CONCEPT OF PEBBLE BED BASED HTGR WITH FAST PEBBLE DISCHARGE SYSTEM

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					2nd International Topical Meeting on HIGH TEMPERATURE REACTOR TECHNOLOGY
Beijing, CHINA, September 22-24, 2004                                                         #Paper D02




     CONCEPT OF PEBBLE BED BASED HTGR WITH FAST PEBBLE
                                           DISCHARGE SYSTEM

                                           DONG Jianling, YU Suyuan
                                INET, Tsinghua University, Beijing 100084 P.R. of CHINA


ABSTRACT: The 10MW high temperature reactor (HTR-10) of China is an advanced pebble bed
modular reactor, which is a high temperature, helium-cooled reactor using a steam generator. The
system is passively safe against the accident of loss-of-forced-cooling. But to get such a safety feature,
low mean core power density, such as 2MW/m3, had been adapted and decay heat has to be
continuously removed from the core through the water wall surrounding the reactor pressure vessel
during the reactor operation. Although the heat power of HTR-10 is only 10MW, the reactor core
cavity has a diameter of 1.8m and mean height of 1.97m, which is surrounded by graphite reflectors
with thickness of 1.0m. Therefore, much larger reactor pressure vessel and relevant surroundings had
to be constructed. This results in higher construction and operation cost. One way to decrease the size
of the pressure vessel and the relevant surroundings is to increase the core power density. But this may
compromise its inherent safety performance.

In this paper, concept of pebble bed based HTGR (high temperature gas cooled reactor) with fast
pebble discharge system against loss-of-forced-cooling accident is put forward to try to make the
reactor has passive safety feature by extracting decay heat of discharged pebbles into pebble catchers,
which are designed and constructed in such a way that no secondary criticality could occur and the
decay heat produced by discharged pebbles is removed by passively safe means, such as water wall,
heat pipes etc.. The preliminary study shows that the concept is feasible in decreasing the size of
reactor pressure vessel and relevant surroundings, and the decay heat continuously removed from the
core.

KEY WORDS: high temperature gas cooled reactor (HTGR), fast pebble discharge system, core
power density, passive safety.



0. INTRODUCTION
The Generation IV International Forum has selected six nuclear energy systems to be developed in
order to meet the goals and criteria for new nuclear systems, based on a coherent methodology. VHTR
(Very-High-Temperature Reactor) is one of them due to its very high temperature. The PMR
(Prismatic Modular Reactor) and PBMR (Pebble Bed Modular Reactor) are put in the frame of the
short-term candidates due to their possible short-term deployment. It is a viable option for future
electricity generation (and hydrogen production based on thermal chemical process for hydrogen
production for VHTR).

The 10MW high temperature reactor (HTR-10) of China is an advanced pebble bed modular reactor,
which is a high temperature, helium-cooled reactor using a steam generator[1-4]. The system is




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CONCEPT OF PEBBLE BED BASED HTGR WITH FAST PEBBLE DISCHARGE SYSTEM                                 # D02


passively safe against the accident of loss-of-forced-cooling. HTR-10 was fully operational in January
2003.

HTR-10 has passive cooling mechanisms and has a negative temperature coefficient. Through natural
radiation, convection and conduction, decay heat is removed passively from the reactor. Therefore,
no melt-down scenario could occur and low fission product release occurs due to loss of coolant
accidents. In HTR-10, to guarantee the inherent safety feature in the event of a loss-of-forced-cooling
accident, two measures had been taken. The first was to limit the amount of energy that would be
deposited in the core. The second was to limit the temperature increase by high thermal inertia and by
a low amount of energy deposited in the core.

For the first measure, the difference of the energy production and energy removal represents the
amount of energy that would be deposited in the core. The decay heat is always a determinant factor of
lower limit of the energy production, since it is impossible to avoid it. The energy production density
of HTR-10 is relatively low because of its low power density. The decay heat that can be removed by
passive means, i.e. temperature limits and heat transfers from the reactor core to the surroundings,
determines the lower limit of the heat removal. In the case of a loss-of-force-cooling accident, against
which no core He cooling is used, decay heat will dissipate through the core structures by means of
heat conduction and radiation to the outside of the reactor pressure vessel, where, on the wall of the
concrete cavity, a water wall cooling system is designed and constructed. This system works on the
principle of natural circulation of water and it takes the decay heat via air coolers to the atmosphere.
Helium that is still present in the core, which is dependent on the type of loss-of-forced-cooling
accident, can take care of the heat transport by natural convection. Due to the good heat conductivity
of graphite and due to the low power density, heat (and energy) can be transferred by conduction. Heat
transfer inside the pebble bed takes place by conduction via pebble-to-pebble contacts and via
radiation across the inter-pebble cavities. The heat transfer from the reactor vessel to the water wall
surrounding the pressure vessel takes place by both radiation and convection.

