studies on open-cycle thorium fuel for present light water

Rainer Salomaa, Nordic-Gen4 Seminar at Risoe, 29-31 Oct. 2012

STUDIES ON OPEN-CYCLE THORIUM FUEL

FOR PRESENT LIGHT WATER REACTORS

R. Salomaa, P. Aarnio, J. Ala-Heikkilä, A. Isotalo,

J. Kuopanportti, L. Rintala, and R. Vanhanen

Aalto University School of Science, Department of

Applied Physics, Espoo, Finland

Contents


• Preface: Why thorium in Finland?

• Nuclear energy in Finland – Generation III+

• Pros and cons of thorium, assessment

• Radkowsky thorium fuel assemblies in PWRs

• Thorium cores of BWRs

• Concluding remarks

Thor from Norse mythology

Iceland,, 10th century

Why (and how) thorium fuel in Finland?

• A Finnish strategy for century-long commitment to nuclear energy:

4 NPP units operating, 1 under construction, 2 in procurement

phase; spent nuclear fuel waste repository under licensing

• Argument: there is not enough 235U to fuel the present LWRs?

• A sustainable solution by 238U-239Pu breeders

• An additional alternative by 232Th-233U breeders (involvement in 239Pu and/or MOX needed during transition phase)

• Long-term options all imply full-scale reprocessing → international

facilities, safeguards, fuel transport, waste management, etc.

• Replace part of uranium by thorium in present-type LWR cores

• Spin-off: E&T and R&D of reactor physics in new parameter ranges

• Domestic mineral resources (side streams inTalvivaara and Sokli)

• Is thorium any asset in Finnish open cycle fuel strategy?



Nuclear energy in Finland

25% of electricity by nuclear (2010)

Rainer Salomaa, Nuclear energy in Finland, 1.6.2012 5

Olkiluoto 1&2

BWR 2880MW

1979, 1982

2008: 14.4TWh, 95%

60a operation time Loviisa 1&2

PWR 2488MW

1977, 1981

2008: 7,7TWh, 90%

50a operation time Helsinki

Triga MkII

1962, Espoo


The 5th unit under construction

Commercial start in 2014?

www.tvo.fi

Environmental Impact Assessments and Applications for Decision

in Principle, Tendering of two further NPP Units

TVO, Olkiluoto 4

www.tem.fi Ministry of Employment and the Economy

Helsinki

Olkiluoto

Loviisa

Pyhäjoki

Fennovoima, Hanhikivi 1

Parliament ratified

DiPs for two NPP

units in 1.7.2010

Both projects under

Procurement phase


?

http://www.tem.fi/

Nuclear waste management in Finland

Olkiluoto Power Plant Loviisa Power Plant

Operational waste

TVO

Operational waste

Fortum

Spent fuel

Posiva

www.posiva.fi

Construction license in 2012, disposal starts in 2020


Posiva Oy


Nuclear involves very long-time commitment.

Time for emerging of Gen 4 and 5?


2000 2020 2040 2060 2080 2100 2120 2140

Fuel disposal of present units OL3fuel disposal

Constr. OL3 operation decommissioning

G4 DEMO PROTO-1 Generation 4

ITER DEMO PROTO COMM. FUSION

Constr.

Generation 3

Generation 4 Generation 5

Figures by tvo, gif, posiva, and iter


• Safety issues

Fuel: neutronics, thermal and chemical properties, burnup 100

MWd/kg, cladding performance, reactivity changes

(233Pa → 233U accumulation, burnable poison, B-shim), licensing

Reactor dynamics: thermal or fast, heat transport, full

Th-usage with different stages from 239Pu to 233U (life-cycle

assessment).

Decay heat for both shut-down and for intermediate fuel disposal.

• Nuclear waste: fission products vs. actinides. Is P&T practicable?

Modification of final repository

• Safeguarding: 232U a radiation barrier, 238Pu production, seed

enrichment (LEU the limit)

• Thorium technology: fuel, reprocessing, reactor, O&M. A new

NPP design is needed. Small nuclear countries have to rely on

international markets.

• Economic viability: price of 235U? BOC and EOC views

Very different opinions. Th is free, price of Pu?

Thorium as nuclear fuel – pros and cons

For the near future Th is not a bargain?


