Preliminary Design Studies on a Nuclear Seawater Desalination System

Proceedings of ICAPP ‘12 Chicago, USA, June 24-28, 2012

Paper 12039

Preliminary design studies on a nuclear seawater desalination system

Andhika Feri Wibisono, Yong Hun Jung, Jinyoung Choi, Ho Sik Kim, Jeong Ik Lee, Yong Hoon Jeong, Hee Cheon NO KAIST

Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology 291 Daehak-ro, Yuseong-gu, Daejeon, 305-701, Republic of Korea

Tel: 82-42-350-3829, Fax: 82-42-350-3810, Email: [email protected], [email protected], [email protected], [email protected], :

[email protected], [email protected], [email protected]

Abstract – Seawater desalination is one of the most promising technologies to provide fresh water especially in the arid region. The most used technology in seawater desalination are thermal desalination (MSF and MED) and membrane desalination (RO). Some developments have been done in the area of coupling the desalination plant with a nuclear reactor to reduce the cost of energy required in thermal desalination. The coupling a nuclear reactor to a desalination plant can be done either by using the co-generation or by using dedicated heat from a nuclear system. The comparison of the co-generation nuclear reactor with desalination plant, dedicated nuclear heat system, and fossil fueled system will be discussed in this paper using economical assessment with IAEA DEEP software. A newly designed nuclear system dedicated for the seawater desalination will also be suggested by KAIST (Korea Advanced Institute of Science and Technology) research team and described in detail within this paper. The suggested reactor system is using gas cooled type reactor and in this preliminary study the scope of design will be limited to comparison of two cases in different operating temperature ranges.

I. INTRODUCTION As world population grows, water crisis becomes a big

threat globally. Among many water shortage areas, arid regions including Middle east and Northern Africa (MENA) need plenty of fresh water for human society and industry, and these areas have to rely on industrial scale desalination for the supply. Multi-Stage Flashing (MSF), Multi-Effect Distillation (MED) and Reverse Osmosis (RO) are mostly used processes for desalination. Most of these processes require thermal energy (heat) and electricity to some extent. Thermal energy currently originates mostly from fossil fuels. Nowadays the unstable nature of fossil fuel is regarded as a common sense. To reduce this risk, cheaper and cost-stable energy resource is needed. Among some candidates, nuclear energy can be considered as promising energy source for large scale desalination. Some researchers have done coupling studies between desalination plant and nuclear power plant previously and there were even some demonstration. There are some options for this coupling, such as co-generation of electricity-desalination plant and desalination heat only (dedicated) nuclear plant. The difference between the co-generation and the dedicated heat is that in co-generation design option, a nuclear reactor produces both electricity and heat while in dedicated heat design option, nuclear

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reactor is focused on producing only heat required for the thermal desalination plant.

Since the cost will be the one of major motivations for choosing a desalination option from others, the cost of each desalination option has to be estimated carefully. Also, since the cost is highly dependent on the geographical location, our discussion in this paper has to be limited to certain location. The United Arab Emirates (UAE) has a strong potential to develop nuclear desalination system due to several reasons. First, the UAE is planning to construct the largest desalination plants near Dubai. Second, recently the UAE has signed a contract with Korean Consortium to build four nuclear power plants in the country. Third, as it will be apparent in the following section, the UAE need for fresh water is growing very rapidly and like electricity the UAE cannot solely rely on fossil fuel anymore. Therefore, the comparison of nuclear and fossil fueled desalination system will be analyzed in this paper based on the UAE condition to perform accurate assessment. To design a dedicated nuclear system for thermal desalination some types of reactor should be considered. One type of reactor that has good potential to supply process heat for thermal desalination is a gas cooled nuclear reactor. The gas cooled

reactor operating at lower temperature (below 750℃) is a proven technology while the very high temperature reactor is considered as the next generation nuclear power plant. The desalination process by MED doesn't require process

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heat at very high temperature so it can be done with the existing gas cooled reactor system. Therefore, coupling of a gas cooled reactor with MED will be studied to find the optimum operating condition and coupling scheme especially to fulfill the UAE demand of desalinated water.

