12.45. Containment structures for PWRs vary to some extent from plant to plant, but they are commonly cylindrical (roughly 37 m diameter) with a domed top (overall height some 61 m). They are usually made of rein­forced concrete, about 1.07 m thick, with an internal steel liner, roughly 38 mm thick. As shown in Fig. 12.3, the entire primary coolant system is enclosed as well as elevated injection tanks.

12.46. A spherical design is shown in Fig. 12.4. Compared with a cy­lindrical design of equivalent free volume, the spherical configuration pro­vides additional operating floor area and efficient placement of auxiliary and maintenance activities. An in-containment refueling water storage tank (IRWST) shown in the figure provides water for both safety injection and severe accident core debris cooling. Sphere diameters vary from about 40 m for a 2600-MW(t) plant to about 60 m for a plant rated at 3800 MW(t). Corresponding free volumes are about 57,000 and 96,000 m3, respectively.

12.47. If there were to be a complete loss of coolant, nearly all the heat content of the coolant and fuel prior to the accident would be released to the containment atmosphere. The volume and strength of the structure are such that it can withstand the maximum containment temperature and pressure that would be expected from the steam produced by the flashing of all the water in the primary circuit and from the effects of the ECCS. Typically, the calculated maximum pressure would be about 280 kPa(g); the containment structure is thus designed to withstand 310 kPa(g) and is tested at 350 kPa(g). At the design pressure, the leakage rate should not

exceed 0.1 percent of the containment volume per day. Spherical contain­ments may be designed for pressures as high as 500 kPa(g).

Example 12.1. Consider a large PWR with a total primary system coolant inventory of 350,000 kg at 15.5 MPa(a) and 320°C. If the contain­ment free volume is 57,000 m3, make an initial approximation of the mag­nitude of the pressure load on the containment following a loss-of-coolant accident.

As a first step, let us determine the amount of steam produced if the final pressure is “guessed” as 200 kPa(a) (about 2 atm). From the steam tables, the specific enthalpy of the subcooled coolant water prior to blow­down is 1462 kJ/kg. At 200 kPa, the specific enthalpy of the saturated liquid is 505 kJ/kg and the latent heat of vaporization is 2202 kJ/kg.

The steam produced would then be approximately

350,000(1462 — 505)
2202

The corresponding specific volume is

57.000

152.000

This corresponds to a pressure of 500 kPa(a), which we use as a second trial. Then, using new properties, the steam produced would be

350,000(1462 — 640)
1922

The new specific volume is then 0.38 m3/kg, which is relatively unchanged. Considering that the total pressure is made up of the partial pressures of air and steam, the gage pressure is roughly 5 atm. However, other effects must be considered in an actual calculation. Thermal energy absorbed by the components, building, and injected water must be considered as well as the heat input by fission product decay. A pressure in the neighborhood of 3 atm is then likely to result.

12.48. In order to cool the containment atmosphere and reduce the pressure by condensing part of the steam after a loss-of-coolant accident, water would be sprayed through nozzles near the top of the structure. The water, which collects in the containment sump, can be recirculated through
the heat exchangers of the residual-heat removal system (§12.29) to provide continuous cooling of the containment atmosphere.

12.49. The containment sprays also serve to remove some of the radio­activity from the atmosphere. Sodium hydroxide or alkaline sodium thi­osulfate in the water facilitates the removal of radioiodines which are generally the determining factor in the environmental hazard that would result from a large radioactive release (§12.160 et seq.). In some PWR installations, the radioactivity level in the containment atmosphere would be reduced by using blowers to circulate the air through iodine absorbers and particulate filters.

12.50. A special type of PWR containment system makes use of an ice condenser to reduce the interior pressure. Instead of the usual steel-lined concrete structure, the steel liner (or shell) is separated from the outer concrete (or shield) building. The annular space between the liner and the concrete, above the level of the reactor vessel, contains cells filled with refrigerated borated ice. In the event of a loss-of-coolant accident, con­densation of the released steam by the ice would limit the pressure in the containment. Consequently, the structure may be designed for a pressure of only 69 kPa(g) and it can have a smaller volume than a conventional PWR containment building. Some fission products would also be removed in the condenser. Furthermore, the borated water formed by the melting ice would collect in the containment sump and would be available for core cooling.