APPROACH FROM THE PFBR TO THE FUTURE FBR 1. Economic factors and design approach

Economic competitiveness is vital for commercial deployment of fast reactors. Significant design efforts are necessary to reduce the capital cost of future FBRs coupled with enhanced safety. Therefore, there is a challenge to identify the critical influential parameters that govern the overall cost and safety, and efforts are channelled into optimizing these with focused R&D, keeping in view the international experience. Lessons learnt from the PFBR in terms of plant layout, civil construction, manufacture of NSSS components, in particular technical specifications, tender packages and regulatory review will be incorpo-rated in the design and construction of future FBRs. A detailed review of the capital cost breakdown of PFBR indicates that the reactor assembly, sodium circuits and fuel handling systems require closer scrutiny for possible cost reduction measures and there is little scope in the balance of plant due to the level of standardization and maturity in its associated systems. Apart from the above, the analysis of unit energy costs of the PFBR reveal further tangible benefits for enhancing plant thermal efficiency, fuel burnup and the plant capacity factor, reducing construction time, using multiple unit construction, and introducing policy measures on financial parameters such as depreciation rate, debt equity ratio, interest rate, etc. (Fig.1). Through the above exercise, and also based on

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experience with construction and operation of FBTR and FBR units worldwide and state of the art R&D with respect to sodium cooled fast reactors (SFRs), well-defined optimization objectives/targets are well-defined for future FBRs, also called the commercial fast breeder reactor (CFBR). These are summarized in Table 1.

There are plans to construct six units each of 500 MW(e) capacity in addition to the PFBR. A systematic roadmap made for the cost reduction of FBRs through the above measures/mechanisms reveals cost benefits of a ~35%

reduction in tariff for the CFBR from 3 to 6 in comparison to the PFBR on a constant money basis and with same debt:equity ratio.

4.2. Measures to enhance safety

Improvements in the shutdown system, decay heat removal system, in-service inspection, sodium purification, steam generator and number of primary sodium pipes are some of the key areas identified for enhancing reactor safety.

In the PFBR, two independent and fast acting diverse shutdown systems, comprising nine control and safety rods driven by individual drive mechanisms (CSRDMs) and three diverse safety rods along with their individual drive mechanisms (DSRDMs) are provided. In both systems, the gravity assisted SCRAM action by dropping the absorber rods (CSR/DSR) into the reactor core is achieved through de-energization of the electromagnets holding the rods. Apart from many distinct differences between electromagnets, the location of the electromagnet is also different. In the CSRDM, the electromagnet is located in an argon atmosphere and is housed in the upper part of the mechanism, while in the DSRDM, it is located in the lower part and is immersed in sodium.

FIG. 1. Factors influencing overall costs.

PLENARY SESSION 1

95 Unprotected loss of flow and unprotected transient overpower are two events leading to severe consequences and are initiating events for a core disruptive accident. For the PFBR, each shutdown system consists of a reactor protection system, actuation system and safety support system. Each shutdown system has a non-availability of less than 10^–3/reactor-year with the overall objective to achieve a non-availability of both shutdown systems of <1 × 10^–6/reactor-year.

For future FBRs, the targeted reliability is 1 × 10^–7/reactor-year for which additional passive/active safety features are considered for implementation in the design. Enhanced reliability of 10^-7/reactor-year provides the potential for keeping a whole core accident as a beyond design basis event.

The temperature sensitive magnetic switch and the temperature sensitive electromagnet are two passive safety concepts currently under development.

Development work on the temperature sensitive magnetic switch is at an advanced stage. As part of an active system, a stroke limiting device has been TABLE 1. BASIC DESIGN FEATURES

Primary circuit Pool with external

purification Pool with no primary sodium outrside pool

Fuel MOX MOX

Fuel burnup (GW·d/t) 100 150 initially and

200 later

Design life 40 calendar years

75% load factor

60 calendar years 85% load factor

Unit Single Twin

Construction time 8 years 8 years

Number of:

Number of SGDHR loops 4 6

Spent fuel storage medium Water Water

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identified for further theoretical and experimental development. Other passive safety features will also be investigated in order to achieve the targeted reliability of shutdown systems.

4.2.1. Decay heat removal system

Accomplishment of smooth decay heat removal, after reactor shutdown, through highly reliable systems is an essential and important safety requirement.

For the PFBR, two different decay heat removal systems are provided, namely, (i) the system operating under normal conditions through the steam–water system, referred to as operation grade decay heat removal system, and (ii) the system whereby decay heat removal occurs by natural convection, intermediate sodium and air paths, and referred to as the safety grade decay heat removal system and designed with adequate diversity. Figure 2 shows the details of the safety grade decay heat removal system in the PFBR.

FIG. 2. The safety grade decay heat removal system in the PFBR.

PLENARY SESSION 1

97 For future FBRs, it is proposed to provide six dedicated loops with a capacity of 6 MW(th) each. To achieve enhanced diversity in sodium and air flow, it is planned to have three loops working in natural convection mode and the other three loops normally working in forced convection mode with an electro-magnetic pump and air blower in each loop, designed to work in natural convection mode with reduced thermal capacity of ~60%.

4.2.2. Arrangement of primary sodium pipes

Four primary pipes feed the primary sodium from the pump to the core in the PFBR (Fig. 3). The pipes are designed as per Class 1 design rules and are made from 316LN stainless steel. Double ended guillotine rupture of one of the primary pipes is considered a Category 4 event as an enveloping case in the