Short-lived Fission Products Research Articles

The transport of short-lived fission products close to the fuel surface

The diffusion equation is an approximate description of fission-product transport in UO 2. Although it is an extremely good approximation for stable and long-lived products its use is suspect for short-lived species when the frequency of diffusive jumps becomes comparable with the decay constant. An alternative treatment of the transport of short-lived fission products is presented in which an integral equation is developed by assuming only that transport takes place by isotropic jumps of a fixed length. This equation is solved numerically. The solution shows both diffusional and knock-out characteristics, and provides a method for linking these processes. A diffusion equation is solved with a “zero inward current” boundary condition to obtain similar features analytically. The formulation explains why diffusion theory should always be valid for thermally activated diffusion at high temperatures. It also offers new prospects for gaining information on the poorly understood in-pile phenomenon of athermal diffusion, and provides a general mathematical framework for discussing near-surface transport.

Rapid removal of Chernobyl fallout from Mediterranean surface waters by biological activity.

The sinking of particulate organic matter from the euphotic zone is an important pathway for the vertical transport of many elements and organic compounds in the sea. Many natural and artificial radionuclides in surface waters are readily adsorbed onto suspended particles and are presumably scavenged and removed to depth on time scales commensurate with both particle sinking rate and retention time of the radionuclide on the particle. Previously, abyssal benthic organisms from the northeast Pacific were found to contain short-lived fission products which entered the sea surface as fallout from nuclear testing. The presence of these radionuclides at great depth could not be explained by Stokesian settling of small fallout particles and it was hypothesized that zooplankton grazing in the surface layers packaged these particle-reactive radionuclides into large, relatively dense faecal pellets which rapidly sank to depth. We report here data from a time-series sediment trap experiment and concomitant zooplankton collections which show conclusively that Chernobyl radioactivity, in particular the rare earth nuclides 141Ce and 144Ce, entering the Mediterranean as a single pulse, was rapidly removed from surface waters and transported to 200 m in a few days primarily by zooplankton grazing.

The determination of uranium using short-lived fission products by cyclic and other modes of activation analysis

The determination of elemental concentration in biological and environmental samples through the detection of short-lived nuclides has been of considerable interest in the last few years. In this context the relative advantages and disadvantages of cyclic activation analysis (CAA) with respect to single conventional one-shot irradiation and counting sequence (for one sample or replicate samples) and what has been termed pseudo-cyclic activation have been discussed with conflicting interpretations. It is the objective of this study to demonstrate through the irradiation of uranium standards by measurement of short-lived fission products how each mode of activation is best utilised. Application of CAA to the problem, in order to enhance signal-to-noise ratio, must also take into account an increasing dead-time with each cycle and therefore mass fractionation of a given sample and standard are investigated for replicate and pseudo-cyclic activation conditions. The variation of timing parameters in the cyclic mode, as well as irradation in a mixed reactor flux and epi-cadmium neutron flux, produces a set of equations from which half-life can be determined to confirm fission product identification in these complex gamma-ray spectra.

Fission product yields in the fast-neutron fission of238U

The fission yields of 38 fission products in the fast-neutron induced fission of238U have been determined using a rapid, multiscaling gamma-ray spectroscopic method. To obtain absolute yields for fission products having half-lives ranging from 32 s to 40 d, a total of 56 multi-scaling gamma-ray spectra were collected using various irradiation and cooling periods. Gamma-rays and photopeak areas of interest were assigned to the fission products by their energies and half-lives. Fission product activities were evaluated from spectral data using growth and decay calculations and fission yields were determined by normalizing the140Ba yield to the average value from reported data. The depleted uranium target, covered with a boron-cadmium thermal neutron shield, was used to keep interference from the fission of235U minimal. Results for the cumulative fission yields, including 17 mostly short-lived fission products measured for the first time, are compared with previous measurements and with the recommended yields in recent evaluations. The agreement, and some discrepancies, in the comparisons are discussed. No explicit even-odd pairing effects are observed in the fission yield data for fast-neutron induced fission of238U.