II. CURRENT STATUS AND PREDICTION OF UAE

DESALINATION

The United Arab Emirates(UAE), having almost one million people and 60 thousand dollars of GDP, is one of the biggest countries doing desalination in the world (almost 835 MIGD at 2013) 1). The water resource demand is growing due to the expansion of agriculture and industry. This water demand reaches 1200MIGD at 20302), as shown in Fig. 1. After 2014, water demand overtakes water supply from pre-existing desalination plant. So, there should be an enlargement of desalination capacity to deal with the water demand. Current UAE’s desalination is mostly depended on cogeneration including MSF, MED or electricity based desalination process, RO. Especially in Abu Dhabi, MSF takes the highest portion of desalination options.

Those types of desalination method cost around 0.5~0.8 dollars per m3 3). This cost can be obtained from plant construction cost, O&M cost, fuel cost and interest. For fossil fuel plant, the fuel cost is the most influencing factor for the unit cost of desalinated water. Therefore, predicting water cost following fuel price fluctuation is needed to effectively handle the water demand and supply.

When estimating annual fuel cost to produce water by cogeneration plant using MSF, fuel cost reached 2 B$/year for Natural Gas (NG) price of 3$/GJ, and for 15$/GJ, the highest natural gas price during the past five decades, total desalination cost reaches up to over 9 B$/year within UAE4). This trend is shown in Fig. 2. The gap between two lines tells the cost instability of water desalination due to

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fuel price fluctuation. Therefore, as stated above, due to fossil power plant’s high dependency on the fuel price, instability of fuel price can be a burden. Therefore, stable energy source for desalination is necessary to reduce the pressure on the increasing water production.

III. ECONOMIC EVALUATION IAEA Desalination Economic Evaluation Program

(DEEP) has been used worldwide for the economic evaluation of various combinations of energy and desalination plants5). User can specify various parameters or the user can use default values as well, which are generally acceptable. In the DEEP code, final water production cost is estimated in two stages as presented in Fig. 3. First, the DEEP code evaluates levelized energy cost in $/kWh using economic models and parameters for an energy plant. The levelized energy cost is presented in cost breakdown which is decomposed into annualized capital, O&M and fuel cost components of the energy plant. The DEEP code then evaluates levelized water cost in $/m3 using economic models and parameters for a desalination plant. The levelized water cost is also presented in cost breakdown which is decomposed into annualized capital, O&M and energy cost components of the desalination plant. The energy cost component means cost of heat or electricity required for desalination. Electricity can be provided either from on-site energy plant, such as in the case of heat, and/or from grid. The heat and on-site electricity cost are calculated based on the levelized energy cost of the energy plant which was estimated in the first stage. When calculating heat cost, the missing revenue due to the lost electricity generation is considered. The cost of grid electricity is calculated based on the related parameter, purchased electricity cos Cpe, specified by user.

Fig. 1. Water demand forecast of UAE.

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Fig. 2. Annual fuel cost estimation following water demand and NG price.

Fig. 3. Two-step water cost estimation of DEEP code.

In this study, economic aspects of dedicated nuclear heat only desalination system were investigated using the DEEP code. First, we evaluated final water costs of various cases including the dedicated nuclear heat only desalination system and then compared them by specifying values of selected major parameters while maintaining all other parameters to the default values provided by the DEEP code.

III.A. Selected DEEP Parameters

As the first step, cases were categorized by

combinations of various desalination technologies (MSF and MED with or without TVC, RO), energy sources (nuclear and natural gas), power conversion systems (heat only and steam cycle) and natural gas prices. Here we set the MED-TVC/Nuclear/Heat-only case as a reference case, which represents the dedicated nuclear heat only

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desalination system. MED-TVC was selected as a reference thermal desalination method since it shows the highest level of performance and economy among the existing thermal desalination methods. Almost all of the electricity in the UAE depends on natural gas6). In lights of such energy situation in the UAE, natural gas was chosen as a representative fossil fuel to compare with the nuclear energy options including the reference case. And we set the power plant employing steam Rankine cycle as a co-generation plant model to compare with the energy plants which only provide heat such as the reference case. An intermediate loop was included for all nuclear cases from the view point of nuclear safety which, in turn, acts as a cost penalty.

Although the DEEP code provides the default values for the most of parameters which are standard data applied widely in the field7), we specified fixed values or ranges of values of some selected parameters to reflect the uniqueness of nuclear heat only desalination plants and current status of the UAE. Default and specified values of selected parameters are summarized in Table I.

