2.0 Reactor Core and Fuel Design

Table 1 gives the basic reactor parameters. The fuel design is based on existing KOFA (Korean Optimized Fuel Assembly) design technology. Most design parameters of fuel rods are the same as those of the KOFA except geometrical arrangement which is changed from the square array to hexagonal array. The hexagonal fuel assembly yields the lower moderator to fuel volume ratio(Vm/Vt) and the hardened neutron spectrum which results in stronger

moderator temperature coefficients and higher plutonium conversion ratio. The effective fuel rod length is reduced to 180cm. Fuel utilizes low enrichment, uranium dioxide, which is operated at a low specific power density(17 kW/kgUCb). The uranium enrichment of the fuel is selected to achieve the 18 months(or longer) operating cycle. The fuel assembly section is a 22.9cm hexagon and the geometry is provided to accommodate control element assembly in each fuel assembly. The fuel assembly consists of 360 fuel rods and 36 guide tubes for control absorbers and/or insertable burnable absorbers and 1 guide tube for central in-core instrument.

The core is rated at 300MWt and consists of 55 fuel assemblies. The average linear heat generation rate is 8.4kW/m which is much lower than that of conventional PWRs. The low power density and increased thermal margins with regard to critical heat flux ensure the core thermal reliability under normal operation and accident conditions. The core is designed to operate without the need for reactivity’ control using soluble boron over the whole power range. The elimination of soluble boron from the primary coolant is a major potential simplification for the advanced light water reactor. From the point of the view of the reactor control and safety, soluble boron free operation offers potential benefits through the presence of a strong negative moderator temperature coefficient over the entire fuel cycle and therefore improves reactor transients and load follow performance. Control rods provide the means of core reactivity control except for long term reactivity compensation for fuel depletion provided by the burnable poison and have enough shut down margin at any time under cold clean conditions including refueling conditions.

Primary Circuit

L. A. ADAMOVICH, G. I. GRECHKO, B. D. LAPIN, V. K. ULASEVICH, V. A. SHISHKIN Research and Development Institute of Power Engineering, Moscow, Russian Federation

Abstract

The paper contains basic conceptual principles used to develop the technical assignment for an autonomous nuclear power plant with integrated nuclear steam supply system (NSSS) designed to provide heat and electricity for areas which are remote with difficult access. The paper also describes technical procedures and equipment, NPP thermal hydraulic flow chart, steam generator design, safety aspects as well as operational and maintenance procedures.

1. Introduction

In areas of Russia which are remote with difficult access, for instance in the Extreme North, Far East, or Siberia a possible reasonable alternative to fossil-fuel energy sources, mainly hydrocarbon, is autonomous nuclear power plants (NPP) of relatively low capacity, shipped to the site in large modules and completely withdrawable from the site upon decommissioning. For the areas in question, small setllements and enterprises with low power demand are typical. The complexity of constructing electric transmission lines, gas pipelines, liquid fuel pipelines, and high cost requires to use local self-contained energy sources. Application of NPPs for power and heat supply may be cost efficient and promising from social and ecological point of view.

A necessity arises to develop new approaches to designing these power sources, naturally, taking into account the experience gained in reactor construction based on modem safety level. Such approaches are to be based on: ‘

• use of well proven technology of water-cooled reactors, in particular, those used at transport facilities with adequate optimisation of their characteristics;

• maximum use of equipment which has operational field-checked prototypes;

• use of designs with inherent safety features; v

• maximum ecological safety of NPP considering nature specifics in the Extreme North and Far East of Russia extraordinarily sensitive to production activities

• preservation of the natural landscape, flora and fauna due to NPP delivery and removal in large plant-manufactured modules which minimize erection and dismantling jobs at the site;

• minimal capital investments and operating costs to make such NPP competitive with conventional fossil power plants.

The paper briefly describes possible implementation of these approaches using a NPP equipped with the integrated nuclear steam supply system.

The following principal requirements were taken into account during choice of structural materials for a cassette type straight-tube steam generator:

1) sufficient level of mechanical properties at operating temperatures taking account of long term operation;

2) corrosion resistance in primary and secondary working media;

3) good weldability and provision of required properties of the welded joints.

