Indirect Cycle Research Articles

Preliminary design and study of the indirect coupled cycle: An innovative option for Gas Fast Reactor

► The study concerns an alternative design for a Gas Fast Reactor. ► It assumes that the primary compressors are driven by the secondary turbogenerators. ► 1st interest: no requirement for external energy for driving the compression system. ► Safety advantages: no LOFA (motor failure) and improvement of grace delay for LOCA. The gas cooled fast reactor (GFR) is one of the six reactor concepts selected in the framework of the Generation IV forum. The main characteristics of the CEA GFR concept are a 2400MW core based on a ceramic pin type fuel as a reference, with an inlet temperature of 400 °C and an outlet temperature of 780 °C. The power conversion system is based on an indirect cycle with helium on the primary circuit, a Brayton cycle with a mixture of nitrogen and helium on the secondary circuit and a steam cycle on the tertiary circuit. In depressurised situations, the use of the gas coolant circulation as the main way to remove the decay heat has been selected. A specific system (DHR system) has been designed: it consists of three loops (3 × 100% redundancy) in extension of the pressure vessel, equipped with heat exchangers and blowers. In the current preliminary viability studies, GFR primary compression system relies on three axial blowers (operating in parallel) driven by 3 electrical motors. The present study concerns an alternative design of the primary compression systems, assuming that the 3 primary compressors are driven by the 3 turbogenerators of the secondary circuits. This new system requires that the 3 shafts connecting the turbines and the compressors of the secondary circuits are also connected to their corresponding primary blowers, via longer shafts crossing the primary circuit vessel. This new cycle is the only new element of complexity in this alternative design. This fact should be put in regards of the advantage of no requirement for external energy for driving the compression system (excepting for start-up) and of the safety advantages: suppression of the loss of flow accident due to a primary motor failure and possibility to use this new cycle to improve the grace delay of the reactor using the turbomachinery to drive the primary blowers during the beginning of an accidental situation.

Preliminary design and study of the indirect coupled cycle: CATHARE2 transient results

Abstract The gas cooled fast reactor (GFR) is one of the six reactor concepts selected in the framework of the Generation IV forum. The main characteristics of the CEA GFR concept are a 2400 MW core based on a ceramic pin type fuel as a reference, with an inlet temperature of 400 °C and an outlet temperature of 780 °C. The present study concerns an alternative design of the primary compression systems, assuming that the primary compressors are driven by the turbogenerators of the secondary circuits. This new system requires that the 3 shafts connecting the turbines and the compressors of the secondary circuits are also connected to their corresponding primary blowers, via longer shafts crossing the primary circuit vessel. This paper presents the transient results obtained with the CATHARE2 code. The transients study concerns various severe depressurised situations caused by a large diameter pipe rupture. The results of the reference break accident confirm the first conclusions obtained with analytical considerations: the behaviour in LOCA situation seems satisfactory. Furthermore sensitivity simulations are performed involving aggravating events and design parameters.

R&D needs for near-term HTRs

Recently some high temperature reactor (HTR) projects evolved towards application to cogeneration of power and industrial process steam. Gas direct or indirect cycles, previously favored, are no longer appropriate for this application. For AREVA and other vendors, the present reference concept is therefore the steam cycle with a core outlet temperature not exceeding 750°C. Because of the large development of this concept up to the 1980s in U.S. and Germany, present developments and risks will be minimized, but not eliminated. R&D and qualification remain necessary.Though many needs are common to different steam cycle designs, this paper focuses on R&D and qualification needs for AREVA block type design.There are different reasons why data from past programs are not sufficient: first, even for components or fuel concepts formerly demonstrated, adequate performance of their present implementation, with recreated design and manufacturing processes, might have to be shown. Moreover increased performance expected in some cases (fuel burn-up, reactor power) for economic competitiveness has to be demonstrated and might require design evolutions that need validation.Materials also require significant R&D. Qualification needs for present commercial graphite grades, different from past ones, and for new materials like composites are well known. But, with operating conditions different from LWR ones, new data might be needed even for usual materials, like the vessel material SA-508.Computer codes experienced large evolutions in last decades: more refined physical modeling, advanced calculation and software techniques. HTR design tools followed the general trend, and now some new experimental data might be needed for their qualification, as the qualification domain evolved (e.g. higher burn-up) and compliance of old data with quality standards is often no more traceable.Though in a first step HTR can simply substitute fossil fuel boilers for industrial steam supply without change in users’ applications, developments of materials, fuel and heat exchanger technologies for higher temperature will allow in the longer term widening the potential application market. But in that perspective some developments on the application side will also be needed: optimization of industrial processes for adaptation to specific features of HTR heat supply, heat transport technologies, heat exchangers for operation in severe environment, etc.Last but not least, the evolution of licensing requirements will force to reconsider some past demonstrations and solutions.There are conditions for this R&D to be useful for HTR industrial development:(1)A close coordination with design, for R&D to focus on relevant issues and to provide data in due time.(2)Availability of appropriate HTR specific R&D facilities, like irradiation rigs for fuel and materials or large helium loops that, for some of them, do not exist yet.

