Kr Release Research Articles

First high temperature safety tests of AGR-1 TRISO fuel with the Fuel Accident Condition Simulator (FACS) furnace

Three TRISO fuel compacts from the AGR-1 irradiation experiment were subjected to safety tests at 1600 and 1800°C for approximately 300h to evaluate the fission product retention characteristics. Silver behavior was dominated by rapid release of an appreciable fraction of the compact inventory (3–34%) at the beginning of the tests, believed to be from inventory residing in the compact matrix and outer pyrocarbon (OPyC) prior to the safety test. Measurable release of silver from intact particles appears to become apparent only after ∼60h at 1800°C. The release rate for europium and strontium was nearly constant for 300h at 1600°C (reaching maximum values of approximately 2×10−3 and 8×10−4 respectively), and at this temperature the release may be mostly limited to inventory in the compact matrix and OPyC prior to the safety test. The release rate for both elements increased after approximately 120h at 1800°C, possibly indicating additional measurable release through the intact particle coatings. Cesium fractional release from particles with intact coatings was <10−6 after 300h at 1600°C or 100h at 1800°C, but release from the rare particles that experienced SiC failure during the test could be significant. However, Kr release was still very low for 300h 1600°C (<2×10−6). At 1800°C, krypton release increased noticeably after SiC failure, reflecting transport through the intact outer pyrocarbon layer. Nonetheless, the krypton and cesium release fractions remained less than approximately 10−3 after 277h at 1800°C.

Bubble formation and Kr distribution in Kr-irradiated UO2

In situ and ex situ transmission electron microscopy observation of small Kr bubbles in both single-crystal and polycrystalline UO2 were conducted to understand the inert gas bubble behavior in oxide nuclear fuel. The bubble size and volume swelling are shown as weak functions of ion dose but strongly depend on the temperature. The Kr bubble formation at room temperature was observed for the first time. The depth profiles of implanted Kr determined by atom probe tomography are in good agreement with the calculated profiles by SRIM, but the measured concentration of Kr is about 1/4 of the calculated concentration. This difference is mainly due to low solubility of Kr in UO2 matrix and high release of Kr from sample surface under irradiation.

Effects of Fuel Oxidation and Dissolution on Volatile Fission Product Release under Severe Accident Conditions

The release of volatile fission products from high-burnup UO2 fuel was examined in a steam atmosphere under severe accident conditions as a part of the VEGA program. The effects of fuel oxidation and dissolution were totally evaluated, by comparing the results with those from previous inert, hydrogen and steam atmosphere tests. It was shown that the oxidation of UO2 to UO2+x by steam generally enhances Cs and Kr release. However, the enhancement becomes smaller above the melting temperature of Zircaloy, about 2030 K, likely due to reduction of UO2+x by molten Zircaloy. The burst release of Cs occurs above about 2300K in the hydrogen atmosphere, while the release rate does not increase so significantly for the examined temperature range (<2800 K) in the steam atmosphere. Analysis of the hydrogen atmosphere test showed that fuel dissolution is apparently connected with the burst release and that a large fraction of Cs is quickly released from the dissolved fuel above 2300 K. It is considered that the fuel dissolution rate in the steam atmosphere is about 1/1000 of that in the hydrogen atmosphere.

Effects of Fuel Oxidation and Dissolution on Volatile Fission Product Release under Severe Accident Conditions

The release of volatile fission products from high-burnup UO2 fuel was examined in a steam atmosphere under severe accident conditions as a part of the VEGA program. The effects of fuel oxidation and dissolution were totally evaluated, by comparing the results with those from previous inert, hydrogen and steam atmosphere tests. It was shown that the oxidation of UO2 to UO2+x by steam generally enhances Cs and Kr release. However, the enhancement becomes smaller above the melting temperature of Zircaloy, about 2030 K, likely due to reduction of UO2+x by molten Zircaloy. The burst release of Cs occurs above about 2300K in the hydrogen atmosphere, while the release rate does not increase so significantly for the examined temperature range (<2800 K) in the steam atmosphere. Analysis of the hydrogen atmosphere test showed that fuel dissolution is apparently connected with the burst release and that a large fraction of Cs is quickly released from the dissolved fuel above 2300 K. It is considered that the fuel dissolution rate in the steam atmosphere is about 1/1000 of that in the hydrogen atmosphere.

Release of Kr, Ag, Sn, I and Xe from UC x targets

The release properties of a UC x thick target associated with a hot plasma ion source have been studied. Measurements have been performed for various elements: Kr, Ag, Sn, I and Xe. The analysis is made assuming pure diffusion or pure effusion as the release processes. The results obtained are compared to the data available in the literature.

