THE MAYAK WORKER DOSIMETRY SYSTEM (MWDS-2013) FOR INTERNALLY DEPOSITED PLUTONIUM: AN OVERVIEW.

The Mayak Worker Dosimetry System (MWDS-2013) is a system for interpreting measurement data from Mayak workers from both internal and external sources. This paper is concerned with the calculation of annual organ doses for Mayak workers exposed to plutonium aerosols, where the measurement data consists mainly of activity of plutonium in urine samples. The system utilises the latest biokinetic and dosimetric models, and unlike its predecessors, takes explicit account of uncertainties in both the measurement data and model parameters. The aim of this paper is to describe the complete MWDS-2013 system (including model parameter values and their uncertainties) and the methodology used (including all the relevant equations) and the assumptions made. Where necessary, Supplementary papers which justify specific assumptions are cited.

Elevated radioxenon detected remotely following the Fukushima nuclear accident.

We report on the first measurements of short-lived gaseous fission products detected outside of Japan following the Fukushima nuclear releases, which occurred after a 9.0 magnitude earthquake and tsunami on March 11, 2011. The measurements were conducted at the Pacific Northwest National Laboratory (PNNL), (46°16'47″N, 119°16'53″W) located more than 7000 km from the emission point in Fukushima Japan (37°25'17″N, 141°1'57″E). First detections of (133)Xe were made starting early March 16, only four days following the earthquake. Maximum concentrations of (133)Xe were in excess of 40 Bq/m(3), which is more than ×40,000 the average concentration of this isotope is this part of the United States.

Toward the framework and implementation for clearance of materials from regulated facilities.

The disposition of solid materials from nuclear facilities has been a subject of public debate for several decades. The primary concern has been the potential health effects resulting from exposure to residual radioactive materials to be released for unrestricted use. These debates have intensified in the last decade as many regulated facilities are seeking viable management decisions on the disposition of the large amounts of materials potentially containing very low levels of residual radioactivity. Such facilities include the nuclear weapons complex sites managed by the U.S. Department of Energy, commercial power plants licensed by the U.S. Nuclear Regulatory Commission (NRC), and other materials licensees regulated by the NRC or the Agreement States. Other facilities that generate radioactive material containing naturally occurring radioactive materials (NORM) or technologically enhanced NORM (TENORM) are also seeking to dispose of similar materials that may be radioactively contaminated. In contrast to the facilities operated by the DOE and the nuclear power plants licensed by the U.S. Nuclear Regulatory Commission, NORM and TENORM facilities are regulated by the individual states. Current federal laws and regulations do not specify criteria for releasing these materials that may contain residual radioactivity of either man-made or natural origin from regulatory controls. In fact, the current regulatory scheme offers no explicit provision to permit materials being released as "non-radioactive," including those that are essentially free of contamination. The only method used to date with limited success has been case-by-case evaluation and approval. In addition, there is a poorly defined and inconsistent regulatory framework for regulating NORM and TENORM. Some years ago, the International Atomic Energy Agency introduced the concept of clearance, that is, controlling releases of any such materials within the regulatory domain. This paper aims to clarify clearance as an important disposition option for solid materials, establish the framework and basis of release, and discuss resolutions regarding the implementation of such a disposition option.

Eliminating bias in routine bioassay when there is an unknown time of intake.

Routine bioassay programmes sometimes find evidence of an unsuspected intake. If there were no workplace indicators of exposure or intake, it is necessary to assume a value for the time of intake. Under these circumstances, the International Commission on Radiological Protection (ICRP) continues to recommend using the midpoint of the interval between routine bioassay measurements (ICRP Publication 78, paragraph 106). The assumption of T/2 as the time of intake, where T is the interval between bioassay measurements, represents the expectation value of the time of intake, (t), assuming uniform probability of an intake at any given time. This assumption results in a modest bias, of the expectation value of the intake, (I), that would have been received by a population of workers who had uniform probability over time of intake. This underestimation leads to a negative or positive bias in dose estimates derived in this fashion. The bias is characterised for realistic, routine urinalysis programs for Pu, U and 3H, as well as for in vivo measurements of 125I, 131I and 137Cs. Simple numerical methods are presented for correcting the bias. The bias is greatest for radionuclides whose half-lives are short with respect to the interval between bioassay measurements. Since the primary concern is estimating intake rather than time, the assumed time of intake should be chosen as t(I) rather than T/2. The ICRP should consider revising some of the tables in its Publication 78 to reflect this.

Spectral emissions and dosimetry of metal tritide particulates.

