Nuclear fission

10.

11.Examples of ACCIDENT PROGRESSION in spent fuel pool

11.1Example from GRS for Spent fuel pool accident in a PWR

GRS has been working on a research project financially supported by the German Federal Ministry of Economics and Technology (BMWi) regarding the extension of probabilistic analyses for SFP. Supporting deterministic analyses of the accident progression inside the SFP were a main part of the project. The accident progression has been analysed for both PWR and BWR pools by using the integral code MELCOR 1.8.6. Objective of the research project was the development of a basic approach for consideration of SFP within Level 2 PSA, the quantification of event trees, and the identification of possible mitigative accident measures. The work has been kindly supported by the utilities of the two reference plants by providing plant data.

General boundary conditions of the MELCOR analyses are:

• 
  the modelling includes spent fuel pool, reactor circuit, containment/reactor building (closed) and relevant compartments of adjacent buildings,
  
• 
  passive autocatalytic recombiners (PARs) are considered as realized in the plants, other SAM measures have not be considered,
  
• 
  different loadings of the pools:
  
  • 
    standard loading during normal power operation (shortly after finishing in-service inspection  highest decay heat for that operating mode),
    
  • 
    partial loading during in-service inspection (one third of the core has been moved into SFP; connection with filled flooding compartment and reactor pressure vessel),
    
  • 
    full loading: inclusion of the whole core from RPV into SFP; pool connected to filled flooding compartment and RPV, and
    
  • 
    full loading: inclusion of the whole core from RPV into SFP; pool separated from flooding compartment (worst case regarding the timing of severe accident sequence).
    
• 
  Postulated initiating event: long lasting station black out (Leaks in the lower part of the SFP are considered as practically eliminated for German NPPs due to the design of the pool (pipe connections only in the upper part of SFP, at least 6 m above the top edge of racks; design against earthquakes between VI and VIII on the EMS/MSK scale).
  

A schematic picture of a SFP inside a German PWR is given in Figure 10.1..

Figure 10.1. Schematic arrangement of SFP in a German PWR with potential containment failure modus

Detailed modelling of the containment and reactor building annulus was applied. 58 passive autocatalytic recombiners (PARs) distributed on 37 control volumes have been considered. Depletion of hydrogen and carbon monoxide is calculated by the PAR model.

Table 10.1. provides summary results for four calculated sequences.

Table 10.1. Summary results of PWR SFD analyses

	Standard Loading (hh:mm:ss)	Partial Loading (hh:mm:ss)	Full Loading, Flooding Compartment filled (hh:mm:ss)	Full Loading, Pool separated*** (hh:mm:ss)
Initiating event	00:00:00	00:00:00	00:00:00	00:00:00
Water Level at top edge of racks	342:08:20	105:41:40	112:58:20	50:28:30
Failure Fuel Assemblies	-	538:34:30	165:42:00	65:29:44
Water Level at lower support plate of racks	677:38:20	239:04:00*	174:02:54	99:03:20
Start of significant relocation	-	538:50:30	166:01:40*	82:26:40*
Water completely evaporated	-	284:51:40*	180:31:40	108:50:00
Failure of steel liner	-	538:53:40	185:22.31	109:25:00
Start of MCCI	-	-	366:15:50	121:20:03
Failure of concrete of the bottom of SFP	-	-	413:36:40	132:06:14
Relocation in compartments below SFP	-	-	413:36:40	132:06:14
Relocation into sump	-	-	-**	134:23:20
Start first venting	-	-	477:48:20	-
Stop first venting	-	-	483:38:20	-
End of calculation	694:26:40	694:26:40	497:28:20	694:26:40

* Calculated point in time cannot be depicted in the right chronological order

** No 4^th cavity (sump) has been used

*** Calculation with four cavities
For the sequence “Full Loading, Flooding Compartment filled” the containment pressure (Figure 10.1.) and the hydrogen masses (Figure 10.1.) are shown.

MCCI SFP

End Venting

Start Venting

Figure 10.1. Containment pressure for sequence SFP Full Loading, Flooding Compartment filled

Oxygen consumed

Figure 10.1. Hydrogen masses for sequence SPP Full Loading, Flooding Compartment filled

The following general conclusions can be drawn:

• 
  MELCOR is, in principle, able to calculate SFP melt scenarios. However, several adjustments have to be made to the input, so that the results need careful interpretation.
  
• 
  Evaporation extends over several days  steam concentrations inside SFP and containment are high  impact of air oxidation is small, hydrogen is generated by Zr-Steam and MCCI chemical reactions.
  
• 
  Only for low decay heat inside SFP, where uncovering of the fuel assemblies is terminated before their heat-up, air oxidation can occur after steam concentration has been depleted.
  
• 
  Heat transfer by thermal radiation has an impact on the containment (calculated by a simple control function model)  strong heat-up of the containment above SFP beyond design temperature.
  
• 
  It might be helpful to initiate filtered containment venting earlier in case of SA inside SFP in order to prevent high containment loads and high venting temperatures later.
  
• 
  It is very likely that SA sequences inside SFP run into filtered containment venting.
  
• 
  During fuel degradation in the SFP (before MCCI begins) the temperatures are lower than in RPV accidents during normal operation  less release of radionuclides from fuel. After MCCI has started, the release fractions from fuel reach levels which are known from accidents in the RPV.
  
• 
  With full loading of the SFP, the fuel melt layer (including material of the racks) at the bottom of the SFP is in the order of 1 m. Such a thick melt layer would probably develop heat transfer mechanisms (convection, steel layer on top) which enhance lateral erosion. Depending on the NPP design, this may lead to different sequences than vertical erosion. In case of the German design, radial melt-through of the containment may be possible. If, on the other hand, corium penetrates through the bottom of the SFP into the sump region, MCCI could be stopped because of the large amount of water in the sump, and because the melt spreads on bigger areas.
  
• 
  For normal loading of the SFP (i.e. in normal operation with RPV fully loaded) the accident evolution in the SFP is much slower than in the RPV.
  

Based on these MELCOR analyses a simple and rough event tree analysis has been performed for SFP accidents. The event tree analysis indicates that there is more than 90% probability for successful containment venting. Late containment failure has approx. 5% probability (e.g. due to venting inoperability, thermal containment failure, containment melt-through). All other failure modes (e.g. open containment, damage due to hydrogen combustion) are insignificant.