Kudel’kina Evgeniya Viktorovna, Russian Federal Nuclear Center — All-Russian Research Institute of Experimental Physics (RFNC-VNIIEF)

Gusev Alexander Leonidovich, Russian Federal Nuclear Center — All-Russian Research Institute of Experimental Physics (RFNC-VNIIEF)

Turhan Nedjat Veziroglu, Clean Energy Research Institute University of Miami Michael Douglas Hampton, University of Central Florida

1. Introduction

Emergency conditions in big cryostats in the circumstances of their long-term operation have been discussed in this review. Emergency conditions in cryostats arise at the appearance of considerable heat flows into a cryoagent, which much more exceed the certificate flows and come in over heat bridges and through heat insulation [1-3]. During a long-term operation of big cryostats, especially in the end of the routine maintenance interval, residual hydrogen gets accumulated in heat-insulating cavities. Hydrogen, as a rule, appears as a result of inter-lattice hydrogen diffusion from thick and warm cryostat casing walls into the vacuum cavity. The residual medium of other gases is mainly formed due to the atmospheric air inflow through microloosenesses. Big cryostats always contain microloosenesses. In the beginning of the cryostat operation, they are, as a rule, insignificant, and then grow due to the processes in welds approaching by magnitude to the maximal permissible value.

The most spread strategies of the cryostat superinsulation operation are built on strictly determined heat insulation routine maintenance intervals. At this stage of cryogenic engineering development, designers, as a rule, determine the routine maintenance interval as one year. However such attitude leads to considerable operational and energy costs. It would be more expedient to build the planning strategy of the routine maintenance interval with the account of changes in the cryostat design condition. In a number of works [4, 5], a possibility is postulated to predict the beginning of the extreme cryostat operation period as well as to plan the optimal duration of routine maintenance intervals. At the same time, as the operational practice has shown, when the routine maintenance interval in cryostat superinsulation is exceeded under certain conditions, some phenomena impeding the normal cryostat operation may appear. However the conduction of extreme planned experiments on full-scale big cryostats has allowed to prove the possibility of management of these effects in order to eliminate a negative effect and obtain a positive effect in case when it is impossible to conduct an emergency superinsulation routine maintenance. In addition, the analysis of these phenomena has permitted to build models, which can be useful at the development of fundamentally new versions of superinsulation embodiments. These processes can be completely stopped and eliminate their extreme danger.

The proposed review covers discussions and analysis of the data being accumulated by the present time on proceeding of electro-sorption processes in screen-vacuum heat insulation (SVHI) layers of big cryogenic reservoirs and cryogenic pipelines [6-7]. Their

influence on the cryogenic products volatility as well as on the safety reduction of thermostatically controlled objects has been demonstrated.

A special attentions has been paid to the field effect, the Bardeen-Brettain-Shockley gas-water cycle, the electro-adsorption effect, metastable states of superinsulation surface, kinetics and dynamics of the residual atmosphere of very big cryogenic reservoirs with insignificant effusion leaks, the cryogenic liquid volatility, the determination of the heat inflows to a cryogenic liquid in the conditions of ambient parameter changes. For the first time, the following recently discovered phenomena in big cryogenic reservoir superinsulation have been described in the references being reviewed [6-9]:

an effect of effusion induced hydrogen superinsulation instability, an effect of effusion induced heat-conductive superinsulation instability in cryogenic-vacuum objects, an effect of multiplication of the number of desorbing hydrogen molecules in respect to the inflowing moist air molecule magnitude.

The effects in heat insulation can be controlled. In order to create new heat insulation samples with a high exergy efficiency and a high safety degree, new heat-insulating structures and designs should be developed [8, 9].

The main tendencies of further superinsulation development have been determined. A fundamentally new approach to the superinsulation designing and calculation has been demonstrated, which, apart from radiation and convection heat conduction mechanisms, takes account of convection component composition variations. In addition, the convection component variations occur due to the change of residual water composition and concentration as a result of the electro-sorption process. The electro-sorption process arises at the availability of clearly expressed hydrogen residual atmosphere on superinsulation heat screens with water concentration changes in the air inflowing through microloosenesses into the heat insulation cavity [6, 7].

Theoretical models for the appearance of effusion induced hydrogen and heat conduction instabilities of the superinsulation have been proposed for the first time in this review. Thermodynamic description of these new effects has been carried out with the use of analytical thermodynamics mechanisms. On the grounds of variational description of heat and mass transfer processes for a heterogeneous system in the continuum approximation and with the account of electro-sorption processes according to the hydrogen-water cycle of Bardeen-Brettain-Shockley, a formulation of the mathematical model of molecular heat and mass exchange in superinsulation has been derived.

A fundamentally new approach to the superinsulation construction and calculation, which, apart from radiation and convection heat conduction mechanisms, takes account of the convection component variation mechanism. In addition, the convection component variations occur due to the change of residual water composition and concentration as a result of the electro-sorption process. The electro-sorption process arises at the availability of clearly expressed hydrogen residual atmosphere on superinsulation heat screens with water concentration changes in the air inflowing through microloosenesses into the heat insulation cavity.