/if STUK-B-YTO 135

Operation of Finnish nuclear power plants

Quarterly report 1st quarter, 1995

Kirsti Tossavainen (Ed.) SEPTEMBER 1995 STUK-B-YTO 135 SEPTEMBER 1995

Operation of Finnish nuclear power plants

Quarterly report 1st quarter, 1995

Kirsti Tossavainen (Ed.) Nuclear Safety Department

FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY P.O.BOX 14, FIN-00881 HELSINKI FINLAND Tel. +358 0 759881 Translation. Original text in Finnish.

ISBN 951-712-062-1 ISSN 0781-2884

Painatuskeskus Oy Helsinki 1995 FINNISH CENTRE FOR RADIATION STUK-B-YTO 135 AND NUCLEAR SAFETY

TOSSAVAINEN, Kirsti (ed.). Operation of Finnish Nuclear Power Plants. Quarterly Report, 1st quarter. 1995. STUK-B-YTO 135. Helsinki 1995, 24 pp. + apps. 2 pp.

ISBN 951-712-062-1 ISSN 0781-2884

Keywords PWR type reactor, BWR type reactor, NPP operating experience

ABSTRACT

Quarterly Reports on die operation of Finnish nuclear power plants describe events and observations related to nuclear and radiation safety which the Finnish Centre for Radiation and Nuclear Safety (STUK) considers safety significant. Safety improvements at the plants and general matters relating to the use of nuclear energy are also reported. A summary of the radiation safety of plant personnel and of the environment, and tabulated data on the plants' production and load factors are also given.

Finnish nuclear power plant units were in power operation in the first quarter of 1995, except for two shutdowns at Loviisa 2, and shutdowns at both TVO units. The shutdowns at Loviisa 2 were due to an abnormal rise in the coolant outlet temperatures of certain fuel bundles. The shutdowns at TVO plant units were due to problems caused by the formation of frazil ice in sea water. Also, electricity generation at both TVO plant units was discontinued for a short period to repair a generator cooling system water leak. The load factor average of all four plant units was 93.9%.

The following events during the first quarter of 1995 were classified level 1 on the International Nuclear Event Scale (INES): At Loviisa 1, one of two isolation valves in the nuclear fuel storage pool cooling system pipeline were found to have been leaking since the 1994 annual maintenance outage. This valve is in a pipeline penetrating the containment building wall. In a potential accident, there would have been only one isolation valve to prevent the release of radioactive substances to the atmosphere via the pipeline in question.

Impurities were observed in certain fuel bundles at Loviisa 2, resulting in abnormally increased coolant outlet temperatures in the bundles in question. Six such bundles were replaced. The impurities might have compromised the cooling of the fuel bundles during certain transients.

In January, frazil ice accumulating in the filters of TVO Is cooling water screening plant caused the plant unit's shutdown by its automation systems. Later the same day, with die plant unit in shutdown state, die filters were again clogged up by ice. Supply of cooling water to systems important to safety is ensured by taking half of die service water required by die TVO plant units from the outlet side in die cold season. If die need arises, even die intake of the other half can be reversed. The reversal was not fully successful this time since some gates in die sumps of the service water pumps could not be opened. The removal of decay heat was ensured the whole time, however, since enough service water was drawn from die outlet side. Also, die service water was cold.

3 FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY STUK-B-YTO 135

During back-up diesel loading tests at TVO plant units in February it was snowing heavily and the air intake channel filters of the diesels were blocked by snow. A decision not to continue the test was made at TVO I. This event disclosed deficiencies in the supply of suction air for the back-up diesels.

Other events in this annual quater were classified level 0 on the INES.

Occupational doses and radioactive releases off-site were below authorised limits. Radioactive substances were measurable in samples collected around the plants in such quantities only as have no bearing on the radiation exposure of the population.

4 FINNISH CENTRE FOR RADIATION STUK-B-YTO 135 AND NUCLEAR SAFETY

CONTENTS

ABSTRACT

1 INTRODUCTION 7

2 OPERATION OF NUCLEAR POWER PLANTS IN IANUARY-MARCH 1995 8 2.1 Production data 8 2.2 Shutdown at Loviisa 2 to remove impurities from fuel bundles 12 2.3 Outage at Loviisa 2 to replace fuel bundles 12 2.4 Cold shutdown at TVO I due to the formation of frazil ice in sea water 12 2.5 Hot shutdown at TVO I due to a generator cooling system leak 12 2.6 Hot shutdowns at TVO II due to the fonnation of frazil ice in sea water and to a generator cooling system leak 12

3 EVENTS AND OBSERVATIONS 14 Loviisa 1 14 3.1 One of two isolation valves in the spent nuclear fuel storage pool cooling line had not been leaktight 14 Loviisa 2 IS 3.2 Impurities caused an abnormal rise in the coolant outlet temperatures of certain fuel bundles IS 3.3 Nuclear fuel cladding leak 16 TVO I 17 3.4 Supply of service water was compromised by frazil ice 17 3.5 Insufficient supply of suction air to back-up diesels during load tests due to a snow storm 18 TVO II 19 3.6 Nuclear fuel cladding leak 19

