Proposed pebble bed modular reactor

Evaluation of the Effect of additional Cooling Water Discharge into the Atlantic Ocean at Koeberg Nuclear Power Station

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Pollution Dilution Potential
Potential Marine Impact

Evaluation of the Effect of additional Cooling Water Discharge into the Atlantic Ocean at Koeberg Nuclear Power Station

The attached report from Cape Weatherwise International provides a full assessment of the impact of the proposed Plant on thermal outflows, separately and in conjunction with the Koeberg NPS.

For theoretical purposes the thermal impact of 10 PBMR Modules were also assessed to determine immediate impact and conclusions drawn.

Executive Summary

In evaluating the effect of additional warm water discharge into the Atlantic Ocean at Koeberg Nuclear Power Station, consideration was given to the possible enlargement and temperature increase of the warm water plume as well as to the potential marine impact.

When calculating the increase in water temperature at discharge point (surf zone) due to one operational Pebble Bed Modular Reactor (PBMR) unit, the result is 0.73°C with one Koeberg unit operational and the rise will be 0.39°C when both units are running^¹³⁷. The theoretical rise in plume temperature at a distance if 1-Km is 0.22°C with one Koeberg unit running and 0.12°C when both are running. The additional outflow from one PBMR unit will cause no change in the dissipation of the plume as the dynamic forces in the ocean govern this.

In assessing the potential marine impact of one PBMR, the additional entrainment of planktonic organisms is calculated to be 2%. The resultant higher mortality of plankton due to entrainment by the operation of one PBMR unit is not, however, considered to be detrimental to the marine environment because of the very localised area affected. For a theoretical 10 PBMR units the additional mortality due to entrainment equals 21%. The theoretical temperature rise along the beach to the south falls well within the natural variability of the temperatures along the Atlantic coast and therefore poses minimal risk to the marine environment.

The cooling water from one PBMR unit will have no detectable effect on the marine environment nor increase the warm water plume to a level where the potential risk increases.

When more than one unit is constructed and operational, detailed studies of the resultant warm plume will have to be undertaken to verify the extent and temperature of the plume. In addition, consideration must be given to upgrade the current marine impact study. Both these studies should be conducted when a 12% increase (6 PBMR units at 1.7m³/s each) of maximum Koeberg operational flow rate^¹³⁸ is reached.

Introduction

Koeberg Nuclear Power Station is situated on the west coast about 30 km north of Cape Town. One of the main reasons for siting the station on the Atlantic coast, is the relatively cold seawater, which is utilised as a condenser-cooling medium. The average sea water temperature is in the region of 13°C with the minimum below 10°C and the maximum exceeding 20°C on rare occasions. At full operation the station pumps just more than 80 cubic meters water per second through the condensers. This water is chlorinated at ± 1 part per million (ppm) before reaching the condensers where the water temperature increases at an average of about 10°C above ambient. Chlorination of the intake water prevents the settlement of young marine fouling organisms inside the plant. This water, warmed and chlorinated, is now returned to the sea into relatively shallow water via the outfall structure. The configuration of the outfall structure causes the water to be jetted in a south-westerly direction at a speed of between 2 and 3 m/s at the outlet of the outfall, depending on tide and sea swell conditions. As the water is more buoyant, a warm water plume is formed.

Studies have been conducted to assess the extent, dissipation and behaviour of this plume under different environmental conditions. In addition, the marine environmental impact of this warm plume has been and still is being researched.

This first focus of the evaluation is to determine the effect on the warm water plume by the additional warm water used by a Pebble Bed Modular Reactor (PBMR). Secondly, the impact on the marine environment is assessed.

A mathematical approach is used starting with the theoretical cooling requirement of one PBMR. The calculations are then continued to determine the possible effects of up to 10 PBMR units.

Assumptions:

The basis for the evaluation is taken as:

A discharge rate of 1.7 cubic meters per second per PBMR.
A PBMR delta temperature of 40 degrees Celsius above ambient at discharge point.
A delta temperature of 10 degrees Celsius above ambient is taken as the Koeberg warm water plume discharge temperature.

The following assumptions are made:

That the PBMR discharge point will converge with the existing Koeberg condenser cooling water discharge.
That the Chlorination concentration of the PBMR cooling water will be below 1 PPM at the condensers.
That no other pollutants are added to the warm water discharge.
That the seawater filtration system will have the same grid spectrum as the Koeberg filtration system.
That the PBMR condenser structures and apertures are in the same physical range as those of Koeberg.
That the discharge of the additional warm water from the first and additional PBMR units flows directly into the Koeberg condenser cooling water outfall channel.
A theoretical multi unit of 10 PBMRs was included to assess cumulative impact should more units be considered at the site.