Identification of Plasmodium relictum causing mortality in penguins ( Spheniscus magellanicus) from São Paulo Zoo, Brazil

This study reports avian malaria caused by Plasmodium relictum in Magellanic Penguins ( Spheniscus magellanicus) from São Paulo Zoo. The disease was highly infective among the birds and was clinically characterized by its acute course and high mortality. The penguins of São Paulo Zoo were housed for at least 2 years without malaria; however, they had always been maintained in an enclosure protected from mosquito exposure during the night period. When they presented pododermatitis, they were freed at night for a short period. São Paulo Zoo is located in one of the last forest remnants of the city, an area of original Atlantic forest. In the winter, the space destined for Zoo birds is shared with migratory species. Hence the possibility exists that the disease was transmitted to the penguins by mosquitoes that had previously bitten infected wild birds. Avian malaria parasites are transmitted mainly by mosquitoes of the genera Aedes and Culex, common vectors in the Atlantic forest. In this study, one Culex ( Cux.) sp . was found, infected with P. relictum. There are diverse problems in housing distinct species of animals in captivity, principally when occupying the same enclosure, since it facilitates the transmission of diseases with indirect cycles, as is the case of Plasmodium spp ., because certain species that cause discrete infections in some bird species can become a serious danger for others, especially penguins, which do not possess natural resistance. Thus, serious implications exist for periodically testing and administrating malaria therapy in captive penguins potentially exposed to mosquitoes during the night period, as well as other captive birds from São Paulo Zoo.

On the Performance of Very High Temperature Reactor Plants With Direct and Indirect Closed Brayton Cycles

The performance of very high temperature reactor plants with direct and indirect closed Brayton cycles (CBCs) is compared and the effects of the molecular weight of the CBC working fluid on the number of stages and sizes of the axial flow, single shaft compressor and turbine are investigated. The working fluids considered are helium (4 g/mole), He–Xe, and He–N2 binary mixtures (15 g/mole). Also investigated are the effects of using low and high pressure compressors with intercooling, instead of a single compressor, and changing the reactor exit temperature from 700°C to 950°C on the plant thermal efficiency, the CBC pressure ratio, and the number of stages in and size of the turbomachines. For plants with direct CBCs, the effect of cooling the reactor pressure vessel with He bled off at the exit of the compressor is also investigated. The present analyses are performed for a reactor thermal power of 600 MW, shaft rotation speed of 3000 rpm, and intermediate heat exchanger temperature pinch of 50°C.

Advanced High Temperature Gas-Cooled Reactor Systems

Three systems have been proposed for advanced high-temperature gas-cooled reactors: a supercritical carbon dioxide (S-CO2) gas turbine power conversion system, a new microchannel heat exchanger (MCHE), and a once-through-then-out (OTTO) refueling scheme with burnable poison (BP) loading. A S-CO2 gas turbine cycle attains higher cycle efficiency than a He gas turbine cycle because of reduced compression work around the critical point of CO2. Considering temperature reduction at the turbine inlet by 30°C through intermediate heat exchange, the S-CO2 indirect cycle achieves an efficiency of 53.8% at a turbine inlet temperature of 820°C and a turbine inlet pressure of 20 MPa. This cycle efficiency value is higher by 4.5% than that (49.3%) of a He direct cycle at a turbine inlet temperature of 850°C and 7 MPa. A new MCHE has been proposed as an intermediate heat exchanger between the primary cooling He loop and the secondary S-CO2 gas turbine power conversion system and as recuperators of the S-CO2 gas turbine power conversion system. This MCHE has discontinuous “S-shaped” fins providing flow channels resembling sine curves. Its pressure drop is one-sixth that of a conventional MCHE with a zigzag flow channel configuration, but it has the same high heat transfer performance. The pressure drop reduction is ascribed to suppression of recirculation flows and eddies that appear around bend corners of the zigzag flow channels in the conventional MCHE. An optimal BP loading in an OTTO refueling scheme eliminates the shortcoming of its excessively high axial power peaking factor, reducing the power peaking factor from 4.44 to about 1.7, and inheriting advantages over the multipass scheme because it obviates reloading in addition to fuel handling and integrity checking systems. Because of the power peaking factor reduction, the maximum fuel temperatures are lower than the maximum permissible values of 1250°C for normal operation and 1600°C during a depressurization accident.

