Thermodynamics And Safety (2)

Technological systems in the real world are either based on or are greatly influenced by the laws of thermodynamics and engineering thermodynamics. Examples include the heat engines (i.e., for example, all internal combustion engines), heat pumps, refrigerators, air conditioners, and cooled electrical components mentioned in the first part of this article (1). It is obvious that some critical technologies or technological processes are in practice very vulnerable in terms of thermodynamics, i.e. sensitive to thermodynamic events (e.g. sensitive to ambient temperature change, dependent on cooling, sensitive to pressure change, etc.). They can themselves cause a thermodynamic reaction or other hazardous behaviour when some thermodynamic parameters are changed, which endangers or directly damages the environment, i.e. surrounding systems, the environment, but also the human system, health and lives. These events that lead to large losses of public assets such as people’s and companies’ property, the environment, people’s lives and health, etc. are called extreme events.

Some extreme phenomena in which thermodynamics plays a large role are given in the following examples.

Storage and transport tanks for liquefied gases

To describe the properties of liquefied gas storage and transport tanks, we draw on the work (J. HORÁK) dealing with the use of thermodynamic data to assess the potential hazard of liquefied gas storage tanks ¹

An extreme phenomenon in this case is the leakage of the contents of storage tanks and tankers used mainly in the chemical and petrochemical industries. Most of the liquefied gases have hazardous properties, some are toxic or corrosive, e.g. chlorine, hydrogen chloride, sulphur dioxide, ammonia, hydrogen sulphide, phosgene, others are flammable, e.g. ammonia, propane, propene, butane, butadiene and vinyl chloride, and can therefore cause major damage to the environment and to human health and life in the event of an accident.

The bins and tanks in question can therefore be considered as a hazardous means that can cause in the event of an accident:

– leakage of a large quantity of a hazardous substance,

– the formation of a mixture of hazardous gas and air,

– in the case of toxic gas, i.e. a toxic cloud threatening the population and the environment,

– in the case of a combustible gas, i.e. a flammable cloud that can ignite under certain conditions, forming a “fireball” that threatens to radiate heat,

– in the case of an explosive substance, a cloud of gas that can threaten the surroundings by its explosion , its destructive wave and the radiation of heat..

The cited work ² uses the following important terms for the analysis of the selected accident scenarios:

Adiabatic boiling – requires pressurized liquefied gas heated above the normal boiling point, when the pressure is released there is a so-called “instantaneous boiling” where the internal heat of the liquefied gas is used for phase transformation, there is no delay caused by heat gain from the surroundings,

Boiling controlled by heat transfer to the liquid – liquefied gases normally have a boiling point below ambient temperature. Therefore, they boil at atmospheric pressure, with the rate of boiling evaporation limited by the rate of heat transfer from the surroundings to the liquid.

The tank accident scenarios have the working space divided into two parts: a liquid space filled with liquefied gas and a gas space filled with compressed gas. The leak scenarios are divided into a liquid compartment leak and a gas compartment leak. In both cases, the so-called slow and fast leakage of gas or liquid from the leak openings of the liquid and gas compartments, respectively, is considered.

Rapid leakage is accompanied by adiabatic boiling, there is a destructive effect of the expansion energy of the steam, the liquid is dispersed in droplets into the gas space and thus accelerates the boiling process, there is also liquid splashing in the form of foam. The leakage rate depends on the parameters of the gas leakage path and whether the leakage path is connected to the liquid and gas parts. A rapid leak is assumed to be an emergency leak.

Slow leakage is accompanied by a boiling-controlled heat transfer to the liquid where there is no liquid leakage, only compressed gas “and the destructive effect of the expansion energy of the steam is not significantly applied”. An example of slow leakage is in the use of cylinders containing a mixture of propane and butane for domestic cooking.

The scenario analysis does not include the case where the tank is ruptured by vapour pressure. Rupture occurs in extreme situations, such as a fire, where the contents of the container are heated to a high temperature. This case is referred to as a physical explosion ³.

