Holger Narrog 23.10.2013

Fast Chloride MCFR vs. Moderated Fluoride MSR

Conclusion: A fast chloride molten salt reactor (MCFR) surpasses a moderated fluoride molten salt reactor significantly. The main difference is the graphite moderator that fills 90% of the reactor vessel of the moderated reactor, reduces the power density, and requires regular replacement. The fast chloride reactor is by far more simple and more compact. Another issue is the internal reprocessing unit. The internal reprocessing is very complex, due to a decay heat that is several kW/g! for fission products fissioned less than a day ago. It requires a very strong cooling if feasible at all. Even if it is feasible it can blow-up any cost calculation. A fast chloride reactor can be operated with a high share of fission products in the fuel and hence requires only degassing plating out noble metal fission products and a replacement of the fuel.

1. Nuclear Evaluation Fast Chloride vs. Moderated Fluoride MSR

Ref. (69)

In opposite to moderated reactors the fast neutron spectrum reactor can use nearly all actinides as 236U, 240Pu, 242Pu as fuel. The build-up of long living high level radioactive waste is by far smaller.

The capture cross sections become very small in the fast neutron spectrum. In the consequence fast breeders can be run with a significant higher fission product concentration as moderated reactors. Actually Fast sodium cooled reactors achieve burn-ups of about 155MWd/Kg hm FBR vs. 55MWd/Kg hm for LWR.

Conclusion: A fast neutron spectrum is an advantage in favor of the fast reactor.

2. Salt Property Evaluation Fast Chloride vs. Moderated Fluoride MSR

Salt Properties 650°C

Unit Moderated Molten Fluoride Salt Reactor

Fast Molten Chloride Salt Reactor

Salt Composition Mol% 71.5%LiF, 15.8%BeF2, 11.8%ThF4, 0.4%UF4,

0.5% Fission Product Chlorides

43% NaCl, 23% KCl, 25.5%UCl3, 4.5% PuCl3, 4% Fission Prod. chlorides

Fuel Costs $/MWh Reprocessing is by far the most significant cost

Reprocessing is by far the most significant cost

Liquidus Temperature

C° 525°C 525-538°C

Heat Capacity cv KJ/m3/K 5038 2780

Density Kg/m3 3250 3230

Heat Conductivity W/m/K 1.1 0.69

Dynamic Viscosity Kg/m/s 0.0071 0.0030

Corrosivity The Fluoride salts are less corrosive. It is expected that nickel based materials work

The chloride salts are more corrosive and require more expensive materials as Mo-TZM.

The data for the chloride salts are based on the study Nuclear Power Plant of the Future and ref. 23

Conclusion: The salt properties of the fluoride salts are due to better corrosion properties and a higher cv very much in favor of a Moderated Fluoride Salt Reactor.

2b. Nuclear Reactions of the Salts used According to the report Reaktorsicherheit und Stahlenschutz in Baden-Württemberg (2000), (ref.

371) the yearly tritium emissions of a PWR are about 30 GBq/yr (Philippsburg II) to 70 GBq/yr (GKN I) in Germany.

The license allows for example the German nuclear power plant Philipsburg II to emit 48000 GBq/yr ( 4.8 x 10 13) of tritium. According to the IAEA Handbook of Nuclear Data for Safeguards ref. 6 the share of 3H is 0.01% for 238U, 0.0142% for 239U and 0,0141% for 241U per fission.

The total yearly tritium production from fission is calculated for a 4444 MWth reactor to 55g or 1.96*1015 Bq/yr. Most of the tritium will form TCl (chloride), TF (fluoride) even if the reactor is run

underfluorinated. Some of the tritium will remain as 3H2. It is assumed that 90% of the tritium and 99% of the TCl/TF is extracted in the degassing. It is further assumed that the very most of the remaining tritium is diffused thru the walls of the reactor, the heat exchangers and emitted in the reactor building. This tritium is absorbed in the charcoal filters of the reactor building. It is estimated in accordance with the (ORNL-4541 ref. 42) MSR project of the 60ies that 0.1% of the tritium is passing the primary and secondary heat exchanger and emitted via the condenser in the cooling water. That means a tritium emission of 2 GBq is well within the tritium emissions of current PWR reactors. In a fluoride reactor using enriched lithium (99% 7Li) the 7Li reaction will take place.

The tritium creation will be x-fold the tritium creation of a chloride salt reactor. It might become an issue for a fluoride salt reactor. A lithium enrichment is necessary anyway. Another topic is the creation of radioactive chlorine in the MCFR. It is due to the excellent hard neutron spectra not necessary to enrich the chlorine. During the operation the following neutron induced reactions occur. 35Cl + n -> 36Cl 75.8% of the chlorine is

35Cl. The

36Cl is radioactive with a half-time of 301.000yr.