Titanium alloys are used as structural materials for steam generating elements, tube sheets and module covers, feedwater tubes, block-section shrouds. Titanium alloys developed in Russia for once-through steam generators possess the abovementioned properties and have lower temperature stresses in comparison with steels under equal operating conditions due to lower values of linear elongation coefficient and modulus of elasticity.

The validity of utilization of titanium alloy as structural materials for cassette steam generator has been confirmed by positive operation experience of a large number of numerous steam generators made of titanium alloys in Russia ship plants with PWRs.

Stainless steel (type 18% Cr, 10% Ni) is used as the structural material for block — section headers and section of feed water tubes and steam tubes welded to them.

Titanium alloy tubes are connected to stainless steel tubes using special strong and leak-tight adapters.

Reactor plant decommissioning starts at unloading of spent fuel and internals. Unloading of fuel assemblies and dismounting of radioactive internals do not require special devices and do not limit methods and the time needed for decommissioning work, because equipment from the reactor is dismounted and unloaded by means of standard plant facilities used during operation (Fig 6).

At the same stage (calculated for 1-3 years) shghth activated and clean equipment is dismounted and disassembled, if it is not needed for the following operation stages.

The secondary stage of work (1-2 years duration) is associated with dismounting, disassembly and removal of reactor plant hot equipment from the plant site.

Slightly activated equipment is disassembled by conventional general industrial methods under control of the radiological safety service, and special facilities and mechanisms are not required.

Hot equipment is broken into fragments by means of remotely controlled manipulators in the concrete silos of the plant

Fragments are packed into protective transport fasks and removed to regional radwaste store.

Spent fuel assemblies in special van-containers are transported to fuel reprocessing plants.

The last stage (1-3 years) is decommissioning of the whole plant including building structures.

• The Integral water-cooled reactor being compact provides better protection from external events due to a reduction in the number of primary pipelines

• Resistance to earthquake is a strong positive feature for integral reactors Other external events have to be treated in a way similar to other reactors

• The reduced probability of internal events increases the relative importance of external events in the safety analysis of integral reactors

5 7 Human factors

The Integral water-cooled reactor provides slower progression of thermal and hydraulic processes in the primary circuit It prolongs the grace period and decreases the human factor effect

5 8 Emergency Evacuation

The enhanced safety of integral water-cooled reactors significantly decreases the probability of accidents which require population evacuation

5 DECOMMISSIONING

Greater attention should be given to the problems of decommissioning in all phases of integral reactor design Decommissioning cost must also be taken into account in technical and economic estimates

XUE DAZHI, LI JICAI, CHANG DAFENG Institute of Nuclear Energy and Technology,

Tsinghua University,

Beijing, China

Abstract

Nuclear heating application has received a wide attention in China due to the favourable economic and environmental aspects. The Nuclear Heating Plant NHR-200 is seen to provide the required energy for district heating, industrial processes and seawater desalination for many sites in China and possibly abroad. The paper summarizes the technical description of the plant and give its main characteristics related to the integral design approach.

1. The development of NHR in China

Because the energy consumption as sensible heat at temperature less than 150°C accounts for about 25% of the total energy consumption in China, therefore, in order to mitigate the energy shortage, environmental pollution caused by coal burning and overburden on coal transportation, great attention has been given to the R& D of the nuclear heating reactor (NHR), which has been as one of the national key projects in science & technology in China since 1980s The NHR could be used in the district heating, air conditioning, sea water desalination and other industrial processes Therefore the NHR could substitute the nuclear energy for fossil fuel and change the energy composition in China This will be of significance m socio-economic development and environmental protection

Research work on possible application of nuclear heat was initiated m early eighties Dunng 1983-1984. the INET used its existing pool type test reactor to provide space heat for the nearby buildings Meanwhile, two types of NHR. i e deep pool type NHR and vessel type NHR. have been developed by INET Based on the heating grid conditions in China and the comparison among various design concepts of the NHR. the vessel type NHR has been selected as a mam development direction As a result, construction of a 5MWt experimental NHR (NHR-5) started in 1986 at INET The reactor was completed m 1989 and has been operated successfully for space heating since then In the meantime, a number of experiments have been earned out to demonstrate the operatmg and safety features of NHR

