GENERAL EVENT DESCRIPTION
On 4 June 2008 at 15:07 there was an event at the nuclear plant Krško. Operators detected an increased leakage of water from the primary system inside the containment. For such cases adequate procedures are in place and they required that emergency of the lowest level - unusual event was declared at 15:56. According to the procedures the plant started to decrease power at a steady rate. The reactor was shut down at 19:50 and the plant was being cooled down after that until the following day. It was found out that the seal degraded on a valve on one of the smaller pipes, which were connected to the primary system. When the working conditions were reached, the valve was replaced and the fault was eliminated. Technical details about the event are enclosed.
During the event there was no release of radioactivity into the environment and it was not necessary to start with any kind of protective measures for the nearby population. All the leaking water was collected in the containment sump and later pumped into the Waste Holdup Tank for future processing by the Liquid Waste Processing System.
At 16.07, that is 11 minutes after declaring the emergency, the personnel of the plant informed about the problem the Slovenian Nuclear Safety Administration (SNSA), which is as an independent nuclear safety oversight authority. Activation of the national emergency response system according to our National Nuclear Emergency Response Plan is not necessary for an event of such level, but the SNSA decided to partially activate the SNSA emergency response organisation. Leakage from the primary system was relatively small and stable, but at that moment the reason for leakage was not known and possible increase of leakage could lead to a more serious event of the loss of primary coolant. Therefore the decision of the SNSA was justified as being conservative.
One of the first duties of the SNSA was also to take care of an appropriate informing of the public abroad and in Slovenia. Slovenia is a signatory of Convention on Early Notification of a Nuclear Accident and as well signed bilateral agreements, which refer to the early notification in the case of radiological emergency with three neighbouring countries (Croatia, Hungary and Austria). As the EU Member State Slovenia is as well liable to report to the European Commission and via EC to all member states in the EU in the framework of ECURIE system. All these agreements prescribe an early notification when it comes to a situation when the state should take measures for the protection of its citizens.
In this case it was already from the beginning of the event possible to exclude with great certainty any threat to the public in the vicinity of the plant, or in any other country. As it was the first time that the National Nuclear Emergency Response Plan was declared on the real event, the SNSA decided to act as "we took measures with the purpose to protect and inform our citizens" and therefore to send appropriate notices abroad via all the channels Slovenia is obliged to report through with the philosophy: it is better to inform too early than not to inform at all and the event could later lead into a serious one.
SNSA decided to send its first notification via the ECURIE system, because this is an European system which provides the earliest notification of all contact points (headquarters) in every member state, including our neighbouring countries. Immediately after the notice was sent for domestic public, some minutes later a message was prepared and sent via the EMERCON system to IAEA and with the same form to our four neighbouring countries.
On this standard IAEA form, a mistake occurred indeed, because when filling out the form it was forgotten to cross off the word EXERCISE, which is printed at the top of the form. IAEA immediately called the SNSA and the latter apologized and asked them to correct the mistake. The next day SNSA prepared a press conference and described how this mistake occurred and how quickly it was corrected. At the time when the neighbouring countries were receiving this form of ours, they already had information via ECURIE system and EMERCON system so that they could quickly find out that probably a mistake occurred and they called us to verify the content of the form.
The European Commission announcement to the public after 7 p. m. launched an outstanding response of media in whole Europe. Based on the lessons learned the system of mutual communication in the European Community will have to be revisited and discussed. Fast informing is obviously working but care should be taken that new problems do not arise along with this.
TECHNICAL DESCRIPTION OF THE EVENT
Course of the event:
1. On 4.06.2008 the main control room (MCR) crew observed an increased leakage from the primary system to the containment. The indication was based on a difference between charging and letdown flow. The estimated leakage was about 3 m3/hr, while normal leakage during operation is about 0,007 to 0,010 m3/hr, i.e. 7 to 10 litres per hour). There were also other confirmations of the leakage (increased readings on the radiation monitors R-11 and R-12. as well as increased humidity in the containment and an increased level of the drainage sump in the containment).
