Effect of PAR structure on hydrogen reduction rate and explosion risk

Transcription

1 , pp Effect of PAR structure on hydrogen reduction rate and explosion risk Myeong Rok Ryu 1, Kweon ha Park 2 1 Engine laboratory, Mechanical Engineering, Korea Maritime and Ocean University, Dongsam-dong, Yeong do-gu, Busan mha1234@naver.com 2 Engine laboratory, Mechanical Engineering, Korea Maritime and Ocean University, Dongsam-dong, Yeong do-gu, Busan Corresponding Author: khpark@kmou.ac.kr Abstract. Nuclear power plant is again concerned about a sever accident due to the Fukushima accident by hydrogen explosions. Hydrogen is produced by the reaction of the fuel rod coating material and water vapor in a very high ambient temperature, which condition is generated with the heat of the fuel. If the hydrogen is not removed sufficiently, that will be a principal cause of a hydrogen explosion. One of countermeasures to remove hydrogen is Passive Autocatalytic Recombiner (PAR). However, it was brought up that the spontaneous ignition on the hot surface inside the PAR could lead to hydrogen explosion. In this study a new design concept of the catalyst structure is proposed and analyzed. The results showed that the proposed structure of a catalyst reduced the hot temperature area which might reduce the hydrogen explosion risk. Keywords: Nuclear power plant, Hydrogen explosion, PAR 1 Introduction Fukushima accident occurred in A massive earthquake of Richter scale 9.0 followed by a tsunami with waves of 10 to 14m struck the Fukushima Daiichi (FD) nuclear power plant operated by Tokyo Electric Power Company (TEPCO). The main power and assistance power of nuclear power plant were lost, so cooling systems did not operate, and then fuel rods overheated caused explosion.[1] If the heat of the fuel rods was not remove enough, the metal, such as Zircaloy used for nuclear fuel covering material, and water vapor would react at high temperature and generate hydrogen. The generated hydrogen is released into the containment, which bring about hydrogen explosion. The shock wave generated by the explosion can cause a complete destruction of a containment building [2]. Due to the recent accident occurred in the Fukushima, a nuclear risk was refocused, and many studies for the accident have been progressed. One of the measures to reduce the hydrogen released into containment is Passive Autocatalytic Recombiner (PAR). There are many studies to improve the performance ISSN: ASTL Copyright 2016 SERSC

2 of PAR.[3]-[6] However the technique takes an issue that PAR can be an igniter of hydrogen explosion instead of preventing when combustion occur in PAR. [7] In this study, a new shape of PAR structure is proposed, and analyzed to improve hydrogen reduction rates and reduce hydrogen explosion risk. 2 Mathematical models and calculation conditions 2.1 Mathematical models Mathematical models applied in this study are given as follows. Momentum equation(1) is δ(ρu) δt + (ρu U) = ρ + τ + S M (1) Energy conservation equation is δ(ρk) δt + (ρuk) = U ρ + U ( τ) + U S M (2) SST(Shear Stress Transport) turbulence model is used for turbulence calculation, which has the accuracy to flow separation phenomenon in reverse pressure gradient.(equation(3)~equation(5)) v t = a 1 k max (a 1 ω, F 2 ) = a 1 k max (a 1,( u y )F 2 (3) F 2 = tanh (arg 2 2 ) (4) arg 2 = max (2 k, ωy y 2 ω ) (5) 2.2 Calculation girds and conditions Fig. 1 shows PAR structures proposed in this study. Fig 1 (a) is a two step PAR, and (b) is a multi-step PAR. The effect of the distance between steps from 50mm to 300mm on the PAR performance is analyzed in the two step case, and the effect of the number of steps is analyzed in the multi-step case. Fig. 2 is a calculation grid. Five hundred thousand grids is used with dense grids near catalyst. Copyright 2016 SERSC 19

3 50mm 100mm 200mm 300mm (a) Two step case 1step 2stpes 3steps (b) Multistep case Fig. 1. New concept structures of catalyst Fig. 2. Calculation grids Table 1 shows calculation conditions. Table 1. Calculation conditions Shapes Initial conditions Items Distance A number of catalyst Pressure Temperature 50(mm) 100(mm) 200(mm) 300(mm) (bar) 300(K) 20 Copyright 2016 SERSC

