Reduction of the collective doses in Almaraz Nuclear Power Plant through source term reduction and the work load reduction

1. Almaraz Nuclear Power Plant Located in the county of Cáceres, 200 Km from Madrid (Spain) Located in the county of Cáceres, 200 Km from Madrid (Spain) 2 water pressurized reactors (PWR) Design Westinghouse 1000 Mw of electric power each one. 2 water pressurized reactors (PWR) Design Westinghouse 1000 Mw of electric power each one. Start up: Start up: –Unit I: 1981 (Cycle 17) –Unit II: 1983 (Cycle 15) Cumulative operation factors: Cumulative operation factors: –Unit I: 84,30% –Unit II: 89,19% Cumulative production: Cumulative production: –Unit I: Gwh –Unit II: Gwh –Total: Gwh

2. Collective dose in the refueling outages in the Nuclear Power station of Almaraz –Unit I: In the 13th refuelig outage a contamination for Antimony (Sb) takes place, whose falling residual effect is appreciable in the following refueling outages. The dose for refueling is about 15-20% of the doses previous to steam generators replacement. The dose for refueling is about 15-20% of the doses previous to steam generators replacement. Year 2003: Unit I – 366 man.mSv and Unit II – 283 man.mSv Year 2003: Unit I – 366 man.mSv and Unit II – 283 man.mSv Actually aprox. 80% of the colective annual dose Actually aprox. 80% of the colective annual dose

3. Decrease of the source term The activity index is a reference value obtained from radiation levels measured in certain points of the primary circuit after the process of decontamination (15 points for loop) The activity index is a reference value obtained from radiation levels measured in certain points of the primary circuit after the process of decontamination (15 points for loop) The decrease of the activity index is observed, in a very significant way, starting from the outage of the SGR, with the exception of the incident with contamination of Sb in the unit 1. The decrease of the activity index is observed, in a very significant way, starting from the outage of the SGR, with the exception of the incident with contamination of Sb in the unit 1.

4. Modifications that affect to the source term Fuel: (Change progressive in materials) coincident in the time with the change of SSGG. Fuel: (Change progressive in materials) coincident in the time with the change of SSGG. Substitution of the steam generators. Substitution of the steam generators. Substitution of the head of the vessel of the reactor (outage of SGR) Substitution of the head of the vessel of the reactor (outage of SGR) RTD’s" By-pass" elimination (outage of SGR) RTD’s" By-pass" elimination (outage of SGR)

4.1 Fuel Change of “ Zircaloy-4” to “ Zirlo” in the cladding and skeleton. Change of “ Zircaloy-4” to “ Zirlo” in the cladding and skeleton. For the same burnt (45 Mw·d/t·U) smaller grade of corrosion: For the same burnt (45 Mw·d/t·U) smaller grade of corrosion: - Zircaloy-4 : mm - Zircaloy-4 : mm -Zirlo : mm -Zirlo : mm The smallest grade of corrosion, together to the great surface ( bars in the nucleus) it supposes an important decrease of contribution of products of corrosion to the primary coolant and, therefore, a decrease of the activation products. The smallest grade of corrosion, together to the great surface ( bars in the nucleus) it supposes an important decrease of contribution of products of corrosion to the primary coolant and, therefore, a decrease of the activation products.

4.2 Steam generators replacement Two decisive factors: Two decisive factors: –Treatment of internal surface (electropulido) to minimize the fixation of the contamination. –Composition of the material of the tubes: The differences in the composition of the materials of the tubes, together with their biggest resistance to the corrosion and the fact that their surface almost constitutes 50% of the surface of the primary circuit, makes the change of SSGG one of the most important contributor to the decrease of the radiation levels. The differences in the composition of the materials of the tubes, together with their biggest resistance to the corrosion and the fact that their surface almost constitutes 50% of the surface of the primary circuit, makes the change of SSGG one of the most important contributor to the decrease of the radiation levels. OldNew MaterialI-600I-800 Ni 72 % mín % Co 0,05 % < 0,015 %

4.3 Substitution of the reactor vessel head Modification of the penetrations alloy, being the current ones of I-690 instead of the I-600 of the old ones. Modification of the penetrations alloy, being the current ones of I-690 instead of the I-600 of the old ones. Smaller susceptibility to the corrosion (this is the reason of the change) Smaller susceptibility to the corrosion (this is the reason of the change) Smaller content in nickel, (58%) instesd of 72% of the old head. Smaller content in nickel, (58%) instesd of 72% of the old head. Smaller content in cobalt impurities. Smaller content in cobalt impurities.

