(UPDATE-2) Of all news outlets, it was Yomiuri who reported the news (two days late) and mentioned the last remaining potential route for the debris pieces - Reactor 3 explosion. From Yomiuri Shinbun (2/24/2014):
The reason [why the debris pieces were there] is unknown; they could have been scattered by the hydrogen explosion in the reactor building, or they could have come from the ocean.
This news continues to be mostly ignored by both the mainstream media and the alternative net media. Very strange.
(UPDATE 2/13/2014) The only news I've found so far about these debris pieces in Naraha-machi is from FNN local Fukushima news. Even that news hides the fact that the radioactivity of maximum 2.92 million becquerels of radioactive cesium IS PER 0.4 GRAM SAMPLE.
Specifically, four small pieces of debris found at the river mouth in Naraha-machi 15 kilometers from the plant may have come from Reactor 3.
The first piece of debris were found in June 2013, but TEPCO didn't mention the discovery until July after three more such high-radiation pieces were found. Even then, they published a half-baked result of the analysis, which was nothing more than the measurement of gamma radiation and beta radiation in microsieverts per hour. (See my post on July 2, 2013 for the first discovery.)
TEPCO disclosed the result of the analysis of the debris done by Japan Atomic Energy Agency (of Monju fame) during the regular February 12, 2014 press conference, and no major news outlet has reported the news so far. Only bloggers took note. (I suppose the mainstream media is busy educating themselves on the intricacies of the State Secrecy Protection Law, even though it hasn't gone into effect yet.)
According to TEPCO, the pieces of debris were not only highly contaminated with radioactive materials released from Fukushima I Nuclear Power Plant but they actually, most likely came from the plant. The degree of contamination from radioactive materials found on the small pieces of debris is similar to that on the debris found around the Reactor 3 building.
From TEPCO's handout for the press, 2/12/2014 (in Japanese):
Very high contamination from cesium-134, cesium-137, and presence of cobalt-60 (in blue rectangle, added by me). Note the unit is Bq per sample, not kilogram. For example, Sample No.3 (0.4 gram) has 2.0 x 10^6 Bq, or 2 million becquerels of cesium-137:
Very high all-beta:
But the composition of radioactive materials (ratio) on the debris pieces shows almost all radioactivity comes from cesium-134 and cesium-137:
Radioactivity of the debris pieces, compared to those of the debris around the Reactor 3 building and of the soil in Naraha-machi and neighboring Hirono-machi. The debris pieces have about the same order of magnitude of contamination as the debris around Reactor 3 for cesium-137, and one to three orders of magnitude higher contamination for cobalt-60. Unit is Bq per gram:
What are the debris pieces made of? Polyethylene, polyolefine, wood:
TEPCO and JAEA's conclusion:
1. These small pieces of debris came from Fukushima I Nuclear Power Plant, most likely from the Reactor 3 building.
2. The high-beta radiation found on the pieces are not from beta-nuclides but from beta radiation from cesium-134 and cesium-137, judging by the composition of the radioactivity.
3. How they got to the river mouth is unknown. It could be by the ocean, or by land. The analysis is inconclusive on that point.
As far as incurious TEPCO goes, this is the end of the analysis. There will be no further analysis whether there is any contribution from MOX fuel that was in the Reactor 3's Pressure Vessel.
During the press conference, TEPCO's PR people could barely answer the questions on this issue, other than to say this was done by JAEA.