atw - International Journal for Nuclear Power | 05.2020

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atw Vol. 65 (2020) | Issue 5 ı May RESEARCH AND INNOVATION 268 The Performance of Low Activation Steel SCRAM on ACPs Source Term in Watercooled Loop of Fusion Reactor ITER Weifeng Lyu, Jingyu Zhang and Shouhai Yang In water-cooled loops of International Thermonuclear Experimental Reactor (ITER), most Occupational Radiation Exposure (ORE) of personnel is due to Activated Corrosion Products (ACPs) in the cooling loops. The corrosion products come from the corrosion of water on steel used in the cooling loops. In order to reduce neutron activation of steel and the corresponding ORE, the Super-clean Reduced Activation Martensitic (SCRAM) steel is recently developed in China to replace the traditional steel SS316, whose performance on ACPs source term needs to be analyzed. In this paper, the corrosion rate of SCRAM under fusion reactor operation condition was measured using a high-temperature flowingwater corrosion experiment loop, which was introduced into ACPs source term analysis code CATE. Then based on LIM-OBB cooling loop of ITER, ACPs activity and dose rate of SCRAM were calculated and compared with that of SS316. The calculation results showed that during the operation phase of the reactor, SCRAM produced higher activity and dose rate than SS316 due to its bad corrosion-resistance, while after shutting down the reactor, SCRAM performed better on ORE decrease than SS316 due to its good activation-resistance. 1 Introduction According to the surveillance data of French PWR plants, more than 90 % of occupational radiation exposure (ORE) of personnel under normal operation is due to the activated corrosion products (ACPs) in the primary coolant circuit [1]. And for the watercooled loops of fusion reactor ITER, the gamma ray from ACPs is also a major contributor to ORE [2]. In the water-cooled loops of ITER, the corrosion is caused by the contact of water and metal material. For instance, the pipe material of heat exchanger is corroded by the coolant and plenty of corrosion products (CPs) are produced. Some CPs are released into coolant, and transported to the regions under neutron radiation by the coolant, such as first wall, blanket, divertor, vacuum chamber, etc. Here, these CPs absorb neutrons and become radioactive, which are called activated corrosion products (ACPs). Some ACPs are transported to the regions out of neutron radiation by the coolant, such as heat exchanger, pipe, valve, pump, filter and so on, where parts of ACPs adhere to the internal surface of the pipe and continuously decay and emit gamma ray. When the workers inspect or repair these devices, they have to bear the radiological dose. As we know, the fission products do not exist in the fusion reactor, so ACPs become the dominant source term and should be reduced as much as possible. Neutron-induced activation of metal components in fusion reactor can be effectively controlled by proper selection of structure materials [3]. For meeting the demand of low activation, the Super- clean Reduced Activation Martensitic (SCRAM) steel is recently developed in China. Considering that it is martensitic steel, although it is developed to obtain reduced activation, it may cause more serious corrosion problem and increase ACPs and ORE, compared with the traditional austenitic steel SS316. This paper focuses on studying the influence on ACPs and ORE from using SCRAM and SS316. The description of the material, the calculation method and code are presented in the following two sections. The description of ITER LIM-OBB loop and the corresponding calculation results and discussions are presented in the fourth section. In the last section, a comprehensive comment is given. 2 Description of material 2.1 The material composition In the files of ITER technical basis [4], SS316 is claimed as structure material widely used in first wall, blanket, divertor, etc. But as we know, the neutron activation of SS316 is serious. So, for decreasing the material radioactivity to a level where economical waste disposal or recycling is feasible in
