Dose Rate Reduction Research Articles

Shielding performance of newly developed boron-loaded concrete for DT neutrons

ABSTRACT In order to enhance the neutron-shielding performance of concrete in neutron-generation facilities from the viewpoint of reduction of effective dose rates in operation and radioactive wastes in decommission, we developed concrete with boron of more than 10 wt%. We performed a neutron-shielding experiment using the mockup of the newly developed boron-loaded concrete and Deuterium Tritium (DT)neutrons at the Fusion Neutronics Source in the Japan Atomic Energy Agency, and measured the reaction rates of 93Nb(n,2n)92mNb and 197Au(n,γ)198Au reactions in the mockup. The calculations were conducted using MCNP-5.14 and FENDL-2.1. The calculation results agreed well with the measured ones, and we confirmed that the accuracy was very good on the atomic composition data of boron-loaded concrete and their nuclear data. In addition, we calculated the neutron and photon effective dose rates and reaction rates of 59Co(n,γ)60Co and 151Eu(n,γ)152Eu reactions, which produce critical radioisotopes in decommission, in boron-loaded concrete and other concretes with DT neutrons, and the experimental condition presently used. The dose rates and reaction rates were drastically reduced by using boron-loaded concrete and it was concluded that boron-loaded concrete had very good shielding performance for DT neutrons.

Performances of Kevlar and Polyethylene as radiation shielding on-board the International Space Station in high latitude radiation environment

Passive radiation shielding is a mandatory element in the design of an integrated solution to mitigate the effects of radiation during long deep space voyages for human exploration. Understanding and exploiting the characteristics of materials suitable for radiation shielding in space flights is, therefore, of primary importance. We present here the results of the first space-test on Kevlar and Polyethylene radiation shielding capabilities including direct measurements of the background baseline (no shield). Measurements are performed on-board of the International Space Station (Columbus modulus) during the ALTEA-shield ESA sponsored program. For the first time the shielding capability of such materials has been tested in a radiation environment similar to the deep-space one, thanks to the feature of the ALTEA system, which allows to select only high latitude orbital tracts of the International Space Station. Polyethylene is widely used for radiation shielding in space and therefore it is an excellent benchmark material to be used in comparative investigations. In this work we show that Kevlar has radiation shielding performances comparable to the Polyethylene ones, reaching a dose rate reduction of 32 ± 2% and a dose equivalent rate reduction of 55 ± 4% (for a shield of 10 g/cm2).

Predictions of space radiation fatality risk for exploration missions

In this paper we describe revisions to the NASA Space Cancer Risk (NSCR) model focusing on updates to probability distribution functions (PDF) representing the uncertainties in the radiation quality factor (QF) model parameters and the dose and dose-rate reduction effectiveness factor (DDREF). We integrate recent heavy ion data on liver, colorectal, intestinal, lung, and Harderian gland tumors with other data from fission neutron experiments into the model analysis. In an earlier work we introduced distinct QFs for leukemia and solid cancer risk predictions, and here we consider liver cancer risks separately because of the higher RBE's reported in mouse experiments compared to other tumors types, and distinct risk factors for liver cancer for astronauts compared to the U.S. population. The revised model is used to make predictions of fatal cancer and circulatory disease risks for 1-year deep space and International Space Station (ISS) missions, and a 940 day Mars mission. We analyzed the contribution of the various model parameter uncertainties to the overall uncertainty, which shows that the uncertainties in relative biological effectiveness (RBE) factors at high LET due to statistical uncertainties and differences across tissue types and mouse strains are the dominant uncertainty. NASA's exposure limits are approached or exceeded for each mission scenario considered. Two main conclusions are made: 1) Reducing the current estimate of about a 3-fold uncertainty to a 2-fold or lower uncertainty will require much more expansive animal carcinogenesis studies in order to reduce statistical uncertainties and understand tissue, sex and genetic variations. 2) Alternative model assumptions such as non-targeted effects, increased tumor lethality and decreased latency at high LET, and non-cancer mortality risks from circulatory diseases could significantly increase risk estimates to several times higher than the NASA limits.