35Cl(n,α) ->32P[β-[14.2days] -> 32S Sulphur is corrosive 36Cl + n -> 37Cl (stable). The reaction is welcome. 37Cl + n -> 2n-> 36Cl The 36Cl is radioactive with a half-time of 301.000yr. 37Cl(n,α) -> 34P(β-[12.34s] ->34S Sulphur is corrosive 37Cl + n -> 38Cl -----37.24min 38Ar (stable) In the MCSFR it is foreseen to reuse the salts in the reactor. As the salts needs to be treated protected from the atmosphere to avoid impurities the radioactivity should not increase the costs significantly. The creation of radioactive sulphur in the chlorine salt is (ref. 46) 1/8 of the oxygen creation 19F[n,α] -> 16O reaction in the fluoride system. The tritium removal at the MSFR requires more efforts and costs than the handling of radioactive chlorine in the reactor. The tritium removal at the MSFR requires more efforts and costs than the handling of radioactive chlorine in the reactor. The tritium removal at the MSR requires more efforts and costs than the handling of radioactive chlorine in the reactor.

Tritium Generation Table AD333

Tritium generation/Kg of fissioned actinides: 0.0351 g %/Fission

Tritium generation/year (g): 55 g 238U 0.01 Ref. 6

Tritium generation/day/Bq: 5.3722E+12 Bq 239Pu 0.0142 Ref. 6

Tritium generation/yr/Bq: 1.9609E+15 Bq 241Pu 0.0141 Ref. 6

Average 0.14 Ref. 6

Tritium Generation by Fission

3. Structure Material

Data 700°C if available Hastelloy N (ref. 161) Molybdenum TZM (ref. 168)

Density (700°C) 8860 Kg/m3 10070 Kg/m3 (ref. 264)

Thermal conductivity(700°C): 23,6 W/mK 112W/m/K (acc. To ref 264)

Young Modulus Gpa (700°C): 169,6 217.9 (acc. To ref. 164)

Mechanical yield

strength/Mpa (700°C):217 700 (acc. To ref. 168)

Accepted mechanical stress in

this design Mpa (700°C):

80 definition based on the

data above

250 definition based on the

data above

Corrosion resistance against

Flibe salts:

acc. To (ref. 61) page 41 less

than 0.025mm/yr at

temperatures of more than

700°C

No Corrosion at 1100°C (ref.

167)

Corrosion resistance chloride

salts:

acc. To (ref. 61) page 42

1.1mm/yr at 850°C No Corrosion expected

Manufacturing: Very Good Challenging, limited experience

Table D Modified Hastelloy N is less expensive and easier to weld than Mo-TZM. That’s why it was developed for fluoride MSR. Two unknown issues with Hastelloy N did surface, one was corrosion induced by the fission product tellurium and the other was irradiation damage caused by (n,alpha) reactions in nickel and boron contaminants (ref. 203). The n, alpha reaction is not observed in the radiation tests with molybdenum alloys. Mo-TZM/TZC allows much higher temperatures, has superior corrosion properties and by far superior mechanical properties. Conclusion: The possibility to use Ni-based materials is an advantage in favor of the moderated fluoride salt reactor even when it limit its potential.

4. The Reactor of the Moderated vs. Fast Chloride MSR

Reactor Design

Unit Moderated Fluoride Molten Salt Reactor

Fast Chloride Molten Salt Reactor

Power Density 24 MW/m3 (core), Peak 70MW/m3 (ref. 42)

492 MW/m3 (fissile zone), 22 MW/m3 (fertile zone)

T(outlet) °C 740°C 800°C

Total Height m 7.57 5.74

Total Diameter m 9.17 4.13

Total zylindric volume

m3 500 77

Complexity Medium complex Inserts made of graphite

No inserts, no complexity

Maintenance The graphite core needs replacement about every 2-4years

No regular maintenance required

Risks Graphite – Salt – Hastelloy materials can cause electrolytical carburization and corrosion ORNL-3626 (ref. 34)

The moderated reactor requires a graphite moderator. The moderator requires nearly 90% of the space in the core. A high neutron flux damages the graphite. It limits the

power density and requires a replacement every 4 years at a peak power density of 70MW/m3

(ref. 42). The combination of graphite and nickel or molybdenum based materials might cause electrolytical carburization and corrosion. The mean thermal expansion coefficient of Hastelloy N is 1,47E-05 vs. 2,50E-06 for graphite that creates some design headaches. Conclusion: Even if molybdenum TZM is more expensive to manufacture the total fast chloride reactor will be by far less expensive. This is a main advantage of the Fast Chloride Molten Reactor.

5. The Primary Heat Exchangers

The heat exchangers of the

MSR are designed

conventionally as tube and

shell heat exchangers.

The new MSR concepts are

based on using compact heat

exchanger designs as micro

channel types as HEATRIX,

plate design types and

others.

The advantages are that

there is less fissile material in

the heat exchangers.

The share of delayed

neutrons in the circuit is

lower.

