The accident at the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) in March 2011 led to
an unprecedented direct input of artificial radioactivity into the marine environment. The
Institute for Radioprotection and Nuclear Safety was requested by the French authorities
to investigate the radioecological impact of this input, in particular the potential
contamination of products of marine origin used for human consumption. This article
describes the close link between the responses provided and the availability of the data,
as well as their nature and ability to meet the requirements of expert investigation.
These responses were needed: (i) to evaluate the inputs of radionuclides into the marine
environment, (ii) to understand their dispersion in seawater, and (iii) to estimate their
transfer to the biota and sediments. Three phases can be distinguished which characterise
these processes during the accident and post-accident periods. The first phase corresponds
to an emergency phase during which no measurements were available on samples from the
marine environment. It involved the formulation of hypotheses based solely on the
expertise of the Institute for Radioprotection and Nuclear Safety. The second phase
started when the Japanese authorities provided measurements of the concentrations of
radionuclides in seawater. Although these data were not yet adapted to addressing the
problems of radioecology, the scenarios could then be refined and the estimates developed
in more detail. During the third phase, the accumulation of data over the course of time
made it possible to study the phenomena in an appropriate way. The chronology of the
events shows that it is essential to have (i) significant measurements of concentration
from samples collected in the various matrices of the marine environment, regularly
updated and sufficiently well-documented, (ii) samples of seawater collected at the
earliest opportunity as close as possible to the damaged site to characterise the source
term, and (iii) a numerical tool allowing rapid modelling of the dispersion of
radionuclides in seawater, as well as their transfer to sediments and the biota,
ultimately for the purpose of estimating the dose to humans.