Autonomous Control Strategies for Very High Temperature Reactor Based Systems for Hydrogen Production

This paper is focused on feasible autonomous control strategies for Generation IV very high temperature reactors (VHTRs)-based systems for hydrogen production. Various burnable poison distributions and fuel compositions are considered. In particular, utilization of transuranium nuclides (TRUs) in VHTRs is explored as the core self-stabilization approach. Both direct cycle and indirect cycle energy conversion approaches are discussed. It is assumed that small-scale VHTRs may be considered for international deployment as grid-appropriate variable-scale self-contained systems addressing emerging demands for hydrogen. A Monte Carlo-deterministic analysis methodology has been implemented for coupled design studies of VHTRs with TRUs using the ORNL SCALE 5.1 code system. The developed modeling approach provides an exact-geometry 3D representation of the VHTR core details properly capturing VHTR physics. The discussed studies are being performed within the scope of the U.S. DOE Nuclear Energy Research Initiative project on utilization of higher actinides (TRUs and partitioned minor actinides) as a fuel component for extended-life VHTR configurations.

Preliminary Design Analysis of Hot Gas Ducts for the Nuclear Hydrogen System

Korea Atomic Energy Research Institute is in the process of carrying out a nuclear hydrogen system by considering the indirect cycle gas cooled reactors that produce heat at temperatures in the order of 950°C. A coaxial double-tube hot gas duct (HGD) is a key component connecting the reactor pressure vessel and the intermediate heat exchanger for the nuclear hydrogen system. Recently, a preliminary design analysis for the primary and secondary hot gas ducts of the nuclear hydrogen system was carried out. These preliminary design activities include a preliminary decision on the geometric dimensions, a preliminary strength evaluation, and an appropriate material selection. In this study, a preliminary strength evaluation for the HGDs of the nuclear hydrogen system has been undertaken. Preliminary strength evaluation results for the HGDs showed that the geometric dimensions of the proposed HGDs would be acceptable for the design requirements.

MIT PEBBLE BED REACTOR PROJECT

The conceptual design of the MIT modular pebble bed reactor is described. This reactor plant is a 250 Mwth, 120 Mwe indirect cycle plant that is designed to be deployed in the near term using demonstrated helium system components. The primary system is a conventional pebble bed reactor with a dynamic central column with an outlet temperature of 900 C providing helium to an intermediate helium to helium heat exchanger (IHX). The outlet of the IHX is input to a three shaft horizontal Brayton Cycle power conversion system. The design constraint used in sizing the plant is based on a factory modularity principle which allows the plant to be assembled “Lego” style instead of constructed piece by piece. This principle employs space frames which contain the power conversion system that permits the Lego-like modules to be shipped by truck or train to sites. This paper also describes the research that has been conducted at MIT since 1998 on fuel modeling, silver leakage from coated fuel particles, dynamic simulation, MCNP reactor physics modeling and air ingress analysis.

