Source Terms for potential NPPs at the Lubiatowo site

In September 2013 a company (PGE) was established in order to construct the first nuclear power plant in Poland. The three designs being considered are the Hitachi GE Advanced Boiling Water Reactor (ABWR), the Westinghouse Advanced Passive (AP) 1000 and the Areva European Pressurized Reactor (EPR).

The present project provides for each of the designs a suite of source terms, taken from open literature. In a second step two specific source terms have been selected for calculations: (1) a source term for a severe accident in which containment integrity is maintained and the containment leaks at the design leakage rate; and (2) a source term in which the containment is assumed to fail, or to assumed to be bypassed.

The selected source terms for the ABWR were the severe accident with intact containment (Case 0) and the Case 13, an accident with loss of all core cooling with vessel failure and passive flooder system operating.

For the AP1000 one accident was a severe accident with containment intact (IC). The second selected accident was a containment bypass scenario (BP).

The EPR source terms were accidents with intact containment and deposition in annulus and building (RC101), and a small interfacing system LOCA, unscrubbed with deposition in building (RC802a

The release fractions for the Iodine and Cesium group in all the selected “intact containment” cases are well below 0.01%. In contrast, the release fractions for selected severe accidents with containment failure are dramatically high, in particular for noble gases, Iodine and the Cesium group.

The results show that “containment failure” sequences can lead to very large releases for each of the reactor designs. Their releases are several orders of magnitude above the releases of the “intact containment” sequences. However, also the calculated frequencies for containment failure sequences are in 1 to 2 orders of magnitude below their “intact containment” counterparts.

Although the cases that are presented here have been analyzed by the respective designer of the various reactor types, one should read the results with caution. Looking just at two accident sequences can provide only an indication what could happen with a certain probability. For an exhaustive evaluation of the risk of a reactor design it is necessary to refer to the full probabilistic and deterministic safety analysis reports.

But even if those sequences seem unlikely, they are (and should be) considered in the safety case of a nuclear reactor. In general these type of calculation results as well as frequencies are subject to huge uncertainties (example: seismic uncertainties). Just looking at accident sequences which are considered as likely (according to current standards) could leave critical weaknesses unattended (as a side note, a multiunit station blackout for more than 12 hour was considered to be almost impossible, before the Fukushima accident happened). Whether the very small accident frequencies published by the reactor designers can withstand a thorough analysis remains to be seen and is beyond the scope of the present study.

Due to its nature the present study does not give any information whether a certain design is better than another. The comparison of safety features was not within the scope of the project. The limitations demonstrate that the accidents and the releases of the selected accidents cannot be compared one to another.

Contact: Müllner Nikolaus

Universität für Bodenkultur Wien