TABLE I

Default and Specified Values of Selected DEEP Parameters

Parameters Default values

Applied casesSpecified values

Total desalination plant capacity Wdrc

100,000 m3/d All cases

330,000 m3/d

Average annual cooling water temperature Tsw

25 ℃ All cases

30 ℃

Total dissolved solids TDS 35,000 ppm

All cases 45,000 ppm

Construction duration of energy plant Le

40 months Ref. case

12-36 months Operational availability of energy plant App

90 % Ref. case

92 %

Specific construction cost (EPC) of energy plant Ce

400 $/kWth Ref. case 320 - 400

$/kWth

Specific O&M cost of energy plant Ceom

2.0 $/MWth·h

Ref. case 1.6 - 2.0

$/MWth·h

Specific fuel cost of energy plant Csf’

- Nuclear cases

2.56 $/MWth·h

- Natural gas L (Min. price)

cases 10.24 $/MWth·h (3 $/MMBTU)

- Natural gas H (Max. price)

cases 34.12 $/MWth·h (10 $/MMBTU)

Purchased electricity cost Cpe

-

All cases 0.01 - 0.05

$/kWe·h

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Total desalination plant capacity Wdrc of 330,000 m3/d is required for a million people city, which is the target of this study. This is based on the daily water usage of 1/3 m3/day for each person. The total desalination plant capacity Wdrc determines the minimum required base energy plant capacity, the lost energy, and the construction and O&M cost of the desalination plant. Average annual seawater condition of Arabian Gulf Sea, which is at average annual seawater temperature Tsw of 30 oC and average annual seawater salinity TDS of 45,000 ppm, was adopted on the basis of the DuPont World Map for Desalination8).

The specific fuel cost Csf is the fossil fuel cost or nuclear fuel cycle cost in $/MWe·h for electricity generating plants or in $/MWth·h for heat only plants. The specific fuel cost Csf is equal to the fuel price based on the heating value Csf’ divided by the net thermal efficiency of the energy plant. For the nuclear energy plants, based on 45,000 MWd/t of burn-up and 2768.4 $/kg-U of specific uranium cost including all the preprocessing cost, Csf’ values for the nuclear energy plants are calculated as 2.56 $/MWth·h9). Therefore, Csf values are 8.27 $/MWe·h for a nuclear power plant employing steam Rankine cycle and 2.85 $/MWth·h for a nuclear heat only plant. Based on the natural gas spot price history at Henry Hub Gulf Coast for the past 14 years, it is reasonable to set the Csf’ values for natural gas at 10.24 $/MWth·h (3 $/MMBTU) minimum to 34.12 $/MWth·h (10 $/MMBTU) maximum10). Csf values are, then, from the minimum of 25.59 to the maximum of 85.30 $/MWe·h for a natural gas power plant employing steam Rankine cycle, and from the minimum of 11.37 to the maximum of 37.91 $/MWth·h for a natural gas heat only plant.

The purchased electricity cost Cpe is the relevant cost in $/kWe·h for electricity supply to the heat only plant and for backup electricity source of RO and thermal desalination plants. At the end of year 2010, both Dubai and Abu Dhabi introduced a dynamic tariff structure on the electricity price that also included slightly raised prices but still retained a large subsidy. Electricity price of Abu Dhabi

ranges from 0.01 to 0.05 $/kWe·h11). In this study, therefore, the purchased electricity cost Cpe was set between 0.01 to

0.05 $/kWe·h for all cases. The DEEP code provides default values of

construction duration Le, specific construction cost Ce and specific O&M cost Ceom for the nuclear heat only plant as well as the nuclear power plant employing steam Rankine cycle. The default values of these parameters for a nuclear power plant employing steam Rankine cycle are 60 months, 4,000 $/kWe and 8.8 $/MWe·h, respectively. On the other hand, default values of these parameters for a nuclear heat only plant are 40 months, 400 $/kWth and 2.0 $/MWth·h, respectively. These values are far less than those for the typical nuclear power plants employing steam Rankine

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cycle due to the absence of electricity generation part. We expect that these values can be less than the current default values since the reference system will be based on low pressure low temperature operation. A low pressure low temperature nuclear system gains economy not only from the construction point of view but also from the operation and maintenance point of view. The fact that the reference system will be constructed in small modular reactor design concept also provides the ground on that expectation. Therefore, values of these parameters for the nuclear heat only plant were set between 12 to 36 months, 320 to 400 $/kWth and 1.6 to 2.0 $/MWth·h in the order mentioned above.