Dynamic model for radionuclide uptake in lichen

Samples of atmospheric particulate material and terrestrial plants, including lichens, were collected in New Brunswick, Canada between 1980 and 1983 and analyzed for a wide range of artificial and naturally-occuring radionuclides, including fission products ( 141Ce, 144Ce, 103Ru, 106Ru, 95Zr and 137Cs) derived from the 16 October 1980 Chinese nuclear test. Activity ratios of some of the short-lived fission products in air particulates and lichens are in reasonable agreement with those predicted from fission product yields for nuclear weapons tests, indicating that only minor fractionation occurred for these radionuclides during their transport through air particulate and lichen environmental phases. The 7Be inventories measured in a suite of lichen (Cladonia rangiferina) samples were used to calibrate each lichen plant for its collection efficiency for atmospheric particulates and fallout radioactivity. A lichen model has been developed to predict lichen inventories of radioactivity for different lichen growth functions and bio-elimination rates. Assuming that lichen growth results in a linear increase in surface area with time, the experimental results yield biological residence times of 1–2 years for 210Pb and Pu and 5–8 years for 137Cs. The more efficient retention of 137Cs is probably due to its physiological uptake in lichen plants as a proxy for potassium, as evidenced by an observed, inverse relationship between 137Cs and 40K activities in lichen.

A gas-jet system for the study of short-lived, light fission products

For the study of short-lived fission products, a gas jet system has been set up in a beam port of the Ford Nuclear Reactor at the University of Michigan. The system consists of a target chamber joined to three polyethylene tubes which serve as gas inlet, fission product outlet and an outlet for pressure monitoring. The target chamber is housed in a 7.6 cm (ID) aluminum tube supported inside a radial beam port. This tube can be withdrawn by ∼1 m, allowing the target chamber to be positioned either close to or ∼1 m from the core. The aluminum tube is surrounded by water, which provides shielding and protects the target when not in use (at ∼1 m position). The target is ∼1 mg of235U, chemically plated on an aluminum planchett. The uranium is covered with 2.7 mg·cm−2 of aluminum. The aluminum covering stops most of the heavy fission fragments; the light fission fragments pass through the foil and are thermalized in a gaseous mixture of nitrogen and ethylene. The transport time of fission products from the target chamber to the detector position was measured to be 970 msec. Preliminary experiments indicate that the transport time can be reduced further. The gas jet system is being utilized for the development of fast, gas-phase chemistry for the separation of bromine from fission products.

Measurement of the short lived cumulative fission yield in u-235

Abstract Three thin metal foils of 93.15% enriched 235U (3mg each) have been irradiated with thermal neutrons at a thermal power of 500 watts, to measure the cumulative fission yield of 148Ce (T½=56s) and some other short lived fission products. Gamma spectra were collected using a 50 cm3 Ge(Li) detector and the yields were calculated using a computer code developed for calculation of cumulative fission yield by first determining the appropriate gamma ray activity as of the end of the irradiation, correcting for detector efficiency and gamma ray branching ratio, and finally, dividing by the number of fission in the sample. Fission rate was determined by fission rack method. The cumulative fission yield of 148Ce, 89Kr, 136mI, 144La, 90Kr, 139Xe, 137Xe, and 140Cs were determined.

Investigation of nonrecoil fission-product release phenomena using multifrequency source-perturbation experiments in EBR-II

A widely-accepted theory that was proposed to explain nonrecoil fission-product release phenomena in LMRs postulated that isotopic transport kinetics are attributable to liquid-state diffusion across a viscous sublayer of Na near the surface of the fuel. A series of multifrequency source-perturbation experiments was recently performed in EBR-II to isolate and quantitatively explore the boundary-layer diffusion mechanism while separating out all other physical variables that affect the isotopic transport kinetics. Detailed analyses of the results using bivariate spectral decomposition and cross-correlation techniques are presented. Results of the analyses provide incontrovertible experimental evidence that boundary-layer diffusion in fact plays no part in the release of short-lived fission products in LMRs. A major conclusion of this investigation is that all nonrecoil fission-product release phenomena originate from mechanisms acting inside the breached element itself. Implications of this and other findings made during this investigation are discussed, and recommendations are made for extending the techniques introduced here in future experiments involving actual breached pins.

