CPPC: Development of a Simple Computer Code for H 2 and CO Combustion in Severe Accidents

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1 CPPC: Development of a Simple Computer Code for H 2 and CO Combustion in Severe Accidents Fernando Robledo (CSN) Juan M. Martín-Valdepeñas (CSN) Miguel A. Jiménez (CSN) Francisco Martín-Fuertes (UPM) CSNI WORKSHOP ON UNCERTAINTIES IN PSA-2 ANALYSES

2 What is CPPC? Developed by Polytechnic University of Madrid for CSN. Stand-alone code for fast calculations on pressure rises in the containment from H 2 and CO combustion in severe accidents. Most recent advances in the field of H 2 and CO combustion. Useful tool for PSA-2 assessments.

3 What is CPPC? INPUT: OUTPUT: Masses of H 2 CO. and Initial environmental conditions in the containment, before burning. Simple geometric data: volume of the enclosure. Combustion completeness. Adiabatic and isochoric combustion pressure. Chapman-Jouguet pressure. Chapman-Jouguet reflected pressure. Effective pressure. Combustion regime.

4 Main Assumptions Ideal gases. Gases homogeneously mixed in containment. Steam-saturated atmosphere previous to the combustion. Water properties from Steam Tables.

5 Flammability Limits Correlation for upward propagation: X H2O = a f + b f X H2 + c f exp (d f X H2 + b f T u ) a f, b f, c f, d f fitted experimentally.

6 Combustion Completeness Pilch et al (1996). Murata et al (1997), taken from CONTAIN 2.0 HECTR 1.5, taken from MELCOR (Gauntt,1997).

7 Combustion Completeness Combustion Completeness CC Pilch (XD=0.0) Pilch (XD=0.3) Pilch (XD=0.6) Pilch Spray (XD=0.0) Pilch Spray (XD=0.3) Pilch Spray (XD=0.6) Murata Murata Spray Gauntt Molar fraction of flammable gases X C

8 Combustion Regimes Regimes considered: Slow deflagrations Flame Acceleration DDT Detonation For each gas mixture CPPC calculates: Fulfillment of criterion for combustion regime. Effective static pressure.

9 Combustion Regimes (Kuznetsov, 2003). m /s, V B R = 0. 6 ( a i r ) f ast flames quasi-detonations 80 mm 9%H2 10% 2 11% 2 13% mm 9%H2 10% 2 11% 2 15% 2 25% mm 9%H2 10% 2 11% 2 slow flames x/d L>7 λ σ > σ * 30 Δ P/Po, t, s t, s t, s

10 Flame Acceleration Criterion Selection of parameter (σ) v σ = b = v u ρ ρ u b Establishing of σ critical σ * = a σ + b σ E T a u c σ

11 Flame Acceleration Criterion Definition of index for FA. i σ = σ σ * Quantification of index for FA = σ σ* i σ 0.92

12 Flame Acceleration Criterion. Dorofeev (2001)

13 DDT Criterion Definition of DDT index i = λ D 7 λ D geometric value D = V 1 / 3 λ: detonation cell size log10( λ) = f ( X H 2, dry, X H 2O, T, p) Quantification of DDT index i λ = D 7 λ 0.57

14 DDT Criterion (CSNI SOAR, 2000).

15 DDT Criterion (Breitung, 2000).

16 Direct Detonation Criterion Air Steam Detonable Flammable Non-flammable H 2 /air stochiometric mixture Hydrogen

17 Pressure Rise Calculation: Slow Deflagrations ( ) ( ) CO CO q H q H A u u A v A A AICC b b A v A q n q n T c n T c n, 2 2,,, + + = ( ) R PM R T D T C T B A c A A A A A va = 3 2 = u b u AICC b u AICC b n n T T p p

18 Pressure Rise Calculation: General Case y ' ' + ( 2 π f ) 2 y = p ( t ) i m Frequency: input data. 5 to 500 Hz as indicated by Breitung and Redlinger (1995b)

