INTEGRA - Integral and separate effects tests on thermal-hydraulic

release, also takes place. However, in this area, still the applicability of modern CFD codes and modern measurements techniques (e.g. PIV, high speed video recording) could be very valuable.

Deliverables in 2018

• The SRV sparger in the PPOOLEX test facility was shortened and the pool water level was increased in order to increase the thickness of the cold stratified layer. The effect on stratification/erosion/mixing behaviour during steam discharge via the sparger pipe was studied in a test and compared to reference tests.

• An extensive test series was carried out in the SEF-POOL separate effect test facility. Data of the characteristics of small-scale phenomena affecting the effective heat and momentum sources was provided to be used for the validation of the simplified EMS/EHS models proposed by KTH. The SEF-POOL results also supported the validation effort of the DCC and interfacial area models of CFD codes for steam injection through spargers at VTT and LUT.

• A literature survey of existing experiment data and models for noncondensible dissolution/release dynamics and of the effects of NCGs in reactor coolant system was done.

• Journal article on pool stratification and mixing induced by steam injection through spargers was published in Nuclear Engineering and Design.

• Journal article on frequency analysis of chugging condensation in pressure suppression pool system with pattern recognition was published in Nuclear Engineering and Design.

2.2.5 INTEGRA - Integral and separate effects tests on thermal-hydraulic problems in

PKL test facility, organize a Programme Review Group and Management Board meeting of the OECD PKL Phase 4 Project in Lappeenranta, report the PWR PACTEL flow reversal due to a pump trip experiments and APROS simulations, carry out the characterizing tests of the passive heat removal test facility, and write a journal article on simulations of the PWR PACTEL nitrogen experiments.

In 2016, the experiments studying the effect of nitrogen in LOCA were carried out with the PWR PACTEL facility. The aim was independently verify the piston effect of nitrogen. In two of these experiments, the line between the upper plenum and downcomer was open and the break location in the cold leg and the accumulator injection point was close to the downcomer.

Therefore, nitrogen escaped through the break and no clear piston effect was observed. In the OECD/NEA PKL Phase 4 project, new experiments are carried out with the PWR PACTEL facility where the break location is in the cold leg near the steam generator and the effect of the line between the upper plenum and downcomer to the piston effect is tested. The experiments require a reference experiment without nitrogen. The reference experiment was carried out and reported as a Quick Look report in 2018. The other two experiments will be carried out in 2019. All the experiments will be analyzed and reported with details together in 2019.

Most of the organizations participating in the OECD PKL Phase 4 Project will do analytical work with thermal-hydraulic codes. It would be beneficial for LUT also to participate in the analytical work of the experiments with the PKL facility. A TRACE simulation model of the PKL test facility was made and tested against the characterizing tests of the facility for that as a master’s thesis.

One meeting of the Programme Review Group and Management Board of the OECD PKL Phase 4 Project took place in Lappeenranta on the week of 22-25 May 2018 and was organized by LUT.

Tripping of a reactor coolant pumps causes asymmetric flow conditions in primary loops as well as in a reactor core. A flow reversal occurs in the affected loop due to the reversal of the pressure distribution in the loop caused by the other running pumps. The final flow conditions are characterized by a slight overflow in the intact loops and a backflow in the loop with the idle reactor coolant pump. In the INTEGRA project, the phenomenon was studied experimentally with the PWR PACTEL facility in 2017. The analyses and reporting of the experiment results and APROS simulations were carried out in 2018.

The SAFIR2018 INTEGRA project introduced a completely new facility, PASI, for the studies of a passive containment heat removal system. The facility was constructed in 2017 and the first experiments were performed in 2018. The reference system for the PASI test facility is the passive containment heat removal system of the AES-2006 type pressurized water reactor.

This type of passive system is designed also for the planned Hanhikivi unit in Finland.

The functioning of the PASI facility is based on natural circulation. With the PASI facility, the goal is to make tests to measure system performance characteristics, and to detect issues that could disturb the operation of a passive system or prevent it from functioning as designed. The PASI test facility consists of a pressure vessel simulating containment conditions, a heat exchanger, a water pool and interconnecting riser and downcomer pipelines. Additional systems are included to provide steam, collect condensate water, remove steam and inject feed water. An aerosol injection system can be added to the system in future.

Figure 2.26. PASI test facility.

Characterizing experiments were performed to analyze, verify and study characteristics that describe this passive heat removal loop. The pressure losses were determined for normal and reverse directions. A heat up and cool down method was used to calculate integral heat losses and heat capacities of the facility over a range of fluid temperatures. The uncertainties of the heat losses and heat capacities are relatively high. There is not any single source for the uncertainties. Mostly those are rising from the approximations made for the analysis and from the uncertainties in the measurements.

A natural circulation experiment was performed to study the normal behavior of the PASI facility and the capability to remove heat. The natural circulation experiment started from atmospheric pressure and low temperature in the vessel. The PASI facility was operating as expected, transferring heat from the vessel simulating containment conditions to the water pool efficiently.

The natural circulation in the loop initiated soon after the steam supply to the vessel simulating containment conditions began. The single-phase period showed steady and smooth operation of the heat removal loop, lasting about 2.5 hours. The oscillation phase started as the hot loop side reached saturation conditions and the single-phase flow turned into two-phase flow. This period was characterized by the strong oscillation of the loop mass flow rate.

Figure 2.27. Loop mass flow rate in the natural circulation experiment. At the end of the experiment, the measuring range of the flow meter had to be changed twice because the peak values of the natural circulation mass flow rates exceeded the maximum value of the flow meter measurement range.

Deliverables in 2018

• Participating in the OECD/NEA PKL Phase 4 project with PWR PACTEL experiments

• TRACE simulation model of the PKL test facility

• Research reports of the flow reversal due to a pump trip experiments and APROS simulations

• The characterizing tests of the passive heat removal test facility

• Journal article “System code analysis of accumulator nitrogen discharge during LOCA experiment at PWR PACTEL test facility” (under review process in Nuclear Engineering and Design)