In a PWR, a pressurizer functions to maintain the pressure of the primary coolant system in a range such that no boiling occurs in the primary system under normal and transient operations. In a current large PWR, the pressurizer is a separate cylindrical tank connected to the reactor coolant system piping by a surge line, nominally 10 inches (25 cm) (Datta and Jang, 2007), and a spray line, nominally 4 inches (10 cm) (NRC, 2012a). Pressure is normally controlled using heaters and a spray valve to maintain the pressure range. A balance of water and steam exist in the pressurizer space. The water level in the pressurizer provides an indication of water inventory in the reactor coolant system.

Some large PWR designs also use a power operated relief valve (PORV) connected to a pressurizer relief tank to assist in controlling pressure. The PORVs will open to reduce pressure prior to the system reaching the RCS safety valve relief set point. In large PWR designs that utilize a PORV, a stuck open PORV is a potential small — break LOCA initiating event.

A PWR pressurizer provides the surge volume for the reactor coolant system. If the RCS temperature increases, less dense reactor coolant system water surges into the pressurizer, compressing the steam space and increasing the primary pressure. Steam will naturally begin to condense to lower the pressure, which may be sufficient for a small or slow transient associated with a minor RCS temperature change. More commonly, however, pressure will rise to the spray valve set point. The open spray valve then admits colder primary water from the RCS cold leg into the steam space to quickly condense steam and subsequently reduce pressure. The spray flow is driven by the discharge pressure of the reactor coolant pump. Conversely, if the RCS temperature decreases, the suddenly denser reactor coolant water causes water to flow out of the pressurizer into the reactor coolant system hot leg, expanding the steam space and decreasing the primary pressure. This is controlled by use of pressurizer heaters, which heat the pressurizer water and subsequently increase pressure (NRC, 2006).

An iPWR integrates the pressurizer into the top of the reactor pressure vessel. A baffle plate or head plate with drilled openings separates the pressurizer from the reactor coolant system and acts as the surge line. Because of the integral nature of the pressurizer design, the 10-inch surge line is eliminated. The volume of an iPWR pressurizer is significantly larger than a current large PWR pressurizer relative to reactor thermal power. In the IRIS iPWR design, the pressurizer volume is about five times larger per unit of power than for a current large PWR design (Ingersoll, 2011). This larger pressurizer volume, coupled with larger reactor coolant system water inventory overall relative to reactor thermal power, provides slower pressure transients in general. This provides a number of operational benefits. First, an operator will have more time to analyze changes in plant operating conditions and respond accordingly. Second, the need for a fast acting spray valve is virtually eliminated because in most cases the natural steam condensation following a slower insurge of coolant into the large volume pressurizer will adequately maintain the RCS pressure under normal operations and expected transients. Finally, the integrated location of the pressurizer in an iPWR design provides a more direct indication to the operator of the water level above the top of the reactor fuel at all times (IAEA, 1995).

The ‘normal’ spray valve is eliminated in many iPWR designs because it is not essential to fine tune the system pressure as discussed above and, in designs with no reactor coolant pumps, there is insufficient driving head in the RCS to provide adequate spray flow. Even in iPWR designs employing smaller reactor coolant pumps, the pressure differential or driving head created in the RCS is much lower than that in a current large PWR. Current large PWRs employ an ‘auxiliary’ spray valve to back up the normal spray valve when the connected reactor coolant pump is unavailable or the normal spray valve is inoperable. The auxiliary spray valve in a current large PWR is typically driven by the charging pump discharge. The NuScale iPWR design plans to use this approach to provide for pressure reduction through pressurizer spray actuation (NuScale, 2012). Other iPWR designs will likely use this approach to include a pressurizer spray function as well. As a result, the spray line is not necessarily eliminated in an iPWR design, but it is generally limited to less than the 4-inch size employed in current large PWRs. Pressurizer heaters in an iPWR design function the same as the heaters in current large PWR designs.

PORVs are not incorporated into the iPWR pressurizer designs. A stuck open PORV was the root cause of the Three Mile Island accident, which is eliminated in all iPWR designs. Safety relief valve discharge is directed to containment or another water storage tank inside containment. Therefore, a pressurizer relief tank is not incorporated into the iPWR designs. In addition, piping to provide a nitrogen cover gas on the pressurizer relief tank and the need for a drain system and an associated pump are eliminated in the iPWR designs.