19 Pressure Rise Calculation: General Case. Pi(t) obtained from typical shape of pressure loads at the different combustion regimes (Breitung and Redlinger (1995b). Upper bound values: P CJ P CJ-R = 1.8 (+0.08) P AICC = 4.1 (+ 0.3) P AICC

20 Pressure Rise Calculation: General case p/paicc p SD p FA p DDT p DET time (s)

21 Pressure Rise Calculation: General case. Calculation of the effective static pressure: y ' ' + ( 2 π f ) 2 y = p m t i ( ) p eff = ( 2 π f ) m y 2 max

22 Pressure Rise Calculation: General Case peff / paicc 4 3 peff SD peff FA peff DDT peff DET frequency (Hz)

23 Validation & Verification Comparison with MELCOR calculations to verify that CPPC provides an upper bound. CPPC code uses combustion completeness = 1. T0: scenarios with CHR activation coincident with vessel failure. T1: scenarios with CHR activation coincident with the maximum of the σ parameter. ESF: Spray + Fan-cooling units. FCL: Fan-cooling units. Full capacity. SPR: Spray system: full capacity. Full capacity.

24 Validation & Verification Scenario dryt0- ESF dryt0- FCL dryt0- SPR wett0- ESF wett0- FCL wett0- SPR Duration (s) MELCOR H2 (CO) mass burnt (kg) Pmax (bar) PAICC (bar) CPPC Regime (229) SD (331) SD (1175) (3145) (3149) (1914) SD FA FA SD

25 Validation & Verification Scenario dryt1- ESF dryt1- FCL dryt1- SPR wett1- ESF wett1- FCL wett1- SPR Duration (s) MELCOR H2 (CO) mass burnt (kg) (2848) (2644) (2635) (3138) (3155) (3129) Pmax (bar) P AICC (bar) CPPC Regime FA FA FA FA FA SD

26 Validation & Verification CPPC results compared with those obtained with other code for AICC calculations in case of slow deflagrations. Satisfactory results, differences in the pressure increase range in the 1%.

27 Validation & Verification. Breitung calculations. XH2 (% vol) XH2O (% vol) Tu (*) (K) Pu (*) (bar) P AICC Breitung (bar) P AICC CPPC (bar) Deviation (%)

28 Validation & Verification. Breitung calculations. Relative errors lie around 1% in wet mixtures. Less than 10% in dry mixtures. Results are considered as acceptable.

29 Plant Applications CSN methodology to calculate the containment failure probability due to hydrogen combustion during the in-vessel phase

30 Plant Applications Obtain containment pressure prior to H 2 combustion. MELCOR calculations. Obtain H 2 mass in the containment. H 2 well mixed. Calculate the containment pressurization. CPPC useful in this step. Overlap the containment pressure distribution with containment fragility curve to obtain containment failure probability. Reflooding considered: 20% additional hydrogen generation (Kuan, 1994).

31 Plant Applications PROBABILITY pdf cdf Zr FRACTION OXIDAZED

32 Plant Applications Results obtained No reflooding scenarios: negligible probability. Reflooding scenarios: significant increase in the containment failure probability and potential for flame acceleration. Safety significance of these results under study.

33 Plant Applications: No reflooding case CUMULATIVE PROBABILITY FRAGILITY 1.39 BARS PRESSURE (BARS)

34 Plant Applications Future applications are planned: Continuation of the verification process. Calculation of the containment failure probability for the ex-vessel phase. Analyses of local hydrogen accumulations.

Development of a Simple Computer Code to Obtain Relevant Data on H 2 and CO Combustion in Severe Accidents and to Aid in PSA-2 Assessments

Development of a Simple Computer Code to Obtain Relevant Data on H 2 and CO Combustion in Severe Accidents and to Aid in PSA-2 Assessments Fernando Robledo Consejo de Seguridad Nuclear (CSN); Spain Juan