Hydrogen: Gen-IV reactors
SCWR-hydrogen plant thermal integration12 September 2011
The temperature range of a thermochemical hydrogen production cycle are the dominant parameters that affect the design of its coupling with a Generation IV supercritical water-cooled nuclear reactor (SCWR). A stream extraction location is suggested. Also, the available heat at most hours of power demand in a day can support an industrial scale steam methane reforming plant if the SCWR power station is operating at full design capacity.
The supercritical water reactor (SCWR) is a Generation IV nuclear reactor with a higher thermal efficiency and has a system configuration that is considerably simpler than a conventional light water reactor (LWR). The working fluid of the SCWR is light water that operates at a supercritical state above the water critical point with a temperature and pressure higher than 375°C and 22.1MPa [1-4], respectively. Since the SCWR uses light water, it can be regarded as a special type of LWR operating at higher pressures and temperatures with a direct, once-through cycle. Therefore, existing and relatively mature LWR technology can be adopted, since there is already extensive worldwide knowledge of LWRs for engineers to design, construct and operate SCWRs.
The SCWR’s direct cycle operation indicates the SCWR’s similarity to a special type of boiling water reactor (BWR). In addition, because steams and liquids can be considered indistinguishable at the supercritical state, only one phase is present in the rector, like the pressurized water reactor (PWR). Therefore, it is anticipated that SCWRs would have a combination of advantages of both BWRs and PWRs [5, 6].
Since the coolant in SCWRs has a high temperature that does not experience phase change in the reactor, and can be directly coupled with energy conversion equipment, SCWRs have a higher thermal efficiency and considerably simpler plant than a conventional LWR. For example, the thermal efficiency of an SCWR could reach 45%, while in comparison, 30-35% are typical values for current LWRs [7-9].
The enhanced power generation efficiency of SCWRs does not imply that SCWRs can reduce the challenges of load balancing. The required load in the power grid changes with the power demand of end users. A typical example is the significant gap between peak- and off-peak-hour power demands . The frequent adjustment of nuclear power output brings issues of safety, reliability and stability into the design, operation and maintenance of a nuclear power station.
To address these challenges, hydrogen production, either by conventional electrolysis or thermochemical water splitting, can be effectively integrated with power generation to buffer the load adjustment, especially at off-peak hours. In this way, SCWRs can operate at a constant power load or at invariable maximum power, independent of the power demand changes on the grid, that is, the power load fluctuations throughout a day or year. When the power load decreases, the SCWR will produce more hydrogen (see also Table 2). Another benefit to couple SCWRs with thermochemical water splitting, rather than steam methane reforming (SMR) to produce hydrogen, is the reduction of greenhouse gases emitted from SMR. It is also anticipated that hydrogen as a clean energy carrier will be a significant driving force for sustainable energy demand in the future [11, 12].
Since thermochemical water splitting can provide much higher thermal efficiency than conventional electrolysis, thermochemical hydrogen production is an emerging technology of growing importance. Currently, the University of Ontario Institute of Technology (UOIT), in collaboration with Atomic Energy of Canada Limited (AECL) and other university and industry partners is developing the Cu-Cl cycle for nuclear hydrogen production. The energy required by thermochemical hydrogen production cycles to split water is predominantly heat, with a lesser portion of electricity if the thermochemical cycle is hybrid [11-13]. From this aspect, therefore, heat exchange between a thermochemical hydrogen production cycle and a SCWR power plant is studied in terms of factors such as the heat extraction location, method, quantity, and hydrogen production scale.
Heat flows in SCWRs
The thermal energy used by thermochemical hydrogen production cycles is the heat extracted from the supercritical water stream exiting the SCWR core. The temperature difference between SCWR and the thermochemical cycle provides the driving force of the heat exchange. Before entering turbines, the water stream is in the form of supercritical water that has the highest temperature to provide the maximum heat transfer. From this perspective, the temperature of the supercritical water upstream of the turbines greatly influences the practicality of coupling SCWRs with thermochemical hydrogen production cycles.
The minimum temperature of supercritical water is determined by the critical point of water, which is 374°C. To reduce the risk of the fluctuation of temperature (that is, the fluctuation caused by flow rate variations of water circulation, nuclear heat release, and power demand change), the temperature of the supercritical water is usually much higher than 374°C. The higher temperature can increase the thermal efficiency of SCWRs. However, the maximum outlet temperature of the reactor core is significantly influenced by reactor materials.
