Nucleophilicity

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The use of liquid fuels avoids some of the technical difficulties such as fuel fabrication for burning actinides a particularly difficult fuel fabrication challenge for the intensely radioactive higher actinides. The traditional MSR has a thermal to epithermal neutron spectrum. Recent work in France and elsewhere has developed the concept of a fast-spectrum MSR. Unlike solid-fuel fast-spectrum reactors, the fast-spectrum MSR has the potential safety advantages of strong negative Doppler and void coefficients traditionally the major safety challenges for solid-fuel fast reactors.

There is a secondary interest in the MSR s use for hydrogen production because of the high-temperature capability.

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In Europe, there is the traditional interest in the MSR as a thermal-neutron breeder reactor. It reflects two contradictory characteristics of MSRs: 1 the two MSRs built in the s and s demonstrated the feasibility of the technology and 2 the new missions and technological options that will require time to determine the best choice of reactor. The scoping and screening phase through confirms the potential capabilities of an AMSR.

The viability phase through is defined as obtaining sufficient information to make a credible determination on the commercial feasibility of an MSR for power generation or viability to meet one of the new missions A It concludes with the selection of one or more reference designs. The performance phase through involves the large-scale integral experiments including molten salt loops in test reactors or a small test reactor.

Molten fluoride salts, the base technology for the MSR, are being considered in multiple nuclear applications see Figure A6. In the other applications, the salts are clean salts without dissolved fuel referred herein as liquid salts. Liquid salts are one of two candidates for this task.

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In Europe, the French Atomic Energy Commission CEA is investigating the use of liquid-salt intermediate loops to replace sodium intermediate loops in sodium fast reactors to reduce costs and eliminate the potential for chemical reactions between the reactor coolant and the power cycle.

The liquid-salt technology is the same basic technology required for the MSR intermediate heat transport loop. These high-temperature heattransport loops are also candidates for use in solar power towers, in-situ recovery of shale oil, and tertiary oil recovery. The intermediate heattransport loop transfers the heat to a Brayton power cycle or a hydrogen production facility.

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The A Fusion Reactors: Liquid salts are major candidates for cooling inertial and magnetic fusion energy systems. Develop a cost estimate for an MSR. Economic performance is an absolute requirement for largescale deployment; thus, a preliminary understanding of MSR economics is required by when preliminary decisions on advanced reactors for fuel production are made. Establish the potential of energy conversion systems to use molten salts as heat transfer agents and the ability to couple the MSR with energy conversion devices.


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Because MSR test reactors have been successfully operated, many technical viability issues have been addressed. Second level viability issues include optimum choice of the power cycle, determination of limits of actinide burning capabilities neutronics and selection of optimum salt for actinide burning , fast or thermal neutron spectrum, salt processing options, design life of reactor materials, noble metal fission product management, licensing strategies, and safeguard strategies.

Since , France has coordinated a European review and reevaluation of the MSR technology that has involved 13 participants. Within the European community, it is part of their examination of MSRs. The activities associated with this specific effort are measurement of selected salt properties and better methods to measure salt properties on line, a cross cutting activity for all salt-related activities. This should enable the program to develop a credible understanding of the economics, capabilities to perform alternative missions electricity, hydrogen, actinide burning, and fuel production , and issues associated with a modern MSR and, thus, provide the basis for a decision on whether to initiate a large-scale developmental program with the goal of deployment.

Background

Since development of detailed conceptual designs for large MSRs in the s, major changes in goals, regulatory requirements, and technologies have occurred that have not yet been integrated into the conceptual design approach for a next generation MSR. Three major changes must be incorporated into a modern MSR design. Second, advances in non-ngnp technologies such as remote operations, robotics, and controls must be incorporated into the conceptual design.

Last, changing mission requirements will simplify plant design. Early MSRs were designed to maximize fuel production; that mission, in turn, required complex, high-capacity, on-line salt processing. For actinide burner and hydrogen missions, there is the potential to eliminate most on-line fuel processing systems and greatly simplify the plant design. A Regulation Liquid fueled reactors use different approaches to reactor safety than solid fueled reactors.

The current regulatory structure was developed with the concept of solid-fuel reactors. The comparable regulatory requirements for this system must be defined. Using current tools, appropriate safety analysis is required followed by appropriate research on the key safety issues. The critical safety requirement for an MSR is that the radionuclides remain dissolved in the molten salt under all conditions. The reactor size, design, and safety systems are dependent upon this property. New applications for MSRs, such as actinide burning, imply higher concentrations of trivalent actinides and noble metals in the salt than were used in the past and may require modification of the salt composition to assure solubility under all conditions.

