Dose distribution measurements and calculations for Dounreay hot particles

Discrete fragments of irradiated nuclear fuel have been discovered on the foreshore at the Dounreay nuclear site in Scotland, offshore on the seabed and at nearby beaches which have public access. The fragments contain mainly (137)Cs and (90)Sr/(90)Y and for particles recovered to date, (137)Cs activities are within the range of 10(3) to 10(8) Bq. The most active particles found at Sandside Beach contain approximately 3 x 10(5)Bq (137)Cs. Direct measurements of the spatial dose distributions from 37 fuel fragments were measured in detail for the first time using radiochromic dye film as part of a national evaluation of the associated potential radiological hazard. Monte Carlo code calculations of the doses are in good agreement with measurements, taking into account variations to be expected due to differences in shape and the increasing importance of self-absorption for the larger, more active fragments. Dose measurements provide little evidence for wide variations in the (137)Cs:(90)Sr/(90)Y ratio between fragments. Specific attention is given to the evaluation of skin dose, averaged over an area of 1 cm(2) at a depth of 0.07 mm, since this is of major radiological concern. There is no obvious dependence of skin dose on the site of origin of the fragments (foreshore, seabed or beaches) for a given (137)Cs activity level. A dose rate survey instrument (SmartION) was shown to provide a rapid and convenient method for skin dose assessment from fuel fragments in the (137)Cs activity range measured (2 x 10(5) to 2 x 10(7) Bq). A conversion factor multiplier of 240 can be applied to the open window SmartION scale reading to estimate the skin dose rate within +/-25%.     

Energy release rate calculations for an interface mode III edge crack based on a conservation integral

The M-integral is applied to the calculation of energy release races for interface edge cracks of the Mode III type. Specifically, for an edge crack along the interface between two elastic wedges of different opening angles and dissimilar elastic properties, and that is subjected to point loads at the apex, a relation is derived among the length of the crack, the energy release race of the crack, the applied loads, the wedge angles and the material parameters.     

Energy release rate calculations for the compact tension specimen using brittle orthotropic materials

An energy release rate calculation is performed using the modified crack closure integral (MCCI) method applied to the compact tension (CT) specimen. Mechanical properties used are typical of short-fiber sheet molding compound (SMC) materials. Using orthotropic linear constant strain finite elements, the classical solution is closely approximated with a relatively coarse grid. The wedge opening of the CT specimen is shown to lower the expected energy release rate by 6·1%, introducing a systematic error. Orientation of the SMC material near the load application point perpendicular to the applied load will raise the energy release rate by 12·0%. The CT specimen results are shown to be insensitive to the distribution of the applied load around the hole periphery. A probabilistic method is developed for converting the distribution of applied failure load to the distribution of critical energy release rate. This method is applied to predict the distribution of failure load for other geometries.     

Dropout-error-rate calculations for a maximum-density study

This paper describes an experimental method for determining the maximum areal density achievable for a given head-to-tape interface (HTI). Using this method, we can calculate amplitude and bit-shift error rates during dropouts for the areal density the HTI is originally designed for, and beyond. First, the read-back waveform is sampled to obtain the amplitude distribution of signal samples below 50% clip level at each areal density. Second, the channel-transfer-function variation is investigated during a dropout to study its effect on the bitshift error rate. Third, empirical relationships between the areal density and an rms-to-rms signal-to-noise ratio (SNR) are obtained. Based on the above information, a theoretical model is presented that predicts the error rate for each areal density after a write skip (defined here as the soft-error rate). The calculated soft-error rate agrees well with its experimental counterpart. The technique presented here is useful in determining the upper limit of the HTI used in the paper.     

Benchmarking of Monte Carlo based shutdown dose rate calculations for applications to JET

The calculation of dose rates after shutdown is an important issue for operating nuclear reactors. A validated computational tool is needed for reliable dose rate calculations. In fusion reactors neutrons induce high levels of radioactivity and presumably high doses. The complex geometries of the devices require the use of sophisticated geometry modelling and computational tools for transport calculations. Simple rule of thumb laws do not always apply well. Two computational procedures have been developed recently and applied to fusion machines. Comparisons between the two methods showed some inherent discrepancies when applied to calculation for the ITER while good agreement was found for a 14 MeV point source neutron benchmark experiment. Further benchmarks were considered necessary to investigate in more detail the reasons for the different results in different cases. In this frame the application to the Joint European Torus JET machine has been considered as a useful benchmark exercise. In a first calculational benchmark with a representative D-T irradiation history of JET the two methods differed by no more than 25%. In another, more realistic benchmark exercise, which is the subject of this paper, the real irradiation history of D-T and D-D campaigns conducted at JET in 1997-98 were used to calculate the shut-down doses at different locations, irradiation and decay times. Experimental dose data recorded at JET for the same conditions offer the possibility to check the prediction capability of the calculations and thus show the applicability (and the constraints) of the procedures and data to the rather complex shutdown dose rate analysis of real fusion devices. Calculation results obtained by the two methods are reported below, comparison with experimental results give discrepancies ranging between 2 and 10. The reasons of that can be ascribed to the high uncertainty on the experimental data and the unsatisfactory JET model used in the calculation. A new dedicated JET benchmark experiment will be performed trying to solve these issues.     