For the operational availability App, the DEEP code provides the same default values of 90 % with no distinction between a nuclear heat only plant and a nuclear power plant employing steam Rankine cycle. The operational availability for the heat only plants is expected to increase due to the absence of electricity generation part which is the major reason for causing unplanned outages, and due to the low pressure low temperature operation, as mentioned above. There have been 140 cases of unplanned outages for 10 years of Korean nuclear power plants operation. Among them, unplanned outages, caused by the secondary system, account for about 60 % of all the cases of unplanned outages12). In light of these facts, the operational availability App of 92 % was adopted for nuclear heat only plants.

III.B. Water Cost Comparison

Water production cost breakdowns of the nuclear heat

only reference case under various conditions are presented in Fig. 4. As shown in the Fig. 4., heat cost decreases as the major energy plant parameters, such as the construction duration Le, specific construction cost Ce and specific O&M cost Ceom, decrease. However, these parameters have no effect on the purchased electricity cost. This is because the energy plant of the reference system is a nuclear heat only plant which provides only heat to the desalination plant. Purchased electricity cost decreases as the purchased electricity cost parameter Cpe decreases. It is obvious that capital and O&M costs show no difference since they are adopting the same desalination technology, MED-TVC, under the same conditions.

Water production cost breakdowns of some important cases are shown in Fig. 5. The result of the nuclear heat only reference case included here is at the construction duration Le of 24 months, specific construction cost Ce of

360 $/kWth, specific O&M cost Ceom of 1.8 $/MWth·h, and

purchased electricity cost Cpe of 0.03 $/kWe·h.

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Fig. 4. Water production cost breakdown of nuclear heat only reference case by varying parameters such as specific construction cost Ce, specific O&M cost Ceom, construction duration Le and purchased electricity cost Cpe.

Fig. 5. Water production cost breakdown of some important cases. (NC, NG-L, and NG-L stand for nuclear, natural gas at minimum price, and natural gas at maximum price, respectively).

When comparing the nuclear heat only reference case to the natural gas cases, natural gas price plays the most important role for determining whether the nuclear heat only reference case is more cost competitive. As you can find in Fig. 6., the nuclear heat only reference case becomes more cost competitive than the cases using natural gas energy if the natural gas price is maintained

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above 4.83 $/MMBTU. In terms of the natural gas price history, this value was real and can be real in the future.

Fig. 6. Water production cost comparison between nuclear heat only reference case and natural gas cases.

As shown in Fig. 7., there may exist a crossing point where water cost of the nuclear heat only reference case becomes even lower than that of the nuclear co-generation cases. Based on the purchased electricity cost Cpe of 0.01 $/kWe·h which is the minimum in the current UAE situation, the reference system can produce water at almost the same cost as the MED-TVC plant with nuclear power plant employing steam Rankine cycle does when the construction duration Le is 12 months, and the specific construction cost Ce and specific O&M cost Ceom decrease to 90 % of default values. Although even at the situation where these parameters decrease up to 80 % of default values, the water cost of the nuclear heat only reference system is still higher than that of the RO plant with nuclear power plant employing steam Rankine cycle for electricity generation. However, the situation can be changed if we consider the cost needed to transport produced fresh water from the system to the end-users. If we can design enhanced nuclear safety features for the nuclear heat only case enabling to construct and operate the system near big cities, this can result in reduction of the water transport cost. The DEEP code provides models and parameters to estimate the water transport cost, as well. We estimated the water transport cost simply by varying values for one parameter, pipeline system length kms, which is the most influential parameter among related parameters, while keeping all other related parameters to the default values. If we assume that the transport pipeline length of the reference system and typical nuclear desalination system is 10 km and 50 km each, then the water transport cost is estimated as 0.01 $/m3 and 0.03 $/m3, respectively. Transport length saving of 40 km results in the water cost reduction of about 0.02 $/m3. When considering the water transport cost saving of 0.02 $/m3, the water cost of the reference system becomes comparable to that of the RO

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plant with nuclear power plant employing steam Rankine cycle. This is a conservative assumption in respect that Braka, which has been selected as a site for the UAE’s first four nuclear power plants, is located in the Western Region of the Emirate of Abu Dhabi, approximately 300 km from Abu Dhabi and 53 km from the nearest city of Ruwais. If we assume ideally direct 300 km length of the transport pipeline installed from the nuclear desalination system at Braka site to Abu Dhabi, then the water transport cost reaches up to 0.16 $/m3.