The cited work considers the following characteristics needed to obtain relevant results:

– the pressure in the reservoir,,

– the ratio of the highest jam in the hopper to the lowest jam,

– for the gas compartment – estimate of the mass of the jam to be released from the vapour compartment of the container when the pressure is released,

– for liquid space – overheating of the liquid above the normal boiling point,

– for liquid space – proportion of liquid evaporable by adiabatic boiling.

To analyse the potential leakage hazard, it is necessary to calculate the expansive work of the vapour in the gas space and the vapour generated by boiling.

Expansion work provides basic information about the punchiness with which steam can escape. The relationships derived for adiabatic expansion can be used for the calculation. “In adiabatic expansion, the vapour does work at the expense of its internal energy, so the expansion cools the vapour and the equality holds”: the expansion work of the vapour is equal to the decrease in the internal energy of the vapour. Two limits of expansion work are usable:

– expansion work of steam in reversible adiabatic expansion (maximum expansion work),

– expansion work of steam in irreversible adiabatic expansion against the ambient (atmospheric) pressure (least work required).

The difference between the stated maximum and minimum necessary work is the so-called unbound work, which is usually transformed into another type of dangerous energy – e.g. destructive pressure wave, dispersive energy dispersing liquid into fine droplets or kinetic energy of flowing liquid and gas stream. This unbound work also affects the rate of mixing of steam or a mixture of liquid and steam with air and significantly affects the formation of a cloud of leaked substance. The transformation of this part of the energy depends on the speed of the process. In case of slow leakage, it may remain untransformed in the form of apparent heat of steam.

The work generated by boiling is considered: volumetric work of steam associated with the expansion of steam in the gas space of the reservoir; expansion work of steam resulting from irreversible adiabatic boiling of a liquid.

The basic design parameter (according to the cited work ⁴), which affects the severity of the consequences of the accident, is the volume and retention of the hazardous substance in it. By organizing the processes, the volume of reservoirs and holdings can be reduced, but in practice very often the poor organization of the company forces the construction of large storages and the maintenance of large holdings.

The most important technological parameter is the difference between the storage temperature and the temperature of the normal boiling point, because it is the energy accumulated in the form of overheating of the content above the normal boiling point that represents a dangerous form of energy, which is then transformed into vapor heat and expansion work. Reducing the mentioned temperature difference means introducing an additional cooling device, which carries additional risks, therefore cooling must be addressed specifically for each type of storage. Furthermore, according to the analysis of the course of the accident, it follows that the rapid phase, when the liquid is forced out of the reservoir by vapor pressure, is more dangerous, therefore, in the event of an accident (or leak), it is expedient to reduce the pressure in the reservoir by controlled release of vapors. “A relatively large amount of energy is accumulated in the form of overheating, but its decisive share is transformed into vapor heat, and only a very small part can manifest itself as destructive expansion work. The main source of the potential danger of reservoirs is therefore their ability to release a large amount of vapor into the environment and create a cloud of a dangerous substance”.

Finally, to illustrate the importance of thermodynamics in reservoirs in safety, we can recommend the following video and document:

Thermodynamics of the steam cycle of nuclear power plants – nuclear accident

There is no need to describe the extreme phenomena caused by a nuclear reaction in more detail, it is enough to look at the most famous events in nuclear energy or even war conflicts, both in the world and in Czechoslovakia.

– the dropping of the atomic bomb on the cities of Hiroshima and Nagasaki in Japan in 1945,

– the nuclear accident of Jaslovská Bohunice A-1 in 1976 and 1977,,

– the Three Mile Island nuclear accident in the USA in 1979,

– the very severe Chernobyl nuclear accident in Ukraine in 1986,

– the very severe Fukushima nuclear accident in Japan in 2011.

The following paragraphs are based on the bachelor’s thesis (T. KOZÁK) ⁵, which does not describe extreme phenomena, but contains a description of the steam cycle of nuclear power plants from the point of view of thermodynamics and a summary overview of nuclear types and their constructions. On the basis of the mentioned work, the importance of thermodynamics in the field of nuclear energy can be demonstrated, as external influences, internal organizational and technological failures can disrupt the thermodynamic cycle of the power plant, which leads to extreme phenomena, i.e. failures and more or less serious nuclear accidents.

