Intermediate Heat Transfer Research Articles

Design and analysis of the improved configuration of the secondary circuit for the EU-DEMO power plant

DEMO is planned to be a prototype fusion power plant capable of demonstrating production of electricity at the level of a few hundred MW. DEMO is considered to be an intermediate step between the ITER experimental reactor and a commercial power plant. Design and assessment studies on the European (EU) DEMO are carried out by the EUROfusion consortium. The Primary Heat Transfer System (PHTS) transfers heat from the breeding blanket (BB) and other reactor heat sources (divertor and vacuum vessel) to the secondary circuit. Two main BB concepts, and the respective PHTSs, for EU-DEMO are considered: the Helium Cooled Pebble Bed (HCPB) BB and the Water Cooled Lithium Lead (WCLL) BB. Two variants for each concept are possible: with or without the Intermediate Heat Transfer System (IHTS), including the Energy Storage System (ESS), between the PHTS and the Power Conversion System (PCS), which generates electricity. The role of IHTS and ESS is to prevent energy pulses obtained by BB PHTS during the plasma burns to be directly transferred to PCS. In 2017 we proposed the first concept of the PCS configuration for the option WCLL BB with the ESS, which enabled almost constant generation of electricity during both plasma pulse and dwell phases. However, we observed some disadvantages of this concept, which had to be cured, i.e., excessive temperature differences between the pulse and dwell phases in several heat exchangers. In the present paper, we describe the GateCycle model of the improved PCS configuration for the WCLL BB with the ESS option, which ensures not only almost constant electricity production during both plasma pulse and dwell phases, but also acceptably small temperature fluctuations |Tpulse -Tdwell| in all the circuit components, which should prevent their failure due to excessive thermal stress.

Thermal-hydraulic modeling and analyses of the water-cooled EU DEMO using RELAP5 system code

The conceptual design of the Primary Heat Transfer System (PHTS) of the water-cooled European (EU) DEMO foresees two independent cooling circuits, the breeding zone PHTS and the first wall PHTS. During the pulse time (120 min) the first delivers thermal power to the turbine, the latter delivers thermal power to the Intermediate Heat Transfer System (IHTS) equipped by an Energy Storage System (ESS). The IHTS delivers partially thermal power to the turbine in pulse so that to accumulate a suitable amount of energy in ESS to operate the turbine during the dwell time (10 min) at almost constant load, despite the EU DEMO pulsation of the generated thermal power. A dynamic model of the primary systems of water-cooled EU DEMO is developed using RELAP5/Mod3.3 system code to verify components sizing and to investigate code predictive capabilities. The model includes the primary and secondary side of the breeding zone and first wall, and in particular: in-vessel (i.e. breeding blanket) and ex-vessel components (i.e. main collectors, hot and cold legs, heat exchangers, steam generators, pumps). Preliminary assessments of the nodalization have been carried out, in particular checking pressure drops along the systems and heat exchanger performances.