Warm Water Plume

Rattey and Potgieter¹ in “Warm Water Plume Report” adequately described the complex dynamic forces acting on the discharged warm water from Koeberg. In assessing the dissipation of a plume with increased volume, the governing factor of path, extent and dissipation of the plume remains unchanged. Only the worst case scenarios need be considered.

Worst Case Scenarios

In defining the worst case scenario, consideration of what the worst plume would be like, should be taken into account. In reality this would be the circumstance in which the least mixing of the warm plume with the Atlantic Ocean occurs. The temperature increase of the buoyant plume would be the highest above ambient at the point where it could have detrimental impact. Two such areas are evident. The first being at the end of the southern breakwater, where a potential re-circulation threat exists and the second on the beaches directly to the south of the Power Station, where higher seawater temperatures could impact on the marine environment.

The first worst case scenario will be the result of light easterly winds with relatively calm sea conditions which periodically occur after a high pressure cell has ridged over the interior. This will be typical just after a south-easterly wind condition. This condition occurs mostly in summer.

The second scenario that can be classified as worst case is the passage of a coastal low or the approach of a frontal system. In these cases, the wind will be from a northerly direction for a period of time, thus causing a southerly long shore current to become prevalent. This condition occurs mostly in winter. A more in depth description of these two conditions is thought to be prudent.

Worst Case Condition 1

Light easterly surface winds with little or no swell conditions:

The wind and swell condition that is associated with the slackening of a southeaster (and a typical summer wind regime), will result in a worst case plume. In this case the southeaster and even the light easterly conditions, cause an up-welling event that result in a sharp drop in sea surface temperatures. This is due to the warmer surface water being driven offshore and the colder water from the bottom being forced to the surface along the coast. The warm water plume will be restricted to a northerly direction, immediately adjacent to the southern breakwater and will have a steep isotherm gradient on the southern flank. This is as a result of the northerly surface current induced by the southeaster. This current has a greater force than the Koeberg ejected plume, that started in a south-westerly direction due to the construction configuration of the outfall structure. The northerly current induced by the wind is of a long shore nature but the physical barrier posed by the breakwater forces the plume in a north-westerly direction.

As Rattey and Potgieter¹ determined, this condition results in a plume of 2 – 3°C above ambient at the northern tip of the southern breakwater, (Warm Water Plume survey no. 15 : 26 September 1986). This was for a plume with Koeberg running at full capacity, thus 82m³/s.

Worst Case Condition 2

Light northerly surface winds with little or no swell conditions:

The warm water plume will move offshore in a south-westerly direction. This is directly against the wave direction and thus the prevailing turbulent forces. As tremendous turbulent forces exist even in small swell heights, dissipation occurs rapidly and no apparent demarcation of plume is observed. Horizontal eddy diffusion also adds in this mixing process. The surface current, being onshore, now tries to swing the plume around to force it on the beaches to the south of the outfall. As the plume is thus mostly parallel to the coast the main body of the plume remains in the turbulent surf zone. Mixing to a greater depth is therefore achieved and the warm plumes to the beaches to the south of the station are thus not as pronounced as in the case with the plumes jetting offshore adjacent to the breakwater. No clear demarcation of the horizontal isotherms can be observed due to the variability, extend and force of the turbulence available close to the shore. A person standing at an elevated position close to the surf zone can easily observe this turbulence.

Plume Temperature rise and associated Risk

To be able to asses and define the potential increase in the temperature of the discharged water, at discharge point as well as of the warm plume in the ocean, a mathematical approach is taken. This will enable the quantification of the associated risk.

The theoretical increase in temperature of the total discharged volume needs to be calculated taking the current Koeberg flow rate and temperature, and combining it with the planned PBMR flow rate and temperature.

This results in:

PpT = (Kv(T) + Av(T)) / Tv (1)

Where PpT = Potential Plume Temperature Increase and

Kv(T) = Koeberg effluent volume at Delta T

Av(T) = Additional effluent volume at Delta T

Tv = Total effluent volume

Applying (1) to the different operating criteria and the volumes and temperatures as defined in the Assumptions, the potential temperature increase for up to 10 PBMR units has been calculated. The conservative assumption that the Koeberg outlet temperature is 10°C above ambient was followed. Operating regimes are defined as A) Koeberg using 42m³/s, B) 62m³/s and C) 82m³/s of cooling water. The results of the calculations is given in Table 47.

