Optimization of the thorium-uranium regime in CANDU heavy-water reactors

Two operating regimes of a heavy-water power reactor operating in a thorium-uranium fuel cycle are examined: accumulation of the required amount of ²³³U and self-fueling with ²³³U fuel. The parameters of 10 variants of the core-lattice cells of a heavy-water power reactor are calculated. The lattice spacing is kept constant and the variants differ by the number of channels, containing fuel elements or targets, and the ²³³U content in ThO₂. Combined channels containing fuel elements and a target at the same time are examined. Preference is given to a cell variant where a cell contains three channels with fuel elements and one channel with targets. For this variant, the fuel burnup is ∼8 MW-days/kg. A large increase of the burnup, i.e., decrease of the amount of reprocessed targets, can be achieved by decreasing the minimum reactivity excess and also by changeing and increasing the complexity of the technology. For example, a large effect is expected from using combined fuel assemblies. In so doing, provisions must be made for performing complicated operation of disassembling highly active fuel assemblies consisting of fuel elements and targets. __________ Translated from Atomnaya énergiya, Vol. 101, No. 5, pp. 327–336, November, 2006.     

Helium cooling of fusion reactors

Basic equations relating temperatures, pressure drop, power generation and geometry are derived for cooling the vaccum wall and blanket regions of a fusion reactor with pressurized helium. Numerical results are presented for two cases which are applicable to use of a direct cycle pressurized helium gas turbine for power conversion in a fusion power reactor system.     

An efficient analytical form for the period-reactivity relation of beryllium and heavy-water moderated reactors

A new mathematical form of the period-reactivity equation for heavy-water- and beryllium-moderated reactors has been developed. This form is represented in a polynomial form with a degree of G+1 for G-th group of delayed neutrons and photoneutrons. A general formula for the coefficients of such polynomial is derived. These coefficients have a linear dependence on the step reactivity insertion. The related constants of this linear dependence are calculated for both types of reactors. In addition, a comparison has been made for the stable reactor periods of an infinite U²³⁵-fueled, D₂O- or Be-moderated reactors, following step reactivity insertion, with and without delayed photoneutrons taken into consideration. Also, a comparison was made for the reactor response to reactivity changes when evaluated using fast and thermal fission delayed neutron group constants combined with and without D₂O- and Be-moderated reactors.     

The feasibility of using organic liquids heated in nuclear reactors as working fluids in turbines,from the thermodynamical standpoint

This article examines, in principle, the possibility of employing heated organic fluids as working fluids for tubines. The organic liquids considered for such applications are diphenyloxide, diphenyl, Dowtherm, etc. Heating takes place inside power-reactor circuits, with evaporation possible in the turbine nozzle. The vapors of these liquids become superheated in adiabatic expansion, although the vapor temperature drops in the process. As an example, we consider data on the thermodynamic cycle of a power reactor using Dowtherm as coolant medium.     

Pu-breeding feasibility in irradiation channels of research reactors

The breeding potential in the irradiation channels of research reactors is of safeguards concern, because of lacking continuous supervision on the type of experiments in all the irradiation channels. Moreover, the irradiation time can be optimized in order to breed high quality weapon grade plutonium. With regard to the safeguards measures currently adopted, IAEA concentrates its efforts on those reactors whose thermal power is greater than 25 MWth, because it was calculated that a 25 MWth LEU-fuelled reactor produces not more than one Significant Quantity of Pu (8 kg)/year in its spent fuel and a HEU-fuelled reactor of this power would require an annual reload of not more than one Significant Quantity of U₂₃₅ (25 kg). In order to investigate whether it would be possible to determine an analogous power level threshold to estimate the clandestine plutonium production capability of different research reactors, the Monte Carlo method was used to determine the neutron flux in the irradiation channels and to calculate the plutonium breeding potential for three different reactor types: (1) a Triga Mark II with 250 kWth, representative for a small size research reactor; (2) a Material Test Reactor (MTR) with 5 MWth, representative for a medium size research reactor; (3) a High Flux Reactor (HFR) with 45 MWth, representative for a large size research reactor. It was observed that the most important factors for plutonium breeding are the neutron flux (to which reaction rates are proportional) and the available space to place irradiation samples. The breeding capability scales fairly well with the reactor power level and from about 10 MWth onwards the proliferation concern raises with increasing power level and available sample space.     