Inference of intakes and doses from inhalation of metal tritide particles has come under scrutiny because of decommissioning and decontamination of US Department of Energy facilities. Since self-absorption of radiation is very significant for larger particles, interpretation of counting results of metal tritide particles by liquid scintillation requires information about emission spectra. Similarly, inference of dose requires knowledge of charged particle and photon spectra. The PENELOPE Monte Carlo radiation transport computer code was used to compute spectral emissions and other dosimetric quantities for tritide particulates of Sc, Ti, Zr, Er, and Hf. Emission fractions, radial absorbed dose distributions, specific energy distributions and related frequency-mean specific energies and lineal energies, and the emitted spectra of electrons and bremsstrahlung photons are presented for selected particulates with diameters ranging from about 0.01 microm to 25 microm. Results characterising the effects of uncertainties associated with the composition and density of the tritides are also presented. Emission spectra are used to illustrate trends in the relationship between apparent and observed activity as a function of particle type and size. Emissions from metal tritide particles are weakly penetrating, and electron emission spectra tend to 'harden' as particle size increases. Microdosimetric considerations suggest that the radiation emitted by metal tritides can be classified as a low linear energy transfer radiation source. For cells less than about 7 microm away from the surface of a metal tritide, the primary dose component is due to electrons. However, bremsstrahlung radiation may deposit some energy tens, hundreds or even thousands of micrometres away from the surface of a tritide particle. The data and analyses presented in this report will help improve the accuracy of dose determinations for particulates of five metal tritides. Future work on the spectral emissions and dosimetry of metal tritide particulates needs to consider the contributions of so-called internal bremsstrahlung, an additional form of bremsstrahlung radiation emitted during beta decay.

Is it useful to assess annual effective doses that are less than 100 mSv?

Questions such as 'How small is small?' and 'How low is low enough?' have long plagued radiation dosimetrists and risk management personnel. Unfortunately, our knowledge about the biological effects of low levels of ionising radiation is scarce and uncertain. If we look to the results of epidemiological studies, we find that it is not easy to arrive at firm conclusions. However, some current radiobiological experiments using microbeams of various radiations, along with improved theoretical models of radiation action, may shed new light on the effects of low levels of ionising radiation. What shall we do in the meantime? Both of our debaters agree that monitoring of radiation workers is necessary, yet careful consideration must be given to the rationale for providing personal monitoring. There is no question that we have done a good job of protecting radiation workers for many years, but we also must be aware of the many implications of our efforts.

Microdosimetric properties of ionizing electrons in water: a test of the PENELOPE code system.

The ability to simulate the tortuous path of very low-energy electrons in condensed matter is important for a variety of applications in radiobiology. Event-by-event Monte Carlo codes such as OREC, MOCA and PITS represent the preferred method of computing distributions of microdosimetric quantities. However, event-by-event Monte Carlo is computationally expensive, and the cross sections needed to transport simulations to this level of detail are usually only available for water. In the recently developed PENELOPE code system, 'hard' electron and positron interactions are simulated in a detailed way while soft' interactions are treated using multiple scattering theory. Using this mixed simulation algorithm, electrons and positrons can be transported down to energies as low as 100 eV. To our knowledge, PENELOPE is the first widely available, general purpose Monte Carlo code system capable of transporting electrons and positrons in arbitrary media down to such low energies. The ability to transport electrons and positrons to such low energies opens up the possibility of using a general purpose Monte Carlo code system for microdosimetry. This paper presents the results of a code intercomparison study designed to test the applicability of the PENELOPE code system for microdosimetry applications. For sites comparable in size to a mammalian cell or cell nucleus, single-event distributions, site-hit probabilities and the frequency-mean specific energy per event are in reasonable agreement with those predicted using event-by-event Monte Carlo. Site-hit probabilities and the mean specific energy per event can be estimated to within about 1-10% of those predicted using event-by-event Monte Carlo. However, for some combinations of site size and source-target geometry, site-hit probabilities and the mean specific energy per event may only agree to within 25-60%. The most problematic source-target geometry is one in which the emitted electrons are very close to the tally site (e.g., a point source on the surface of a cell). Although event-by-event Monte Carlo will continue to be the method of choice for microdosimetry, PENELOPE is a useful, computationally efficient tool for some classes of microdosimetry problem. PENELOPE may prove particularly useful for applications that involve radiation transport through materials other than water or for applications that are too computationally intensive for event-by-event Monte Carlo, such as in vivo microdosimetry of spatially complex distributions of radioisotopes inside the human body.

Making it safe, making it legal, and creating peace of mind.

The job of a medical or academic radiation safety officer has three parts: keeping it safe, keeping it legal, and helping people feel that they are safe. Absence of peace-of-mind about radiation protection matters can create very real health effects, even when there is little or no radiation exposure involved. Frightened people may make decisions such as changing jobs (and losing health insurance), terminating a pregnancy, or moving, all of which impact health. Furthermore, frightened people who choose to stick with it may suffer from anxiety, stress, insomnia, and weight loss or even weight gain. Genuinely listening to the concerns of those who benefit from radiation safety services can help to provide peace-of-mind and minimize decisions that are risky to health.