Fig. 7. Water cost comparison between nuclear heat only reference case and nuclear co-generation cases at purchased electricity cost Cpe of 0.01 $/kW(e)·h.

In conclusion, from the view point of economics, we found that the dedicated nuclear heat only desalination system based on the UAE local condition can be more cost-competitive than other desalination systems under certain conditions. We found how much the economic gap between each option exists and varies as the major parameters change. The dedicated nuclear heat only desalination system in the UAE region may not only be the most economically feasible option, but also the best option in consideration of safety and operational advantages, which were not converted into economic values in this study.

IV. GAS COOLED REACTOR FOR SEAWATER

DESALINATION

To supply the water needs of the UAE with nuclear seawater desalination, KAIST research team is studying the possibilities of coupling a nuclear reactor with MED desalination plant. The gas cooled reactor is considered as an option since it has some advantages over the water or

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liquid metal system. By using gas as a coolant, the reactor operation will always be maintained in the pure single phase operation even during an accident that causes the reactor temperature to increase. Noble gas, such as helium, is also known as an inert for chemical reaction and is quite transparent to neutron so we can be less concerned about the radioactive leak when there is a leak in the system. Another advantage is the impossibility of total coolant loss, whenever the loss of coolant accident happened, the system will be depressurized until the pressure is equal with the surrounding pressure13).

IV.A. Previous Studies

Some previous studies have been done regarding the

coupling of a gas cooled reactor with a seawater desalination plant. The considered reactors are the helium cooled Pebble Bed Modular Reactor (PBMR) and also helium cooled Gas Turbine Modular Helium Reactor (GTMHR)14). Both PBMR and GTMHR main functions are to produce electricity and they used the waste heat from the cycle for the desalination process. Both of the reactors are a graphite moderated helium cooled reactor which operate at high temperature (around 850 oC to 900 oC) and produce waste heat around 100 oC. This temperature is sufficient enough for MED desalination plant. MED desalination plant usually operates at maximum top brine temperatures below 80 oC to reduce the risk of scaling on the equipment during operation. Principle of the coupling of a GT-MHR to MED process is shown in Fig. 815). The technical document of IAEA on Optimization of the coupling of nuclear reactors and desalination systems stated the amount of water produced by utilizing waste heat from GT-MHR with MED is only about 43,500 m3/day16). Another study is done by Mabrouk Methnani17), 2006. He compared the cost of nuclear desalination system using HTGR with fossil fuel combined cycle system by using the DEEP code. The result shows that the nuclear desalination option with HTGR for both RO and MED process are lower than the fossil fuel desalination system.

Compared to the previous study of waste heat utilization of gas cooled reactor for seawater desalination, KAIST research team decided to study the possibility for designing a dedicated gas cooled reactor for thermal desalination system. Though it's called dedicated heat system but the reactor will still produce electricity since the operating temperature of gas cooled reactor is too high for direct use to MED. The electricity produced by this designed reactor doesn't have to be very high and will be used to supply the electricity needed by the desalination plant only. By doing so, the reactor operating temperature and pressure can be reduced. The operating temperature can be reduced since a desalination system doesn't require high temperature process heat and the operating pressure can be reduced because the main purpose of the system is

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Fig. 8. Principle of the coupling of GT-MHR to MED process, utilizing the waste heat. (Dardour et al.15)).

not to produce electricity but supply heat for the desalination process. Therefore, the system doesn't need to use high pressure turbine for the cycle. However, there are some aspects that will give lower limit of the operating condition below which the system operation is impossible. The main aspect that influences the system pressure is the heat transfer capability of the gas coolant. Gas becomes better heat transfer media as the operating pressure goes up. The main aspect that influences the system temperature is the Wigner effect from the graphite moderator. Wigner effect occurs in graphite irradiated below 300 oC and becomes a problem for reactor operation below 150 oC. The first prototype of gas cooled reactor (Magnox) had found a way to deal with the Wigner effect which was by doing annealing process on the refueling period. All Magnox reactors were operated at inlet temperature above 150 oC18). Therefore, it is better to consider that 150 oC is the lower limit of the operating temperature for graphite moderated reactor. IV.B. Coolant Selection and Reactor Core Thermal Power

Based on the operational experience there are two

gasses suitable for the gas cooled reactor which are CO2 and helium. Helium is preferable as a coolant since CO2 can disintegrate and react with graphite structure at high temperature. CO2 can also cause oxidation of structural

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materials at certain temperatures (350 oC - 450 oC for steel)13). Helium is a good choice since it will not become radioactive when it is irradiated and it also has good heat transfer properties and good chemical properties.

