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Over the years, numerous evaluations of material attractiveness have been performed for conventional light water reactors to better understand the nature of the spent fuel material and its desirability for misuse at different points in the nuclear fuel cycle. However, availability of such assessments for newer, Generation IV reactors such as Molten Salt Reactors is rather limited. In the present study we address the gap in knowledge of material attractiveness for molten salt reactor systems and describe the nature of irradiated fuel salts which the nuclear safeguards community might be faced with in the near future as more and more such reactors enter commission and operation. Within the scope of the paper, we use a large database of simulated irradiated fuel salt isotopics (and other derived quantities such as gamma activity, decay heat, and neutron emission rates) developed specifically for a molten salt reactor concept in order to shed some light on possible weapons usability of uranium and plutonium present in the irradiated fuel salts. This has been achieved by proposing a new attractiveness metric that is better suited for quantifying attractiveness of irradiated salts from a model molten salt concept. The said metric has been computed using a database that has been created by simulating the irradiation of molten fuel salt in a concept core over a wide range of operational parameters (burnup, initial enrichment, and cooling time) using the Monte-Carlo particle transport code, Serpent. With the help of this attractiveness metric, the findings from this study have shown that in relative terms, molten salt spent fuel is more attractive than spent fuel produced by a conventional light water reactor. The findings also underscore the need for strengthened safeguards measures for such spent fuel. These results are expected to be useful in the future for regulatory authorities as well as for nuclear safeguards inspectors for designing a functional safeguards verification routine for irradiated fuel of such unique nature.

AbstractThere exist elements apart from uranium and plutonium that could potentially be used to construct the core of a nuclear explosive device. These belong to the so-called minor actinides (MAs), which exist in nonnegligible amounts in spent nuclear fuel (SNF) and are in nearly all cases not covered by international safeguards. Future reprocessing of SNF could result in significant separation of these elements, potentially leading to new proliferation concerns. In this work, a methodology for a transparent assessment of the barriers against proliferation of MAs has been developed and applied to the case of neptunium, americium, and curium separated from spent fuel from pressurized water reactors. In this methodology, openly available data and Monte Carlo simulations have been used to assess the barriers posed by a number of parameters relevant to the production of a nuclear explosive device from SNF. The evaluation shows that the properties of neptunium present low barriers to proliferation and that it should be discussed within the context of future nonproliferation treaties and possibly be placed under international safeguards. The properties of americium and curium present higher barriers to proliferation, meaning that these elements require less focus in the nonproliferation context. [ABSTRACT FROM AUTHOR]

Nuclear fuel cycle advancements will result in new types of fissile material, including nuclear wastes, that require security and safeguards. Nuclear wastes may be more vulnerable for diversion by non-state actors, and chemical processing to recover fissile material is not an insurmountable challenge. Previous work has applied a figure of merit (FOM) to assess material attractiveness and security risks. This analysis applies the material attractiveness FOM to wastes produced by fuel cycles from the Fuel Cycle Evaluation and Screening (FCES) study. Two aspects of security risk are studied: (1) the time before the fissile material in the waste becomes attractive and (2) the number of waste packages required to obtain a critical mass of fissile material. Two fuel cycles are presented to highlight detailed results: (1) once-through use of low-enriched U in light water reactors (LWRs) and (2) continuous recycle of Pu in sodium fast reactors (SFRs). Increasing LWR used nuclear fuel (UNF) package loading increases the time to attractiveness, but the larger packages contain enough Pu for multiple critical masses. The high-level waste (HLW) from processing the SFR fuels has similar FOM behavior but longer time to attractiveness due to the concentration of fission products. More HLW packages are required to obtain a critical mass; that number can be further increased by increasing the separation efficiency. Extended to all FCES fuel cycles, the minimum time before attractiveness is generally lower for UNF than for HLW because radioactivity is concentrated in HLW. For nearly all fuel cycles that produce UNF, only one package is required to recover enough fissile material for a critical mass. Notably, some advanced fuel cycles produce HLW, of which only two packages need to be recovered to obtain a critical mass, even when the target fissile material is recycled. Going forward, an assessment of the security risks posed by fissile material in nuclear wastes will need to quantify the challenge posed by separations. Ultimately, the assessment could inform security and response measures; whether any of the observations might affect these measures could be an area for future work. Finally, future analysis could study whether different fuel cycle wastes are more attractive for use in radiological dispersal devices or radiological exposure devices. [ABSTRACT FROM AUTHOR]

