Portoroz Workshop – Session 2 – Rocher – 1/7 Impact of Main Radiological Pollutants on Contamination Risks (ALARA) Optimisation of Physico Chemical Environment and Retention Technics during Operation and Shutdown A. Rocher, Electricité de France/GPR - J.L. Bretelle, Electricité de France/GDL M. Berger, Electricité de France/SEPTEN 1- Introduction The goal of this paper is to precise the behaviour of different radiochemical species in the primary coolant of PWR plants. Managing these pollutants must lead to limit Reactor Coolant System (RCS) walls “over-contamination” to decrease the dose rates during the maintenance operations (ALARA). In French Plants, Co 60, silver and antimony represent the major radiochemical pollutants which require a good knowledge of the different phenomena to ensure the lowest contamination risks. The stakes deal with the control and the optimization of collective and individual doses including waste treatment with low costs. These stakes represent primordial elements of nuclear acceptability. 2- Normal contamination of circuits In French PWR plants : − More than 90% of integrated doses are due to contaminated walls in contact with the primary coolant because of activated corrosion products. − 80% of total dosimetry are integrated during outages. In the absence of specific pollution, the mean contribution of out of core deposits to dose rates is as follows (results expressed as percentage equivalent dose rates in Sv/h) : − Co 60 = 50% (the older the plant, the greater the Co 60 quantity); − Co 58 = 30% (with a higher level in 1300 MW units); − Ag 110m + Sb 124 + Cr 51 + ... = 10%. Usual contaminants come mostly from materials/coolant interactions leading to soluble, particulate, colloidal products transport and corrosion products activation (table 1). Nuclides / Half-life Gamma energy keV (% emission) Beta energy keV (% emission ) Co58 / 71 days 811 (100%) Co60 / 5.3 years 1173 (100%) 1333 (100%) 318 (100%) Ag110m / 253 d 658 (100%) 885 (80%) 86 (55%) Sb124 / 60 d 603 (98%) 610 (51%) Sb122 / 2.7 d 564 (66%) 1400 (63%) Table 1: Gamma and beta energies and emission percentage (> 50%) of major pollutant nuclides The dose rates can increase in case of more important corrosion products transport [1] or of incidental pollution. In the latter case, the ratio of species contributing to dose rates is modified.Portoroz Workshop – Session 2 – Rocher – 2/7 3 - Incidental over-contamination The major pollutants responsible for over-contamination in EDF plants are : − Co 60 coming from stellite degradation ; − Ag 110 m coming from control rod perforation and probably from materials like seals ; − Sb 122 / Sb 124 coming from pumps of auxiliary system. Given the importance of over-contamination with respect to dosimetry, representing 10 to 30 % of outages dosimetry in the concerned units, a specific program is being set up in France, in the context of the ALARA project. The goal is to optimize diagnosis, surveillance, prevention and remedies to reduce pollutions even though priority is set on prevention. 3.1 – Cobalt 60 contamination – “hot spots” 3.1.1 – Hot spots sources In most cases, hot spots are due to particles of cobalt activated by a neutron flux (Co 60) mainly from hard facing surfaces equipments (stellite, rich in cobalt) in the RCS (valves, pumps, internals, etc.). 3.1.2 – Impact on dosimetry The contribution of hot spots to shutdown dosimetry may appear to be marginal in French PWR reactors (2 to 4 %), but becomes more significant (15 to 25 %) for the units affected. This excess dosimetry has to be taken into account, particularly for the most exposed workers. Approximately ten French PWR units have been affected by this phenomenon over the last 15 years [2]. 3.1.3 – Hot spots behaviour - Indicators Surveillance is designed to inform the site as early as possible, of the presence of hot spots (mapping) in order to take the appropriate measures to prevent their propagation and/or to eradicate them. During unit operation, most hot spots will remain fixed to the fuel. Others may fall, by gravity, to the bottom of the pool or the low points of the primary coolant system or be trapped in the special devices. The most common locations are as follows: − Thermal sleeves of the pressuriser − Steam generator packing glands − Valves of the primary cooling system. − etc. After connection of the Residual Heat Removal System (RHRS), some hot spots may migrate into this circuit and be deposited or fixed. The most common locations are: the pumps, heat exchangers and valves of the circuit. An underwater pool cleaner should pass through the pool out after discharging. In this case, particularly high equivalent dose rates, equal to or greater than 1 Sv per hour, measured in contact with the filters, represent the last indicator of the possible presence of hot spots, before draining of the pools. Since no warning signs have been identified yet, to indicate the occurrence of hot spots, it was decided to concentrate on preventive filtering, trapping hot spots as close as possible to their source to eliminate them. 3.1.4 – Preventive strategy Stellite limitation It concerns all the units by performing the following actions: − limiting the use of cobalt based components in contact with the primary coolant ; − removing stellite particles produced during some maintenance operations (e.g. lapping). Preventive filtration with specific devices The simplified diagram below illustrates the principles of the preventive filtration methods which are proposed (Fig. 1). It consists on the filtration of all the effluents which could transport hot spots outside the Reactor Building.Portoroz Workshop – Session 2 – Rocher – 3/7 RCS/RHRSPOOLSNIVDSTANKNIVDSSUMPNUCLEARAUXILIARIESPREVENTIVE ACTION N°2 :FILTRATION OF NIVDSPRIMARY SIDEPREVENTIVE ACTION N° 3 :FILTRATION OFPOOL DRAIN LINESPREVENTIVEACTION N° 1 :FILTRATION OFPOOL BOTTOMSREACTOR CONTAINMENTFUEL POOLCOOLING ANDPURIFICATION SYSTEM Fig. 1: Preventive filtration methods The drains of pools are important routes for hot spots, before they spread through the systems. The installation of fine filters, an initial containment barrier, is proposed for the drain orifices of each pool : − refuelling cavity located in the reactor building ; − Spent fuel pit, located in the fuel building. Appropriate filters are