PKL facility. Photo: FRAMATOME, Germany

International Standard Problem (ISP) on Primary Coolant Loop Test Facility (PKL) experiment on Multiple SG U-tube rupture

In the framework of the WGAMA/WGFS activity "Status Report on Good Practices for Analyses of Design Extension Condition without Significant Fuel Degradation (DEC-A) for Operating Nuclear Power Plants" there was a call to organise an analytical benchmark activity related to the assessment of code capabilities in predicting a DEC-A experiment. In this context, the ETHARINUS project participants initiated an ISP and evaluated that the PKL III J5.1 run2 (MSGTR) test is suitable for such an activity. The test results will be kept closed to both project members and participants until the submission of the blind calculations.

The proposal is justified by the necessity to assess the thermal–hydraulic code capabilities in predicting a DEC-A scenario.  

The activity will also:

  • Contribute to a better understanding of postulated and actual events;
  • Compare and evaluate the capability of best estimate computer codes in predicting controlled experiments and actual plant transients, and thus improve confidence in them as assessment tools for safety questions;
  • Highlight code deficiencies, if present;
  • Help users to evaluate the limits of their simulation process and thus improve their skills;
  • Provide information for quantifying code uncertainties;
  • Suggest necessary experiments to reduce technical ambiguities brought out;
  • Evaluate the efficiency and practicability of the codes (in addition to their accuracy);
  • Evaluate the effectiveness of the procedures for a challenging DEC-A scenario;
  • Address the user effect, since the user will have to make some parameter choices in the model.

This ISP will assist in improving thermal-hydraulic code modelling of DEC-A scenarios.

A blind and open phase exercise of ISP is conducted on Multiple SG U-tube rupture related to DEC-A scenario that may occur in the hypothetical scenario of a combined MSLB and MSGTR, by using the PKL experiment data obtained by Framatome GmbH in Erlangen, Germany, to clarify code predictive capabilities as well as user effect.

The PKL test facility replicates the entire primary system and most of the secondary system of a 1300 MWe 4-loop PWR plant with elevations scaled to 1:1 and diameters reduced by a factor 12. The facility detailed design was based to the largest possible extent on specific data of the Philippsburg nuclear power plant, unit 2. The scaling concept aims at simulating the overall thermal-hydraulic behaviour of the full-scale power plant. One of the main conditions to meet these requirements is to respect a full scale hydrostatic head correspondence with the reference PWR, in order to properly capture natural circulation (NC) which is under gravitational control. The power, volume and cross-sectional area scaling factor is 1:145. A full-scale correspondence of the frictional pressure loss distribution in the system is reproduced. The four primary loops are simulated independently to enable all possible primary loop coolant asymmetries during transient conditions. Full height fuel rods, spacers and internal structures are used in the RPV simulator, and the rods in the core are electrically heated. Also a full height U-tube bundle with a scaled number of tubes is used to simulate the primary side of each SG.

A description of PKL (drawing showing the overall facility layout, major components and their characteristics, and the overall scaling of the facility, instrumentation and its measurement errors) will be provided, as well as tabular data and material data necessary for the creation of input decks.

Boundary and initial conditions of the PKL III J5.1 Run2 test will be provided to participants for the execution of the blind phase. This will constitute the minimum set of information for simulating the thermal hydraulic response of a Multiple SG U-tube rupture accident in the hypothetical scenario of a combined MSLB and MSGTR. Then, the experimental test data will be distributed to participants for performing the post-test calculations, during the open phase.

Although the occurrence frequency of above postulated scenario is extremely low, it is meaningful and worthwhile the comprehension of the thermal-hydraulic phenomena and of the accident progression from experimental point of view, when the MSGTR simultaneously happens in each loop. This can be achieved by experiment using the PKL.

The objectives of the PKL III J5.1 test are to:

  • Investigate effectiveness of procedure on primary depressurisation and core cooling under simultaneous multiple failures during MSGTR (4 failed U-tubes in each of 4 SGs);
  • Obtain Break flow between primary and secondary side in the individual SGs; and
  • Find influences of Primary Side Depressurisation via safety relief valves of the PRZ (fill level in RPV).

The detailed list of relevant phenomena as well as the Figure of Merits (FoMs) (scalar and time trends) is detailed in the benchmark specifications.



CSNI International Standard Problem Procedures - CSNI Report N°17 - Revision 4


CSNI International Standard Problems (ISP): Brief Descriptions (1975-1999)


International standard problems (ISP): brief descriptions (1975-1997), 1997.


International standard problems (ISP): brief descriptions (1975-1994), 1994.


Task leaders: Simon Schollenberger (Framatome), Alessandro Del Nevo (ENEA).

Member countries and participating organisations: Belgium, China, European Commission, Finland, France, Germany, Hungary, Italy, Japan, Korea, Poland, Spain, Sweden, Switzerland, United Arab Emirates

Member's area



Martina ADORNI