It is more compact.

6. The Power Plant Due to the bigger reactor the moderated fluoride reactor has a bigger reactor building. It requires space and equipment like a crane to replace the graphite once every 2 – 4 years.

Ref. 42

The reactor outlet temperature of the fast chloride reactor is 40° higher which compensates slightly the lower cv of the chloride salts. It allows as well a better thermal efficiency. The real advantage is that the fast chloride salt reactor structure material Mo-TZM gives room for development to significantly higher operation temperatures.

Ref. 42

Both concepts require a 3 - circuit system with its complexity. It is a major disadvantage of most of the MSR concepts. The usage of Mo-TZM in the fast chloride reactor allows the usage of liquid bismuth lead as intermediate coolant. The advantage is the very low m.p. of 125°C. Conclusion: The usage of Mo-TZM gives the Fast Chloride Molten Salt Reactor the potential of using higher temperatures and intermediate coolants as Bi/Pb with a low melting point of 125°C.

7. The Reprocessing Unit All MSR concepts need a regular degassing of the fuel. In the moderated MSR it is done by a helium bubbling. It seems a simple method to get out the gaseous fission products. In the fast MCFR it is done by a 10mbar vacuum distillation at 950°C. The method is more complex but seems suitable to extract about 35% plus due to further decay of fp to such with a low bp. in total 40% of the fission products. Another 20% of the fission products are extracted as metal by gravity.

The fast MCFR does not have a complete and complex reprocessing unit. A fraction of the fuel is taken out and shipped to an external reprocessing unit about 2 years after it is taken from the reactor. The moderated MSR includes a complete and very complex reprocessing unit.

Ref. 42

Corrosion risk!

Corrosion risk! Cooling of the fission

products

Multistage

Processes

The degassing of the fuel takes place in a

helium bubbling process. Gaseous fission

products as Xe, Kr, I are separated in a

helium gas flow.

The system is simple and would not create

technical or economic challenges.

Conclusion: The gas extraction system is required in all MSR reactors to extract gasses and perhaps even noble metals. The complete reprocessing unit with its challenges of the heat creation and its complexity is part of the political promise to avoid waste. It is complex if feasible and would blow up any cost calculation.

Calculation Moderated MSR:

1970 MSR Recalculation 2000 MW Version of this reactor

1 feet/m: 0,3048 1 in/m: 0,0254

Core Diameter: 18ft 5,49 Acc. To ref. Core Diameter/m: 7,22

Total Diameter/m: 6,77 Estimate Total Diameter 9,24

Core Height: 13ft 3,96 Acc. To ref. Core Height (ornl 4541)/m: 5,23

Total Height Reactor/m 6,41 measured Total Height Reactor/m: 8,23

Core Volume/m3: 93,63 Calculated Core Volume/m3 214

Total Mass Graphite/to 304 Acc. To ref. Total Mass Graphite/to 780

Density Graphit/Kg/m3 1750 Acc. To ref 195 Density Graphit/Kg/m3 1750

Volume Graphit/m3 174 Acc. To ref. Volume Graphit/m3 445,50

Side Reflector Thickness/m 0,76 Measured Side Reflector Thickness/m 0,76

Side Reflector Volume/m3 59,07 Calculated from estimatesSide Reflector Volume/m3 99,60

Top/Bottom Reflector estimate av./m 0.3 - 0.73m Average est.: 0,55 Top/Bottom Reflector av./m 1,2

Top/Bottom reflector Volume: 33,69 est.from calc. Top/Bottom reflector Volume: 160,85

Graphit in core/m3: 80,96 est.from calc. Graphit in core/m3: 185,05

Total cylindric volume/m3: 247,08 Calculated Total cylindric volume/m3: 551,59

Electrical Power/MW 1000 Acc. To ref. Electrical Power/MW 2000

Efficiency% 44,40% Acc. To ref. Efficiency% 44,00%

Thermal Power/MW 2252,25 Calculated Thermal Power/MW 4545,45

Average Power density W/cm3: 22,00 Acc. To ref. Average Power density W/cm3: 21,24

Specific heat capacity KJ/Kg/K: 1,55 ref. (64) Specific heat capacity KJ/Kg/K: 1,55

T inlet/°C: 600 T outlet/°C 740 T inlet/°C: 600 T outlet/°C 740

Mass Flow/Kg/s: 10379 calculated Mass Flow/Kg/s: 20947

Density/Kg/L: 3,25 ref. (64) Density/Kg/L: 3,25

Volume Flow/m3/s: 3,19 calculated Volume Flow/m3/s: 6,45

velocity in the Reactor/m/s: 2,6 Acc. To ref. velocity in the Reactor/m/s: 3

Flow Area/m2: 1,23 calculated Flow Area/m2: 2,15

Ref.: (24)

1 inch fuel channels acc. to ORNL – 3626

Ref.: (24)

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