D. DOMANSKI, P. ZANOCCO and M. GIMÉNEZ
Assessments of a Passive Heat Removal System in an integral reactor
D. B. DOMANSKI
Comisión Nacional de Energía Atómica
S. C. de Bariloche, Argentina
Email:
P. ZANOCCO, M. GIMÉNEZ
Comisión Nacional de Energía Atómica
S. C. de Bariloche, Argentina
Abstract
This work is oriented to develop knowledge for supporting engineering tasks regarding the design of a "Passive Heat Removal System" (PHRS) of an integral-type reactor (CAREM-25), and analyze the plant evolution in case of Loss of Heat Sink (LOHS). The mission of this system is to reduce the pressure on the primary system by means of decay heat removal, by condensing steam from the primary system in condensers immersed in containment pools. The condensate returns to the reactor vessel establishing a two-phase natural circulation circuit. A model of PHRS is developed using RELAP5 code. Some studies are made with the system isolated, by setting boundary conditions, to characterize its phenomenology under nominal operation. Different configurations of tubes are analyzed. Then, this model is integrated to the plant model, with the aim to analyze the performance of this system. Two stages are observed: initially, when the PHRS is triggered the primary system pressure decreases sharply because the steam is condensed while the liquid remains subcooled. Then, the primary system continues with saturated depressurization until to reach the grace period (36 hours). Finally, the PHRS demand ensures cooling and decay heat removal without requiring power energy or human actions during 36 hours, avoiding the safety valves demand. Pressure is decreased allowing the action of normal shutdown systems.
1. INTRODUCTION
The introduction of passive safety systems in advanced reactor designs is one of the topics that pose challenges to the safety demonstration. The present work consists in analyze the response of CAREM reactor with the actuation of the Passive Residual Heat Removal System (PHRS) in case of Loss of Heat Sink (LOHS).
The PHRS condensers are located in a pool filled with cold water inside the containment building. Due to systems assembly and lay-out issues, it is of particular interest the study of different configurations of tubes. Some particular studies arise regarding the impact on of this construction issues in the system capacity.
2. rEACTOR DESCRIPTION
A brief description of the reactor and the passive safety system that are related with the present work is presented [1] [2]. CAREM is an Argentine project to achieve the development, design and construction of an innovative, simple and small Nuclear Power Plant (NPP).
CAREM is an indirect cycle reactor with some distinctive features that greatly simplify the design and also contributes to a high safety level. Some of the high level design characteristics are: integrated primary cooling system, self-pressurized primary system, safety systems relying on passive features, primary cooling by natural circulation.
2.1. Primary System and its main characteristics
CAREM NPP design is based on a light water integral reactor. The whole primary system, core, steam generators, primary coolant, steam dome and control rod mechanism, are contained inside a single pressure vessel. For low power modules (below 150 MWe), the flow rate in the reactor primary systems is achieved by natural circulation (Fig.1). Several innovative features can be observed within the primary system, besides their passive safety systems, such as the self-pressurization (result of the liquid-vapour equilibrium), reactivity control without boron in the coolant, and as mentioned above, primary system coolant driven by natural circulation induced by the location of the steam generators above the core.
2.2. Passive Residual Heat Removal System
CAREM safety systems are based on passive features and must guarantee no need of active actions to mitigate events during the grace period (36 hours). They are duplicated to fulfill the redundancy criteria.
The Passive Residual Heat Removal System (PHRS) has been designed to reduce the pressure on the primary system and to remove the decay heat in case of Loss of Heat Sink (LOHS) with unavailability of the active safety systems. It is a simple system (Fig.2) that operates condensing steam from the primary system in condensers. The condensers consist of an arrangement of parallel horizontal U tubes between two common headers which are located in a pool filled with cold water inside the containment building.
The top header is connected to the steam dome, while the lower header to the reactor vessel in a position below the reactor water level. The inlet valves in the steam line are always open, while the outlet valves are normally closed, therefore in stand-by mode the tube bundles are filled with condensate. Each PHRS contains two modules with two condensers tubes.
