Description of TRIGA Reactor
 

M. Ravnik

 

Provided are basic technical and operational characteristics of TRIGA Mark II reactor at J. Stefan Institute. More detailed description is found in the following documents:

1. Varnostno porocilo za reaktor TRIGA Mark II v Podgorici, Revizija 3, IJS-DP-5823, junij 1992

2. I. Mele, M. Ravnik, A. Trkov, “TRIGA Mark II Benchmark Experiment, Part I: Steady-State Operation,” Nuclear Technology 105, 37-51, 1994.

3. I. Mele, M. Ravnik, A. Trkov, “TRIGA Mark II Benchmark Experiment, Part II: Pulse Operation,” Nuclear Technology 105, 52-58, 1994.

4. M. Ravnik, T. Žagar, A. Peršič, Fuel Element Burnup Determination in Mixed TRIGA Core Using Reactor Calculations", Nuclear Technology 128, 35-45, 1999.
 
 

General Description of the Reactor

The reactor is a typical 250-kW TRIGA Mark II light-water reactor with an annular graphite reflector cooled by natural convection. The side and the top views of the reactor are shown in Figures 1 and 2.
 
 

Reactor Core

The core is placed at the bottom of the 6.25-m-high open tank with 2-m diameter. The core has a cylindrical configuration (Figure 3).

In total there are 91 locations in the core, which can be filled either by fuel elements or other components like control rods, a neutron source, irradiation channels, etc. The core lattice has an annular but not periodic structure (see Figure 3).

Elements are arranged in six concentric rings: A, B, C, D, E and F, having 1, 6, 12, 18, 24 and 30 locations, respectively. Each location corresponds to a hole in the aluminum upper grid plate of the reactor. The distances between locations in a given ring are equal. The distances between the center of the core and the rings are given in Table 1 and Figure 4. Note that the ring passes through the center of the fuel elements of that ring.

Location A1 is normally filled with an empty irradiation tube made of aluminum. Fuel elements in locations D8, E10, E11 and D17, E22, E23 can be removed in groups of three. Triangular cutouts in the upper grid at these locations enable inserting big vertical irradiation tubes (approximately triangular shape with dimensions of three fuel elements). In pulse mode operation these channels are removed to avoid local power peaking.
 
 

Table 1. Number of fuel elements in rings and diameters of the rings.

Ring

Positions

Diameter [inch]

A

1

0

B

6

3.2

C

12

6.3

D

18

9.4

E

24

12.5

F

30

15.7


 

Core Support Grids

The core is supported by a bottom grid plate that in addition provides accurate spacing between the fuel elements. The top grid plate also provides accurate lateral positioning of the core components. They are both made of aluminum of the same thickness (0.75 inch) but of different diameters (Figure 4).

Small holes (approx. 10mm diameter) for inserting flux measuring probes are provided in the upper plate. They are positioned between the fuel elements' holes in straight lines in radial direction. There are 16 holes in three radial lines, first from B-3 to F-11, second from B-2 to F-9 and third from B-6 to F-24 (See Fig. 8, Additional material 3).
 
 

Graphite Reflector

A graphite reflector enclosed in aluminum casing surrounds the core. A 2.5-inch-wide annular groove in the upper part of the reflector body is provided to contain a special irradiation facility (rotary specimen rack). It is made of aluminum and consists of 40 holes with inner diameter of 38 mm. The dimensions of the reflector, the casing, and the rotary specimen-rack groove are provided in Figure 4 and in Table 2.

There are two horizontal irradiation channels running through the graphite reflector. The radial irradiation channel stops at the inner radius of the reflector and is positioned 2.75 inches below the horizontal mid-plane. The tangential irradiation channel is also positioned 2.75 inches below the horizontal mid-plane and passes the core 12.77 inches from the center. Radial positions of the channels can be seen in Figure 2. Both irradiation channels are filled with air and are clad with aluminum. Other horizontal channels extend only to the reflector outer edge.
 

Table 2. Dimensions of the reflector.

Component

Dimension [inch]

Material

Reflector   Graphite
Outer diameter

41.8

 
Inner diameter

17.9

 
Height

21.2

 
Cladding  

Aluminum

Thickness (inner, top)

0.25

 
Thickness (outer, bottom)

0.50

 
Rotary specimen-rack groove  

Air

Outer diameter

28.9

 
Inner diameter

23.8

 
Height

10.2

 
Specimen rack  

Al

Diameter of holes

38 mm

 
Irradiation channels  

Al

Outer diameter

8.0

 
Inner diameter

6.0

 

Fuel Elements

A fuel element is a cylindrical rod with stainless steel (SS-304) cladding. Its total length is approximately 28 inches with 1.5-inch diameter. Fuel material in each element is 15 inches long. There are 2.6-inch-long and 3.7-inch-long cylindrical graphite slugs at the top and bottom ends, respectively, which act as axial reflectors. In the center of the fuel material is a 0.25-inch-diameter hole which is filled by a zirconium rod. Between the fuel meat and the bottom graphite end reflector is a 1/32-inch-thick molybdenum disc.

The fuel is a homogeneous mixture of uranium and zirconium hydride. In these experiments, only one type of fuel element will be used: standard stainless steel-clad fuel elements with 12 wt.% uranium of 20% enrichment (uranium is 20 wt.% 235U). The geometry is shown in Figure 5 and dimensions are given in Table 4.
 

