The characteristics of fission gas release from monocrystalline uranium dioxide during irradiation

Release rates for ^85mKr, ⁸⁷Kr, ⁸⁸Kr, ¹³³Xe, ¹³⁵xe and ¹³⁸Xe were measured in the temperature range 700–1550°C. The data were analysed in terms of diffusion of the rare gases and their halogen precursors. The diffusion coefficients for xenon and iodine were found to be similar whilst krypton also had a similar mobility at ~1200°C but otherwise diffused more slowly. Bromine had a high mobility compared with the rare gases (X 200).     

Effect of spring water on the radon concentration in the air at Masutomi spa in Yamanashi Prefecture,Japan

The concentrations of ²²²Rn existing in air have been studied by using a convenient and highly sensitive Pico-rad detector system at Masutomi spa in Yamanashi Prefecture, Japan. The measurements in air were carried out indoors and outdoors during the winter of 2000 and the summers of 1999 and 2005. The concentrations of ²²²Rn in spring water in this region were measured by the liquid scintillation method. The concentrations of natural radionuclides contained in soils surrounding spa areas were also examined by means of the γ-ray energy spectrometry technique using a Ge diode detector to investigate the correlation between the radionuclides contents and ²²²Rn concentrations in air at each point of interest. The atmospheric ²²²Rn concentrations in these areas were high, ranging from 5 Bq/m³ to 2676 Bq/m³. The radon concentration at each hotel was high in the order of the bath room, the dressing room, the lobby, and the outdoor area near the hotel, with averages and standard deviations of the concentration of 441 ± 79 Bq/m³, 351 ± 283 Bq/m³, 121 ± 5 Bq/m³, and 23 ± 1 Bq/m³, respectively. The source of ²²²Rn in the air in the bath room is more likely to be the spring water than the soil. The spring water plays carries the radon to the atmosphere. Our measurements indicated that the ²²²Rn concentration in the air was affected by the ²²²Rn concentration in spring water rather than that in soil.     

Air concentration estimation of radionuclides discharged from Fukushima Daiichi Nuclear Power Station using NaI(Tl) detector pulse height distribution measured in Ibaraki Prefecture

Air concentrations of ¹³³Xe, ¹³¹I, ¹³²I, ¹³³I, ¹³²Te, ¹³⁴Cs, ¹³⁶Cs, and ¹³⁷Cs in the early stage of the Fukushima Daiichi Nuclear Power Station (FNPS1) accident were estimated for six locations in Ibaraki Prefecture from pulse height distributions measured with NaI(Tl) scintillation detectors continuously operated as a part of monitoring station systems. Air concentration of ¹³³Xe in Ibaraki was estimated for the first time, which showed the maximum value of 150 kBq m^?3 in the plume arrived at the southern part of Ibaraki around noon of 15 March 2011. The plume was found to consist mainly of noble gases without other nuclides at significant level and to correspond to the hydrogen explosion at Unit 3. Spatiotemporal distributions of ¹³¹I and other nuclides were discussed for the plumes detected during the periods of 15–16 and 20–21 March. Variations in radionuclide composition among the plumes and within each plume were also discussed.     

Indirect assessment of 135Cs activity in the ventilation system of the Ignalina NPP RBMK-1500 reactor

The accumulation of long-lived ¹³⁵Cs isotope in the ventilation system components of the Ignalina NPP Unit 2 was investigated by spectrometric measurements and mathematical modeling. Volumetric activities of fission noble gas and other short-lived isotopes (⁴¹Ar, ^85mKr, ⁸⁷Kr, ⁸⁸Kr, ⁸⁸Rb, ¹³³Xe, ^133mXe, ¹³⁵Xe, ¹³⁸Xe, ¹³⁸Cs) have been measured by gamma spectrometric technique. Modeling of radionuclide transport in the ventilation system provides possibility of determining essential transport parameters: effective gas flow, mean gas retention time, deposition rate of aerosols. Estimated parameters were used for indirect evaluation of difficult to measure ¹³⁵Cs isotope activity in the ventilation system components: a delay chamber and aerosol filters. The results show that the major part of ¹³⁵Cs activity is accumulated in aerosol filters, whereas the total surface activity of the delay chamber is considerably lower. Specific activities of the ventilation system components of the Ignalina NPP Unit 2 are below the clearance levels for ¹³⁵Cs.     