The Optimization of Radiation Protection in the Design of the High Temperature Reactor-Pebble-Bed Module

The optimization of radiation protection is an important task in both the design and operation of a nuclear power plant. Although this topic has been considerably investigated for pressurized water reactors, there are very few public reports on it for pebble-bed reactors. This paper proposes a routine that jointly optimizes the system design and radiation protection of High Temperature Reactor-Pebble-Bed Module (HTR-PM) towards the As Low As Reasonably Achievable (ALARA) principle. A systematic framework is also established for the optimization of radiation protection for pebble-bed reactors. Typical calculations for the radiation protection of radioactivity-related systems are presented to quantitatively evaluate the efficiency of the optimization routine, which achieve 23.3%~90.6% reduction of either dose rate or shielding or both of them. The annual collective doses of different systems are reduced through iterative optimization of the dose rates, designs, maintenance procedures, and work durations and compared against the previous estimates. The comparison demonstrates that the annual collective dose of HTR-PM is reduced from 0.490 man-Sv/a before optimization to 0.445 man-Sv/a after optimization, which complies with the requirements of the Chinese regulatory guide and proves the effectiveness of the proposed routine and framework.

The impact of cobalt-60 source age on biologically effective dose in high-dose functional Gamma Knife radiosurgery.

OBJECTIVE Functional Gamma Knife radiosurgery (GKRS) procedures have been increasingly used for treating patients with tremor, trigeminal neuralgia (TN), and refractory obsessive-compulsive disorder. Although its rates of toxicity are low, GKRS has been associated with some, if low, risks for serious sequelae, including hemiparesis and even death. Anecdotal reports have suggested that even with a standardized prescription dose, rates of functional GKRS toxicity increase after replacement of an old cobalt-60 source with a new source. Dose rate changes over the course of the useful lifespan of cobalt-60 are not routinely considered in the study of patients treated with functional GKRS, but these changes may be associated with significant variation in the biologically effective dose (BED) delivered to neural tissue. METHODS The authors constructed a linear-quadratic model of BED in functional GKRS with a dose-protraction factor to correct for intrafraction DNA-damage repair and used standard single-fraction doses for trigeminal nerve ablation for TN (85 Gy), thalamotomy for tremor (130 Gy), and capsulotomy for obsessive-compulsive disorder (180 Gy). Dose rate and treatment time for functional GKRS involving 4-mm collimators were derived from calibrations in the authors' department and from the cobalt-60 decay rate. Biologically plausible values for the ratio for radiosensitivity to fraction size (α/β) and double-strand break (DSB) DNA repair halftimes (τ) were estimated from published experimental data. The biphasic characteristics of DSB repair in normal tissue were accounted for in deriving an effective τ1 halftime (fast repair) and τ2 halftime (slow repair). A sensitivity analysis was performed with a range of plausible parameter values. RESULTS After replacement of the cobalt-60 source, the functional GKRS dose rate rose from 1.48 to 2.99 Gy/min, treatment time fell, and estimated BED increased. Assuming the most biologically plausible parameters, source replacement resulted in an immediate relative BED increase of 11.7% for GKRS-based TN management with 85 Gy, 15.6% for thalamotomy with 130 Gy, and 18.6% for capsulotomy with 180 Gy. Over the course of the 63-month lifespan of the cobalt-60 source, BED decreased annually by 2.2% for TN management, 3.0% for thalamotomy, and 3.5% for capsulotomy. CONCLUSIONS Use of a new cobalt-60 source after replacement of an old source substantially increases the predicted BED for functional GKRS treatments for the same physical dose prescription. Source age, dose rate, and treatment time should be considered in the study of outcomes after high-dose functional GKRS treatments. Animal and clinical studies are needed to determine how this potential change in BED contributes to GKRS toxicity and whether technical adjustments should be made to reduce dose rates or prescription doses with newer cobalt-60 sources.