Formula omitted] and [formula omitted] monitoring at MRI, Tsukuba and its importance

Atmospheric 85 Kr and 133 Xe have continuously been monitored since 1995 (weekly averages) at the Meteorological Research Institute, Tsukuba. Elevated 85 Kr concentrations observed in spring and fall of 1995–1997 are the result of two factors, the operation of the PNC Tokai nuclear plant and secondly the seasonal change of prevailing wind directions. Elevated 85 Kr levels arising from releases of the Tokai plant are interpreted in a semi-quantitative fashion using the daily 85 Kr release data and available meteorological data. During periods without the influence of the Tokai plant, 85 Kr background concentrations (1.3 Bq/m 3 of air) are similar to those for latitudes between 35 and 40°N. Background concentrations show a continuous increase with time and pronounced seasonal variations. The yearly increase rate of about 2.5% observed between 1996 and 1998 is very similar to European observations. The features of the seasonal variations are minimum values during summer and maximum values during winter. 133 Xe concentrations are not influenced by major local sources. One observation of a high 133 Xe concentration in June 1996 can be interpreted as originating from a Chinese underground nuclear test conducted earlier in the same month.

Release of ion-implanted Kr from (Th, U)O 2 — effect of matrix oxidation

The effect of matrix oxidation on diffusion and trapping of gas atoms in (Th, U)O 2 was studied. Krypton was ion-beam implanted into the (Th, U)O 2 and its release in air was measured. Trapping of gas atoms during migration had been previously observed in pure ThO 2 at the implantation doses selected. Before implantation, the specimens were annealed at 1400°C in either air or hydrogen to remove polishing damage. Kr release was measured in air at temperatures increasing from 700°C to 1500°C. The H 2-annealed (Th, U)O 2, stoichiometric at the time of implantation, became oxidized early in the heat treatments. The Kr release from these oxidized specimens showed only mild or weak trapping. That is, the fractional release was significantly larger than that from ThO 2 which had shown much stronger trapping at the corresponding doses. This enhanced release is in line with theoretical predictions of a lowered solution energy of noble gases in the anion-excess oxide. Specimens oxidized before implantation suffered uranium depletion at the surface by preferential vaporization. As a result, the release from the pre-oxidized specimens showed a behavior of strong trapping.

Structure and Thermal Stability of Amorphous Ni84Y8Kr8 Alloy

The structural studies of amorphous Ni84Y8Kr8 alloy have been performed by means of X-ray diffraction combined with DTA/TG, TEM and SEM experiments. The local structure of this amorphous alloy appears to be well represented by a mixture of noncrystalline microstructures with short-range interatomic distances similar to crystalline Ni9.4Y, Ni and Kr. The X-ray diffraction pattern at 300°C demonstrated the occurrence of crystallization. Initially, the diffraction peaks can be indexed as a mixture of Ni9.4Y and Ni phases, and Ni9.4Y decomposes into Ni and Ni5Y at higher temperatures. X-ray diffraction measurements exhibit the formation of solid fcc Kr bubbles or precipitates above 540°C. The rapid release of Kr does not occur below 700°C.

Annealing behaviour of copper and nickel containing high concentrations of krypton studied by positron annihilation and other techniques

Bulk copper and nickel samples containing 3 and 5 at. % krypton respectively have been studied by conventional positron annihilation techniques. Most of the emphasis has been placed on the changes in positron lifetime and angular correlation parameters during isochronal annealing from ambient up to near the metal melting points. The study was complemented by transmission and scanning electron microscopy, together with macroscopic measurements of weight and dimensions. These techniques, combined with previous studies, provide a fairly detailed picture of both the as-prepared materials, where the krypton is present as a high density of solid phase precipitates (solid bubbles), and the subsequent marked response of the substructure to annealing. A number of features are of particular interest. The onset temperature for bubble coalescence events is correlated with the melting temperature of the solid Kr inside the bubbles. At higher temperatures extensive swelling is observed prior to and simultaneously with the release of the majority of the krypton. The structure after the Kr release contains micrometre-sized pores and approximately 10 nm bubbles. This structure partly recovers at higher temperatures, but sufficient krypton is retained in the pores to maintain a large swelling up to close to the metal melting points. Positrons are found to become trapped at the Kr-metal interface in Kr bubbles, and there is clear evidence for a quantitative relation between lifetime and Kr density. Both these features are in agreement with the recent theory of Jensen and Nieminen (1987). Finally, positron trapping into dislocations is observed to occur at a rate much lower than that predicted from published specific trapping rates.