Studies of Ceramic Fuels with the Use of Fission Gas Release Loop, (II)

The γ-ray spectra of fission gases released from UO₂-graphite pellets under neutron irradiation were measured. With and without separating fission gases into xenon and krypton, 25 kinds of γ-ray were observed and assigned to nine nuclides, ^85mKr, ⁸⁷Kr, ⁸⁸Kr(⁸⁸Rb), ¹³³Xe, ¹³⁵Xe, ^135mXe and ¹³⁸Xe (¹³⁸Cs). A value of 15 min is proposed for the half-life of ¹³⁸Xe, based on analysis of the decay curves. Discussion is given on problems related to determination of the release rate of each fission product through measurement of the height of each peak in the γ-ray spectrum.     

Measurements of 131I in the thyroids of employees involved in the Fukushima Daiichi nuclear power station accident

The Great East Japan Earthquake Disaster on 11 March 2011 caused an unprecedented accident at the Fukushima Daiichi nuclear power station operated by Tokyo Electric Power Company (TEPCO). Nuclear Fuel Cycle Engineering Laboratories of Japan Atomic Energy Agency performed internal dose measurements of 560 employees involved in the accident during the period from 20 April to 5 August in 2011 at the request of TEPCO. The present paper describes our measurements of ¹³¹I in the thyroid that is the predominant contributor to the internal dose. These measurements were carried out using an HPGe detector installed in a low-background shielded chamber made of 20-cm-thick steel and the detector was placed adjacent to the subject's neck. The typical minimum detectable activity of this technique was 10 Bq for a counting time of 10 min; however, this sensitivity made it difficult to identify a residual thyroid content of ¹³¹I corresponding to a committed effective dose of 20 mSv for late subjects. This paper discussed technical issues experienced through the measurements such as the influence of ¹³¹I in the rest of the body, the calibration phantom of use, and so on.     

The mechanisms of in-pile fission gas release from UO2

In-pile release mechanisms of fission gas from UO₂ at low temperatures were studied. The release of ¹³³Xe, ¹³⁵Xe, ¹³⁸Xe, ^85mKr, ⁸⁸Kr and ⁸⁷Kr from a sintered UO₂ pellet was measured at temperatures ranging from 250 to 930°C using a graphite specimen holder. The release from the holder, in which a fraction of fission gas was recoil-implanted, was subtracted to obtain the net release from the UO₂ pellet. Knock-out release from the UO₂ was measured directly, and it was found that it was not the main release mechanism, at least not for short-lived nuclides. A ‘pseudo-recoil’ release model is proposed to explain the low temperature release under irradiation. In the model, some of the defects produced by fission fragments act as short-lived carriers for fission gas.     

Source term estimation of atmospheric release due to the Fukushima Dai-ichi Nuclear Power Plant accident by atmospheric and oceanic dispersion simulations

The source term of the atmospheric release of ¹³¹I and ¹³⁷Cs due to the Fukushima Dai-ichi Nuclear Power Plant accident estimated by previous studies was validated and refined by coupling atmospheric and oceanic dispersion simulations with observed ¹³⁴Cs in seawater collected from the Pacific Ocean. By assuming the same release rate for ¹³⁴Cs and ¹³⁷Cs, the sea surface concentration of ¹³⁴Cs was calculated using the previously estimated source term and was compared with measurement data. The release rate of ¹³⁷Cs was refined to reduce underestimation of measurements, which resulted in a larger value than that previously estimated. In addition, the release rate of ¹³¹I was refined to follow the radioactivity ratio of ¹³⁷Cs. As a result, the total amounts of ¹³¹I and ¹³⁷Cs discharged into the atmosphere from 5 JST on March 12 to 0 JST on March 20 were estimated to be approximately 2.0 × 10¹⁷ and 1.3 × 10¹⁶ Bq, respectively.     