4 RADIATION SAFETY 20 4.1 Occupational exposure 20 4.2 Radioactive releases 20 4.3 Environmental monitoring 20

5 SAFETY IMPROVEMENTS AT NUCLEAR POWER PLANTS 23 No significant safety improvements

6 OTHER MATTERS RELATING TO THE USE OF NUCLEAR ENERGY 24 6.1 Regulations 24

Appendix 1: Regulatory control of nuclear facilities 25 Appendix 2: Plant data 26

5 FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY STUK-B-YTO 135 FINNISH CENTRE FOR RADIATION STUK-B-YTO 135 AND NUCLEAR SAFETY

1 INTRODUCTION

According to die Nuclear Energy Act (990/87), plants' production and availability factors are regulatory control of die use of nuclear energy given and die radiation safety of plant personnel belongs to die Finnish Centre for Radiation and and die environment is summarised. Safety Nuclear Safety (STUK). The Centre's functions improvements at die plants and general matters also include regulatory control of physical relating to die use of nuclear energy are also protection, emergency preparedness and of reported. The fourth Quarterly Report contains a nuclear material safeguards. The scope of nuclear summary of information reported earlier in die power plant regulatory control and inspections is year. given in Appendix 1 and general information about die Finnish plants in Appendix 2. The report is based on information submitted to die Centre by die utilities and on observations STUK publishes quarterly a report on die made by die Centre during its regulatory operation of Finnish nuclear power plants. In this activities. The events described in die report are report, plant events and observations during each classified on die International Nuclear Event quarter are described, tabulated data on die Scale ONES).

7 FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY STUK-B-YTO 135

2 OPERATION OF NUCLEAR POWER PLANTS IN JANUARY-MARCH 1995

The Finnish nuclear power plant units were in power operation in the first quarter of 1995. except for two shutdowns at Loviisa 2 and shutdowns at TVO plant units. There was also a brief break in production at both TVO plant units to repair a generator cooling system leak.

2.1 Production data Detailed production and availability figures are given in Tables I and II. Nuclear's share of total electricity production in Power diagrams describing electricity generation Finland was 27.8 %. The load factor average of at each plant unit and the causes of power the plant units was 93.9 %. reductions are given in Figs 1—4.

Table 1. Plant electricity production and availability.

Electricity production Availability Load (gross, TWh) factor (%) factor (%)

First 1994 First 1994 First 1994 quarter quarter quarter 1995 1995 1995 Loviisa 1 1.00 3.68 100.0 91.9 100.0 90.3 Loviisa 2 0.81 3.30 82.5 81.6 81.0 80.9 TVOI 1.55 6.21 97.4 96.9 97.8 96.4 TVOII 1.54 5.95 97.7 92.8 96.9 92.4

generator synchronized (h) Availability factor = •100% calendar time (h)

Load factor = gross electricity production . m % rated power - calendar time (h)

8 FINNISH CENTRE FOR RADIATION STUK-B-YTO 135 AND NUCLEAR SAFETY

Table 11. Nuclear energy in Finnish elearicity produaion.

First 1994 1993 quarter 1994

Nuclear electricity production 4.7 18.3 18.8 (net, TWh)*

Total electricity production 17.4 62.1 58.1 in Finland (net, TWh)"

Nuclear's share of total 27.0 29.5 32.3 electricity production (%)

Load factor averages 93.9 90.0 92.8 of Finnish plant units (%)

a Source: Statistics compiled by the Association of Finnish Electric Utilities.

9 FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY STUK-B-YTO 135

f5jf /. Daily average gross power of Loviisa 1 in January-March 1995. 1 The low pressure section of one of two turbines One primary circulation pump tripped from a was repaired; a turbine trip due to a controller spurious temperature signal, the reactor to 77% malfunction occurred during power raising; the power. reactor was at 51 % power. A primary circulation pump tripped again due to the same spurious temperature signal, the reactor to 75% powc.

Fig 2. Daily average gross power of Loviisa 2 inJanuary-March 1995. 1 One control rod dropped to the reactor core in (see chapters 2.3 and 3.2.). consequence of a low-frequency converter 5 One control rod dropped to the reactor co. • in failure, the reactor to 92% power. consequence of a low-frequency converter 2 Power restriction due to elevated temperatures in failure; the reactor was at 88% power from zirconium-spacer fuel bundles (see chapter 3.2). which it decreased to 84%. 3 Cold shutdown to clarify the cause for elevated 6 Two control rods dropped to the reactor core. temperatures in zirkonium-spacer fuel bundles One because of a low-frequency converter failure (see chapters 2.3 and 3.2). and the other due to the tripping of a protective 4 Outage to replace zirconium-spacer fuel bundles switch; the reaclrr to 84% power.

10 FINNISH CENTRE FOR RADIATION STUK-B-YTO 135 AND NUCLEAR SAFETY

Fig 3. Daily average gross power ofTVO 1 in January-March 1995. 1 Reactor :cram and cold shutdown due to the 2 Generator cooling system water leak was formation of frazile ice in the sea water (st« repaired; the plant unit was in hot shutdown state chapters 2.4 and 3.4). (see chapter 2.S).

