Investigation of "dry" recriticality of the melt during late in-vessel phase of severe accident in Light Water Reactor D.Popov, KNPP, BG O.Runevall, KTH, Se International Conference “Nuclear power for the people”, 18 – 21 September 2013, Sl.Briag, Bulgaria

Content 1.History of the problem 2.Working task 3.Methodology 4.Results 4.1. From MCNP 4.2. From ASTEC calculations 4.3. From Thermocalc calculations 4.4. Calculation of Keff 5.Conclusion This work has been performed in , but first reported now 2

3 HISTORY of the problem - cont’d After issuing the results from MASCA (Fig.1) project, and especially about the formation of a layer of pure metals (U, Zr, Fe…) at the bottom of the experimental nstallation The explanation is that at high temperatures, Zr and possibly other metals like Fe, Cr, Ni are more chemically active to recombine the Oxygen than heavy metals like Uranium: at T>2300K: UO 2 +Zr->ZrO 2 +U ….(1) Fig.1 MASCA experimental results

The MASCA project has shown that in late stage of in-vessel phase of SA, the formed pool is stratified at least at three levels: - upper – metallic – layer - medium – oxidic – layer - lower – heavy metals (including U and Zr) The oxidic layer is the largest one among the three ones. It also can be sub-stratified depending on the density of some oxides (shown in the table in the right). 4 HISTORY of the problem - cont’d CompoundMelting point,  C Density, d [g/cm 3 ] (at 20  C) Different iron (incl. non- stoichiomeric) oxides – 5.74 ZrO UO More complex compounds, e.g. FeZrO g/mol Pu 2 O PuO PuO

5 Referent: the 11th cycle of Unit 5, KNPP, VVER-1000: Result from ORIGEN: 362 kg of Pu-239 at EOC, T=261.4 efpd, but also: 6 kg of Pu kg of Pu kg of Pu kg of Pu-242 -Total amount of Pu – 517 kg, from kg UO 2 (61881 kg U) at average burn-up ~26 MW.d/kgU HISTORY of the problem - cont’d

6 At the end of cycle in a VVER (LWR) it will be formed a considerable amount of Pu, mainly due to the following thermochemical reaction at T>2000: PuO 2 +Zr->ZrO 2 +Pu….(2) This quantity will be present in the pool formed in late in-vessel phase of SA of LWR, when all the water is evaporated (the pool is “dry”) Could it form some critical configuration? 7- 8 kilograms of liquid Pu metal accumulated in the bottom of the reactor be enough for recriticallity on fast neutrons? There was no information about the investigation of Pu behavior in the molten pool in the open literature

7 Working task Inside of the deep pool formed during in-late phase, the plutonium could be formed due mainly to reaction (1) and to exist in pure metallic form HOW MUCH plutonium can be accumulated? Can it form critical mass at given geometry? This problem had to be checked further by numerical simulations

8 Methodology 1.Run MCNP calculations for a typical core loading at 40 MWd/tU 4,4% enrichment and new Zirconium content for TVSA assemblies to calculate the fission products 2.Run ASTEC (v.1.3 at the time) for: –assessment of Pu decrease in form of FP release of Pu oxides –quantitative assessment of pool components 3.Run Thermocalc code for Thermochemical equilibrium calculations (through solving the Gibbs equation) to assess the chemical form of Pu and the mixture of heavy metals in the bottom layer 4.Run again MCNP with the Pu and othe heavy metals formed after achievement of thermochemical balance in the pool to calculate Keff

9 Results from ASTEC calculations Run for VVER-1000: - slow accident progression: SB LOCA + Total Loss of Feedwater (TLFW), Safety systems no available Result: The rate of PuO 2 release as FP is <1.E-9 kg/s Conclusion: One can estimate that all Pu remains in the pool assessment of Pu decrease in form of FP release of PuO 2

quantitative assessment of pool components after the core and RPV internals degradation 10 Results from ASTEC calculations –cont’d Initial mass of components for the Thermocalc calculations (kg) Total mass of molten material ~142 t (not taking into account some other components like B 4 C, Cr etc. for the next calculations) UFeZrO

11 - There were quite rescently published solution models for the U-O-Zr and the Pu-O-Zr systems -These were used for thermochemical calulations to estimate the composition of the different layers -The time scale allowed the system to reach equilibrium layers out of thermodynamic equilibrium layers in thermodynamic equilibrium Radiative heat transfer Molten oxide pool Heavy metal layer Light metal layer Results transition from ASTEC to Thermocalc calculations

Results from Thermocalc calculations 12 3 sensitivity calculations for different amount of Fe in the thermodynamic equilibrium metallic layer of the lower part of pool before oxidation and minimization of Gibbs energy:

Results from Thermocalc calculations – weight percents of metals in the thermodynamic equilibrium ionic/oxide layer after oxidation and minimization of Gibbs energy 13

Results from MCNP calculations - k eff 14

CONCLUSION There is no probability to formation of critical mass during severe accident even with formation of heavy metallic layer at the bottom of RPV Thank you 15