Conceptual nuclear design of a 20 MW multipurpose research reactor

Nuclear Science and Technology, Vol. 4, No. 1 (2014), pp. 26-35

Conceptual Nuclear Design

of a 20 MW Multipurpose Research Reactor

Nguyen Nhi Dien, Huynh Ton Nghiem, Le Vinh Vinh, Vo Doan Hai Dang

Reactor Center – Nuclear Research Institute – Vietnam Atomic Energy Institute

01 Nguyen Tu Luc, Dalat, Lamdong, Vietnam

Seo Chulgyo, Park Cheol, Kim Hak Sung

Korean Atomic Energy Institute,

150 Dukjin-dong, Yuseong-gu, Taejeon 305-353, Korea

(Received 5 March 2014, accepted 10 March 2014)

Abstract: This paper presents some of studied results of a pre-feasibility project on a new research

reactor for Vietnam. In this work, two conceptual nuclear designs of 20 MW multi-purpose research

reactor have been done. The reference reactor is the light water cooled and heavy water reflected

open-tank-in-pool type reactor. The reactor model is based on the experiences from the operation and

utilization of the HANARO. Two fuel types, rod and flat plate, with dispersed U₃Si₂-Al fuel meat are

used in this study for comparison purpose. Analyses for the nuclear design parameters such as the

neutron flux, power distribution, reactivity coefficients, control rod worth, etc. have been done and the

equilibrium cores have been established to meet the requirements of nuclear safety and performance.

Keywords: HANARO, AHR, MTR, MCNP, MVP, HELIOS, dispersed U₃Si₂-Al, open-tank-in-pool,

equilibrium core, BOC, EOC, shutdown margin.

has considerable experience in the research

reactor technology through the design,

construction, operation and utilization of the

High-flux Advanced Neutron Application

Reactor (HANARO) of 30 MWth. Therefore,

in the framework of the joint study on the pre-

feasibility of MRR with KAERI, a model of

Advance HANARO Reactor (abbreviated as

AHR) has been developed to meet the

requirements for use in the future [3,4]. Based

on the model of AHR, a similar reactor model

with plate fuel type MTR (abbreviated as

MTR) has been also developed for the purpose

of comparison between the two fuel types.

I. INTRODUCTION

Research reactor has been widely

utilized in various fields such as industry,

engineering,

medicine,

life

science,

environment, etc., and now its application

fields are gradually being expanded together

with the development of its technology. The

utilization of a research reactor is related to the

necessary and essential technologies of

information technology, nano-technology,

biotechnology, environmental technology and

space technology. Hence, R&D in the area of

research reactor utilizations has a large effect

on the growth of a national industry.

II. NUCLEAR DESIGN REQUIREMENTS

A. General

Vietnam has a plan to construct a high

performance multipurpose research reactor

(MRR) to satisfy increasing utilization

demands. So, the pre-feasibility studies to build

a new MRR have been set [1,2]. The Korea

Atomic Energy Research Institute (KAERI)

A research reactor should be designed in

conformity with user's requirements. The

reactor type, power, and core configuration,

systems and the installed experimental

NGUYEN NHI DIEN, HUYNH TON NGHIEM, LE VINH VINH, VO DOAN HAI DANG

facilities depend on the application purposes

and on the construction and operation costs as

1) The neutron flux variation at the irradiation

sites and the nose of the beam tubes should be

stable with a 5% variation regardless of a

loading or unloading of samples.

well. Hence,

a

flexible design is an

indispensable feature when considering a

future expansion of its experimental facilities.

2) The axial neutron flux gradient in the

reflector region should be within ±20% over a

length of 50 cm.

The major basic principles to develop

models of the conceptual design are as follows.

1) Multipurpose research reactor with a

medium power

3) The maximum fast and thermal neutron

fluxes at an irradiation site inside the core

should be greater than 1.3x10¹⁴and 4.0x10¹⁴

n/cm²-s, respectively. The maximum thermal

neutron flux at the reflector region should be

greater than 4.0x10¹⁴n/cm²-s.

2) High ratio of flux to power

3) High Safety and Economics

4) Sufficient spaces and expandability of the

facility for various experiments

4) The maximum local power peaking factor

should be less than 3.0.

Fundamentally, a research reactor should

be designed to achieve the established safety

objectives such as the IAEA standards. The

nuclear design requirements for the AHR and

MTR are considered in two parts, functional

and performance requirements.

5) The average discharge burn-up of the fuel

assembly should be higher than 50% of the

initial fissile heavy material, U-235.

