Nuclear fission

4.10Radioactive release issues (e.g. PH control in the containment, source term assessment)

The limitation of radioactive release into the environment is one of the main SAM strategies. The most efficient way to limit these releases is to maintain the containment integrity and tightness from the beginning of the accident to the NPP stabilization (at atmospheric level). Any uncontrolled failure or bypass of the containment during a fuel melt accident can induce catastrophic offsite consequences, in particular during the early phase of the accident. Maintaining the containment integrity is the first priority of SAM strategies.

As objective during severe accident on the first place is to minimise radioactive releases into the environment from the very high source term available inside the containment. Some of the most important challenges for limitation of radioactive releases into the environment are:

1. 
   Failures of containment isolation valves to close properly upon request. Respectively measures for assurance of the containment isolation valves closure are needed in SAMG. These have to be applied also to the personnel hatches and hatches for observation and maintenance.
   
2. 
   Leakages on a protection systems during sump recirculation mode in equipment located outside containment. Respectively measures for prophylaxis and maintenance of equipment, control of seals and prevention of their violation, etc. are of high importance.
   
3. 
   Failures of filtered venting system (FVS). Respectively measures for prophylaxis and maintenance of the FVS and assurance of thermal conditioning and other preparatory operation of FVS system are needed.
   

Iodine is a significant contributor to the doses caused by a severe reactor accident. This is due to the fact that iodine can exist in a highly volatile form that cannot be easily removed from the containment atmosphere.

pH in the sump is one important parameter in determining the formation of volatile iodine. The lower the pH of the sump water, the higher the fraction of volatile iodine. The sump water pH can be controlled by adding suitable chemicals to the sump. The equipment for feeding the chemical to the containment, as well as the storage tanks, should be protected against the hazards that may lead to, or result from, a severe accident.

4.10.1EDF&IRSN, FRANCE

4.10.1.1Status

EPR (PWR): pH control of the sump is achieved by Sodium Hydroxide injection (either safety or spray injection).

French Fleet (PWR): pH control of the sump is achieved by Sodium Hydroxide injection by spray system. On 1300 MWe and N4 reactors a pH control of the sump is also achieved by passive dissolution of sodium tetra borate. For 900 MWe, Silver concentration inside the control rod will fix iodine into the sump once melted, so no additional passive dissolution of sodium tetra borate is necessary.

4.10.1.2EDF L2 PSA modelling

EPR (PWR): Sodium Hydroxide injection (either safety or spray injection) are modeled in the L2 PSA. Failure of these systems leads to specific release category.

French Fleet (PWR): For 1300 MWe and N4 reactors the Sodium Hydroxide injection system used to be modeled in the L2 PSA, leading to specific release category, but it is not relevant any more considering the new passive dissolution of sodium tetra borate.

4.10.1.3IRSN L2 PSA modelling (for 900 and 1300 MWe PWRs)

The APET distinguishes cases with and without spray system (for sodium hydroxide injection). Source term calculations take into account the sump pH control, the impact of silver in sump, and main chemical reactions associated to iodine.

Radioactive releases are calculated for all type of accidents. A very fast running code (MER) is used for that purpose. That gives a possibility to present the risks in function of frequencies and consequences of each accident. Efficiency of each measure that helps reducing the release can be assessed (aerosol deposit by spray, pH control, containment tightness control …).

For example, the pH control can:

• 
  delay the gaseous halogens release (I2) from containment sumps;
  
• 
  reduce significantly the molecular and organic iodine releases.
  

4.10.2JSI, Slovenia

4.10.2.1Status

For Slovenian reactor, PWR type, the pH control is obtained by trisodium phosphate (TSP) in crystalline form. The TSP is stored in baskets, placed on the containment building floor and distributed along the wall to ensure a uniform distribution and mixing in the post-LOCA recirculation water. The long-term pH is raised by dissolution of solid TSP. Adjusting the containment sump pH greater than 7.0 will result in retention of iodine in the containment sump water and may mitigate iodine releases. In the SAM the containment sump pH is of long term concern. Namely, reduced pH levels might also degrade the long-term retentive capacity of fission products of the accumulated water. In a case of low pH the SAM recovery action is injection of buffer solution into containment.

4.10.2.2L2 PSA modelling

The pH control is passively achieved therefore these equipment is protected against hazards. There is no system used for pH control, therefore no PSA modeling is provided.

2. Tractebel, Belgium

The pH control of water in the containment sumps is done by the addition of NaOH.

The control of pH has an impact on the source term evaluation, as it allows a better retention of FPs in the liquid phase in the containment. Indeed, the sump water pH control along with the containment sprays are the two essential means to retain more FPs inside the containment and reduce the FP present in the containment atmosphere.

According to SAMG, the actions to perform the pH control by adding NaOH in both the Early and the Late phases are implemented in the L2 PSA model. The potential reduction of the FP inventory in the gas phase inside the containment depends strongly on the success of the SAM actions of pH control and of containment spray.