Fig 4. Daily average gross power of TVOII in January-March 1995. Periodic tests which require power reduction, the Nuclear fuel cladding leak was located; the reactor at 65 % power. reactor at 90* (see chapter 3.6). Hot shutdowns due to the formation of frszile ice Periodic tests which require power reduction; in the sea water and to a generator cooling reactor stability measurements were made and a system water leak during start-up (see chapter fuel leak was located; lowest reactor power was 2.6). 47%.

II FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY STUK-B-YTO 135

2.2 Shutdown at Loviisa 2 to While in shutdown state, repairs pending a suitable shutdown were carried out. Upon remove impurities from fuel completion of the work, and after the weather bundles had improved, the plant unit was brought back on-line on 22 January 1995. Loviisa 2 was placed in a hot shutdown state on 14 January 1995 to find out what caused an 2.5 Hot shutdown at TVO I due abnormal rise in the coolant outlet temperature of fuel bundles and to remove possible impurities to a generator cooling system from the bundles (see chapter 3.2). leak

In inspections conducted while in hot shutdown, A water leak was observed at TVO I on 30 a minor primary circuit leak was located and die January 1995 which was assumed to originate plant unit was placed in cold shutdown to repair from the generator cooling system. The plant unit it. The leak was in a welded seam joining the was operating at 100% power. The unit was body of a main gate valve and an out-of-service stopped the same day to locate the leak. The pipe fitting connecting to small-diameter piping. water came through a flanged joint in the cooling- Plant unit start-up was commenced on 16 January water tube of the generator's st&tor. The water in 1995. It was interrupted and the plant unit was the cooling water system of the generator is not brought back to cold shutdown, as it took longer radioactive. The leak was repaired a "id the plant than expected to remove from the primary unit was brought back on-line on 31 January coolant impurities which dislodged during the 1995. shutdown phase. While the unit was in cold shutdown, some flange seals of primary circuit 2.6 Hot shutdowns at TVO II due main safety valves were also replaced. The plant unit was brought back on-line on 19 January to the formation of frazil ice 1995. in sea water and to a generator cooling system leak 2.3 Outage at Loviisa 2 to replace fuel bundles Sea water was unusually cold and prone to freezing. The power of TVO II was therefore reduced on 20 January 1995. Reduced sea water Loviisa 2 was shut down on 28 of January 1995 inlet tunnel level indicated thai the coarse screen to replace six zirconium-spacer fuel bundles with in the tunnel was close to blocking, so the plant steel-spacer bundles (see chapter 3.2). Some unit was placed in hot shutdown. repairs, tests, inspections and maintenance were carried out during the outage. After replacement of the fuel bundles, the plant unit was brought With the reactor at 45% power, a partial reactor back on-line on 9 February 1995. scram was tripped by hydraulic insertion of eight control rods into the reactor. At the same time, slow insertion of other control rods into the 2.4 Cold shutdown at TVOI due reactor by means of motors was initiated. The to the formation of frazil ice total number of control rods is 121. When the in sea water slow insertion of control rods at over 30% power is started, an automatic reactor scram occurs to At 10 o'clock on 20 January 1995, a reactor avoid high local power at the top section of the scram occurred at TVO I when frazil ice blocked reactor during the insertion. Owing to the partial some service water filters (see chapter 3.4). The scram, the reactor power had not decreased plant unit's reactor remained in hot shutdown sufficiently before the start of control rod state after the scram. In the afternoon the plant insertion and a reactor scram occurred. unit was cooled to cold shutdown state because no quick improvement in weather was expected.

12 FINNISH CENTRE FOR RADIATION STUK-B-YTO 135 AND NUCLEAR SAFETY

While in the shutdown state, a previous The plant unit was stopped, the leak repaired and temporary repair of a generator cooling system start-up resumed on 22 January 1995. pipe was modified among other things, and other work was made which was pending hot When power-raising was started at the plant unit, shutdown. When these had been completed, and an error was made which affected the reactor when the weather improved, the plant unit was pressure control; a pressure transient occurred brought back on-line on 21 January 1995. which initiated the reactor automatic scram function. An actual scram did not occur because On thf- same day, with the plant unit at 70% all control rods were still in the reactor core. The power, an incorrectly planned temporary repair usual post-event inspections were made and start­ of the generator cooling system caused vibrations up was Fesumed. The plant unit was brought back in the pipe in question, creating a new crack on-line on 23 January 1995. through which water entered the turbine building.

13 FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY STUK-B-YTO 135

3 EVENTS AND OBSERVATIONS

Loviisa 1

// was detected at Loviisa 1 during the first quarter of1995 that an isolation valve in a pipeline penetrating the containment building had not been leaktight. This event is level 1 on the INES scale.