Energy release rate calculations by the finite element method

The present paper outlines a finite element method for calculating the energy release rate. The method is based on a continuum mechanics formulation of the virtual crack extension principle and can be used with linear elastic materials as well as materials following the deformation theory of plasticity. The formulation is easily incorporated into a finite element program, but can also conveniently be used as part of a post-processing program, which uses stress and displacement data from a finite element analysis to calculate the energy release rate. The present formulation can be used as a unified approach for 2-D and 3-D fracture problems and includes the effect of body forces and traction loading on the crack faces.     

Effect of delamination face overlapping on strain energy release rate calculations

The strain energy release rate is often used as a fracture mechanics parameter to describe delamination propagation and onset in composites, and is conveniently evaluated using finite element analysis. A common problem encountered in analysis is overlapping (interpenetration) of the delamination faces if these faces are not constrained. This paper examines the effect of overlapping on strain energy release rate calculated using the virtual crack closure method in conjunction with 3-dimensional anisotropic finite element analysis. A bilinear gap element was used between the delamination faces to prevent these faces from overlapping. Three problems were studied: (1) laminates with an embedded crack and embedded delaminations; (2) laminates with free edge delaminations; and (3) end-notched flexure specimens. The results of this investigation indicated that overlapping has a significant effect on the component values of G_I and G_II as well as on the total strain energy release rate. It was also found that end effects can create non-uniform energy release rates along the crack front in edge delamination problems due to twisting of the sublaminate with unsymmetrical lay-up, so that 3-dimensional finite element analysis is required.     

Analytical shielding calculations for a proton therapy facility

The University of Pennsylvania is building a proton therapy facility in collaboration with Walter Reed Army Medical Center. The proposed facility has four gantry rooms, a fixed beam room and a research room, and will use a cyclotron as the source of protons. In this study, neutron shielding considerations for the proposed proton therapy facility were investigated using analytical techniques and Monte Carlo simulated neutron spectra. Neutron spectra calculations were done using the GEANT4 (v6.2) simulation code for various materials: water, carbon, iron, nickel and tantalum to estimate the neutron production at proton beam energies of 100, 175 and 250 MeV. Dose equivalent calculations were performed using analytical methods at various critical points within the facility, by folding the GEANT4 produced neutron spectra with dose equivalent rate data from the National Council on Radiation Protection and Measurements (NCRP) Report #144.     

Dose rate dependency of electronic personal dosemeters measuring X- and gamma-ray radiation

Three models of electronic personal dosemeters (EPDs)-Siemens Mk 2.3, Rados RAD-60S and Vertec Bleeper Sv-were irradiated with seven photon beam qualities: 60Co, 137Cs and the ISO narrow spectrum series X-ray qualities N-250, N-200, N-150, N-60 and N-20. The personal dose equivalent rates delivered to the devices varied between 0.002 and 0.25 mSv s(-1). Measurements were made with the EPDs mounted free-in-air as well as against Lucite and water phantoms. Results for all models of EPDs showed differences in personal dose equivalent energy response for different energies covered by this range of radiation qualities, with different models showing variations from 15 to 65%. In some cases, the personal dose equivalent rate response of these devices varied by a factor of 3 between irradiations at typical calibration dose rates and those normally encountered by nuclear energy workers.     

Two-dimensional position recovery for a free-ranging automatedguided vehicle

An absolute position recovery method for an automated guided vehicle (AGV) which can move in any direction (free-ranging) on the floor using vision is introduced. The floor is permanently encoded with the terms of a pseudorandom binary array requiring only one bit of code per quantization interval, independent of the desired resolution     

Loading constraints for a multi-compartment vehicle routing problem

OR Spectrum - Multi-compartment vehicles (MCVs) can deliver several product segments jointly. Separate compartments are necessary as each product segment has its own specific characteristics and...     

The effect of freezing rate on the accuracy of numerical freezing calculations

The accuracy of finite difference and finite element freezing time predictions was examined. The accuracy of the predictions was shown to depend on the rate of freezing, with the extent of underprediction increasing as the rate of freezing increased. A feasible explanation for this is that the deviation of the actual physical behaviour from the models used is more significant at higher rates. The accuracy of predictions is therefore limited more by the physical approcimations than by numerical approximations. Only a marginal improvement, if any, would be expected by elaborating the numerical schemes. Until the physical behaviour of food freezing can be more accurately quantified, predictions to better than ±10% appear, therefore, impracticable. Such accuracy can be obtained by a simple prediction method.     

Dose rate assessment of terrestrial gamma radiation in the Delta region of Nigeria

In situ gamma spectroscopy has been employed to characterise natural radiation in the soil at 32 locations in the Delta region of Nigeria. The outdoor gamma dose rates in the air due to 40K, 238U and 232Th in the soil were 1.5 +/- 0.9 nGy h(-1), 6.9 +/- 1.6 nGy h(-1) and 16.3 +/- 3.1 nGy h(-1) respectively. The estimated total annual outdoor effective dose to the people in the region was 31.6 microSv y(-1).     

Dose rate effect on micronuclei induction in cytokinesis blocked human peripheral blood lymphocytes

The effect of dose rate on the induction of micronuclei (MN) in human peripheral blood lymphocytes was investigated over a range of dose rates from 0.125 Gy h(-1) to 178.2 Gy h(-1). The response of MN induction was fitted with a linear quadratic model and the alpha and beta coefficients were estimated. It was found that beta values decrease with decreasing dose rate as in the case of chromosomal aberration. At the dose rate of 0.125 Gy h(-1), pure linear response of MN induction was observed. An attempt was made to simulate the calibration curve for the purpose of biological dosimetry at different dose rates and exposure times. The yields when simulated with the exposure time or the dose rate are in agreement with experimental results.