Radiation Shielding Requirements Research Articles

An Optimal Radiation Shield Design of a Small Modular Nuclear Power Reactor by Genetic Algorithm

One of the critical concerns in developing small modular nuclear reactors (SMRs) is employing efficient radiation shielding consistent with the design and implementation requirements. Besides observing the radiation shielding requirements, the design process entails developing compact, lightweight shielding as well as ensuring the safety of the staff and radiation-sensitive equipment around the reactor under different operating conditions. Conventional methods of radiation shielding design are based on experiments conducted. Therefore, the resultant design involves trial and error, rarely achieving an optimal, effective, and efficient design. In this study, a multi-objective method based on the genetic algorithm coupled with the MNCP calculation code has been used to enhance the radiation shielding system of a SMR. Using this method, the thickness of different shielding layers has been optimized to minimize the total dose (neutrons and gamma) at the output, the weight, and the overall volume of the shielding. To assess and compare the method used to an implemented conventional method, a sample design of the MRX nuclear power reactor is considered. Using the optimization technique, the radiation shielding of the reactor has been redesigned and compared with the original design. This paper discusses the existing deficiencies in the initial design of the primary shield of the reactor as a design implemented based on conventional methods. Similar to the original design, the main materials considered for radiation shielding in consecutive layers were water and steel, and the final layer contained lead. The thickness of the different layers and their arrangements have been optimized to assess the upgraded method. The calculations indicate that the overall thickness of the proposed shielding is suggested as 93.8 cm compared to 140 cm to obtain the total dose of neutrons and gammas, which is suggested as less than 10 µSv/h. The results imply a 38.56% reduction in volume and a 17.24% reduction in weight of the radiation shielding compared to the reference reactor design.

Simulation of radiation environment and design of multilayer radiation shield for orbital exploration of Jupiter

The harsh radiation environment of Jupiter imposes significant constraints on the shield design of spacecraft. Due to these constraints, traditional aluminum shields cannot meet the radiation shielding requirements needed for the exploration of Jupiter. In this paper, the design for a highly efficient radiation shield structure is proposed that utilizes a combination of high-Z/low-Z elements. A typical exploration orbit of Jupiter is selected, and the energetic particle radiation environment is calculated using the D&G83 model. The Geant4 toolkit is used to evaluate the shielding performance of materials, and quantitative simulations are performed to analyze the shielding efficiency of bilayer and trilayer shield structures under an areal density constraint of 4.5 g/cm2. Based on the simulations, an optimal shield structure is determined. The simulation results show that the integral electron flux with energy greater than 3 MeV is approximately three orders of magnitude greater than that of geostationary orbit with an orbit of 4000 km at the perijove and 5420000 km at the apojove, resulting in an enormous total dose effect. The optimal shield structure is a bilayer structure that comprises of a tantalum outer layer with a density of 3.5 g/cm2 and a polyethylene inner layer with a density of 1.0 g/cm2. This shield has the ability to reduce the total radiation dose to 1559 Gy per year, which is 32 % lower than that of traditional aluminum shields. Findings in this research are critical for design of radiation shields, and can be used for performing reliability analyses and predicting the lifespan of spacecraft during the development process.

Nuclear scoping analysis of ITER bioshield top lid toward its preliminary design review

During ITER operations, electronics located in the crane hall, which is above the tokamak, will be exposed to neutron and photon fields from both the plasma and the activated water. To protect the electronics, the implementation of dedicated shielding on the crane hall platform and the bioshield top lid is required. The design demands optimisation attending to constructability, weight limits, and radiation shielding requirements. This work evaluates eight shielding configurations by assessment of the neutron flux and dose accumulated over 4700 h of operation at 500 MW for electronics protection. This corresponds to a neutron wall load of 0.3 MW a/m² as specified in the ITER Project Specification. An intermediate-source approach has been followed with SRC-UNED, considering all relevant radiation sources while minimising the computational time required. Results were presented at the top lid Conceptual Design Review aiming to support decision-making. Further optimisation has since been performed to reach a top lid proposal for its Preliminary Design Review. All outcomes show that radiation levels above the north and south crane hall platforms are compatible with the critical electronics requirements.

