Deep-Earth reactor: nuclear fission, helium, and the geomagnetic field.
Geomagnetic field reversals and changes in intensity are understandable from an energy standpoint as natural consequences of intermittent and/or variable nuclear fission chain reactions deep within the Earth. Moreover, deep-Earth production of helium, having (3)He/(4)He ratios within the range observed from deep-mantle sources, is demonstrated to be a consequence of nuclear fission. Numerical simulations of a planetary-scale geo-reactor were made by using the SCALE sequence of codes. The results clearly demonstrate that such a geo-reactor (i) would function as a fast-neutron fuel breeder reactor; (ii) could, under appropriate conditions, operate over the entire period of geologic time; and (iii) would function in such a manner as to yield variable and/or intermittent output power. (+info)
Summary of the JCO criticality accident in Tokai-mura and a dose assessment.
A criticality accident occurred on September 30, 1999, in a conversion test facility at the JCO Tokai site. The accident was triggered by pouring an 18.8% enriched uranyl nitrate solution into a precipitation vessel beyond the critical mass. The accident continued for about 19 hours before the criticality could be stopped. during which time neutrons and gamma-rays were emitted continuously due to fission reactions. The total number of fission reactions was 2.5 x 10(18), which was estimated by an activity analysis of the fission products in the solution of the precipitation vessel. The accident gave serious radiation dose to 3 employees and fatal dose to 2 of them. Neutrons and gamma-rays emitted by the accident caused meaningful doses to the residents of the surrounding area of JCO. The dominant dose to the residents and JCO employees was brought by neutrons and gamma-rays from the precipitation vessel, while the contribution of radioactive plume was negligible. The individual dose was estimated for 234 resident, 169 JCO employees and 260 emergency personnel. The maximum doses were 21 mSv for the residents, 48mSv for the JCO employees, and 9.4mSv for the emergency personnel, respectively. No deterministic effect, however, has been observed, except for the 3 workers. (+info)
Analysis of a uranium solution for evaluating the total number of fissions in the JCO criticality accident in Tokai-mura.
The uranium solution in the precipitation tank in the JCO's uranium conversion facility was analyzed in order to evaluate the total number of fissions in the criticality accident. Two analytical groups at JAERI performed chemical analyses independently in order to check the validity of the results: the concentration of the fission products (95Zr, 99Mo, 103Ru, 131I, 140Ba, etc), uranium, boron and impurity elements in the solution. The analytical results obtained by the two groups were almost in agreement within the analytical error. The number of fissions per one gram of uranium in the accident was determined to be (1.5 +/- 0.1 ) x 10(14). Also, the total number of events was evaluated to be (2.5 +/- 0.1) x 10(18) fissions using the total amount of uranium (16.6 kg) fed into the precipitation tank at the accident. (+info)
Transport calculation of neutrons leaked to the surroundings of the facilities by the JCO criticality accident in Tokai-mura.
A transport calculation of the neutrons leaked to the environment by the JCO criticality accident was carried out based on three-dimensional geometrical models of the buildings within the JCO territory. Our work started from an initial step to simulate the leakage process of neutrons from the precipitation tank, and proceeded to a step to calculate the neutron propagation throughout the JCO facilities. The total fission number during the accident in the precipitation tank was evaluated to be 2.5 x 10(18) by comparing the calculated neutron-induced activities per 235U fission with the measured values in a stainless-steel net sample taken 2 m from the precipitation tank. Shield effects by various structures within the JCO facilities were evaluated by comparing the present results with a previous calculation using two-dimensional models which suppose a point source of the fission spectrum in the air above the ground without any shield structures. The shield effect by the precipitation tank, itself, was obtained to be a factor of 3. The shield factor by the conversion building varied between 1.1 and 2, depending on the direction from the building. The shield effect by the surrounding buildings within the JCO territory was between I and 5, also depending on the direction. (+info)
Determination of radionuclides induced by fast neutrons from the JCO criticality accident in Tokai-mura, Japan for estimating neutron doses.
