Assessment of Molten Salt Blankets for Power Plants

S. Malang (Consultant)

C.P.C. Wong (GA), M. Sawan (UW) M. Dagher (UCLA), P. Fogarty (ORNL)

S. Smolentsev (UCLA)

Evaluated Blanket concepts:

A. Self-cooled FLiNaBe Blanket

B. Dual coolant blanket with helium cooled steel structure and self cooled FLiBe breeding zone, based on:

o Be pebble beds as neutron multiplier o Pb-layer as neutron multiplier

University of California at Los Angeles

February 23-25, 2004

GOALS OF THE STUDY

● Identify attractive Molten Salt (MS) breeder blanket concepts which: —

—

Do not depend on advanced ferritic steel as structural material, Can be developed, qualified, and tested in the time frame of ITER.

● Make scoping designs, layouts and first analyses of the candidate concept

● Prepare for the selection of one MS blanket concept for tests in ITER.

CANDIDATE MS BLANKET CONCEPTS

● Basically, there are three different candidate concepts under consideration: — Self-cooled FLiNaBe blanket with Be

pebble beds as neutron multiplier and RAFS structure. (This concept replaces a self-cooled FLiBe blanket with advanced ferritic steel as structural material since this combination is not feasible in the time frame of ITER) Dual coolant blanket with self-cooled FLiBe breeding zone, helium-cooled RAFS structure, and Be pebble beds as neutron multiplier,

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— Dual coolant blanket with self-cooled FLiBe breeding zone, helium-cooled RAFS structure, and a liquid Pb layer as neutron multiplier.

● MS dual coolant blanket concepts had been suggested at the end of the last APEX meeting by C.P.C. Wong as a combination of Molten Salt breeder material with lead lithium Dual coolant blanket concepts.

GEOMETRY AND WALL LOADS OF A POWER PLANT SERVING AS A COMMON BASIS FOR ALL CANDIDATE BLANKET CONCEPTS

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Geometry and size similar to ARIES-RS,

Torus subdivided into 16 sectors,

Breeding blankets subdivided into segments with a height of ~8 m,

Coolant access tubes attached to the bottom of the blanket segments,

Maximum neutron wall load 5.45 MW/m

Max. surface heat flux at the FW 1 MW/m

SELF-COOLED FLiNaBe BLANKET

● A self-cooled FLiBe blanket with re-

circulating flow was the reference MS concept in the APEX Task IV. It allows coolant exit temperatures up to 700ºC, maximum neutron wall loads of 5.45 MW/m, and maximum surface heat fluxes of 1 MW/m. The high melting point of FliBe (459ºC), however, requires a structural material with an allowable maximum temperature of 800ºC, and maximum interface temperature breeder/ structure of 700ºC. These temperature limits are anticipated for advanced ferritic steels, for example oxide dispersion strengthened (ODS) steel with nano-sized oxide particles. Such steels are under development, but it will not be possible to test and qualify them in the time scale of ITER blanket tests. For this reason FLiNaBe with a lower melting point (340ºC) has been selected to replace FLiBe for blanket tests in ITER, allowing the use of more conventional ferritic steels, for example F82H.

IMPLICATIONS OF THE REPLACEMENT FLiBe ⇒ FLiNaBe

FLiBe FLiNaBe Min. MS temp. 500ºC 370ºC Max. MS temp. 700ºC 650ºC Max. steel temp. 800ºC 550ºC Max. interface temp. 700ºC 550ºC Thickness of Be 37 mm 70 mm

pebble bed

● Basic blanket geometry and flow scheme are kept constant with two exceptions: —

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Radial depth of breeding blanket increased from 300 mm to 500 mm, Secondary breeding blanket in outboard region eliminated.

RE-CIRCULATION BLANKET CONCEPT WITH FLiNaBe FLOWING THROUGH THE

BERYLLIUM PEBBLE BEDS

Advantages

• Combination of high flow rate in FW/multiplier region with low flow rate in the large central channel results in effective cooling of FW and beryllium pebble beds, and, at the same time, in maximized coolant exit temperatures.

Flow rate through the pebble bed about a factor of three larger than in a once-through concept, resulting in increased heat transfer from beryllium to FLiBe.

Pressure drop in the pebble bed still small (<0.1 MPa) since flow path length short (~0.07 m) and velocity low (<0.03 m/s).

Mechanical stresses in perforated plates containing pebble beds are low.

FLOW SCHEME OF THE RE-CIRCULATION BLANKET CONCEPT

Main flow goes down from top as well as sides and back flows

Cross-section at mid-plane of the self-cooled FLiNaBe module

FLiNaBe routing at the First Wall

beryllium pebble bed

6

mid plane

bottom

top

qsurface

pol.

rad.

rad.

tor.

DUAL COOLANT BLANKET WITH HELIUM-COOLED STEEL STRUCTURE AND SELF-

COOLED FliBe BREEDING ZONE

Introduction ● Dual coolant (DC) blankets with

helium-cooled steel structure and a self-cooled lead-lithium breeding zone have been under development in the US and the EU for many years (ARIES-ST, PPA, PPCS). Such concepts are based on the following ideas: ● Cool the steel structure with helium

since cooling by the liquid metal would require insulating coatings.

● Use flow channel inserts made of SiC to decouple electrically and thermally the flowing LM from the load carrying walls.

● Use the volumetric heat generation in the breeder material to achieve an exit temperature ~100 K higher than the maximum structure temperature.

● Cool the FW box in toroidal direction, leading together with poloidal/radial grid plates to a strong segment box.