For the second measure, HTR-10 contains large amounts of graphite. Since graphite has a high
specific heat capacity that increases with increasing temperature, the energy deposition in the core
leads to a relatively small increase of the temperature.

As a result of the two measures, the HTR-10 reactor core cavity has a diameter of 1.8m, mean height
of 1.97m, which is surrounded by the graphite reflectors thickness of which is 1.0m. In the cavity,
there are 27000 pebbles. In order to produce an environment in favor of convection, ratio of  height
and diameter of the pressure vessel has to be great, which limits the application of other type of
pressure vessel, say spherical pressure vessel. The reactor thermal power is 10MW and the mean
power density of the core is 2MW/m3. The working pressure of the helium coolant in the primary
system is 3.0MPa and mean core inlet and outlet temperature of 250oC and 700 oC respectively. Figure
1 gives the structure layout of the reactor.

The HTR-10 feature is its inherent safety against the loss-of-forced-cooling accident. But this feature
also causes high construction and operating cost. The core power density and dimensions of graphite
reflector layer determines size of the pressure vessel. The pressure vessel has a height of 11.15m,
diameter of 4.2m and weight of 167,000kg. This vessel is much larger than that of PWR used to
produce more than 10 times that power. It can be imagined that if the power is further increased, the
pressure vessel will have to be enlarged. Furthermore, in order get inherent safety feature, decay heat
in the nominal operational conditions has to be removed through the water wall surrounding the


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HTR2004                                                                            Beijing, CHINA, 2004. 9.


pressure vessel from the core during the reactor operation. This is a loss of energy. For HTR-10, one
way to decrease the dimensions of the core pressure vessel is to increase its core power density. But
this may compromise its inherent safety performance.

As we know, reactor cores may be classified as rigid or movable cores. As far as a movable core is
concerned, it is possible for pebbles to be removed by gravity to a passively cooled storage place
outside the core[5-6]. Obviously, the core in HTR-10 based on pebble bed is a movable core. By using
such a feature, concept of fast pebble discharge system against loss-of-forced-cooling accident in high
power density gas cooled reactors based on pebble bed is put forward to try to make high power
density high temperature gas cooled reactors have features of passive safety.




                               FIGURE 1 Structure layout of HTR-10[1]


1 CONCEPT OF FAST PEBBLE DISCHARGE                                         SYSTEM          AGAINST
LOSS-OF-FORCED-COOLING ACCIDENT
The basic idea of fast pebble discharge system against loss-of-forced-cooling accident in high power
density high temperature gas cooled reactor based on pebble bed is to use a fast pebble discharge
system to discharge part of pebbles into a pebble catcher at the bottom part of the reactor pressure
vessel cavity to decrease the power density in the core by means of the fluidity of the pebble bed so
that the decay heat can be partly or whole removed outside the core by passive way and avoid that the
fuel particle temperature exceeds 16000C. The pebble catcher is designed and constructed in such a
way that no secondary criticality could occur and the decay heat produced by discharged pebbles is
removed by passively safe means, such as water wall, heat pipes etc.

There are two methods to discharge the core pebbles. One is to install a dam board on the bottom of
the core. When the temperature in the core exceeds the set value, the dam board melts away and the
pebbles drop into the pebble catcher. The other is to combine a ball valve or something that has the
same functions as the ball valve and the relevant valve-opening device to form a safety pebble



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CONCEPT OF PEBBLE BED BASED HTGR WITH FAST PEBBLE DISCHARGE SYSTEM                                    # D02


discharge valve and to install it in the discharge piping system. In the case of loss-of-forced-coolant
accident, the ball valve is passively opened by the valve-opening device utilizing gravity (or pressure
supplied by a gas storage tank), the pressure of the inlet of the helium circulator and the pressure of the
outlet of the helium circulator in the primary loop.

The concept might bring us merits as follows:
1) It permits HTGR based on pebble bed to have high core power density, while still keep passive
   safety feature;
2) Size of the reactor pressure vessel and the relevant surroundings can be reduced;
3) Spherical pressure vessel may find application in HTGR with a feature of passive safety;
4) Decay heat that needs being continuously removed from the reactor core can be reduced, so output
   heat of the core can be increased.