A criticality accident occurred at a uranium conversion facility in Tokai-mura, Japan on September 30, 1999, and fission neutrons were continuously emitted for about 20 hours. Materials of stainless steel or iron, and chemical reagents were collected at places between 2 m and 270 m from the criticality accident site on October 25 and 26, 1999, November 27, 1999 and February 11, 2000. Neutron-induced radionuclides. such as 54Mn and 58Co, in the materials exposed to fast neutrons from the accident were measured to estimate the neutron fluences and energy distributions. Highly sensitive y-ray spectrometry with a well-type Ge detector was performed after radiochemical separation of Mn and Co from the materials. An instrumental neutron activation analysis was mainly applied for determinations of the target elements and chemical yields. The concentrations of 54Mn and 58Co in a mesh screen of stainless steel collected at a location 2.0 m from the accident site were determined. The total number of fission events was evaluated to be 2.5 x 10(18) by Monte-Carlo calculations of neutron transfer by considering the observed values of 54Mn and 58Co. The results presented here are fundamental to estimate the neutron doses at various distances. (+info)
Activation of soil and chemical reagents exposed to the neutrons released by the JCO criticality accident in Tokai-mura.
Specific activities (Bq/g-element) of residual neutron-induced radionuclides by the JCO criticality accident were measured for soil, concrete block and chemical reagent samples collected in the JCO campus. Induced radionuclides such as 24Na, 46Sc, 54Mn, 59Fe, 60Co, 65Zn, 82Br, 122Sb, 134Cs and 140La were detected in the samples, depending on the ground distance from the accident point and the sampling date. Apparent thermal, epi-thermal and fast neutron fluences, which reached the sample at each point, were roughly estimated from the specific activities and cross sections of the target nuclides taken from a literature. The present data are believed to be important as validation data for a three-dimensional neutron transport model calculation. (+info)
Dosimetry of fission neutrons in a 1-W reactor, UTR-KINKI.
The energy spectrum of fission neutrons in the biological irradiation field of the Kinki University reactor, UTR-KINKI, has been determined by a multi-foil activation analysis coupled with artificial neural network techniques and a Au-foil activation method. The mean neutron energy was estimated to be 1.26 +/- 0.05 MeV from the experimentally determined spectrum. Based on this energy value and other information, the neutron dose rate was estimated to be 19.7 +/- 1.4 cGy/hr. Since this dose rate agrees with that measured by a pair of ionizing chambers (21.4 cGy/hr), we conclude that the mean neutron energy could be estimated with reasonable accuracy in the irradiation field of UTR-KINKI. (+info)
Relative biological effectiveness of fission neutrons for producing micronuclei in the root-tip cells of onion seedlings after irradiation as dry seeds.
The relative biological effectiveness (RBE) of mixed neutron and gamma-ray radiation emitted at a 252Cf source at the Research Institute for Radiation Biology and Medicine, Hiroshima University, compared with 60Co gamma-ray radiation was determined. The tissue-absorbed dose contribution of the accompanying gamma radiation was about 35.7% to the total tissue-absorbed dose from the 252Cf mixed radiation. The 252Cf mixed radiation and 60Co gamma rays produced approximate linear changes in the frequency of micronuclei induced in root-tip cells of Allium cepa L. onion seedlings after irradiation as dry dormant seeds with varying absorbed doses in onion seeds. Therefore, the RBE for radiation-induced micronuclei was calculated as the ratio of the slopes for the 252Cf mixed radiation and the 60Co gamma rays. The deduced RBE value of 252Cf mixed radiation to 60Co gamma rays to induce micronuclei in dry dormant onion seed cells was about 90.5 +/- 3.6 (+/- 1sigma); the RBE of neutrons from the 252Cf mixed radiation was about 150 +/- 6 (+/- 1sigma). Furthermore, the sensitivity ratio of the induction rate of micronuclei in dry dormant seeds to that in seedlings by neutrons from 252Cf mixed radiation was significantly different from that by 60Co gamma rays. From these results, we concluded that the repair efficiency of DNA damage induced by neutrons may be different from that by gamma rays. (+info)