Philippine Journal of Science 149 (3-a): 791-799, October 2020 ISSN 0031 - 7683 Date Received: 28 Jan 2020

Radiation Dose in a Reactor Service Area of the AP-1000 Based on the Fukushima Accident

Rokhmadi1*, Ardani1§, Taswanda Taryo1, Muhammad Subekti1, Toshikazu Takeda2, and Arturo Failuga Salih3

1Center for Technology and Safety – BATAN Kawasan PUSPIPTEK Gedung No. 80 Serpong Kota Tangerang Selatan 15310 Indonesia 2Nuclear Research Center, Fukui University, Fukui-ken, Japan 3Philippine Nuclear Research Institute, Department of Science and Technology Commonwealth Ave., Diliman, Quezon City, Philippines

Cooling water as a radiation shield from a reactor core originates from the reactor core. In the case of an accident, when the cooling water level inside a pressure vessel decreases, the function of water as a radiation shield decreases as well. After the time passage of the reactor shutdown, the strength of gamma (γ) radiation source from the reactor core decreases. The combination of the decrease of the water that reduces the function of radiation shielding and the decrease of the source strength will affect the pattern of dose rate in the reactor service area (RSA) after an accident takes place. This paper discusses γ dose rate in an RSA of an advanced pressurized water reactor (PWR) (AP- 1000) plant (NPP) based on the accident of Fukushima No. 1 reactor. The referred accident is the decrease in the level of cooling water until the it reached half of the height of the reactor core. The reactor core was not damaged since the γ source is still trapped in the matrix of the reactor core. The highest dose rate occurs when the water level decreases to the upper level of the reactor core. The dose rate above the pressure vessel at the RSA prior to the decrease of the water level was 0.7 rad/h, while the dose rate reached 740 rads/h when the water level reaches to the middle of the reactor core. Indeed, the dose rate at a point of 17.5 m horizontally from the center of the core tank was 0.057 rad/h. This condition requires highly selective control to enable entry to the area of the accident. Indeed, a large γ radiation rate due to a decline in the surface horizontal distance is around 6,404 times greater than those of cosmic and natural γ rays.

Keywords: AP-1000 reactor, dose rate, Fukushima No-1 reactor, reactor service area, water cooling level decrease

INTRODUCTION al. 2012). A nuclear accident that occurred in Fukushima No. 1 reactor becomes a basis to increase the efforts to Natural γ rays originate from cosmic radiation and from overcome the impact of the accident. This is due to the the earth itself and, according to the United Nations use of nuclear energy for the production of electricity, Scientific Committee on Atomic Radiation Securities, the which is one of the few options for future energy. While average dose rate is 89 nGy/h or 8.9 μrad/h (Hazrati et nuclear energy demand increases in the world (Krikorian *Corresponding Author: [email protected] and Evrensel 2017), fossil fuel supply is limited and has §Retired a serious impact to the environment (Purwadi 2010). To