From the previous study it can be seen that by utilizing waste heat from 600 MWth reactor core we can produce about 43500 m3/day fresh water via thermal desalination. The dedicated heat gas cooled reactor should minimize the reactor core thermal power while increasing water production capacity. It is a feasible idea since the main function of the reactor is for process heat and not electricity production. From the previous section, the amount of desalinated water needed for a city with one million of population in UAE will require about 300,000 m3of fresh water per day. The thermal power needed to produce this amount of water by MED is calculated to be about 850 to 900 MWth. This amount of power can be supplied by 3 to 4 gas cooled reactors with each reactor having thermal power of 300 MWth. Therefore, for the preliminary study the reactor system is specified to produce thermal power of 300 MWth.

IV.C. Heat and Electricity Production

For the primary side design, there are 2 options

available for the heat and power cycle. The first option is using helium direct Brayton cycle and the other option is

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using helium indirect cycle with either helium Brayton cycle or steam Rankine cycle. The first option is preferable from the economical point of view since it can reduce the amount of component needed by the system. To optimize the usage of reactor thermal power for the process heat, the system needs to have a recuperator.

Two cases are made for the preliminary design calculation. The first case uses Magnox Calder Hall19) operating temperature and the second case with a higher temperature. The pressure of 6 to 6.5 MPa is set as the compressor outlet pressure. The reactor core pressure drop is estimated to be 125 kPa using the GTMHR core as the reference20).

To define mathematical equation for designing the primary system, some data will be needed such as turbine efficiency, compressor efficiency, recuperator effectiveness, heat exchanger pressure drop, and pressure drop along the pipeline. This data can be found from Brayton cycle assessment21) which are shown in Table II.

TABLE II

Brayton Cycle Parameter

Variable Value εrec 95% ηT 93% ηC 89%

As the pressure drop value in heat exchanger, 0.1 bar

or 100 kPa is a reasonable value for gas22). The pressure drop along the pipeline is negligible in the preliminary design. Reactor:

The mass of helium coolant needed can be obtained by

Q m H H (1)

m (2)

The enthalpy data for a given temperature and pressure can be found from NIST property database23). Turbine:

To find the turbine outlet operating condition, first we have to assume isentropic process in the turbine.

T , T γ (3)

where turbine outlet pressure (P1) can be calculated after estimating turbine pressure ratio. The actual turbine outlet temperature (T1) can be calculated by

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Δ , T T , (4) Δ η Δ , (5) T T Δ (6) Recuperator:

Heat transfer equation in recuperator can be formed as Q Q (7) m H H m H H (8)

ε (9)

Intermediate Heat Exchanger

The heat that can be extracted from the primary side to secondary side is then Q m H H (10) Compressor

The mathematical equation in compressor side is similar with turbine which is

T , T γ (11)

Δ , T T , (12) Δ η ∗ Δ , (13) T T Δ (14)

Electricity Production

To calculate how much electricity is produced, the turbine power, compressor power, and generator efficiency is necessary. The other factors that needed for calculating how much the electricity is produced are mechanical losses (1%), parasitic losses (2%), and switchyard losses (0.5%)24). Therefore,

P . Q Q (15)

The operating condition of each case is shown in Table

III while the heat and electricity production is shown in Table IV. The condition of every stream in Case I and Case II are shown in Fig. 9 and Fig. 10 respectively.

TABLE III

Operating Parameter for 2 Design Cases

Parameter Case I Case IIReactor inlet temperature (oC) 167 287.3 Reactor outlet temperature (oC) 345 490 Helium mass flow (kg/s) 324.9 285.3 Turbine pressure ratio 1.6 1.6 Max Pressure (MPa) 6 6.5

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TABLE IV

Heat and Electricity Production

Parameter Case I Case IIHeat to secondary side (MWth) 265.84 240.64Electricity produced (MWe) 31.26 56.11

Fig. 9. Block diagram of primary system design for case I.

