Primary Coolant Loop Test Facility (PKL) Project
Joint project

The Primärkreislauf-Versuchsanlage (Primary Coolant Loop Test Facility (PKL)) is a facility is owned and operated by Framatome GmbH (formerly AREVA NP) in Erlangen, Germany to perform experiments on the thermal-hydraulic behaviour of pressurised water reactors (PWRs) during operational transients in order to try to solve PWR safety issues.

For a number of years, Framatome GmbH has conducted experiments on reactor thermal hydraulics at the PKL facility, including earlier experiments carried out in the framework of the SETH Project  (2001‑2003) and in earlier phases of the PKL Project. Complementary tests have also been performed at the PMK test facility in Budapest, Hungary and at the PACTEL test facility in Lappeenranta, Finland).

PKL Project phases

First phase (2004-2007)

PK-1 investigated pressurised water reactor (PWR) safety issues, specifically:

  • boron dilution accidents
  • loss of residual heat removal (RHR) in mid-loop operation (during shutdown conditions).

In the first category of tests which dealt with inherent boron dilution during a small break loss-of-coolant accident, the key issue was whether there were plant thermal-hydraulic conditions that could produce reactivity insertion into the core, and thus a potential accident situation. The experiments tried to reproduce different plant and system configurations, especially those that were believed to be potentially the most serious, thus encompassing an envelope of conservative cases. Measures that could remedy the boron dilution issue were also investigated.

The second category of tests was to assess accident management operations of the primary circuit after a loss of residual heat removal capability during mid-loop operation. Tests with closed reactor circulation system explored safe procedures for re-establishing heat removal and avoidance of significant reactivity insertion into the core. Tests with open circulation systems aimed to explore measures for preventing fuel damage and maximising safety margins provided by different means of coolant injection.

Among other things, the test matrix focused on the following items received great attention within the international reactor safety community:

  • boron dilution events after small-break loss-of-coolant-accidents
  • loss of RHR during mid-loop operation in closed reactor coolant systems (RCS), potentially also leading to boron dilution
  • loss of RHR during mid-loop operation with open RCS.

Two tests were performed in 2004 and two in 2005. A workshop was also organised in 2005 covering an analytical exercise with code predictions related to the PKL tests. The 2005 tests focused on the loss of RHR in a three-quarter loop operation with the reactor cooling system closed and on the effect of pressuriser temperature on the RCS inventory distribution. The two tests were carried out to address the following scenarios:

  • Run A: Loss of RHR with RCS filled, initial level mid loop, pressuriser cold
  • Run B: Loss of RHR with RCS filled, initial level mid loop, pressuriser hot.

Three final tests for phase one were performed and reported on in 2006. The first phase of the project ended with a final meeting in May 2007.

Project data

The project data abstract is public. Computer Program Services of the Data Bank provides access to Data Bank participating countries.

External article

Umminger, K., Schoen, B., Mull, T., PKL Experiment on loss of Residual Heat Removal under Shutdown conditions in PWR, Paper 6440, 2006 International Congress on Advances in Nuclear Power Plants (ICAPP'06), Reno, NV, USA, 4-8 June 2006.

Second phase (2008-2011)

PKL-2 investigated safety issues relevant for current PWR plants as well as new PWR design concepts. It also focused on complex heat transfer mechanisms in the steam generators and boron precipitation processes under postulated accident situations. 

The first category included tests addressing the heat transfer mechanisms in the steam generators (SGs) in the presence of nitrogen, steam and water, in both vertical and horizontal steam generators. Also covered were cool down procedures in the case where steam generators had partly dried out on the secondary side. An additional test addressed heat transfer in the steam generators under reflux condenser conditions (e.g. fast secondary side depressurisation). Fast cool down transients (with a water-filled reactor coolant system) such as main steam line break, completed by tests that mixed of hot and cold water in the reactor pressure vessel (RPV) downcomer and the lower plenum were also considered in this phase. Further investigation addressed boron precipitation processes in the core following large break loss of coolant accidents (LB-LOCA).

The program included eight integral experiments at the PKL test facility covering the following topics:

  • G1: Systematic investigation of the heat transfer mechanisms in the SGs in presence of nitrogen, steam and water (two tests performed in July and August 2008)*
  • G2: Cool-down procedures with SGs isolated and emptied on the secondary side (one test with three runs performed in December 2008)
  • G3: Fast cool-down transients (main steam line break) (one test performed in July 2009)**
  • G4: Accident situation under reflux condenser conditions for new PWR design concept (one test with two runs performed in December 2009)
  • G5: Boron precipitation following large break loss of coolant accidents.
  • G6: RCS cool down with void formation in RPV upper head (one test performed in April 2011)
  • G7: Counterpart test with the rig-of-safety assessment/large-scale test (ROSA/LSTF) facility on a small break loss-of-coolant accident (LOCA) with accident management procedures (one test performed in July 2011)

* The tests on the heat transfer mechanisms in the SGs in the presence of nitrogen were complemented by tests in the PMK test facility in Hungary for horizontal SGs.

** The tests on fast cool down transients were completed with tests at the Rossendorf Coolant Mixing Model (ROCOM) test facility in Germany on mixing in the RPV downcomer and the lower plenum.

