Archive for September, 2011

UNDERSTANDING JAPAN’S NUCLEUR CRISIS

Following the events at the Fukushima Daiichi nuclear reactors in Japan has been challenging. At best, even those present at the site have a limited view of what’s going on inside the reactors themselves, and the situation has changed rapidly over the last several days. Meanwhile, the terminology involved is somewhat confusing—some fuel rods have almost certainly melted, but we have not seen a meltdown; radioactive material has been released from the reactors, but the radioactive fuel currently remains contained.

Over time, the situation has become a bit less confused, as cooler heads have explained more about the reactor and the events that have occurred within it. What we’ll attempt to do here is aggregate the most reliable information we can find, using material provided by multiple credible sources. We’ve attempted to confirm some of this information with groups like the Nuclear Regulatory Commission and the Department of Energy but, so far, these organizations are not making their staff available to talk to the press.

It will take up to nine months before the crippled Fukushima Daiichi Nuclear Power Station in Japan is stabilized, facility owner Tokyo Electric Power Co. (TEPCO) said last week. Radiation has been leaking from the complex since a massive earthquake-triggered tsunami inundated Japan’s Pacific coast and knocked out cooling systems at the plant’s six reactors just over a month ago.

At an April 17 news conference, TEPCO officials unveiled a phased “road map” to bring the crisis under control. In the first three months of the plan, the company hopes to cool the reactors and gradually reduce the level of leaking radiation.

Three to six months after that, the utility expects all of the reactors to achieve “cold shutdowns,” a stable condition in which temperatures drop and radiation leaks substantially decline.

“It is a sign of the extraordinary seriousness of the Fukushima nuclear disaster that TEPCO anticipates it may take nine more months before cold shutdown can be achieved,” Daniel O. Hirsch, lecturer on nuclear policy at the University of California, Santa Cruz, tells C&EN. “Radioactive releases could continue for a long time.”

TEPCO said the plan involves installing a concrete cover over the three badly damaged reactor buildings to contain the radiation. Hydrogen explosions in the buildings in days immediately after the quake blew off their roofs and scattered radioactive debris. The company has been pumping nitrogen gas into plant reactors to help prevent additional hydrogen blasts.

“We will do our utmost to curb the release of radioactive materials by achieving a stable cooling state at the reactors and spent-fuel pools,” TEPCO Chairman Tsunehisa Katsumata told reporters at the briefing. “The company has been doing its utmost to prevent a worsening of the situation. We have put together a road map and will put all our efforts into achieving these goals.”

Robots that are being used to measure the levels of radiation in the nuclear complex have reported radioactivity readings of up to 57 millisieverts per hour. Workers cannot enter these areas because the upper limit for nuclear workers in Japan is 250 millisieverts per year.

Meanwhile, very low levels of radioactive material linked to the damaged nuclear plant in Japan have reached the U.S. West Coast. But analyses of air and water samples for radiation contamination show that the U.S. remains safe, EPA Administrator Lisa P. Jackson told the Senate Environment & Public Works Committee earlier this month (C&EN Online Latest News, April 14).

“Let me be clear. EPA has not seen and does not expect to see radiation in our air or water reaching harmful levels in the U.S.,” Jackson said. “All of the data that we have seen, which we continue to make public and available on our website, indicates that while radiation levels are slightly elevated in some places, they are significantly below problematic levels.”

EPA has 164 monitoring stations spread throughout the 50 states, and the agency measures radioactive substances in air, precipitation, drinking water, and milk.

Recent air samples have contained very small amounts of iodine, cesium, and tellurium, which are consistent with possible releases from the damaged Japanese reactors, Jackson testified. The largest amounts were found in samples from Alaska, she said, but all of the radiation levels detected “are hundreds of times below levels of concern.”