6) The reactor operating cycle should be

longer than 30 days.

B. Funtional Requirements

The functional requirements aim to

ensure the safety of the reactor and ready to

operate in all conditions.

III. CORE CONCEPT

The basic concepts of the reactor are the

light water cooled and moderated, heavy water

reflected, open-tank-in-pool type research

reactor and 20 MW power cores loaded with

two typical geometric kinds of fuel elements as

rod or flat plate.

1) The power coefficient and temperature and void

coefficients of the reactivity should be negative for

all operational and accident conditions.

2) The shutdown margin should be at least 10

mk (1mk = k/k  1/1000) regardless of any

changes in the reactor condition.

A. Fuel

3) The second reactor shutdown system should

be prepared to improve the reactor safety and

its shutdown margin should be at least 10 mk

for all relevant design basis fault sequences.

Fuels selected for the design are

commercial or commercial available. The fuel

meat is fabricated by a dispersion of high

density U₃Si₂particles into pure Al with its

uranium enrichment 19.75 wt%. Two kinds of

fuel assemblies in the core are standard fuel

assembly and control fuel assembly (including

control rods inside fuel assembly). Some

specifications of the fuel elements and

assemblies are listed in Table I and their cross

sectional views are showed in Figure 1.

4) The excess reactivity should be at least 10

mk at the end of cycle for conducting

experiments and 15 mk for the Xe override.

C. Performance requirements

The performance requirements aim to

ensure meeting the requirements of use and

high economic efficiency.

27

CONCEPTUAL NUCLEAR DESIGN OF A 20 MW MULTIPURPOSE RESEARCH REACTOR

Table I. Specifications of the fuel element and assembly

Fuel element

66.0w/%U, 5.2w/%Si,

28.8w/%Al

72.8w/%U, 6.0w/%Si,

21.2w/%Al

Meat content

Fuel length (mm)

700.0

Fuel diameter/widththickness (mm)

6.35/5.49 (In/Out)

64.0/51.4x0.61 (S/C)*

Fuel density (g/cm³)

Cladding thickness (mm)

Cladding material

Fuel assembly

6.06

0.76/1.19 (In/Out)

Al

6.6

0.37/0.445 (In/Out)

Al

Square

21/17 (S/C)

Shape

Element number

Hexagonal

36/18 (S/C)

* S/C: Standard fuel assembly / Control fuel assembly

a) AHR standard

b) AHR Control

c) MTR standard d) MTR control

Fig. 1. Cross sectional view of AHR and MTR standard and control fuel assemblies

B. Core Arrangement

core. The reactor regulating system shares

control rods with the reactor protection system.

Fig. 2 shows the horizontal cross sectional

view of the AHR and MTR cores. Some

specifications of the cores are listed in Table II.

The core has 23 lattices that consist of

fourteen standard assemblies, four control

assemblies and three in-core irradiation sites.

The heavy water reflector tank of 200 cm in

diameter and 120 cm in height surrounds the

Fig. 2. The horizontal cross sectional view of the AHR and MTR cores

28

NGUYEN NHI DIEN, HUYNH TON NGHIEM, LE VINH VINH, VO DOAN HAI DANG

Table II. The specifications of the cores

Reactor type

AHR

MTR

3

1199.5 x 70

1527.7 x 70

Core volume (cm )

Fuel assembly Number

Control rod Number

Absorber material

Total weight U-235 (kg)

In-core irradiation sites

16 S + 4 C

4

Hf

9,87

3

4

Hf

10,12

3

Fig. 3. The layout of the experimental sites of the AHR and MTR

IV. NUCLEAR ANALYSIS

distribution, the reactivity of the core and the

reactivity worth of control rods were also

assessed to meet the requirements. Two core

configurations with one and three in-core

irradiation sites were proposed. Although the

first configuration (with one irradiation site) is

better in the fuel saving point of view, the

configuration with three in-core irradiation

sites was selected to meet predicted utilization

of in-core irradiation in the future.

To confirm that the conceptual cores

satisfy the functional and performance

requirements, nuclear analyses are performed

for fresh core and equilibrium core with

several code systems such as MCNP [5], MVP

[6], HELIOS [7], etc.

A. Fresh Core

The basic analysis of the core

characteristics was performed for the fresh core

with and without irradiation facilities.

