Fluoride-Salt Cooled High Temperature Reactors (FHR)

Raluca O. Scarlat Nuclear Engineering, UW Madison [email protected] | HEATandMASS.ep.wisc.edu

Engineering Physics Colloquium UW Madison 21 April 2015

FOR PUBLIC DISTRIBUTION Outline

1. Overview of FHR technology 2. Tritium management in the FHR 3. Tritium transport in the salt-graphite system 4. Salt freezing and overcooling transients 5. Future work

Raluca Scarlat | HEATandMASS.ep.wisc.edu 2 FHRs: Solid Fuel, Salt Cooled Reactors

Liquid ﬂuoride salt coolants Coated particle fuel Excellent heat transfer (TRISO Fuel) Transparent, clean ﬂuoride salt Boiling point ~1400ºC Reacts very slowly in air No energy source to pressurize containment But high freezing temperature (459oC) And industrial safety required for Be

FHRs have uniquely large fuel thermal margin

Raluca Scarlat | HEATandMASS.ep.wisc.edu 3 900 MWth 400 MWth PB-FHR PBMR Nuclear Air-Brayton Combined Cycle (NACC)

Fluoride Salt Coolants Were Conventional combined cycle gas Developed plants achieve efﬁciencies of 50-60% for the Aircraft Nuclear Propulsion Program

! Aircraft Reactor Experiment, 1954 Nuclear Air-Brayton Combined Cycle (NACC)

Modified GE 7FA Turbine for Co-fired NACC

Base-load power: 42% efficiency (100 MWe)

Gas co-firing: 66% efficiency (242 MWe)

FHR Physical Plant Arrangement Coiled Tube Air Heaters (CTAHs)

P.V. Gilli et al., “Radial Flow Heat Exchanger,” U.S. Patent Number 3712370, Filing date: Sept. 22, 1970, Issue date: 1973 PB-FHR Mark 1 Core Design

Andreades, C., Cisneros, A. T., Choi, J. K., Chong, A. Y. K., Fratoni, M., Hong, S., … Peterson, P. F. (2014). Technical Description of the “Mark 1” Pebble-Bed Fluoride-Salt-Cooled High-Temperature Reactor (PB-FHR) Power Plant. Outline

1. Overview of FHR technology 2. Tritium management in the FHR 3. Tritium transport in the salt-graphite system 4. Future work

Raluca Scarlat | HEATandMASS.ep.wisc.edu 10

Tritium Source Term

! The main sources of tritium production are Li-6 and Li-7 1 6 3 4 7 1 4 1 3 0 n+3Li→1H +2 He 3 Li+0 n→2 He+0 n+1H ! Natural Li isotopic composition: 7.5% 6Li – 92.5% 7Li ! Target 6Li start-up concentration: 60 ppm 6Li ! Steady state 6Li concentration: 8 ppm 6Li ! Lithium-6 is continuously created: 1 9 6 4 6 6 0 0 n+ 4 Be→2 He+ 2 He 2 He→3 Li+ −1e ! Natural Be isotopic composition: 100% 9Be

Cross Sections for (n,α) Reactions 10000$

1000$

100$

10$

Cross%Sec)on%(b)% 0.1$

0.01$

0.001$

0.0001$ 0.00001$ 0.0001$ 0.001$ 0.01$ 0.1$ 1$ 10$ 100$ 1000$ 10000$ 100000$ 1000000$10000000$ Energy%(eV)% Tritium Source Term

Estimated Tritum Production Rate in the Mk1 PB-FHR Core

14 0.18 0.16

12 0.14 10 0.12 8 0.10 6 0.08 0.06

EFPY) EFPY) 4 0.04 T/EFPD) (mol 2 0.02 0 0.00 0 1 2 3 4 5 6 7 8 9 10 11 Tritium Production Rate (Ci/ Production Tritium Effective Full Power Years

g/year per GWe Ci/year per GWe PB-FHR 176 1.7 x 106 PWR (average) 0.08 0.73 x 103 CANDU (Darlington) 576 8.0 x 106 PBMR 512 4.9 x 106 MSR 92 0.65 x 106

1. http://www.nrc.gov/reactors/operating/ops-experience/tritium/faqs.html#normal 2. Ohashi, Hirofumi and Sherman, Steven R. Tritium Movement and Accumulation in the NGNP System Interface and Hydrogen Plant. s.l. : Idaho National Laboratory, 2007. INL/EXT-07-12746

FHR Tritium Emission Target

(Based on tritium production at steady state flibe isotopic composition, per GWe) ! FHR production: (1.7)106 Ci/yr (176 g/yr) ! If 100% released to air: 10-5 Ci/kg air (0.5x10-7 NRC limit) ! Reduce by a factor of 200 ! PWR emissions: (0.7)103 Ci/yr (0.075 g/yr) ! Reduce by a factor of 2000