In China it has been, decided to construct a 200MWt NHR demonstration plant to realize the NHR commercialization The feasibility study of the first DaQing demonstration plant has been completed and approved by the respective authonties

It is worth to mention that dunng the last few years the INET has closely cooperated with Siemens-KWU. German and the former EIR, Switzerland on the R&D of the NHR, and recently the INET also has information and personnel exchange on the matter with the respective institutions m Russia and other countnes

At the chosen parameters the gross thermodynamic efficiency for V-500 SKDI equals 38.1% and the net efficiency equals 37.0%. The absence of main circulation pumps (MCP) reduces the auxiliary power requirement. The main equipment masses for VVER-1000 and V-500 SKDI are presented in Table 3.

V-500 SKDI capital costs relative to those of VVER-1000 will decrease by the reduction of the necessary expensive equipment (absence of primary piping, MCP, accumulators and outside SG). The use of spectral reactivity control gives the possibility of improving the fuel cycle in comparison with that in current LWRs. According to estimations the natural uranium requirements in V-500 SKDI will be 1.1 times less than in VVER-1000. The reduction of the number of equipment units and the plant layout simplification will lead to a decrease of concrete specific expenditures and construction costs. The proposed safety systems and the guard vessel prevent steam and fission products from being released into the containment volume. Consequently the main goal of the V-500 SKDI containment is the defense of the plant against external effects. This simplifies the containment design as compared with an LWR containment. Accordingly in estimates made for beyond design basis accidents the values of population exposure dose limits are not exceeded at a distance of 500-600 m from the NPP. This permits us to consider the opportunity of an NPP location close to large cities and its use for nuclear co-generation.

The factors enumerated above should lead to the improvement of V-500 SKDI technical and economical characteristics as compared with those of current middle and large sized nuclear power plants.

Alongside with development of the V-500 reactor design possible other ways of using water of supercritical pressure in nuclear power were considered. It seems promising to develop integral plants of small power for which the problems of reactor vessel manufacturing are simplified with the acceptable decrease in costs. For example, for a reactor of 250 MW (el) power the vessel OD is 3,5 m with the wall thickness being 240 mm.

From the ecological point of view it seems interesting to use the economically justified possibility, in the case of development of reactors with water of supercritical pressure, of the application of dry and wet water cooling towers to remove heat from the condensers. It will be especially important in the case of using reactors with water of supercritical pressure a for nuclear power-and-heating plants and its location in the vicinity of towns. With the thermodynamic efficiency of the cycle being of the order of 38% the specific amount of steam coming to the turbine is approximately 25% less than this value for WWER-1000. During operation of NPPs with reactors using water of supercritical pressure under the conditions of nuclear power-and-heating plant the steam flow rate into the condenser will reduce by 20% more.

6. Conclusion

Integral NPP with an electrical capacity of 500MW with natural circulating coolant may be created in a vessel with a diameter of less than 5 m with the use to a supercritical primary pressure. The V-500 SKDI safety level satisfies the requirements of the new generation to NPPs. The V-500 SKDI economic characteristics are not inferior to those of current NPPs due to the reduction of capital costs, improved fuel cycle characteristics, plant layout simplification and simpler operation. The NPP may be created on the existing Russian industry base without any significant change of modern LWR technology. It is necessary to construct a V-500 SKDI prototype reactor for testing of the accepted design and to make new decisions. The possible ways of using water of supercritical pressure in nuclear power have been considered.

REFERENCES

[1] Objectives for the development of advanced nuclear plants. JAEA, 1993.

[2] V. S.Protopopov, Teplofysica wisoky temperatur 4 (1977) 815-821.

[3] V. S.Protopopov and V. A.Silin, Teplofysica wisoky temperatur 2 (1973) 445­447.

[4] V. A.Silin, V. A.Voznesensky and A. M.Afrov, Nuclear Engineering and Design 144 (1993) 327-336.

