LABGENE CONTAINMENT FAILURE MODES AND EFFECTS ANALYSIS

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1 LABGENE CONTAINMENT FAILURE S AND ANALYSIS F. B. NATACCI Centro Tecnológico da Marinha em São Paulo São Paulo, Brasil Abstract Nuclear power plant containment performance is an important issue to be focused when developing an extended Level 1 Probabilistic Safety Assessment (PSA). The main reason for this resides on the fact that the containment plays a fundamental role on accidental sequences since it is the most relevant barrier on the mitigation of consequences of postulated severe accidents. LABGENE is a prototype nuclear power plant. This paper presents a first approach of the failure modes and effects analysis (FMEA) for the LABGENE containment, identifying possible failures and the associated causes that can compromise containment performance. The initial plant damage states, which are the input for this analysis, are based on the event trees developed for LABGENE Level 1 PSA. It should be emphasized that this is a preliminary study in accordance with the state of the art of the corresponding PSA and is subjected to further completion. The evaluation of the qualitative analysis presented herein allows a concise and broad knowledge of the development of accidental sequences related to the LABGENE containment. 1. INTRODUCTION The main objective of the whole PSA together with the containment reliability analysis is to complement the well known deterministic accident analysis in order to assure the safe operation of a nuclear facility in a degree as high as possibly achievable. As stated in reference [1], PSA can be developed in three distinct levels. Level 1 PSA focuses evaluation for identification of all possible scenarios that can lead to reactor core damage. The plant damage states of the accidental sequences identified in Level 1 together with the containment response define the scope of Level 2 PSA which lead to the determination of the source terms and the containment release frequencies. The focus of Level 3 PSA is to evaluate off-site plant consequences and calculate public risks based on the results of Level 2 PSA and on mitigating measures. Level 3 PSA is also used to delineate emergency response planning. This paper relates to a first approach of the evaluation of the LABGENE containment performance which is the link between Level 1 and Level 2 PSA. Brief descriptions of the plant and of the containment are presented in section 2. Section 3 contains a condensed FMEA of the containment. Section 4 brings up some final considerations based on what was identified during the development of the analysis. 2. LABGENE DESCRIPTION The LABGENE plant is an experimental prototype pressurized water reactor (PWR) with the main purpose of being a training facility in order to gain operating experience as well as to acquire plant specific data. After consolidation, a similar plant is intended to be used for submarine propulsion. It is a two-loop PWR with a thermal power capacity of approximately 48 MWth. Each loop has a steam generator which produces steam that is directed to four turbines: two propulsion turbogenerators and two auxiliary turbogenerators. The propulsion turbogenerators drive an electric motor which drives a shaft with a hydraulic brake intended to simulate a marine propulsion. The auxiliary turbogenerators provide electrical F. B. NATACCI 1

2 power to the plant. The ultimate heat sink at LABGENE is a shielding pool which simulates a very large water source. The reactor core consists of twenty-one fuel assemblies. These assemblies contain fuel rods, control and safety rods, and burnable poison rods. The fuel rods contain uranium dioxide enriched at about 5% and are cladded by stainless steel. The burnable poison helps control reactivity because the LABGENE reactor coolant is not borated. Different from other pressurized water reactors, control rods in LABGENE are used to control reactivity. During normal operation, the safety rods are completely withdrawn from core and are used only to shutdown, along with the control rods LABGENE containment The LABGENE plant has basically the same barriers as in other nuclear facilities against any possible release of radioactive fission products from core to the environment, namely: the fuel matrix; the fuel cladding; coolant pressure boundary which includes vessel; and the containment. The LABGENE containment is composed of the containment itself, the shielding pool,, and other specific s and devices important to allow the correct operation of the containment and the plant as a whole, in order to avoid radiation release to the environment. This concept of the containment differs from conventional reactor containments because in LABGENE design there is the need of having a special structure to simulate the hull of a submarine. Figure 1 shows schematically the LABGENE components important for the containment analysis [2]. Figure 1. General arrangement of the LABGENE containment The containment comprises the containment barrier and its internal environment. The containment barrier is a metallic cylindrical structure assembled inside which is intended to behave as a submarine hull and house all equivalent equipment. The F. B. NATACCI 2