The United States and the Russian Federation have agreed to dispose of their excess weapons-grade plutonium, with consuming the material as nuclear fuel in light water reactors for electricity generation often discussed as the best option. Lightbridge Corporation has several thermal reactor fuel designs that offer very high burnups, in the range of 21 at. % or approximately 190 900 MWd/tonne of heavy metal, which make them well suited for consuming excess weapons-grade plutonium. MCNP6.2 computer simulations were performed to quantify the mass of plutonium consumed in a Lightbridge-designed fuel rod compared to traditional mixed-oxide (MOX) fuel, as well as the attractiveness of the plutonium in the used fuel for weapons purposes. The results of these simulations show that the Lightbridge plutonium disposition fuel variant consumes approximately 5.5 times more plutonium per fuel rod than MOX fuel and that the material attractiveness of the Lightbridge-used plutonium is noticeably less than that of MOX fuel. [ABSTRACT FROM AUTHOR]

The attractiveness for weapons usage of the proposed fuel cycle for the PT-SCWR was evaluated in this study using the Figure-of-Merit methodology. It was compared to the attractiveness of other fuel cycles namely, Low Enriched Uranium (LEU), U/Th, Re-enriched Reprocessed Uranium (RepU), and Pu/Th/U. The optimal content of natural uranium, which can be added to Pu/Th to render the produced U-233 unattractive, was found to be 9%.A ranking system to compare the attractiveness of the various fuel cycles is proposed. RepU was found to be the most proliferation resistant fuel cycle for the first 100 years,while, the least proliferation resistant fuel cycle was the originally proposed Pu/Th one.The reactivity feedback coefficients were calculated for all proposed fuel cycles. All studied reactivity coefficients have the same sign implying that all the fuel cycles will behave neutronically in a similar way. The Pu/Th/U fuel was found to have the most negative value of the Coolant Void Reactivity which will help to restore the core to a safe status faster in case of a loss-of-coolant accident. The fuel and moderator temperature coefficients did not show significant differences between the fuels studied.

Due to excess reactivity, fresh nuclear fuel often contains burnable poisons. This research looks at six different burnable poisons and their impacts on reactivity, material attractiveness, and waste management. An MCNP simulation of a PWR fuel pin was performed with a fuel burnup of 60 GWd/MTHM to determine when each burnable poison fuel type would decrease below a k∞ of 1. For determining the plutonium material attractiveness in each burnable poison fuel type, the plutonium isotopic content of the used fuel was evaluated using Bathke's Figure of Merit formula. For the waste management analysis, the thermal output of each burnable poison fuel type was determined through ORIGEN decay simulations at 100 and 300 years after being discharged from the core. The performance of all six burnable poisons varied over the three criteria considered and no single burnable poison performed best in all three considerations.

A comparative study was performed with different designs of the fast breeder reactor (FBR) blanket to evaluate the performance of plutonium breeding accompanied with denaturing, and trans-uranium (TRU) transmutation for enhancement of the proliferation resistance and actinide management in the future fuel cycle. Four designs of the FBR blanket fuel were intercompared, conventional depleted uranium (DU) oxide fuels, DU with TRU oxide fuels, DU with TRU oxide fuels with homogeneous/heterogeneous moderators. As results, with heterogeneous moderator in the axial and the radial blanket fuels, the greatest TRU transmutation and plutonium denaturing performance were attained without deteriorating fuel breeding. Design optimization was performed as for ZrH 1.65 moderator rod/pellet arrangement in the axial and the radial blanket fuel assemblies. The optimum number of moderator rods per assembly was 25 (11% of the number of total rods) with proper mutual interval (2–3 cm) and symmetrical arrangement in the radial blanket, and insertion of 1 cm thick ZrH 1.65 moderator pellets to 4 and 10 cm from the top and the bottom planes of the active core was the optimum design in the axial blanket, in order to achieve the highest TRU transmutation ratio and plutonium denaturing performance without deteriorating fuel breeding. From the non-proliferation aspect, material attractiveness of plutonium were evaluated comparing with the criterion based on decay heat, and required TRU doping ratio in DU with TRU oxide fuel were proved to be as 1.6–2.5 wt.% with heterogeneous moderator, 2.1–3.0 wt.% with homogeneous moderator, and 4.1–5.0 wt.% without moderator. The heterogeneous loading of moderation showed the great advantages in fuel treatment in clear the heat generation limit criterion, as well as mass requirement to the initial and multi-cycle phase. [ABSTRACT FROM AUTHOR]

Highlights: [•] The paper analyzes the plutonium production of recycling nuclear fuel option. [•] To evaluate material attractiveness based on intrinsic feature of material barrier. [•] Evaluation based on isotopic plutonium composition of spent fuel LWR and FBR. [•] Even mass number of plutonium gives a significant contribution to material barrier, in particular Pu-238 and Pu-240. [•] Doping MA in FBR blanket is effective to increase material barrier from weapon grade plutonium to more than MOX fuel grade. [Copyright &y& Elsevier]

