decided or limited. Evacuation can be a bad choice for adults having special responsibilities in the crisis.
The best waste is the waste that does not exist, and soil is a wealth to preserve. This raises the fundamental question: must we concentrate or dilute radioactivity in the case of accident?For cesium, the specific activity reference value for exemption in normal conditions is 10 Bq.g-1. This value is very near the value of 8 Bq.g-1 used by Japanese authorities to define responsibilities between local and national levels for cesium, which is a good value. We can see that this level corresponds to 3 MBq.m-2. Soils below 10 Bq/g of cesium should not be considered as radioactive, and should not be removed. This means that areas where deposition is less than 3 MBq/m² should be treated by plowing, at a large extend, by the farmers and owners of gardens.It is not useful to let grow plants which have a much higher activity than the commercial criterion. The livestock is a part of the life and capital of the farmer. For the pastures, it is useful to mow some meadows to get new grass 2 months later.Large supplies are needed from other regions. Mowing produces organic matter which can be incorporated in the manure and later in the soil. At last plowing is a time-consuming work, it is recommended to begin the plowing with the plots of land near the house. Garden green garbage should be separated from ordinary household garbage and oriented to municipal landfills.
Tritium is present in CANDU and ITER. It is an isotope of hydrogen, (12 y), very mobile, a major element that does not concentrate in the food chain, and is always naturally diluted [6][7]. After a contamination, plants activities decrease by a factor 100 to 1000 in 2 days. Discussions occurred about organic compounds, but have little interest in case of accidental release, and total tritium measurement is enough. Although a release of 10 g of tritium (HTO) has a small radiological impact, some limited intervention close to the release point may be necessary (food ban for some days). The concentrations in foodstuffs recommended by the Codex Alimentarius are nevertheless much too low, and reference value should be at the level of 106 to 107 Bq.L-1.
Plutonium is coming from fuel reprocessing. It is a dense metal, non-volatile, alpha emitter with no external irradiation, and a non-metabolized element. Inhalation exposure would remain less than 10 mSv in the early phase of a1 gram 239Pu accidental release [8]. But the food restriction intervention for leaf vegetable would concerns an area of 10 km length.There is no significant transfer from soil, no transfer between leaves and fruits, and transfers to animal products (meat and milk) are relatively low. Analysis of Chernobyl accident has shown thatresuspension is of minor importance. Standard theoretical impact assessments are probably much overestimated. Nearly no waste should be produced. Incineration is possible.
[1]COUNCIL REGULATION (Euratom), 2016/52 - laying down maximum permitted levels of radioactive contamination of food and feed following a nuclear accident or any other case of radiological emergency. (2016).
[2] WHO,Codex general standard for contaminants and toxins in food and feed - CODEX STAN 193 (1995).
[3] ICRP,The 2007 Recommendations of the International Commission on Radiological Protection Publication 103, Ann. ICRP (2007).
[4] IAEA, Guidelines for remediation strategies to reduce the radiological consequences of environmental contamination. — TRS 475(2012).
[5] GUETAT Ph.et Al., Iodes radioactifs et impacts environnemental et sanitaire : étude bibliographique et quantification – Rapport CEA –R-6065 (2004).
[6] IAEA, TECDOC 1738, Transfer of tritium in the environment after accidental releases from nuclear facilities, ISBN 978–92–0–102814–3 (2014).
[7] LE GOFF P. et al, Tritium levels in milk in the vicinity of chronic tritium releases, Journal of Environmental Radioactivity, 151 (Part 1):282-292; (2016).
[8] GUETAT Ph. et Al, Plutonium in the environment: key factors related to impact assessment in case of an accidental atmospheric release Pu Future 2008 Radiochemical Acta 97, 257-260 (2009).