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Analysis of ARC System for Gas-cooled Fast Reactor

Filip Osuský

Slovak University of Technology in Bratislava,

Faculty of Electrical Engineering and Information Technology

Ilkovičova 3

812 19, Bratislava, Slovakia

filip.osusky@stuba.sk

Lenka Dujčíková, Štefan Čerba, Gabriel Farkas, Branislav Vrban, Jakub Lüley

Slovak University of Technology in Bratislava,

Faculty of Electrical Engineering and Information Technology

Ilkovičova 3

812 19, Bratislava, Slovakia

lenka.dujcikova@stuba.sk, stefan.cerba@stuba.sk, gabriel.farkas@stuba.sk,

branislav.vrban@stuba.sk, jakub.luley@stuba.sk

ABSTRACT

The paper is focused on application of an assembly reactivity control (ARC) system for

gas-cooled fast reactor (GFR). The ARC system provides negative reactivity feedback

without damaging the neutron economy. Liquid/liquid system is used and the idea is that the

separate liquid pushes 6Li in to the core region after temperature increase. Potassium is

currently the best choice for the expansion liquid with a low solubility with lithium, a large

thermal expansion coefficient, a low neutron absorption cross-section, a low corrosion with

the cladding materials and is chemically stable under irradiation. Nowadays, second

recriticality of the fast reactor core is discussed based on the steady state neutronics

calculations. It is assumed that the molten core is relocated within fixed core boundaries and

new core compaction is responsible for second recriticality of the nuclear system. The

purpose of the ARC system is to mitigate such event occurrence and to overcome the issue of

too positive coolant temperature feedback and too large positive coolant void worth. The

analysis provides reactivity worth of system with different number and type of the ARC rods

within the fuel assembly by SCALE code. The investigated cases are after overpowered

transient where control rod devices are located above the core and during rapid temperature

increase.

1 INTRODUCTION

System safety for fast reactors is discussed within the framework of self-controllability

and self-terminability. Self-controllability of the fast reactor core is evaluated for the

abnormal events such as unprotected transient overpower (UTOP); unprotected loss of

primary coolant flow (ULOF) and unprotected loss of heat sink [1,2]. Overall reliability of

shutdown systems depends on the well-conceived design, manufacture, quality control,

prototype testing, on-line monitoring and surveillance [3].

Another important issue is to underline the difference between inherent safety and

passive safety. Inherent safety refers to the achievement of safety through the elimination or

exclusion of inherent hazards through the fundamental conceptual design choices made for

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Proceedings of the International Conference Nuclear Energy for New Europe, Portorož, Slovenia, September 5-8, 2016

the nuclear power plant. Reactor design in which one of the inherent hazards is eliminated is

inherently safe with respect to the eliminated hazard. On the other hand, the concepts of active

and passive safety describe the manner in which engineered safety systems, structures, or

components function are distinguished from each other by determining whether there exists

any reliance on external mechanical and/or electrical power, signals or forces. The absence of

such reliance in passive safety means that the reliance is instead placed on natural laws,

properties of materials and internally stored energy [4].

The paper continues previous work [5] and describes implementation of the ARC

passive safety systems for the gas-cooled fast reactor in the next sections.

2 DESCRIPTION OF COMPUTATIONAL MODEL

For the purpose of calculations, three-dimensional hexagonal model of the GFR was

developed using the KENO VI computational tool [6]. Cross section of the model is shown in

the Fig. 1. The model consists of the core, radial and axial reflector and safety rods system.

The vore is divided into two parts - inner core and outer core. Core height is 165 cm from

total reactor height, which is 500 cm. Gas plenum consisting of homogeneous mixture of

helium, rhenium, wolfram and silicon carbide is placed below and above fuel part. Height of

the upper gas plenum is 75 cm and height of the lower part is 50 cm. Core and the gas

plenums are surrounded by the axial and radial reflectors composed from the mixture of

Zr3Si2 and helium, wherein helium volume fraction reaches 40 % in the case of axial reflector

and 20 % in radial reflector. Height of axial reflector is 100 cm on the both sides of the model.

The radial reflector is arranged into 6 rings of hexagonal assemblies with height 500 cm.

Fuel zone is divided in two parts – the inner zone and the outer zone. Both consist of

heterogeneous hexagonal assemblies. The inner zone contains 264 fuel assemblies and the

outer zone 252 fuel assemblies. Array of 217 fuel pins is placed inside each assemblies and it

is enclosed in SiC tube with thickness of 0.2 cm. Space between pins is filled by helium

coolant. Pitch of fuel assembly is 17.529 cm (Fig. 2).