The said work presents a brief introduction and history of nuclear research and energy, and also presents an introduction to thermodynamics, i.e. thermal cycles, Carnot cycle and steam thermodynamics (see thermodynamic laws and cyclic phenomena). In the second part of the thesis, he describes the ideal and real Rankin-Clausian steam cycle, the increase in thermal efficiency and the carnotization of the cycle.

An ideal Rankin-Clausian cycle converts the energy stored in the steam into mechanical work. It consists of four thermodynamic events or of the states shown in the figure, the course of which is recorded in the T-s diagram of water and water vapor.

Termodynamika: ***Ideální Rankin-Clausiův oběh*** dle ⁶

Event 1-2: adiabatic compression, i.e. temperature of saturated liquid increased by pressure.

Event 2-3: isobaric heating; the liquid is heated in the boiler (steam generator, reactor) under constant pressure and transformed into saturated or superheated steam.

Event 3-4: adiabatic expansion; i.e. the steam expands through the turbine and converts its energy into work (rotation of the turbine),

Event 4-1: Condensation; i.e. the vapor condenses back to a saturated liquid.

An ideal thermal cycle neglects losses that have a direct effect on the decrease in thermal efficiency. These are primarily heat and pressure losses in the boiler and condenser, then irreversible processes occurring in the turbine and pump.

True Rankin-Clausian cycle – “The most significant decrease in the thermal efficiency of the cycle is caused by the irreversible events that occur when the steam passes through the turbine. In the case of an ideal circulation, we assume that the expansion in the turbine is isentropic and the corresponding heat drop (difference in steam enthalpies at the inlet and outlet of the turbine) corresponds to it. However, when the steam passes through the turbine, expansion losses occur, as a result of which the expansion curve deviates in the direction of increasing entropy. The enthalpy of the steam leaving the turbine is higher than in the case of ideal isentropic expansion. Therefore, the actual heat drop will decrease, which means the turbine output as well. Also, as entropy increases, the amount of heat that must be removed in the condenser will increase. Losses similar to those in the turbine also occur in the pump, where also due to the increase in entropy during liquid compression, the work required to drive it will increase. Other no less significant losses include pressure losses caused by friction and heat leakage to the surroundings as the working substance flows through the pipe, boiler (steam generator) and condenser.” A comparison of the ideal and actual circulation is in the next picture.

Termodynamika: ***Skutečný Rankin-Clausiův oběh*** dle KOZÁK, Tomáš. *Bakalářská práce. Termodynamika parního cyklu jaderných elektráren* Brno: VUT v Brně, Fakulta strojního inženýrství, Energetický ústav 2013.

To increase the thermal efficiency (T.HORÁK) he mentions two options: reducing the pressure in the condenser and changing the pressure and temperature of the steam. The above is related to the price and quality of technology, equipment construction, and the quality and price of fuel. The normal pressure in condensers ranges from approximately 2.5 kPa to 9 kPa depending on the price of fuel, for nuclear power plants it is in the range of 8 to 9 kPa, so nuclear fuel is considered cheap from this point of view. The pressure and temperature of the steam cannot be changed arbitrarily due to the possibilities of the materials.

For highly alloyed austenitic materials, the temperature has stabilized at 550°C for fossil blocks, but this material has a number of disadvantages, such as high cost, poor thermal conductivity and high thermal expansion. “At nuclear power plants, the pressure and temperature of the steam is limited by the type of reactor used and its coolant. The design of most power reactors does not allow steam to be superheated, they only produce saturated steam with a temperature of around 280-290°C“.

Termodynamika: Nahrazení Rankin-Clausiova oběhu trojicí Carnotových oběhů dle⁷

During carnotization, the Rankin-Clausian cycle is divided into three parts (see figure): I – heating of compressed water into a saturated liquid; II – heating of saturated liquid to saturated steam; III – superheat of steam.