The temperature range in Japan  for a high-temperature large fast reactor cooled by supercritical water (SCFR-H) is 467-537°C, with blankets cooled by ascending and descending flow. The SCFR-H adopts a radial heterogeneous core with zirconium-hydride layers between the driver core and the blankets for making the coolant void reactivity negative. The highest temperature range cited at the University of Tokyo  was 593-600°C due to the corrosion and duration of reactor material operation. The design of an SCWR in China was planned to be at a temperature of 500°C , using reduced activation ferritic-martensitic steels as the reactor core materials.
The Idaho National Engineering and Environmental Laboratory (INEEL) cited a temperature range of 565-620°C based on the existing engineering experience in supercritical water power plants , such as supercritical water turbine technology. AECL is developing an SCWR with an outlet temperature in the range of 625-650°C [17, 18]. This temperature allows it to also produce hydrogen by thermochemical water splitting. AECL is collaborating with UOIT (University of Ontario Institute of Technology, Canada) and ANL (Argonne National Laboratory, USA) to develop the technology of coupling the CANDU-SCWR with thermochemical hydrogen production.
Hydrogen production cycles
Around two hundred thermochemical water splitting cycles have been reported in past literature [19, 20]. Among these cycles, the sulfur-iodine (S-I, [12, 22-25]) and copper-chlorine (Cu-Cl) cycles are two leading examples [19-22]. There are several types of S-I cycles and three-step cycle is commonly accepted as the most typical loop [23-25]. In the three-step loop, steps 2 and 3 are endothermic and their minimum temperature requirements are 850°C for step 2 and 450°C for step 3.
|Step (type of reaction)||Reaction|
|Step 1: Hydrolysis of iodine and sulfur dioxide (exothermic)||I2(l+g) + SO2(g) + 2H2O(g) --> 2HI(g) + H2SO4(l), 120°C (1)|
|Step 2: Oxygen production (endothermic)||H2SO4(g) --> SO2(g) + H2O(g) + 1/2O2(g), 850°C (2)|
|Step 3: Hydrogen production (endothermic)||2HI(g) --> I2(g)+ H2(g), 450°C |
For the Cu-Cl cycle, there are also several types with various numbers of steps from 2 to 6 depending on reaction types and conditions [20, 21, 24]. For similar reasons as the S-I cycle, the four-step Cu-Cl cycle (as laid out in Figure 1) will be considered in this paper.
|Step I: Hydrogen production step (endothermic)||2CuCl(aq) + 2HCl(aq) --> 2CuCl2(aq) + H2(g), in aqueous solution of HCl, 80~100°C (I)|
Step II: Drying step (endothermic)
CuCl2(aq) + nfH2O(l) --> CuCl2.nhH2O(s) + (nf - nh)H2O, nf >7.5, nh = 0~4, at 30~80°C (crystallization) or 100~260°C (spray drying) (II)
Step III: Hydrolysis step (endothermic)
2CuCl2.nhH2O(s) + H2O(g) --> CuOCuCl2(s) + 2HCl(g) + nhH2O(g), nh is 0~4, at 375°C
Step IV: Oxygen production step (endothermic)
|CuOCuCl2(s) --> 2CuCl(molten) + 1/2O2(g), 500~530°C|
Figure 1 shows the temperature levels of SCWRs and temperature requirements of S-I and Cu-Cl cycles. It can be found that most SCWRs can cover the temperature requirements of all steps in the Cu-Cl cycle. The driving process for the heat exchange provided by CANDU-SCWR of Canada is larger than 100°C.
For the S-I cycle, step 2 requires a temperature of 850°C, which cannot be reached with current technology of SCWRs. Although the temperature of SCWRs can cover step 3, the heat quantity at 450°C required by step 3 occupies less than 10% of the total heat required by the S-I cycle. In comparison, the heat quantity required at 850°C by step 2 accounts about 90% of the total heat requirement.
In theory, the water stream of the SCWR coolant loop, either in a supercritical or gaseous state, can be directly extracted as the reactant and heating fluid for the Cu-Cl cycle. However, this extraction method is not suggested. The chemical reaction pressure is lower than 2.4 MPa in the Cu-Cl cycle [21, 24]. In comparison, the water stream exiting the SCWR core is usually higher than 25 MPa if it is supercritical water. This means the water pressure leaving the SCWR must be significantly decreased to the lower pressure of the Cu-Cl cycle. In addition, if the supercritical water stream is open to chemicals of the Cu-Cl cycle, it may lead to chemical contamination. Therefore, indirect heat exchange is suggested, whereby the water stream in the SCWR coolant loop serves as the heat carrier rather than as the reactant source of Cu-Cl cycle. The term ‘water stream’ in this paper refers to the water stream of the SCWR coolant loop.
The temperature levels of SCWR and the Cu-Cl cycle also have significant influence on the location of the heat extraction from an SCWR, and the Cu-Cl cycle in turn will influence the water flow arrangement of the SCWR.