As the ultimate goal of a research reactor

is its utilization, the irradiation facilities should

be designed in conformity with the user's

requirements. The required irradiation facilities

should be located at proper positions to

maximize neutron utilization and minimize

reactivity effect. Based on the neutron flux

distribution of the reflector region, the

arrangement by their purposes has been studied

to achieve the objectives above. Their

The core configuration should be

designed to meet the functional and

performance requirements. The neutron flux at

the in-core irradiation sites and the reflector

region of the cores without irradiation facilities

was calculated by the MCNPX code [8] using a

mesh tally. On the other hand, the power

29

CONCEPTUAL NUCLEAR DESIGN OF A 20 MW MULTIPURPOSE RESEARCH REACTOR

reactivity worth is considered as a priority

because of the influence to the reactor core.

Various layouts of the irradiation facilities

were proposed, and one of them was selected.

To evaluate the stability of neutron flux at the

irradiation sites, their neutron fluxes were

calculated when the control rods are located at

300 mm and fully withdrawn.

The reactivity effect by the irradiation

facilities was estimated to be 20.2 mk and 28.9

mk and the total control rods worth 182.4 mk

and 217.7 mk for AHR and MTR, respectively.

Table III shows the neutron fluxes at the

irradiation facilities. Figure 4 presents the

thermal and fast neutron distribution of the

AHR fresh core.

Table III. Neutron fluxes at the experimental sites

Neutron flux [n/cm2/sec]( Thermal<0.625eV, Fast>1.0MeV)

AHR MTR

Maximum

Thermal Fast

Average

Thermal Fast

Maximum

Thermal Fast

Average

Thermal Fast

CT 4.46E+14 1.46E+14 3.04E+14 9.80E+13 4.01E+14 1.13E+14 2.87E+14 8.06E+13

IR1 3.21E+14 1.18E+14 2.23E+14 8.29E+13 3.37E+14 9.31E+13 2.49E+14 6.76E+13

IR2 3.16E+14 1.20E+14 2.23E+14 8.26E+13 3.33E+14 9.16E+13 2.46E+14 6.65E+13

CNS 8.71E+13 1.15E+12 7.01E+13 8.76E+11 8.49E+13 1.69E+12 6.48E+13 1.24E+12

ST1 1.37E+14 1.96E+12

ST2 2.40E+14 3.47E+12

NR 1.28E+14 3.20E+11

-

1.40E+14 3.23E+12

1.79E+14 1.01E+13

1.28E+14 1.32E+12

-

NTD1 4.74E+13 1.13E+11 4.31E+13 8.12E+10 4.93E+13 4.19E+11 4.26E+13 3.19E+11

NTD2 4.63E+13 9.91E+10 4.24E+13 7.60E+10 5.29E+13 4.84E+11 4.57E+13 3.56E+11

NTD3 5.16E+13 2.43E+11 4.70E+13 2.04E+11 4.64E+13 5.21E+11 3.93E+13 3.78E+11

HTS1 6.96E+13 3.42E+11 5.96E+13 2.71E+11 7.02E+13 6.30E+11 5.79E+13 5.03E+11

HTS2 2.23E+13 2.07E+10 1.93E+13 1.39E+10 2.25E+13 2.81E+10 1.97E+13 2.29E+10

NAA1 1.39E+14 4.96E+11 1.20E+14 3.88E+11 1.22E+14 8.15E+11 1.05E+14 6.27E+11

NAA2 4.11E+13

NAA3 1.74E+13

-

3.59E+13

1.52E+13

-

4.00E+13

1.53E+13

-

3.55E+13

1.35E+13

-

RI1 3.53E+14 1.47E+13 2.60E+14 9.05E+12 2.31E+14 1.49E+13 1.69E+14 9.28E+12

RI2 3.44E+14 1.42E+13 2.57E+14 8.91E+12 2.18E+14 1.47E+13 1.58E+14 9.13E+12

RI3 2.46E+14 4.03E+12 1.85E+14 2.54E+12 2.10E+14 1.53E+13 1.58E+14 9.56E+12

RI4 2.48E+14 4.23E+12 1.86E+14 2.82E+12 2.03E+14 1.45E+13 1.52E+14 8.92E+12

RI5 2.24E+14 3.12E+12 1.67E+14 2.10E+12 2.15E+14 1.55E+13 1.58E+14 9.51E+12

30

NGUYEN NHI DIEN, HUYNH TON NGHIEM, LE VINH VINH, VO DOAN HAI DANG

a) Thermal Neutron Flux

Fig. 4. Neutron flux profile at the AHR fresh core

b) Fast Neutron Flux

B. Equilibrium Core

6-batch core) are assessed. The 9-batch cores

show a high discharge burnup and a good fuel

economy, but the cycle lengths are less than 30

days. They look proper for a low utilization

condition of the reactor. The 6-batch cores with

a cycle length greater than 30 days are suitable

for the design requirements, so they are

selected for evaluating in detail. In the 6-batch

core, three of the standard fuel assemblies or

two of the standard fuel assemblies and two of

the control fuel assemblies are replaced for an

operation cycle, so the whole core will be

replaced for 6 cycles according to the loading

strategy. There are many loading patterns that

they depend on the fuel management strategy.