The main difference between discharges through the safety valve and a pipe break from the steam plenum is the position of the discharge orifice. The pipe from the steam plenum is located lower than the safety valve. In normal regimes it opens into the steam volume. But during the discharge process, it may some times be covered by water or two phase liquid. In this case the total drain quantity of water depends on the opening is covered or not.

Discharge from different heights ( distance to the initial water level) has been tested.

In the case of small heights at the beginning of the discharge process the level of two phase liquid ( cause by the vaporization ) may be higher than the discharge orifice. It causes a mixed discharge of water and steam, and increased the total drain quantity. Specially for the bigger orifice diameter the discharge process is stronger, the total drain quantity is increased notable. This can be seen in Fig. 4. For the case of mixed discharge, the total drain quantity may be increased from 19 % to 25 % of initial inventory ( orifice diameters are increased from 4. 3mm to 8. 5mm respectively ) . The less the height difference between the initial level and discharge orifice, the more the total drain water quantity.

Tests showed that when the water inventory increased from 71% to 84% of total volume the total drain water quantity increased from 17.1 % to 20.3% of the total water inventory.

The estimated preliminary investment of the double-reactor plant is about 211 million dollar. The plant completed investment is 441 million dollar.

The plant load factor can reach 80%. The main reasons are as follows:

(1) Simplified systems;

(2) The primary circuit pressure is reduced from ISMPa to lOMPa;

(3) The component reliability is increased obviously, especially the water-water heat exchanger is more reliable than a water-steam one in the primary circuit

In calculating the amount of oil replaced, 7008 hours operation time is considered.

The double-reactor has 8 pumps and total consumption of power is 1600KW in the hot state. The third circuit feed water pump can save power consumption by 20%, but the middle circuit pumps need more power. So the saved offsets the loss. But the pressurizer, wastes disposal system, boron recycling system and high pressure injection system can save some electric power. So the electrical power used in the plant is estimated as 16.5MW. In our estimation, the oil replaced capacity is about 5.15x 105t/a.

The following is a brief plant economic analysis:

(1) Electricity and heat co-generation cost

The estimated plant operation cost is 23.5 million dollar each year. It produces 0.798 billion KWhr electricity (about 0.683 billion KW. hr for users), 3558 billion Kcal heat If the price of one million Kcal heat is considered as 2.95 dollar, the heat supply can obtain 10.5 million dollar profit Thus, the gross cost of the electricity is 0.016 dollar/KW. hr and its net cost is 0.019 dollar/KW. hr.

(2) Retrieval funds

The production cost is about 23.5 million dollar, but the cost of the oil which would be replaced every year, is about 60.6 million dollar. The difference is 37.1 million dollar. The time needed to recover capital is about 12 years. If the oil price is 176.5 dollar/t, the time is 7 only years.

(3) Thermal efficiency

The thermal efficiency of a large nuclear power plant is about 33%, while the nuclear co-generation plant is about 80%.

In summary, the small-sized nuclear co-generation plant is more economic than the coal or oil fuel co-generation plant

Steam Generator (SG) is one of the most important components of the reactor system. It is important as the core itself, because reliability with a minimum amount of maintenances is mandatory for a long time period of time. Indeed, the availability of the plant will be determined by the SG reliability. Specifically, in integral type reactor, enhancement of the thermal effectiveness and resistance against corrosion is of utmost concern to increase the plant performance.

Seismic Design Criteria

The internal reactor pressure vessel structure of integral type is completely different from the existing design, i. e. by introduction of SGs in RPV just above the core. Such configuration requires certainly higher requirements to seismic design.

Needs in the Future

In order to support the development of integral concept extensive experimental program have been undertaken. However, the following research and development (R&D) are considered necessary to develop the reactor more efficient:

• An efficient and reliable SG including the material development related to corrosion protection.

• Reactor dynamics including the accidental conditions using large scale thermal-hydraulic test facility.

• A demonstration/experimental plant might be necessary to be constructed to demonstrate the operability and maintainability.

Beside this, to reach proveness of integral design technology the international promotion must be supported by creation of industrial consortium. The work must be developed together between designer, vendor, utility and the government.