3.1 One of two isolation valves in not cancelled. The new control's adjustment was not made, however. The actuator's performance the spent nuclear fuel storage test was conducted based on the control pool cooling line had not been previously in use, and the error thus was not leaktight noticed. The incorrect adjustment meant mat the valve did not close tightly after having received In a leaktightness test conducted at Loviisa 1 on the closing signal. A leaktightness test of the 28 February 1995, the leakage of the isolation isolation valve was not conducted because die valve of the fuel pool cooling system exceeded valve had not been opened and those responsible the acceptance limit. for the leaktightness tests were not aware of the adjustments made in the actuator's control. The valve is in a pipeline penetrating the containment building wall. The line has two The leaktightness of pipelines penetrating the successive isolation valves which close containment building is monitored by annual automatically during transients or accidents, and tests. The test results are acceptable when the prevent the release of radioactive substances to measured values are below valve-specific limits. the atmosphere. These pipelines belong to the In addition, the sum of individual leaktightness cooling system of the spent fuel storage pool. test results must be below the leak limit based on Water is circulated in this system to remove from the allowable containment leakage rate. This the pools decay heat generated inside spent fuel combined leakage rate limit ensures that the bundles. allowable containment overall leakage limit is not exceeded if the leakage of several individual In the 1994 annual maintenance outage, valves is close to their acceptance limit. In the modifications to improve reliability were due in event in question, both the valve-specific leakage the isolation valve and its actuator. There was an approval limits and the combined leakage rate of error in the production plan of one of the valve's the valves were exceeded. Had its operation been mechanical parts, therefore, modifications required, the isolation valve would not have planned in the valve were given up in this stage. performed according to design. The cooling line's Although the mechanical modifications were other isolation valve was leaktight the whole cancelled, some cables were replaced according time and, if so required, would have prevented to the electrical and instrumentation plans. The the spreading of radioactive substances from the replacement was done because old cables containment building via the line in question. connecting to the valve's actuator had already been disconnected. The leaking isolation val.e was readjusted and leak-tested, with results clearly below the leak Modifications originally planned in the actuator's limits. Imatran Voima Oy is looking into control weir made because the work order was measures to prevent recurrence.

14 FINNISH CENTRE FOR RADIATION STUK-B-YTO 135 AND NUCLEAR SAFETY

Loviisa 2

During the first quarter of1995, accumulated impurities were detected in certain fuel bundles at Loviisa 2, resulting in an abnormal rise in the coolant outlet temperatures of these bundles. The event is classified level 1 on the INES scale. Abo a minor nuclear fuel cladding leak was observed at Loviisa 2 which is level 0 on the INES.

3.2 Impurities caused an An abnormal rise was observed in the outlet coolant temperature of fuel bundles with abnormal rise in the coolant zirconium spacers. The average fuel bundle outlet temperatures of certain coolant outlet temperature is about 300 °C. The fuel bundles temperatures vary about 10 "C around the average due to various bundle powers. As Towards the end of 1994, a slight increase in die regards the bundles with zirconium spacers, mere coolant outlet temperatures of some fuel bundles was a difference of 3—5 °C as compared with the was observed at Loviisa 2. The gradual temperatures anticipated by calculations. Also the deposition of corrosion products on the surfaces bundles with steel spacers exhibited a slight of the fuel assemblies was considered the most temperature increase. Because of the rise in the likely cause for the elevated temperatures. The temperatures, the plant mr *s power was reduced temperatures did not exceed the limit set in the in January 1995 by a ft ~ percentage units in Technical Specifications, however. The limit accordance with the Technical Specifications. was originally established to ensure the safety of a clean reactor core. The plant unit was placed in hot shutdown on 14 January 1995 to clarify the matter. The changes Primrj-y circuit surfaces were chemically cleaned in water chemistry and temperatures during in the 1994 annual maintenance outage. The post- shutdown and start-up were expected to dislodge startup solubility of corrosion products in the impurities into the cooling water from where cooling water was unusually high after the their removal by intensified operation of the cleaning. The temperatures continued to rise cool'ng water clean-up systems was intended. As although water-chemistry adjustments were made planned, corrosion products dislodged into the during plant operation in an attempt to affect the cooling water during die shutdown, bom in behaviour of corrosion products. dissolved and in solid form. The plant unit was brought back on-line on 19 January 1995. The reactor core at Loviisa plant units comprises 313 nuclear fuel bundles, each containing 126 Despite the clean-up measures conducted during fuel rods. Each fuel bundle is encased in a fuel the shutdown, no clear change was observed in channel, open at both ends. The bundles are the coolant outlet temperatures of fuel bundles laterally supported by "spacers", honeycomb-like with zirconium spacers. That is why a decision structures, assembled of very thin-walled metal was made to replace these with bundles having pipe. During the 1994 annual maintenance steel spacers. The plant unit was in shut-down outage, six test bundles were loaded into »he state from 28 January to 9 February 1995 to centre of the reactor core. These bundles had carry out the replacement. zirconium metal spacers; so far, only stainless- steel spacers had been used. Cooling water flows One fuel bundle with zirconium spacers was through the reactor core, entering from the disassembled and examined. The examination bottom and exiting at the top. The reactor's confirmed previous doubts about the partial operation is monitored i.a. by measuring the fuel blocking of the bundle's spacers. Deposits were bundle coolant outlet temperatures. There are 210 observed on the spacers' lower surfaces; their such measurements. upper surfaces were clean, though. The most deposits were on the surfaces of the lowermost