Numerical Modeling of a Novel Solar Driven Sulfuric Acid Decomposition Reactor

An efficient and cost-effective approach to produce hydrogen sees the direct use of thermal power to drive thermochemical water splitting processes. Such cycles break the hydrogen-oxygen bounds through a series of heat-driven chemical reactions, with recirculation of intermediate substances of different type. Among the various water splitting processes, the thermo-electrochemical Hybrid Sulfur (HyS) is one of the most appealing cycles. It has only fluid reactants and is comprised of only two global reaction steps: a low temperature electrochemical exothermic section, operating at temperatures on the order of 100 °C, and a high temperature endothermic thermal section operating at max temperatures of about 800 °C. In the electrochemical section SO2 and H2O are combined to produce electrochemically H2 at the electrolyzer cathode and H2SO4 at the electrolyzer anode. Sulfuric acid is recycled inside the plant, concentrated and decomposed, in the high temperature thermal section, into SO2, O2 and H2O. The endothermic decomposition of sulfuric acid takes place in two main reactions: H2SO4 → SO3 + H2O (1) SO3 → SO2 + 0.5 O2 (2) The required thermal power is provided by external source and by internal heat recovery from the SO2-O2 stream product of the decomposition reactions. Solar driven HyS process is being studied by the Greenway Energy and the University of South Carolina within the DOE HydroGEN program (part of the DOE Energy Material Network initiative) [1], partnering with the Idaho National Laboratory, the Savannah River National Laboratory and the National Renewable Energy Laboratory. The project focuses on the high temperature sulfuric acid decomposition section. One of the main tasks of the project is to develop and fabricate an effective and low cost high temperature solar reactor for the decomposition of the sulfuric acid into sulfur dioxide. Previous work was carried out within the DOE-NHI Initiative and resulted in identifying a reactor concept based on the bayonet heat exchanger as the baseline reactor configuration [2]. The bayonet reactor was tested both from a modeling point of view and from an experimental point of view, demonstrating the potential to achieve almost the reaction equilibrium conditions. At temperatures of about 800 °C and pressures of 14 bar, an SO2 production of approximately 19 mol fraction % was achieved, which is essentially the equilibrium value at the same operating conditions [3,2]. However, a solar driven bayonet concept presents several drawbacks, mainly relative to the need for an additional intermediate heat exchanger to transfer the solar power to the reactive sulfur mixture. The removal of the intermediate solar heat exchanger will result in increased energetic and exergetic efficiencies and decreased capital and operating costs. A novel reactor concept is proposed, described and discussed, based on a direct solar heated cavity receiver system. It realizes the solar driven sulfuric acid decomposition and internal heat recovery in a single unit, directly heated by the incident solar radiation without the presence of any intermediate heat transfer fluid. The reactor includes catalytic structures to decompose H2SO4 and foamed structures in the internal heat recovery channels to increase the overall thermal conductivity. A reactor unit, made of SiC material, and directly heated from solar radiation has preliminarily been demonstrated from a modeling point of view. A detailed transport phenomena model was developed using Finite-Volume approach and run in STAR-CCM+. The kinetics parameters reported in Reference [3] were adopted for the current simulations. Results showed excellent conversion rates and reduced temperature gradients and pressure drops inside the device, achieving better performance than the bayonet configuration. The cavity receiver-reactor also allows: (1) efficient internal heat recovery from the decomposition products and (2) connections with the metallic interfaced HyS equipment (e.g. tubing, valves, etc.) at low temperatures. Results obtained from a first conceptual scale up analysis of the reactor concept also demonstrated the actual feasibility for integration in both centralized and distributed solar tower based plants.

Design and analysis of a new configuration of secondary circuit of the EU-DEMO fusion power plant using GateCycle

Conceptual and design studies on the European DEMOnstration Fusion Power Plant (EU-DEMO) are carried out under the lead of the EUROfusion Consortium. The Primary Heat Transfer System (PHTS) transfers heat from the nuclear heat sources, i.e. the breeding blanket, divertor, and vacuum vessel, to the secondary circuit called Power Conversion System (PCS) which generates electric energy. To mitigate undesirable transient effects resulting from the pulsed EU-DEMO operation, the Intermediate Heat Transfer System (IHTS) with the Energy Storage System (ESS) filled with molten salt is added between the PHTS and PCS. One of the four considered options for the blanket cooling and the related PHTS is the Water-Cooled Lithium-Lead Breeding Blanket (WCLL BB). Recently new cooling concept for the WCLL BB has been proposed. This work is focused on the design of the new configuration of the steam/water PCS for the option WCLL BB with the ESS. The detailed GateCycle model of the considered circuit is created and its operation during the plasma burn and during the dwell phase is simulated. The proposed model of the PCS cycle is used to study the effect of the operating conditions on the cycle power and efficiency, and its basic operating parameters.

Study of EU DEMO WCLL breeding blanket and primary heat transfer system integration

The Water-Cooled Lithium Lead breeding blanket is a candidate option for the realization of European DEMO power plant. One of the main functions of the breeding blanket is to recover the thermal power from the first wall and the breeding zone and to drive it to the Primary Heat Transfer System. Moreover, due to the DEMO pulsed operation, an Energy Storage System is foreseen in order to ensure thermal energy availability and reduce cycling loading during dwell time. The blanket design must interface with the Primary Heat Transfer System and the Intermediate Heat Transfer System in an integrated solution with the aim of delivering feasible power plant, with high conversion efficiency.A thermal-hydraulic model of the main components of the water cooled Primary Heat Transfer System is developed, using RELAP5 system code, to support the design and sizing. A RELAP5/Mod3.3 extended version has been set-up to implement PbLi and molten salt (HITEC) fluid proprieties, as well as the relevant heat transfer correlations. The results of the preliminary applications provide a comprehensive set of thermo-hydraulic parameters (i.e. coolant inventory, pressure drops), relevant for the assessment of the design integration.

