atw Vol.

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

466

AMNT 2018 | YOUNG SCIENTISTS' WORKSHOP

| | 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

connections.

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

channel.

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

atw Vol. 63 (2018) | Issue 8/9 ı August/September 466 AMNT 2018 | YOUNG SCIENTISTS' WORKSHOP | | 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 connections. 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 channel. 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

atw Vol. 63 (2018) | Issue 8/9 ı August/September pictured. Typically, four control rods are required for reactivity control in TRIGA reactors with thermal power levels of less than 1 MW [IAEA2016B]. Further, graphite elements are at the outer positions. For MTR research reactors, there are often empty places at the centre of the core grid for radiation samples. If the input number of fuel assemblies or elements does not match the number of grid positions, the implemented algorithm considers these typical core characteristics. The assemblies or elements are positioned in respect of this information. Furthermore, the free flow path is calculated as a function of total core area and number of fuel assemblies, elements and other components inside the research reactor core. 3 Generated input decks of exemplary MTR and TRIGA reactors In this part, first functionality of the new modelling system is demonstrated by generating an exemplary MTR and TRIGA research reactor model. For this purpose, two reference research reactors were chosen. Providing technical details in [ABD2008A] and comparative data in [ABD2008B], the ETRR-2 was identified as a MTR reference facility. The ETRR-2 is a multipurpose research reactor located in Inshas, the Arab Republic of Egypt. It corresponds to the rightmost research reactor design in Figure 2-1. The ETRR-2 reactor consists of 29 fuel assemblies of MTR type with 19 fuel plates each and has 22 MW nominal power. Further description is presented in [ABD2008]. The main nodalisation of the generated ETRR-2 model in ATHLET is pictured in Figure 3-1. On the left side, the coolant loop is presented in bright blue. The reactor pool is modelled with two pipes interconnected by cross-connections. The inner pool pipe is connected to the reactor chimney, which is marked in brown, by a single junction pipe. The reactor core is modelled with two representative assemblies. Each is composed of 18 core cooling channels. One assembly is representing 28 grouped average assemblies. The other assembly considers a hot channel factor on the 19 fuel plates plus one extra penalised fuel plate. The nodalisation of both assemblies is identically and shown in Figure 3-2. To check the capability of the nodalisation to reproduce the thermal hydraulic plant conditions, steady state calculations were performed. | | Fig. 2-5. MTR core layout (left) and TRIGA core layout (right), generated by software for input deck generation. | | Fig. 3-1. Overview of whole Nodalisation of the ETRR-2 (left) and one fuel assembly (right) with 18 core channels generated by the software for input deck generation. Power [MW] Loop mass flow [kg/s] The initial conditions of the experiment and the calculated parameters are compared in Table 3-1. The experiment was performed at 9.5 MW thermal power. There is good agreement between the calculated and experimental stationary data. As an exemplary TRIGA research reactor, the IPR-R1 was identified. The IPR-R1 is a TRIGA Mark I model, installed in Belo Horizonte in Brazil and operated since 1960. Several analytic and experimental studies were performed and published. As reference data, experimental results in [REI2009] were used. The IPR-R1 corresponds to the leftmost research reactor design in Figure 2-1. It is operating at 250 kW and consists of 63 fuel elements of TRIGA type. Further description is presented in [REI2009]. The main nodalisation of the generated IPR-R1 model in ATHLET is shown in Figure 3-2. On the left side, the coolant loop is Core mass flow [kg/s] Core outlet temperature [°C] Core pressure drop [bar] | | Tab. 3-1. Overview of whole Nodalisation of the ETRR-2 (left) and one fuel assembly (right) with 18 core channels generated by the software for input deck generation. presented in bright blue. The reactor pool is modelled with two pipes interconnected by cross-connections. The inner pool pipe is connected to the core entrance and core outlet. 13 core channels, interconnected by crossconnections, with 63 fuel elements represent the reactor core (see Figure 3-2 right). The core nodalisation based on the nodalisation presented in Figure 2-4. The experiment was performed at 50 kW thermal power. In Table 3-2, the calculated steady state results are compared to measured core inlet and outlet temperatures. At different positions, measuring devices were installed (see [REI2009]). There are small deviations but overall the results are consistent. Further, the ATHLET simulation is compared to published RELAP steady state calculation in [REI2009], which reaches steady state conditions after about 2000 s simulation time. Reference pressure [bar] Calculation 9.5 309.24 302.86 35.01 0.42 2.2 Reference [ABD2015] 9.4 309.24 302.87 34.9 0.31* 2.0 *in [ABD2015] core pressure drop of 3.1 bar is mentioned, but in /IAEA2005/ 0.6 bar pressure drop at 100 % core power is referred 467 AMNT 2018 | YOUNG SCIENTISTS' WORKSHOP AMNT 2018 | Young Scientists' Workshop Heuristic Methods in Modelling Research Reactors for Deterministic Safety Analysis ı Vera Koppers and Marco K. Koch