Containment Chemistry Modeling

M. Salay

D.A. Powers

R.Y. Lee

Overview and Status

European Review Meeting On Severe Accident Research (ERMSAR) 2015

Objectives and Motivations

• Revise and refine MELCOR containment

chemistry model to ‘scale’ findings from

Phébus-FP on iodine chemistry to reactors:

– Old model hastily constructed for ISP dealing with Canadian RTF tests

– Change in paradigm by Phébus-FP

• Raw water issue

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Old Paradigm

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Mitigation easy under old paradigm

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Phébus-FP Tests Included a Sump

• Iodine did not behave as expected

– Iodine concentration fell to a steady state level

– Steady state persisted for ~90 hours

• ‘steady-state’ gaseous iodine concentration persisted despite changes in sump pH and temperature

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Aerosol Phase Chemistry

Phase

Degradation phase

Washin

g p

hase

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Evaporating Sump

Condensing Sump

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New Paradigm for Iodine

• Iodine chemistry in containment still object of research – Basic iodine chemistry understood – Interactions with other materials in

reactor is the problem

• New paradigm focuses on interactions of iodine with painted surfaces in containment – Iodine binds to paint, evolves under

irradiation – Evolved gaseous iodine oxidizes to IOx

particles – Particles deposit back on the paint

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New Paradigm Derived from

PHÉBUS-FP Results

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‘Scaling’ the Phébus-FP Results

• Geometry and mass transport – ‘steady-state’ gaseous iodine chemistry

depends on balance of gaseous iodine sources and sinks

• These will differ in reactors from those in tests

– Iodine release possible depends on iodine that reaches painted surfaces versus pools

• Released iodine has less access to sumps in reactors

• Chemical simplicity – Final model intended for system code

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Elements of the Modeling • Primary objective still

understanding ‘gaseous’ iodine under DBA and beyond design basis accidents

• Elements – Gas phase chemistry – Aqueous phase chemistry – Heterogeneous chemistry

• Gas <->Liquid • Gas <-> Solid • Both <-> Surfaces

Surfaces

Gas Phase Modeling

Aqueous Modeling

Heterogeneous Modeling

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‘Raw’ water issue

• Fukushima accident management activities reminded us that US plants will use raw water as a coolant of last resort – Seawater – Lakes and ponds – Rivers

• Water recirculated from sumps to RCS which may be pressurized – Higher water temp and pressure than considered in past – Sump screen blockage

• Corrosion during plant recovery – Mild steel of vessel and containment liner – Clad and exposed fuel

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Other materials

• Many solutes may be present: – Core degradation products

– Degradation of organic materials in containment

– Raw water

• Solutes: Borate, hydrazine, phosphate, calcium, zinc, aluminum, iron, uranium, and organic species

• Solutes in raw water include: Na+, Mg2+, Ca2+, K+, Sr2+, Cl-, SO4

2-, Br-, F-, HCO3-, B(OH)3, and biota

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Strategy

• More chemistry possible than tolerable in a

systems level accident progression code like MELCOR

• Many technical issues still being resolved

– Fukushima information may be crucial

• Strategy

– Standalone code developed

– Use to define important chemistry to carry into

MELCOR

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Gas Phase Modeling • Thermal and radiolytic conversion of gaseous

iodine into iodine oxide particles I2(gas) or CH3I + O3 or OHo I2O4 or I2O5

• Reactions taken from JPL and Mätzing (~2000)

• Solved using the quasi-steady state method in connection with aqueous radiolysis

• Sump acidification by formation of HNO3

• Boundary condition for gas solubility in aqueous phases H2, O2, CO2, CO, O3, H2O2, organic vapors

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Liquid Phase Modeling

• Primary objective is to treat the partitioning of iodine from aqueous phase to gas phase – Thermal: 2I- + O2(aq) + H2O → I2(aq) + 2OH- – Radiolytic: I- + OHo ↔ HOI- → OH- + Io Io + I- → I2

- 2I2- ↔I3

- + I- I3

- ↔ I2(aq) + I- I2(aq) ↔ I2(gas) – Formation of volatile organic iodides: CH3I

• Additional objectives – Corrosion – Precipitation

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Aqueous Modeling Approach

• Thermochemical treatment for most non-radionuclides

• Kinetic approach where necessary – Mass transport to and from surface – Radiolysis – Leaching

• Non-ideal behavior – Calculate activity coefficients using electrolyte-

specific, non-electrolyte-specific, and seawater-specific models

– Validate against existing data

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Radiolysis • Yields for α, β, γ, and no