Post-irradiation studies on knock-out and pseudo-recoil releases of fission products from fissioning UO2

By using post-irradiation techniques, in-pile releases of ¹³³Xe, ^85mKr, ⁸⁸Kr, ⁸⁷Kr and ¹³⁸Xe from UO₂ fissioning at low temperatures below about 200° C are studied: these are analyzed into a time-dependent knock-out and time-independent pseudo-recoil releases. For the latter, a “self knock-out” mechanism is proposed: when a fission fragment loses thoroughly its energy near the UO₂ surface and stops there, it will knock out the surface substances and accordingly the fragment (i.e. the fission product) will be released. The effective thickness of the layer where the self knock-out occurs is found to be ~7Å. As for the knock-out release, the following is estimated from its dependence on various factors: the knock-out release of fission products occurs from the surface layer with the effective thickness of ~20Å: the shape of UO₂ matrix knocked out by one fission fragment passing through the surface is equivalent to a cylinder ~32Å diameter by ~27Å thick, (i.e. the knock-out coefficient for UO₂ is ~660 uranium atoms per knock-out event). On the basis of the above estimations, the conclusions derived from the past in-pile studies of fission gas releases are evaluated.     

Production of Fission Product Xenon

Radioactive Xe isotopes were produced by in-pile melting of irradiated uranyl nitrate hexali yd rate crystals. An aluminum pipe closed at the end was placed adjacent to the reactor core of the TTR-1 (swimming pool type reactor), and uranyl nitrate hexahydrate crystals in a stainless steel capsule were irradiated therein at a neutron flux of 2×10¹¹/cm²-sec. After in-pile cooling for 10–15hr, the capsule was heated to melt the crystals. The rare gas activity released from the crystals was led out of the reactor by helium sweep and collected in a charcoal trap cooled by liquid nitrogen. More than 99% of the rare gas activity was recovered. The composition of the activity at the time of separation, while depending on cooling time and target irradiation history, was in the present case ¹³⁵Xe(–80%), ¹³³Xe(–10%) and small amounts of Kr isotopes. The remaining uranyl nitrate, crystallizes again upon removal of heat, and can serve for succeeding irradiations. About 30 mCi of radioactive Xe isotopes were produced by 4 hr irradiation using 5 g of 13% enriched U. Radioactive Xe produced in this way is being used for medical purposes.     

Performance evaluation of a resonance ionization mass spectrometer developed for the FFDL system of fast reactors

To prevent a fuel failure event from becoming a serious radiation accident, sodium-cooled fast reactors are equipped with a system for failed fuel detection and location (FFDL). The FFDL instrument employed in the prototype fast breeder reactor Monju is based on the gas tagging method, in which precise and accurate measurements of krypton and xenon isotope ratios (⁷⁸Kr/⁸⁰Kr, ⁸²Kr/⁸⁰Kr and ¹²⁶Xe/¹²⁹Xe) must be performed in a short time. Burnup measurements also contribute to accurate determination of ⁸²Kr/⁸⁰Kr. We have developed a highly sensitive resonance ionization mass spectrometer for the isotopic analyses, which uses resonance ionization of Kr and Xe atoms by a pulsed laser at wavelengths of 216.7 and 249.6 nm, respectively. In evaluating the performance of our spectrometer, we find that systematic errors caused by isotope shifts can be reduced to negligible levels, and that statistical errors of 3% at a nuclide concentration of 7 ppt can be achieved with a single measurement time of about 40 minutes for each Kr and Xe isotope ratio. This means that, within one hour, about 200 fuel assemblies can be individually identified with a probability of 99%, verifying the applicability of our spectrometer to the FFDL system of fast reactors.     

Preliminary Study on Production of Xenon-133 from Neutron-Irradiated Uranium Metal and Oxides by Oxidation

A method was developed for recovering the fission product ¹³³Xe from several kinds of reactor-irradiated U targets, including Al-U alloy, metallic U, and uranium oxides.

In order to observe the release of ¹³³Xe from U compounds at high temperatures, irradiated targets were heated at 500°~1,000°C in vacuum or under gas flow. The liberated ¹³³Xe was trapped on charcoal beds, and the release rate of ¹³³Xe from the compounds was determined by measuring the activity accumulating in the traps.

More than 90% of the ¹³³Xe was liberated from the alloy upon melting and from metal and uranium oxide upon oxidation.

