Fault-tolerant digital feedwater control in nuclear power plants

Nuclear plants, such as PWRs and CANDU-PHWRs (Pressurized Heavy-Water Reactors) have experienced both controllability and reliability problems with analog controls for steam generator level. This has been a major operability problem on the KANUPP PHWR, and is the limiting factor in the maneuverability of the Pickering A 4 x 540 MW PHWR units in Canada. In the United States, the Nuclear Regulatory Commission (NRC) and the Institute for Nuclear Plant Operation (INPO) have focussed their attention on feedwater control because the operability problems during startup and low power operation on most PWRs have caused a large number of scrams. Prompted by the INPO and NRC interest and by the good operating experience CANDU-PHWRs with Digital Feedwater Control Systems (DFCS), EPRI launched a pair of demonstration DFCS projects for U.S. LWRs; one BWR - Monticello and one for a PWR - Sequoyah. Since then a number of U.S. plants have initiated programs to retrofit digital feedwater control. This paper addresses the identified issues of importance to LWR utilities looking to retrofit a DFCS. Examples of approaches in design are drawn from the dFCS for Philadelphia Electric Co.'s Peach Bottom Atomic Power State and from CANDU-PHWRs such as Pt. Lepreau, both designed by AECL