BWR water chemistry guidelines and PWR primary water chemistry guidelines in Japan – Purpose and technical background

After 40years of light water reactor (LWR) operations in Japan, the sustainable development of water chemistry technologies has aimed to ensure the highest coolant system component integrity and fuel reliability performance for maintaining LWRs in the world; additionally, it aimed to achieve an excellent dose rate reduction. Although reasonable control and diagnostic parameters are utilized by each boiling water reactor (BWR) and pressurized water reactor (PWR) owner, it is recognized that specific values are not shared among everyone involved.To ensure the reliability of BWR and PWR operation and maintenance, relevant members of the Atomic Energy Society of Japan (AESJ) decided to establish guidelines for water chemistry. The Japanese BWR and PWR water chemistry guidelines provide strategies to improve material and fuel reliability performance as well as to reduce dosing rates. The guidelines also provide reasonable “control values”, “diagnostic values” and “action levels” for multiple parameters, and they stipulate responses when these levels are exceeded. Specifically, “conditioning parameters” are adopted in the Japanese PWR primary water chemistry guidelines. Good practices for operational conditions are also discussed with reference to long-term experience.This paper presents the purpose, technical background and framework of the preliminary water chemistry guidelines for Japanese BWRs and PWRs. It is expected that the guidelines will be helpful as an introduction to achieve safety and reliability during operations.

Temporal variation of dose rate distribution around the Fukushima Daiichi nuclear power station using unmanned helicopter

Aerial radiological survey using an unmanned aerial vehicle (UAV) was applied to measurement surface contamination around the Fukushima Daiichi nuclear power station (FDNPS). An unmanned helicopter monitoring system (UHMS) was developed to survey the environmental effect of radioactive cesium scattered as a result of the FDNPS accident. The UHMS was used to monitor the area surrounding the FDNPS six times from 2012 to 2015. Quantitative changes in the radioactivity distribution trend were revealed from the results of these monitoring runs. With this information, we found that the actual reduction of dose rate was faster than the one calculated with radiocesium physical half-life. It is indicated that the attenuation effect of radiation by radiocesium penetration in soil is dominant as for reason of reduction of dose rate.

P3. The influence of the anti-scatter grid in paediatric screening

Introduction The anti-scatter grid is of great use when imaging in fluoroscopy. The grid absorbs the scattered radiation in an attempt to improve image quality. The use of the grid is however accompanied with an increase in dose to the patient. The aim of this study was to assess if there is a significant difference in image quality and dose rate for small patient imaging if the anti-scatter grid is in the beam as to if it were removed. Materials and methods Slabs of 1 cm of Perspex were used to simulate different thicknesses of patients. The TOR F18 Leeds test object was placed between the Perspex slabs and used to assess the resolution and the contrast in the image. The dose rate on the surface of the patient was also measured for different thicknesses of Perspex with a PTW Nomex multimeter. Resolution, contrast, dose rate to the surface of the Perspex, kV and mA were measured for Perspex thickness intervals of 1 cm and for the grid in and out of the beam. Results Results show that that when removing the grid for Perspex thicknesses up to 5 cm the relative contrast loss is 20% and lower, for thicknesses 6–10 cm, 20–40% and above 10 cm, more than 50%. Spatial resolution loss of 10% and lower was registered for thicknesses of smaller than 8 cm. The percentage difference in mA for 5 cm of Perspex was 44.6% at a constant kV. Conclusion For patients with thickness of 5 cm or less, average patient of age 1 year, the anti-scatter grid may be removed with an inconsequential reduction in image quality and a 44.6% reduction in dose rate.