2. DISCHARGE TIME
Although the dynamic process of HTGR is relative slow due to its large heat inertia and negative
coefficient, whether the pebble discharge system is effective or not in reducing the core power density
in the case of the loss-of-forced-cooling accident still depends on discharge time. It should be short
enough so that pebbles drop into the pebble catcher by gravity before the temperature of fuel particles
in pebbles exceeds a limit value, say 16000C.

The pebble flow is of discrete flow. Discharge time is a function of geometry of the conical-shaped
core bottom, density of pebble material, pebble surface friction coefficient under the high temperature
and the diameter of discharge pipe. Besides, it is an experimentally determined fact that the size of the
discharge opening is a significant factor for pebble. If diameter of discharge opening is less than a
certain proportion of the pebble diameter, the pebble at the exit section forms an arch and forbids
further pebble motion[7]. In order to discharge pebbles smoothly, the diameter is at least larger than 5
times diameter of pebbles.




                       Figure 2 Sketch of the core cavity and the discharge pipe



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HTR2004                                                                               Beijing, CHINA, 2004. 9.


Figure 2 is the sketch of the core cavity and the discharge pipe of the fast pebble discharge system. As
it is stated in section 1, the HTR-10 core cavity has a diameter of 1.8m, the mean height of 1.97m, and
holds 27000 pebbles, diameter of which is 60mm. In the conical-shaped bottom, the cone angle is 1200
and diameter of the discharge pipe is 500mm. Obviously, the diameter of the discharge pipe is greater
than 5 times diameter of pebbles, so arch bridge can not be formed during the pebble discharge
process.

Discharge flow of coarse free-flowing materials through the circular opening of the conical-shaped
bottom of a cylindrical bunker can be calculated by expressions given in reference [8]. For HTR-10,
by the expressions and all the known values, it can be got that the pebble numbers discharged from the
core per second for the face-centered cubic lattice piling and for the identical layer piling equal 1279
pebbles/s and 899 pebbles/s, respectively. Because the total number of pebbles in the core is 27000,
the time needed to discharge all the pebbles is roughly 22 seconds and 30 seconds, respectively. In
order to be conservative, the discharge time is taken as 30 seconds.

It should be mentioned that the calculations above do not give consideration into the high temperature,
such as the high temperature of greater than 10000C, and helium environment during discharge.
Reference 9 tells that the mean coefficient of friction of graphite materials in vacuum (after
precalcination) is 0.7-0.75 when the temperature is the room temperature and reduces to 0.1-0.15 at
temperatures exceeding 15000C. Admission of air after heating into the vacuum would sharply reduce
the friction coefficient by a whole order of magnitude (from about 0.75 to about 0.08). High friction
would quickly resume after subsequent air evacuation (at 10-1 torr). Factors obtained during friction in
helium differ comparatively little from those obtained in vacuum. So for HTR-10, the pebble number
discharged from the core per second should be greater than that calculated above.



3. INTEGRITY OF FUEL PARTICLES IN A PEBBLE DURING THE FAST
DISCHARGE PROCESS
In HTR-10, fuel element is a sphere pebble. It consists of the fuel zone and the non-fuel zone. The
coated fuel particles are scattered in the graphite base to compose the fuel zone, thickness of which is
5.0cm. The non-fuel zone, also called graphite shell, consists of the pure graphite, wall thickness of
which is 0.5cm. Before the pebbles are discharged as spent fuel element, half of the fission products
are gaseous. So at a high burn-up, a high pressure will exist inside the particle pressure vessel. The
high pressure exerts a tensile force on the pressure vessel wall.

When pebbles drop into the pebble catcher, there are two processes during which pebbles and the inner
fuel particles may be damaged. The first one is the temperature gradient. The second one is the mutual
impact between pebbles and between the pebbles and the catcher bottom during the discharge process.