Ohashi, Hirofumi and Sherman, Steven R. Tritium Movement and Accumulation in the NGNP System Interface and Hydrogen Plant. s.l. : Idaho National Laboratory, 2007. INL/EXT-07-12746 Tritium Sinks and Sources

Raluca Scarlat | HEATandMASS.ep.wisc.edu 14 Alternative Tritium Sinks

! Fuel elements (pebbles: 100 days to full BU) ! Reflector pebbles ! Graphite particle absorber: annular cartridges of ~2.0-mm carbon spheres ! Gas sparging ! Double-wall heat exchangers Tritium Permeation Barriers for CTAH tubes

Barrier Base Metal PRF SS316, MANET, 10 to Al O TZM, Ni, 2 3 >10,000 Hastalloy-X TiC, TiN, SS316, MANET, 3 to TiO2 TZM, Ti >10,000 Cr2O3 SS316 10 to 100 Si Steels 10 BN 304SS 100 N Fe 10 to 20

Er2O3 Steels 40 to 700

Barriers to release through CTAH • 4x lower mass convection coefﬁcient than the pebble bed • Aluminum Oxide Tritium permeation barrier: Kanthal AF or Alkrothal 14; ~0.1 mm • Ensure low tritium concentration in the ﬂibe at CTAH inlet: 10-12 to 10-23 wt fraction (10-5 to 10-17 g/m3)

1. Fluoride-Salt-Cooled High Temperature Reactor (FHR) Materials, Fuels and Components White Paper. UCBTH-12-003. Berkeley, CA. fhr.nuc.berkeley.edu SINAP Gas Sparging Studies

Raluca Scarlat | HEATandMASS.ep.wisc.edu 17 Outline

1. Overview of FHR technology 2. Tritium management in the FHR 3. Tritium transport in the salt-graphite system 4. Future work

Raluca Scarlat | HEATandMASS.ep.wisc.edu 18 Tritium Transport Scales of Interest

Core Fuel Element

Raluca Scarlat | HEATandMASS.ep.wisc.edu 19 Perfect Sink Calculations

Table I. Perfect Sink Calculation Input and Outputs Inputs Inputs Parameter Value Units Parameter Value Units Salt Average Carbon Molecular 923.15 K 12 g/mol Temperature Weight Tritium Molecular Reynolds Number 500 - 3 g/mol Weight Diffusivity (Tritium in 3.90E-09 m2/s Graphite Density 1.70E+06 g/m3 FLiBe) Pebble Diameter 0.03 m Pebble Life 1.4 yr Salt Viscosity 6.78E-03 kg/(m*s) FLiBe Volume 46.82 m3 Salt Density 1.96E+03 kg/m3 Outputs Mass Transfer Salt Kinematic Viscosity 3.45E-06 m2/s 5.74E-05 m/s Coefficient Bulk Salt Schmidt Number (ν/D) 885.25 - Concentration Steady 2.38E-06 mol/m3 State Bulk Salt Steady State Production 2.66E-07 mol/s Concentration 1.15E-05 mol/m3 Rate Startup Graphite Saturation Startup Production Rate 1.28E-06 mol/s 0.27 g T/g C Steady State μ Graphite Saturation Pebble Surface Area 1945.27 m2 1.28 μg T/g C Startup

Raluca Scarlat | HEATandMASS.ep.wisc.edu 20 Background Information: MSRE

! ORNL-4865: Graphite stringers (CGB) from MSRE were analyzed for tritium with depth profiling. Contains information on FP distributions and transport.

! ORNL-5011: Small graphite samples were

exposed to T2 gas and analyzed.

! Briggs (1972): Measured and calculated distributions of tritium in the MSRE.

! Does this data lend itself to useful benchmark exercises?

! If so, what assumptions about the MSRE and its operations limit application to FHR?

21 What data is relevant?

! 750 C for 6.5 hr in P_T2 = 0.14 Pa on Strehlow, R. A. (1986). Chemisorption of tritium on graphites 1 cm^3 specimens. at elevated temperatures. Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 4(1986), 1183. ! POCO AXF-5Q graphite taken as the doi:10.1116/1.573434 example (non-oxidized).

Michael Young 22 Modeling of MSRE Depth Profile

Trapping Model

Raluca Scarlat | HEATandMASS.ep.wisc.edu 23 We need to understand the transport and trapping mechanisms

! Porous diffusion with adsorption occurring on the edge of crystallites near grain boundaries. ! Intra-granular diffusion ! Intracrystalline sorption ! Chemisorption on pore or grain surfaces

Redmond, J. P., & Walker, P. L. (1960). Hydrogen sorption on graphite at elevated temperatures. The Journal of Physical Chemistry, 64(9), 1093–1099.

