ID :
184942
Fri, 05/27/2011 - 18:36
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Shortlink :
https://oananews.org//node/184942
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'Recriticality' unlikely at Fukushima plant: nuclear reactor expert
TOKYO, May 27 Kyodo - An expert on nuclear reactors on Friday downplayed the possibility that melted nuclear fuel believed to be at the bottom of the pressure vessels for the crippled nuclear reactors at the Fukushima Daiichi power plant will resume fission, a phenomenon otherwise called ''recriticality.''
''As there are a large number of impurities inside the reactors, including neutron-absorption materials that prevent fission, the chances (of recriticality) should be lower than calculated,'' Hisashi Ninokata, professor at the Tokyo Institute of Technology, said in reference to his calculation of the possibility of the fuel developing nuclear chain reactions again.
While neutron moderation by means of water is needed to maintain nuclear chain reactions, Ninokata assumed that granules of uranium fuel, each about 5 millimeters in diameter, have accumulated to an extent of 3 meters in diameter and 20 to 50 centimeters deep at the bottom of a pressure vessel, with each granule exposed to water.
Ninokata then made a number of calculations under different conditions, such as the presence of fission components, like cesium, or constituents of capsular vessels for fuel rods in the accumulation. As a result, he calculated that fission would occur under such conditions as the presence of water approximately in the same amount as the fuel and the absence of fission components and other materials in the fuel.
Under more realistic conditions such as the presence of fission components equal to 1 percent of the fuel or constituents of capsular vessels to 10 percent, Ninokata calculated that no fission would occur due to the absorption of neutrons or a drop in the ratio of water.
The assessment by Ninokata backs up the Atomic Energy Society of Japan's view on the situation of reactors after their possible meltdowns resulting from the March 11 earthquake and tsunami disaster that crippled the Fukushima nuclear plant.
On the controversial question of whether the injection of seawater to prevent the nuclear crisis from worsening increased the possibility of recriticality, Ninokata said the nuclear fuel is unlikely to resume fission thanks to the use of seawater containing more impurities than fresh water.
''As there are a large number of impurities inside the reactors, including neutron-absorption materials that prevent fission, the chances (of recriticality) should be lower than calculated,'' Hisashi Ninokata, professor at the Tokyo Institute of Technology, said in reference to his calculation of the possibility of the fuel developing nuclear chain reactions again.
While neutron moderation by means of water is needed to maintain nuclear chain reactions, Ninokata assumed that granules of uranium fuel, each about 5 millimeters in diameter, have accumulated to an extent of 3 meters in diameter and 20 to 50 centimeters deep at the bottom of a pressure vessel, with each granule exposed to water.
Ninokata then made a number of calculations under different conditions, such as the presence of fission components, like cesium, or constituents of capsular vessels for fuel rods in the accumulation. As a result, he calculated that fission would occur under such conditions as the presence of water approximately in the same amount as the fuel and the absence of fission components and other materials in the fuel.
Under more realistic conditions such as the presence of fission components equal to 1 percent of the fuel or constituents of capsular vessels to 10 percent, Ninokata calculated that no fission would occur due to the absorption of neutrons or a drop in the ratio of water.
The assessment by Ninokata backs up the Atomic Energy Society of Japan's view on the situation of reactors after their possible meltdowns resulting from the March 11 earthquake and tsunami disaster that crippled the Fukushima nuclear plant.
On the controversial question of whether the injection of seawater to prevent the nuclear crisis from worsening increased the possibility of recriticality, Ninokata said the nuclear fuel is unlikely to resume fission thanks to the use of seawater containing more impurities than fresh water.