2. The main control room activated Abnormal Operating Procedures to: (a) verify the available equipment for the inventory control, (b) to determine leak location and its possible isolation. After it was found out that the leakage can not be isolated and that the leakage exceeded Technical Specification (TS) limit, the plant declared first level of emergency (i.e. »Unusual event«) at 15:56 CEST. At this time also the SNSA was informed about the event – a message from the NPP was sent out on 16:07 CEST.
3. The technical director of the plant assumed position of the Emergency Director from the Shift Supervisor at 16:39. CEST. Since the leakage exceeded TS limit for »unidentified leakage«, which is 0,227 m3/hr (for »identified leakage« this limit is 2,27 m3/hr), the MCR crew started decreasing power from the full power (686 MWe) with a rate of 5 MWe/min by using General Operating Procedures. The plant was disconnected from the grid at 19:30 CEST and the reactor was subcritical at 19:50 CEST. At 19:55 was the first entrance to the containment. After the first assessment that the leak location was in the vicinity of the reactor coolant pump No. 2, the more exact location was determined later on: it was a leakage from one of the valves of the resistance temperature detectors (RTD) manifold. At this stage »unidentified leakage« became »identified leakage«.
4. To perform a valve repair, the cooldown of the system from the operating temperature and a decrease of pressure were required. The temperature was: < 177 ºC at 01:37, <93 ºC at 07:55, and the pressure was 70 kp/cm2 at 00:22, 3 kp/cm2 at 15:00 (all times CEST on 05.06.2008). At 12:40 on 05.06.2008 there was the end of »Unusual event« declared by the Krško NPP. The long term of core cooling was established via the residual heat removal (RHR) system.
5. Plant personnel performed replacement of the leaking valve with a new one.
6. Before reconnecting to the grid, the NPP carried out detailed analysis of the situation and the corrective actions taken. After successful inspection of all the RTD manifold valves the plant was on 8 June 2008 afternoon ready for start-up. The NPP was reconnected to the electrical grid on 9 June 2008, at 3:38 p.m. local time. The plant was operating at full power by the end of the morning, 10 June 2008.
7. The SNSA has analyzed the situation and corrective actions as well and carried out a special regulatory inspection for this purpose. The results of the inspection showed that there were no reasons to delay the plant start up.
Event analysis:
1. Current maintenance experience: The Krško NPP maintains the valves of the RTD manifold loop by the programme of »Preventive Maintenance of Hand-operated Valves« (ADP-1.4.215). The valve 8067B, which experienced leakage, was exempted from the programme based on the EPRI experience and documentation (EPRI – Electrical Power Research Institute). All these (RTD manifold) valves are a subject of visual inspection during walk-down of the containment after each plant shutdown or start-up. If non-conformances were observed (traces of boron on the valve housing), then:
in case of a smaller leak, the valve seal package was checked and tightened by additional torque controlled by torque wrench,
in case of a larger leak, the valve was replaced entirely.
2. Corrective action: After the event on 04.06.2008 the valves 8067B and 8063B were both replaced due to easier repair, also a seal package on valve 8073A was replaced (it was on a »back-seat« after outage in 2007 – i.e. last outage) and all seal packages of all valves on RTD manifolds were checked.
3. Possible cause: The most probable reason for leak on the valve 8067B was degradation of the seal package. During the outage of 2006 a leakage on this valve was detected and it was repaired by applying additional torque on seal package. During outage 2007 there was no leak observed on this valve and there were also no other corrective actions on the other RTD manifold valves. Actually the seal package (six seal rings) of valve 8067B was found degraded during the outage 2006, then the degradation propagated further and the leakage through the seal package occurred. During this process the nuts of seal package become loosened and also the valve stem was eroded due to water/steam leakage.
4. About the consequences:
There was no leakage of radioactive substances to the environment,
Reactor safety systems were not activated neither was a need for their activation,
There were no health effects, neither to the employees nor to the population.
5. Long term actions:
Checking of the industry practice and existing recommendations about the maintenance of RTD manifold valves.
Paying more attention to RTD manifold valves during each outage (more thorough checking and inspection of valves).
Possible replacement of this measuring system (RTD manifold) by an up to date system performing the same function.