In case of primary system overpressure, the reactor protection system demands automatic opening of the outlet valves. The water drains from the tubes and steam from primary system enters the tube bundles and is condensed on the cold surface of the tubes. The condensate is returned to the reactor vessel establishing a Natural Circulation (NC) circuit. In this way, heat is removed from the reactor coolant. During the condensation process the heat is transferred to the water of the pool by a boiling process. This evaporated water in condenser pools is conducted to the containment suppression pool, where condenses. This way the decay heat is stored within the containment during the grace period [3].
FIG. 1. CAREM primary system FIG. 2. PHRS layout
3. MODEL DEVELOPMENT
In order to perform the calculations with RELAP5 a one-dimensional nodalization of CAREM reactor has been developed. The model involves the Primary System and the PHRS.
First some studies are made with the system isolated by setting boundary conditions and then the PHRS model is integrated to the reactor model to analyze its performance during a LOHS.
A brief description of reactor and PHRS models are presented. The models are developed within an integral platform of data nodalization and management, implemented for RELAP code for simulation of reactor transients. The system includes geometry and process input data, calculation of related parameters, data processing for nodalization development, and automatic generation of the input file for the RELAP code, supporting quality assurance and minimizing input errors. The primary circuit nodalization has been set-up dividing it into the most relevant components: RPV dome, steam generators (SG), down comer, riser, core and lower plenum. The SG secondary side is also included in the model, the rest of the secondary system and process systems have been modelled as boundary conditions.
The PHRS nodalization includes the following system components: steam line (steam line piping and inlet header), condensers (heat structures have been taken into account), condensate line (return line piping, outlet header and valves) and system pool.
4. PHRS efficiency curve
As it was described before, the PHRS condensers are located in a pool inside the containment building. In order to analyze important aspects for the assembly and location of equipment, some studies are made with the system isolated by setting boundary conditions. In this case, the removal power in function of primary pressure is analysed for several configurations, considering different pipe lengths.
Design conditions are modelled by means of boundary conditions: primary system pressure of 12.25 MPa and a pool temperature of 100°C (saturation state). The PHRS removal power is 2MW per redundancy (1MW per module) at primary pressure of 12.25 MPa. To analyze the efficiency of the equipment against possible changes in the effective length of the heat exchangers, three simulations are performed: DL (design length), DL/2 (half of design length) and x*DL/2 (including x as a length compensation factor) (Fig.3).
Heat removal capacity must be maintained in order to fulfil design requirements. Thus, a reduction in the length of the tubes is compensated increasing the quantity of tubes: the case corresponding to the design length represents two tubes while the others, four tubes.
The heat transfer coefficients in condensers are observed in Fig. 4. The heat transfer mode recorded along the tube is filmwise condensation for the primary side and subcooled nucleate boiling for pool side. The tube coefficient (an equivalent coefficient due to the conduction process) is the one that will govern in greater proportion the heat exchange. So power will depend mainly on heat transfer area. However, the condensation coefficient is reduced with length, as result of liquid formation that make more difficult the condensation process. This would decrease heat transfer capacity in the last section of the tubes.
Due to aspects related to the equipment assembly, it is required to analyse the performance of the system with shorter tubes; in particular, it is proposed to use tubes with a half-length, and at the same time to double the number of tubes. This configuration was studied, and it is observed that it is not enough to remove a power of 1MW at the operating pressure of 12.25 MPa. This happens because, despite the condensation coefficient is higher near the inlet region, the overall condensation coefficient decreases for shorter tubes (Fig.4). This is because the flow mass decreases, as long it has to split in higher number of tubes. So the tubes length must be increased to compensate this phenomenon, to x*DL/2.
When the primary pressure decreases, the efficiency of the equipment is reduced mainly due to the decrease in the associated primary side temperature. Nevertheless, the decrease is (slightly) higher for shorter tubes, mainly due to the increased sensitivity on condensation coefficients (Fig.3).