Space neutron radiation shielding property of continuous fiber and functional filler reinforced polymer composite using Monte Carlo simulation

For radiation shielding and lightweight requirements, fiber-reinforced polymer (FRP) composite has advantages in serving as a structure-shielding multifunctional material due to low-Z element nature and excellent mechanical properties. This study established the Monte Carlo simulation model of continuous fiber and filler reinforced epoxy composite to research neutron radiation shielding, which contains the microstructure of the composite. For validation, the numerical results of the proposed model were compared with the experimental results and found in good agreement. Then, the effects of fiber and filler distribution were studied. The neutron linear and mass attenuation coefficients of various composites were simulated to evaluate the shielding performance and lightweight effect. It shows that the boron compound is critical for shielding thermal neutrons, and the shielding effectiveness for fast neutrons mainly depends on the type of fiber. The structural design and stacking sequence of fibers and fillers significantly affect neutron shielding performance. Compared with Al-alloy, obvious low-weight advantage is obtained for shielding neutrons. The shielding mechanism and energy deposition inside composites were also analyzed.

A short Review of the ALARA Principle in Radiation Protection

Any activity involving radiation should be planned and conducted in such a way that the exposures to individuals are optimally low, taking into consideration the economic and cost aspects. This principle of optimization is called ALARA. ALARA stands for “As Low as Reasonably Achievable”. The ALARA principle states that the radiation exposures should be limited to a minimum level since the Ionizing radiation may produce biological effects in the exposed person. Background of ALARA Concept: The introduction of ALARA was a great philosophical change, as instead of working up to the permissible exposure limits, the idea was to keep exposures to levels as far below the limits as could reasonably be done, considering economic and technical factors. The linear-no-threshold (LNT) model, labeled is the basis of the ALARA protocol; if even small doses lead to slight increases in the risk of cancer than doses should be kept as low as possible. The concept of ALARA was first introduced in 1954 in the National Council on Radiation Protection & Measurements (NCRP) Report, and later in International Commission on Radiological Protection (ICRP) Publication 1 where it was initially called ALAP, the acronym for As Low As Practicable (ICRP 1959b). As per Atomic Energy Regulatory Board (AERB), Govt of India, all radiation work should be carried out in a preplanned and controlled manner so that the exposure to the workers and persons in and near sites of radiation use is kept as low as reasonably achievable (ALARA) and does not exceed the recommended limits. Suitable control measures shall be employed to minimize radiation exposure so that maximum benefits are derived with minimum radiological risk. Discussion: The ALARA principle appears simple but is often misinterpreted and inappropriately applied in radiation protection. It is often interpreted as “as low as possible” without taking into consideration the balancing factors. Considering radiation shielding requirements as an example excessive unnecessary shielding with materials such as lead, steel, or concrete is a waste of global resources. This in turn can cause other damages such as chemical harm and pollution to the environment, which have proven damages to human health. It is more meaningful to make use of the wasted resources for service improvements, such as reducing the charges to patients, investing in better medical technologies, and training of staff. Conclusion: The aims of radiation protection are (i) to avoid the occurrence of deterministic effects and (ii) to limit the risk of stochastic effects. ALARA protocol has successfully limited the exposure of radiation workers to impressively low levels of dose around 1 mSv. Since significant costs are incurred to comply with ALARA and the validity of the LNT model is unclear below doses of 100 mSv, more research on the health effects of low radiation doses is necessary before alternatives to ALARA can become viable.

Evaluation of Radiation Shielding Requirements and Self-shielding Characteristics for a Novel Radiosurgery System.

In order to provide safe operating conditions for radiation workers and the public at large, the radiation shielding for radiotherapy treatment rooms will have to be determined by an expert physicist according to the applicable radiation control regulations. A new radiosurgery system with integrated self-shielding, called the Zap-X, obviates the need for costly and complex radiation bunkers. Radiation levels in the vicinity of the ZAP-X radiosurgery system were acquired for a number of different operating conditions, and a 3D radiation dose distribution was measured for better visualization of hot spots and the general dose distribution. The radiation shielding requirements were evaluated according to the International and Chinese standards, respectively. While the integral self-shielding of the Zap-X was designed in accordance with international standards, it was found to be insufficient for Chinese standards. In order to meet the needs of the growing new generation of radiotherapy equipment, several suggestions for improvement of Chinese standards are proposed.

Radiation Shielding for Helical Tomotherapy Vault Design.

