1. Approach to Practical Elimination in Finland
M-L. Järvinen, N. Lahtinen and T. Routamo
International Conference on Topical Issues in Nuclear Installation Safety: Safety Demonstration of Advanced Water Cooled Nuclear Power Plants
6th – 9th June 2017
Vienna, Austria

2. Content
Finnish safety requirements stipulating practical elimination concept
The implementation of the requirements for NPPs
Examples of application:
Pressure vessel rupture
Severe accident management
MlJ

3. Evolution of the Finnish safety requirements
Consti-tution
Laws, Decrees
STUK Regulations
YVL Guides
Standards
References in the requirements
General Design Principles of a Nuclear Power Plant, 1976
55 criteria
Based on 10CFR50, Appendix A
YVL 1.0 Design Principles of a Nuclear Power Plant, 1982
State of the Art requirements, e.g. severe accident management requirements
Consideration of the IAEA Safety Series after midd-1980
EURATOM regulations, directives
WENRA reference levels and safety goals for new reactors
A new level, STUK mandatory requirements, 1st January 2016
MlJ

4. Safety requirements related to practical elimination concept
In STUK regulation Y/1/2016 items with extremely low possibility include
for reactor design
criticality accidents (§10, 3. a) iii .); (nuclear reactor's physical feedback characteristics (§11, 1) )
loss of integrity of (major) pressure bearing components (§10, 3. b) i.);
containment leaks as a consequence of reactor pressure vessel failure (§10, 3. c) iii);
and in connection of fuel handling and storage
criticality accidents (§12, 4.);
severe accidents (§12, 5.).
Further detailed requirements in YVL Guides, especially such as Guide YVL B.1 (design), Guide YVL A.7 (PSA) and Guide YVL C.3 (radioactive releases)
Nuclear Energy Decree Section 22 b states that;
“The release of radioactive substances arising from a severe accident shall not necessitate large scale protective measures for the public nor any long-term restrictions on the use of extensive areas of land and water.
In order to restrict long-term effects the limit for the atmospheric release of cesium-137 is 100 terabecquerel (TBq). The possibility of exceeding the set limit shall be extremely small.
The possibility of a release requiring measures to protect the public in the early stages of the accident shall be extremely small. ”
MlJ

5. Basic features of the YVL requirements
YVL B.1
deterministic analyses complemented by probabilistic risk assessments and expert assessments
cannot be based solely on compliance with a cut-off probabilistic value
all practicable measures shall be taken to reduce the risk
examples of events
YVL C.3
any release of radioactive substances in a severe accident shall not warrant the evacuation of the population beyond the protective zone or the need for people beyond the emergency planning zone to seek shelter indoors
Reference to Guide YVL A.7
YVL B.1
YVL C.3
YVL A.7
YVL A.7
a) the mean value of the frequency of a release of radioactive substances from the plant during an accident involving a Cs-137 release into the atmosphere in excess of 100 TBq is less than 5∙10-7/year;
b) the accident sequences, in which the containment function fails or is lost in the early phase of a severe accident, have only a small contribution to the reactor core damage frequency.
MlJ

6. The Defence-in-Depth levels, event categories, frequencies and relation to the phenomena to be practically eliminated in the Finnish context. *)
*) and the accident sequences, in which the containment function fails or is lost in the early phase of a severe accident, have only a small contribution to the reactor core damage frequency.
MlJ

7. Continuous improvement of safety, implementation of YVL Guides issued in 2013
Section 7 a § Guiding principles
The safety of nuclear energy use shall be maintained at as high a level as practically possible. For the further development of safety, measures shall be implemented that can be considered justified considering operating experience and safety research and advances in science and technology.
New YVL guides are applied as such to the new NPPs i.e. those for which Decision in Principle was made in 2010
For the operating NPPs or other nuclear facilities and the NPP under construction an implementation decision is made
the nuclear facility is reviewed against new YVL Guides
the modifications considered reasonably practical are implemented
The application of Section 7a of the Nuclear Energy Act and exemptions are explicitly described in building regulations.
Operating NPPs
Implementation of updated regulations has been completed in 2015 – 2016 and resulting safety enhancement projects are ongoing.
licensees are required to analyse a wider spectrum of initiating events compared to examples given in Guide YVL B.1.
Olkiluoto 3 NPP unit
in operating license phase
reviewed as part of licensing process
Hanhikivi 1
in construction license phase
MlJ