4 3 Results and discussion 3.1 Evaluation methods for hydrogen explosion risk Fig. 3 shows the test method of hydrogen explosion risk. Spontaneous combustion takes place in the condition of over 4 vol.% of hydrogen and over K. Therefore, the hydrogen explosion risk is proportional to the thickness of B-A which is the possible area of spontaneous combustion. A is the thickness of the hydrogen concentration under 4 vol.%, and B is the thickness of the temperature over K. Fig. 3. Evaluation method for possibility of hydrogen spontaneous combustion 3.2 Calculation results Distance between steps Fig. 4 shows the areas of spontaneous combustion with distance variation. The areas of the all two step PARs are reduced rapidly comparing with one step PAR. But the areas of two steps are not much different each other. Fig. 5 shows initial hydrogen concentrations being able to occur spontaneous combustion and hydrogen reduction rates. The initial hydrogen concentration increases until 100mm, and then vibrates. (a) 50mm (b) 100mm (c) 200mm (d) 300mm Fig. 4. Spontaneous combustion area with distance variation Copyright 2016 SERSC 21

5 (a) Initial hydrogen concentrations (b) Hydrogen reduction rates Fig. 5. Initial hydrogen concentrations and hydrogen reduction rates Number of steps Fig. 6 shows the areas of spontaneous combustion with number of steps. The area of the two step case is reduced rapidly comparing one step PAR, and is continuously reduced with increasing steps. The initial hydrogen concentrations of spontaneous combustion in the cases of one, two, and three step are 5.2%, 8%, 8.7%, respectively. (a)1 step catalyst (b)2 step catalyst (c)3 step catalyst Fig. 6. Spontaneous combustion area with the number of steps 4 Conclusion This study proposing and analyzing new structure of passive auto catalytic recombiner (PAR) is summarized as; In the two step structure, the initial hydrogen concentrations being able to occur spontaneous combustion and hydrogen reduction rates are rapidly increased comparing with one step structure. The initial concentration increases until 100mm, and then vibrates. In the multistep tests, the initial hydrogen concentrations in the cases of one, two, and three step are 5.2%, 8%, 8.7%, respectively. And the spontaneous ignition area are reduced with step increase. The above results show that the multistep structure of PAR will be better way to reduce the hydrogen explosion risk caused by a spontaneous ignition. References 1. The American Nuclear Society Special Committee on Fukushima.: Fukushima Daichi: ANS Committee Report, American Nuclear Society(2012) 22 Copyright 2016 SERSC

6 2. Kim, J.T, Hong, S.W, Kim, S.B.: Numerical analysis of the hydrogen-steam behavior in the APR1400 containment building a hypothetical total loss of feed water accident. Journal of the Korean Society for Computational Fluids Engineering, Vol.10, No.3, pp.9-18(2005) 3. Deng, J, Cao, X.W.: A study on evaluation a passive autocatalytic recombiner PARsystem in the PWR large-dry containment. Nuclear engineering and design, Vol. 238, No. 10, pp (2008) 4. Park, J.W, Koh, B.R, Suh, K.Y.: Demonstrative testing of honeycomb passive autocatalytic recombiner for nuclear power plant. Nuclear engineering and design, Vol. 241, No. 10, pp (2011) 5. Gera, B, Sharma, P.K, Singh, R.K, Vaze, K.K.: CFD analysis of passive autocatalytic recombiner interation with atmosphere. Journal of Kerntechnik, Vol. 76, No. 2, pp (2011) 6. Meynet, N, Bentaib, A, Giovangigli, V.: Impact of oxygen starvation on operation and potential gas-phase ignition of passive auto-catalytic recombiners, Combustion and flame, Vol. 161, No. 8, pp (2014) 7. Reinecke, E.A, Bentaib, A, Kelm, S, Jahn, W, Meynet, N, Caroli, C.: Open issues in the applicability of recombiner experiments and modelling to reactor simulations. Progress in Nuclear Energy, Vol. 52, pp (2010) Copyright 2016 SERSC 23