791 Philippine Journal of Science Rokhmadi et al.: Radiation Dose in AP-1000 Reactor Service Area Vol. 149 No. 3-a, October 2020 construct a new NPP, nuclear safety aspects should be NPP’s RSA above the (RPV). The AP- implemented and the impact of radiation from the plant 1000 is an NPP designed and sold by Toshiba-Westinghouse, to working areas, people operating the plant, and the and the AP-1000 is a PWR (a 1000-MWe PWR or PWR- environment should be determined. 1000) with the improvement of using passive nuclear safety. In addition, the fourth of the Chinese AP-1000 was launched Various studies have been conducted to examine the into operation in January 2019 (WNN 2019). Table 1 shows phenomena of accidents that occur, aimed at quantifying the main parameters of AP-1000. nuclear reactor safety against accidents to evaluate the effectiveness of reactor accident management actions. . Main parameters of AP-1000 (Schulz 2008). According to the IAEA guidelines, planning for a reactor Table 1 accident management strategy requires input from the Parameter 2 SG/4 RCPs analysis of accident phenomena and analysis of their Net electric power, MWe 1,117 consequences (Udiyani et al. 2013). A nuclear accident Reactor power, MWt 3,400 at Fukushima No. 1 reactor decreased the water level in Hot leg temperature, °C 321 the reactor pressure vessel and, hence, this also implies a reduction of the radiation shield. The addition of the coolant Number of fuel assembly 157 in the primary cooling system of the reactor enhances the Type of fuel assembly 17 x 17 function of water as a radiation shield, especially for an Active fuel length, m 4.27 RSA, i.e. the area above the pressure vessel within the reactor Linear heat rating, kW/ft 5.71 containment. In addition, since the reactor automatically shut Control/gear rods 53/16 down due to the accident, there is no γ source formation from 3 the radiative-catch reaction and spontaneous fission reaction Vessel flow, m /h 68.1 in the reactor core. What remains is the decaying γ associated SG surface area (each), m2 11.1 with activation and fission products, and a higher level of Pressurizer volume, m3 59.5 (Ardani 2007). SG – steam generator; RCPs – reactor cooling pumps The deteriorating aspect of the radiation shield function due to the decreasing water level and the strong decline The AP-1000 is a PWR-type reactor with two cooling in γ source activity inside the core is a significant reason loops to produce a net power output of 1,117 MWe. It is to determine the factor of radiation dose rate in the RSA an evolutionary improvement on the AP600 – essentially, a of an NPP. The Fukushima No. 1 nuclear reactor accident more powerful model with roughly the same footprint. The was categorized as a Level 7 disaster in the International design decreases the number of components – including Nuclear and Radiological Event Scale (Samet and Chanson pipes, wires, and valves. Because of its simplified design 2015). Although a Fukushima Daiichi NPP (Fukushima compared to a Westinghouse generation II PWR, the AP- No. 1) is a (BWR), it is relevant 1000 has 50% fewer safety-related valves, 35% fewer to investigate a similar nuclear accident that may occur pumps, 80% less safety-related piping, 85% less control in PWR-1000 (a 1000-MWe PWR) or AP-1000 due to cable, and 45% less seismic building volume. In addition, similar general structure of the reactor. The chronology of probabilistic risk assessment was used in the design of the the Fukushima No. 1 reactor accident had been reported by plants and this enabled minimization of risks and calculation the NPP operator (Ardani 2007). The accident took place of the overall safety of the plant. From the safety calculation, since the occurrence of a power blackout of the reactor plant the AP-1000 has a maximum core damage frequency 5.09 brought about by an earthquake of 8.9 intensity (Richter × 10−7 per plant per yr. Power reactors principally remain scale). A 14-m-high tsunami generated by the earthquake to yield heat from radioactive decay products even after hit the plant with wave heights that temporarily inundated the main reaction is shut down; hence, it it is imperative to the station up to about 14 m above sea level, whereas the remove this heat to avoid a meltdown of the reactor core. In reactors were designed to withstand wave heights of 5.7 m. the AP-1000, Westinghouse's passive core cooling system As a result, the turbine building was flooded with salty sea uses a tank of water situated above the reactor. When the water, damaging the diesel-powered backup generator as passive cooling system is activated, the water flows by well as other emergency accessories of the reactors (Aliyu gravity to the top of the reactor where it evaporates to 2015). The cooling water level in the pressure vessel then remove heat. Although the reactor operators take no action, decreased to half of the height of the core. the system uses multiple explosively-operated and DC- As part of the lessons learned and review of nuclear operated valves, which must operate within the first 30 min. technology, particularly in radiation safety at the reactor, this The safety design is also intended to passively remove heat piece of accident information was applied to the AP-1000 for 72 h, after which its gravity drain water tank must be NPP (Schulz 2008) to predict the radiation dose rate in the topped up for as long as cooling is required.

792 Philippine Journal of Science Rokhmadi et al.: Radiation Dose in AP-1000 Reactor Service Area Vol. 149 No. 3-a, October 2020

a) PWR-1000 b) AP-1000 Figure 1. Safety systems of PWR 1000 and AP-1000 (Schulz 2008).

