S.MELHEM and M. RABABAH
Investigation of performance of Passive heat removal
system for advanced nuclear power reactors under severe conditions
S. MELHEM
Jordan Atomic Energy Commission (JAEC)
Amman, Jordan
Email:
M. RABABAH
Jordan Atomic Energy Commission (JAEC)
Amman, Jordan
Email:
Abstract
The work presented in this paper discusses issues related to post shut-down cooling of advanced nuclear power reactors by totally relaying on passive processes. The paper investigates the Passive Residual Heat Removal System (PRHRS) capability for removing all the core decay heat without any AC power source or even any operator intervention to provide a significant grace time, during which essentially passive natural processes can provide adequate cooling, before any operator action is needed. It also investigates the compliance of the design of PRHRS with IAEA requirements (SSR2/1 and GS-R-4). The reactor which was investigated is the VVER-1000 AES-92 model which is the proposed design to be constructed in Jordan. A deterministic safety code will be used to model the essential components of the primary, secondary loop and passive safety systems with corresponding control systems for AES-92 model to analytically address the performance of the PRHRS taking into the severe environmental conditions of the NPP site in Jordan.
1. INTRODUCTION
The reliable removal ofdecay heat, after shutdown following some fault or external event, is one of the major challenges in designing adequately safe nuclear plant. Fukushima accident has highlighted the desirability of making the plant robust against events that have led to the loss of active, powered systems to perform this function. In particular, there is a great benefit in engineering the plant such that it provides a significant grace time, during which essentially passive natural processes can provide adequate cooling, before engineered, powered intervention is needed, if, indeed, it ever does become necessary.
In the VVER-1000 AES-92 design, Passive safety features are widely implemented to deal with design basis and design extension conditions. One of these passive safety features is the PRHRS which removes the residual heat in the event of non-availability of active heat removal systems on account of complete loss of normal and emergency AC Power supply (station blackout condition).
A review against IAEA requirements (SSR-2/1 and GSR, Part4) for passive safety systems has been addressed. This paper also discusses the deterministic safety analysis of PRHRS using RELAP 5 MOD3.2 and its behaviour under SBO.
2. Desicription of PRHRS, aes-92 design
The system consists of four independent closed circuits with natural circulation (4x33%); each one is connected to the secondary side of each steam generator in the primary loop. Each circuit has three air-steam heat exchangers modules which are located outside the containment building, pipelines of steam supply and removal of the condensate; air ducts that supply and discharge air, air flaps and control or adjusting devices. (See Figure 1 which represents one of these Circuits).
As shown in figure 1-A, the steam taken from the secondary side of steam generators is condensed in the finned tube heat exchangers of these circuits, the condensed liquid returns back through down-coming pipelines to the secondary side of the steam generators by gravity and establish a continuous natural circulation; the cooling media for the PRHRS heat exchangers is atmospheric air that flows through the air ducts. During Station black out accident, the air flaps, located at air path upstream and downstream of the PRHRS heat exchangers open due to release of holding electromagnets which allow the heat to be transferred to the atmospheric air. Inside each heat exchanger module, the heat is transferred form the steam to the air which enters the draught parts of the air ducts that end with common collector-deflector at the top of the containment building as shown in figure 1-B. [1]
PRHRS has a special controller with two operating modes: SG pressure maintaining mode and cool down mode; SG pressure maintaining mode is conducted by the passive drive of the controller, which is driven by the pressure from SG to maintain the hot standby parameters of the reactor plant; cool down mode is conducted by the active drive which is powered from DC batteries to cool down the reactor plant.