Similarly, the behavior of noble metal fission products in the salt and their ultimate disposition is required. Under some conditions, fission product noble metals may plate out on heat exchangers resulting in high decay heat loads and limited equipment lifetimes. However, there are multiple fuel cycle challenges. Some are common to other reactors and their associated fuel cycles, and some are unique to the MSR.

Specifically, because the system is a molten fluoride salt system, there are unique chemical issues not associated with other reactors. There is a need to develop a fluoride high-level waste form and an integrated fuel recycle strategy. Since earlier MSR efforts, there have been major advances in separation technologies and proposals for highly innovative separation systems unique to fluoride salts.

Preliminary exploration of these systems is appropriate because of their potential.

Table of contents

More detailed efforts will be required in the future. This requires the development of high-pressure helium to lowpressure salt heat exchangers.

The same technology is required to transfer heat from the MSR to a helium power cycle except the heat is transferred from the molten salt to the helium in the power cycle. Most, but not all, of the components in this system are very similar to those required for the NGNP program.

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It is also necessary to develop corrosion control and coolant monitoring strategies for protecting the reactor vessel and primary piping alloys. This temperature limit was largely due to the coupling required for steam cycle operations and did not represent a fundamental limit.

Thus, there is a natural base to build on to extend candidate materials for the higher temperature objectives of the Generation IV Program. The MSR and NGNP both use graphite as a moderator and various carbon-carbon composites for multiple structural applications; consequently, graphite and carboncarbon research will be coupled to the NGNP.

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A Survey and Selection of Candidate Salt and Structural Materials Candidate salts and materials will be selected based on literature survey, system design requirements, and investigation of materials use in industrial applications. In an MSR, the designer selects both the specific molten fluoride salt composition and the materials of construction. The demands of actinide burning may result in a choice of non-radioactive salt constituents that is different from previous applications. However, most of the work is not strongly dependent on the salt composition. Materials testing will take place over the range of temperatures, flows, and stresses expected in the MSR system.

A Irradiation Testing of Candidate Salts and Structural Materials Candidate materials and salts will be irradiated under expected neutron spectrum conditions to extend the existing knowledge base to meet the Generation IV MSR requirements. Following irradiation, A A Materials Modeling Advanced, mechanistically-based models for radiation performance will be developed.

Materials modeling is expected to be a crosscutting activity. Table A6. FY budget includes FY carryover funds. Consequently, the schedule to determine the viability of an AMSR is determined by these larger programs. The European programs that are part of GIF the largest and best funded MSR programs plan to complete scoping and screening activities by and establish the viability of the MSR by and to optimize performance by The activities of the NGNP program, NHI program, and the GIF program planning are expected to provide a more detailed program schedule within the next one to two years.

Traditional thermal and epithermal neutron spectrum MSRs are moderated by unclad graphite with the graphite-to-fuel ratio adjusted to provide the optimum neutron balance. Fast-spectrum MSRs, a relatively new development, have no core internals. In the core, fission occurs within the flowing fuel salt, which then flows into a primary heat exchanger where the heat is transferred to a secondary molten salt coolant.

The reactor and primary system are constructed of modified Hastelloy-N or a similar alloy for corrosion resistance to the molten salt. Volatile fission products e. The secondary coolant loop transfers the heat to the power cycle or hydrogen production facility. The secondary heat transport loop also uses a liquid salt that may be the same molten salt used in the A The secondary coolant 1 provides isolation between the low-pressure reactor and either the power cycle if electricity is being produced or a hydrogen production facility and 2 chemically reacts and traps tritium that escapes from the primary system.

With a fluid-fuel reactor, the tritium is not trapped in the solid fuel and tends to migrate across hot heat exchangers. A small cleanup system removes the tritium from the secondary coolant.

These parameters are for a large MWt U-thorium, liquid-fuel breeder reactor designed for the production of electricity using a steam cycle. Addm A Table 1. Design characteristics of the s MSBR. Unlike solid-fuel reactors, MSRs operate at steady-state conditions with no change in the nuclear reactivity of the fuel as a function of time.

Fuel is added as needed; consequently, the reactor has low excess nuclear reactivity. No excess fuel is needed at reactor startup to compensate for fuel depletion, and no excess reactivity is required to override xenon poisoning. No significant buildup of xenon occurs over time because the xenon gas continuously exits via the off-gas system. There is a negative temperature coefficient because increased temperatures lower the fuel-salt density and push fuel out of the reactor core.