The isolated Xe was purified by a system embodying cold traps and cryogenic distillation.

The final products were sealed in ampoules. They proved to possess radiochemical purity exceeding 99.9%, and less than 1 μ/ampoule (1 ml) of non-radioactive gaseous contaminants.     

Estimation of release rate of iodine-131 and cesium-137 from the Fukushima Daiichi nuclear power plant

The atmospheric release rates of I-131 and Cs-137 from the Fukushima Daiichi nuclear power plant in March 2011 were estimated by comparing environmental monitoring data of air concentration and deposition rate on a regional scale with calculated values from an atmospheric dispersion model. Although the release rates were not estimated for all days after 11 March, because of lack of monitoring data, temporal changes in the release rates were reasonably estimated with estimated uncertainties in a factor of 3.3 and 2.9 for I-131 and Cs-137, respectively. A large release was estimated from the night of 14 March to at least the afternoon of 15 March, with maximum values of 7.2 × 10¹⁵ Bq h^?1 for I-131 and 1.5 × 10¹⁴ Bq h^?1 for Cs-137. The release rates during other periods were estimated at one- to two-orders of magnitude smaller than the largest release rate on 15 March. Uncertainty in the estimated release rate for 15 and 20 March was larger than for other periods. The significant release during 14 and 15 March and the trend of the release rate by the end of March were consistent with previous reports. This agreement, despite using different datasets, shows robustness of the temporal changes estimated in the studies.     

基于AST方法的核电厂LOCA释放源项计算分析

根据AST方法建立了AP1000LOCA放射性核素活度计算模型,研究事故后安全壳及环境中放射性核素活度的变化。结果表明:事故后安全壳气空间内各核素的放射性活度呈先增大后减小的趋势,40min时达到最大。根据核素性质,将其分为不考虑母核衰变的核素和考虑母核衰变的核素。事故发生40min后,前者在安全壳内的活度指数减小,典型核素有131~135I、83 Krm等,后者由于母核衰变的影响导致其在安全壳内的活度减小趋势放缓,典型核素有85 Kr、133 Xem、133 Xe和135 Xe等。I和Cs由于受自然去除机制的去除作用,事故几小时后其向环境的累积释放量增长非常缓慢;对于Kr和Xe,半衰期较长的核素向环境的累积释放量不断增大,半衰期较短的核素在事故几小时后向环境的累积释放量趋于平衡。     

Dosimetry in Krypton-85 Gas Irradiation Vessel

Dose rates obtained in a cylindrical vessel filled with gaseous ⁸⁵Kr source were measured using blue cellophane dosimeter. Besides obtaining the dose rate distribution within the vessel, the dose rate at vessel center was correlated to the ⁸⁵Kr pressure, to the shape and size of the vessel, and to the material forming the inner wall surface. The ⁸⁵Kr source used in the experiment amounted to 300 Ci (11.1 TBq), with a radioactive concentration of 35.9 mCi/ml NTP (1.33GBq/m/NTP).

The dose rate in the vessel was found to rise in direct proportion to the ⁸⁵Kr pressure, and also to increase in keeping with the size of the irradiation vessel and with the atomic number of the material lining the inner wall. Fairly good uniformity of the dose rate was noted, a ratio of only 1.33 being registered between the values obtained at the locations of maximum and minimum dose rate. The observed relationship between the dose rate and ⁸⁵Kr pressure agreed fairly well with a theoretical expression derived for spherical β source. A dose rate of 4.8×10⁷rad/h (480 kGy/h) can be expected to be realized with a ⁸⁵Kr source charged to 760 Torr (1.01×10⁵Pa) in a vessel measuring 1,710 mm×570 mm diameter, lined with Pb.     