Fluid-to-fluid scaling of heat transfer in circular tubes cooled with supercritical fluids

Experimental investigations of heat transfer at prototypical conditions of supercritical water cooled reactors (SCWRs) are strongly limited due to their huge technical and financial efforts required. One of the possible solutions is the application of model fluids, which have much lower critical pressure and critical temperature. Model fluid technique has been widely applied in the thermal-hydraulic studies of nuclear engineering. In spite of growing activities of heat transfer at supercritical conditions using model fluids, there does still not exist any reliable fluid-to-fluid scaling methods, to transfer the test data in model fluids directly to the conditions of prototype fluid. This paper presents a fluid-to-fluid scaling method for heat transfer in circular tubes cooled with supercritical fluids. Based on conservation equations and boundary conditions, one set of dimensionless numbers and the requirements of a complete scaling are determined. Scaling of pressure and temperature ensures the similarity of thermo-physical properties of various fluids. A new dimensionless number, presenting the product of the so-called pseudo Boiling number, Reynolds number and Prandtl number, is applied to scale heat flux. The distortion approach is used to scale mass flux. The scaling of heat transfer coefficient is based on Nusselt number. In addition, a new approach is introduced to validate the scaling law. The validation results show good feasibility and reasonable accuracy of the proposed scaling law. Assessment of scaling factors of various parameters indicates the high feasibility of Freon-134a as model fluid for SC water. Some guidelines can be derived for the future experimental investigations on heat transfer at supercritical pressures using model fluid techniques.     

Condensation heat transfer with noncondensable gas for passive containment cooling of nuclear reactors

Noncondensable gases that come from the containment and the interaction of cladding and steam during a severe accident deteriorate a passive containment cooling system's performance by degrading the heat transfer capabilities of the condensers in passive containment cooling systems. This work contributes to the area of modeling condensation heat transfer with noncondensable gases in integral facilities. Previously existing correlations and models are for the through-flow of the mixture of steam and the noncondensable gases and this may not be applicable to passive containment cooling systems where there is no clear passage for the steam to escape. This work presents a condensation heat transfer model for the downward cocurrent flow of a steam/air mixture through a condenser tube, taking into account the atypical characteristics of the passive containment cooling system. An empirical model is developed that depends on the inlet conditions, including the mixture Reynolds number and noncondensable gas concentration.     

Discussion on the stability of heated channels with different fluids at supercritical pressures

The paper extends previous work on the stability of heated channels with fluids at supercritical pressure as predicted by available models.A set of dimensionless numbers proposed to predict the threshold of instabilities is further discussed to highlight their capabilities and possible improvements. In particular, it is shown that the choice made for the reference value of the derivative of specific volume with respect to specific enthalpy is justified as an extension of classical formulations adopted for boiling channels. Moreover, the degree of universality to be expected by the use of these dimensionless numbers while using different fluids is clarified; in this aim four different fluids are considered: water, carbon dioxide, ammonia and refrigerant R23.In order to provide a clear perspective of the usefulness of the proposed dimensionless numbers for dealing with different fluids, linear stability maps generated by a previously developed in-house code, making use of balance equations in dimensionless form, are then compared with the results obtained by computations performed in dimensional terms. In this aim, both an in-house code and RELAP5 are used. The reference considered system is a long circular channel with uniform heating and no singular pressure drops, already addressed in previous analyses, here assumed both in vertical upward and in horizontal flow conditions. The comparison of the predictions obtained for the different fluids allows to ascertain the level of applicability of the dimensionless numbers and, as an interesting by-product, confirms the possibility to encounter static instabilities also in systems at supercritical pressure.     

