Development of a Source Term Estimation System in Power Plant Emergencies: SOUTERM

Transcription

1 Development of a Source Term Estimation System in Power Plant Emergencies: SOUTERM S. De Maria 1, J.González 1, J. Bustillo 2, R. Martínez 3, I. Peñalva 4, F.Legarda 4, J.I. Serrano 5 1 Iberdrola Ingeniería Consultoría, Avda. de Burgos 8 B Madrid 28036, Spain; smc@iberdrolaingenieria.es jgk@iberdrolaingenieria.es 2 Nuclenor, Calle Hernán Cortés, 26 Santander Spain; javier.bustillo@nuclenor.es 3 Tecnatom, Ingeniería de Operación. Avda. Montes de Oca, 1. San Sebastián de los Reyes Madrid, Spain; rmfanegas@tecnatom.es 4 EHU-UPV, Departamento de Ingeniería Nuclear y Mecánica de Fluidos. Escuela Superior de Ingenieros, Alda. Urquijo Bilbao, Spain; inppebei@lg.ehu.es inpleibf@bi.ehu.es 5 Consejo de Seguridad Nuclear, Justo Dorado Madrid, Spain; jisr@csn.es Abstract. In the framework of the Investigation Project PR-10, Source Term Estimation in Nuclear Power Plants Emergencies, Ed. 1, January 2000", of the Co-ordinated Plan of Investigation CSN-UNESA regarding Nuclear Safety and Radiological Protection, IBERINCO, TECNATOM and the EHU-UPV have developed a Windows PC Program (SOUTERM) that allows the source term evaluation in an emergency situation in the Spanish Nuclear Power Plants and, also, to estimate the possible derived consequences. This programme has been done in two steps, first a generic model was developed and later, a specific model was made to fit the generic model to each power plant. The activities of the Project are the following ones: A. Summary and revision of the whole available documentation B. Definition of a methodology (generic model) C. Generation of the specific models of each power plant D. Functional and design analysis of the tool E. Development of the PC Program F. PC Program Validation G. Set up of the program and training in the power plants H. Edition of the documentation The assessment of dose projections during the response to radiological emergencies needs, as input data, the source term. SOUTERM will allow the user to get this source term using real data from the power plant. The purpose is to obtain a source term as realistic as possible. Once the source term has been calculated, an assessment of dose is possible by means of the code RASCAL. The output of this code will picked up by SOUTERM, so the user will be allowed to visualise the results and to carry out a valuation of the PLABEN (Basic Plan of Nuclear Emergency). 1. Introduction In the framework of PCI Projects sponsored by CSN-UNESA, the PR-10 Source Term Estimation in Spanish Nuclear Power Plants Emergencies" a project has been developed by the group formed by IBERINCO, TECNATOM and the University of the Basque Country (EHU-UPV). During an accident, predicting or characterising the composition and timing of the release (source term) is the first step for estimating doses [1, 2, 3]. Errors associated with predicting the source term have a direct impact on the accuracy of dose estimates. The aim of the project is based on the supply of a computer system which will allow the evaluation of the source term during a situation of emergency, as well as the estimation of the activity that is released to the outside of the plant and the classification of the generated situation. The tasks included in the project are listed below: 1

2 (a) (b) (c) (d) (e) (f) (g) Compilation and review of the available documentation: analysis of the achieved information relating to the approaches that have been used in the main countries of our nuclear environment. The documentation has been analyzed in detail and has set up the basis for the development of a general methodology of the source term estimation. Definition of the generic methodology for the estimation of the source term which will be put into practice in the Spanish nuclear power plants. This generic model consists of the generation of several diagrams with the calculation methodology to apply in their different steps. Development of specific models of the plants, trying to specify the general method that was developed during the previous step to each Spanish nuclear power plants. The specific analytical models are established, identifying the particular parameters of each plant. This task includes the identification of the release pathways, both monitored and no monitored ones, as well as their features. Functional analysis of the tool, by means of creating the functional and in-depth design of the computer tool. The requirements and functions of the system and the data conceptual model are clearly defined. Computer development of the system by means of carrying out all these tasks oriented to develop the software program or code of the tool. Tool validation by means of carrying out several tests, performing the combination of the different modules that are members of the system. These tests have already been established in the appropriate Validation Plan. Implementation of the tool in each of the power plants, as well as in the CSN. Once the correct operation of the tool is verified, a training course will be given to the users. 2. Methodology The bibliographic compilation work had the aim of getting a precise knowledge of the approaches that had been taken by other countries when evaluating the source terms in situations of emergencies in nuclear power plants. Such process of compilation and the subsequent analysis was focused on 14 countries. As a conclusion of the analysis, the implementation of the methodology described by T.J. McKenna and J.G. Giitter [4] was recommended, although some specific features will be incorporated to improve the tool. After the mentioned bibliographic analysis, a methodology approach was set out in order to determine the way the tool will work, in general terms, when an emergency situation is declared at any nuclear power plant (FIG. 1 and FIG. 2). The development of this generic methodology leads to several diagrams for both PWR and BWR, so that a description was written for each one, giving details of the calculation methodology that is going to be used. Once this methodology was defined and approved, the specific features of each nuclear power plant were ready to be included. The tool will start up as soon as the emergency is declared, which normally happens due to diverse symptoms and when there is a prediction of a release to the plant environment. Once the emergency is declared, the tool follows the steps that are listed below: (a) (b) Calculation of the available activity to discharge. If the core of the reactor is origin of the emission an initial estimation of the core damage is carried out by means of a module called VALORA. This will provide a first evaluation of the state of the fuel elements that make up the core, so that the available activity to be released can be estimated. Identification and analysis of the origin and the course of the emission by means of the instrumentation available in plant [5], distinguishing between: Ongoing release, when a diagnosis of the situation is made; this release occurs through a monitored way or through a no-monitored way. 2