POTENTIAL APPLICATIONS FOR NUCLEAR ENERGY BESIDES ELECTRICITY GENERATION: A GLOBAL PERSPECTIVE

2 Energy supply is increasingly showing up as a major issue for electricity supply, transportation, settlement, and process heat industrial supply including hydrogen production. Nuclear power is part of the solution. For electricity supply, as exemplified in Finland and France, the EPR brings an immediate answer; HTR could bring another solution in some specific cases. For other supply, mostly heat, the HTR brings a solution inaccessible to conventional nuclear power plants for very high or even high temperature. As fossil fuels costs increase and efforts to avoid generation of Greenhouse gases are implemented, a market for nuclear generated process heat will be developed. Following active developments in the 80's, HTR have been put on the back burner up to 5 years ago. Light water reactors are widely dominating the nuclear production field today. However, interest in the HTR technology was renewed in the past few years. Several commercial projects are actively promoted, most of them aiming at electricity production. ANTARES is today AREVA's response to the cogeneration market. It distinguishes itself from other concepts with its indirect cycle design powering a combined cycle power plant. Several reasons support this design choice, one of the most important of which is the design flexibility to adapt readily to combined heat and power applications. From the start, AREVA made the choice of such flexibility with the belief that the HTR market is not so much in competition with LWR in the sole electricity market but in the specific added value market of cogeneration and process heat. In view of the volatility of the costs of fossil fuels, AREVA's choice brings to the large industrial heat applications the fuel cost predictability of nuclear fuel with the efficiency of a high temperature heat source free of Greenhouse gases emissions. The ANTARES module produces 600 MWth which can be split into the required process heat, the remaining power drives an adapted prorated electric plant. Depending on the process heat temperature and power needs, up to 80 % of the nuclear heat is converted into useful power. An important feature of the design is the standardization of the heat source, as independent as possible of the process heat application. This should expedite licensing. The essential conditions for success include: Timely adapted licensing process and regulations, codes and standards for such application and design An industry oriented R&D program to meet the technological challenges making the best use of the international collaboration. Gen IV could be the vector Identification of an end user (or a consortium of) willing to fund a FOAK

The Supercritical Carbon Dioxide Power Cycle: Comparison to Other Advanced Power Cycles

This paper consists of three parts. The first part presents a mostly thermodynamic comparison of the supercritical carbon dioxide (S-CO2) cycle to helium Brayton, superheated steam, and supercritical steam cycles. Issues that contribute to plant cost are discussed. The second part presents an economic comparison of a gas-cooled reactor coupled to S-CO2 direct, helium Brayton direct, and superheated steam indirect cycles. The results indicate savings of up to 30% if the steam indirect cycle is replaced with the direct S-CO2 cycle. Compared to the helium direct cycle, the savings can reach 15%. The third part describes the optimization and potential of the indirect S-CO2 cycle and the effect of reheating. The indirect cycles of helium to S-CO2 and lead bismuth to S-CO2 are studied to assess the performance of gas-to-gas and liquid metal or liquid salt indirect cycles, respectively. It is shown that although the indirect cycle of helium to S-CO2 is feasible, it poses challenges in the intermediate heat exchanger design and suffers efficiency losses due to the large power consumption of the main circulators. Gas indirect cycles are well suited for liquid metal or liquid salt reactors. Further, the study indicates that employing reheat is economically unattractive for the indirect cycle of helium to S-CO2 because of efficiency reduction from pressure losses in reheaters and interconnecting ducting and additional capital cost. A similar conclusion was also reached for the indirect cycles of liquid metal or liquid salt to S-CO2 even though pumping power is very small. This is because of the additional cost of an intermediate liquid metal (or liquid salt) loop, which needs to be added since it is not possible to place all heat exchangers for reheat inside the reactor vessel.

ANTARES: The HTR/VHTR project at Framatome ANP

Framatome ANP is developing a very high temperature reactor (VHTR), relying on its previous experience with high temperature reactor concepts, from its participation in the MODUL and the GT-MHR designs. While being a major actor in the nuclear reactor business with proven light water technology, AREVA wishes to be ready to meet the new challenges calling for small grid requirements, high temperature process heat and cogeneration. The Framatome ANP VHTR design for electricity production is based on an indirect cycle coupled to an “off-the-shelf” combined cycle gas turbine. Although direct cycle HTRs are being promoted for their high efficiency, preliminary evaluations show that the Framatome ANP design efficiency is on par with a direct cycle while avoiding power generation system (PGS) developments and keeping the PGS contamination free. Moreover, the nuclear heat source of the indirect cycle could also be used to meet the heat supplies from a standard design for multiple applications.

Parametric studies on different gas turbine cycles for a high temperature gas-cooled reactor

The high temperature gas-cooled reactor (HTGR) coupled with turbine cycle is considered as one of the leading candidates for future nuclear power plants. In this paper, the various types of HTGR gas turbine cycles are concluded as three typical cycles of direct cycle, closed indirect cycle and open indirect cycle. Furthermore they are theoretically converted to three Brayton cycles of helium, nitrogen and air. Those three types of Brayton cycles are thermodynamically analyzed and optimized. The results show that the variety of gas affects the cycle pressure ratio more significantly than other cycle parameters, however, the optimized cycle efficiencies of the three Brayton cycles are almost the same. In addition, the turbomachines which are required for the three optimized Brayton cycles are aerodynamically analyzed and compared and their fundamental characteristics are obtained. Helium turbocompressor has lower stage pressure ratio and more stage number than those for nitrogen and air machines, while helium and nitrogen turbocompressors have shorter blade length than that for air machine.