Figure 1: Model of GFR2400 Core [7]

Fuel pellets consist of mixture of plutonium carbide PuC and natural uranium carbide

UnatC. The inner and the outer fuel part differ in plutonium enrichment in the fuel – the inner

fuel consists of 14.12 % PuC in volumetric fraction and the outer fuel from 17.65 % of PuC.

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Proceedings of the International Conference Nuclear Energy for New Europe, Portorož, Slovenia, September 5-8, 2016

Higher content of plutonium in the outer core leads to higher breeding ratio in this part of the

core. Radius of the fuel pellet is 0.335 cm and it is wrapped with layer of wolfram and

rhenium with the thickness of 0.005 cm. The layer is supposed to retain fission products

inside the fuel assembly. The gap with the diameter of 0.0145 cm is located between the fuel

and the layer to prevent contact of the fuel and structural materials. The gap is filled by

helium. Fuel pin cladding is made from silicon carbide layer with the thickness of 0.103 cm

[8].

a) Fuel Assembly b) Fuel Pin

Figure 2: Design of the fuel

Computational model of GFR2400 includes system of the control and safety rods as

well. The system consists of 31 control assemblies, divided into 18 control safety devices

(CSD) and 13 diverse safety devices (DSD). Both CSDs and DSDs have the same material

composition and structure, they differ just by the location in core and by the method of use.

Control safety devices are used to control the reactivity during normal operation, while

diverse safety devices are used just to shutdown reactor in the case of emergency. Material

composition of the control devices can be found in Tab. 1. It should be noted that the sum of

material fractions is less than 1. The reason is that the rod follower will be inserted into the

control device during scram. Control rod pitch is same as for the fuel rod – 17.529 cm.

Control rods are arranged in three rings. Positions of all devices are shown in the Fig. 3,

where red assemblies stand for DSD and white assemblies for CSD.

Figure 3: Positions of CSD and DSD Assemblies [7]

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Proceedings of the International Conference Nuclear Energy for New Europe, Portorož, Slovenia, September 5-8, 2016

Table 1: Homogeneous Composition of Control Rod

Component Material Volume Fraction

Absorber B4C 0.3026

Cladding SiCf 0.1058

Coolant He 0.4057

Structural materials AIM1 steel 0.1122

In the model, helium is used as a coolant. Average temperature of the helium during

normal conditions is 913.6 K and fuel average temperature reaches value of 1263.16 K.

Pressure during the normal operation is 7 MPa.

3 ARC SYSTEM DESCRIPTION

3.1 General description

The main idea of the ARC safety system (Fig. 4.) is replacement of one or more fuel

pins by the ARC injection rods with minimal change to each fuel assembly. Liquid reservoir

with neutronically transparent liquid is located in the upper part of the fuel assembly. The

ARC injection rod consists of two concentric tubes where the inner tube is filled with

potassium and the outer tube with argon. The lower reservoir contains a dual-layer of liquids

with floating 6Li on the potassium. The absorber in the form of 6Li is pushed in to the outer

tube with the temperature increase by the thermal expansion of potassium (Fig. 5.) [8].

Different speed of the control system actuation can be achieved by changing of diameter for

inner and outer tube.

ARCs design is cost-effective and minimizes the inventory of neutron-absorbers. If the

failure of the system occurs, there is no positive reactivity insertion into the core [9].

Figure 4: Concept of ARC [10]

Figure 5: ARC system during

operation [10]

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Proceedings of the International Conference Nuclear Energy for New Europe, Portorož, Slovenia, September 5-8, 2016

3.2 Proposed cases

The 60-degree symmetric core includes 516 fuel sub-assemblies each with 217 pins.

The proposed cases consist of 4-6 ARC pins and the rest are the fuel pins, shown in Fig. 6 and

Fig. 7. Different radius of the concentric cylinders in the ARC pins is also used for the

achievement of different actuation speed of the safety system. Each case with different

number of the ARC pins was calculated with following dimensions of the concentric

cylinders (Fig. 8):

1) 1 mm effective radius for 6Li: r1 = 1.85 mm, r2 = 2.55 mm, r3 = 3.55 mm,

2) 0.7 mm effective radius for 6Li: r1 = 2.15 mm, r2 = 2.85 mm, r3 = 3.55 mm,

3) 0.4 mm effective radius for 6Li: r1 = 2.45 mm, r2 = 3.15 mm, r3 = 3.55 mm,

where purple colour represents potassium expansion liquid, grey colour stands for

cladding and white colour is area where during normal operation is argon and during

temperature increase beyond normal operation, the expansion of potassium inserts liquid

absorber in the form of 6Li into the core region.

