This research work started 2 years ago, with a very limited number of 3 samples [1] -[3] , as a consequence of a first work about radioactivity in sea water salts [4] . Since then, the general aim is to establish one radioactive contamination factor (RCF) in many places as possible from world’s marine sediments; it has been increased to 10, all of them in Mexican coasts. The reason for this scarcity is of course always present difficulties in oceanography research, even when the results obtained are enough encouraging in the sense that today radioactive contamination, at least in Mexican waters, shows minimal values compared to natural radioactivity, but additionally and not less important, is the fact that we dispose now with one suitable indicator to evaluate some future radioactive contamination at planet scale, no matter what source might be. However, these results might acquire greater interest, only if they are compared with those obtained by other countries, in samples taken up from a large number of sites, as much as possible, and very well distributed all over the sea surface, which represents about 80% of that of entire planet.

Samples of marine sediments were taken up along the Gulf of Mexico coasts, from north to southeast, about 60 - 150 km out of sea, at 50 - 2500 m depth (Figure 1). Situation of sampling were quoted, and samples were dried out in the laboratory by heating them on a flat recipient. They were ground in a glass mortar, conditioned in Marinelli containers and carefully weighed, in order to be detected by 12 - 24 hours in one low background semiconductor HPGe detector, coupled to a PC charged with Maestro program II of radioactive detection, in which background counts had been previously detected at same time span than samples. Radioactive contamination factor (RCF) has been obtained as percentage of contaminant radioactivity from ¹³⁷Cs (662 keV γ rays), related to that of natural radioactivity from ⁴⁰K (1462 keV γ rays), both expressed as Bq/g. In this way, RCF is established as follows: RCF = Bq ¹³⁷Cs × 100/Bq ⁴⁰K. In this equation ⁴⁰K radioactivity is the total one (11% EC γ rays ⁴⁰K-⁴⁰Ar + 89% β⁻ particles ⁴⁰K-⁴⁰Ca). So, counts obtained from γ spectra have been divided by 0.11.

Table 1 shows the 10 more significant accidents happened till now in the whole history of nuclear reactors.

Figure 2 shows the background radioactivity in our HPGe detector.

Figure 3 shows the spectrum of ¹³⁷Cs and ⁶⁰Co known activities in an epoxy matrix conditioned in a Marinelli container, used to obtain the efficiency in our detection conditions for γ rays emitted by ¹³⁷Cs (0.45%).

Figure 4 shows the spectrum of ⁴⁰K known activity in a weighed sample of KCl conditioned in a Marinelli container, used to obtain the efficiency in our detection conditions for γ rays emitted by ⁴⁰K-⁴⁰Ar (0.22%).

Figure 5 shows the superimposed spectra of 7 sediment samples taken up during the last campaign.

Table 2 shows RCF values for ten samples treated till now (7 in last campaign and 3 in previous one).

In our samples we found one very small peak of ²⁰⁸Tl, (2614 keV, half life 3.1 m), and some other small peaks of radioactive heavy metals, members of the radioactive chain of ²³²Th (half life 1.4 × 10¹⁰ years), which is evidence of the presence of ²²⁸Ac (1459 keV, half life 6.1 hours). So, peaks of ⁴⁰K and ²²⁸Ac should not be distinguishable [5] . However, counts from ⁴⁰K have always been 10 - 15 times larger, and then in the ratio of contaminant vs. natural radioactivity, this tiny contribution goes into the ⁴⁰K natural one. Therefore, it seems that marine sediments represent the most suitable type of sample to measure the radioactive contamination at planet scale, by means of RCF, since the sea occupies the greatest portion of earth surface. One sediment taken up from Mexican lake (Avandaro) has been treated also at same conditions with marine ones, but has not been found in neither ⁴⁰K nor ¹³⁷Cs [3] . Therefore, it also seems that marine sediments are contaminated at present by more than 2000 nuclear tests performed in the world since 1945, while 10 significant nuclear reactors accidents happened in the last 50 years represent just a negligible contribution to radioactive contamination at planet scale. Moreover, these accidents happened till now confirmed the statistical results obtained by Rasmussen report in 1976 [6] , which predicts that danger imposed by nuclear research and power reactors is definitely smaller than those caused by a lot of human activities, such as cars and airplanes circulation, fires, high voltage, dams, and of course air and water contamination, as well as natural disasters, such as earthquakes, rays, tsunamis and floods.

^*Three first samples.

So, it seems quite reasonable to continue this research, by taking up samples as many as possible all over the world, in order to measure the conditions of radioactive contamination at optimum scale, avoiding panic with the comprehensive fact that nuclear power installations are not an uncontrollable danger, now that nuclear energy represents the most promissory means to continue the mankind evolution. Also, the proposed high number of RCF results should lead to some suitable average, with small positive or negative variations, by no means unreliable, since everyone should be always different, due to marine currents (studied by limnology), as well as chosen depth to take up the samples.