Brayton Cycle for High-Temperature Gas-Cooled Reactors

This paper describes research on improving the Brayton cycle efficiency for a high-temperature gas-cooled reactor (HTGR). In this study, we are investigating the efficiency of an indirect helium Brayton cycle for the power conversion side of an HTGR power plant. A reference case based on a 250-MW(thermal) pebble bed HTGR was developed using helium gas as a working fluid in both the primary and power conversion sides. The commercial computer code HYSYS was used for process optimization. A numerical model using the Visual-Basic (V-B) computer language was also developed to assist in the evaluation of the Brayton cycle efficiency. Results from both the HYSYS simulation and the V-B model were compared with Japanese calculations based on the 300-MW(electric) Gas Turbine High-Temperature Reactor (GTHTR) that was developed by the Japan Atomic Energy Research Institute. After benchmarking our models, parametric investigations were performed to see the effect of important parameters on the cycle efficiency. We also investigated single-shaft versus multiple-shaft arrangements for the turbomachinery. The results from this study are applicable to other reactor concepts such as fast gas-cooled reactors, supercritical water reactors, and others.The ultimate goal of this study is to use other fluids such as supercritical carbon dioxide for the HTGR power conversion loop in order to improve the cycle efficiency over that of the helium Brayton cycle. This study is in progress, and the results will be published in a subsequent paper.

ACACIA: A small scale power plant for near term deployment in new markets

A 60 MWth, 23 MWe nuclear plant design with an indirect Brayton cycle is proposed for application on markets other than large-scale electricity production on the short to medium term. The reactor will be a simplified pebble bed high-temperature reactor, whereas for the secondary cycle nitrogen is proposed as a heat carrier. In this way, a conventional air based gas turbine can be applied, while at the same time excluding the scenario of air ingress in the reactor core through a heat exchanger leak. Two variations of cycle design will be presented: co-generation and maximised electricity production. Economic aspects of the small-scale plant will be discussed. For the reactor core, a cartridge design without fuel recirculation or other form of on-line fuelling is proposed, in order to obtain maximum plant simplicity. Excess reactivity is being adjusted to a constant low level over the entire core lifetime by using burnable poison (BP). A design with an annular core has been opted for with an inner reflector where the burnable poison could be placed in the form of B 4 C pins. This positioning of the burnable poison has been chosen in order to maximise the depletion of the burnable poison as well as not to reduce the control rod worth. A flat reactivity curve during the entire core lifetime with an excess reactivity below 5% can be achieved. As main tool in the study a model with coupled neutronics/thermal-hydraulics has been developed using the code system PANTHERMIX. Analysis of the temperature profile over the reactor and its evolution during the cartridge lifetime shows large variations associated mainly to the depletion process of the burnable poison. Therefore, the inclusion of thermal-hydraulic feedback proved to be essential in the core design. Also, the two most important safety transient scenarios for pebble bed HTRs have been analysed: pressurised and depressurised loss of forced cooling without scram. The results demonstrate the inherent safety characteristics of the design, since the maximum fuel temperatures remain below the level where fuel damage becomes important.

The helminth community of the wood mouse, Apodemus sylvaticus, in the Sierra Espuna, Murcia, Spain.

The helminth community of the wood mouse, Apodemus sylvaticus, in the Sierra Espuna was characterized after a complete analysis of its helminth community component and infracommunity structure relative to host age, sex and year of capture. The helminth community comprised 13 species: one trematode, four cestodes and eight nematodes. The cestode Pseudocatenotaenia matovi and the nematode Syphacia frederici were the most prevalent and abundant helminth species, respectively. Sixty four percent of mice analysed presented helminths with a direct cycle and 42% presented helminths with an indirect cycle. The helminth community presents a low diversity with infracommunities usually made up of only one or two helminth species. Host age and year of capture seem to play a major role in determining species richness and helminth diversity, but not in determining the abundance of helminths. Host sex does not seem to affect the infection rate nor the diversity. Further studies on more samples of wood mice and other small mammal species in this regional park are needed to explore any possible interactions between helminth communities in the host populations.