Purpose:The purpose of the present study is to carry out radiation shielding calculations to find out adequate thicknesses of protective barriers such as walls and ceiling based on minimum space required to house helical tomotherapy unit. This study also aim to derive expression for use factor and estimation of patient workload for tomotherapy facility for optimizing radiation shielding requirements.Materials and Methods:The basic definitions and formulae given in NCRP/IAEA reports were referred and modified for tomotherapy machine to calculate optimized shielding thicknesses requirements. Workload is estimated based on observations of patient treatments on tomotherapy machine and analysis of their treatment plan data. A mathematical expression is derived for calculating use factor in terms of beam divergence angle at source corresponding to field length, angle of source rotation about isocenter, and distance of primary barrier from isocenter. Radiation shielding requirement of protective barriers such as walls and ceiling of helical tomotherapy vault is calculated based on minimum room dimensions as specified by the manufacturer, permissible dose limit (s), and values of optimizing parameters such as workload, use factor etc. for tomotherapy machine.Results:Using derived mathematical expression for use factor in this study, it was found that value of use factor varies with distance of primary barrier from isocenter and its value was found to be 0.093 for given minimum room dimensions. Radiation shielding requirements for protective barriers (walls/ceiling, etc.) were arrived and reported in this paper.Conclusions:A typical helical tomotherapy vault design is proposed based on the calculated shielding thicknesses of protective barriers. Further, it is also concluded that tomotherapy machine can be installed in a vault designed for 6 MV conventional linear accelerator with minor modification.

Radiation shielding requirements for the full power operation of the linear IFMIF prototype accelerator (LIPAc) at Rokkasho

The detailed shielding analysis has been performed in order to find a configuration of the radiation shielding in the accelerator building necessary for the operation of the Linear IFMIF Prototype Accelerator (LIPAc), which is under construction in Rokkasho, Japan, at the full power of 1.125 MW to satisfy the radiation dose limit prescribed by Japanese regulations. The final design of the beam dump constructed by CIEMAT, Spain was fully taken into consideration. The impact of all the features potentially causing neutron streaming were investigated and the optimization of local additional shieldings were conducted. As the result, the most important one was found to be the air ventilation duct shield, which completely covers the ducts and creates an additional labyrinth. By considering the additional shielding configuration determined in the present study, the legal criteria at the controlled area boundary can be completely satisfied.

Multiple-Apogee Highly Elliptical Orbits for Continuous Meteorological Imaging of Polar Regions: Challenging the Classical 12-h Molniya Orbit Concept

AbstractA novel type of multiple-apogee highly elliptical orbits termed as MAP HEO with a period of rotation between 14 h and 15 h is introduced. These orbits are designed to achieve continuous geostationary (GEO)-like imaging of the polar regions in an optimum way. The combination of GEO and HEO satellites would then offer continuous monitoring of weather from space at any point of the globe. This capacity would represent a breakthrough for short- and long-term weather forecasting and narrowing uncertainties in the knowledge of the Earth’s climate through better sampling and more accurate characterization of the diurnal cycle. MAP HEO systems can be launched at critical inclination and are characterized by a local minimum of ionizing radiation. These features simplify the process of orbit maintenance, reduce radiation shielding requirements, and favor a longer lifetime of the mission. Unlike previously considered HEO systems implemented for communications, such as 12-h Molniya and 24-h Sirius radio systems, a MAP HEO constellation achieves a uniform geometrical sampling, which reduces view angle dependent biases. These observational conditions with complete coverage of the diurnal cycle, diverse range of solar illumination, and viewing observational conditions are beneficial for high-latitude meteorological and climate applications, such as the retrieval of Essential Climate Variables (ECV).

MO‐F‐213CD‐10: PShield: An Algorithm for Optimization of PET/CT Shielding

Purpose: Calculation of radiation shielding requirements for high‐workload PET installations using the methods proposed by AAPM TG‐108 can be difficult. The principle challenge that makes PET shielding design more complex than other diagnostic imaging modalities, aside from the higher photon energy, is that it is a multisource problem for which no unique solution exists. The PShield algorithm incorporates three‐dimensional numerical methods to optimize PET shielding and deliver a cost‐optimized solution while making no approximations. Methods: PShield uses a sequential quadratic programming routine to optimize PET shielding by minimizing a cost function in 3‐dimensions using extrapolations of the TG‐108 formulas. PShield makes no approximations and accounts for the contribution of every radiation source to the dose rate at every location in the problem using a discrete mesh. We used two simple examples of shielding problems to compare PShield with the TG‐108 methods. Results: The benefit of applying an optimization routine to an indeterminate problem is the identification of the only solution to the problem that minimizes the desired cost function. Choosing a poorly optimized solution can Result in a shielding design that requires as much as 50% more shielding than an optimized design to reach the same dose rate at a given control point. The increased accuracy afforded by P Shield ensures that dose rates at every point in a control area never exceed the design dose, whereas a reasonable design based on the TG‐108 methods may have hot spots where the dose rate exceeds the design dose by a factor of 2 or more. Conclusions: PShield is an exact three‐dimensional numerical solution for optimal PET shielding which identifies a singular solution which is cost‐optimized. This is especially important for modern PET/CT suites, where increases in scanner capabilities have resulted in more complex shielding problems and the potential for high occupational doses.

