CORE DESIGN FOR NEUTRON FLUX MAXIMIZATION IN RESEARCH REACTORS

Federico E. Teruel and Rizwann-uddinDepartment of Nuclear, Plasma and Radiological Engineering

University of Illinois at Urbana-ChampaignUrbana, IL 61801, USA

teruel@uiuc.edu; rizwan@uiuc.edu

Objective

Study different core configurations of a pool type multipurpose research reactor to:

• maximize the neutron flux delivered to the reflector,• produce an acceptable life cycle (~ 40 days),• allow irradiation positions in the inner reflector.

Reason

The intensity of neutron sources in a research reactor defines types of applications and rates of production. Competitiveness of the reactor.

Outline

Modern research reactors examples and neutron flux requirements.

Comments about FRM-II design.

Tools to model neutronic behavior in research reactors.

Our simplified model of FRM-II.

Alternative model and goals.

Compromise solution.

Neutron flux levels and life cycle of compromise solution.

Summary and conclusions.

Modern research reactors

FRM-II (Germany) OPAL (Australia)*

*Under construction

Research reactors overview

1.62020%U3Si2-Al3.2OPAL**

0.7100<20%UMo77.4JHR*

3.01020%U3Si23.0JRR-3M

1.73020%U3SiAl5.0HANARO

4.02093%U3Si2-Al8.0FRM-II

3.085N/AU3O8-Al25.5HFIR

Flux(th) *1013/MW

Power (MW)

EnrichmentFuelFlux(th)*1014

*Projected**Under construction

FRM-II

• Single, cylindrical fuel element (5.9cm to 12.15cm, 70cm height).• 113 involuted, curved fuel plates. • uranium silicide-aluminium dispersion (1.5g/cm3 and 3.0g/cm3). • Enrichment factor 93%. • Life cycle 52 days.

Computational tools to accurately model neutronic behavior of research reactors

Ready to use (BOC):

Detailed geometry

Cross sections

Accurate neutron fluxes

MCNP

Neutron Flux Maximization

Geometric constrains

ORIGEN

MONTEBURNS

Burnup calculations

Life cycle

Design

1/4 real core250 cycles of 1000 p.SD in keff < 0.0025

40 internal steps / 2 days

36 burnup regions,External steps / 2 days1 predictor corrector

MCNP simplified model of FRM-II

AL Meat HW

CR

• Core modeled as an homogeneous mixture,• Reflector modeled without any facilities,• Angular symmetry (non-θ dependence).

0.0E+00

2.0E+14

4.0E+14

6.0E+14

8.0E+14

1.0E+15

1.2E+15

1.4E+15

0 6 12 18 24 30 36 42

Radius (cm)

Neu

tron

Flux

(n/c

m2/

sec)

g

gg

ThermalFastFRM-II

Basic variations of FRM-II model

Constrains

• Inner irradiation zone.• Relatively long fuel cycle (> 40 days).• 20 % enrichment

Assumption

• 10 MW of power.• Meat U3Si2 of 4.8 gr/cm3, 20 % enrichment (fresh).• Outer reflector heavy water.• Core height 70 cm.• Core modeled as a homogeneous mixture.

Goals

• Unperturbed thermal peak greater than 4E14 n/cm2/sec (equivalent to 8E14 n/cm2/sec for 20 MW).• Inner irradiation zone greater than 350 cm2 (10 cm radius).• Life cycle greater than 40 days.

MeatBerylliumLight waterHeavy water

z = 0

z = 35z = 35

z = 70

Conclusion: for acceptable life cycles (i.e. greater than 40 days), maximum thermal neutron fluxes are far lower than the value expected as a goal.