Thorium assemblies in present LWRs


Reactor physics calculations

• Academic problems with professional tools. Monte Carlo codes MCNP,

Serpent, FLUKA, and burnup codes CASMO, Serpent, Monteburns, DeTra

• Educational and training objectives

• Code development and validation

• A soft transition from present generation systems to a fully new fuel

structure: Th in single rods, fuel assemblies, and full core loads

• A single Radkowsky type seed blanket unit (SBU) in a PWR.

Comparision between Serpent and CASMO. Neutronics and burnup

calculations, comments on proliferation. Main author: Jaakko Kuopanportti

• Full Th-U core in a BWR. Possibilities for 3D enrichment and spectral

shifts. Core load optimization . Main author: Risto Vanhanen


Radkowsky thorium assembly in a PWR

• Use of thorium fuel in a commercial PWR UO2 core (4% enriched)

• Performance of a single Radkowsky thorium fuel assembly: 17×17 Westinghouse

PWR, SBU = separate blanket (BSA) and seed sub-assemblies (SSA); details as in

Todosov and Kazimi (2004)

• Seed blanket changed in 54 month cycles, blanket part for 254 mos

• Calculations by CASMO-4E and by a Monte Carlo code Serpent

• Role of soluble boron, typical values used here

tvo.fi


Neutron flux and fission rate distribution


150d/1st cycle/SBU 1500d/1st cycle/SBU

UO2 SBU

Relative power production of RTF and LWR assy


Relative power shares of the blanket and seed Relative power shares of the left and right side

of the UO2 assembly next to SBU. Average

relative power for assembly equals unity.

Second cycle in red.

Burnup and fissile production


Burnup for a 54-month cycle. Discharge

burnups (MWd/kg):

seed (1st cycle) 130.9 (129,5)

seed (2nd cycle) 124.4 (122,5)

blanket (1st cycle) 37.2 (37,7)

blanket (2nd cycle) 38.9 (39,7)

Casmo (Serpent)

Serpent predicts slightly more U233 production

than Casmo-4E; very little difference in

U235 depletion

Fuel integrity a challenge!

Proliferation aspects: Th has some visible advantages


UO2 discharge burnup is 45MWd/kg

and that of SBU 64.8 MWd/kg.

Spontaneous neutron rates 7.23,

4.46, and 4,12 (in 105/kgs) of seed,

Blanket, and UO2, respectively.

Isotopic mass fractions of the discharged uranium

Linear mass production per refueling Pu mass fractions at

discharge


Thorium fueled core in a BWR

Thorium simulation in a BWR


Parameters for a ”least change Th-scenario”


An equivalent Th BWR core load

• Challenges: spectral changes due to burn-

up and void distribution. A true 3D variation

of nuclide compositions. Absorbable poison

(Gd) common.

• Core load optimization is an extremely

demanding task. Here instead an

equilibrium cycle and control rod drive

pattern is used for reference.

• The goal to get the same energy and

neutron production during life-time

• Casmo-4E was used for lattice calculations,

Simulate-3 for full core calculations, nuclear

data comparisons by Serpent

• Six Th assemblies analysed : #0 reference,

#1 ’least changes’, #2 1010, #3 1212, #4

homogeneous and #5 heterogeneous low-

thorium assemblies, and the pure UO2 assy


Axial power distribution gentler in Th assemblies


Hot excess k-effective and shutdown margin


Smoother keff in Th scenarios

Shutdown margins improved in one Th assy

All full core reactivity feedbacks negative

Axial power peaking versus burnup at two vertical positions



Some findings

• Thorium fuel cycle provides interesting possibilities for a soft transition into

current LWRs. To obtain full benefits of Th-cycle a complete redesign of the

whole nuclear energy structure is needed.

• Single Radkowsky SBU technically feasible for PWRs: no excessive flux

variations. Reasonable breeding of 233U but at very high burnup → fuel

integrity, licenciability? Advantages concerning safeguards and waste

management demonstrated.

• Optimization of BWR Th fuel assemblies yet under development. Case

studies, however, demonstrate several interesting properties: less Gd

needed, reduction of power peaking and improved safety margins. Novel

axial enrichement designs worth further studies. However, at the

considered small burnups insufficient 233U breeding → need of highly

enriched 235U, no fissile savings!

• Th economy and sustainability require international reprocessing facilities.

Technically using thorium fuel in LWRs appears feasible, but

economically non-feasible - fuel costs are not a critical factor.

Such studies, however, provide useful R&D and E&T tasks during

the long term commitment to nuclear energy

Thank You!

Acknowledgements to Fortum, TVO and Academy of Finland


studies on open-cycle thorium fuel for present light water

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