3. review of PHRS against IAEA requirements
According to IAEA Glossary, it is not absolutely clear what is considered as a passive system. However, in the IAEA glossary the definition of passive component is “A component whose functioning does not depend on external input, such as actuation, mechanical movement or supply of power.” [2] Referring to EUR glossary, the passive safety system is defined as “ A system which is essentially self-contained or self-supported, which relies on natural forces, such as gravity or natural circulation, or stored energy, such as batteries, rotating inertia, and compressed fluids, or energy inherent to the system itself for its motive power, and check valves and non-cycling powered valves (which may change state to perform their intended functions but do not require a subsequent change of state nor continuous availability of power to maintain their intended functions).”. [3]
There are not much specific requirements or guidance on passive systems in the IAEA Safety Standards, however the safety requirements for active safety systems are vast and comprehensive, some of these requirements should be apply on passive safety systems such as single failure criterion. After Fukushima the requirements for ultimate heat sink and associated heat transfer chain to the ultimate heat sink have been significantly strengthened. The table below shows the compliance of PRHRS with IAEA requirements.
TABLE 1. Compliance of PRHRS design with IAEA requirements [4, 5, 6]
Requirement / Paragraph / Review10: Assessment of engineering aspects / 4.29: where innovative improvements beyond practices current practices have been incorporated in the design… / · Design of the PRHRS was tested on a dedicated facility at OKB GP. References to experimental documentation (design, scaling, and experiments) are provided. The tests were performed in summer and winter conditions. Detailed analyses of the PRHRS were performed using GAMBIT code. The code was extensively validated using experimental data. [6]
· Performance of the PRHRS (with steam generators) during beyond design accidents was addressed using analytical methods that were validated using integral experiments conducted at FEI's GE2M-SG facility including effects of non-condensable gases.
· Performance of the PRHRS under different wind conditions was addressed by performing wind tunnel experiments on a model of the reactor building.
15:
Deterministic and probabilistic approaches / 4.53:
Deterministic and probabilistic approaches have been shown to complement one another … / · DSA and PSA methods were used to assess safety of the AES-92 design.
· In the design provide the highest impact on the PSA results. It was shown that elimination from the design of only PRHRS would result in an increase in CD frequency by a significant number.
· In this paper the deterministic analysis is elaborated.
16:
Postulated initiating events / 5.11:
Where prompt and reliable action is necessary in response to postulated initiating events…. / · The automatic response of active safety systems is complemented by an “automatic” start of the passive safety systems of the hydraulic accumulators of stage I, II, III and the PRHRS.
· Other passive safety systems specifically designed to cope with BDBA need manual operator action for initiation (passive filtering system of the inter-containment space (KLM), melt trap (1 JKM)).
17:
Internal and external hazards / 5.20:
The design shall be such as to ensure that items important to safety are capable of withstanding the effect of external events… / · The AES-92 design provides an effective protection against all types of external initiators that have limited potential or low probability of damaging the reactor building and reactor unit items. This can be explained by implementation of PRHRS, which does not require any active system operation and can be automatically actuated in black-out conditions.
· The most important is the PRHRS that is designed to operate under extreme environmental conditions. For example due to its passive actuation and layout (four independent natural circulation loops connected to the steam generators secondary sides) this system can function in event of external fires with very low failure probability.
25:
Single failure criterion / 5.39:
Spurious action shall be considered to be one mode of failure… / · The PRHRS has twelve heat exchangers, where nine are sufficient. Hence, there are two redundancies. Concern: in case of a SBLOCA, however, one loop out of four is inoperable, which means that three HXs are lost. If we assume single failure in one channel, only three heat exchangers (HXs) are operable, which may not be sufficient. Similarly, in case of SGTR, three HXs are in the affected loop - their operation under this condition has not been analyzed. In case of a single failure in one channel, there are only three HXs operable.
32:
Design for optimal operator performance / 5.58:
The design shall be such to promote the success of operator actions…. / Measures have been taken in the design to promote the success of operator actions and to prevent errors occurring. The main measures are as follows:
· Passive systems have been incorporated to carry out the safety functions that need to be performed after the occurrence of an initiating event. These systems do not require operator actions to actuate them or for their operation;
· the active safety systems and actions are actuated automatically; and
· Interlocks are incorporated into the design to prevent the operators carrying out incorrect actions.