A concluding joint workshop with the ROSA-2 Project was organised from 15-19 October 2012 at the OECD Conference Centre and attracted 65 participants from 14 countries and 2 international organisations. 45 presentations were given covering the general overview of PKL and ROSA projects, analyses of the counterpart test at the PKL and ROSA/LSTF facilities, the results and analyses related to PKL-2/ROCOM tests, the results and analyses related to other PKL and ROSA tests, analytical studies on core exit temperature (CET) behaviour and general analyses related to PKL-2 and ROSA-2 tests. The conclusion of this workshop prepared the session chair together with the final summary integration report of the two projects was issued as a public report in 2013.

Third phase (2012-2016)

PKL-3 began in 2012 and investigated safety issues relevant for current PWR plants as well as for new PWR design concepts by means of transient tests under postulated accident scenarios and systematic parameter studies on thermal hydraulic phenomena.

The first category included tests that addressed safety issues related to beyond design basis accident transients with significant core heat-up, i.e. station blackout scenarios or LOCAs in connection with the failure of safety systems. Without adequate accident management (AM) procedures, the postulated course of events would lead to a severe accident scenario with core damage. In the tests, the efficiency of very late initiated AM measures were established so that the safety margins could be explored. Both scenarios were connected with a perfomance assessment of the core exit temperature (CET) which is normally used as criterion for the initiation of AM measures involving emergency operating procedures and/or severe accident management measures.

With respect to current safety issues, events in cold shut down state (i.e. failure of residual heat removal (RHRS)) were also covered by the PKL-3 test phase. Findings from the the first two phases on thermal hydraulic phenomena (e.g. pressure evolution following failure of RHRS, boron dilution) were compared to the transients test conducted with an open RCS.

The second category of tests addressed some issues investigated in the previous PKL phases that could not be completed. Parameter variations from tests conducted in PKL-2 were implemented in PKL-3, either to provide an extension to already existing databases on cool down procedures under asymmetric natural circulation, or to determine the sensitivity of boron precipitation in the core following LB-LOCA for specific parameters. As with previous phases, complementary tests at ROCOM, PMK and PWR PACTEL facitiles were also considered for phase three..

In 2015, two meetings were held and in November, final tests were completed. A counterpart test was performed to compare with a test conducted at the ROSA facility in Japan. The ATLAS Project also condidered reproducing a PKL-3 test for comparison.

With all tests completed, the operating agent, Framatome GmbH, conducted analysis of the results and produced the final report in July 2016. Project participants and Framatome GmbH completed discussions on a follow-up phase to cover gaps in knowledge. There was a benchmark workshop with two PKL-3 benchmarks outcomes in collaboration with an ATLAS Project benchmark activity in Pisa on 13-16 April 2016.

PKL-3 Members' area (password protected | reminder)

Fourth phase (2016-2020)

PKL-4 investigates safety issues relevant for current pressurised water reactor (PWR) plants as well as for new PWR design concepts and focuses on complex heat transfer mechanisms under two-phase flow, boron dilution and precipitation, and on cool-down procedures.

The issues are investigated by means of thermal-hydraulic experimentsconducted at the PKL facility in Erlangen, Germany. In addition to the tests that conducted at the PKL facility, additional tests will be performed at the PMK facility in Budapest, Hungary and at the PACTEL facility in Lappeenranta, Finland.

Phase four began on 1 July 2016 and will end on 30 June 2020. It focuses on parametric studies on thermal-hydraulic procedures for model development and validation of thermal-hydraulic system codes, and on experimental verification of cool-down procedures and operation modes for different incidents and accidents.

The first category of experiments addresses subjects related to current safety issues that either suffer from the lack of a dedicated database for analysis and validation of computer codes, or from uncertainties in the safety evaluation stemming from open issues or questions. The extension of already existing databases related to these subjects is the most important goal of the first category of tests.

The second category of experiments contain mostly transient tests either on subjects already investigated in previous PKL phases as answers to questions that could not be completed, or on subjects that represent current topics from the international debate on PWR safety. Complementary tests at PMK and PWR PACTEL are considered in this phase.

Finally, two test subjects remain open for decision by the programme partners following the results of preceding experiments (either confirmation tests or tests addressing specific participants' requests).

PKL-4 members' area (password protected | reminder)


PKL-1: Belgium, Czech Republic, Finland, France, Germany, Hungary, Italy, Japan, Korea, Spain, Sweden, Switzerland,  United Kingdom and United States

PKL-2: Belgium, Czech Republic, Finland, France, Germany, Hungary, Japan, Korea, the Netherlands), Spain, Sweden, Switzerland, United Kingdom and United States

PKL-3: Belgium, Czech Republic, Finland, France, Germany, Hungary, Italy, Japan, Korea, Spain, Sweden, Switzerland,  United Kingdom and United States

PKL-4: Belgium, China, Czech Republic, Finland, France, Germany, Hungary, Japan, Korea, Spain, Sweden, Switzerland and United States.

Project period

PKL-1: Jan 2004 to May 2007
PKL-2: April 2008 to Sept 2011
PKL-3: April 2012 to April 2016
PKL-4: July 2004 to June 2020


PKL-1: USD 1.2 million per year, with German parties covering 50% of this cost
PKL-2: ~ EUR 1 million per year, with German parties covering 50% of this cost
PKL-3: EUR 3.9 million
PKL-4: EUR 4.78 million