We must anticipate that the day is approaching when details of nuclear weapons design and fabrication will become common knowledge. On that day we must be particularly certain that all special nuclear materials (SNM) are adequately accounted for and protected and that we have a clear understanding of the utility of nuclear materials to potential adversaries. To this end, this paper examines the attractiveness of materials mixtures containing SNM and alternate nuclear materials associated with the plutonium-uranium reduction extraction (Purex), uranium extraction (UREX), coextraction (COEX), thorium extraction (THOREX), and PYROX (ah electrochemical refining method) reprocessing schemes. This paper provides a set of figures of merit for evaluating material attractiveness that covers a broad range of proliferant state and subnational group capabilities. The primary conclusion of this paper is that all fissile material must be rigorously safeguarded to detect diversion by a state and must be provided the highest levels of physical protection to prevent theft by subnational groups; no "silver bullet" fuel cycle has been found that will permit the relaxation of current international safeguards or national physical security protection levels. The work reported herein has been performed at the request of the U.S. Department of Energy (DOE) and is based on the calculation of "attractiveness levels" that are expressed in terms consistent with, but normally reserved for, the nuclear materials in DOE nuclear facilities. The methodology and findings are presented. Additionally, how these attractiveness levels relate to proliferation resistance and physical security is discussed. [ABSTRACT FROM AUTHOR]

Abstract: Material attractiveness evaluations based on isotopic Plutonium barrier compositions have been investigated for evaluating intrinsic features of proliferation resistance such as decay heat (DH), spontaneous fission neutron (SFN), as well as attractiveness concepts of figure of merit (FOM) and “attractiveness” (ATTR) as a function of diluted fraction of even mass Plutonium to Pu-239 composition. The DH and SFN compositions increase significantly with increasing diluted fraction of even mass Plutonium, except for DH of Pu-242. It shows that diluted fraction of Pu-238 contributes most significantly to the DH ad SFN compositions at certain diluted fractions. Only diluted fraction of Pu-238 can reach a DH composition level of MOX fuel for small diluted fraction and an SFN level of MOX fuel can be reached by all diluted fractions of even mass Plutonium. It requires more than 80% of diluted Pu-238 fraction to achieve a low material attractiveness based on FOM1 formula and there is no available diluted fraction for Pu-240 and Pu-242 for reaching it. It is easier to reach a low material attractiveness by using FOM2 than that of FOM1 caused by additional contribution of the SFN component, as well as ATTR, which also considers both DH and SFN components. The material attractiveness of FOM2 and ATTR as a function of diluted even mass Plutonium fractions are shown in similar trend, which requires some diluted fraction depending on even mass Plutonium for achieving low material attractiveness of FOM 2 and practically unusable level of ATTR. [Copyright &y& Elsevier]

Abstract: Thorium (Th) oxide fuel offers a significant advantage over traditional low-enriched uranium and mixed uranium/plutonium oxide (MOX) fuel irradiated in a Light Water Reactor. The benefits of using thorium include the following: 1) unlike depleted uranium, thorium does not produce plutonium, 2) thorium is a more stable fuel material chemically than LEU and may withstand higher burnups, 3) the materials attractiveness of plutonium in Th/Pu fuel at high burnups is lower than in MOX at currently achievable burnups, and 4) thorium is three to four times more abundant than uranium. This paper quantifies the irradiation of thorium fuel in existing Light Water Reactors in terms of: 1) the percentage of plutonium destroyed, 2) reactivity safety parameters, and 3) material attractiveness of the final uranium and plutonium products. The Monte Carlo codes MCNP/X and the linkage code Monteburns were used for the calculations in this document, which is one of the first applications of full core Monte Carlo burnup calculations. Results of reactivity safety parameters are compared to deterministic solutions that are more traditionally used for full core computations. Thorium is fertile and leads to production of the fissile isotope 233U, but it must be mixed with enriched uranium or reactor-/weapons-grade plutonium initially to provide power until enough 233U builds in. One proposed fuel type, a thorium-plutonium mixture, is advantageous because it would destroy a significant fraction of existing plutonium while avoiding the creation of new plutonium. 233U has a lower delayed neutron fraction than 235U and acts kinetically similar to 239Pu built in from 238U. However, as with MOX fuel, some design changes may be required for our current LWR fleet to burn more than one-third a core of Th/Pu fuel and satisfy reactivity safety limits. The calculations performed in this research show that thorium/plutonium fuel can destroy up to 70% of the original plutonium per pass at 47 GWd/MTU, whereas only about 30% can be destroyed using MOX. Additionally, the materials attractiveness of the final plutonium product of irradiated plutonium/thorium fuel is significantly reduced if high burnups (∼94 GWD/MTU) of the fuel can be attained. [Copyright &y& Elsevier]