The loading pattern showed in Table IV is

evaluated in detail.

An equilibrium core is dependent on an

operation strategy, so there may be various

equilibrium cores according to a reactor

operating strategy. In this report, an

equilibrium core is proposed and analyzed to

meet the established design requirements.

Fuel Management

A candidate model for an equilibrium

core can be easily obtained by considering

target discharge burnup, cycle length and

excess reactivities at begin of cycle (BOC) and

end of cycle (EOC). There are many candidate

models according to the number of reloaded

fuel assemblies and the loading pattern. The

equilibrium cores with 2 or 3 fuel assemblies

reloaded for an operation cycle (the 9-batch or

Table IV. Loading location of the fuel assemblies for 6-batch cores

Cycle

Assembly Number

(standard+control)

Loading Location

MTR

AHR

H14,H16,C1,C3 H9,H12,C1,C3

H8,H10,H12 H14,H15,H7 (move H14,H15,H7 to

H2,H4,H6)

1

2

2+2

3+0

31

CONCEPTUAL NUCLEAR DESIGN OF A 20 MW MULTIPURPOSE RESEARCH REACTOR

3

3+0

H7,H9,H11

H13,H10,H16 (move H13,H10,H16 to

H3,H5,H1)

4

5

2+2

3+0

H13,H15,C2,C4 H8,H11,C2,C4

H2,H4,H6

H14,H15,H7 (move H14,H15,H7 to

H2,H4,H6)

6

3+0

H1,H3,H5

H13,H10,H16 (move H13,H10,H16 to

H3,H5,H1)

Once a cycle length and a loading

pattern are determined, an equilibrium core is

obtained by numerical iterations. The initial

core is loaded with the new FAs then the

burnup calculations are iterated by the loading

pattern until the parameters of burnup and

reactivity are stable over 6 cycles. Table V

presents the calculated results of the average

burnup and reactivity of 6 cycles for different

cycle lengths. From these results, it can be

concluded that the 36 days cycle for AHR and

34 days cycle for MTR meet the performance

requirements.

Table V. Burnup and reactivity of the equilibirum cores

Reactor type

AHR

36

MTR

34

Cycle Length (days)

Average Burnup (%U-235)

- BOC

35

37

33

35

23.43 24.02 24.61 22.38 23.04 23.70

31.82 32.65 33.47 29.08 29.94 30.81

50.35 51.77 53.18 48.65 49.91 51.17

- EOC

- Discharge

Reactivity (mk)

- BOC (no Xe)

- Fuel Depletion

- Xenon Buildup

- Power Defect

- EOC (eq. Xe)

- Shutdown Margin

111.9 109.9 107.8 87.8 85.8 83.6

37.5

38.1

3.0

38.7

38.1

3.0

39.9 15.1 16.7 18.3

38.0 36.2 36.3 36.3

3.0

33.4

15.0

30.1

17.1

26.9 33.5 29.8 26.0

19.6 22.2 24.2 26.4

Power Distribution

6 cycles at a 300 mm position of the control

rods was calculated. Table VI shows maximum

total peaking factors for the 6 cycles equilibrium

cores and Table VII shows the power

distributions and peaking factors at the cycle

that total peaking factor reaches the maximum

value. The maximum local power peaking factor

for AHR and MTR are 2.56 and 2.79

respectively.