It is noted that the structures of the building and pressure and photons that result directly from the fission process vessel, as well as the cooling water, of the PWR-1000 when the reactor is at power are referred to as prompt (AP-1000) reactor pressure vessel are very similar to and photons, decayed neutrons and photons that of the Fukushima No. 1 reactor, although the latter were also considered. is a BWR-type reactor. In our calculation, only the water level decrease – both in Fukushima No. 1 and the AP-1000 Radiation Dose Formula – was considered. Therefore, a model of radiation dose It is assumed that a point source releases particles of rate in the RSA of the AP-1000 reactor is considered to energy E in a random direction at a rate of S Bq (1 Bq = be very similar to that of the Fukushima No. 1 accident. 1 disintegration/s). At a distance of r from the source, it is In this developed scenario, the AP-1000 reactor was placed a detector that has a cross-sectional aperture (area) extinguished by an accident after operating near the of L2. The number of particles per second entering the end of the cycle, i.e. the reactor had been operating for detector is then given by the following formula (Warden 540 d with 3400 MW thermal capacity – the same as 2009): the Fukushima No. 1’s power level during the accident. This accident caused water to drop to a half-high level (1) of fuel without incurring damage to the structure of the fuel element. The source of radiation was still intact in the reactor core and in the confinement of fuel elements, 2 2 and had not yet been scattered elsewhere. The purpose of where 4πr is the area of a sphere with radius r. Hence, L 2 this study is to demonstrate the level of radiation safety in / (4πr ) is the probability that a particle from the source the RSA after the half-reduction of reactor core cooling will reach the detector. A dose is defined as the energy water. The radiation dose rate analysis in the space was deposited into a mass of material. In the SI system of units, performed by using the MCNPX code program (Fensin the unit of dose is the Sievert (Sv), which is measured in 2010) based on the strongest source simulation generated Joules/kg. The older unit that used to be the standard was by the ORIGEN2.1 code program (Ardani 2010). the Röntgen equivalent in man (or mammal) or rem. It has units of 100 ergs/g and, thus, 1 Sv equivalent to 100 rem. A dose rate in the detector is defined as the energy rate entering the detector ER divided by the mass of the METHODOLOGY detector ρL2t, where ρ is the density of the detector and t is the depth to which the particles penetrate. The dose MCNP-compatible sources were prepared to represent rate is then given by (Warden 2009): and photon radiation from a typical AP-1000 core (Pelowitz 2008). Source descriptions were prepared (2) to represent conditions while the reactor is at power, and also at various times following shutdown. While neutrons

793 Philippine Journal of Science Rokhmadi et al.: Radiation Dose in AP-1000 Reactor Service Area Vol. 149 No. 3-a, October 2020

Equation 2 is the basic formula for the radiation dose lengths (Warden 2009): rate. The cross-sectional area of the detector drops out of Equation 1, and D is expressed in SI units. Therefore, (6) the unit of E is in Joules, length in m, and weight in kg. However, in terms of energies of particles like electrons, protons, and nuclei, a Joule is a prohibitively large unit. Assuming x is equal to ρt, the equation then becomes: A more useful unit of energy is the electron volt (eV), (7) the energy that a particle with a charge of one electron acquires falling across an electrical potential of 1 V Thus, the conversion of 1 MeV is 1.602 x 10–13 Joules in order Now, instead of a beam of particles, assume that this to convert MeV to Joules. In addition, cm is a much more becomes a source of γ rays with an initial decay rate of convenient unit. The density ρ is usually expressed in g/ So (Bq) and an attenuated rate of S after traversing t cm cm3. The dosage is not usually expressed in Sv/s but in of material. Thus, the preceding expression could be Sv/h. That is an additional factor of 3600. Finally, the Sv substituted into the dosage rate formula with S substituted is a huge unit for dose. A millionth of an Sv, a micro-Sv for B but, before doing this, it must reflect on what the or μSv, is much more convenient. Thus, expressing D in dosage rate formula means. The dosage rate formula units of μSv/h using units of MeV, g/cm3, etc. requires the calculates the energy deposited per mass. Now, the energy conversion factor as follows (Warden 2009): deposited per second between the surface of a material and x = ρt) is the number of γ rays per s that were absorbed (3) times the energy of each γ E. This is: (8) Based on Blakeman et al. (2007), the dosage rate formula now becomes: and can be substituted for SE in the dosage rate formula. (4) Thus the final formula for the γ radiation is as followed (Warden 2009):