Kanashenko, S. L., Gorodetsky, A. E., Chernikov, V. N., Markin, A. V., Zakharov, A. P., Doyle, B. L., & Wampler, W. R. (1996). Hydrogen adsorption on and solubility in graphites. Journal of Nuclear Materials, 233-237, 1207–1212. doi:10.1016/ j.jnucmat.2009.03.033

Atsumi, H., Tanabe, T., & Shikama, T. (2011). Hydrogen behavior in carbon and graphite before and after neutron irradiation – Trapping, diffusion and the simulation of bulk retention–. Journal of Nuclear Materials, 417(1-3), 633–636. doi:10.1016/ j.jnucmat.2010.12.100

Michael Young 24 Matrix Graphite and Nuclear Graphite

Raluca Scarlat | HEATandMASS.ep.wisc.edu 25 Irradiation Effects

Raluca Scarlat | HEATandMASS.ep.wisc.edu 26 MITR FHR Irradiations

! Two in-core irradiations completed: 300 and 1000 hours at 700°C ! Double-encapsulation with nickel inner vessel, graphite liner, titanium cold-wall ! 100-300g of MSRE secondary flibe ! Nuclear heating with ±3°C temperature control (He/Ne) ! Separate gas flows in each containment, tritium collected

" Future irradiations for IRP-2 o Tritium uptake from flibe into irradiated graphite (reflector, fuel matrix, etc.) o Transport and release of gaseous tritium and activation products (from flibe and corrosion) o Tritium diffusion from salt through metals (heat exchangers) o Incorporate electrical heating for temperature control independent of reactor power

27 Summary: FHR Tritium Transport - Ongoing Work

! Goals: ! How much tritium will be retained in an FHR fuel pebble? ! How long will it take for the pebble to reach equilibrium?

! Studies: ! What is the mechanism of tritium transport and trapping in matrix graphite? ! What data from nuclear graphite is relevant? ! What data from higher temperature is relevant? ! Characterize irradiation effects ! What is the effect of salt intrusion? ! What are the desorption characteristics – important for fuel handing

Michael Young 28 Outline

1. Overview of FHR technology 2. Tritium management in the FHR 3. Tritium transport in the salt-graphite system 4. Salt Freezing and overcooling transients 5. Future work

Raluca Scarlat | HEATandMASS.ep.wisc.edu 29 FHR Decay Heat Removal Systems

PB-FHR Mark 1 Design Report. UC Berkeley. UCBTH-14-002 Similitude and the use of simulant fluids

Tmelt, ﬂibe Tin Tout

Tmelt, Dowtherm

Flibe Coolant 67%LiF – 33% BeF2

Dowtherm® A Oil

26.5% diphenyl 73.5% diphenyl oxide Freezing phenomenology: phase diagram Freezing phenomenology: uncertainty in salt composition Freezing Phenomenology: Phase Diagram

O. Beneš and R. J. M. Konings, ‘Thermodynamic properties and phase diagrams of fluoride salts for nuclear applications’, J. Fluor. Chem., vol. 130, no. 1, pp. 22–29, (2009). Freezing Phenomenology: Supercooling

Flibe

Simulant fluids

Raluca Scarlat | HEATandMASS.ep.wisc.edu Summary: FHR Overcooling Transients - Ongoing Work

! How does the passive safety system perform under over-cooling transients? ! What is the freezing phenomenology of flibe salt? ! Is it possible to use simulant fluids for experimental studies of overcooling transients? What are the distortions and what are the limitations?

Michael Young 36 Summary: FHR

! FHR is a reactor technology that has the potential to change the economics of nuclear power production ! It aims at a short commercialization timeline, but using already available technologies ! The salt coolants, the particle-encapsulated fuel, and the open-air Brayton turbine lead to inherent safety features for FHR, and ease of design of passive safety systems ! Tritium transports is an important area of study that has unique characteristics for the FHR system ! Overcooling transients are an emerging area of study, unique to high temperature salts. The ability to use simulant fluids has a significant impact on technology development timeline and cost.

Michael Young 37 Fluoride-Salt Cooled High Temperature Reactors (FHR)

Raluca O. Scarlat Nuclear Engineering, UW Madison [email protected] | HEATandMASS.ep.wisc.edu

Engineering Physics Colloquium UW Madison 21 April 2015

FOR PUBLIC DISTRIBUTION Raluca Scarlat | HEATandMASS.ep.wisc.edu 39