Target and ion source developments for a helium-jet type ISOL at the Kyoto University reactor

Abstract A target system and ion sources for a He-jet type ISOL have been successfully constructed and the characteristics have been studied. The beam intensities of short-lived fission products have been much improved by increasing the He gas pressure owing to a range effect of fission products in the He gas and a short transport time effect of aerosols in a target chamber and a capillary. An oscillating electron ion source has been coupled with the He-jet system and ten kinds of elements have been ionized with efficiencies of 0.4–2.4%. A high temperature thermal ion source has been studied to ionize the alkali, alkaline-earth and rare-earth elements. The effects of the He-jet on the ion source have been discussed.

Short-lived fission product release from UO 2: Annular versus solid pellets

We have irradiated, in a “dual sweep” test assembly, one element containing solid UO 2 pellets with four, 1 × 1 mm surface slots and one element containing annular UO 2 pellets with a central hole two mm in diameter. Data are reported for a linear power range of 17–57 kW/m; thermocouples monitored operating temperatures. He-2%H 2 carrier gas swept short-lived fission products from both elements past independent spectrometers for identification and measurement. The releases from the solid and annular fuel were power-dependent from 17–57 kW/m, with the annular fuel release up to eight times greater than that for solid fuel. The annular fuel also showed a larger released iodine inventory in fuel and sheath areas with access to the carrier gas.

Postirradiation Dimensional Stability and Fission Product Behavior of Deliberately Defected UO2Fuel at 200 and 400°C

Twenty-four-hour sweep tests have been carried out in flowing air at 200 and 400C on deliberately defected UO2 fuel elements with 2.5-yr discharge times. At 200C there was no diametral change, but at 400C, swelling and severe sheath cracking were observed. Neither short-lived fission products nor TUCs, TXCs, or WRu were detected above background. Maximum YVKr release was less than or equal to 7.4 X 10U Bq (less than or equal to 2 X 10 W Ci).

Authors

Authors

Release of short-lived fission products from UO 2 fuel: Effects of operating conditions

To support accident “source-term” studies, we have carried out a series of in-reactor tests to determine the behaviour of short-lived fission products (Xe, Kr, I) in operating UO 2 fuel under a variety of conditions. The linear power range examined was 1762 kW/m to a maximum burnup of about 220 MW. h/kg U. Under normal operating conditions, release of the short-lived xenons and kryptons was described by a λ(decay constant) −0.5 relationship, characteristic of diffusion; there was a strong temperature-dependence of release in the thermally-activated diffusion range. Release during shutdown and startup transients was dependent on prior operating temperature, while under oxidizing conditions enhancement of release up to a factor of 4.5 was observed. Under loss-of-coolant conditions, release was described by a λ −1 relationship, characteristic of movement of stored inventory. Most release accompanied the re-wet portion of the transient, consistent with enhancement due to fuel cracking. No iodines exited from the fuel elements under normal, oxidizing or loss-of-coolant conditions.

Authors

Authors

Natural and man–made radionuclide concentrations in lichens at several locations in Austria

Regions of elevated natural radioactivity (Gastein–valley, Waldviertel) were compared to regions of low natural radioactivity (surroundings of Salzburg City). The natural radionuclides 238U, 40K and 7Be and the man–made nuclides 137Cs, 144Ce, 95Ze95Nb were measured by gamma ray spectrometry. No statistically significant difference was found between samples of the Gastein–valley, Waldviertel, Salzburg, Grossarl and Dachstein concerning the content of natural radionuclides. The amount of the short–lived fission products 144Ce, 95Zr–95Nb as well as of the natural 7Be increased with altitude. The specific nuclide concentration varies with species but was almost independent of growth form and substrate.

Release of Short‐Lived Fission Products from Intact and Defected UO2 Fuel Elements

The rates of escape of short‐lived isotopes of xenon and krypton to the coolant from a deliberately defected UO2 fuel element are at least 100 times greater than the release rates for identical species within an intact element. The enhanced escape is consistent with accelerated transport mechanisms under defect conditions. The rate of escape to the coolant of radioiodines is less than that for the short‐lived gases, reflecting strong bonding between the iodine species and internal UO2 and zirconium alloy surfaces.