Stability of Krypton Fixed in Zeolite-3A and -5A

The fixation operation was carried out at 450–650°C, 1,000 atm for 24 h. The amount of fixed Kr was measured using thermal neutron activation analysis. For zeolite-3A the amount of Kr fixed decreased from 20 to 5 w/o as the fixation temperature rose. In the case of zeolite-5A, Kr fixation was observed at 550°C and above, and the amount increased to 15 w/o as the temperature rose to 650°C. The diffusion coefficient of Kr in the zeolite was determined from the result of heating tests. The value obtained was substituted in a diffusion equation, enabling us to predict the Kr release behavior at any temperature in a dry atmosphere. Both the samples of zeolite-3A fixed below 525°C and of zeolite-5A below 625°C showed an intensive Kr release in water that would not be expected from the release behavior under dry conditions. However, such unexpected release was not observed in either sample fixed above these respective temperatures. This could be accounted for by the fact that zeolite cations move easily when zeolite is moisturized.

Stability of krypton fixed in zeolite-3A and -5A.

The fixation operation was carried out at 450650°C, 1, 000 atm for 24 h. The amount of fixed Kr was measured using thermal neutron activation analysis. For zeolite-3A the amount of Kr fixed decreased from 20 to 5 % as the fixation temperature rose. In the case of zeolite-5A, Kr fixation was observed at 550°C and above, and the amount increased to 15 w/o as the temperature rose to 650°C. The diffusion coefficient of Kr in the zeolite was determined from the result of heating tests. The value obtained was substituted in a diffusion equation, enabling us to predict the Kr release behavior at any temperature in a dry atmosphere. Both the samples of zeolite-3A fixed below 525°C and of zeolite-5A below 625°C showed an intensive Kr release in water that would not be expected from the release behavior under dry conditions. However, such unexpected release was not observed in either sample fixed above these respective temperatures. This could be accounted for by the fact that zeolite cations move easily when zeolite is moisturized.

Measurement and Interpretation of CO and Kr + Xe in Irradiated ThO2‐Containing HTGR Fuel Particles

The irradiation of microspheres of ThO2 and Th0.81U0.19O2in the form of high‐temperature gas‐cooled reactor (HTGR) particles resulted in the buildup of CO as well as Kr + Xe within the particle. The particles were later equilibrated at 1325T2270 K, crushed, and the CO and Kr + Xe contents were measured. Oxygen released as CO can be represented by the expression O/f=74.6F exp (–7400/T), where O/f is the moles of oxygen released per mole of fissioned actinide, F is fission per initial heavy‐metal atom, and T is the equilibration temperature. This oxygen release is reasonably consistent with that predicted from thermodynamic data for the oxides of the fuel and fission products. The burnup dependence of the release of Kr and Xe to the void space within the particle was consistent with published data and approached the theoretical value for the 233u‐235U system, (Kr+Xe)/f0.32.

The thermal release of krypton from thin layers of gold

The release of Kr from thin layers of Au was investigated by thermal annealing of Kr-bombarded Au-discs. Large fractions of the gas were released below 900°K within 200 min. The gas emission occurred with a spectrum of activation energies, and it was mainly controlled by about 30 kcal/mole for 1 keV of bombarding energy, 50 kcal/mole for 5 keV, and for 10 keV. The release behaviour showed that temperature measurements with kryptonated Au-samples are sufficiently reliable only for those temperatures which correspond to the ascending part of the spectrum.

The sputtering of trapped gas from tungsten

0.5 and 2 kev Kr+ ions were injected into polycrystalline tungsten. The gas-loaded target was then bombarded with Ar+ ions with energies between 0.1 and 4 kev and the rate of release of Kr and the fraction of Kr released after a given Ar+ ion dose were measured. Effective gas sputtering coefficients were then deduced and a comparison of the form of their variation with Ar+ ion energy with the known variation of the sputtering coefficient of tungsten suggests that the gas release is not conditioned wholly by target erosion. It is believed that some of the gas release is a result of localized heating effects which occur near the point of ion impact (thermal spikes) and cause an enhancement of thermally induced gas desorption.

The release of kr 85 from irradiated uranium dioxide on post-irradiation annealing

Release of Kr 85 from irradiated uranium dioxide heated to temperatures above 1800° C is more rapid than predicted by simple extrapolation of data obtained at temperatures below 1600° C. The release has been found to occur in three distinct stages, an initial burst on reaching the annealing temperature, a steady state release at constant temperature and a release on cooling that starts at approximately 1450° C, irrespective of the cooling rate. This behaviour has been interpreted in terms of substoichiometry of uranium dioxide and segregation of gas bubbles to grain, boundaries above 1800° C.

The release of krypton 85 from irradiated uranium on thermal cycling through the phase changes

The release of Kr 85 from high burnup uranium on cycling through the phase changes has been studied by a gas entrainment and counting technique. It has been shown that significant releases occur, especially at the α—β phase transformation. The magnitude of the release depends upon the previous history of the uranium.