Water circulation in a posited SCWR coolant stream with hydrogen co-generation from the Cu-Cl thermochemical cycle has the typical features of a single-reheat system that uses a preheater and two types of turbines, that is, a high pressure (HP) turbine and low pressure (LP) turbine (Table 1).
|Component||State of water produced||Temperature (°C) produced||Pressure (MPa) produced|
|Hydrogen cycle heat exchanger*||Supercritical||>600||25-26|
*An additional output of 525°C, 15 MPa steam goes to the preheater
**Hydrogen loop uses helium pressurised to 2 MPa; temperature varies between 600°C (after heat exchanger) to 250°C (after all steps of hydrogen cycle)
Table figures are based on existing fossil fuel power plants using a supercritical water turbine. Temperature and pressure values calculated according to water stream enthalpy change and expansion ratio [3, 26]
It can be found that a point downstream of the nuclear reactor but upstream of the turbines can provide around 100°C of a driving temperature difference for step IV of the Cu-Cl cycle (which requires 530°C). At this location, a bypass line of the supercritical water stream equipped with heat exchanger can be designed to enable heat extraction from supercritical water to the Cu-Cl hydrogen production cycle.
For step III of the Cu-Cl cycle, the temperature requirement is 375°C. The heat for this step can be supplied from the stream at 530°C after leaving step IV.
The water stream exiting the Cu-Cl cycle still has a temperature higher than 375°C, but the pressure has strong dependence on the dimensions and structures of heat exchangers inside the Cu-Cl cycle. The water stream exiting the Cu-Cl cycle can be directed to a steam turbine, heat regenerator, or the nuclear reactor core, depending on the pressure. Detailed studies of the pressure drops of the supercritical water passing through the chemical reactors will be performed in future research.
The water stream can also be arranged to pass through HP and LP turbines in parallel. In this type of layout, the water stream flows directly back to the SCWR core after leaving the HP turbine, rather than entering the LP turbine. Therefore, there are two water streams at 625-650°C leaving the core of the SCWR. This provides multiple choices for the heat extraction bypass lines and the water stream circulation layout.
The hydrogen production scale is significantly influenced by the heat quantity required by the Cu-Cl cycle, and the heat quantity that can be extracted from the SCWR. Table 2 lists the hydrogen production scale at different times of a day. The calculation is based on the design capacity of 1000 MWe. The ratio of the generated power to its design capacity is assumed to be equal to the ratio of power demand to power supply capacity on the power grid. The ratio is based on data from the provincial Ontario government, Canada . The available heat for hydrogen generation is the difference between the design capacity of the SCWR and the power actually generated in real time. The SCWR operates at its maximum capacity with cogeneration of hydrogen. The heat transferred to the Cu-Cl cycle from SCWR is assumed to have a loss of 40%, and only 50% of the heat released by exothermic reactions of the Cu-Cl cycle is reused inside the cycle. (Although there are no exothermic chemical reactions in the Cu-Cl cycle, the processing of some intermediate products is exothermic. Exothermic physical processes include molten CuCl heat recovery, oxygen gas heat recovery and HCl gas heat recovery). Table 2 shows that a single SCWR can produce 170-320 tons of hydrogen each day, depending on the peak and off-peak power demand hours of the grid. This hydrogen production scale is equivalent to that of an industrial steam methane reforming plant .
|Energy used for power generation*||Available heat for hydrogen production||H2 production|
|Time||% peak hour||% of design capacity MWe||MWe**||Unused power capacity MWe||Unused heat quantity, MWt (at 45% thermal efficiency)||Tons/day***||Tons/specified period|
|Daily H2 generation||230.6|
*Percentages from power consumption statistics of Ontario, Canada 
**Designed full power capacity for single SCWR assumed to be 1000 MWe
***Calculations assume 20% heat loss from SCWR-hydrogen plant transfer. Net heat input required by the Cu-Cl cycle is 227 MJ/kgH2, assuming 50% of heat from exothermic reactions is reused inside the Cu-Cl cycle [24, 28]
Z.L. Wang, G. F. Naterer and K. S. Gabriel, Faculty of engineering and applied science, University of Ontario Institute of Technology (UOIT), 2000 Simcoe St North, Oshawa, Ontario L1H 7K4. Sam Suppiah, AECL, Chalk River Laboratories, Chalk River, Ontario, K0J 1JO.
Financial support from Atomic Energy of Canada Limited and the Ontario Research Fund is gratefully acknowledged.
This article is based on a paper presented at the 2nd Canada-China joint conference on super-critical water-cooled reactors, Toronto, Ontario; April 25-28, 2010.
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