Multigamma-ray calibration sources

We have calibrated a self-consistent set of multigamma-ray standards using the automated multi-spectrometer γ-ray counting facility at LLNL's Nuclear Chemistry Division. Pure sources of long-lived activity were produced by mass separation and,or, chemical purification. The sources were counted individually and in combination on several different calibrated spectrometer systems. These systems utilize various detectors ranging from small (x-ray) detectors to large volume high-purity Ge detectors. This has allowed the use of the most ideal individual detector-efficiency characteristics for the determination of the relative γ-ray intensities. Precise energy measurements, reported earlier (Meyer, 1976) have been performed by an independent method. Both the energy and γ-ray-emission probabilities we determine compare well with independently established values such as the recent ICRM intercomparison of 152Eu. We discuss our investigations aimed at resolving the shape of the efficiency response function up to 10 MeV for large volume Ge(Li) and high-purity Ge detectors. Recent results on the γ-ray-emission probabilities per decay for 149Gd and 168Tm multigamma-ray sources are discussed. For 168Tm, we deduce a 0.01% β − branch to the 87.73-keV level in 168Yb rather than the previous value which was a factor of 200 greater. In addition, we describe current cooperative efforts aimed at establishing a consistent set of data for short-lived fission products. Included are recent measurements on the bromine fission products with γ rays up to 7 MeV.

Extraction of Ag(I), Cd(II), In(III), Sn(II), Sn(IV), Sb(III), and U(VI) from aqueous solutions by ketone solutions using single-step batch and continuous SISAK methods1,2

The extraction properties of Ag(I), Cd(II), In(III), Sn(II), Sn(IV), Sb(III), and U(VI) from aqueous KI/H2SO4 solution into a mixture of 4-methyl-2-pentanone (methyl isobutyl ketone, MIBK) and cyclohexanone (CHO) were studied. Both single-step batch and SISAK2 methods were used. The oxidation of Sn(II) to Sn(IV) by iodine and complexation of Sn(IV) by 2,3-dimercapto-propanol-1 (BAL) were also investigated. A method for rapid and continuous separation of indium from tin was developed for investigation of short-lived indium fission products.

The Spallator and APEX Nuclear Fuel Cycle a New Option for Nuclear Power

A new nuclear fuel cycle is described which provides a long term supply of nuclear fuel for the thermal LWR nuclear power reactors and eliminates the need for long-term storage of radioactive waste. Fissile fuel is produced by the Spallator which depends on the production of spallation neutrons by the interaction of high energy (1 to 2 GeV) protons on a heavy metal target. The neutrons are absorbed in a surrounding natural uranium or thorium blanket in which fissile Pu-239 or U-233 is produced. Advances in linear accelerator technology makes it possible to design and construct a high beam current continuous wave proton linac for production purposes. The target is similar to a sub-critical reactor and produces heat which is converted to electricity for supplying the linac. The Spallator is a self-sufficient fuel producer, which can compete with the fast breeder. The APEX fuel cycle depends on recycling the transuranics and long-lived fission products while extracting the stable and short-lived fission products when reprocessing the fuel. Transmutation and decay within the fuel cycle and decay of the short-lived fission products external to the fuel cycle eliminates the need for long-term geological age storage of fission product waste.

The Use of Beta-Strength-Function Data to Obtain Average Decay Energy Values for Short-Lived Fission Product Nuclides

In this paper, the authors point out that data from earlier experiments carried out to measure beta-strength functions for short-lived fission products can also be used to provide average beta- and gamma-decay energy values for these nuclides. In their evaluation of decay data for the ENDF/B-V fission product file, the authors have used this approach as a means of deducing average decay energy values for a number of these isotopes for which experimentally based average values would otherwise not have been available. The methods employed are discussed, and the results for the average beta-decay energies per decay, (E..beta..), are presented. Where available, (E..beta..) values deduced from decay scheme studies and from direct beta-spectrum measurements are given for purposes of comparison. Evidence is presented that suggests that the conventional decay scheme studies may not be a reliable source of average decay energy data for nuclides with large Q..beta.. values. The authors propose that different types of experimental measurements, possibly involving total absorption techniques (of which the beta-strength work treated here might be considered as one example), may provide a better means of producing this important information.