atw Vol. 65 (2020) | Issue 5 ı May Parameter Value Average coolant temperature (°C) Dissolved oxygen contents (mg/kg) Coolant pH Pressure (MPa) 1 Average coolant velocity (m/s) The experiment contained seven time points for sample measurement, which are 100 h, 200 h, 400 h, 800 h, 1,000 h, 1,200 h and 1,500 h. At these time points, parts of the samples were taken out of the experiment loop, and then several times cleaning respectively with hydrochloric acid, acetone and water were performed to dissolve and remove the corrosion products in the samples. After drying process, the residual mass of base metal in the samples was weighed. The corrosion rate can be measured through monitoring the mass decrease of base metal in the samples, and the power model of non-linear regression [7] was adopted to fit the curve of corrosion rate, as follows. The corrosion rate of SS316 is quoted from Reference [8]. It can be seen that the corrosion rate of SCRAM is obviously higher than SS316, which is an expected weakness of ferritic/martensitic steel compared with austenitic steel. SCRAM: SS316: 150 less than 0.01 7 (20 °C) | Tab. 2. The operation parameters of the experiment loop. 3 Method and code of calculation (1) (2) 3.1 The ACPs calculation The code CATE [9] is developed by North China Electric Power University. It is capable to analyze the nuclide composition and spatial distribution of ACPs along the cooling loops. The European Activation File EAF-2007 [10,11] is introduced into CATE, which includes the nuclear data of 2231 nuclides and makes CATE capable to calculate any activation product of any material. The simulation of ACPs transport in CATE is based on the theory that 6 | Fig. 1. Schematic of ACPs transport in the cooling loop. the main driving force is the temperature change of the coolant throughout the loop and the resulting change in metal ion solubility in the coolant, which is presented in Figure 1. The pipe surfaces with high neutron flux and resulting high temperature, such as first wall, blanket, diverter, are named “In-Flux” surface node, while the other pipe surface without neutron flux and with relative low temperature, such as pipe, pump, valve, heat exchanger, are named “Out-Flux” surface node. Considering the velocity of coolant is as fast as 6 m/s, ACPs in the coolant will be mixed rapidly, so it can be assumed that the coolant is a homogeneous node. And the model above for ACPs transport is named three-node model. 3.2 Dose rate calculation In the field of radiation protection, dose rate is an intuitive parameter to evaluate material behavior. The ARShield code is developed by North China Electric Power University to calculate the dose rate caused by ACPs. ARShield is an advanced version of the point kernel integration code QAD-CG developed by Los Alamos National Laboratory. It provides the pre-job for visualization of large-scale radiation field and virtual roaming in nuclear plant, by breaking the restrictions of the traditional point kernel integration code. The detailed characteristics of ARShield can be seen from Reference [12]. The composition and activity of ACPs calculated by CATE are introduced into ARShield, and then the dose rate is calculated using the point kernel integration method, which is as follows. (3) where, r, point at which gamma dose rate is to be calculated; r', location of source in volume V; V, volume of source region; μ, total attenuation coefficient at energy E; , distance between source point and point at which gamma intensity is to be calculated; K, flux-to-dose conversion factor; B, dose buildup factor. 4 Calculation of activity and dose rate of ACPs in ITER LIM-OBB loop 4.1 Description of ITER LIM-OBB loop The schematic of the cooling loop is shown in Figure 2. The main equipment includes blanket module, heat exchanger, hot leg pipe, cold leg pipe, pump, resin and filter. The blanket module belongs to the In-Flux region, others belong to the Out-Flux region. The main design parameters of ITER LIM-OBB loop is presented in Reference [8]. The calculations are based on the ITER SA1 operation scenario [13], corresponding to 432 full power day operation with several dwell and burn periods. Because of the limitation of electric field and magnetic field, the plasma can’t sustain for a long time, and has to be operated under pulse mode. In the calculation process of neutron activation, the time step should be smaller than the pulse time, which makes the simulation time-consuming. In RESEARCH AND INNOVATION 269 Research and Innovation The Performance of Low Activation Steel SCRAM on ACPs Source Term in Water- cooled Loop of Fusion Reactor ITER ı Weifeng Lyu, Jingyu Zhang and Shouhai Yang
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