(9) where S is in Bq, E is in MeV, ρ is in g/cm3, and t is in cm. Note that r is still expressed in units of m. Experimental Method for Calculation As explained in the introduction, the accident model Formula for γ Radiation discussed referred to the reduction of the water level in γ rays do not uniformly slow down and eventually stop in the RPV. Water in the pressure vessels – especially at the a material medium. γ rays are ultrahigh-energy photons top, and in addition to functioning the transfer of heat – and always travel at the speed of light. They mostly pass also serves as a radiation shield in the reactor room. If the right through matter unless they interact and are absorbed water level in the reactor vessel decreases, the radiation by annihilation. There are three possible interactions shield function becomes reduced; hence, the radiation depending on the energy of the incident γ photon, such as dose rate in the RSA changes as well. In the reactor core, pair production, Compton scattering (absorption and re- the photon radiation source radiates isotropically with emission), and photoelectric effect (ionization by causing the same probability in all directions. A photon will react an electron to be emitted). A beam of photons or γ rays with a material that keeps the media through a three-way is then attenuated when passing through matter as the reaction, such as photoelectric effect for low energy photons in the beam are absorbed by the particles in the photon, Compton scattering effect for all energy levels, matter medium. If the photon has a cross-section of σ to and effect of pair production for photon energy > 1,022 interact with particles in the medium, the attenuation of MeV. As a result of the reaction, a photon will experience the beam will then be (Warden 2009): a decrease in flux and energy. (5) To estimate the photon flux in the RSA, the MCNPX code was utilized. The program works based on particle where NA is Avogadro's number expressed as AMU/g, m travel simulation and γ photon particles radiate from the is the mass of absorbing particles in AMU, and t is the reactor core. The superiority of the Monte Carlo-based depth to which the beam of γ rays has penetrated. Bo and B theory program is that the code can be used to estimate are the initial and surviving number of γ rays respectively. the particle flux in multi-type media in geometric shapes This formula is sometimes presented in terms of collision that are not necessarily symmetrical.

794 Philippine Journal of Science Rokhmadi et al.: Radiation Dose in AP-1000 Reactor Service Area Vol. 149 No. 3-a, October 2020