3 main functions of the containment are: to shelter equipment and s of the primary circuit, protecting them from external hazards; to withstand pressure peaks caused by any of the postulated design accidents, specially large loss of coolant accidents; to collect, retain and provide controlled release of any radioactive material; and to provide means to monitor the containment environment. The shielding pool is a water tank that surrounds the containment and serves as a radiation shielding, complementing all other shielding barriers such as the shielding tank installed around. It is also the ultimate heat sink for the plant, supplying water to the emergency cooling for residual heat removal. The is a seismic concrete structure. Its main functions are: to withstand external natural and technological events such as earthquakes, explosions and external missiles, protecting the containment and the integrity of all components inside it; and to provide a secondary confinement to fission products, allowing a controlled radioactive release to the environment, if necessary. Other specific s and devices related to the containment comprise mainly the gas detection and control, the radiation monitoring, the containment parameter monitoring, the ventilation and air conditioning, the detection and firefighting, the containment isolation and the leakage detection. The internal containment environment is monitored and parameters such as pressure, temperature, fluid level, humidity, radiation, hydrogen concentration and noise are some of the most important ones which are constantly monitored. 3. LABGENE CONTAINMENT FAILURE S AND ANALYSIS The technique used for the first approach of LABGENE containment qualitative reliability analysis was the failure modes and effects analysis - FMEA [3]. This analysis is summarized in Table 1. Table 1. LABGENE containment FMEA FAILURE Containment isolation failure (including process piping, ventilation, electrical cables and instrumentation penetrations, and access hatches) Failure to isolate or inadvertent opening of containment penetrations in case of accident Interface loss of coolant accident outside the containment High radioactivity Process parameter reactor coolant High radioactivity release from primary to secondary confinement release from the primary coolant to the secondary confinement Remote isolation by the operator, when possible Secondary confinement and ventilation F. B. NATACCI 3

4 FAILURE Steam generator tube rupture Process parameter reactor coolant Plant shutdown and radioactive contamination of the secondary Residual heat removal by the emergency cooling Containment rupture External causes, such as earthquakes, tornadoes, structural failures, flooding, external missiles, external fires and external explosions. Containment or structural failure is considered. The simultaneous failure of the core and the reactor vessel is postulated Overpressure and high temperature due to direct containment heating immediately after high pressure ejection of melted core debris. This can occur when vessel ruptures and the melted core is instantaneously atomized, reacting with oxygen and steam, releasing a high amount of chemical energy High radioactivity secondary Visual detection by the operator, together with several process alarms, including high radioactivity alarm High radioactivity, release to the external environment release to the secondary confinement Reactor failure can occur, with the consequent radioactive release to the external environment None Possible relief by the ventilation F. B. NATACCI 4

5 FAILURE Overpressure and high temperature due to hydrogen fire and/or explosion in the containment, due to vessel failure. In case of explosion, the containment can also rupture due to the impact of missiles Overpressure and /or missile impact due to vapor physical explosion, due to the direct contact of water with the melted core, inside or outside the reactor vessel, immediately after its failure Overpressure and/or missile impact due to reactor vessel structural failure (during normal operation) Overpressure due to noncondensable gas and vapor buildup in the containment Overpressure and/or high temperature due to uncontrolled fire in the containment or in High hydrogen concentration containment, and high radioactivity, High radioactivity, Gas detection and control Hydrogen recombiners None Regular tests and inspections of the reactor vessel integrity High pressure containment High temperature containment or in, identified by the detection and firefighting Possible relief by the ventilation Detection and firefighting, and possible relief by the ventilation F. B. NATACCI 5

6 FAILURE Overpressure, high /or missile impact in the containment due to catastrophic failure of vessel, when it is operated beyond design limits. This can occur in accidental sequences that lead to core damage, when the reactor coolant is still operating under high pressure Structural failure of vessel support, by erosion or melt through, due to direct attack of the containment, caused by core debris resultant from reactor vessel failure High radioactivity, None None 4. FINAL CONSIDERATIONS The FMEA of the containment led to the identification of two main failure modes, namely: containment isolation failure and containment rupture. As one can see, these failures result in undesirable effects, so, their causes, detection methods, and compensation measures have to be accurately studied. As it was already emphasized, the analysis presented herein is a first approach of the assessment of the LABGENE containment response, after the identification of plant damage states as a result of accidental sequences. Such sequences are determined in Level 1 PSA. Further development of Level 2 PSA requires the quantitative containment event tree analysis which embraces the response of all support s and devices available to ensure the correct operation of the containment. REFERENCES [1] Procedures for Conducting Probabilistic Safety Assessments of Nuclear Power Plants (Level 1) (IAEA Safety Series No. 50-P-4, Vienna, 1992). [2] GENPRO Engenharia, Prédio do Reator Sistema da Contenção Descrição do Sistema (Technical Report R MS-0001, São Paulo, 2011). [3] CTMSP, Análise de Modos de Falha e Efeitos e Árvores de Eventos do Sistema de Contenção (Technical Report R RA-06/00, São Paulo, 2001). F. B. NATACCI 6