PShield: An exact three‐dimensional numerical solution for determining optimal shielding designs for PET/CT facilities

Calculation of radiation shielding requirements for high-workload positron emission tomography/computed tomography (PET/CT) installations using the methods proposed by Task Group 108 (TG 108) of the American Association of Physicists in Medicine can be a complex task. The principal challenge that makes PET shielding design more complex than other diagnostic imaging modalities, aside from the higher photon energy, is that it is a multisource problem for which no unique solution exists. Although many solutions may meet shielding design dose limits, each solution has a different cost for materials and construction according to the type, thickness, weight, and location of the specified shielding. Here, the authors describe PShield, an algorithm that incorporates three-dimensional (3D) numerical methods to optimize PET shielding and delivers a cost-optimized solution while making no approximations. The PShield algorithm uses a sequential quadratic programming routine to optimize PET shielding by minimizing a cost function in three-dimensions using extrapolations of the formulas published by TG 108. PShield makes no approximations and accounts for the contribution of every radiation source to the dose rate at every location in the problem using a discrete mesh. The authors used two simple examples of shielding problems to compare PShield with the TG 108 methods. The benefit of applying an optimization routine to an indeterminate problem is the identification of the only solution to the problem that minimizes the desired cost function. Choosing a poorly optimized solution can result in a shielding design that requires more shielding than an optimized design to reach the same dose rate at a given control point. The increased accuracy afforded by PShield ensures that dose rates at every point in a control area never exceed the design dose, whereas approximations often used to evaluate TG 108 methods may result in hot spots where the dose rate exceeds the design dose. The PShield algorithm is an exact 3D numerical solution for optimal PET/CT shielding based on the methods proposed by TG 108. Selection of the optimum shielding design can minimize the total material cost and structural burden of installed shielding or achieve other goals desired by a specific site. This is especially important for modern PET/CT suites, where increases in scanner capabilities have resulted in more complex shielding problems and the potential for high occupational doses.

Radiation Shielding Analyses for the ITER Upper Port ECRH Launcher

The International Thermonuclear Experimental Reactor (ITER) will use an electron cyclotron resonance heating (ECRH) system in the upper port of the device for plasma stabilization, heating, and current drive by injecting millimeter wave beams into the plasma chamber. The millimeter waves are transmitted to the plasma through long and narrow waveguide channels. The required plasma wall openings could result in enhanced neutron radiation loadings to the ECRH launcher and neighboring reactor components. The analyses aimed at proving that the shielding requirements and all related nuclear design limits specified by ITER can be met for the proposed ECRH launcher design concepts. The nuclear criteria included human safety issues, nuclear waste regulation aspects, and radiation shielding requirements. The proof was conducted by calculating the radiation loads to sensitive components such as the diamond window of the ECRH launcher, the vacuum vessel, and the superconducting magnets and assessing the potential radiation doses to work personnel during shutdown periods. Dedicated computational approaches were developed to handle the related neutron streaming and shielding problems on the basis of three-dimensional Monte Carlo calculations by the MCNP code. Suitable MCNP models of the launcher were generated by the automatic conversion of the underlying computer assisted design models using a newly developed interface program. The results of the analyses show that all radiation design limits can be safely met for the considered launcher and shield designs.