Evaluation of forest decontamination using radiometric measurements

An experiment has been conducted to evaluate the additional dose reduction by clear felling contaminated forestry in Fukushima Prefecture, Japan, and using the timber to cover the areas with wood chips. A portable gamma spectrometry system, comprising a backpack containing a 3 × 3″ NaI(Tl) detector with digital spectrometer and GPS receiver, has been used to map dose rate and radionuclide activity concentrations before, after and at stages during this experiment. The data show the effect of the different stages of the experiment on dose rate at different locations around the site. The spectrometric data have allowed the assessment of the contributions of natural and anthropogenic radionuclides to the dose rate at different parts of the site before and after the experiment. This has clearly demonstrated the value of radiometric methods in evaluating remediation, and the effect of other environmental processes. The value of spectrometric methods which directly measure radionuclide concentrations has also been shown, especially through the identification of the contribution of natural and anthropogenic activity to the measured dose rate. The experiment has shown that clearing trees and applying wood chips can reduce dose rates by 10–15% beyond that achieved by just clearing the forest litter and natural redistribution of radiocaesium.

Characterization of the radiation environment at the UNLV accelerator facility during operation of the Varian M6 linac

The bremsstrahlung photon flux from the UNLV particle accelerator (Varian M6 model) was determined using MCNP5 code for 3MeV and 6MeV incident electrons. Human biological equivalent dose rates due to accelerator operation were evaluated using the photon flux with the flux-to-dose conversion factors. Dose rates were computed for the accelerator facility for M6 linac use under different operating conditions. The results showed that the use of collimators and linac internal shielding significantly reduced the dose rates throughout the facility. It was shown that the walls of the facility, in addition to the earthen berm enveloping the building, provide equivalent shielding to reduce dose rates outside to below the 2mrem/h limit.

Preventing Complications from High-Dose Rate Brachytherapy when Treating Mobile Tongue Cancer via the Application of a Modular Lead-Lined Spacer.

PurposeTo point out the advantages and drawbacks of high-dose rate brachytherapy in the treatment of mobile tongue cancer and indicate the clinical importance of modular lead-lined spacers when applying this technique to patients.MethodsFirst, all basic steps to construct the modular spacer are shown. Second, we simulate and evaluate the dose rate reduction for a wide range of spacer configurations.ResultsWith increasing distance to the source absorbed doses dropped considerably. Significantly more shielding was obtained when lead was added to the spacer and this effect was most pronounced on shorter (i.e. more clinically relevant) distances to the source.ConclusionsThe modular spacer represents an important addition to the planning and treatment stages of mobile tongue cancer using HDR-ISBT.

Space Radiation Quality Factors and the Delta Ray Dose and Dose-Rate Reduction Effectiveness Factor.

In this paper, the authors recommend that the dose and dose-rate effectiveness factor used for space radiation risk assessments should be based on a comparison of the biological effects of energetic electrons produced along a cosmic ray particles path in low fluence exposures to high dose-rate gamma-ray exposures of doses of about 1 Gy. Methods to implement this approach are described.

Study of shielding options for lower ports for mitigation of neutron environment and shutdown dose inside the ITER cryostat

Mitigation of the neutron environment inside the cryostat, and of the subsequent decay gamma dose field from activated materials, is necessary in order to reduce heating of coils and occupational exposure, thereby facilitating smooth operation and maintenance of ITER. Several lines of action are currently being explored to mitigate crucial contributions, such as the leakage through the lower ports. Results are presented here for the two types of lower ports in ITER: cryopump ports and remote-handling ports. Different shielding configurations and material options are investigated and compared in terms of neutron attenuation, coil heating and shutdown dose rate reduction, whilst also considering other engineering constraints such as weight or pumping power. Results enable informed decision-making of best compromise solutions for subsequent design and integration.

Effect of patient bladder voiding on radiation dose rates measured around patients undergoing PET/CT imaging using 18F-FDG

Accurate dose rate estimates is important for radiation protection specialists conducting risk assessments and performing dose reconstruction in cases of accidental exposures. Objectives: The objectives of this work was to experimentally evaluate the bladder voiding factor effect on the dose rate measured from patients undergoing PET/CT imaging studies using 18F-FDG by directly measuring the dose rate immediately before and after voiding, and compare the results with the current scientific literature. Results: The bladder voiding effect had a dose rate reduction factor of about 12% between dose rates measured before and after voiding. This measured reduction factor agreed with the 15% reported by the AAPM Task Group 108. We have also measured dose rates at one meter from 50 patients and found an average dose rate per unit activity of 93.7 μSv/hr/GBq. Our dose rate results were in excellent agreement with the results of current published data (92 μSv/hr/GBq, AAPM Task Group 108). Conclusions: The presented data can be applied in radiation protection optimization procedures, especially for the protection of the care givers from patients undergoing 18F-FDG PET/CT imaging when they are considered as external radiation source or hazard to others. The provided information will benefit medical physicist working in nuclear medicine and radiation safety policy makers.

