Coolant mixing under natural circulation conditions following a postulated small break loss-of-coolant-accident

Funded from 01.06.2004 to 30.11.2006 by the Nuclear Special Committee "Plant engineering" of VGB PowerTech (Germany) under the registration number: SA"AT" 23/04.


Partial depletion of the primary circuit during a hypothetical small break loss of coolant accident can lead to the interruption of one-phase flow natural circulation. In this case, the decay heat is removed from the core in the reflux-condenser mode. Experiments for this scenario carried out at the integral test facility PKL (AREVA Erlangen) showed that low-borated water preferably accumulates in those two loops, which are nit receiving safety injection. After refilling of the primary circuit, natural circulation re-establishes. The low-borated coolant shifts towards the reactor pressure vessel. Mixing in the downcomer and the lower plenum is an important phenomenon mitigating the reactivity insertion into the core in this postulated scenario. Mixing experiments at the ROCOM test facility were carried out to quantify these effects.

ROCOM experiments for a similar scenario, which boundary conditions are based on calculations using the thermal hydraulic code ATHLET can be found here.

Boundary conditions

The boundary conditions for the ROCOM experiments were derive from one of the corresponding experiments at the test facility PKL. Low borated slugs were prepared in two neighboring loops of the ROCOM test facility. The volume of these slugs corresponds to the volume of the loop seal and a part of the steam generator outlet chamber. The natural circulation in the PKL experiment started with a time shift and a different mass flow rate in the two loops. For conservative reasons, the mass flow curve from the loop with the higher value was used in identical manner for both loops in the ROCOM experiments. In the given scenario, emergency core cooling water is injected through one loop into the vessel. For modeling this feature an accurately modeled nozzle was connected to loop 3 of the ROCOM test facility. The corresponding amount of emergency core cooling water is injected through this nozzle.

Density differences arising from the temperature differences between slugs, ambient coolant and injected emergency core cooling water have a significant influence on the coolant mixing under natural circulation conditions. Due to the fact, that the facility cannot be heated up, the necessary density differences were created by adding sugar to the water.


The main mixing processes take place in the downcomer of the test facility. A new sensor was developed for an improved visualization and quantification of the mixing in the downcomer. This sensor consists of a grid with 64 radial and 32 axial measuring points, which allows deriving a nearly complete figure of the mixing processes in the downcomer.

Fig. 1 shows two snapshots of the unwrapped downcomer, the left part shows the boron concentration at the selected time point, the right one the distribution of the emergency core cooling water. From the first snapshot becomes clear, that the slugs, which have a less density, replace the emergency core cooling water and spreads out in the upper part of the downcomer forming a ring around the core barrel. In the second snapshot is to be seen, that the lower borated water moves slowly down. It is transported into the lower plenum by admixing to the emergency core cooling water streak.

Schnappschüsse der Verteilung der Borkonzentration und des Notkühlwassers im Downcomer

Fig.1: Snapshots of the boron concentration and emergency core cooling (ECC) water distribution in the downcomer

At about 75 s after the start of the transient first lower borated coolant reaches the core inlet plane. This first reduction of the boron concentration is detected below the angular position of the loop with the active emergency core cooling injection. This confirms the statement made above, that the lower borated coolant is transported with the emergency core cooling water streak in the first phase of the transient. Within the next 150 s the minimum boron concentration in the core inlet plane reduces further. At t=204 s the minimum is reached (1688ppm). After this, the boron concentration in the core inlet plane rises again. Fig. 2 shows the measured boron distribution in the core inlet plane for different time points.

Zeitsequenzen der Borkonzentration in der Kerneintrittsebene

Fig.2: Time sequences of the boron concentration distribution in the core inlet plane

The distribution of the boron concentration is heterogeneous. Reduction of the boron concentration below 2300 ppm (that means a reduction of more than 200 ppm) is only observed in the three outer rows of the fuel assemblies, the position of the overall minimum itself is a fuel element in the outermost row.

This experiment showed a considerable increase of the boron concentration of the initial nearly boron-free slugs during the transport from the cold leg to the core inlet plane.




S. Kliem