Fig. 10. Block diagram of primary system design for case II.

Table IV shows that case I provides more heat to the secondary system but case II produce more electricity. From the system main purpose point of view, case I provides a better system than case II. However, there is another aspect, which should be considered, for choosing the operating condition: the Wigner effect. As it was stated before, the Wigner effect should be concerned of when the graphite moderated nuclear reactor is operated below 300 oC. Since case I reactor inlet temperature is 167 oC, the amount of energy stored in the graphite due to the Wigner effect should be carefully estimated. In this sense, case II is a better system in terms of operation since the Wigner effect is less worrisome.

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IV.D. Secondary Side Design & Coupling Scheme

For the secondary side design, water system is the best option to connect the primary system with desalination system. Open loop system using seawater and closed loop system using cooling water are the two most commonly adopted systems for connecting a nuclear reactor with a seawater desalination system. An open loop system using seawater has a risk of radiation leak from the primary system and also has a scaling problem that could occur in high temperature. Therefore, from the safety point of view, closed loop system using cooling water is preferable. The next step for the secondary side design is designing the secondary side operating condition. MED top brine temperature is usually up to 70 oC. The operating condition of the intermediate loop should be arranged to make it possible for seawater feed temperature to reach the appropriate condition.

The other part of the design that should be considered for the intermediate loop is whether to use a steam generator to change the water phase from liquid to steam and send it to MED plant or using a flash tank and send the vapor portion to MED plant. By using the steam generator, the amount of water needed by system will be smaller compare to using the flash tank but the steam temperature will be much higher. By using the flash tank, steam temperature will be lower but the vacuum system is needed. Besides that, higher amount of water also means larger pipeline size. Further economical analysis is needed for accurate comparison.

Coupling of the nuclear reactor with a MED desalination plant is usually done with one nuclear reactor coupled to several MED plants. The capacity of a MED unit that has been demonstrated in the market is about 20,000 m3/day25). From the DEEP code calculation, the thermal power required for providing 20,000 m3/day is around 50 MWth. Based on this data, the heat provide by the KAIST designed system will be sufficient for 4 MED plants. As to provide 300,000 m3/day desalinated water, 4 reactors will be needed which will result in maximum capacity of 320,000 m3/day.

V. SUMMARY

In this paper, we studied the potential of developing

nuclear desalination system with thermal desalination method for the UAE. Based on the economical analysis using the UAE local condition, the nuclear desalination plant is economically favored over the fossil fuel (natural gas) desalination plant. The reason is because fossil fuel desalination system depends on natural gas price which is unstable parameter while nuclear energy offers a stable and fairly low fuel price which results in stable and low water production cost. Therefore, it can be concluded that the nuclear desalination system has a high chance to be developed in the UAE.

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A dedicated nuclear system for desalination which focused on supplying process heat for a MED system is being studied by KAIST research team. One type of reactor under consideration is helium cooled graphite moderated reactor. The low operating pressure of the reactor is preferable to ensure the safety of the proposed reactor system. Operating at lower temperature can produce more heat and less electricity than operating at higher temperature. However, the high temperature operating case is a better option if we consider the graphite Wigner effect. To provide 300,000 m3/day desalinated water, the best coupling scheme would be 4 reactors with each reactor coupled to 4 MED plants with 20,000 m3/day capacity.

The further works for developing helium cooled graphite moderated nuclear reactor which is dedicated for the fresh water production only are: optimizing the operating condition which can result in higher performance of the system, selecting the operating condition for intermediate loop , and designing the fuel assembly and safety system for the proposed nuclear reactor.

.

ACKNOWLEDGMENTS

This work is supported by KUSTAR-KAIST Research Project Proposal on "Development of Highly Passive Small Modular Reactor System for Large Scale Seawater Desalination".

NOMENCLATURE

H enthalpy (kJ/kg) m mass flow (kg/s) P pressure (MPa) Q heat flow (kW) T temperature (oC) Greek symbols ε effectiveness η efficiency ΔT temperature difference Subscripts C compressor T turbine in reactor inlet out reactor outlet

REFERENCES

1. Mohammed Al Hajjiri, Einar Al Hareeri, “ADWEC Winter 2011/2012 Electricity & Water Demand Forecasts”, Abu Dhabi Water & Electricity Company.

