26.08.2015   Nuclear Reactors 1

Sodium cooled fast reactors

In the second half of 2010, there were only two operating sodium cooled fast reactors worldwide, the BN-600 in the Russian Federation and the restarted MONJU in Japan. In the past, there were more sodium cooled fast reactors (the last of the two units in France was shut down early in 2010), and several such reactors are expected to start operation in the coming years (in China, India and the Russian Federation) [4.4].

There are two advanced SMR designs in the sodium cooled fast reactor category — the Japanese 4S of 10 MWe and the US PRISM reactor of 311 MWe (840 MWth). The 4S is a pool-type reactor with an intermediate heat transport system and metallic U-Zr fuel. The basic characteristics of the 4S and PRISM are given[27] in Table 4.5. The PRISM reactor is intended to be fuelled with metallic UPuZr fuel using plutonium and depleted uranium from used light water reactor fuel.

The 4S is different from typical past and present sodium cooled fast reactors in that it is being designed for:

• 30 years of continuous operation on a site without reloading or shuffling of fuel;

• whole core refuelling on the site after the end of a 30-year operation cycle.

Подпись: Box 4.3. Sodium fast reactors Sodium has high heat capacity, allowing linear heat generation rates in the reactor core as high as 485 W/cm, but reacts exothermically with air and water. For this reason all sodium cooled fast reactors incorporate an intermediate heat transport system with secondary sodium as a working fluid. Primary sodium delivers heat generated in the reactor core to an intermediate heat exchanger located within the reactor vessel (pool type reactor) or outside (loop type reactor). Secondary sodium delivers core heat to the steam generators located reasonably far from the reactor in a dedicated premise to localise the impacts of still possible steam-sodium reaction. Indirect Rankine cycle on superheated steam is used for power conversion. Using three circuits is not favourable to the plant’s economics, but operation at relatively high temperatures (~530oC at core outlet) gives higher thermodynamic cycle efficiency in sodium cooled fast reactors, ~42% compared to 32-36% in PWRs. Primary and intermediate sodium circuits operate at a very low pressure of ~0.3 MPa. The space over the sodium pool surface in the reactor pressure vessel is typically filled with a low pressure inert gas, such as argon. Conventional sodium cooled fast reactors use forced circulation of the primary and secondary sodium in normal operation. The systems of decay heat removal are typically active. To be competitive economically, sodium cooled reactors target high fuel burn-ups of up to 130 MWday/kg. In most cases they are being designed in view of operation with the future closed nuclear fuel cycles. Positive experience of operation of fast sodium cooled reactors with oxide and metallic (U-Zr, U-TRU-Zr) fuel exists [4.12].

Although it has a very long core lifetime, the 4S offers a very small linear heat rate of 39 W/cm in the core and yields an average fuel burn-up of only 34 MWday/kg at the end of a long operation cycle. Correspondingly, the Rankine cycle efficiency is only 33% compared to 42% reached in other sodium cooled fast reactors.

The 4S uses non-conventional mechanisms of reactivity control in operation and reactor shut down, and utilises decay heat removal systems that are all passive and operate continuously. These mechanisms and safety design features of the 4S are described in Section 8.6.

The reactor vessel is thin and tall (3.55^24 m) and the containment, provided by the guard vessel and the concrete silo with a top dome in which the reactor is located, is compact.

Table 4.5. Basic characteristics of advanced SMR designs — sodium cooled fast reactors

SMR Design Principal designer, Country

Thermal/Electric output, MW (gross)

Availability/ Plant lifetime

Construction period/ Land-based or floating

Mode of refuelling/ Refuelling interval

Mode of deployment/ Plant configuration

4S

30/10

50 MWe option

12 months on the site

Distributed or concentrated

Toshiba Corporation,

95%/ 30 years

Whole core/30 years

Japan [4.2]

PRISM,

General Electric, USA [4.44]

840/311 WMe

*

In batches/12-24 months

The 4S is designed for both distributed or concentrated deployment. Different from other known sodium cooled fast reactors, the 4S provides for an option of hydrogen (and oxygen) co-production with high temperature electrolysis.