Table 4. Fuel element data

Component

Dimension

[inch]

Material

Density

[g/cm3]

Fuel element      
Outer diameter

1.5

   
Element length

28.4

   
Fuel material  

U-ZrH

6.0

Outer diameter

1.4

   
Inner diameter

0.25

   
Height

15.0

   
Zr rod

Zr

6.5

Diameter

0.25

   
Height

15.0

   
Axial reflector  

Graphite

1.6

Diameter

1.4

   
Height upper

2.6

   
Height lower

3.7

   
Supporting disc  

Molybdenum

10.2

Thickness

0.03125

   
Cladding  

SS-304

7.9

Thickness

0.02 

   
Top and bottom ends  

SS-304

7.9

Height top 

4.1

   
Height bottom

3.0

   


 

Mass of uranium [g]

278

Mass of 235U [g]

55.4

U in U-ZrH [wt.%]

11.9

Enrichment [wt.%]

19.9

H/Zr atom ratio

1.6

Control Rods

Three control rods of fueled-follower type are used in the reactor: regulating (R), shim (C), and safety (S). Their locations are indicated in Figures 3 and 4. They are identical in geometry and composition. (See Table 5 and Figure 6.a.). When the control rods are in completely up position their position indicator is set to indicate 200 steps. It indicates 900 steps at completely down position.
 

Table 5. Description of the fuelled follower control rods.

Component

Dimension

[inch]

Material

Fueled-follower control rod    
Outside diameter

1.4

 
Element length

43.75

 
Fuel material  

U-ZrH

Outer Diameter

1.3

 
Inner Diameter

0.25

 
Height

15.00

 
Zr rod  

Zr

Diameter

0.25

 
Absorber  

B4C

Diameter

1.3

 
Length

15.00

 
Voids  

Air

Top void, length

3.75

 
Bottom void, length

5.5

 
Cladding  

SS-304

Thickness

0.02 

 
Top and bottom fittings  

SS-304

.

Mass of uranium [g]

235.7

Mass of 235U [g]

46.9

U in U-ZrH [%]

11.9

Enrichment [%]

19.9

H/Zr atom ratio

1.6

Transient Rod

Similar to the fueled-follower control rods, the transient rod (T in Figures 3 and 4) consists of the absorber part and the so-called air follower, which replaces the fuel part in the fueled-follower control rods. (See Table 6 and Figure 6.b.) The purpose of the air-follower, which is in fact an empty tube, is to reduce power peaking that could appear when the transient rod is in its fully withdrawn position. The transient rod has a guide tube that is the only structural component of the reactor that extends into the active volume of the core. Vertical dimensions of the transient rod are approximately the same as of the control rods shown in Figure 6.a.

Transient rod is equipped with pneumatic system for rapid withdrawal. When the pneumatic valve is open by pressing the "Fire" signal, the air pressure pulls the rod from its completely inserted position to its preset final position defined by the transient rod drive mechanism. When the transient rod is in completely up position its position indicator is set to indicate 0 steps. It indicates 900 steps at completely inserted position.
 

Table 6. Dimensions of the transient rod.

Component

Dimension [inch]

Material

Transient rod

   

Outside diameter

1.25

 

Element length

43.75

 

Air follower

 

Air

Diameter

1.2

 

Height

21.75

 

Absorber

 

B4C

Diameter

1.2

 

Length

15.00

 

Cladding

 

Aluminum

Thickness

0.028

 

Top and bottom ends,

fittings

 

Aluminum

Transient-rod guide tube

 

Aluminum

Outside diameter

1.50

 

Thickness

3 mm

 

Neutron Source

The neutron-source element (Figure 7) contains a Ra-Be neutron source with activity of 106 neutrons/s. The outer dimensions of the source element are similar to the fuel element.
 

Core Configuration

Core configuration is adjusted to pulse operation. It contains 52 fuel elements in compact arrangement (no in-core irradiation channels except central one, no empty positions, approximately circular shape). Excess reactivity is approximately 3$ and slightly exceeds the transient rod worth. Shutdown margin is approx. 7$. Maximum allowed pulse reactivity is 2.5$.
 

Reactor instrumentation

The reactor is equipped with 5 independent nuclear channels. Their characteristics are provided in Table 7.

Table 7. Nuclear instrumentation channels
 

Name

Detector type

Range

Start channel

fission counter

0.05mW-50W

Linear channel

compensated ionization chamber

100mW-300kW

Logarithmic channel

compensated ionization chamber

1W-1MW

Safety channel

ionization chamber

100W-300kW

Pulse channel

ionization chamber

10MW-2GW

Signal from the safety channel is used in reactivity measurements. The ionisation chamber is compensated for this purpose.

Fuel temperature is measured in two fuel elements instrumented with thermocouples. The thermocouple tip is in the center of the fuel element where the temperature is normally the highest. Instrumented fuel elements are inserted in the locations with maximum power density (normally B ring).
 

Figures
 
 

Figure 1. Side View of the TRIGA Reactor.
 
 


Figure 2. Top View of the TRIGA Reactor.
 
 

Figure 3. Core Configuration with Rod Locations Labeled.
 
 

Figure 4. Schematic Top and Side Views of the Reactor Core and Graphite Reflector.
 
   
 
 


 

Figure 5. Fuel Element.
 
 

Figure 6.a. Fueled-Follower Control Rod.
 
 

Figure 6.b. Transient Rod.
 
 


 

Figure 7. Neutron Source.