New Lessons of the Fukushima Accident for Severe Accident Management Guidance - Some Important Short Term and Long Term Aspects

The accident at the Fukushima-Daiichi nuclear power plants (NPPs) has been studied extensively for lessons to be learned for severe accident management guidance (SAMG). As a consequence, as has been reported in various publications, revisions have been made to a number of PWR and BWR SAMG programmes. They include such effects as deviating instrumentation behaviour under severe accident conditions, leaking hydrogen from the containment to adjacent compartments, role of turbine driven systems, spent fuel pool aspects. Forensics of the damaged reactors at Fukushima has also provided other important insights that warrant specific attention in accident management programs. They are both relevant for the short term as well as the long term - where there is no evidence that this latter part has received appropriate attention in most SAMG approaches, as they focus on achieving a stable safe condition on the shorter term. Many aspects, however, warrant longer term attention, which may also already influence the short term measures. Injection onto a degraded core can have substantial negative effects: it can repressurise the Reactor Pressure Vessel and thereby interrupt the injection by low pressure (e.g. portable) pumps, which in turn can even promote core melting by the heat produced by the Zr-water reaction; the hydrogen generated this way can be a direct threat for the containment integrity that relies on hydrogen control measures. Prime objective must be to avoid any interruption of the injection flow. This matter is the subject of another paper by the authors at this conference. Primary Containment Penetration Failures - Severe accident guidance should stress the importance of assuring that reactor vessel pressure is low whenever reactor vessel lower head fail-ure is imminent and that any ex-vessel core debris is quickly covered with water to prevent excess-save heating and consequential failure of containment penetrations. Long Term Core Cooling (beyond SAMG) for a Severe Accident - Guidance is needed for preparation for maintaining adequate long-term core cooling when severe accident guidance is exited. For example, high rates of water injection after core cooling is recovered are counter-productive and cannot be continued in the long term. Long term water management to collect and process water that escapes from leakages from the various components containing radioactive material or from spraying fission product release points. Integration of SAMG with procedures for Site Disruptive Accidents (SDA) - to restore/maintain command and control after an SDA - and integration with procedures for the use of portable equipment to meet Station Black-Out (SBO) or Extended Loss of AC Power (ELAP); potential adaptation to meet SAMG needs. (author).