PROSPECTS OF APPLYING HEAT PUMPS UPON CARRYING OUT COMBINED MASS TRANSFER PROCESSES

The prospects of using heat pumps for the separation of mixtures by combination of different mass transfer processes are considered, which is associated with the supply and removal of thermal energy. The heat released in some stages can be successfully used at other stages of separation. Using heat pumps makes it possible to change the temperature potentials of heat fluxes, which greatly improves the efficiency of regenerative heat transfer. If a vapor phase forms when carrying out mass transfer processes, it can be used as the working fluid of the heat pump of the open type. If mass transfer processes are carried out without the formation of vapor phases, it is necessary to apply heat pumps of the closed type, in the circuit of which a variety of intermediate heat transfer fluids circulate. The article briefly describes the features of mixtures separation by a combination of fractional crystallization by evaporation, dissolution, fractional melting, distillation and rectification with the use of compressor heat pumps of open and closed type. In order to assess the energy efficiency of different variants of combined processes and to compare them with traditional separation it is suggested to use the relative equivalent consumption, which makes it possible to take into account the different cost of thermal and electric energy required for carrying out the considered process of separation. Different variants of organizing the combined separation are compared. It was found that their energy efficiency significantly depends on the composition of the initial mixture, on requirements for the separation products, as well as on the position of the eutectic point on the liquid - solid phase charts. It was shown that the use of heat pumps allows reducing several times energy costs, as well as the consumption of refrigerants for combined separation.

Experimental study of downward facing boiling on a structured hemispherical surface

A heating system with liquid metal as the intermediate heat transfer medium was introduced into a scaled three-dimensional reactor vessel. The liquid metal was heated by heaters and then circulated inside a hemispherical vessel. Then, the outer surface of the lower head was cooled by boiling water. The objective of this study is studying the boiling regimes and heat fluxes on the outer surface of the hemispherical lower head. The boiling heat transfer was investigated on a hemispherical plain surface and on a surface with interconnected grooves with triangular cavities surface using saturated deionized water at atmospheric pressure. The critical heat flux (CHF) on the plain surface at an inclination angle of 85° was 857.3 kW/m2, with no boiling crisis observed on the structured surface up to the highest heat flux of 1366.9 kW/m2 at the inclination angle of 85°, with the liquid metal temperature higher than 400 °C. Thus, the CHF on the structured surface was more than 59% greater than on the plain surface at an inclination angle of 85° with the liquid metal temperature inside the pressure vessel reduced by 80–100 °C for the same heating power. The structured surface forms a liquid-vapor conversion path with the cavities as stable nucleation sites and the interconnected grooves as cooling water supply pathways. Thus, the structured surface significantly enhances the boiling heat transfer and the CHF.

Techno-Economic Analysis of ORC in Gas Compression Stations Taking Into Account Actual Operating Conditions

Gas compressor stations represent a huge potential for exhaust heat recovery, currently under-exploited. Typical installations consist of multiple gas turbine units in mechanical drive arrangement, operated, most of the time, at part-load conditions and with limited conversion efficiency. In this context, this paper investigates the energetic-economic potential of ORC application in typical gas compression facilities, as innovative contribution with respect to literature. The ORC is designed to convert the gas turbines wasted heat into useful power. Additional power output can be used either inside the compression facility, reducing the amount of consumed natural gas and, consequently, the environmental impact, or delivered to the electrical grid. Taking into account real operation of gas turbines in a natural gas compression station, located in North America, additional generated energy and CO2 avoided, thanks to ORC operation, are quantified. Two ORC arrangements, namely with and without intermediate heat transfer fluid, are proposed and the design performance are identified. Influence of topper cycle part load operations on bottomer section are quantified through an off-design thermodynamic evaluation. The goal of the performed analysis is to obtain a detailed scenario of the integrated system operation on yearly basis. Results, for a reference 50 MW compression station, show that the direct heat exchange configuration guarantees up to 66 GWh/year of additional electrical energy, saving up to 36*103 tons/year of CO2, while ORC investment costs can be recovered within 7 years of operation. The performed comprehensive investigation assesses the ORC as a techno-economic profitable technology to recover wasted heat in natural gas compression facilities.