15 FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY STUK-B-YTO 135 spacers. The other five bundles with zirconium Uranium dioxide, the fuel employed in the spacers were visually examined without Loviisa reactors, is encased in 2.5 m-long tubes. disassembling them. A distinct layer of impurities Both Loviisa plant units have about 39 000 such was seen on the surfaces of their fuel channels. rods assembled into 126-rod bundles. During handling the fuel is in the form of whole bundles Even the temperatures of steel-spacer bundles of which there are 313 in the Loviisa reactors. which remained in the reactor had somewhat The fuel rod wall forms a gas-tight cladding, increased; this change is not expected to increase preventing the transfer to the cooling water of significantly by the annual maintenance outage, fission products generated during reactor however. That temperature changes occurred operation. The radioactivity levels in cooling also in these bundles indicates that the water are regularly monitored by laboratory accumulation of corrosion products is related not measurements. only to spacer material but also to the location of the bundles in the reactor. The increase in the primary coolant noble gas concentration was detected in a laboratory The impurity concentrations of reactor cooling measurement. The activity concentration of the water must be kept low because, fc example, noble-gas isotope xenon-133 first increased to corrosion products attaching to fuel bundle about 50-fold, as compared with the pre-leak spacers would block the flow channels of the level of 10* Bq/kg. In about a week from the first bundles, reducing the flow of cooling water leak detected, the noble-gas concentrations through them. With too much reduction in the further increased slightly and the xenon-133 flow, during potential further transients or activity concentration in primary water was about accidents the fuel rods might undergo a heat 10s Bq/kg after the leak had stabilized. The transfer crisis, i.e. sufficient contact of cooling activity concentration of the isotopes of iodine water with the entire fuel rod surface would be increased only in the second phase. At its highest, prevented. Fuel cladding temperatures would rise the activity concentration of iodine-131 was about and, in the worst case, the cladding would be 10-fold as compared with the pre-leak level. This damaged Corrosion products depositing on concentration soon decreased to the pre-leak level cladding surfaces also locally reduce heat transfer of 10* Bq/kg. The fission-product concentrations from fuel to cooling water and increase fuel rod had been less than one percent of the limits set in cladding temperatures. With too much increase in the Technical Specifications. the cladding temperatures, fuel rod damage may result even during normal plant operation. A minor fuel leak such as this does not prevent the safe operation of the plant unit. The leak is 3.3 Nuclear fuel cladding leak monitored by laboratory measurements and by continuously operating analysers. The bundle A slight increase was observed in the fission containing the leaking rod will be identified and product activity concentrations of primary coolant removed from the reactor in the next annual at Loviisa 2 on 15 February 1995. The increase maintenance outage. was due to a minor nuclear fuel cladding leak.

16 FINNISH CENTRE FOR RADIATION STUK-B-YTO 135 AND NUCLEAR SAFETY

TVOI

In the first quarter of 1995, frazil ice compromised the supply of service water at TVO I. In another event, a snow storm blocked the air intake filters of back-up diesels, revealing shortcomings in the supply of suction air. These events are classified level 1 on the INES scale.

3.4 Supply of service water was condenser were tripped by automatic devices. A turbine trip followed, and, immediately after that compromised by frazil ice a scram at 10 o'clock. All the plant unit's systems operated according to design during the scram. So much frazil ice was accumulating in the screens of the cooling water screening plant at The building up of frazil ice in the sea v.„ inlet TVO I on 20 January 1995 that the plant unit was t tunnel continued and, at about 18 o'clock, the stopped by its automation systems. The water level downstream of die band screens accumulation of ice continued and the screens began to decrease although die intake of water to blocked up again later compromising the supply die turbine plant had already ended. Intake of sea of sea water to the plant unit. water to the right auxiliary building (to cool safety systems) was to be reversed to occur from Both TVO plant units have their own sea water die outlet tunnel and die water was to be returned cnannels and tunnels. Sea water is cleaned to die inlet tunnel. Intake of sea water for mechanically in coarse screens, fine screens and systems in the left auxiliary building (which cool band screens. The band screens are kept in safety systems) had been reversed to occur from operation when necessary. Sea water is pumped die oudet tunnel already in die autumn. This is not only to the turbine plant but also to cool down done by gates in die sumps of service water the components of several systems ensuring pumps. All die necessary gates would not open safety. The service water systems employed for this time. In about half an hour one otiier gate the cooling have several redundant sub-systems; was opened and die water level downstream of their components are in two separate buildings, die band screens stopped decreasing, and normal oie right and the left auxiliary building. From the water level was restored. The gates were first viewpoint of safety, the most important items to suspected inoperable due to die stiffness of die be cooled are the heat exchangers of systems for bearings of tiieir drives. The cause for die decay heat removal and the heat exchangers of inoperability is still under investigation. back-up diesels.