Energetic Analysis of Experimental Behavior of Molten SnxLiy When Impacted by a Vertical Column of Water

The SnxLiy and PbxLiy, alloys are being considered as liquid breeding materials for fusion reactor applications. Thus, it is important to understand the safety implications associated with inadvertent contact with water used in an indirect cycle. In an effort to study this interaction, experiments have been conducted with these molten alloys when impacted with a vertical 2.4-m-tall column of water at 30 and 60°C. The qualitative behavior of Sn75Li25 was compared under similar conditions with other candidate molten metals, specifically tin, lead, and a lead-lithium alloy, Pb83Li17. Multiple pressure spikes were produced with Sn and Pb, while essentially only one initial pressurization followed by a few strongly damped minor peaks was observed with the different alloys containing lithium. Results with tin-lithium are quite similar to pure tin and lead behavior. Dynamic pressure traces from the physical and chemical reactions are discussed and used to compare the energetics associated with the two different alloys. The pressure traces were used to calculate experimental impulses, which represent the energetics of the reactions. The impulse ratio of the experimental and the theoretical values increased in the tests of alloys containing lithium. (Hydrogen production from lithium-water reactions was quite rapid and copious.) In contrast, hydrogen production with tin-lithium was modest and quite similar to the lead-lithium alloy. It was found that the metal-water interactions of Pb83Li17 and Sn75Li25 are quite similar and have significantly reduced energetics from those of pure lithium and other reactive metals being considered.

Conceptual design of a fast breeder reactor cooled by supercritical steam

The steam-cooled fast breeder reactor with negative reactivity characteristics against voiding and flooding is feasible under the supercritical pressure. The breeding ratio is 1.04. A flat core with a zirconium hydride layer is adopted for mitigating the void reactivity. The thermal efficiency of the indirect cycle system is improved 9% relatively from current PWRs. The core should be cooled in 10 s after the large break LOCA. The coast down time should be larger than 30 s to overcome the LOF by the trip of all blowers.

Xylem-to-Phloem Transfer of Organic Nitrogen in Young Soybean Plants

Xylem-to-phloem transfer in young vegetative soybean (Glycine max [L.] Merr.) plants (V4 stage) was identified as the difference in the distribution of [(14)C]inulin, a xylem marker, and [(14)C]aminoisobutyric acid (AIB), a synthetic amino acid, fed via the transpiration stream. Since [(14)C]AIB was retained in the stem to some extent, whereas [(14)C]inulin was not, the distribution of these marker compounds in each leaf was expressed as a percentage of the total [(14)C] radioactivity recovered in the foliage. The developing third trifoliolate was a consistent and reliable indicator of xylem-to-phloem transfer. The phloem stream provided to the developing trifoliolate up to fourfold the relative proportion of solute received from the xylem stream; this was markedly reduced by increased light intensity and consequently water flow through the xylem. Evidence from heat girdling experiments is discussed with respect to the vascular anatomy of the soybean plant, and interpreted to suggest that direct xylem-to-phloem transfer in the stem, in the region of the second node, accounted for about one-half of the AIB supplied to the developing trifoliolate, with the remainder being provided from the second trifoliolate. Since AIB is not metabolized it seems likely that rapid transfer within the second trifoliolate occurred as direct veinal transfer rather than indirect cycling through the mesophyll. This study confirmed that xylem-to-phloem transfer plays a major role in the partitioning of nitrogen for early leaf development.

Synthèse et étude du réarrangement des diazoles-1,3: alkyl-1 alkylthio-2 (allylthio, arylthio, cycloalkylthio) imidazoles. Partie II. Réarrangement et réactions parasites

The kinetics of the [Formula: see text] rearrangement reaction for a series of 1-alkyl-2-alkylthio (or allylthio, arylthio, cycloalkylthio) imidazoles has been studied at various temperatures. The rearrangement and hydrolysis results have been compared with those of 1-alkyl-2-methylthio-Δ-2-imidazolines and 1-alkyl-2-methylthio-Δ-2-tetrahydropyrimidines. Electronic and steric effects are discussed and a parallel between pKa and rearrangement is proposed. The limit of this parallel is either a structural effect such as direct (ortho effect) or indirect (cycle deformation) steric interactions or geometric modifications of the heterocycles studied (five- or six-membered cycles). All these results permitted the coherent choice of the amplification compound of a nonconventional photographic process.