a) 4 ARC pins b) 5 ARC pins

Figure 6: Design of the fuel assembly with the ARC system

Figure 7: Design of the fuel assembly with 6

ARC pins

Figure 8: Design of the ARC pin

The assumed inner pressure of the ARC system is 3 MPa to prevent collapse caused by

outer coolant pressure of 7 MPa. The densities of particular materials are shown in Tab. 2.

r1

r3

r2

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Proceedings of the International Conference Nuclear Energy for New Europe, Portorož, Slovenia, September 5-8, 2016

Table 2: Densities of particular materials

Material Density [g/cm3]

6Li 0.475 39K 0.695

40Ar 0.00178

4 RESULTS

The used neutronic code is SCALE with continuous energy spectrum of neutrons. The

used cross-section library is ENDF/B-VII.0. The number of simulated neutron generations

were 500 with 10000 generated neutrons. The results of the calculation are shown in Tab. 3.

The case with all control rods withdrawal above the core was taken as a reference case. The

reason is better statistics parameters of calculations. As expected, the highest negative

reactivity insertions for particular number of the ARC pins is achieved for the case 1) with

effective radius 1 mm. On the other hand, the lowest negative reactivity insertion is achieved

for the case 3) with effective radius 0.4 mm. However, the actuation speed of the case 3) is the

highest.

Table 3: Results of k-eff for various cases

Number of the ARC pins: 4

Effective radius[mm] k-eff

Δρ [pcm] Reference case Safety system fully actuated

1 1.0137 ± 0.00037 0.99558 ± 0.0003 1795

0.7 1.01348 ± 0.00029 0.99984 ± 0.00032 1346

0.4 1.01328 ± 0.00033 1.0048 ± 0.00027 832

Number of the ARC pins: 5

Effective radius [mm] k-eff

Δρ [pcm] Reference case Safety system fully actuated

1 1.0118 ± 0.00027 0.99014 ± 0.0003 2162

0.7 1.01173 ± 0.00028 0.99519 ± 0.00031 1643

0.4 1.012 ± 0.00032 1.00175 ± 0.00033 1011

Number of the ARC pins: 6

Effective radius [mm] k-eff

Δρ [pcm] Reference case Safety system fully actuated

1 1.01031 ± 0.00032 0.98412 ± 0.00028 2634

0.7 1.01063 ± 0.00031 0.99068 ± 0.00031 1993

0.4 1.01054 ± 0.00031 0.99817 ± 0.00028 1226

In Fig. 9 is shown a sum of proportional neutron flux distribution within each

incorporated ARC pin for some selected cases. The main changes of the neutron flux withing

the cases are located in the energy region 73 keV - 400 keV. This region is highly dependent

from resonances on 40Ar and 39K (Fig. 10). There are no resonances on 39K for energies higher

than 200 keV. The absorption of neutrons is higher for 40Ar in the energy region higher than

200 keV and it can result in the positive reactivity insertion. This effect is reflected in cases

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Proceedings of the International Conference Nuclear Energy for New Europe, Portorož, Slovenia, September 5-8, 2016

with 5 and 6 ARC pins during reference case where lowering of the 40Ar content increases the

reactivity of the system. The reactivity increase when the effective radius decrease is almost

negligible. In the case for 5 ARC pins when the effective radius decreases from 0.7 to 0.4 mm

is 26 pcm and in the case with 6 pins, the increase is 31 pcm when the effective radius

decreases from 1.0 to 0.7 mm.

Figure 9: Proportional neutron flux distribution (RC - reference case)

Figure 10: Total cross-section for 40Ar and 39K at 300 K [11]

5 CONCLUSION

Application of the ARC system was shown. The highest insertion of negative reactivity

is achieved in the case with biggest volume insertion of 6Li as expected. Different actuation

speed can be achieved with different radius of the concentric cylinders in the safety systems.

The application of the ARC system may be difficult due to complexity of the safety systems.

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Proceedings of the International Conference Nuclear Energy for New Europe, Portorož, Slovenia, September 5-8, 2016

Also monitoring of the availability of the systems can be an issue. However, there is no

positive reactivity insertion during the failure of the ARC system.

In the future is necessary to evaluate actuation speed of proposed cases together with

thermo-hydraulic and pressure evolution within the system. Proposed passive system is very

promising, however there is still need for improvement and simplification of the system.

ACKNOWLEDGMENTS

Authors thank to the STU Grant scheme for Support of Young Researchers No. PoDi

1333 for support.

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