When the plant unit is not in operation decay heat Sea water for the equipment in the auxiliary generated in die reactor is transferred to die sea buildings is usually taken from the inlet tunnel. by die heat exchangers of various systems. If But, if necessary, for example at times when the necessary, sea water is also used to cool inlet tunnel is prone to freezing, the intake of sea components important to safety such as die plant water can be reversed to occur from the outlet unit's four back-up diesels. During die event in tunnel. question, die sea water inlet for die components in die right auxiliary building was not available At the time of the event, the temperature of sea quite according to design. Decay heat removal water was very low, -0.36 °C, and frazile ice was ensured die whole time, however, since began to form. The screening plant's band enough sea water was taken from die oudet side. screens were in operation and warm water sprays Also, die sea water was cold. were coupled to them to prevent freezing. The band screens were still so much blocked by ice Teollisuuden Voima Oy has launched an that the level of sea water downstream of the investigation to eliminate tiiese problems. As an band screens began to decrease. Due to this immediate measure to ensure die reliable decrease, pumps injecting sea water to the turbine performance of die gates of die pump sumps,

17 FJNNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY STUK-B-YTO 135

regular test manoeuvring of the gates in the right Suction air for the diesels is drawn from outside auxiliary building is started. The reliability of the building. Besides the air-intake grid in die corresponding gates in the left auxiliary building outer wall, there is a filtering unit in die air- is ensured when water intake is reversed from the intake channel of die diesel turbochargers. inlet to the outlet side every autumn and in spring During a diesel's operation, the condition of its when the original arrangement is restored. What filter is monitored by measuring the pressure is more, die preliminary maintenance programme difference between the air-intake channel and the for the pump sumps will be reassessed. diesel room. If the pressure difference deviates from its normal range of fluctuation an alarm So much frazil ice was forming also in the sea indication appears in the control room based on water inlet channel of TVO II on 20 January which the necessary measures can be taken. 1995 that the coarse screen began to clog up. The plant unit was placed in hot shutdown at about 17 Diesel load tests were carried out on 1 February o'clock. Like at TVO I, the intake of sea water 199S. There was a heavy snow storm in Olkiluoto for systems in the left auxiliary building had at mat time and a strong, turbulent wind was previously been reversed to take place from the blowing, lifting snow in the air. During the test at outlet tunnel. The intake of water to the right TVO I, so much snow ended up in die filters of auxiliary building was reversed after the plant the air intake channel of the diesels tiiat an alarm unit was stopped. Normal sea water intake to the on pressure difference actuated in die control right auxiliary building was restored the next room. A decision was made to stop the test morning after an improvement in the temperature although there was noUiing out of the ordinary in conditions of sea water. Sea water intake reversal the operation of the diesels. The filter was partly was unproblematic at TVO II blocked by a compact layer of snow which had accumulated in its lower part. The filter was A similar type of reactor scram due to frazil ice replaced and the load test was completed. A occurred at TVO I in 1988 (STUK-B-YTO 52, diesel load test was also carried out at TVO II 1988). In 1992, the band screens at TVO II were during which an alarm on a blocked filter clogged up by seaborne algae and a reactor scram actuated in die control room. This blocking was occurred (STUK-B-YTO 101, 1992). not serious and mere was nodiing out of die ordinary in me diesel's operation, so die test was 3.5 Insufficient supply of suction completed.

air to back-up diesels during This being a test, the plant unit's safety was not load tests due to a snow storm affected. The event disclosed deficiencies in die supply of suction air to die back-up diesels. During back-up diesel load tests conducted at TVO plant units, the filters of the air intake As an immediate measure to prevent recurrence channels of the diesels began to clog due to a and to ensure the supply of suction air, the strong wind and a snow storm. operating personnel was advised about die quick removal of die filter unit once it begins to show Both TVO plant units have four back-up diesels signs of blocking. Furthermore, Teollisuuden submitted to load tests every four weeks. These Voima Oy looks into measures to ensure die diesels supply power to safety-important supply of suction air to die back-up diesels under components when the plant unit does not generate abnormal conditions without removing die filter electricity and the national grid has been lost. units.

18 FINNISH CENTRE FOR RADIATION STUK-B-YTO 135 AND NUCLEAR SAFETY

TVO II

In the first quarter of 1995, a minor nuclear fuel cladding leak was observed at TVO II. The event is level 0 on the INES scale.

3.6 Nuclear fuel cladding leak fission gas leak rate has been within the range of 370-580 MBq/s, corresponding to die maximum A minor activity increase in the off-gases of leak rate of one rod. The leak rate of the noble turbine steam condensers was observed on 26 gas isotope xenon-133, generally employed for January 1995. Laboratory measurements leak-rate estimation, in March increased to 100 confirmed it was due to a minor fuel leak. Mbq/s. The coolant has not been in contact with the uranium dioxide since the activity The reactor of TVO II has 500 fuel assemblies concentrations of iodine-131 and of actinides most of which contain 64 fuel rods. The fuel rods have not increased. Mesurements show that the arc thin tubes filled with pellets compressed of sand tanks and carbon columns employed for tile uranium dioxide. The fuel rod wall forms a gas- delay and filtering of off-gases have performed tight cladding, preventing the transfer from fuel according to design and that the activity of the air to reactor cooling water of fission products discharged through the main stack has not generated during reactor operation. Gaseous and increased. water-dissolved radioactive substances are monitored regularly by laboratory measurements. In March the fuel leak was tentatively located in Also the activity of reactor-to-turbine steam lines one cell comprising four fuel assemblies. This and off-gas lines is monitored by continuous- was done by manoeuvring the control rods while operation radiation measurements which quickest operating the reactor at about 50% power. Before indicate potential fuel leaks. annual maintenance outage it is ensured that there are no fuel leaks elsewhere in the reactor core. This fuel leak has remained a minor cladding Actual leak-locating will be carried out in the defect: only fission gases forming inside the rod next annual maintenance outage in May and the are released to the primary circuit. The combined leaking assembly will be removed from service.

