CONTROVERSES NUCLEAIRES !
SEISMES ET ENERGIE NUCLEAIRE
JAPON Fukushima Dai-Ichi (11 mars 2011)
SUIVI 2011
1er avril
Fukushima I plutonium likely not from MOX fuel, says ASN official

     The plutonium found by Tokyo Electric Power Co. at the Fukushima I nuclear power plant site very probably did not come from the mixed-oxide fuel elements loaded in unit 3 of the six-unit plant, Michel Bourguignon, a commissioner of France’s nuclear safety authority ASN, said March 29.

     ASN officials including Bourguignon said at a Paris press briefing, however, that it will not be possible to identify the source of the plutonium until information about its isotopic composition has been acquired.
     Tokyo Electric Power Co. announced the morning of March 29 Japan time that it had detected traces of plutonium in samples of soil collected from the site on March 21 and 22, but the utility said the concentrations were the same as in the "usual environment" and the plutonium "is thought not to be harmful to human health." Tepco said it would "strengthen environmental monitoring of power station and surrounding environment."
     The report of plutonium was greeted with alarm in Europe, where the relationship was immediately drawn with the presence of the MOX fuel in unit 3 at Fukushima I.
     At the press briefing, ASN commissioner Marie-Pierre Comets said that the plutonium concentrations at Fukushima I were "on the order of those from nuclear weapons test fallout." She explained that the isotopic makeup seemed to indicate, however, that at two of the site’s five measuring points, the plutonium was related to the accident, not to lingering weapons test fallout.
     Olivier Gupta, ASN deputy director general, said "the presence of plutonium confirms the hypothesis of major fuel damage" because the temperature of the fuel in one or more of the Fukushima I reactors had risen sufficiently to liberate not only volatile elements like iodine and cesium, but also heavy elements like plutonium.
     Bourguignon said, however, that he doubted the plutonium had melted, because its melting point is 3.100 °C and it evaporates at 4.100 °C.
     Bourguignon said he didn’t think the MOX fuel was responsible for the plutonium found at Fukushima I because there was "very little MOX fuel" in that reactor compared to the amount of uranium fuel in all three reactors.
     Rather, he said, the plutonium created on the rim of uranium fuel pellets through irradiation had likely been washed down by the tons of water poured on the cores over the past days. The pellets would have been exposed after fuel cladding failed, he said.
     The presence of MOX fuel in unit 3 does not change the order of magnitude of the environmental or health impacts of the releases, ASN officials have said repeatedly.
     As it struggles to cool the reactors and spent fuel pools, Tepco is faced with large quantities of contaminated water that it will have to manage, the ASN officials said.
     Units 1, 2 and 3 at Fukushima I have suffered significant core damage, the ASN officials said. They said the cores are still partly uncovered despite more than a week of cooling with seawater and now fresh water. It is likely that containment and perhaps reactor vessel integrity had been lost at up to three units, given the continued escape of radioactive substances from the reactors, the officials said.
     Tepco also reported March 29 that radioactive water was accumulating in the basements of turbine buildings of the three units and in tunnels leading to them. It said the water had high levels of contamination.
     Tepco has been pumping what it calls the “puddle water” out of the turbine buildings and into the condensers, but some of the condensers are now full, the company said March 29. In those cases, Tepco is pumping the 
contaminated water to the suppression pool surge tanks, it said.

(suite)
 suite:
     It needs to pump the water out to allow work to restore electricity supply to the reactors to resume, Tepco said.
     Tepco also reported that three workers who had been laying cable in the turbine building of unit 3 on March 24 had been confirmed to have received doses of approximately 170 millisieverts from wading through contaminated water. It said it had measured the dose rate at the surface of the water at approximately 400 mSv/hour.
     Japanese media reported March 29 that the workers had been discharged from the hospital and were doing well. The reports spoke of doses of 3.000 mSv (three Sieverts), saying that was much lower than had originally been thought. It didn’t say whether they had sustained radiation burns or burns from hot water, or both.
     But Bourguignon, a professor of medicine, said that because leg tissue is not very sensitive to radiation, the effective dose to the men from that contamination was equivalent to a whole-body dose of only about 3 mSv — 
three times higher than the annual regulatory limit for the public but well below the 20 mSv/year dose allowed for radiation workers in France. In Japan, the occupational dose limit is 50 mSv/year.
     Marie-Pierre Comets, another ASN commissioner, said that there are "positive" developments at Fukushima I, such as success in starting freshwater cooling to replace sea water cooling of the reactors and partial success in restoring regular electric power supply.
     Also positive, said Comets and Gupta, is the apparent stabilization of the situation at the Fukushima I spent fuel pools, which last week were feared to be on the verge of emptying and exposing fuel elements. As of midevening 
March 30 Japan time, however, the Japan Atomic Industrial Forum, the nation’s nuclear industry group, reported that "pool cooling capability" had been restored only at units 5 and 6. Seawater injection was being considered” for the unit 1 pool, seawater injection continued at the unit 2 pool, and seawater spraying continued at the pools for units 3 and 4, JAIF said.
     Comets and Gupta said that the discovery of radioactive water in the tunnels poses new problems for Tepco.
     It is not known precisely how the water is getting from the reactor building into the turbine building, Gupta said, but there is a lot of it — about 1 meter deep across the vast expanse of the turbine building.
     And Tepco is faced with the challenge of collecting tons of water as waste, finding an acceptable way to dispose of it, and finding a way to connect the reactors to a permanent, closed-circuit cooling system.
     “That will take time, given how damaged the installations are,” Gupta said.
     Tepco "is going to have to manage the consequences of cooling [the reactors] in an open circuit," he said, but added the utility has had little choice but to douse the cores with water to prevent extensive core melt and potential vessel and containment failure.
     Japanese media reported March 29 that the government was considering nationalizing Tepco, one of the country’s largest companies, so it can meet the challenge of recovering from the Fukushima I accident. Trading in Tepco’s stock was suspended March 29; it has lost about three-quarters of its value since the accident began March 11.
 
 Ann MacLachlan, Paris