Radiation shield analyses in support of the FS design for the ITER ECRH launcher

This paper is devoted to the radiation shield analyses for the front steering (FS) design of the electron cyclotron resonance heating (ECRH) launcher installed in the ITER upper port. The neutron fluxes, nuclear heat density, helium production, and neutron displacements rate have been calculated by Monte Carlo N-Particle (MCNP) transport code. Three-dimensional MCNP models of the FS ECRH launcher have been produced with high accuracy directly from the CAD geometry models by means of the McCad interface programme. A shielding analysis has been performed to estimate the material composition and optimum length required for the launcher's internal shield. Neutron streaming calculations have been done using the MCNP point detectors method. Variance reductions in nuclear responses of particles track-length estimations were achieved by the optimized weight windows, particles splitting, and Russian roulette techniques. Nuclear design parameters of the FS ECRH launcher have been evaluated by the neutronics analyses with dedicated computational procedures. The analyses show that safety issues and radiation shielding requirements in the FS design are fully satisfied to radiation design limits specified for ITER project.

Comparison of radiation shielding requirements for HDR brachytherapy using and sources

has received a renewed focus lately as an alternative to sources for high dose rate (HDR) brachytherapy. Following the results of a recent work by our group which proved to be a good candidate for HDR prostate brachytherapy, this work seeks to quantify the radiation shielding requirements for HDR brachytherapy applications in comparison to the corresponding requirements for the current HDR brachytherapy standard. Monte Carlo simulation (MC) is used to obtain and broad beam transmission data through lead and concrete. Results are fitted to an analytical equation which can be used to readily calculate the barrier thickness required to achieve a given dose rate reduction. Shielding requirements for a HDR brachytherapy treatment room facility are presented as a function of distance, occupancy, dose limit, and facility workload, using analytical calculations for both and HDR sources. The barrier thickness required for is lower than that for by a factor of 4–5 for lead and 1.5–2 for concrete. Regarding HDR brachytherapy applications, the lead shielding requirements do not exceed , even in highly conservative case scenarios. This allows for the construction of a lead door in most cases, thus avoiding the construction of a space consuming, specially designed maze. The effects of source structure, attenuation by the patient, and scatter conditions within an actual treatment room on the above‐noted findings are also discussed using corresponding MC simulation results.

The development of the intense positron beam at Washington State University

A High Voltage Engineering K-4000 Van de Graaff ion accelerator has been acquired by Washington State University. This accelerator will be used to create a positron source through the 12 C(d, n) 13 N reaction. The accelerator is capable of accelerating protons or deuterons up to 4 MeV with a maximum beam current between 200 and 300 μA. During normal operation, a 3 MeV deuteron beam will impinge on a diamond or graphite target producing ∼3.3 Ci of 13 N from which a ∼1×10 7 s −1 monoenergetic positron beam will be created. In addition to creating positrons, the source will also emit a large number on neutrons (∼1.2×10 11 s −1), producing a serious radiation hazard. The development of this 13 N based positron beam is discussed including the expected positron yield and radiation shielding requirements.

Radiation Shielding for Fusion Reactors

Radiation shielding requirements for fusion reactors present different problems than those for fission reactors and accelerators. Fusion devices, particularly tokamak reactors, are complicated by geometry constraints that complicate disposition of fully effective shielding. This paper reviews some of these shielding issues and suggested solutions for optimizing the machine and biological shielding. Radiation transport calculations are essential for predicting and confirming the nuclear performance of the reactor and, as such, must be an essential part of the reactor design process. Development and optimization of reactor components from the first wall and primary shielding to the penetrations and containment shielding must be carried out in a sensible progression. Initial results from one-dimensional transport calculations are used for scoping studies and are followed by detailed two- and three-dimensional analyses to effectively characterize the overall radiation environment. These detail model calculations are essential for accounting for the radiation leakage through ports and other penetrations in the bulk shield. Careful analysis of component activation and radiation damage is cardinal for defining remote handling requirements, in-situ replacement of components, and personnel access at specific locations inside the reactor containment vessel.