The efficiency of the Carnot cycle depends only on the temperatures between which it operates. Region I of the Carnot cycle has the lowest efficiency, we try to increase this by heat recovery. “With heat regeneration, we try to reduce, ideally eliminate, area I, in which we supply heat to the working substance at the lowest temperature, and thus reduce the influence of this area on the overall thermal efficiency of the circulation. Heat regeneration is carried out by controlled steam extraction, most often after expansion in the high-pressure part of the turbine. After expansion of the steam in the high-pressure part of the turbine, part of the steam is removed and fed to the heater (regeneration exchanger), where the steam transfers heat to the feed water before it is heated by the reactor heat. Regeneration steam condenses in the heater and the resulting condensate is led to the condenser.” “The resulting thermal efficiency of the circulation is higher, by reducing the area I there was an increase in the mean temperature at which heat is supplied to the circulation. The so-called carnotization of the circulation was carried out.’ ⁸.

To improve the efficiency of area III. steam reheating is used. “In nuclear power plants, ensuring steam heating is difficult. In the single-circuit arrangement of the nuclear power plant, where the turbine works with radioactive steam, steam reheating is not used, there is a problem with the location of the reheater. In two- and three-circuit arrangements, steam heating is carried out in several ways. For gas- and liquid-metal-cooled reactors, heating is carried out with the primary coolant in a special exchanger or in a reheater built into the steam generator. In PWR type reactors, the steam is heated mainly by the input steam to the turbine or by the extraction steam from the turbine. However, with this heating method, the steam cannot be heated to a temperature higher than the saturation temperature corresponding to the input steam pressure. Separation of excess moisture in the separator is usually carried out at the same time as the steam is heated. The main reason why the heating of saturated steam is carried out is the need to ensure the dryness of the steam after expansion. ⁹.

Nuclear power plants use a core reactor in which a controlled fission chain reaction takes place. Fission in the entire volume of fuel heats the fuel and the nuclear energy bound in it is converted into thermal energy. The production of electrical energy then takes place using the Rankin-Clausian cycle, possibly using the Brayton cycle for gas-cooled reactors. The thermal output of the active zone is primarily limited by the thermal output that we are able to remove from the active zone of the reactor (it is not theoretically limited by the neuron flow). From the point of view of thermodynamics, the most interesting is the design of the reactor, the arrangement and cooling of its active zone, the type and parameters of the coolant, or the overall concept of the nuclear power plant, see the next picture.

Provozní schéma jaderné elektrárny dle KOZÁK, Tomáš. *Bakalářská práce. Termodynamika parního cyklu jaderných elektráren* Brno: VUT v Brně, Fakulta strojního inženýrství, Energetický ústav 2013.

“Especially due to the possible leakage of radioactivity, or due to the specific design of the reactor, the heat generated in the active zone cannot be directly used to produce steam or heat gas and drive a steam or gas turbine. It is necessary to structurally separate equipment and operational units that may be in direct contact with radioactivity or may be affected by it, and equipment that may not come into contact with radioactivity. That is why the layout of nuclear power plants is generally multi-circuit, a closed primary circuit with radioactive coolant usually only removes heat from the reactor and transfers it to other thermal circuits. Nuclear power plants can be divided into three groups – single-circuit, double-circuit and three-circuit nuclear power plants.”

Due to the above-mentioned facts, the thesis further describes individual types of reactors and the technical solution of the circuits. Another result of the work is the description of the program for drawing and calculating the Rankin-Clausian circulation in the MATLAB program.

The conclusion is a statement and reflection on the energy trends and attitudes of some countries towards nuclear energy and the current effectiveness of replacing the core with renewable sources. The author admits the possibility and applicability of using smaller nuclear reactors for individual technological units or electricity supply. energy of smaller residential units, cities. As already mentioned, the work does not directly focus on extreme phenomena that can occur in nuclear power plants due to environmental and internal disturbances, but due to a comprehensive view of technologies and the use of thermodynamics, it can be observed that this is a device that works with high thermodynamic parameters and the safety of operation due to the thermodynamic events taking place inside is very sensitive to the quality of materials, technological solutions and organization of construction, operation, etc.

In conclusion, to illustrate the importance of thermodynamics in nuclear energy safety, we can recommend the following videos:

Thermodynamics and safety (2)

Storage and transport tanks for liquefied gases

Thermodynamics of the steam cycle of nuclear power plants – nuclear accident

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