The BWR differs from the PWR in two very important respects:

• The water is allowed to boil as it flows through the core.

• The steam thus evolved passes directly to the turbine; that is, the BWR is a direct-cycle design and the need for intermediate steam generators is eli­minated (Table 1.8 and Fig 1.29).

It follows that the coolant pressure is only slightly higher than that required at the turbine, typically about 80 bar, and the temperature 300°C. The lower coolant pressure means a lower core power density (about 50 MW/m3) and is generally about half that for the PWR. Hence the core volume of a BWR is correspondingly larger, being about double that of a PWR to give the same thermal output.

The BWR pressure vessel however is more than double that of the PWR because of the additional need to accommodate steam separators and dryers above the core and a number of jet pumps located in the annulus between the core and the vessel wall. The steam separators take out most of the water phase, allowing it to return for recirculation via an annular downcomer which surrounds the core. The steam, which leaves the separators with a low mois­ture content still entrained, passes through mesh type dryers at the top of the pressure vessel before leaving, >99.5% dry, to drive the turbine. The jet pumps, numbering twenty or more, form part of the coolant recirculation system. This system normally consists of two independent loops, each having a recirculation pump and associated control valves located external to the pressure vessel. Although the pressure vessel for a BWR is appreciably larger than for the PWR, the lower operating pressure results in a wall thick­ness about half that of the PWR vessel.

The turbine steam is radioactive, mainly due to the radionuclide N-I6 formed from an (n, p) reaction with the 0-16 in the water, requiring the high pressure end of the turbine to be shielded. Fortunately, N 16 has a short half life of 7.1 seconds and maintenance work during shutdown is not unduly hampered as the activity soon decays away.

The BWR fuel rod is similar to that of the PWR but it can tolerate a larger diameter because of the lower power density. A fuel bundle is based on an 8 x 8 array. The whole is encased in a square zircaloy channel to prevent lateral flow between adjacent as­semblies, in contrast to the open array of the PWR fuel assembly.

Control rods are cruciform in shape and move in the interspace between four fuel assemblies. They are driven hydraulically from below the core and thus counter neutron flux distortion due to steam voids in the upper region of the core. Also, as refuelling is off-load (burn-up values approaching 30 000 MWd/t) and requires access to the core from above, bottom entry for the control rods does not interfere with the annual refuelling. Reactivity is also regulated by varying the coolant flow through the core and hence the value of the void fraction as the degree of boiling is increased or decreased.

BWRs are generally housed in primary and second­ary buildings. The former, termed the ‘drywell’, is a steel pressure vessel surrounded by reinforced concrete and is designed to withstand LOCA pressure transients. The secondary containment building, termed the ‘wet — well’, completely encompasses the drywell. The base of the wet well contains a pool of water connected to the drywell via large submerged ducts. In the event of a LOCA, high pressure steam in the drywell is directed into the pool where it condenses. For this reason the wetwell is sometimes called the pressure suppres­sion chamber. The pool of water is also available for use by the Emergency Core Cooling System (ECCS).