The advantages of gas turbine power plants in general and closed cycle systems under gas pressure in particular for waste heat recovery are well known. A satisfactory efficiency for electric power generation and good conditions to obtain a significant amount of hot water above 100°C lead to a high fuel utilization. However, as in most of projects, it is not much possible to produce high temperature steam or water without significantly decreasing the electricity production. A new method for an additional generation of high quality process or domestic heat is proposed. The basic feature of this method lies in arranging one or two steam generators or preheaters in parallel with the low pressure side of the recuperator. The high total efficiency and the noteworthy flexibility of this system are emphasized. This arrangement is suitable for any kind of heat source, but the applications presented in this paper are related to helium direct cycle nuclear power plants the main features of which are a single 600 MW(e) turbomachine, a turbine inlet temperature of 775°C, no or one intermediate cooling and a primary circuit fully integrated in a pre-stressed concrete reactor vessel.

Time delayed mechanical borehole stability is mostly depending on the pore pressure consolidation process. Establishment of pore pressure equilibrium in shale is a time dependent process which is characterized by shale intrinsic properties i.e., porosity, permeability, fluid and rock stiffness parameter etc. In shale, water movement is greatly restricted by the low permeability of shale which may cause pore pressure storage. The Influence of induced pore water pressure and its dissipation is critical for the evaluating of time delayed borehole stability. This paper discusses and presents a sensitivity analysis of the impact of shale intrinsic properties on transient pore pressure and its impact on time delayed mechanical borehole instability. The aims are to establish pore pressure trend, material plasticity, and time delayed borehole collapse risk. In an attempt to minimize transient pore pressure related instability problems, detailed and careful analyses are highly dependent on the constitutive models adopted for the shale. In this study, a physical model was used to introduce hypotheses of time delayed stability and both analytical and numerical models were developed to verify the hypotheses. The analytical model is based on poroelastic constitutive model, coupled with pressure diffusivity formulation. A numerical material model is developed based on finite elements. The analytical model quantifies shale intrinsic properties vs time delayed stability; whereas, the numerical model diagnoses pore pressure storage effect during underbalanced drilling in shale and its impact on mechanical instability. The undrained condition (immediately after the wellbore is drilled) as well as the drained condition was analyzed. The integrated approach in this study may give a clear picture on shale complexicity and its adverse effect on time delayed stability. Wellbore stability models that include some aspects of coupled have already been developed. However, time delayed borehole instability in shales in UBD condition is a new research area; adjustments are required in existing model. The analytical simulation results show that shale permeability higher than 40 hD, the difference of the consolidation time for different finite characteristic time are insignificant and converge to zero while permeability 100 hD. In addition to shale permeability, other parameters (i.e. pore fluid viscosity, porosity and fluid bulk modulus) are influencing the consolidation processes. This paper demonstrates the effects of such parameters also. The numerical results implies that a M-C elastic - plastic model is capable of evaluating plasticity and material deformation effects in UBD conditions along with accounted time delayed transient pressure trend in shale. It was shown that the shale behaviour during UBD is a transient problem, and cannot be described without a fluid diffusion process. It is also noted that shale intrinsic parameters are playing a significant role in time delayed borehole stability analysis.

Considering the concern about a more efficient, rational use of heat sources, and a greater location flexibility of power plants owing to cooling capability, closed gas cycles can offer new solutions for fossil or nuclear energy. An efficient heat conversion into power is obtained by the combination of a main non-intercooled helium cycle with a flexible, superheated, low-pressure bottoming steam cycle. Emphasis is placed on the matching of the two cycles and, for that, a recuperator bypass arrangement is used. The operation of the main gas turbocompressor does not depend upon the operation of the small steam cycle. Results are presented for a conservative turbine inlet temperature of 750 C. Applications are made for a coal-fired power plant and for a nuclear GT-HTGR. Overall net plant efficiencies of 39 and 46 percent, respectively, are projected.

A now unavoidable concern is the best possible use of a given energy source in order to meet the demand of power and heat. The most efficient, rational, thermodynamically satisfactory way of producing heat is to cogenerate it with power and to handle it as a by-product. Another basic condition is to supply heat without disturbing the operation of main turbogenerators. Closed helium cycle, non-intercooled, gas turbine coal-fired and nuclear power plants which meet these requirements are presented in this paper. Low-pressure steam and hot water are recovered from the precooler; medium, high pressure steam and steam superheating are generated by means of a bypass arrangement of the recuperator. The concept offers attractive overall energy conversion coefficients, very flexible heat production conditions and significant potentialities of reducing oil consumption. Results are given for the conservative 750 C gas turbine inlet temperature.

The concept of a helium cycle nuclear power plant with a single turbomachine located outside the P.C.R.V. in a leaktight basement is being studied. Main features of the system are pointed out, as are the favorable aspects it offers for a satisfactory design of the plant, a high safety level, a good efficiency for electricity generation associated with a dry cooling tower and heat recovery in good conditions for heating or industrial purposes. Above all, the high technological credibility due to the temperature of 750 C is worth being emphasized. In this paper, a 750 C/800 MWe power plant (2000 MWt) with one intercooler is presented as an example of a possible first generation of gas turbine HTGR. Development prospects for higher power levels, 1000/2000 MWe, are also investigated, at first with one but also with two intermediate coolers, taking into consideration the somewhat higher-helium temperature. The net plant efficiencies, for electricity generation only, range from 40 to 45 percent.