Dimensionless parameters in stability analysis of heated channels with fluids at supercritical pressures

The paper proposes dimensionless parameters for the analysis of stability in heated channels with supercritical fluids. The parameters are devised basing on the classical phase change and sub-cooling numbers adopted in the case of boiling channels, proposing a novel formulation making use of fluid properties at the pseudo-critical temperature as a function of pressure. The adopted formulation for dimensionless density of a given fluid provides a unique dependence with respect to dimensionless enthalpy, in a reasonably wide range of system pressures, thus giving generality to the predictions of unstable conditions obtained as a function of dimensionless parameters. It is shown that these parameters allow setting up quantitative stability maps for a single heated channel with imposed overall pressure drop, in analogy with the ones proposed in previous work concerning boiling channels. Similarities with the boiling channel stability phenomena are pointed out, also supporting the conclusions with system code predictions.     

Physicochemical study on supercritical fluids and their industrial use

In this study, we confirm a tetrahedral structure for a Co²⁺ complex in supercritical water instead of the octahedral structure in normal water. The absorption spectra of Ru²⁺ complexes were observed for only stable species such as [Ru(phen)₃]²⁺ in supercritical water. The photochemistry of uranyl(VI)-tributylphosphate(TBP) complex in supercritical carbon dioxide(scCo₂) was studied in the pressure range of 8-40 MPa and from 289 to 336K. In order to examine the hydrogen-bond structures and molecular motions of supercritical water and non-aqueous solvents, their ¹H spin-lattice relaxation times were measured under sub- and supercritical conditions by using high-pressure NMR. NiMo/Al₂O₃ was chosen as a catalyst because of its catalytic activity for hydrocracking aromatic structures.     

Application of serpan's damage function to heavy-water moderated power reactors and its control with surveillance data of the karlsruhe power reactor,MZFR

For calculating $\text{ΔNDT}$, the irradiation-induced shift of the nil ductility transition temperature, for the design of pressurized water reactor vessels, literature data envelope curves are used. In the case of light-water moderated reactors no correction of different spectra influence is considered. Calculations with the damage functions, published by Serpan et al., demonstrate that neglecting spectrum influence yields conservative design data. For heavy-water moderated reactors an adjusting factor is calculated from Serpan's damage function. The $\text{ΔNDT}$ measured with samples of the ASTM reference steel A302-B, irradiated in the heavy-water reactor, MZFR, is compared to irradiation results of samples from the same steel plate irradiated in the Oak Ridge low intensity test reactor (LITR) which are published by NRL. From this comparison an experimental adjusting factor is derived. Both calculated damage function and experimental factors agree very well. This result demonstrates that Serpan's damage function is applicable also to MZFR. surveillance samples. The neutron spectrum in the MZFR surveillance location is extremely soft. Thermal neutron population is 900 times the fast flux ($\text{E > 0.9}\text{MeV}$). Its damage contribution, calculated with damage function, amounts to 73%.     

Economic feasibility of high-temperature reactors for industrial cogeneration: an investor's perspective

This paper studies the economic potential of using high-temperature nuclear reactors (HTRs) for cogeneration of industrial process heat and electricity. A reference case HTR is found to deliver cost-competitive process heat with temperatures of ≥200 °C, rendering the chemical and pulp and paper industries potential candidates. The reference case investment yields a positive net present value of €304 million. Real options analysis is employed to account for the uncertain environment and the resulting managerial flexibilities of the project. A real option model for optimal investment timing is adapted to HTRs for industrial cogeneration. The value of the option to invest in an HTR is determined at €667 million and the electricity price threshold for an optimal investment at 79 €/MWh. Though the option to invest in an HTR represents a significant value for a utility, the investment should be delayed until the electricity price has reached the threshold value. We also propose a model to calculate the option value of switching between two different operating modes (cogeneration vs. electricity only). For the reference case, this option value turns out to be €85 million.     