Evaluation of eight decision rules for low-level radioactivity counting.

In low-level radioactivity measurements, it is often important to decide whether a measurement differs from background. A traditional formula for decision level (DL) is given in numerous sources, including the recent ANSI/HPS N13.30-1996, Performance Criteria for Radiobioassay and the Multi-Agency Radiation Survey and Site Investigation Manual (MARSSIM). This formula, which we dub the N13.30 rule, does not adequately account for the discrete nature of the Poisson distribution for paired blank (equal count times for background and sample) measurements, especially at low numbers of counts. We calculate the actual false positive rates that occur using the N13.30 DL formula as a function of a priori false positive rate a and background Poisson mean mu = rhot, where rho is the underlying Poisson rate and t is the counting time. False positive rates exceed a by significant amounts for alpha < or = 0.2 and mu < 100 counts, peaking at 25% at mu approximately equal to 0.71, nearly independent of alpha. Monte Carlo simulations verified calculations. Currie's derivation of the N13.30 DL was based on knowing a good estimate of the mean and standard deviation of background, a case that does not hold for paired blanks and low background rates. We propose one new decision rule (simply add 1 to the number of background counts), and we present six additional decision rules from various sources. We evaluate the actual false positive rate for all eight decision rules as a function of a priori false positive rate and background mean. All of the seven alternative rules perform better than the N13.30 rule. Each has advantages and drawbacks. Given these results, we believe that many regulations, national standards, guidance documents, and texts should be corrected or modified to use a better decision rule.

Determining parameters of lognormal distributions from minimal information.

The lognormal distribution has a number of properties that do not lend themselves to simple "back-of-the-envelope" calculations. Mathematical relationships are presented for the basic parameters of the large population lognormal distribution as a function of characteristics available to, or needed by, the risk analyst. A freeware computer program called LOGNORM4 has been written to take the tedium out of determining various characteristics of lognormal distributions, given 1 of 15 sets of values that uniquely specify a lognormal distribution.

Doses to workers in the United States nuclear weapons program due to external irradiation at the dawn of the atomic era (1940-1960).

Radiation doses to workers at the Manhattan Engineer District (MED) and Atomic Energy Commission (AEC) sites due to external irradiation during 1940-1960 are reviewed. Categorized radiation dose data were available from AEC annual reports for some years. Annual individual radiation dose data for nine MED/AEC sites for all years were available from the U.S. Department of Energy's Comprehensive Epidemiologic Data Resource. These data are combined to produce an estimate of external collective dose equivalent to 1,720 person-Sv for 1940-1960. During this period there were 19 criticality incidents; 41 persons in a workforce of several hundred thousand were accidentally overexposed in these and other incidents, including three men who died due to acute radiation syndrome.

Ten principles and ten commandments of radiation protection.

For decades, the phrase "time, distance, and shielding" has been presented as summarizing the "basics" of radiation protection. Indeed, for protection from external radiation sources, these three principles are probably the most important ones on which a worker can make decision and take actions. however, these principles do not address protection against intakes of radioactive materials or "ontakes" (skin contamination), other risk-limiting measures, or other important protective measures taken by governments, public health agencies, regulators, and institutional programs (measures such as performance standards, health education, facility engineering requirements, and administrative procedures), I have identified ten principles and ten accompanying commandments of radiation protection: time, distance, dispersal, source reduction, source barrier, personal barrier, decorporation, effect mitigation, optimal technology, and limitation of other exposures. Corresponding non-technical forms of the commandments are hurry (but don't be hasty); stay away from it; disperse it and dilute it; use as little as possible; keep it in; keep it out; get it out or off of you (after intake or skin contamination); limit the damage; choose the best technology (perhaps a non-radiation technology); and don't compound risks (don't smoke). Technical versions of the commandments are also provided using the verbs "optimize," "maximize," or"minimize." Not all commandments can be applied at the same time, and application may be different for workers and members of the public. Advantages, disadvantages, and implementation of these principles and commandments are discussed, and numerous examples provided. The application of the principles and commandments must be based on knowledge of the radiological conditions to be managed.

Minimum detectable activity when background is counted longer than the sample.

This note discusses the use of blank or background counting data that are measured for times that differ from times used for the sample counts. The correct formula for the minimum detectable activity, under this condition, is given as follows: MDA = [3 + 3.29 square root of Rbtg(1 + tg/tb)]/epsilon tg, where Rb denotes background count rate, tb and tg denote background and gross count times, and epsilon denotes counting efficiency. Counting backgrounds for a long time reduces decision levels, uncertainties, and minimum detectable activities. These benefits are fully available only when there is no other source of variability than random fluctuations in count rates.