4.4 elimination of the By-pass of RTDs The pipes of By-pass of the RTDs and the valves and components associated of the pipes of the primary circuit have been eliminated. The pipes of By-pass of the RTDs and the valves and components associated of the pipes of the primary circuit have been eliminated. Substituted by a system of measure of temperature by direct immersion. Substituted by a system of measure of temperature by direct immersion. 13 valves and aprox 30 meters of pipe have been eliminated in each loop. 13 valves and aprox 30 meters of pipe have been eliminated in each loop. Decrease of contribution of corrosion products coming from the seats of the valves (Stellite with high content in cobalt) Decrease of contribution of corrosion products coming from the seats of the valves (Stellite with high content in cobalt) Disappearance of accumulations of Crud in the system (geometry complex and many valves). Disappearance of accumulations of Crud in the system (geometry complex and many valves). Additional advantages in normal operation (higest availability of the plant when being avoided the maintenance necessities by leaks in the valves of the system). Additional advantages in normal operation (higest availability of the plant when being avoided the maintenance necessities by leaks in the valves of the system).

4.5 Other factors Operatives: Operatives: –Fuel integrity (minimization of activation products) –Chemistry of the primary in normal operation (coordinated chemistry of high pH) to avoid deposits of Crud in the reactor. –Decontamination of the primary in each refueling outage for the elimination of Crud. Structurals: Structurals: –Change the control rods for defects in the integrity (Ag-110m) –Elimination of secondary neutronic sources to avoid incidents of loss of integrity of them (Sb-122 / 124). –Shielding of lines or systems ( cavitydrainage line).

4.6 Quantification of the source term(1/2) A good indicator is the activity retained in the desmineralizadores of the system CVCS after the decontamination of the primary circuit in each refueling: A good indicator is the activity retained in the desmineralizadores of the system CVCS after the decontamination of the primary circuit in each refueling: Unit 1 R.O.Co-58Co-60Sb-122Sb-124Total 11 (1) (2) (3) 4036 (3) Activity in Ci (1) Steam Generator Replacement (2) Sb Incident (3) Activity CVCS + BTRS

4.7 Quantification of the source term (2/2) A decrease of the extracted activity, consequence of a decrease of the products of corrosion of the primary circuit is observed. A decrease of the extracted activity, consequence of a decrease of the products of corrosion of the primary circuit is observed. Unit 2 R.O.Co-58Co-60Total 10 (1) Activity in Ci (1) Steam Generator Replacement (2) Activity CVCS + BTRS

5. Decrease of the work load in refuilig outage Standard refueling before the modifications: Standard refueling before the modifications: – man-hours in Controled Area Standard refueling after the changes: Standard refueling after the changes: – man-hours in Controled Area Decrease: Decrease: – man-hours (43%)

6. Causes of the decrease Smaller inspection and maintenance requirements in SSGG Smaller inspection and maintenance requirements in SSGG (1 SG for refueling outage) (1 SG for refueling outage) Smaller requirements of inspection of penetrations of the head of the vessel of the reactor (less susceptibility to the corrosion) Smaller requirements of inspection of penetrations of the head of the vessel of the reactor (less susceptibility to the corrosion) Decrease of maintenance works in valves of RTD’s system. Decrease of maintenance works in valves of RTD’s system. Decrease of decontamination works in the loops (No leaks). Decrease of decontamination works in the loops (No leaks). Better radiological conditions for execution of the works (higher effectiveness in the works) Better radiological conditions for execution of the works (higher effectiveness in the works)

7. Summary The modifications implanted in the last years have produced a drastic change of the radiological characteristics of the power station, with dose rates much lower, and far from being temporary,will continue in the next outages. The modifications implanted in the last years have produced a drastic change of the radiological characteristics of the power station, with dose rates much lower, and far from being temporary,will continue in the next outages. The changes have also cause a very important decrease of the work load in controlled area during the outages. The changes have also cause a very important decrease of the work load in controlled area during the outages. Both circumstances have caused that the collective doses during the refueling outages are around 15-20% of the received ones before the changes. Both circumstances have caused that the collective doses during the refueling outages are around 15-20% of the received ones before the changes. The maximum individual doses in each outage are below 5 mSv, what allows to fulfill the new legal requirements of dose without problems. The maximum individual doses in each outage are below 5 mSv, what allows to fulfill the new legal requirements of dose without problems. Objectives of collective dose for C.N.de Almaraz : <300 mSv-p/year.reactor Objectives of collective dose for C.N.de Almaraz : <300 mSv-p/year.reactor