Assessment of flow stability boundaries in a heated channel with different fluids at supercritical pressure

This paper presents results on the stability of a simple heated channel containing fluids at supercritical pressure with an external imposed pressure drop. Basing on a recent work that discussed stability characteristics in a fluid-to-fluid comparison perspective, additional data are presented in order to discuss relevant parametric effects, also including a more accurate fluid-to-fluid comparison.Three different analysis tools, including a system code and in-house linear and transient analysis programs, were adopted to evaluate stability thresholds at different channel throttling conditions and orientations; four different supercritical fluids were considered. The diversity of the adopted tools and the good level of agreement observed in the comparison of their results provide adequate confidence on the general reliability of the obtained information.Among the addressed phenomena, both Ledinegg and density-wave oscillations are considered, pointing out a fundamental continuity between the two phenomena that occur in adjoining regions of the parameter space. Numerical effects are also highlighted, quantifying the impact of truncation error occurring in the use of system codes in the analysis of flow instabilities. The results obtained by different fluids provide further support to the conclusion that, when an appropriate dimensionless formalism is used, the differences obtained in the stability behaviour at imposed heat flux are relatively small for a number of fluids of interest for experimental analyses.     

On the analogies in the dynamic behaviour of heated channels with boiling and supercritical fluids

In this paper, the origin and the consequences of a new dimensionless formalism recently introduced for analysing the dynamic behaviour of heated channels with fluids at supercritical pressures are discussed. A unified view of boiling and supercritical fluid instabilities is proposed, basing on the argument that, despite the strong differences in their operating conditions, in both cases the relevant dynamics results from the changes in density that the fluid undergoes along the channel.The discussion is presently supported only by modelling, performed both by a simplified program and a system code; however, considering the complete independence of these modelling tools, the close agreement observed between their results provides a reasonable level of confidence in the proposed conclusions.Interesting thermodynamic relationships, devised as a by-product of the introduction of the new dimensionless parameters, are also presented, in the aim to stimulate further studies capable to provide greater insight into the fascinating aspects raised by recognising the intimate similarity of these two classes of phenomena, which have such a remarkable relevance for present and future nuclear reactor technology.     

Numerical modeling and design of supercritical CO2 pre-cooler for fusion nuclear reactors

One of the key issues of fusion technology is the efficient recovery of the fusion power extracted by heat transfer fluids in the breeding blanket. The Spanish National Program TECNO_FUS is exploring a dual-coolant breeding blanket design concept and its plant auxiliary systems for a future power reactor (DEMO), with liquid lead–lithium as main primary nuclear power recovering fluid. Supercritical CO₂ is chosen for the secondary circuit, since its high efficiency at significantly lower required temperatures than for the Brayton helium cycle, due to low compression work near the critical point and also because its additional major benefits in terms of tritium control. Use of printed circuit heat exchangers (PCHE) is suggested in literature due to its highly compact design and robustness for the high pressures found. This work deals with the heat exchanger devoted to release the thermal energy of the power cycle to the thermal sink. The aim of this work is analyzing how the nearness of the CO₂ to its critical point affects the performance of the heat exchanger. Computer Fluid Dynamics (CFD) simulations that include the complex thermal behavior of CO₂ properties at supercritical conditions are used in order to achieve an accurate approach to the design of this heat exchanger. These results are compared with others obtained through correlations found in the open literature. The behavior of CO₂ close to its critical point results in an inefficient use of the exchange area, giving a temperature profile in CO₂ which remembers a condensation process and an overall heat transfer coefficient 1.4 times higher than the one achieved with literature correlations design.     

Discussion of heat transfer phenomena in fluids at supercritical pressure with the aid of CFD models

The paper discusses heat transfer enhancement and deterioration phenomena observed in experimental data for fluids at supercritical pressure. The results obtained by the application of various CFD turbulence models in the prediction of experimental data for water and carbon dioxide flowing in circular tubes are firstly described. On this basis, the capabilities of the addressed models in predicting the observed phenomena are shortly discussed.