Implications of the ARIES Study

for Steady State Tokamaks

Farrokh Najmabadi

for The ARIES Team

International Workshop on Steady-State Tokamaks

Princeton Plasma Physics Laboratory

January 5 – 7 1993

ARIES Is a Community-Wide Study

RPI

PPPL

ANL

Collaborations

ARIES

UCLA

GA

LANL

ORNL

U. Wis.

INEL

MDA

MIT

There Are Four ARIES Tokamak Reactor Designs

• ARIES-I is based on modest extrapolations in physics and ontechnology which has a 5 to 20 year development horizon often byprograms outside fusion (Completed 6/90).

• ARIES-III is an advanced fuel (D-3He) tokamak reactor(Completed 8/91).

• ARIES-II/IV is based on greater extrapolations in physicse.g., 2nd stability (To be completed 10/92).

Goal: A combination of economic competitiveness, a high level ofsafety assurance, and attractive environmental features.

The ARIES-II/IV Activity Is Examining

Reasonably-Consistent Advances in Physics

to Improve the Economics of the Reactor

and to Reduce

the Required Extrapolation in Technology.

ARIES-II/IV Designs Will Be Based on the SamePlasma Core but Two Distinct Fusion Power Cores

ARIES-II will use vanadium as the structural material and liquidlithium as the coolant to assess the potential of low-activationmetallic blankets.

ARIES-IV will use SiC composite as the structural material, Heas the coolant, Li2O as the solid breeder (instead of Li2ZrO3 inARIES-I). The aim is to further improve the safety features ofARIES-I and achieve an inherently safe reactor.

The ARIES-I Design Activity Has HighlightedHigh Leverage Areas

• The three major cost items are: (1) magnets, (2) current drive andrecirculating power, and (3) blanket and shield.

• The trade-off among MHD, bootstrap, and current drivedetermines the optimum steady-state reactor:

� To achieve a reasonable current-drive cost, a steady-state first-stability reactor should have high A and low I to achieve a highbootstrap current fraction.

� Because β is low, B should be high to have a reasonable fusionpower density (blanket and shield cost).

The ARIES-I Design Activity Has HighlightedHigh Leverage Areas

In order to improve the reactor economics over ARIES-I:

1. Increase β to reduce magnet cost =⇒ 2nd stability.

2. ∼100% bootstrap-current fraction to reduce current-drive cost=⇒ 2nd stability.

3. Higher fusion power density (wall loading) to reduce blanket andshield cost =⇒ First-wall neutron and heat flux concerns?

4. Improve safety and environmental features to achieve LSA 1=⇒ For ARIES-IV, eliminate W armor of divertor plate, usealternate breeders, and eliminate Be.

The Trade-off Among MHD, Bootstrap, andCurrent Drive Determines the Optimum Reactor

• ARIES-II: Second stability operation (qo = 2.).

⇓

• Requires profile control (FWCD and NBCD)

⇓

• Maximize bootstrap fraction, aiming at IBS/Ip = 1 (ε βP � 2.2√

ε).

⇓

• Beta value would be modest (4% to 6%) even though normalizedbeta is high (βN ∼ 6).

Major Parameters of ARIES Reactors

ITER-CDA ARIES-I ARIES-IV

Aspect ratio 2.8 4.5 4.0

Plasma major radius (m) 6.0 6.75 6.0

Plasma minor radius (m) 2.15 1.5 1.5

Toroidal field on axis (T) 4.85 11.3 7.6

Toroidal field on the coil (T) 11.5 21 16

Plasma current (MA) 22 10 6.6

Plasma beta 4% 1.9% 3.4%

Fusion power density (MW/m3) 1 3.8 4.0

Neutron wall loading (MW/m2) 1 2.5 2.8

Internal of ARIES-IV Tokamak Fusion Core

Equilibrium and Stability

• Stability to n = ∞ ballooning requires:qo = 2, q∗ >∼ 2.5, and δ95

>∼ 0.4.

• All examined equilibria appear to be unstable to n = 1 kink modeswithout a conducting wall.

• ARIES-II-IV equilibria is stable to n = 1 kink modes with aperfectly conducting wall at b/a <∼ 1.25

• It is assumed that plasma rotation would compensate for the finiteconductivity of the wall (V in ARIES-II, Be in ARIES-IV).