3 (c) (d) (e) Potential release or identification of the real situation with a preanalyzed sequence, when a prognosis of the situation will be made. In case of a potential release, a discharge to some area will be identified but with no release to the outside. When measurements of the instrumentation of the plant are not accurate enough to make an estimation of the source term, a database containing preanalyzed accidents will be used. Identification of the pathway and its reduction mechanisms and reduction factors. Calculation of the released flow. Estimation of the source term. BLOCK 1 Emergency Start t = 0 SOUTERM APPLICATION INSTRUMENTATION Thermocouples Spray Level ARM shutdown time Pressure Hydrogen VALORA Core specific inventory RCS activity CORE AVAILABLE INVENTORY First estimation of emergency Available activity FIG.1. Generic methodology, estimation of available activity From the starting point of the philosophy of the generic model, the methodology has developed through a more specific proposal for the existing reactor models in Spain: PWR and BWR. 3

4 Initially, two generic models were developed, one for PWR reactors and the other one for BWR reactors, taking as a reference C.N. Almaraz and C.N. Cofrentes respectively. However, the final product of the project is a tool for each of the Spanish nuclear power plants including their specific model. BLOCK 2 Available Activity EMISSION SOURCE IDENTIFICATION Diagnosis Prognosis ONGOING RELEASES FORESEEN RELEASES MONITORED PATHWAYS NO-MONITORED PATHWAYS POTENTIAL RELEASES PREANALYSED ACCIDENT SEQUENCES REACTOR CONTAINMENT STEAM GENERATORS OTHER SYSTEMS OR BUILDINGS FIG.2. Generic methodology for PWRs, emission source identification 3. Features of the tool As mentioned before, SOUTERM program allows the calculation of the fraction of the fission product inventory released from the core as a result of nuclear power plant accidents. This inventory after being partly removed on the way to the environment becomes the potential radionuclide off-site release. Once source term has been calculated, SOUTERM will calculate off-site dose using RASCAL 2 for WINDOWS (Rascal 2.2 is included in). A presentation screen, during which the program database is loaded, is the beginning of a SOUTERM running. After the presentation screen, the main program window shows up. This window has a frame in the left side containing the emergency tree. The emergency tree shows the accident evolution and it 4

5 is the main element of the program. The user will be able to access to all the functions included in SOUTERM through this tree. The emergency tree elements are shown below: (a) (b) (c) Emergency. It is the root element. Emergency is represented by means of a danger sign (red triangle with a yellow exclamation mark inside). Inventory. The available inventory to be released is showed as the first result obtained of the emergency. The icon that represents inventory is an atom. Inventory origin depends on the accident type. It could be the reactor core, the spent fuel pool, the waste tanks, etc. Starting from inventory, the user can access to VALORA module. VALORA will evaluate the core damage. This module is different for PWR and BWR power plants. In the case of a PWR, VALORA uses core exit thermocouples and containment radiation monitors to evaluate the core damage. On the other hand, in the case or a BWR, evaluation of core damage is based on containment radiation (dry and wet well), containment hydrogen inventory and water level of the reactor pressure vessel. VALORA incorporates another function that allows the assessment of spent fuel damage. Discharge. One single emergency can be composed of as many discharges as needed. There are four different types of discharges: preanalyzed accidents, discharge forecast, monitored discharges and no-monitored discharges. All discharges, except preanalyzed accidents, allow the insertion of data in order to calculate the source term: discharge related events, discharge origin, emission point, specific activity and flow. Once the discharge has the whole needed data to perform a source-term calculation, two elements are added to the emergency tree: source-term and dose. The first one shows the released radionuclides with their activity rate and the second one will allow to run RASCAL program for dose assessment. Results from RASCAL are showed on a power plant map (FIG. 3). FIG.3. Results for dose assessment 5

6 4. Conclusions SOUTERM represents a computer program that allows the potential radionuclide release assessment as a result of nuclear power plant accidents. The assessment is made in a dynamic way with the capability to adapt itself to the ongoing emergency. SOUTERM is aimed at radiological protection staff in Spanish power plants, making use of several plant parameters. The evolution of these parameters during an emergency can be observed. SOUTERM provides a method that can be applied in every Spanish nuclear power plant, including a specific model for each one, which involves an unquestionable improvement in the calculation of the activity released to the offsite during emergencies, as well as a better training for the involved staff. Nowadays, the specific models for the different nuclear power plants are completed, previous to the final implementation in each power plant and CSN. 5. References 1. T. McKenna, J. Trefethen, K. Gant, J. Jolicoeur, G. Kuzo and G. Athey, RTM-96: Response Technical Manual, NUREG/BR-0150, US Nuclear Regulatory Commission, March TMI Emergency Dose Calculation Manual, 6610-PLN Rev.12, September Soffer. L. et al., Accident Source Terms for Light-Water Nuclear Power PlantsAccident Source Term for LWR plants, NUREG-1465, U.S. Nuclear Regulatory Commission, February T.J. McKenna and J.G. Giitter Source Term Estimation during Incident Response to Severe Nuclear Power Plant Accidents, NUREG- 1228, US Nuclear Regulatory Commission, October Off-Site Dose Calculation Manual (MCDE) for the Spanish NPPs. 6