19 FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY STUK-B-YTO 135

4 RADIATION SAFETY

Individual doses to nuclear power plant personnel were below the dose limit. Also environmental releases were well below the release limits. Only such quantities of plant-based radioactive substances were measurable in samples collected around the nuclear power plants as have no bearing on the radiation exposure of the population.

4.1 Occupational exposure limits are given. During this report period, releases into the environment were well below The highest individual dose at a Finnish nuclear authorised limits. power plant in this annual quarter was 4.8 mSv and it was received at Olkiluoto nuclear power 4.3 Environmental monitoring plant. The Radiation Decree stipulates that i>. effective dose caused by radiation work to a Radiation safety in the vicinity of Finnish nuclear worker shall not exceed SO mSv in any single power plants is monitored by regular sampling year. The dose may not exceed 20 mSv per year and analysis programmes. This monitoring aims as an average over five years. This monitoring of to follow the environmental dispersion of the annual average was started at the beginning of radioactive releases and to ensure that the 1992. Radiation doses have been below releases remain below authorised limits. During authorised limits both at Loviisa and at Olkiluoto this quarter, a total of 109 samples were analysed nuclear power plants. according to the monitoring programmes.

Most doses are incurred during annual Radioactive substances originating in Loviisa maintenance outages of which there were none nuclear power plant were measurable only in during this quarter. samples of fallout. The January sample contained radioactive isotopes of cobalt and silver (cobalt- The individual dose distribution of nuclear power 60 and silver-llOm) and the February sample plant personnel is given in Table II which radioactive isotopes of manganese and cobalt specifies the number of exposed individuals by (manganese-54, cobalt-58 and -60). In two dose range and plant site. This information is samples of fallout, the radioactive isotope of from STUK's central dose register. cobalt (cobalt-60) originating in TVO nuclear power plant was detected. All the measured Collective occupational dose at Loviisa plant units concentrations were very low. was 0.13 manSv and 0.08 manSv at TVO during this quarter. According to a STUK Guide, the Radioactive isotopes of strontium and caesium collective dose limit for one plant unit is 2.5 (strontium-90, caesium-134 and -137) and of manSv per one gigawatt of net electrical power plutonium (plutonium-238 and 239+240) averaged over two successive years; this means a originating in the Chernobyl accident and in the total annual collective dose of 2.22 manSv/year fallout from nuclear weapons tests are still and of 3.56 manSv/year for the Loviisa and measurable in environmental samples. Natural Olkiluoto units respectively. radioactive substances (such as beryllium-7 and potassium-40) are also detected. Their 4.2 Radioactive releases concentrations are usually higher than those of substances detected in samples from power plant In Table IV, the releases of radioactive effluents releases or from fallout. measured at each plant site and the annual release

20 FINNISH CENTRE FOR RADIATION STUK-B-YTO 135 AND NUCLEAR SAFETY

Table III. Occupational dose distribution in the first quarter of 1995 and in 1994.

Dose range Number of persons by dose range (mSv)

First quarter 1995 In 1994 Loviisa TVO Total* Loviisa TVO Total' <0,5 88 92 180 210 522 674 0,5-1 35 32 69 143 237 356 1-2 29 15 50 158 178 2% 2-3 14 2 18 115 105 201 3-4 3 2 6 68 73 134 4-5 - 2 2 68 58 116 5-6 - - - 53 29 79 6-7 • - - 22 24 53 7-8 - - - 12 14 39 8-9 - - 1 13 8 29 9-10 - - - 9 11 30 10-11 - - 2 7 11 21 11-12 - - - 10 3 19 12-13 - - - 4 8 12 13-14 - - - 5 8 17 14-15 - - - 2 3 7 15-16 - - - 2 3 8 16-17 - - - 1 - 3 17-18 - - - - 1 5 18-19 - - - - - 2 19-20 - - - - 1 1 20-21 - - - - 1 3 21-25 - - - I - 2 >25 • " " " ~

a These data also include Finnish workers who have received doses at Swedish nuclear power plants. The same person may have worked at both Finnish nuclear poweir plants and inSweden . In the firstquarte r of 1995, the highest dose to a Finnish nuclear power plant worker was 10.8 mSv, and it was received in Sweden.