For the calculation of γ flux with the MCNPX code, it requires few inputs in the form of geometry representing the object and the material composition, which contains the object and the power of the photon source (Westinghouse Electric Company 2004). Out of the core, the available medium includes cooling water, pressure vessel from SS material, radiation shield, and room over an air-filled vessel. Inside the core, the composition of the starting material consists of fresh fuels and cooling water. During the reactor operation, it will occur the formation Figure 2. Reactor plan and RSA and point of calculation. and shrinkage of material in the core due to fission and activation reactions between the core material and Material Composition neutrons. Therefore, the material composition in the The initial material composition of the AP-1000 reactor reactor core changes continuously. After the reactor is presented in Table 2; The composition is also equipped has been shut down, the material composition in the with the total weight of the core material, i.e. 143,432 kg reactor core would have changed due to the decay of the (Ardani 2010), since the ORIGEN-2 package program radioactive material. requires the absolute weight of each element. The reactor core material composition was taken into account from To determine the material composition in the reactor time to time starting from the reactor operation to 1,744 core and the source strength of photons in the core, the min after the reactor blackout. This was determined ORIGEN-2 reactor core depletion simulation program through simulation of the core depletion using the package was used,. The ORIGEN-2 program package ORIGEN-2 package program, within the assumption of requires the input of reactor core material composition – 540 days (one operation cycle of AP-1000) and 3,400 time representing the duration of the reactor operation and MW thermal power. power as a representation of reactor power in operation. The burning core material is the fresh fuel element of the Table 2. Material composition of fresh fuel (% w/o). storage plus the reactor primary cooling water passing Biological Reactor Pressure Nuclide Water through the reactor core volume. shield core vessel The ORIGEN-2 output is a γ photon source strength with H 0.02430 0.0252 0.1112 an energy range between 0–11 MeV, which is subdivided O 1.14000 0.1874 0.8888 into 18 groups of photon energy. The type and quantity Mg 0.00396 of material composition in the reactor core changes with reactor operation time due to fission, activation, Al 0.01830 absorption, and decay. γ flux output can be directly Si 0.03520 converted into the radiation dose rate by inserting a dose S 0.36100 conversion factor into the input and, in this paper, flux- Ca 0.16830 to-dose conversion factors based on ICRP-2 were used. Fe 0.15917 The determination of γ flux at any point is represented Ba 1.55100 by placing the detection points in the MCNPX code Zr 0.1486 (Fensin 2010). Figure 2 shows the pattern of composing U-235 0.0212 the object geometry approach and the detection points representing the calculated location of the flux. Point 1 U-238 0.6101 is the central point located on the RSA and just the floor Mn 0.06154 above the center of the core. Points 2, 3, 4, 5, and 6 are Cr 0.61538 located on the horizontal floor of a plot, with Point 1 Fe 0.32308 having a distance of 2 points as far as 350 cm. Points 7, 8, 9, 10, and 11 are located in a vertical line while Dots 12, 13, 14, 15, and 16 are located on the horizontal and vertical line of the diagonal. The dose rate at the detection Geometry of the Core points is a simulated result of a normalized MCNPX The geometry of the reactor is respectively approached calculation to a photon source. To obtain an absolute by a cylindrical shape, a cylindrical press vessel with a radiation dose rate, this simulated dose is multiplied by half-spherical cap, a bi-cylindrical shield, and a cylindrical the total predetermined total photon source by ORIGEN-2 RSA (Ardani 2010). The geometry of each object is calculation. presented in Table 3.

795 Philippine Journal of Science Rokhmadi et al.: Radiation Dose in AP-1000 Reactor Service Area Vol. 149 No. 3-a, October 2020

Table 3. Geometry of each object. Inner diameter Outer Height Object (cm) diameter (cm) (cm) Core 304.04 426.72 Pressure vessel 404.98 447.64 1220.65 Biological 447.64 1092 1420.65 shield RSA 3,962 3,962

RESULTS AND DISCUSSION Figure 3. Water level decrease as a function of time.

Decreasing Surface Level of the Water of the Core Powerful Photon Sources At the Fukushima No. 1 reactor, the accident preceded the Figure 4 shows a strong pattern of γ photon sources from earthquake on 11 Mar 2011 at 14.46 PM and caused the the ORIGEN-2 program simulation that resulted from reactor to extinguish. The water level decreased on 12 Mar the time of extinguishing (0 min until the vessel pressure 2011 at 11:04 AM to 50 cm from the surface of the reactor drops to the center level of the core, i.e. 1744 min). The core; at 12:05 PM, the water level has reached down to the energy of γ is categorized as some groups, in which level of 150 cm below the reactor-core surface. This means the γ energy varies from 0.01–9.5 MeV. It is seen that that the water level drops approximately 1.6 cm/min, there is a strong decrease in the group of photon sources which is the same assumption as occurring in Fukushima starting from 2.25 MeV. Generally, the source strength No. 1 accident. The data shown in Table 4 was utilized in will decrease all of the time, but that steadily declined for energy levels greater than 7 MeV. This takes place Table 4. Correlation of time and water level. very quickly since there is a photon emitted following the decay of the rapidly decaying Rb-92 and Rb-94 emitting Time of after Water level decrease No. reactor accident (min) (cm) material composition. Meanwhile, the other long-lived radionuclidesdo not contribute photons within the high 1 1,183 0 energy range, i.e. between 7 and 9.5 MeV. At lower 2 1,223 64 energy levels of at most 7 MeV, the short and long-lived 3 1,263 128 make the source strength decrease but not 4 1,303 192 as fast as those of the photon energy more than 7 MeV. 5 1,380 315 As described previously, one of the programming inputs of 6 1,457 438 the MCNPX program is a cell that represents the geometry 7 1,534 561 and composition of the material occupied. Constant cells were used for pressure vessels, biological shields, and 8 1,611 684 air on a pressure vessel in the reactor's containment roof, 9 1,655 755 and the cell core changes their material composition due 10 1,699 826 to material decay. 11 1,744 897