3

ADVANTAGES OF REPLACING LEAD LITHIUM IN THE DC BLANKET BY FLiBe

● No need for electrical insulation between breeder and structure since the electrical conductivity of the molten salt is orders of magnitudes lower than in the LM.

● No need for thermal insulator since the heat transfer from the MS flowing slowly in the large ducts is so low that the heat losses are negligible small.

● Better compatibility between breeder and steel anticipated.

● Lower fluid density results in lower static pressure.

DISADVANTAGES OF REPLACING LEAD LITHIUM IN THE DC BLANKET BY FLiBe

● Tritium breeding in FLiBe lower than in Pb-17 Li, requiring an additional neutron multiplier close to the FW. Candidate materials: Be or Pb

● Tritium solubility in the MS considerably lower than in Pb-17 Li, leading to higher T-partial pressure and permeation losses.

● Sufficient compatibility between FLiBe and ferritic steel under irradiation may require difficult REDOX control.

BASIC GEOMETRY OF THE DC BLANKET

● Torus subdivided into 16 sectors (shape and size similar to ARIES-RS)

● Each sector subdivided into one inboard and two outboard segments.

● Poloidal helium manifolds arranged behind the breeding zone.

● Entire steel structure cooled by helium, flowing first in the FW in toroidal direction and than in separation wall, grid plates and back plate in poloidal direction

● Design of FW box and FW cooling scheme very similar to the EU DC blanket

● FLiBe flowing with low velocity through poloidal ducts where volumetric heating rises the temperature at least 100 K above the maximum structure temperature.

● Two basic versions of the MS dual coolant blanket, depending on the neutron multiplier used.

DUAL COOLANT BLANKET WITH HELIUM-COOLED STEEL STRUCTURE, SELF-

COOLED FLiBe BREEDING ZONE, AND BERYLLIUM PEBBLE BEDS SERVING AS

NEUTRON MULTIPLIER

● Arrangement of the neutron multiplier: —

—

—

Use of Be as pebble beds to improve tritium release and allow for large swelling under neutron irradiation.

Arrange the pebble beds with a required thickness of 5 cm close to the FW.

Cool the Be pebbles directly with FLiBe flowing through the bed in radial direction (To extract the heat generated in the Be from the bed surface, at least one additional cooling plate immerged in the bed would be required).

(PRINCIPLE ARRANGEMENT OF Be PEBBLE BEDS AND ITS COOLING SIMILAR TO THE CASE OF SELF-COOLED FliNaBe

BLANKET, HOWEVER THE SINGLE-WALLED FW REPLACED BY A PLATE WITH

TOROIDAL COOLING CHANNELS)

FLiBe FLOW SCHEME

FLiBe Down

Be pebbles

FLiBe Up

MID-PLANE CROSS-SECTION OF THE DC Be PEBBLES MULTIPLIER CONCEPT

MODIFICATIONS OF THE MULTIPLIER REGION FROM Be TO Pb

● 5 cm thick lead layer arranged

directly behind the FW, enclosed and cooled by the FW plate and at the backside by a separation plate,

● All heat generated in the Pb flows into the helium, increasing the fraction of heat to be removed by helium,

● Helium flow scheme identical to the concept with beryllium multiplier,

● FLiBe flow scheme modified, up-flow in the two outer channels and down-flow in the central channel.

MID-PLANE CROSS-SECTION OF THE DC LIQUID Pb-LAYER MULTIPLIER CONCEPT

COMPARISON OF THE THREE CANDIDATE MOLTEN SALT BLANKETS, CONCLUSIONS

● Tritium self-sufficiency can be

achieved with all three concepts ● Re-weldability of the VV and sufficient

shielding of the coils requires for all three concepts the same combined thickness of blanket/shield/VV — —

1.20 m outboard 1.05 m inboard

● All three concepts allow for max. neutron wall load of 5.45 MW/m² and max. surface heat flux of 1 MW/m².

● Temperature limits for F 82 H ferritic steel (550ºC max and 550ºC at interfaces multiplier/steel as well as MS/steel) obeyed in all three concepts.

● Beryllium multiplier leads to higher energy multiplication

FliNaBe blanket with 7 cm Be: 1.27 FliBe (DC) blanket with 5 cm Be: 1.21 FliBe (DC) blanket with 5 cm Pb: 1.13

COMPARISON OF THE THREE CANDIDATE MOLTEN SALT

BLANKETS, CONCLUSIONS (Continued)

● Achievable MS exit temperature highest for DC concept with lead multiplier (T > 700°C) and lower for the two cases with Be multiplier (~650°C).

● Which of the three concept leads to the highest electricity generation for a given fusion power remains to be seen. The concept with lead multiplier allows for the highest MS exit temperature, but has on the other hand the smallest energy multiplication and extracts about 40% of the total heat with helium at lower temperatures. The self-cooled FLiNaBe concept achieves probably the lowest exit temperature, but has the highest energy multiplication, and the highest “low temperature” in the BRAYTON cycle. The guess is, that the product of the energy multiplication and the thermal efficiency will not be much different for the three concepts.

COMPARISON OF THE THREE CANDIDATE MOLTEN SALT

BLANKETS, CONCLUSIONS (Continued)

● MS flow scheme most simple in the case of DC with Pb multiplier, more complicated for the case of DC with Be multiplier, and most demanding for the self-cooled FLiNaBe blanket.

● However, the direct contact of MS with Be may facilitate the chemistry control (REDOX process).

● High pressure He-cooling of the steel structure in the DC blankets requires two different heat extraction systems. It offers, however, the inherent advantage to use the helium flow for pre-heating, as guard heating in case of idling, and as independent heat removal system in case of malfunctions in the MS loops.