Cobalt Radioactivity Behaviors in a BWR Environment and Countermeasures for Dose Rate Reduction

While under normal water chemistry without any specific metal ions in reactor coolant a high electrochemical corrosion potential caused by highly oxidizing species such as hydrogen peroxide promotes the formation of hematite film on piping surfaces with a densely packed film structure, the presence of a certain amount of nickel ions prevents the magnetite film from changing to hematite by forming a nickel ferrite. This formation of nickel ferrite instead of hematite accelerates cobalt buildup, and this is especially notable for carbon steel. The observed reduction of radioactivity concentration in reactor water by zinc injection or by nickel/iron ratio control can be explained by the role of zinc or nickel in preventing the film on the fuel rod surfaces from changing to hematite, thereby stabilizing the cobalt activity on this surface. A thermodynamic evaluation suggests that zinc ferrite is more stable than cobalt ferrite only when the ratio of cobalt to zinc divalent ions, [Co2+]/[Zn2+], is <0.011 in molar units. This ratio is consistent with the ratio of 60Co activity to zinc concentration commonly used in industry to control reactor water zinc levels for a dose rate reduction under the hydrogen water chemistry condition. Based on the present understanding of radioactivity behaviors, the actual radiation dose reduction methods are classified into the several groups and summarized from the viewpoint of the interaction between the oxide and various metal ions.

Assessing cost and effectiveness of radiation decontamination in Fukushima Prefecture, Japan

Despite the enormous cost of radiation decontamination in Fukushima Prefecture, it is not clear what levels of reduction in external radiation exposure are possible in the Special Decontamination Area, the Intensive Contamination Survey Areas and the whole of Fukushima. The objective of this study was to evaluate the cost and effectiveness of radiation decontamination in Fukushima Prefecture in its entirety. Using a geographic information system, we calculated the costs of removal, storage containers, transport, and temporary and interim storage facilities as well as the reduction in air dose rate for a cumulative external exposure for 9000 1 km × 1 km mesh units incorporating 51 municipalities. The decontamination cost for the basic scenario, for which forested areas within 20 m of habitation areas were decontaminated, was JPY2.53–5.12 trillion; the resulting reduction in annual external dose was about 2500 person-Sv. The transport, storage, and administrative costs of decontamination waste and removed soil reached JPY1.55–2.12 trillion under this scenario. Although implementing decontamination of all forested areas provides some major reductions in the external radiation dose for the average inhabitant, decontamination costs could potentially exceed JPY16 trillion. These results indicate that technologies for reducing the volume of decontamination waste and removed soil should be considered to reduce storage costs and that further discussions about forest decontamination policies are needed.

A new approach to reduce uncertainties in space radiation cancer risk predictions.