Material attractiveness levels based on the composition of plutonium isotopes have been investigated by adopting attractiveness (ATTR) and figure of merit (FOM) formulas as well as decay heat and spontaneous fission compositions. FOM formula requires relatively high doping rate of even mass plutonium than ATTR formula and it shows much lower material attractiveness level can be obtained when spontaneous fission neutron component is included as additional proliferation resistance component, which is combined with decay heat component. To denature plutonium based on Kessler's criterion for high technology level, it requires 2% MA doping in the blanket regions and loading 1% doping MA shows higher enough to denature plutonium of medium technology and low technology levels. Attractiveness level of weapon grade or high attractiveness level is shown by blanket fuel regions of Fast Breeder Reactor (FBR) with adopting ATTR1 and FOM1 formulas, while adopting ATTR2 and FOM2 formulas show core regions reach practically unusable level or low attractiveness level and blanket regions obtain weapon grade level or high attractiveness level for no MA doping case. MA > 1.6% of ATTR2 and MA > 3.6% of FOM2 in the blanket regions are enough to reach practically unusable level or low attractiveness level. Image 1 • Material attractiveness analysis of attractiveness (ATTR) and Figure of Merit (FOM) has been used. • Evaluation of material attractiveness in FBR blanket region is adopted based on plutonium composition. • Decay heat and spontaneous fission neutron are significantly effective to reduce the level of material attractiveness. • Increasing MA doping are effective to denature plutonium into low level attractiveness and less MA doping rate for ATTR. [ABSTRACT FROM AUTHOR]

• The material attractiveness of Lightbridge's metallic fuel design is analyzed. • Doping of 238Pu, 237Np, and 241Am in fresh fuel increases the concentration of 238Pu in spent fuel. • The material attractiveness of each plutonium vector is quantified using a Figure of Merit analysis. This paper investigates the material attractiveness for an innovative fuel design (IFD) based on Lightbridge's metallic fuel (MF) design. The used MF will have a plutonium vector that offers lower material attractiveness than traditional used uranium oxide fuel because of its high 235U concentration (19.7%), allowing for higher fuel burnup. An IFD was modelled using published information from Lightbridge. This paper presents analysis carried out on the IFD to assess its material attractiveness. The IFD was burned for 21 atom percent, equivalent to 190.9 GWd/MTU, for 1150 days – two 18-month cycles— which translates to 88.32 kW/fuel rod. This IFD power level is a 13% uprate to a traditional uranium oxide fuel burned at a level of 44 GWd/MTU for 1150 days, a power of 67.94 kW/fuel rod. This configuration of the IFD offers a one-to-one replacement of the traditional uranium oxide fuel rod in a pressurized water reactor (PWR). Additionally, the fresh IFD was doped with three different actinides (238Pu, 237Np, and 241Am) from 0.1 to 1 wt% and the resulting plutonium vector's material attractiveness were evaluated using Bathke's Figure of Merit 1 and 2. The figure of merit (FOM) analyses showed that the non-doped used IFD has medium attractiveness as compared to the high attractiveness of traditionally used uranium oxide fuel. The doped IFDs had medium attractiveness with the FOM values slightly decreasing with increasing dopant concentration. In the FOM estimations, the radiation dose term was neglected as its contribution was insignificant due to the relatively small gamma signature from plutonium isotopes. Overall, the IFD analyses highlights that the actinide doped MF can yield used fuel which is unattractive. [ABSTRACT FROM AUTHOR]

• 232U is introduced into natural uranium. • The enrichment process is simulated to determine the weapons grade vector. • The material attractiveness of the weapons material is determined. • The decomposition of UF 6 molecules is analyzed. The effect of 232U on enrichment and material attractiveness provides a significant firewall against proliferation. Mixing 232U with natural uranium creates major obstacles to the weaponization process. The decay heat of 232U is significant and increases over time by a factor greater than 5 through the decay of 232U daughter products. The dose rate also becomes significant as 208Tl builds up and emits an energetic gamma ray of 2.615 MeV. These factors, along with the bare critical mass and spontaneous fission neutron rate are used to determine the material's attractiveness through a Figure of Merit analysis. Considering 232U is lighter than 235U, attempting to enrich a mixture of natural uranium and 232U to weapons grade will also increase the concentration of 232U and thus increase the material's unattractiveness. This study simulates natural uranium with a 232U concentration of 0.03 at.% being enriched to weapons grade and determines the attractiveness of the resulting uranium vector. The results show that the material becomes unattractive within a year of initial separation. In addition, the alpha decay of 232U in gaseous UF 6 will result in the decomposition of UF 6 molecules. The rate of this decomposition in the initial natural uranium mixture, the mixture enriched to 3 at.% 235U, and this mixture enriched to 90 at.% 235U is calculated. These rates are notable and may pose a significant firewall to obtaining weapons grade uranium. [ABSTRACT FROM AUTHOR]