2. T. Mezher, H. Fath, Z. Abbas, and A. Khaled, "Techno-economic assessment and environmental

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impacts of desalination technologies," Desalination 255, p. 263-273 (2011).

3. Yuan Zhou, Richard S.J.Tol, “Evaluating the cost of desalination and water transport”, WATER RESOURCES RESEARCH, VOL. 41, W03003, p. 10(2005).

4. O. A. Hamed, H. A. Al-Washmi, and H. A. Al-Otaibi, "Thermoeconomic analysis of a power/water cogeneration plant," Energy 31, p. 2699-2709 (2006).

5. K. C. Kavvadias and I. Khamis, “The IAEA DEEP desalination economic model: A critical review,” Desalination, Vol. 257, p. 150-157 (2010).

6. International Energy Agency, Beyond the OECD –

United Arab Emirates, http://www.iea.org/, (2011).

7. International Atomic Energy Agency, Desalination Economic Evaluation Program (DEEP) User’s Manual, Computer Manual Series No.14, p. 32, IAEA, Vienna (2000).

8. International Atomic Energy Agency, Examining the

economics of seawater desalination using the DEEP code, IAEA-TECDOC-1186, p. 14, IAEA, Vienna (2000).

9. World Nuclear Association, The Economics of Nuclear

Power, http://www.world-nuclear.org/, (2011).

10. U.S. Energy Information Administration, Natural Gas Prices, http://www.eia.gov/, (2011).

11. L. El-Katii, Interlinking the Arab Gulf: Opportunities

and Challenges of GCC Electricity Market Cooperation, p. 8, Oxford Institute for Energy Studies (2011).

12. Korea Ministry of Education, Science and Technology

and Korea Institute of Nuclear Safety, 2010 White Paper on Nuclear Safety, p. 106-108, Korea Ministry of Education, Science and Technology and Korea Institute of Nuclear Safety (2010).

13. D. E. Shropshire, "Lessons Learned From GEN I Carbon Dioxide Cooled Reactors." Proceedings of ICONE 12, Virginia (2004).

14. International Atomic Energy Agency, "Status of design concepts of nuclear desalination plants," IAEA-TECDOC-1326, IAEA, Vienna (2002).

15. S. Dardour, S. Nisan, and F. Charbitt, "Utilisation of waste heat from GT-MHR and PBMR reactors for

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nuclear desalination," Desalination 205, p. 254-268 (2007).

16. International Atomic Energy Agency, "Optimization of the coupling of nuclear reactors and desalination systems," Final report of a coordinated research project 1999-2003, IAEA-TECDOC-1444, IAEA, Vienna (2005).

17. M. Methnani, "Influence of fuel costs on seawater desalination options," Desalination 205, p. 332-339 (2007).

18. B. J. Marsden, "Irradiation Damage and Annealing," Nuclear Graphite Research Group, School of Mechanical, Aerospace and Civil Engineering, The University of Manchester.

19. HM Nuclear Installations Inspectorate, Report on the results of Magnox Long Term Safety Reviews (LTSRs) and Periodic Safety Reviews (PSRs).

20. Idaho National Laboratory and General Atomics, "Prismatic HTGR Thermal-Fluid Behaviour," HTGR Technology Course for the Nuclear Regulatory Commission (2010).

21. R. Schleicher, A. R. Raffray, and C. P. Wong, "An Assessment of the Brayton Cycle for High Performance Power Plants," Proceedings of 14th Topical Meeting on the Technology of Fusion Energy, Utah (2000).

22. R. Mukherjee, "Effectively Design Shell-and-Tube Heat Exchangers," CHEMICAL ENGINEERING PROGRESS, American Institute of Chemical Engineers (1998).

23. E. W. Lemmon, M. O. McLinden, and D. G. Friend, "Thermophysical Properties of Fluid Systems" in NIST Chemistry WebBook, NIST Standard Reference Database Number 69, Eds. P. J. Linstrom and W. G. Mallard, National Institute of Standards and Technology, Gaithersburg MD, 20899, http://webbook.nist.gov, (retrieved January 16, 2012).

24. V. Dostal, "A Supercritical Carbon Dioxide Cycle for Next Generation Nuclear Reactors," Doctoral Thesis, MIT (2004).

25. J. Goossen, "Nuclear Process Heat Desalination", GCEP - Fission Energy Workshop, Stanford (2007).

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