EPCO reported that “most part of the fuel is considered to be submerged in the bottom of reactor pressure vessel and some part exposed.” TEPCO also reported that leakage of cooling water from the reactor pressure vessel is likely to have occurred. However, TEPCO considers that the actual damage to the reactor pressure vessel is limited, on the basis of the temperatures now being measured around the reactor pressure vessel.

With regard to the status of the reactor core of Unit 1, TEPCO believes that because the fuel has been being cooled continuously by means of water injection, it is unlikely that the situation could result in a future release of large amounts of radioactive material.

The results of the analysis are provisional; TEPCO will continue to conduct investigations. Similar analyses will be conducted for Units 2 and 3.

Nitrogen gas is still being injected into the containment vessel in Unit 1 to reduce the possibility of hydrogen combustion inside the containment vessel.

In Units 1, 2 and 3 fresh water is being continuously injected into the reactor pressure vessel; temperatures and pressures remain stable.

To protect against potential damage as a result of future earthquakes, TEPCO started work on 9 May to install a supporting structure for the floor of the spent fuel pool of Unit 4.

Fresh water is being injected as necessary into the spent fuel pools of Units 1 – 4.

Stagnant water with high levels of radioactivity in the basement of the turbine buildings of Units 1, 2 and 3 is being transferred to the condensers, the radioactive waste treatment facility, the high-temperature incinerator building and temporary storage tanks. Stagnant water in the basement of the turbine building of Unit 6 is being transferred to a temporary tank. Countermeasures against the outflow of water to the sea and to prevent and minimize the dispersion of radionuclides in water have been put in place.

Radiation Monitoring

Deposition in 47 Prefectures

The daily monitoring of the deposition of caesium and iodine radionuclides for 47 prefectures is continuing. Since 12 May negligible deposition has occurred. I-131 was reported in only one prefecture and Cs-137 was reported in three prefectures, with a value of 4.8 Bq/m2 for I-131 and a range of from 4.7 to 10 Bq/m2 for Cs-137.

Gamma Dose Rates in 47 Prefectures

Gamma dose rates are measured daily in all 47 prefectures. The Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan reports values on the basis of data collected from each prefecture. On 18 May the value of gamma dose rate reported for Fukushima prefecture was 1.6 µSv/h. In all other prefectures, reported gamma dose rates were below 0.1 µSv/h, with a general decreasing trend.

Gamma Dose Rates in Areas More Than 30 km from Fukushima Daiichi Plant

Gamma dose rates reported specifically for the monitoring points in the eastern part of Fukushima prefecture, for distances of more than 30 km from the Fukushima Daiichi plant, showed a general decreasing trend, ranging from 0.1 µSv/h to 17 &nicro;Sv/h, as reported for 17 May.

Maps of gamma dose rates, deposition of Cs-134 and deposition of Cs-137 within the 80 km zone around the Fukushima Daiichi plant were produced by means of aerial gamma ray monitoring by the Nuclear Safety Technology Centre of MEXT and the United States Department of Energy.

The map of the deposition of radiocaesium is presented in Fig. 1. The values represent the sum of Cs-134 and Cs-137. The areas in green show a deposition of these two radionuclides of between 0.6 and 1 MBq/m2. The areas in yellow indicate a deposition of between 1 and 3 MBq/m2. The areas in red indicate a deposition of between 3 and 30 MBq/m2. All are normalized to 29 April 2011.

The map shows that the results obtained are consistent with all previous reported measurements of deposition in soil and of gamma dose rates.

Air Concentrations of Radionuclides On-site at Fukushima Daiichi Plant

On-site measurements at the west gate of the Fukushima Daiichi plant indicate the presence of I-131 and Cs-137 in the air in the close vicinity of the plant (within approximately 1 km). The values observed in the previous days show daily fluctuations with an overall decreasing tendency.

Concentrations of Radionuclides in Drinking Water

As of 10 May, the restriction on the consumption of drinking water relating to I-131 – which had been applied since 1 April as a precautionary measure for one remaining location (the village of Iitate in Fukushima prefecture), and only for infants – was lifted.