Waste heat recovery by means of Organic Rankine Cycle (ORC) system coupled with two-phase closed thermosyphons

This paper aims at assessing the performance of a waste heat-to-power plant by means of an ORC (Organic Rankine Cycle) system coupled with two-phase closed thermosyphons (or gravity-assisted heat pipes or wickless heat pipes). The heat exchanger, made up of two-phase closed thermosyphons, is used for conveying heat from exhaust stream to ORC working fluid. In reality, a hot oil loop or a pressurized hot water loop or a saturated steam loop or even a direct evaporator is often used to transfer heat from the heat source to the ORC system. However, installing a heat exchanger directly in the hot gas paths of ORC system evokes the concerns for the flammability and/or toxicity of organic working fluids especially when the heat source has a relatively high temperature. Also the use of an intermediate heat carrier loop such as thermal oil or saturated steam or pressurized water loop is costly and involves installation of comparatively heavy components. In principle, the use of two-phase closed thermosyphons for capturing and transporting heat from a waste heat source to organic working fluid is less expensive than utilizing an intermediate heat transfer loop and eliminates safety issues as in the case of direct installation of an ORC evaporator in the hot gas paths.

Experimental investigation of a LBE-helium heat exchanger based the ADS

Accelerator driven sub-critical system (ADS) has been universally regarded as the most promising approach to dispose the long-lived nuclear wastes. Lead bismuth eutectic (LBE) is the suitable coolant and spallation target material for ADS. But some theoretical models and empirical correlations of LBE are different from that of the existing conventional fluid. The performances of intermediate heat exchanger and heat transfer characteristics of LBE-helium two-loop cooling system are still not fully understood. The experimental investigation of the LBE-helium heat exchanger is performed in the present research. The effectiveness-NTU correlation of LBE-helium heat exchanger is verified experimentally in the LBE-helium experimental loop of ADS. The experimental results indicate that the effectiveness of LBE-helium heat exchanger is 0.8–0.88 at Rc = 0.45–0.77. The Moody correlation of friction factor is in reasonable agreement with the experimental data. The overall heat transfer coefficient and actual heat transfer rate increase with increasing LBE inlet temperature, and decrease with increasing helium inlet temperature. As helium inlet flowrate increases, overall heat transfer coefficient and actual heat transfer rate increase remarkably.

Simultaneous heat integration and techno-economic optimization of Organic Rankine Cycle (ORC) for multiple waste heat stream recovery

In the past decades, the Organic Rankine Cycle (ORC) has become a promising technology for low and medium temperature energy utilization. In refineries, there are usually multiple waste heat streams to be recovered. From a safety and controllability perspective, using an intermedium (hot water) to recover waste heat before releasing heat to the ORC system is more favorable than direct integration. The mass flowrate of the intermediate hot water stream determines the amount of waste heat recovered and the final hot water temperature affects the thermal efficiency of ORC. Both, in turn, exert great influence on the power output. Therefore, the hot water mass flowrate is a critical decision variable for the optimal design of the system. This study develops a model for techno-economic optimization of an ORC with simultaneous heat recovery and capital cost optimization. The ORC is modeled using rigorous thermodynamics with the concept of state points. The task of waste heat recovery using the hot water intermedium is modeled using the Duran-Grossmann model for simultaneous heat integration and process optimization. The combined model determines the optimal design of an ORC that recovers multiple waste heat streams in a large scale background process using an intermediate heat transfer stream. In particular, the model determines the optimal heat recovery approach temperature (HRAT), the utility load of the background process, and the optimal operating conditions of the ORC simultaneously. The effectiveness of this method is demonstrated with a case study that uses a refinery as the background process. Sensitivity of the optimal solution to the parameters (electricity price, utility cost) is quantified in this paper.

Підземні газифікація та спалювання бурого вугілля з використанням підземних вод

The energy issue is one of the most significant issues for Ukraine due to its lack of energy supplies. Coal remains are the main fuel and power resource, which will fulfill the need for chemical raw material for the next few centuries. The problem of coal excavation is priority for Ukraine. One of the main aspects is rationalization of mining and protection the environment. Underground coal gasification (UCG) and underground coal incineration (UCI), which unites mining and their underground processing in one technical process which can solve all difficulties, but the implementation is problematic due to fundamental disadvantages. One of ways of improving the underground mining and optimizing of these processes is using groundwater as an intermediate heat-transfer carrier.Key words: coal, underground coal gasification, underground coal incineration, ground water.