The U.S. National Research Council's views of the radiation hazards in space

The author was the Chairman of a Task Group on the Biological Effects of Space Radiation formed as a result of discussions between NASA and the U.S. National Research Council's Committee on Space Biology and Medicine — a committee under the U.S. National Research Council's Space Studies Board. The Task Group was asked to review current knowledge on the effects of long-term exposure to radiation in space and to consider NASA radiation shielding requirements for orbital and interplanetary spacecraft. The group was charged with assessing the adequacy of NASA planning for the protection of humans from radiation in space and with making recommendations regarding needed research and/or new shielding requirements. This manuscript is a summary of the findings and recommendations of the Task Group. Beyond the protection of the Earth's atmosphere and its magnetosphere, the exposure to ionizing radiations far exceeds that on Earth. Of all the risks astronauts may face, this one is probably the most straightforward to control — by providing adequate shielding. However, because shielding adds weight, cost and complexity to space vehicles, it is important for designers to have a good quantitative understanding of the true risk and its degree of uncertainty so as not to under- or overshield spacecrafts. The extrapolations from our knowledge of ionizing radiation effects of low linear energy transfer (LET) to the risks from high-atomic-number high-energy energetic (HZE) cosmic rays are very uncertain because the necessary experiments on the effects of such particles have not been carried out and the extrapolation from low-LET to very high-LET has great uncertainties. These uncertainties were enumerated by the Task Group, and the types of experiments needed to minimize the uncertainties were described. The report found that, because of the small amounts of available time for biological research at HZE accelerators, it would take more than a decade of effort to obtain the answers to a narrow set of key questions that would facilitate reduction in risks and identification of the types of shielding needed.

Role of recovery pass beam phase error in RF system design for same cell energy recovery FELs

Recovery of residual energy in the electron beam leaving the FEL interaction region allows considerable improvement in two problem areas of particular concern in high average power designs: (1) the RF power required to generate a given average optical output power is reduced, and (2) the power and energy of the beam which must be dumped are reduced, with concomitant reductions in the amount of heat which must be removed and in the radiation shielding requirements. Recirculation of the beam for a second pass through the linac allows the residual beam power to be recovered in the same RF structure used for acceleration, minimizing the investment in structure and yielding a compact layout. If the energy recovered from the beam is adjusted so that the part which interacted with the FEL optical fields is reduced to the same energy as the part of the beam which did not (“differential” energy recovery), then a relationship between the RF power required, the power delivered to the FEL optical mode, the beam current, and the linac structure's external coupling coefficient is established. For a 100 kW optical output example using eight TESLA-type super conducting cavities, a minimum of 250 kW of RF power is required if Q e is adjusted to 2×10 6. This would be less power than that required for beam loading in the injector linac.

Large solar flare radiation shielding requirements for manned interplanetary missions

As the 21st century approaches, there is an ever-increasing interest in launching manned missions to Mars. A major concern to mission planners is exposure of the flight crews to highly penetrating and damaging space radiations. Beyond the protective covering of the Earth's magnetosphere, the two main sources of these radiations are galactic cosmic rays and solar particle events. Preliminary analyses of potential exposures from galactic cosmic rays (GCR's) were presented elsewhere. In this Note, estimates of shielding thicknesses required to protect astronauts on interplanetary missions from the effects of large solar flare events are presented. The calculations use integral proton fluences for the February 1956, November 1960, and August 1972 solar particle events as inputs into the NASA Langley Research Center nucleon transport code BRYNTRN. This deterministic computer code transports primary protons and secondary protons and neutrons through any number of layers of target material of arbitrary thickness and composition. Contributions from target nucleus breakup (fragmentation) and recoil are also included. The results for each flare are presented as estimates of dose equivalent [in units of roentgen equivalent man (rem)] to the skin, eye, and bloodforming organs (BFO) behind various thicknesses of aluminum shielding. These results indicate that the February 1956 event was the most penetrating; however, the August 1972 event, the largest ever recorded, could have been mission- or life-threatening for thinly shielded (< or = 5 g/cm2) spacecraft. Also presented are estimates of the thicknesses of water shielding required to reduce the BFO dose equivalent to currently recommended astronaut exposure limits. These latter results suggest that organic polymers, similar to water, appear to be a much more desirable shielding material than aluminum.

Radiation shielding requirements for manned satellites.

The radiation shielding requirements for the protection of the crews of manned satellites are investigated. Two diverse types of missions are studied: a long-duration, high-altitude mission above the Van Alien Belt, and a short-duration, low-altitude, polar mission below it. Radiobiological tolerance criteria are considered, and a criterion based on partial recovery of sustained somatic damage is examined for the long-duration mission. Model solar flare, cosmic, and Van Alien Belt radiation environments are postulated. Radiation transport calculations are carried out to obtain the biological doses due to the various environmental components. Curves of dose vs shielding thickness, together with the model environments and the postulated radiobiological tolerance criteria, are used to calculate minimum amounts of shielding. For the high-altitude mission, results show that considerable savings in shielding weight are obtained if a radiation recovery criterion, rather than a straight cumulativedose criterion, is used. For the low-altitude mission, the effects of different geomagnetic field models in modifying the free-space dose and the results of using different target models are illustrated.