Food Monitoring

From 12 to 18 May, the Ministry of Health, Labour and Welfare reported results of continued monitoring for radioactivity in food. Over this period, results for 503 food samples from fifteen different prefectures were reported. Most of this monitoring continues to be concentrated within Fukushima prefecture (39% of samples reported for 12 – 18 May). The majority of results were below regulation values, but 28 out of these 503 samples (fewer than 6%) were found to have radioactivity levels above the Japanese regulation values for radiocaesium. These samples were collected in three prefectures (Fukushima, Ibaraki and Kanagawa). None of the 503 samples was found to have radioiodine in excess of the regulation values.

In Fukushima prefecture, 175 of the 194 samples (more than 90%) had radiocaesium levels below the regulation values set by the Japanese authorities. However, 19 of the 194 samples (fewer than 10%) exceeded the regulation values for Cs-134/Cs-137. Samples above the regulation values were bamboo shoots (ten samples), shiitake mushrooms (five samples), and four samples of fish (two samples of whitebait, one sample of ayu and one sample of Japanese smelt).

In Kanagawa prefecture, 6 out of 33 samples (18%) were found to exceed the regulation values set by the Japanese authorities for Cs-134/Cs-137, these were six samples of unprocessed tea leaves (an additional ten samples of unprocessed tea leaves were found to have levels below this regulation value).

In Ibaraki prefecture, 3 of the 66 samples (4%) reported were above the regulation values set by the Japanese authorities for Cs-134/Cs-137. These were unprocessed tea leaves (two samples) and parsley (one sample).

Food Restrictions

As of 18 May, the only food restrictions remaining are in Fukushima prefecture and for the cities of Kitaibaraki and Takahagi in Ibaraki prefecture.

In Fukushima prefecture there are restrictions on the distribution and consumption of sand lance fish. In specified areas of Fukushima prefecture there are also restrictions on the distribution of raw unprocessed milk, turnips, bamboo shoots, ostrich ferns and shiitake mushrooms, and restrictions on the distribution and consumption of specific non-head type and head-type vegetables (e.g. spinach and cabbage), flowerhead brassicas (e.g. cauliflower) and shiitake mushrooms.

In Ibaraki prefecture there is a continuing restriction on the distribution of spinach produced in the cities of Kitaibaraki and Takahagi.

Marine Monitoring

The marine monitoring programme is carried out both near the discharge areas of the Fukushima Daiichi plant by TEPCO and at off-shore stations by MEXT. The increase in the radioactivity in the marine environment had occurred by aerial deposition and by continuing discharges and outflow of water with high levels of radioactivity from the Daiichi plant.

Seawater Monitoring

The activity concentrations of I-131, Cs-134 and Cs-137 in seawater close to the Fukushima Daiichi plant at the screen of Unit 2 have been measured every day since 2 April. Concentrations of Cs-134 and Cs-137 decreased from initial values of more than 100 MBq/L to less than 5 kBq/L on 7 May, but increased to levels of around 20 kBq/L on 16 May, and to about 10 kBq/L on 17 May. There was a significant increase in levels of I-131 from about 8 to 80 kBq/L from 10 to 11 May, in parallel with the increase for both radiocaesium isotopes. This indicates that there is still some production of fission products. The I-131 levels decreased to about 20 kBq/L on 17 May.

Monitoring of the marine environment is performed by TEPCO in the near field area and by MEXT at off-shore sampling positions. The monitoring of MEXT also includes: measurement of ambient dose rates in air above the sea; analysis of ambient dust above the sea; analysis of surface samples of sea water; and analysis of samples of sea water collected at 10 m above the sea bottom and in a mid-layer, as well as at several locations for sediments. At most of the offshore stations, I-131, Cs-134 and Cs-137 reached levels below the detection limit of 10 Bq/L.

 

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