Experimental study on a novel liquid metal fin concept preventing boiling critical heat flux for advanced nuclear power reactors

The public has expressed concern about the safety of a nuclear power plant after the Fukushima Daiichi nuclear disaster that was caused by a tsunami on March 11, 2011. There are enough safety systems in nuclear power plants, but the accident occurred owing to an expected, complex natural disaster. If inevitable nuclear accidents take place, we have to consider the best way to mitigate public damage. The leakage of radioactive material from nuclear power plants should be prevented under all circumstances. The integrity of the reactor vessel can be maintained by flooding the coolant around the reactor vessel, which means that the radioactive materials are retained in the vessel. This cooling strategy is called in-vessel corium retention through external reactor vessel cooling (IVR-ERVC). A feasibility study of this cooling strategy was conducted with full-size facilities simulating AP600 and AP1000. The stable cooling of a high-power reactor is uncertain in heat removal capacity. We propose a new idea to resolve this issue by inserting liquid metal between the reactor vessel and the flooded water. The liquid metal will function as an intermediate heat transfer medium. A large heat transfer area could thus be ensured for heat removal uncertainty. The likelihood of a steam explosion is strongly prevented because it is due to direct contact of water with the molten fuel. In the current study, this cooling concept is called “liquid metal fin.” An experimental test and a computational fluid dynamics (CFD) simulation confirmed that heat transfer was improved. The heat transfer between the coolant and the heated structure was enhanced when the liquid metal was flooded. The highest heat flux of a possibly dangerous region was reduced to 1/3 when the liquid metal was used. IVR-ERVC with liquid metal can be established successfully at a high-power reactor through these results.

Waste Heat Recovery from Castings and Scrap Preheating by Recovered Heat Using an Intermediate Heat Transfer Medium

Energy conservation is a major topic of concern since our energy sources are exhausting exponentially. This paper focuses on waste heat recovery using which scrap preheating is done in metal castings using sand molds. During solidification of molten metal, most of the heat is lost to the sand. The proposal is to prepare the sand mould with aluminium shots surrounding the mold cavity. These shots absorb some of the heat from the solidifying metal in the mold cavity. The heated shots are separated from the mold and they are allowed to transfer their heat energy to the metal scrap by conduction. The experiments indicate that at least 6.4% of heat recovery is achievable. This will be instrumental in reducing the enormous amount of energy spent to melt the metal considering the fact that casting is the most widely used manufacturing process globally.

Hybrid gas turbine–organic Rankine cycle for seawater desalination by reverse osmosis in a hydrocarbon production facility

Despite water scarcity, the use of industrial waste heat for seawater desalination has been limited in the Middle East to date. This study evaluates the technical and economic feasibility of integrating on-site gas turbine power generation and reverse osmosis equipment for the production of both electricity and fresh water in a coastal hydrocarbon production facility. Gas turbine exhaust gas waste heat is recovered using an intermediate heat transfer fluid and fed to an organic Rankine cycle evaporator, to generate mechanical power to drive the reverse osmosis high pressure pump. Six candidate organic working fluids are evaluated, namely toluene, benzene, cyclohexane, cyclopentane, n-pentane and R245fa. Thermodynamic and desalination performance are assessed in the harsh climatic and salinity conditions of the Arabian Gulf. The performance metrics considered incorporate electric power and permeate production, thermal and exergy efficiency, specific energy consumption, system size, and permeate quality. Using toluene in the bottoming power cycle, a gain in power generation efficiency of approximately 12% is achieved relative to the existing gas turbine cycle, with an annual average of 2260m3/h of fresh water produced. Depending upon the projected evolution of local water prices, the investment becomes profitable after two to four years, with an end-of-life net present value of 220–380million USD, and internal rate of return of 26–48%.

DRACS thermal performance evaluation for FHR

Direct Reactor Auxiliary Cooling System (DRACS) is a passive decay heat removal system proposed for the Fluoride-salt-cooled High-temperature Reactor (FHR) that combines coated particle fuel and a graphite moderator with a liquid fluoride salt as the coolant. The DRACS features three coupled natural circulation/convection loops, relying completely on buoyancy as the driving force. These loops are coupled through two heat exchangers, namely, the DRACS Heat Exchanger and the Natural Draft Heat Exchanger. In addition, a fluidic diode is employed to minimize the parasitic flow into the DRACS primary loop and correspondingly the heat loss to the DRACS during normal operation of the reactor, and to keep the DRACS ready for activation, if needed, during accidents.To help with the design and thermal performance evaluation of the DRACS, a computer code using MATLAB has been developed. This code is based on a one-dimensional formulation and its principle is to solve the energy balance and integral momentum equations. By discretizing the DRACS system in the axial direction, a bulk mean temperature is assumed for each mesh cell. The temperatures of all the cells, as well as the mass flow rates in the DRACS loops, are predicted by solving the governing equations that are obtained by integrating the energy conservation equation over each cell and integrating the momentum conservation equation over each of the DRACS loops. In addition, an intermediate heat transfer loop equipped with a pump has also been modeled in the code. This enables the study of flow reversal phenomenon in the DRACS primary loop, associated with the pump trip process. Experimental data from a High-Temperature DRACS Test Facility (HTDF) are not available yet to benchmark the code. A preliminary code validation is performed by using natural circulation experimental data available in the literature, which are as closely relevant as possible. The code is subsequently applied to the HTDF that is under construction at the Ohio State University.