One-year time variations of anthropogenic radionuclides in aerosols in Tokyo after the Fukushima Dai-ichi Nuclear Power Plant reactor failures

We report on the results of monitoring of environmental radiation for one year (13 March 2011 to 12 March 2012), including air dose rates and the concentrations of radionuclides in aerosols in Tokyo, after the reactor failures at the Fukushima Dai-ichi Nuclear Power Plant. The air dose rates began to increase at 4:00–5:00 JST on 15 March 2011, and the maximum rate was observed at 10:00–11:00 JST. Two peaks were observed before 23 March 2011, and then the air dose rates decreased until March 2012. The time variations of concentrations of radionuclides in aerosols showed tendencies similar to those of air dose rates. Short-lived radionuclides (⁹⁹Mo (^99mTc), ^129mTe (¹²⁹Te), ^131mTe, ¹³²Te (¹³²I), ¹³³I and ¹³⁶Cs) were under the detection limit during April 2011. Iodine-131 was detected until early June 2011, and long-lived radionuclides (¹³⁴Cs and ¹³⁷Cs) were detected intermittently for one year. Based on our results, gamma doses and committed effective doses resulting from inhalation were estimated.     

Storage of Krypton-85 by Adsorption Method

Several fundamental experiments were performed on adsorption process of ⁸⁶Kr storage at the ordinary condition of temperature and pressure. A long-term test for storage was then performed with radioactive ⁸⁵Kr and the safety was confirmed by hot run of 38 days. Of many adsorbents adopted in preliminary experiments, the largest amount of adsorbed Kr was AW-500 (16.4 cc STP/g) among molecular sieves and Tsurumi HC-30E (33.2 cc STP/g) among charcoals. The amount of adsorbed Kr decreased slightly with the addition of Rb and with the irradiation of the order of 10⁵ Mrad. It was found that adsorption rate is effected larger by Xe than by air. In ⁸⁵Kr storage test of 100 Ci, the pressure in storage vessel varied in the range of about ±4%, and the ⁸⁵Kr leak rate from the vessel was the order of 10^?12cc·atm/s. It was found that there is no considerable temperature rise due to β disintegration. From these results, the adsorption process may be useful for storage of ⁸⁵Kr for long-term.     

Method for Determining Degassing Constant of Xenon in Liquid Sodium

A method for determining the xenon degassing constant from liquid sodium to the cover gas by measuring the ¹³⁵Xe and ^135mXe release rate ratios, which were produced by the decay of ¹³⁵I in liquid sodium, was investigated using the Toshiba inpile fission product loop. Release rate ratios of ¹³⁵Xe to ^135mXe showed values of 9.9~7.6 at a sodium temperature range of 240~390°C and degassing constant of 1.2 × 10^?3-2.6 × 10^?3 sec^?1, which correspond to degassing half-life of 9.7~4.4 min, were obtained. This method is found to be effective for the determination of fission gas degassing constant, which has a corresponding half-life between several minutes to 40 min and will be useful for the study of fission gas transport phenomena in liquid sodium systems.     

低温下活性炭吸附分离Kr和Xe

研究了低温下活性炭吸附分离Kr和Xe的方法。Kr和Xe混合气在-78 ℃活性炭吸附柱上进行富集,根据Kr、Xe在活性炭柱上脱附条件的差异实现了Kr和Xe的分离。结果表明,Kr和Xe的回收率均大于90%,Kr样品中Xe的去污系数达10⁴以上,Xe样品中Kr的去污系数达10³以上。     

Production of high-purity medical radio isotope 64Cu with accelerator-based neutrons generated with 9 and 12 MeV deuterons

We conducted a feasibility study for producing a high-purity medical radioisotope ⁶⁴Cu from natural zinc with accelerator-based neutrons. ⁶⁴Cu isotopes were produced via the ⁶⁴Zn(n,p) reaction. The accelerator-based neutrons were generated via the C(d,n) reaction using low-energy deuterons of 9 and 12 MeV on a 1-mm-thick carbon target. First, the production purity was estimated using the evaluated nuclear data library JENDL-4.0 and our previously measured thick target neutron yield. We found that even when natural zinc was used as the starting material, significantly high-purity ⁶⁴Cu could be obtained. Next, irradiation experiments for producing ⁶⁴Cu using natural zinc were conducted at Kyushu University Tandem Laboratory, with the amounts of ⁶⁴Cu isotopes and other gamma-emission nuclides measured by a high-purity germanium detector. As a result, high-purity ⁶⁴Cu isotopes of 1.11(49) × 10⁰ and 3.70 (17) × 10⁰ Bq/g/μC were produced with incident deuteron energies of 9 and 12 MeV, respectively.