21 FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY STUK-B-YTO 135

Table IV. Radioactive releases at each plant site, first quarter 1995. Gaseous effluents (Bq)'

Plant site Noble gases Iodines Aerosols Tritium Carbon 14 (Krypton-87 (Iodine-131 equivalents) equivalents)

Loviisa Report period 1.9-10" b) 9,5 10* 6.9-107 4.9 1010 3.5 10'° In 1994 1.0-10 "b) 1,7-105 2.3-10* 2.110" 1.710"

Olkiluoto Report period 6.3-10'° 4.4-10* 4.3 10 s 3.3 1010 c) In 1994 3.5-10 n 1.1 10' 1.3-10" 3.1 10" c)

Annual release limits Loviisa 2.2-10" e) 2.2-IO"e) Olkiluoto 1.8 10 '* 1.1 10"

Liquid effluents (Bq)"

Plant site Tritium Other nuclides

Loviisa Report period 4.3-10'2 d) In 1994 1.1-10" 4.1 10*

Olkiluoto Report period 3.1 10" 4.2-10' In 1994 2.8-1012 1.1-10 ,0

Annual release limits Loviisa 1.5-10 M 8.9 10"e) Olkiluoto 1.8-10,5 3.0-10"

a The unit of radioactivity is Becquerel (Bq); 1 Bq = one nuclear transformation per second. b The calculatory release of argon-41 from Loviisa 1 and 2 in krypton-87 equivalents was 3.9 • 10 " Bq in the release period and 1.5 10 " Bq in 1994. c The carbon-14 release-estimate based on experimental data was 1.6 • 10 " Bq in Olkiluoto in the report period, and 6.4 10 " Bq in 1994. d Below the detection limit, e The numerical value shows the release limit for the Loviisa plant site, assuming the ram of various types of release limit shares shall be smaller than or equal to 1.

22 FINNISH CENTRE FOR RADIATION STUK-B-YTO 135 AND NUCLEAR SAFETY

5 SAFETY IMPROVEMENTS AT NUCLEAR POWER PLANTS

During the first quarter of 1995, no significant safety improvements were made at the nuclear power plants. Modifications are usually focused on annual maintenance outages of which there were none during this quarter.

23 FINNISH CENTRE FOR RADIYTION AND NUCLEAR SAFETY STUK-B-YTO 135

6 OTHER MATTERS RELATING TO THE USE OF NUCLEAR ENERGY

During the first quarter of 1995, regulations concerning nuclear power plants were revised.

6.1 Regulations Guide, requirements for ensuring safety during the shutdowns are given and the Centre's control During this quarter, STUK published YVL Guide procedures during the shutdowns are described. 1.13, shutdowns at nuclear power plants. In this

24 FINNISH CENTRE FOR RADIATION STUK-B-YTO 135 AND NUCLEAR SAFETY

APPENDIX 1 REGULATORY CONTROL OF NUCLEAR FACILITIES

Regidat. / control and inspection* CouncBofStato by the Finnish Centre for Radiation and Decisions Nuclear Safety

i <*• i

PreinMaiy plans fur aw plant and softly principles Location and environmental imped of eSe plant Arrangements for nudar fuol and nudeer watte Decftfon in Principh management

PrejMninary safoty analysis loport on he planned is inline aisd opoiaJkm offtio plant plus ftie piofcninaiy safely analysis Sanity classificeoon of cunyonotiti and structures Quaity assuranco plan Plans for nudoar fuel and nudaar waste manaoommt Physical protection and omonjsncy propsixsms Construction Rama

• Construction play, rnanutadurm^ wtlalapon of components and structures • Porformanco tests of systems • Final safety analysis n^ort on to stuctur» arid operaacn of fha plant and tho final safoty analyst* • Probabilistic safety tnolysis • Coniposoton end cot npetenco of fie operating • Tachnical Spadfications » Nuctear fuel management arid safegiiaro* • Methods of nuclear was» management • Physical protadion and amanjaneypraparadnasa Operating licence

Start-op tasting at various power »vale Maintenance, inspections and tasting of componantsand structuraa Operation of systems and the whole plant Tha oparatfng organisation and management Training of personnel Qualifications of indMdiials Operafonal incidents Repairs and modfflcations Refuelling Nudear fuel management and safeguards Nuclear waste management Radiation protection and safety of the environment Physical protection and emergency preparedness Fire protection

25 FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY STUK-B-YTO 13S

APPENDIX 2

PLANT DATA

Plant Start-up Commercial Rated power Type, supplier unit operation (gross/net,MW)

Loviisa 1 8 Feb. 1977 9 May 1977 465/445 Pressurized water reactor (PWR), Atomenergoexport

Loviisa 2 4 Nov. 1980 5 Jan. 19»! 465/445 Pressurized water reactor (PWR), Atomenergoexport

TVOI 2 Sept. 1978 10 Oct. 1979 735/710 Boiling water reactor (BWR), Asea Atom

TVO II 18 Feb. 1980 1 July 1982 735/710 Boiling water reactor (BWR), Asea Atom

Imatran Voima Oy owns the Loviisa ] and 2 plant units in Loviisa and Teollisuuden Voima Oy the TVO I and II plant units in Olkiluoto, Eurajoki.

26 FINNISH CENTRE FOR RADIATION STUK-B-YTO 135 AND NUCLEAR SAFETY

CONTRIBUTORS

STUK/Nuclear Safety Department'. Päivi Salo Jarmo Konsi Seija Suksi Pauli Kopiloff Keijo Valtonen Pekka Lehtinen Jouko Mononen STUK/Research Department: Matti Ojanen Tarja K Ikäheimonen Mervi Olkkonen (translation) Seppo Klemola Veli Riihiluoma

27 ISBN 951-712-062-1 ISSN 0781-2884 Painatuskeskus Oy Helsinki 1995