32:
Design for optimal operator performance / 5.59:
The need for intervention shall be minimum… / The design aim is that no operator actions should be required in the first 30 minutes following an initiating event. This is achieved by the incorporation of passive systems and automatic initiation of active systems.
53:
Heat transfer to ultimate heat sink / No paragraph / For DBAs and also for DECs without loss of primary circuit integrity, heat removal to the ultimate heat sink is provided for an indefinite time. If the active systems are available, then the service water acts as the ultimate heat sink, to which heat is transferred through the intermediate circuit. If the active systems are unavailable, then the outside atmosphere acts as the ultimate heat sink, to which heat is transferred via heat exchangers of the PRHRS.
61:
Protection system / 6.32:
The protection system shall be designed with fail-safe… / In addition to the inherent scram feature in case of power loss, passive safety systems have functional capability to cool the plant even in the case of complete loss of power.
68:
Emergency power supply / 6.44:
The combined means to provide emergency power… / One of the set of the batteries of each train powers required power for thee monitoring of the operation of PRHRS during 24 hours (with possibility of 72 hours) without recharging.
4. Modelling of PHRS for AES-92 design
In this paper, analysis of station black out (SBO) accident scenario with complete loss of all AC power sources including emergency diesel generators is performed. The main purpose of the analysis is to investigate the performance of the PRHRS at a high temperature of Jordan NPP site and to check if the safety acceptance criteria are met. In order to model and study the natural circulation characteristics of PRHRS, RELAP5 MOD3.2 is used to develop the models of the Primary Loop, Secondary loop and the PRHRS.
4.1. RELAP Mod 3.2
RELAP5 3.2 is best-estimate system analysis code designed especially for the modeling of a wide range of operational, emergency and transitional processes that may occur in systems equipped with nuclear or electric heat sources and using as the main heat transfer medium with water in one- or two-phase state. [7]
Basic characteristics RELAP5 computer code are as follow:
· A one-dimensional model of two-phase flow, including:
o 2 mass conservation equations,
o 2 energy equation,
o two equations of conservation of momentum
· One-dimensional neutron kinetics model.
· Hydrodynamic modeling system using the following basic components:
o pipe ,
o simple volume ("single volume")
o boundary condition ("time-dependent volume" and "time-dependent junction")
o simple connection ("single junction")
o "branch" (branching flow models)
o pump,
o valve (various types)
4.2. Boundary conditions
In normal operation of the VVER-1000 AES-92 design, the initial and boundary conditions are given in the Table 2:
TABLE 2. Initial and boundary conditions[1]
Parameter / ValueThermal Power, MW / 3000
Coolant temperature at the reactor inlet, C° / 291.0
Coolant Pressure at the reactor outlet, MPa / 15.7
Coolant flow rate through the reactor, m3/h / 86000
Pressurizer level , m / 8.17
Collapsed level in SG, m / 2.356
Steam pressure at the SG outlet, MPa / 6.27
Feed water Temperature, C° / 220.0
Air Ambient Temperature, C° / 41
In the analysis, the following assumptions were considered:
· All PRHRS channels are available with delay of 30s in order to be connected from the moment of losing all AC Power sources.
· A delay of 30 s until the PRHRS channels reach the full power capacity.
· The PRHRS power characteristics (taken from the experimental data) are assumed at ambient air temperature of 41 C°.
4.3. RELAP nodalization
Nodalization of the primary loop is shown in figure 2-A. The core model is presented by 4 channels and one bypass channel. One of the four channels represents the hot channel with radial peak factor of 1.81 for the hottest fuel rode. Elevation between SG outlet and heat exchangers module is about 15m. The air-steam heat exchangers modules for each PRHRS channel are modelled as one active heat structure (see figure 2-B) based on PRHRS power characteristics which are taken from the experimental data at ambient air temperature of 41 C°.
4.4. Results and discussion
The analysis was made to see if the PRHR is capable of removing the decay and residual heat after reactor scram, and to ensure the cooling is maintained, and to ensure that all parameters are within the safety margins. The main parameters analysed are the pressure, the fuel temperature, and cladding temperature.