8. Example Loviisa unit 1 and unit 2
Reactor pressure vessel
mainly based on standards that were in force in the Soviet Union
sufficient safety factor
ISI-inspections, qualified system
embrittlement margins
Severe accident
top level critical safety functions of SAM strategy
Containment isolation
Primary system depressurisation
Absence of energetic events (mitigation of hydrogen combustion)
Reactor core cooling (RPV lower head coolability and melt retention)
Containment pressure control (long-term containment cooling)
in addition
Subcriticality of the core
Cooling of refuelling pools
Severe accident safe state (SASS) in the long-term
SAM safety functions and the SAM systems of the Loviisa NPP.
MlJ

9. Example Olkiluoto unit 1 and unit 2
Reactor pressure vessel
ASME Boiler and Pressure Vessel Code
sufficient safety factor
ISI-inspections, qualified system
lifetime for 60 year needs to be demonstrated
Severe accident
Ex-vessel SAM strategy
Automatic primary system depressurisation
Gravity driven flooding of lower drywell for ex-vessel core melt cooling
penetrations in lower drywell have been shielded
steam explosions must not endanger containment integrity
Containment water filling from external source
Filtered containment venting
a venturi scrubber with filter retention of 99.9 % for aerosols and 99 % for elemental iodine
Containment pH control
NaOH solution can be injected to the containment spray to prevent formation of molecular and organic iodine
Containment filled with inert nitrogen gas during power operation 
SAM safety functions and the SAM systems of the Olkiluoto NPP units 1&2.
MlJ

10. Example Olkiluoto unit 3, (EPR under construction, operating license phase)
Reactor pressure vessel
ASME Boiler and Pressure Vessel Code
sufficient safety factor
ISI-inspections, qualified system
60 year design lifetime
Severe accident
SAM provisions integrated already in the basic design
Ex-vessel SAM strategy
Primary system depressurised prior to pressure vessel failure
Core melt cooling in a separate compartment. Passive flooding
Containment heat removal by sprays
Hydrogen mixing and removal by passive autocatalytic recombiners
Containment filtered venting
a controlled way to decrease the containment pressure in the long-term
a diverse path to release heat in specific accident scenarios during refuelling outages
SAM safety functions and the SAM systems of the Olkiluoto NPP units 3.
MlJ

11. Example Hanhikivi unit 1 ( DiP assessment 2014)
Reactor pressure vessel
design objectives and design principles of the main nuclear components of the AES-2006 are mainly in compliance with the Finnish safety requirements
effects of reactor pressure vessel material (radiation embrittlement )
Severe accident
STUK finds that it is possible to implement the systems and strategy for managing severe accidents in compliance with the Finnish safety requirements. The implementation of the functions for severe accidents by independent systems in compliance with the Finnish requirements shall be verified in the construction licence phase.
MlJ

12. Conclusions
The ”practical elimination” of early or large releases consist of two aspects
Defence-in-Depth effectively in design and operation of a reactor
specific accident sequences, provisions ensure no need to be included mitigation in the design
The idea of practically eliminated
reactor pressure vessel rupture, fast increase of reactivity already in early designs
conditions arising during severe accident needs to be assessed as part of demonstration
severe accident management systems have been implemented also for operating NPPs
The Finnish approach for practical elimination of large and early releases
is driven by limiting the overall frequency of accidents
addressing sequences for which mitigation is not feasible, extremely low probability
addressing specific sequences supports also achieving the probabilistic overall safety goal
MlJ

13. Thank you!
MlJ