the simulation of the water level reduction in the AP-1000 reactor pressure vessel. This paper describes the rate of accidental doses with the impact of decreasing the water level in the pressure vessel to the center of the core, i.e. 705.64 cm from the full core water level in which the extrapolation was found. It was revealed that the water level began to fall in 1,183 min from the starting of the reactor to exhaust and reached the center of the core at 1,744 min after the reactor shutdown. The correlation between the decrease in the water level and the time after the accident reactor is shown in Table 4 and Figure 3. Figure 4. Decay of photon source intensity after reactor shutdown.

796 Philippine Journal of Science Rokhmadi et al.: Radiation Dose in AP-1000 Reactor Service Area Vol. 149 No. 3-a, October 2020

Figure 5 shows the division of cells in a reactor pressure of the core. This situation lasted until the water level went vessel divided into 10 cells. These cells were originally the down at the top of the reactor core at 1,611 min. Starting reactor pressure vessel containing water at normal reactor from the beginning to 1,611 min, water degradation conditions, alternating with air as the water surface in the occurred next to the core and the dose rate decreased since pressure vessel descended. The uppermost cell, initially the decrease in water has no effect anymore on the RSA. filled with water, begins to alternate with air within 1,183 min after the reactor extinguished and filled with air at 1,223 min after the reactor shutdown. The successive cells below that indeed lose water and are occupied by air, as indicated in Figure 5. With the loss of water alternating air, the radiation shield function becomes lessened.

Figure 6. Radiation dose of RSA in vertical direction.

Figure 5. The MCNPX model for the division of cells in a pressure vessel. Figure 7. Radiation dose on RSA in diagonal direction.

The calculation results of the radiation dose rate in the RSA chamber after the reactor shutdown, i.e. until the water level decline inside the pressure vessel reached the middle of the fuel element, using the MCNPX program can be seen in Figures 6, 7, and 8, respectively. While Figure 6 shows the radiation dose rate with vertical direction at 1, 7, 8, 9, 10, and 11 points, Figure 7 depicts that to the diagonal direction at an angle of 45° from the horizontal line at the detection points of 1, 12, 13, 14, 15, and 16. Finally, Figure 8 shows the radiation dose rate in the horizontal direction at the detection points of 1, 2, 3, 4, 5, and 6. The three graphs – as respectively seen in Figures 6, 7, and 8 – have the same tendencies that the radiation dose Figure 8. Radiation dose on RSA in slab direction. decreases after 1,183 min of the reactor shutdown. This is due to the strong radiation source decreasing, as shown in Figure 4, while the water conditions in the pressure Influence of Biological Shields on Dose Rates in the RSA vessel are still at the top of the reactor core. After that, the Up to the same height as the vessel cylinder, the water level in the pressure vessel descended and resulted biological shield is assumed to be attached to the in an empty space above the water, so the water function outside of the pressure vessel (diameter: 447.64 cm) as a radiation holder decreased. Although the strength of at the top on a ball-shaped vessel biological shield to the photon source is decreased, the radiation dose rate the level of RSA with 892 cm diameter. The effect of increased. At this time, the effect of radiation decrease is biological shielding on the dose rate in the RSA was still more dominant than that of the strong decline of the analyzed as followed. photon source resulting from its rapid drop in water level

