Core melt accident

While there is active core melt accident research, the TMI-2 accident demon.strated what happens in an accident that ends in partial core melt. Such a partial melt is in contradiction to the RSS assumption that if a melt starts, it will propagate to a full melt which were the only accidents considered in the RSS. 

Bun/, H. et al., 1983, NAUA Mod 4 A Code for Calculating Aerosol Behavior in LWR Core Melt. Accidents Karlsruhe, Germany KfK-3554, August. 

Either to reduce the size so much that core melt accidents almost certainly can be contained by the vessel used (this involves maximum unit sizes of 50-100 MW in a traditional design, while the pebble-bed reactor may circumvent this limitation, if the integrity of the pebbles can be guaranteed),... 

Experimental information on the transient behaviour of CAPRA pins, diluents and pools with inert materials is urgently needed. The proof that inherent features and special measures/devices will work under the various circumstances of a core-melt accid its needs extensive code calculations. Experiments have to show the basic operability under specific accident conditions. 

The PLEXUS code is used to calculate dynamics mechanics structural response due to various accidents such as sodium-water reaction or mechanical energy release following a core melt-accident. 

C. W. Forsberg and P. Peterson, Making Core Melt Accidents Impossible in a Large 2400-MW(t)... 

How Large Can Reactors Be Built Without The Possibility Of A Core-Melt Accident ... 

The evaluation is based on an assumption of a core melt accident. 

When the design review of the BWR 90 was initiated, regulatory developments indicated a need to strengthen the capability of the reactor containment to withstand the effects of a core melt accident. Such requirements are now codified in Finland and Sweden. The essential features of the BWR 90 containment to achieve enhanced environmental safety, including protection during a degraded core accident, are ... 

When source terms for a complete core melt accident (in which the melt progress could not be stopped within the reactor pressure vessel) have to be calculated, the aerosol production during the core - concrete interaction phase also has to be taken into consideration. In the Reactor Safety Study (US NRC, 1975), an empiric approach was used with respect to the fission product release during this phase, in recognition of the fact that during this stage the environment is chemically oxidizing and that a metallic iron phase is present. From this approach, it was concluded that the remainder of the volatile fission products still present in the molten corium... 

The retention of fission products by scrubbing the steam flow in a BWR pressure suppression pool is based on the exchange of matter between gas and liquid phases. Similar scrubbing processes are also at work in other accident situations, such as in a PWR steam generator tube rupture event (see Section 6.2.3.), as well as in the Venturi scrubbing process during controlled depressurization of the containment after a core melt accident (see Section 7.3.4.4.). 

All the results discussed above were relative to the pure system starting from I2 + H2O. As has been mentioned, in the situation of a core melt accident one has to deal with solutions containing a mixture of 1 and I2, usually showing a large excess of initial I". At the containment temperatures to be considered, no measurable volatility is to be expected for 1 from aqueous solutions. Aim et al. (1979) have derived from measurements made on pure iodide solutions in dilute boric acid, an I" partition coefficient of about 10 (pH 5, 100 °C, 10 gI/1) and have explained this value by the volatility of hydrated 1 ions. However, possible alternative explanations such as 1 carry-over by droplets (entrainment), volatility of HI or the presence of trace amounts of I2 in the iodide solution due to air oxidation may be more probable according to a rough estimate, the apparent I" partition coefficient mentioned above could be caused by an I2 fraction in the test solution of about 0.05%. 

Table 7.23. Parameters affecting Fission product release in the Chemobyl-4 accident and in LWR core melt accidents (Albrecht, 1987)...

Parameter Chernobyl accident Core melt accident... 

Reactor coolant system boundary isolation failure could result in coolant blowdown and overpressurization of the low pressure piping. This can lead to a loss of coolant accident and containment bypass that, if combined with failures in the emergency core cooling systems, would result in a core-melt accident with significant off-site radiation releases. 

Malfunctioning of power operated valves could create unacceptable situations during accidents and contributes to the risk associated with postulated core-melt accident sequences. 

A complete loss of the ESW system could potentially lead to a core-melt accident, posing a significant risk to the public. 

Improvements of the starting reliability of onsite EDGs will reduce the probability of events which could escalate into a core melt accident. 

The end-state of each path on the containment event tree describes the effectiveness of the containment to mitigate offsite doses for that accident sequence. The radiological consequences of the core-melt accident are largely determined by three major considerations ... 

The in-containment refuelling water storage tank water is also used to flood the contaimnent during refuelling operations and core melt accidents. 

The disadvantage of choosing the in-vessel retention option is that there is a shght risk that the natural movement of the in-containment water over the reactor vessel in a real core melt accident sequence could be insufficient to prevent core melt-through. 

Two years after the 10 October 1957 Windscale military reactor fire and core melt accident formal external regulation of the UK civilian nuclear power industry through the Nuclear Installations Act was enacted in the UK in 1959, under the auspices of the Nuclear Installations Inspectorate. A government press release at the time nevertheless stated that the fire had no bearing on the safety of nuclear power stations being built for electricity authorities . One substantial later implication of the accident was the establishment of the UK National Radiological Protection Board (NRPB) in 1971. 

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