atw 2018-09v3


atw Vol. 63 (2018) | Issue 8/9 ı August/September



| | Fig. 2-4.

Nodalisation of MTR Fuel Assembly.

| | Fig. 2-3.

Nodalisation of MTR Fuel Assembly.

The main system boundary to be

modelled in the input deck is defined

at the pool with the inlet and the

outlet pipe. The reactor pipework is

composed of different pipes that are

built up by pipe segments (horizontal,

vertical, etc.). The pipes may also

contain valves and pumps. The modularisation

process is used as the basis

for object-oriented software design.

2.2 Applied nodalisation

rules for selected MTR and

TRIGA types

To realise the transformation and

exportation of reactor data into

ATHLET- format, nodalisation schemes

have to be developed and their rules

have to be implemented in the software.

For different research reactor

types, different nodalisation rules

have to be applied. Within the system

code ATHLET, the thermal hydraulic

nodalisation is represented by

thermo-fluiddynamic objects (TFOs).

TFOs are classified into pipes, branches

and special objects. Pipe objects

simulate one-dimensional fluid flow,

branch objects represent major

branching, and special objects are

used for simulation of components

with special requirements, e.g. cross


Focusing on the core geometry of a

MTR research reactor, each assembly

has several separated cooling channels

between the fuel plates. To cover

different postulated initial events, e.g.

blockage of one cooling channel in a

fuel element, the reactor core is

considered in detail and for each

cooling channel one representative

pipe is used. To reduce calculation

time, it is possible to group assemblies,

if they have identical characteristics.

Otherwise, there are modelled

separately. In Figure 2-3, the applied

nodalisation scheme for MTR fuel

assemblies is presented. Every fuel

assembly is linked to a common

branch before entering and leaving

the reactor core. The fuel plates are

modelled as Heat Conduction Objects

(HCOs). Internal fuel plates are

coupled on both sides to corresponding

TFOs. External fuel plates are

coupled one-sided to a TFO representing

a core channel and the other

side is coupled to a common bypass


Focusing on the TRIGA research

reactor, the core is composed of

several fuel rods in one tank. In contrast

to the MTR core, the fuel rods

have no separated cooling channels.

Therefore, the determination of

nodalisation depends on the core

layout. Based on typical TRIGA core

grid structures (Mark I and II), heuristics

are derived and realised in a

simple algorithm to determine the

linkage of TFOs. This approach

reduces the required input data to the

number of grid positions n in the first

circle around the centre point and the

number of grid positions along the

radius r (starting at the centre point)

– see Figure 2-4. Further, the length

of r is required. In radial direction, the

cooling area is divided into rings starting

at the centre point. In tangential

direction, the cooling area is divided

into segments.

The number of segments depends

on the number of grid position in the

first circle. The algorithm also computes

the belonging cross connections

and geometrical data. In the pictured

nodalisation in Figure 2-4, there are

13 pipes connected by cross connection

objects (6 grid positions along

r-direction and 6 grid positions in the

first circle). As already applied for

MTR core design, the pipes are linked

to a common branch before entering

and leaving the reactor core. The fuel

rods are modelled as cylinders and

­defined adiabatic at the inner side.

The outer side is coupled to the

corresponding TFO.

As default setting, the axial power

profile for both core designs (MTR

and TRIGA) follows a sinus curve.

While the geometry of guide boxes

and control plates/rods are not considered,

the external reactivity is

modelled by a signal in the general

control simulation module of ATHLET.

In the following Figure 2-5, the

generated core layouts by the software

for input deck generation is

presented. Only fuel assemblies with

fuel plates (MTR) and fuel rods

( TRIGA) are shown. Other components

or empty positions are not

AMNT 2018 | Young Scientists' Workshop

Heuristic Methods in Modelling Research Reactors for Deterministic Safety Analysis ı Vera Koppers and Marco K. Koch

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