FIG. 3. PHRS efficiency curves for different tube lengths (results per module)
FIG. 4. Heat transfer coefficients for different tube lengths
5. Loss of Heat Sink SIMULATION
5.1. Modeling considerations
Previous to the transient code run, a steady-state has been performed with reactor operating at 100% power, maintaining the reactor pressure at the nominal value (12.25MPa).
On the other hand, a series of considerations and hypotheses are postulated to simulate the event:
— Failure of all control and regulation systems of the plant, in addition to process systems, is considered. Therefore, it must be demonstrated that the safety functions are effectively fulfilled by the Safety Systems.
— Activation of only one redundancy of PHRS.
5.2. Event description
The transient to be analyzed is a LOHS: in this case the initiating event proposed is an abrupt loss of SG feedwater. For a better description and to establish a criterion for transient analysis four phases has been identified: Phases 1-2 are stages taking place before PHRS demand and Phases 3-4 after system actuation (Fig.5).
Phase 1: Due to the LOHS, down-comer temperature increases. This causes, on the one hand, a reduction in the buoyant force (decreases in the water density), decreasing the flow. On the other hand, coolant expansion occurs increasing the liquid level, the steam in the dome is compressed increasing the pressure system.
First Shutdown System (FSS) is triggered when temperature at the exit of the SG reaches its correspondent set-point (high primary pressure signal is almost simultaneously). As a consequence of the power reduction, heat flux within the core is reduced decreasing subcooled boiling and core void generation stops. Thus, a temporal pressure decrease is verified.
Phase 2: Once finalized this brief depressurization stage, due to there is no power removal, the temperature goes on increasing in the down-comer and in the whole circuit driving again to the primary circuit coolant expansion with the subsequent primary system pressure increase. The rate of increased pressure is reduced compared with Phase 1, because the generated power drops to decay values.
During this pressurization phase, the primary system remains in a sub-cooled condition.
Phase 3: When the system pressure reaches the PHRS set-point, the system is activated by opening the outlet line valves. As it was explained before, the sub cooled water drains from the tubes and steam from primary system enters to the tube bundles condensing on them.
Immediately after the PHRS actuation a sharp depressurization phase takes place. This behaviour is mainly due to imbalance coming from the steam condensation in the dome without liquid boiling in the primary system. This condition is also accompanied, in a less extent, by the sub-cooled water (at pool temperature) coming into the RPV from condensers tubes immediately after the PHRS actuation.
Phase 4: Once the primary system reaches again its saturation condition the sharp depressurization phase ends and from this moment on, pressure begins to be ruled by saturation conditions, steam reposition into the steam dome (generated in the core) and steam condensation in the PHRS. Pressure continues decreasing steadily until the end of the grace period, allowing the action of normal shutdown cooling systems.
FIG. 5. Short-term pressure and primary system temperatures evolution for LOHS
6. CONCLUSIONS
The reactor cooling function in safe conditions is guaranteed with PHRS demand, fulfilling with safety margins and design safety limits. PHRS ensures cooling and decay heat removal without requiring energy or human actions during 36 hours (grace period), avoiding the safety valves opening. Moreover, this occurs by using only one redundancy. Pressure is decreased allowing the action of normal shutdown cooling systems.
The efficiency of the equipment varies with effective length of the heat exchangers: a reduction of the tubes length, together with an increase in number of tubes, causes a local flow mass decrease so lower removal power. Therefore it is observed that, as the pressure decreases, the efficiency decreases comparatively to a greater extent for shorter tube configurations, due to the greater weight in the variations of the heat transfer coefficient. Nevertheless, as long as the transferred power is ruled mainly due to the conduction phenomena through the tube structure, a relatively low impact is observed.
References
[1] “Small and Medium Sized Reactor: Status and Prospects”, International Seminar, Cairo-Egypt, IAEA (2001).
[2] “IAEA, Status of innovative small and medium sized reactor designs 2005: Reactors with conventional refuelling schemes”, TECDOC 1485, IAEA, Vienna (2005).
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