Reactivity accident scenarios could originate from internal boron dilution in the primary system of a nuclear pressurized water reactor type (PWR or VVER). In essence the phenomena is caused by boron dilution following vaporization and condensation within the primary coolant system as a result a reduction in primary system mass inventory, e.g. during a loss of coolant accident (LOCA). When the liquid level in the reactor vessel falls below the hot leg elevation, steam begins to flow to the steam generators and condenses there. This steam carries no boron, thus boron concentration in the cold leg loop seals begins to decrease. If for some reason this water plug with low boron concentration enters the core without any major mixing with the borated coolant, the result is a positive reactivity insertion. The paper presents an analysis using the RELAP5 Mod 3.3 for a small break LOCA in one cold leg pipe of a Russian-type VVER-1000 nuclear reactor. The boundary conditions consider, as far as possible, a realistic situation in which the four accumulator tanks (SIT) and the three low pressure injection systems (LPIS) are available for core cooling but one of the three high pressure injection systems (HPIS), not serving the damaged loop, is not available. A range of break sizes (from 30 cm2 to 120 cm2) have been investigated in order to identify a range of potentially adverse conditions. From the results obtained, in some calculations boron dilution is observed in some loop seals. The worst situation was found to occur at the 40cm2 break size. However, due to ECCS injection and mixing an increase of boron concentration is observed in the upper parts of the downcomer. Additionally RELAP codes have been used to perform analyses of the balance of boron mass within the reactor circuit.
Addressing Boron Dilution Scenario Through RELAP5/3.3 Analysis of VVER-1000 SB LOCA
D'AURIA, FRANCESCO SAVERIO;
2008-01-01
Abstract
Reactivity accident scenarios could originate from internal boron dilution in the primary system of a nuclear pressurized water reactor type (PWR or VVER). In essence the phenomena is caused by boron dilution following vaporization and condensation within the primary coolant system as a result a reduction in primary system mass inventory, e.g. during a loss of coolant accident (LOCA). When the liquid level in the reactor vessel falls below the hot leg elevation, steam begins to flow to the steam generators and condenses there. This steam carries no boron, thus boron concentration in the cold leg loop seals begins to decrease. If for some reason this water plug with low boron concentration enters the core without any major mixing with the borated coolant, the result is a positive reactivity insertion. The paper presents an analysis using the RELAP5 Mod 3.3 for a small break LOCA in one cold leg pipe of a Russian-type VVER-1000 nuclear reactor. The boundary conditions consider, as far as possible, a realistic situation in which the four accumulator tanks (SIT) and the three low pressure injection systems (LPIS) are available for core cooling but one of the three high pressure injection systems (HPIS), not serving the damaged loop, is not available. A range of break sizes (from 30 cm2 to 120 cm2) have been investigated in order to identify a range of potentially adverse conditions. From the results obtained, in some calculations boron dilution is observed in some loop seals. The worst situation was found to occur at the 40cm2 break size. However, due to ECCS injection and mixing an increase of boron concentration is observed in the upper parts of the downcomer. Additionally RELAP codes have been used to perform analyses of the balance of boron mass within the reactor circuit.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