Parameter: inner and outer core radii

Basic variations of FRM-II model

2.772.583.113.062.742.31MTF (*1014)0.91.10.70.81.11.4U5 ratio to FRM-II (Kg/Kg)1.071.121.021.031.101.15Multiplication factor202017181920Outer core radius (cm)171614151515Inner core radius (cm)VIVIVIIIIIICase

3.883.204.474.132.55MTF (*1014)0.10.50.20.31.0U5 ratio to FRM-II (Kg/Kg)

0.861.000.780.871.12Multiplication factor19.5-2019-20162023Outer core radius (cm)15-15.515-16151920Inner core radius (cm)

XIXIXVIIIVIICase

A compromise solution, asymmetric core model

• Thin cores produce high fluxes,• Thick cores produce acceptable life cycles,• Multipurpose Reactor → high thermal fluxes in one region of the reflector,• Other applications do not require such high thermal fluxes.

Asymmetric coremay produce theseresults Meat

BerylliumLight waterHeavy water

Asymmetric core modelParameters:• Thick core section inner radius• Thick core section outer radius• Thin core section inner radius• Thin core section outer radius• Angular aperture thin section

MeatBerylliumLight waterHeavy water

4.053.864.43.784.043.573.01MTF center thick section (*10E14)

0.871.561.731.741.962.122.2MTF center thin section (*10E14)

1.121.111.071.151.081.091.11Multiplication factor

18018018015015012090Angular aperture thin section ( º )

1719.51819.5181818Thin core outer radius (cm)

1618.51718.5171717Thin core inner radius (cm)

22222022202020Thick core outer radius (cm)

16161516151515Thick core inner radius (cm)

VIIVIVIVIIIIIICase

Asymmetric core (case VIII)

7External radius (cm)Inner irradiation zone

4.71U5 (kg)-20%70Height (cm)

28700Volume (cm3)Total core17Outer radius (cm)16Inner radius (cm)Core thin section22Outer radius (cm)16Inner radius (cm)Core thick

section

MeatBerylliumLight waterHeavy water

1.231Heavy Water from 0-7 cm1.230Be from 0-7 cm1.080Black absorber from 0-7 cm1.162Base case1.000Critical with Hf-CR from 11.0 to 11.4 cm

keffCase

Life cycle of 41 days withmarginal reactivity of 6.8% (Δk/k)

Thermal neutron flux for asymmetric model (z = 0 plane)

Distance (cm)

Ther

mal

neu

tron

flux

(n/c

m2/

sec)

-60 -30 0 30 60

Detailed thermal neutron fluxfor asymmetric model

MeatBerylliumLight waterHeavy water

East

South

North

0.0E+00

5.0E+12

1.0E+13

1.5E+13

2.0E+13

2.5E+13

3.0E+13

3.5E+13

4.0E+13

0 5 10 15 20 25 30 35 40 45 50 55 60Radius (cm)

TNF

per u

nit p

ower

(n/c

m2/

sec/

MW

)

ffffff

ffffff

FRM-II modelEast AM-VIIISouth AM-VIIINorth AM-VIII

Asymmetric compact core with shorter life

3.12U5 (kg)-20%70Height (cm)

19022Volume (cm3)Total core7Outer radius (cm)6Inner radius (cm)Core thin section14Outer radius (cm)6Inner radius (cm)Core thick

section

1.1418 holes of 2 cm of diameter of light water1.152Base case1.000Critical with Hf-CR from 11.0 to 11.4 cm

keffCase

Life cycle of 25 days withmarginal reactivity of 5.8% (Δk/k)

M e a tB e r y l l i u mL i g h t w a t e rH e a v y w a t e rA l u m i n u mH a f n i u m

M e a tB e r y l l i u mL i g h t w a t e rH e a v y w a t e rA l u m i n u mH a f n i u m

Thermal neutron flux for asymmetric compact core (z = 0 plane)

Distance (cm)

-60 -30 0 30 60

Ther

mal

neu

tron

flux

(n/c

m2/

sec)

Detailed thermal neutron flux forasymmetric compact core

MeatBerylliumLight waterHeavy water

East

South

0.0E+00

5.0E+12

1.0E+13

1.5E+13

2.0E+13

2.5E+13

3.0E+13

3.5E+13

4.0E+13

4.5E+13

5.0E+13

0 5 10 15 20 25 30 35 40 45 50 55 60Radius (cm)

TNF

per u

nit p

ower

(n/c

m2/

sec/

MW

)

v

FRM-II modelEast AM-IXSouth AM-IXNorth AM-IX

Summary and conclusions:

• The asymmetric design allows to reach an unperturbed thermal peak per unit power comparative to the FRM-II one. In addition, using L.E.U, the life cycle is conservatively estimated in 41 days.