Koeberg Operating Regime	Outfall Temperature increase per number of Pebble Bed Modular Reactors
Koeberg Operating Regime
1	2	3	4	5	6	7	8	9	10
A	0.73	1.35	1.89	2.36	2.77	3.14	3.47	3.77	4.04	4.29
B	0.50	0.96	1.37	1.74	2.08	2.39	2.67	2.93	3.18	3.40
C	0.39	0.74	1.07	1.38	1.66	1.92	2.17	2.40	2.62	2.82

Table 47: Maximum Temperature increase in degrees Celsius.

Taking into consideration that the maximum expected plume temperature increase is 0.73°C or 7% at the outfall, with one additional unit operating, this increase will have no significance on the plume path, extend or dissipation. The fractional increase in buoyancy will not influence physical behaviour of the plume in the turbulent ocean.

Utilising the above data, the potential increase in temperature of the plume at 1-Km can now be calculated. The conservative approach is followed that the Koeberg warm plume is 10°C above ambient at outfall and 3°C above ambient at 1-Km distance from the outfall. To calculate the theoretical new plume temperature at 1-Km, we use (1) but add the Koeberg delta plume temperature at 1-Km.

PpT^(1-Km) = PpT/(TK/TK^(1-Km) (2)

Where TK = Koeberg Outlet Temperature above ambient and

TK^(1-Km)= Koeberg Plume Temperature above ambient at 1-Km

Koeberg Operating Regime	Temperature at 1-Km increase per number of Pebble Bed Modular Reactors
Koeberg Operating Regime
1	2	3	4	5	6	7	8	9	10
A	0.22	0.40	0.57	0.71	0.83	0.94	1.04	1.13	1.21	1.29
B	0.15	0.29	0.41	0.52	0.62	0.72	0.80	0.88	0.95	1.02
C	0.12	0.22	0.32	0.41	0.50	0.58	0.65	0.72	0.79	0.85

Table 48: Maximum Temperature increase in degrees Celsius.

Pollution Dilution Potential

A practical mathematical approach to the potential dilution of any additional pollution into the discharged water is taken.

For any dilution, the following will hold true:

Ci/Cf

Where: Ci = Initial Effluent Concentration and

Cf = Final Effluent Concentration,
and the result will be the calculated dilution.

When the additional discharged water is taken, per assumptions, as 1.7 m³/s per PBMR, and the different operating regimes of Koeberg with the associated cooling water discharge rates of 42 m³/s, 62 m³/s and 82 m³/s we get dilution factors of:

A (Koeberg cooling water discharge volume of 42 m³/s) 1.7/42 = 0.040

B (Koeberg cooling water discharge volume of 62 m³/s) 1.7/62 = 0.027

C (Koeberg cooling water discharge volume of 82 m³/s) 1.7/82 = 0.021

In the operation of a single PBMR unit, which will have 1.7 m³/s of cooling water added to the Koeberg discharge, any impurities resulting from the operation, such as chlorination, need to be assessed utilising the above factors. As Koeberg chlorinates their cooling water at 1 part per million, chlorination might be a special case. For any other impurities, the above will hold.

In practical terms, the dilution of impurities from the PBMR will be a factor of approximately 25, 37 and 48 times for the different operating regimes (A, B and C) of Koeberg.

As the assumptions preclude any additional pollution, no calculation is done for any specific pollution but the above factors can be utilised for this purpose.

Potential Marine Impact

Various factors need to be taken into consideration to determine the potential marine impact of any additional pollution in the form of warm water as a result of any cooling process using seawater.

Temperature

Water temperature is a parameter that influences the physical ability of water to dissolve gasses that sustain marine macro fauna as well as micro fauna. Any increase in this temperature decreases the solubility of gasses over time therefore decreasing the capability of water to hold life giving dissolved oxygen. In addition, micro and macro organism metabolic rates are increased due to an increase in temperature. Due to the increase in metabolism, organism development is speeded up and consequently more dissolved oxygen is required to maintain existence. Changes in temperature can also effect the life cycles of various organisms as the mating and spawning of some are triggered by certain water temperature regimes. The overall effect of increased thermal pollution may therefore be a reduction in the number and species of marine fauna in the area.

