Effects of process-generated hydrogen on RPV walls

7. Hydrogen pressure in PWR

(a) Zoom on time dependent concentra- tion profile close to the steel-water inter- face.

(b) Zoom on the time dependent concen- tration profile at the lining-base material interface in the RPV wall.

Figure 7.9: Concentration profile in the RPV wall as a result of the typical cooling path for the reactor coolant of a pressurized water reactor during a cold shutdown considering only radiolysis generated hydrogen.The lines show the concentration profile for different moments after the start of cooling with an interval of 1 hour up to 50 hours. 7.3 Pressure during incidental or accidental conditions 7.3.1 Radiolysis generated hydrogen During the lifetime of reactor, several abnormal events may occur. These abnormal events, or accidents, can trigger the extra cooling system to prevent the fuel in the reactor from overheating. Such events can be a pipe break in the primary circuit, a large steamline break, . . . . As the water level in the reactor drops due to such a break, extra water is provided to the system to cool the reactor core. The water temperature is 40 ◦ C and therefore results in a big thermal shock for the structural materials and the RPV. This event is called a pressurized thermal shock (PTS). This rapid cooling of the internal wall of the RPV prevents the hydrogen in the wall from diffusing and escaping. This results in a combination of a high hydrogen concentration in the RPV wall together with a low temperature. Therefore, high hydrogen fugacities will be reached during a PTS. [84] One can simulate the evolution of the hydrogen fugacity in the RPV wall when a PTS occurs. The problem can be simplified to an instant cooling of the internal RPV wall to 40 ◦ C. The initial conditions, as for the cold shutdown, are taken to be the steady state hydrogen concentration and temperature profile during hot in-service condition. The variation of temperature in the RPV wall as a function of time after the initiation of the PTS is shown in Figure 7.10. The lines indicate the temperature with an interval of 10 minutes and are drawn up to 150 minutes after the start of the PTS. One can see that after approximately 120 minutes or 2 hours, the temperature over the complete thickness of the wall has reached its final value of 313 K or 40 ◦ C, 88