• The asymmetric model produces a high thermal neutron flux zone ( ∼ 3.9E14 n/cm2/sec), moderate thermal neutron flux zone ( ∼ 2.4E14 n/cm2/sec), and low thermal neutron flux zone ( ∼ 1E14 n/cm2/sec).

• The design also presents a inner irradiation zone to irradiate materials that could be designed to be characterized by a higher fast/thermal flux ratio than the one obtained in the reflector.

• The asymmetric compact core produces a high thermal neutron flux zone ( ∼ 4.9E14 n/cm2/sec), moderate thermal neutron flux zone ( ∼ 4.0E14 n/cm2/sec), and low thermal neutron flux zone ( ∼ 3.2E14 n/cm2/sec). The life cycle is estimated in 25 days.

Usage of Research Reactors

Reactor theory, dosimetry, instrumentation

Teaching and training applications

In core and out core testing locations

Test and design of materials for use in fission and fusion reactors.

Material testing and irradiation

Location for huge crystals, stable and homogeneous flux

Neutron transmutation dopingIndustrial processing

Irradiation positionsProduction of radioisotopes for medical, military, and scientific purposes.

Isotope production

High flux, hot source or fissile material, beams

Fast neutron irradiation, neutron therapy

Fast and hot neutron sources

High flux, beamsStructure research, radiography, neutron therapy

Thermal Neutron Source

High flux, CNS, beamsStructure research, molecular motionCold Neutron Source

RequirementsPurpose

Burnup calculations I

1.06

1.07

1.08

1.09

1.1

1.11

1.12

1.13

1.14

1.15

1.16

1.17

0 5 10 15 20 25 30 35 40 45 50Time (days)

Mu

ltip

licat

ion

fac

tor

(kef

f)Core divided in 36 different regions,Steps for MCNP calculation every 2 days,Error in keff = 0.005, two SD

Burnup calculations II

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

0 5 10 15 20 25 30 35 40 45Radius (cm)

Th

erm

al n

eutr

on

flu

x ra

tio

South - 21d

South - 41d

North - 21d

North - 41d

East - 21d

East - 41d

Flux in cubes of 3 cm edges (too big),Critical calculation, then the peak is due to CR withdraw,Power in south region is slightly decreasing and power in north region isincreasing with burnup

Fast flux I (z = 0 plane)

Fast flux II

0.0E+00

1.0E+13

2.0E+13

3.0E+13

4.0E+13

5.0E+13

6.0E+13

0 5 10 15 20 25 30 35 40 45 50 55 60Radius (cm)

Fast

neu

tron

flux

per

uni

t pow

er (n

/cm

2/se

c/M

W)

East AM-VIII

South AM-VIII

North AM-VIII

-1378->4.0E13 n⋅cm-2s-1MW-1

10625842413>3.5E13 n⋅cm-2s-1MW-1

10742103921>3.0E13 n⋅cm-2s-1MW-1

11082367439>2.0E13 n⋅cm-2s-1MW-1

AM-IX areas as % of FRM-IIAM-IX

FRM-II simplified

model

Area in z = 0 plane (cm2)

Thermal flux comparison

1.04

1.05

1.06

1.07

1.08

1.09

1.1

1.11

1.12

1.13

1.14

1.15

1.16

0 5 10 15 20 25 30 35Time (days)

Mul

tiplic

atio

n fa

ctor

(kef

f)Burnup calculation compact core