When the pebble discharge valve is opened, at first, pebbles in the discharge pipe drop, where the
temperature is low, then high temperature pebbles in the core fall into the discharge pipe and the
pebble catcher. During the process, pebbles in the core will withstand a great temperature gradient
because the surface temperature of the pebble can be as high as 14000C, while the environment
temperature in the catcher is only 2500C or even much lower. But when pebbles are discharged out
of the core and drop into the pebble catcher, the graphite shell will contract under the temperature
gradient. The graphite shell contraction cause compressive stresses on fuel particles. If the integrity of
the graphite shell is guaranteed, then the graphite shell contraction is helpful to prevent fuel particles


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CONCEPT OF PEBBLE BED BASED HTGR WITH FAST PEBBLE DISCHARGE SYSTEM                                # D02


from crack. This stress distribution is better than that of pebble charging process. When new pebbles
are charged into the core, pebbles also withstand high temperature gradient, but the graphite shell
expands. The HTR-10 operation experience has shown that when new pebbles are charged into the
core, fuel particle coating integrity of pebbles can be guaranteed. So it seems that the temperature
gradient cannot damage the particles. But it should be pointed out that the inner pressure of particle
coatings of high burn-up pebbles is much high than that of particle coatings of low burn-up pebbles.
For those pebbles, whether their fuel particle integrity can be guaranteed remains uncertain during the
fast discharge process and needs further studying.

As far as the pebble in HTR-10 is concerned, numbers of drop from the height of 4m to the pebble bed
before being broken are at least 50 times[10]. So pebble graphite shell and the inside fuel particles
should not be damaged by the mutual impact between pebbles and the impact between pebbles and the
bottom of the catcher during the discharge process.


4. CONCLUSIONS
The preliminary study above shows that the discharge time could be very short and the integrity of
pebbles could be guaranteed. Although studies and experiments have to be done further, it seems
feasible to use such concept of fast pebble discharge system to increase the core power density of
HTGR while to maintain its passive safety feature. The HTGR using this concept might have
advantages as follows:

    1    Greater core power density, while still keep passive safety feature;

    2    Size of the reactor pressure vessel and the relevant surroundings can be reduced;

    3)   Spherical pressure vessel may find application in HTGR with feature of passive safety;

    4 Decay heat that needs being continuously removed from the reactor vessel (water wall) can be
    reduced and useful output heat of the core can be increased.


ACKNOWLEDGMENTS
This paper includes many contributions. Particular acknowledgments are addressed to Mr. Alain
CONTI, Mr. Delpech MARC, Mr. Jean Claude GARNIER, Mr. Dominique BARBIER and Mr.Yves
LEJAIL in CEA-Cadarache, France.


REFERENCES
[1] Xu Yuanhui. The HTR-10 project and its further development. Proceedings of the Conference on
    High Temperature Reactors, Petten, NL, April 22-24, 2002.

[2] Zuoyi Zhang and Suyuan Yu. Future HTGR Developments in China after the Criticality of the
    HTR-10, Nuclear Engineering and Design, Vol. 218 No.1-3, 2002, pp249-257

[3] Zhong Daxin, Qin Zhenya. Overview of the 10MW High Temperature Gas-Cooled Reactor Test
    Module, Proceedings of celebration for the HTR-10 criticality, Beijing, CHINA, March 19-21,
    2001.




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HTR2004                                                                        Beijing, CHINA, 2004. 9.


[4] Dong Jianling, MENG Yang, YU Suyuan, FU Jiyang. Feasibility Analysis of Method to Avoid
    Tube Rupture in HTR-10 Steam Generator, Proceedings of SMiRT-16, Washington, USA, 2001

[5] Farhang Sefidvash. Status of the Small Modular Fluidized Bed Light Water Nuclear Reactor
    Concept , Nuclear Engineering and Design 167( 1996) 203-214.

[6] J.R. Powell, H. Ludewig, M. Todosow. SAFETY ASPECTS OF PARTICLE BED REACTOR
    PLUTONIUM BURNER SYSTEM, Proceedings of the Global '93 Conference, Seattle, 1993. pp.
    652-655 (http://www.nea.fr/html/trw/docs/global93/)

[7] Andrew Kadak. Pebble dynamics in PBMR: experiments and modeling, Project report, Course
    22.033/22.33

[8] T. M. Verghese and R. M. Nedderman. The discharge of fine sands from conical hoppers,
    Chemical Engineering Science, Vol. 50, No. 19, pp.3134-3153, 1995.

[9] A.P. Semenov. Tribology at high temperature, Tribology International Vol. 28. No.1, pp. 45-50,
    1995.

[10] Tang Chunhe, Zhu Junguo, Qiu Xueliang, Xu Zhichang, Zhang Chun and Ni Xiaojun. Fabrication
    of the first loading fuel of 10 MW High Temperature Gas Cooled Reactor-Test Module.
    Proceedings of celebration for the HTR-10 criticality, Beijing, CHINA, March 19-21, 2001




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