Heat exchanger network design of large-scale industrial site with layout inspired constraints

This paper presents a systematic approach for the synthesis of the heat recovery network in total site using a Mixed Integer Linear Programming model. This model returns a near-to-optimal network configuration with minimum utility cost while allows to select geographically closest matches. The Heat Load Distribution is the subproblem of the network design and has been reported to be quite expensive to solve for large-scale problems. The computational complexity of HLD resides in the number of streams and the feasible networks. An additional challenge, raising particularly in industrial problems, has been the intermediate heat transfer network which aggravates the combinatorial complexity. The presented methodology deals with those difficulties by priority consideration based on the location of process units. It helps significantly reducing the computational time and also comes with a realistic network sketch with respect to the plant layout. Several examples are discussed along with a real industrial case study.

A catalytic combustion-type CO gas sensor incorporating aluminum nitride as an intermediate heat transfer layer for accelerated response time

Abstract. A catalytic combustion-type carbon monoxide gas sensor exhibiting good sensing performance even at moderate temperatures was previously developed by employing a Pt loaded CeO2–ZrO2–SnO2 solid solution as the CO oxidizing catalyst. The addition of aluminum nitride as an intermediate heat transfer layer between the Pt coil and the CO oxidizing catalyst drastically accelerated the response of this device to CO at temperatures as low as 70 °C.

Simulation of some performance-related events of KALIMER-600

A sodium-cooled fast reactor named KALIMER-600 has been under development at KAERI. Since the characteristics of the performance-related events of KALIMER-600 were expected to be different with those of a conventional light water power plant, some performance-related events were suggested and simulated in this study. The postulated performance-related events were one feedwater pump stop, one pump trip in the primary pool with a shaft seizure, one pump trip in the primary pool with a loss of electric power, one pump trip in the intermediate heat transfer loop, and the isolation of one intermediate heat exchanger. In addition, a fast-running performance analyzer was developed by modifying the results of previous studies to simulate the performance-related events.Most of the suggested events could be determined as performance-related events, which can be continuously operated without threatening the safety of the KALIMER-600 except for the event of a one pump trip with a shaft seizure in the primary pool. Through this study, the KALIMER-600 was proven to be a safe plant to accommodate various performance-related events.

Theoretical and experimental modelling of a heat pipe heat exchanger for high temperature nuclear reactor technology

High temperature heat sources are becoming an ever-increasing imperative in the processing industries for the production of various plastics, fertilisers, coal-to-liquid fuel and hydrogen generation. Current high temperature reactor technology is capable of producing reactor coolant temperatures in excess of 950 °C. At these temperatures, tritium which is a radioactive contaminant found in the reactor coolant stream, is able to contaminate the secondary stream by diffusing through the steel retaining wall of the heat exchanger between the reactor coolant and secondary process coolant stream. Current regulations therefore require an extra intermediate heat transfer loop to ensure no cross contamination. A novel heat pipe heat exchanger design is presented which circumvents the need for an intermediate coolant loop. This is done by physically separating the reactor coolant and secondary coolant by two pipe walls and a vapour section and a liquid section. A theoretical transient heat transfer model of such a device is presented. The model uses separate hot gas heating fluid and cold water cooling fluid control volumes, and for the internal working fluid a control volume consisting of a liquid and its vapour in equilibrium with each other. A 2 kW rated experimental model was constructed and tested, using Dowtherm-A as working fluid, to validate the heat pipe heat exchanger theoretical model and design. By determining the boiling heat transfer coefficient through the use of an experimentally formulated correlation it was shown that the theoretical model is indeed able to simulate the characteristic chaotic behaviour, due to the boiling and condensation processes, of the device to within a reasonable level of accuracy. It is concluded that the theoretical simulation model can be used to predict the performance of a higher temperature sodium-charged heat pipe heat exchanger, provided suitable boiling and condensation heat transfer coefficients are used.