Rattey and Potgieter⁴ determined that the natural standard deviation of surf zone temperature is in the order of 0.46°C on a daily basis. With Koeberg operational this increases to 0.62°C. One PBMR will add a conservative additional delta temperature of 0.12°C. The additional 0.16 for Koeberg and 0.12 for PBMR can also be described within the natural deviation.

It was found by Cook² and in subsequent years by Cook³, that no detrimental effect on the marine life around Koeberg can be found due to the influence of the warm plume. The reports state that no settlement by opportunistic warm water species or a reduction of Species Diversity Index could be found.

Entrainment Process

With one PBMR unit operating, the total sea water volume used for one day will be approximately 150 thousand cubic meters. For Koeberg this volume exceeds 7 million m³. For 10 PBMR's the volume will increase to 1.5 million m³. This water will be pumped and forced through filter systems and condensers. This huge volume of water contains vast numbers of planktonic organisms, all less than 3mm in size, which then get subjected to heat, physical stress, mechanical damage, pressure changes, turbulence as well as chlorination. This entrainment process poses a risk that the planktonic biomass might be reduced.

Utilising the pollution factors calculated for the different operating regimes, the reduction in phytoplankton biomass can be calculated. The average phytoplankton biomass reduction for Koeberg was calculated to be 53% by Cook³from measurements made. He also found the reduction in zooplankton mortality to be 22% due to entrainment.

For a PBMR, the grid sizes of the marine filtration system and the physical process through the condensers units is taken to be the same as for Koeberg. Similar forces in the PBMR cooling system to marine animals such as hytoplankton will exist, thus the quoted reduction in biomass and mortality rates will apply.

In the entrainment process, only a very localised area and volume of the Atlantic Ocean is under consideration, thus the effect of biomass reduction and higher than normal plankton mortality is not deemed to be significantly detrimental to the marine environment.

Conclusion

In evaluating the effect that the additional warm water from one, then up to ten, PBMR units will have on the warm water plume as well as the potential impact on the marine environment, a number of conclusion are made:

With one PBMR, the maximum expected outfall temperature rise is 0.73 Degrees Celsius with only one Koeberg unit operational and 0.39 Degrees Celsius with both Koeberg units running.
With ten PBMR's the maximum increase of temperature at outfall with both Koeberg units running is 2.82 Degrees Celsius.
With one PBMR, the maximum expected temperature rise at a distance of one Kilometre is 0.22 Degrees Celsius with only one Koeberg unit operational and 0.12 Degrees Celsius with both Koeberg units running.
With ten PBMR's the maximum expected temperature rise at a distance of one Kilometre is 0.85 Degrees Celsius with both Koeberg units running.
The fractional increase in buoyancy to elevated temperature will not influence the physical behaviour of the plume in the turbulent ocean.
The additional cooling water volume from a PBMR will not cause any changes in the dissipation of the Koeberg plume as the dynamic forces in the ocean governs this.
The theoretical temperature rise at 1 Kilometre falls well within the natural variability of the Atlantic Ocean and therefore poses a very low to insignificant risk to the marine environment.
The plankton mortality and limited biomass reduction due to the entrainment process has an effect only on a very localised area of the Atlantic Ocean, thus the influence will be of a very low significance.
It was found that no detrimental effect on the marine life around Koeberg could be proved, thus one PBMR will cause no settlement of opportunistic warm water species nor will it reduce the number of species found in the area.

It can be concluded with a high level of confidence, that the warmed water from one PBMR unit will have no detectable effect on the marine environment nor increase the size or temperature of the current warm plume in any significant way.

Recommendations

Should more than one PBMR be considered, further studies should be considered:

The discharge temperature of 40 degrees Celsius per PBMR will result in an increase of 2 Degrees Celsius at outfall with 5 additional units. The extend, dissipation and dilution of the resultant warm plume need to studied when this stage is reached.
The mortality of phytoplankton at the higher stress temperatures needs to be studied.

References:

Warm Water Plume Report, D Rattey and F Potgieter, 1987, Koeberg Nuclear Power Station (Internal Eskom Report)
Final Report, Marine Environmental Monitoring Programme, 1989, P A Cook, Zoology Department, University of Cape Town.
Marine Environmental Reports, Marine Environmental Monitoring Programme, 1990-2001, P A Cook, Zoology Department, University of Cape Town.
Interpretation of Physical Oceanographic Data for Koeberg (1985-1988), D Rattey and F Potgieter, 1989, Koeberg Nuclear Power Station (Internal Eskom Report)

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