Current Drive

• 90% of plasma current (6.6 MA) is driven by self-sustainedbootstrap.

Method Power

Seed Current 0.48 MA ICRF Fast Wave (124 MHz) 20 MW

Overdrive Current 0.38 MA Lower Hybrid (8 GHz) 32 MW

Edge Current 0.04 MA Lower Hybrid (8 GHz) 15 MW

• A large current-drive power is still needed mostly to drive thecurrent at the plasma edge.

Comparison of Bootstrap and Equilibrium CurrentDensity Profiles

Transport and Divertors

• Confinement scaling (τE = HτL , H ∼ 3.0) consistent with DIII-DVH mode and high-βp discharges in TFTR and DIII-D.τp/τE ∼ 10 is assumed.

• Double-null gas-target divertors are utilized. Because of low coreplasma temperature and high density, high-recycling poloidaldivertors are alternatives.

Engineering Features of ARIES-II

• ARIES-II blanket is made of vanadium alloy (V-5Cr-5Ti)structural material with liquid lithium as the coolant and tritiumbreeder.

• ARIES-II blanket utilizes an insulating layer (TiN) which willreduce the MHD pressure drop by a factor of 15 (< 1 MPa).

• Because of the low pressure drop, the blanket design has beenoptimized toward heat transfer and simplicity.

• Because of high coolant outlet temperature, a gross thermalefficiency of ∼45% is estimated.

Engineering Parameters of ARIES-II FPC

Coolant inlet temperature (◦C) 330

Coolant outlet temperature (◦C) 610

Maximum structure temperature (◦C) 680

Coolant pressure (MPa) 0.5

Energy multiplication 1.4

Tritium breeding ratio 1.1

Recirculating power fraction 15%

Net plant efficiency 39%

Mass power density (kWe/tonne of FPC) 100

Engineering Features of ARIES-IV

• ARIES-IV blanket has used the ARIES-I design experience.

• ARIES-IV blanket is a He-cooled design with SiC/SiC compositestructural material, Li2O solid tritium breeder, and Be neutronmultiplier.

• SiC bulk material and B4C are used as the shield material.

• An advanced Rankine power conversion cycle as proposed forfuture coal-burning plants (49% gross efficiency).

• By eliminating the Li2ZrO3 breeder and W coating of the divertorplate, and minimizing Be inventory, a high level of safety assuranceis projected for ARIES-IV.

Engineering Parameters of ARIES-IV FPC

Coolant inlet temperature (◦C) 350

Coolant outlet temperature (◦C) 750

Maximum structure temperature (◦C) 950

Coolant pressure (MPa) 10

Energy multiplication 1.2

Tritium breeding ratio 1.1

Recirculating power fraction 19%

Net plant efficiency 40%

Mass power density (kWe/tonne of FPC) 120

Cost Comparison of ARIES Reactors (1992 U.S.$)

Cost of electricity (mills/kWh)

Without WithSafety Credits Safety Credits

ARIES-I 107 94

ARIES-II 82 71

ARIES-IV 85 63

Implications of ARIES Research for a Next-Step,Advanced Tokamak Experiment

• The current drive cost and recirculating power has a major impacton the attractiveness of a reactor. The only “non-speculative” andefficient current drive is bootstrap current and it is not totally free!(trade-off against plasma beta).

• Plasma parameters are inter-dependent. An attractive reactorrequires optimization of a self-consistent plasma as opposed tooptimization of a single plasma parameter (i.e., a plasma withhighest β does not necessarily result in an optimum reactor).

⇓

• Best tokamak is one with an MHD state with highest β that isconsistent with the constraint of approaching ∼100% bootstrapcurrent, acceptable confinement, acceptable ash exhaust, etc.

Implications of ARIES Research for theTechnology Program

• Achieving the potential safety and environmental features of fusionshould be the fundamental goals of the fusion technology andmaterial research.

• Material development program should emphasize materials thathave other applications (e.g., aerospace and automotive industries)to ensure that these materials are developed and have reasonablecosts.

• An extensive fusion technology and material R&D should beginNOW in order for these technologies to be tested in near-full scalein ITER and be ready for the fusion DEMO.