DBA accident analysis

6.4.1 Steady state calculation

Before to simulate the DBA accident scenario a stationary input deck for CESAR was built to establish the initial conditions of the transient. The restart file generated by this run was used as initial condition for the transient simulation. This input deck was constrained by introducing different controllers by means of structures ‘REGU’ [6] (Controlled Steady State Phase). The task of the structures ‘REGU’ is to adjust the different parameters in order to fit the computed response with the expected one. In this way, they were set to the desired physical values for PZR level and pressure, SG flow rate, SG steam produced, SG water level, water flow across the core, just to name a few. This stationary transient was run for 30000 s to verify that the calculated conditions were steady and the actual initial conditions of the simulation were achieved. After the desired steady-state conditions were achieved, all the structures ‘REGU’ were removed to verify the stability of the modelling (Free Steady State Phase). As further, confirm of the parameters steadiness, a “null transient” was performed for another 20000 s. The comparison between the steady-state calculated results

obtained by the ASTEC code and those computed by RELAP5- GOTHIC coupled codes is summarized in Table 6-1. All the selected RELAP code steady state calculated values have been found in [7]. As it can be seen, the calculated values were in good agreement with each other.

Table 6-1: IRIS Steady state predicted result

6.4.2 The DBA scenario

The DBA scenario considered in this analysis is the guillotine rupture of the direct vessel injection (DVI) line Figure 6-3

Parameters ASTEC RELAP5/GOTHIC

TIME(s) TIME(s) Pressurizer Pressure (Pa) 1.556E7 1.555E7 Core inlet Temperature (K) 566.15 564.15 Core outlet Temperature (K) 602.14 602.15 Core mass flow/(by pass) (kg/s) 4707/(203) 4517/(213)

Core power (MW) 1000 1000

SG inlet temperature (K) 497.15 497.15 SG outlet temperature (K) 588.2 590.40 SG steam pressure (Pa) 5.86E6 5.80E6

SG collapsed level (m) 1.84 1.95 Feedwater flow (kg/s) 64.22 62.85 RCS water mass 324580 324500 Primary to secondary heat transfer 998.24 1000

EBT A/B water mass (kg) 12795 12400 (x2) EBT A/B water Temp. (K) 303.05 322.05 RWST A/B water mass (kg) 625000 (x4) 1194200 (x2) RWST A/B water Temp. (K) 290.05 293.15 RWST A/B water level. (m) 8.6 9.1 PSS A/B water mass (kg) 300000 (x1) 145300(x2) PSS A/B water Temp. (K) 322.05 322.05

PSS water level (m) 2.06 3.00

LGMS A/B water mass(kg) 100000 9890 (x2) LGMS A/B water Temp. (K) 322.05 322.05 Containment atm. Temp (K) 322.05 322.05 Environment atm. Temp (K) 308.15 308.15

Figure 6-3: DVI DEG LOCA schematic description

Although this is the smaller line connected to the vessel, it requires a particular analysis due to its position nearer to the top of the active core (TAF). The DVI line is connected to the reactor coolant system in the annular region near the bottom of the steam generators. Another important feature of this line is that, it is the discharge line of the LGMS, EBS and reactor cavity (when completely flooded). The double-ended break can be considered conservative from the point of view of liquid level and containment pressurization. A SBLOCA transient for IRIS reactor can be divided in three distinct phases (Figure 6-4). The first one, denominated the blow-down phase is defined as the period during which the reactor coolant system pressure is reduced and the containment pressure increases until the reactor coolant system and containment pressures equalize. The containment pressure in this phase is limited by the PSS and the reduced break flow due to the EHRS heat removal from the vessel. This phase, due to the relatively small dimensions of the break and of the ADS stage-1 lines, can be long for IRIS, in the order of 2000 seconds for the considered case. The

blow-down phase is considered concluded when the pressures in the vessel and in the containment are in equilibrium with a containment vessel peak lesser than 10E5 Pa. The break flow stops and the gravity makeup from the LGMS becomes available.

Figure 6-4: Overview of IRIS response to SBLOCA sequence

The blow down phase is followed by a depressurization phase. This phase can be further divided in two portions, before and after the opening of the ADS-stage2 lines. In the first one the pressure in the vessel remains lower than the pressure in the containment and the break flow reverses since heat is removed not from the containment, but directly from inside the vessel. During this phase the drywell pressure is reduced following the steam condensation on the containment walls and by the cool water discharged from the LGMS through the DVI break line. As the drywell pressure is reduced faster than the PSS pressure, a portion of suppression pool water is pushed out through the vent lines and assist in flooding the vessel cavity. After the opening of the ADS stage-2 lines the drywell and reactor vessel pressures are coupled and the coupled system is depressurized by the EHRS. The depressurization phase is followed by the long-term cooling phase, where the containment and vessel pressure is slowly reduced as the core decay heat decreases. During this phase of the accident recovery, gravity makeup from reactor cavity is available as required. Since decay, heat is directly removed from within the vessel and the vessel and containment are thermodynamically coupled, the long-term break

flow does not depend on the core decay heat, but it is in fact limited to only the containment heat loss.

6.4.3 Transient analysis

The transient starts after 30000 s of steady state. Break opening is initiated at t=0 s transient time. The first 86400 s (1 day) of accident are calculated and the main parameters are compared. All times of the events are given with respect to the break time assumed as time 0 s. The comparison of the main event during the DBA DEG transient is illustrated in Table 6-2.

Table 6-2: Main event cronologies