Water is the most commonly used boiling coolant, and about 30% of the world’s nuclear reactors are boiling-water reactors (BWRs). These reactors were de­scribed in Section 2.4.

Many of the features of water as a boiling coolant are identical to those of water as a liquid coolant, which were described in Section 3 5. It should be noted that BWRs operate at much lower pressures than PWRs (7 rather than

15.5 MPa, 1000 rather than 2300 psia).

Using water as a boiling rather than a liquid coolant entails the additional im­portant problem of radiolysis, whereby the water is decomposed into its con­stituent elements, hydrogen and oxygen, which are released into the vapor during the boiling process. The rate of recombination of the hydrogen and oxy­gen is much slower than in a system operated purely in the liquid phase, lead­ing to higher concentrations of oxygen in the circuit fluid. Since the circuit is under stress due to the high pressure, a form of corrosion called stress corrosion cracking can occur, and this has presented a major difficulty in the operation of BWRs. It can be overcome by using more resistant materials, but replacing pipework in existing reactors is obviously an expensive process.