The power distribution is strongly

dependent on the positions of the control rods

and it was checked for all possible positions at

5 cm intervals. The largest maximum linear

power of the equilibrium cores was observed at

a 300 mm position of the control rods. The

power distribution for the equilibrium cores of

30

32

NGUYEN NHI DIEN, HUYNH TON NGHIEM, LE VINH VINH, VO DOAN HAI DANG

Table VI. Maximum total peaking factor for the equilibrium cycles

Cycle

Reactor

type

Parameter

1

2

3

4

5

6

Position of FA

H02

C2

H01

2.5

H03

2.46

H11

2.76

C3

H04

2.49

H01

2.79

AHR

MTR

Fq(peaking factor) 2.47

2.56

H04

2.77

2.56

H04

2.76

Vị trí FA

H09

2.69

H01

2.74

Fq

Table VII. Power distribution and peaking factor for equilibrium cores

(cycle 5 for AHR, cycle 6 for MTR)

AHR

Total Power (kW)

MTR

Total Power (kW)

Location

Fq

H01

H02

H03

H04

H05

H06

H07

H08

H09

H10

H11

H12

H13

H14

H15

H16

C1

1117

1061

1314

897

1064

1305

811

1071

1329

1063

1088

1298

1224

991

1184

1050

577

1.93

1.72

2.21

1.58

1.73

2.18

1.38

1.64

2.37

1.76

1.65

2.29

1.78

1.4

1.57

1.42

2.44

2.04

2.56

1.95

1167

1158

1229

1092

1223

1143

1033

1179

985

1066

1213

987

1137

1093

1170

1174

445

2.79

1.82

1.96

2.51

2.02

1.82

2.12

2.21

1.67

2.31

2.26

1.66

2.01

1.91

2.05

1.96

1.28

1.55

1.29

1.56

C2

C3

C4

491

589

476

527

449

531

Reactivity Coefficients

region is to cool the fuel assemblies, and so

called a ‘coolant’ and the light water in the

gaps of the flow tubes is called a ‘moderator’.

Nuclear characteristics of these two light water

regions are somewhat different, and a heat

transfer between them is small. Therefore, their

temperature variations following a power

change are also different, thus the respective

temperature coefficients were computed

separately. The effect of a spectrum hardening

To affirm the inherent safety, the

reactivity coefficients should be determined.

They include temperature coefficients of fuel,

light water and heavy water. Physical changes

of water due to a temperature change could be

considered in two ways: one is a density

change, and the other is a cross section change

for a nuclear reaction. There are the gaps of the

flow tubes for AHR. The light water in the fuel

33

CONCEPTUAL NUCLEAR DESIGN OF A 20 MW MULTIPURPOSE RESEARCH REACTOR

of neutrons following a temperature increase

for heavy water is so small that it can be

negligible. Table VIII presents the result of

temperature and void coefficients. From this

result, they are negative (except temperature

coefficient of moderator. where almost of

arriving neutrons are slowed down) and meet

the functional requirements. The temperature

variation of moderator is so small, therefore its

contribution to power coefficient is small.

Table VIII. Reactivity coefficients of temperature and void

Parameter

Fuel temperature coefficient (mk/K)

Light water temperature coefficient (mk/K)

- Coolant

AHR

<-0.002 <-0.02

MTR

-0.059

0.06

-0.11

- Moderator

Light water void coefficient (mk/%)

0 - 5 %

-1.23

-1.37

-1.48

-1.79

-1.97

-2.25

5 - 10 %

10 - 20 %

Heavy water void coefficient (mk/%)

0 - 5 %

-1.26

-0.79

V. CONCLUDING REMARKS

the temperature coefficients are negative

showed the inherent safety feature. The

parameters for utilization and for the safety

aspects of the reactor respectively meet the

performance and functional requirements.

From the functional and performance

requirements, two reactor models AHR and

MTR were proposed and investigated. The

reference reactors are the light water cooled

and moderated, heavy water reflected and

open-tank-in-pool type research reactors with a

20 MW power.

The comparison of cores loaded with 2

different fuel types, AHR and MTR, shows that

the AHR fuel type core has a little longer

operation cycle and higher discharge burn up as a

result. In the safety point of view, the MTR core

has an advantage because of shutdown margin,

temperature and coolant void coefficients are

higher compared to those of AHR core.

The maximum fast and thermal neutron

flux in the core region are greater than 1.0×10¹⁴

n/cm²s and 4.0×10¹⁴n/cm²s, respectively. In

the reflector region, the thermal neutron peak

occurs about 28 cm far from the core center

and the maximum flux is estimated to be

4.0×10¹⁴n/cm²s.

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KAERI/VAEC pre-possibility study on a new

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average discharge burnup is greater than 50%.

For the proposed fuel management scheme, the

maximum peaking factor Fq is less than 3. The

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shutdown systems are greater than 10 mk and

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NGUYEN NHI DIEN, HUYNH TON NGHIEM, LE VINH VINH, VO DOAN HAI DANG

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35