797 Philippine Journal of Science Rokhmadi et al.: Radiation Dose in AP-1000 Reactor Service Area Vol. 149 No. 3-a, October 2020

Table 5. Dose rate variation for vertical direction and ratio to the analytical results. Point detection Distance from source center Doses using MCNPX Ratio to point detection Ratio to point detection using (m) (μRad/h) using MCNPX analytical method

1 13.5 0.71 0.59 0.63 7 17.0 0.40 0.67 0.69 8 20.0 0.28 0.71 0.73 9 24.0 0.20 0.74 0.76 10 27.5 0.15 0.76 0.79 11 31.0 0.11

Figure 6 depicts the dose rate toward the vertical graph Further from Point 1, the decrease of radiation dose rate of the dose rate at each detection point – quite similar is very significant. For comparison with the maximum specifically at the water level between 620 and 900 point – at 1,611 min – the dose rate at Point 1 is 740 rad/h, cm. At those times, the dose rates vary between 0.01 the vertical dose rate at Point 11 is 32 rad/h, the diagonal and 10.0 mrad/h, and the dose rate can be analytically directional dose rate at Point 16 is 3.8 rad/h, and the estimated at a certain distance from the source of radiation horizontal directional dose rate at Point 6 was 0.07 rad/ in a homogeneous material. This, indeed, is inversely hr. Indeed, a large γ radiation rate due to a decline in the proportional to the square of the distance from the source surface horizontal distance (17.5 m) is 6,404 times greater center (Warden 2009). Doses at two points of detection than those of cosmic and natural γ rays. 2 each of r1 and r2 have a dose ratio of (r1/r2) (Jemii 2013). The dose ratios in vertical detection points can then be compared to those resulted using MCNPX calculations, as shown in Table 5. It is noted that the relative error CONCLUSIONS resulting from MCNPX calculation varies from 0.05–0.1. The variables affecting the dose rate in the RSA are the Table 5 shows the ratio of dose comparison between the strong decline of the γ photon source activity on the detection points using the MCNPX and the comparison core and the decrease of the water level above the upper of doses using the analytical method. While the analytical boundary of the core in the pressure vessel. The decrease method uses a one-dimensional approach (not counting in the water level surface in the pressure vessel becomes the biological shield around the reactor), the MCNPX dominant in affecting the dose rate. The strength of the γ calculation applies 3-D geometry – taking into account source decreases relatively less, but the increase in dose the existence of a biological shield around the reactor. rate is very sharp due to the influence of decreasing the This adjacent value indicates that the dose rate to the water level in the pressure vessel. vertical is not much affected by that around the reactor. The water level decrease in a pressure vessel next to the The difference of calculation results in a 6% maximum for core does not affect the dose rate in the RSA because, in the analytical method approach (assuming an empty space the event of a water level decrease, the dose rate decreases and no scattering), while MCNPX calculation results are due to the influence of a strong decrease in the source based on the real condition of air-filled objects. of γ radiation. The dose rate in the horizontal direction The difference takes place at doses toward the diagonal is strongly influenced by the existence of the radiation ones. The large decrease between the doses at Point 1 shield and the photon scattering of the RSA itself. In this and Point 11 does not show the correlation ratio of the scenario accident at AP-1000, the dose rate during the distance, as seen in Table 5. This is due to the existence Fukushima No. 1 reactor accident at a distance of 17.5 m of the influence of biological shields especially at great towards the horizontal is around 6,404 times greater than distances, which block the traveling particles. Doses at those of cosmic and natural γ rays. the detection points on the floor of the RSA appear more extreme. At Points 1 and 2, the dose rate is seen to coincide – its value at the same order number – since the biological shield at Point 2 is not significant. However – for Points ACKNOWLEDGMENTS 3, 4, 5, and 6 – there is no direct exposure from the core The authors are firstly very appreciative to the Head source, and only scattering from the RSA takes place. of Center for Nuclear Reactor Technology and Safety,

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