The prediction of space radiation induced cancer risk carries large uncertainties with two of the largest uncertainties being radiation quality and dose-rate effects. In risk models the ratio of the quality factor (QF) to the dose and dose-rate reduction effectiveness factor (DDREF) parameter is used to scale organ doses for cosmic ray proton and high charge and energy (HZE) particles to a hazard rate for γ-rays derived from human epidemiology data. In previous work, particle track structure concepts were used to formulate a space radiation QF function that is dependent on particle charge number Z, and kinetic energy per atomic mass unit, E. QF uncertainties where represented by subjective probability distribution functions (PDF) for the three QF parameters that described its maximum value and shape parameters for Z and E dependences. Here I report on an analysis of a maximum QF parameter and its uncertainty using mouse tumor induction data. Because experimental data for risks at low doses of γ-rays are highly uncertain which impacts estimates of maximum values of relative biological effectiveness (RBEmax), I developed an alternate QF model, denoted QFγAcute where QFs are defined relative to higher acute γ-ray doses (0.5 to 3 Gy). The alternate model reduces the dependence of risk projections on the DDREF, however a DDREF is still needed for risk estimates for high-energy protons and other primary or secondary sparsely ionizing space radiation components. Risk projections (upper confidence levels (CL)) for space missions show a reduction of about 40% (CL∼50%) using the QFγAcute model compared the QFs based on RBEmax and about 25% (CL∼35%) compared to previous estimates. In addition, I discuss how a possible qualitative difference leading to increased tumor lethality for HZE particles compared to low LET radiation and background tumors remains a large uncertainty in risk estimates.

Study of neutron attenuation properties of super alloys with added rhenium

In this paper, the new generation of rhenium enriched super alloys with enhanced neutron attenuation properties was investigated. These materials provided improved neutron shielding capabilities compared to other shielding materials such as water, concrete, etc. The GEANT4 code was used to simulate transport of Am–Be source neutrons in new shielding materials. Neutron equivalent dose rate measurements were carried out to obtain the dose rate reduction. It was demonstrated that new rhenium enriched alloys are feasible for neutron shielding applications.

Radiobiology of tissue reactions.

Tissue effects of radiation exposure are observed in virtually all normal tissues, with interactions when several organs are involved. Early reactions occur in turnover tissues, where proliferative impairment results in hypoplasia; late reactions, based on combined parenchymal, vascular, and connective tissue changes, result in loss of function within the exposed volume; consequential late effects develop through interactions between early and late effects in the same organ; and very late effects are dominated by vascular sequelae. Invariably, involvement of the immune system is observed. Importantly, latent times of late effects are inversely dependent on the biologically equieffective dose. Each tissue component and--importantly--each individual symptom/endpoint displays a specific dose-effect relationship. Equieffective doses are modulated by exposure conditions: in particular, dose-rate reduction--down to chronic levels--and dose fractionation impact on late responding tissues, while overall exposure time predominantly affects early (and consequential late) reactions. Consequences of partial organ exposure are related to tissue architecture. In 'tubular' organs (gastrointestinal tract, but also vasculature), punctual exposure affects function in downstream compartments. In 'parallel' organs, such as liver or lungs, only exposure of a significant (organ-dependent) fraction of the total volume results in clinical consequences. Forthcoming studies must address biomarkers of the individual risk for tissue reactions, and strategies to prevent/mitigate tissue effects after exposure.

Low-radiation environment affects the development of protection mechanisms in V79 cells.

Very little is known about the influence of environmental radiation on living matter. In principle, important information can be acquired by analysing possible differences between parallel biological systems, one in a reference-radiation environment (RRE) and the other in a low-radiation environment (LRE). We took advantage of the unique opportunity represented by the cell culture facilities at the Gran Sasso National Laboratories of the Istituto Nazionale di Fisica Nucleare, where environment dose rate reduction factors in the underground (LRE), with respect to the external laboratory (RRE), are as follows: 10(3) for neutrons, 10(7) for directly ionizing cosmic rays and 10 for total γ-rays. Chinese hamster V79 cells were cultured for 10months in both RRE and LRE. At the end of this period, all the cultures were kept in RRE for another 6months. Changes in the activities of antioxidant enzymes (superoxide dismutase, SOD; catalase, CAT; glutathione peroxidase, GPX) and spontaneous mutation frequency at the hypoxanthine-guanine phosphoribosyl transferase (hprt) locus were investigated. The results obtained suggest that environmental radiation might act as a trigger of defence mechanisms in V79 cells, specifically those in reference conditions, showing a higher degree of defence against endogenous damage as compared to cells grown in a very low-radiation environment. Our findings corroborate the hypothesis that environmental radiation contributes to the development of defence mechanisms in today living organisms/systems.