– Function of T, pH

• Quasi-steady state solution method

– Avoid solving very large number of very stiff differential equations – Will not handle abrupt (~minutes) transients – H2(aq), O2(aq), H+ constant in time step – Depletions accumulated and fed to equilibrium calculation

• Account for ionic strength on rates of reaction – Generally, ionic specie activities decrease in electrolytes – Generally, neutral specie activities increase in electrolytes

• Six reaction sets considered for water radiolysis: LIRIC; INSPECT; Ershov & Gordeev; Palfi, Wojnáfovits & Takács; Sunaryo & Domae; and RISØ

• Include radiochemical reactions of other solutes: Chloride, Carbon monoxide & Carbon dioxide, Hydrazine, Iodine, Silver, Organic species

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Additional aqueous modeling

• Iodine Reaction schemes: LIRIC, INSPECT, AIMS, IODE, PSI

• Precipitation – Lots of solids can precipitate, e.g: AgI,

AgCl,Al(OH)3, Fe(OH)3,CaCO3, CaSO4.nH2O,calcium phosphate, borate

• Difficult to predict – Solid solutions especially challenging

– Currently just ‘flag’ when solubility product exceeded for a small list of the many possible solids

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Thermochemical Modeling Challenges

• Adopt key species potential approach to the minimization of Gibbs Free Energy – Allows extensive speciation

• Extend data and properties to ~550 K and P ~150 bar – Water properties from IAWPS Industrial Formulation

2007 • Augment with additional parameters relevant to SA

modeling: viscosity, Kw, self diffusion etc.

– Modified HKF method to calc. properties

• Activity coefficients of solutes at ionic strengths at least up to 1 molal

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Heterogeneous Modeling

• Gas exchange between liquid and atmosphere

• Ion absorption on precipitates

• Degradation of paint and cables

• Corrosion

• Precipitation

• Aerosol formation (IOx)

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Mass Transport at the Liquid-Gas Interface

• Gas-water exchange essential aspect of modeling – Iodine partitioning from aqueous phase to atmosphere – Acidification by radiolytic HNO3 and HCl from cables – Oxygen and hydrogen concentration in water

• Water present in a variety of configurations – Droplets – Falling films – Spills – Water pools

• Updating from two-film model to surface renewal and surface penetration modeling – Can include effects of chemical reactions

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Remaining Heterogeneous Issues • Liquid diffusion of reactive species

– I2(aq) + H2O ↔ HOI + I- + H+ – Diffusion of neutral influenced by ion diffusion – Treat now only as single solute

• Multi-component diffusion in gas phase boundary layer – Evaporating and condensing steam couples to fluxes of

other dissolving gases

• Uptake coefficients – Receives much attention in oceanography literature

• Solute enrichment at still liquid surface – Not considered

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Simple Corrosion Scheme

• Short term corrosion

– Nucleation and growth of ferrous hydroxide

– Influence of other solutes

• Different types of “green rust”

• Longer term corrosion

– Anaerobic corrosion

– Attributed to bacteria

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Degradation of Organic Materials in Containment

• Aqueous organic species poorly understood – Can affect radiolytic reaction schemes – Can react to form volatile organic iodides

• Organic vapors can be produced by – Pyrolysis during energetic accident phases – Continued radiolysis – Synergism between the effects of temperature and radiation

observed in cable insulation

• Focus on two sources of organic vapors – Cable insulation – Paint – Biota not considered

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Problem of Including Organics in Model

• There are a lot of possible organic species – Aliphatic: CH4 – Ketones: methyl ethyl ketone, acetone – Alcohols: methanol – Aromatics: benzene – Chlorinated species: vinyl chloride

• Radiolysis over the long term leads to even more: CH4 + OH → CH3 + H2O 2CH3 → C2H6 C2H6 + OH → C2H5 + H2O etc.

• Currently anticipate including limited organics in model – Where to stop? (Chose Ethyl)

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Status and Information Needs • Individual models developed for all elements • Need to combine models • Remaining information needs

– Effect of paint aging – Competition with other species

• Cl2 and HCl are likely competitors – Any other potential competitors for iodine adsorption?

• Cl/I ratio expected ~ 100 – For similar surface affinities expect 100/1 Cl to I on surface

• Need to ensure that our conclusions on iodine would not be completely altered by competition

– Understand features about paint that affect • Iodine adsorption • Organic iodide production

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