The most serious accident considered for the design basis of a B"^TC begins with the rupture of one of the pipes connecting the (external) circulating pump with the reactor vessel as illustrated in Figure 4.28 This initial rupture produces a more gradual depressurization than is the case in a P"^WR since the pipe is con­siderably smaller than the main pipework in a P"^TC system (50 cm compared with 80 em in a P"^WR Other factors restricting the depressurization rate are the facts that the reactor vessel contains about 40% steam by volume and that the steam line is shut off within a few seconds, isolating the vessel from the main heat sink (the turbine) so that the system coolant can escape only from the break. Although the flow in the damaged loop would reverse due to the break, core cooling is maintained during the early part of the accident since the feed pump continues to rotate (coast down) for some time, feeding water to the ves­sel, and circulation continues in the undamaged loop. Eventually the feed pump flow stops and the suction of the jet pumps (which circulate liquid in the

vessel) becomes uncovered, causing the core flow rate to drop to zero (Figure 4.29). Due to this core flow stoppage, the core begins to dry out and increase in temperature after about 10 s from the initiation of the break. The flow at the break switches mainly to steam, the water in the annular space containing the jet pumps being completely discharged, and steam formation occurs in the lower plenum as the system pressure decreases more rapidly. Vaporization oc­curring in this way because of depressurization is often referred to as flashing, and the effect of lower plenum flashing is illustrated in Figure 4.30. The flash­ing effect causes a two-phase mixture to flow up through the jet and the core, resulting in enhanced core heat transfer during this period.

After about 30 s, the emergency core cooling system is triggered and the au­tomatic depressurization system operates, reducing the vessel pressure and al­lowing the LPCI and LPCS to come into operation. In the boiling-water reactor, the fuel is in the form of fuel elements consisting of a number of fuel pins mounted in a shroud, i. e., a rectangular box open at the upper and lower ends. The systems inject water above the core, and this water flows into the lower

plenum down the shroud surrounding each fuel element. The existence of this water near the fuel elements causes them to heat up much more slowly, and, eventually, water passing down the shroud into the lower plenum floods the lower plenum and water begins to rise through the core, quenching it in much the same way as in the P’^TC. Just as in the P’^TC, during this reflooding phase, the reflood rate is limited by the rate at which the generated steam can es­cape—the steam binding effect. This phase of the LOCA event is illustrated in Figure 4.31. Figure 4.32 shows a typical calculated temperature response of the shroud (channel) and fuel rods during a LOCA.