20.08.2015   Fast Reactor Safety. (Nuclear science. and technology)

Protective System

As can be seen from Fig. 1.31 the plant protective system (PPS) which detects a failure and shuts down the reactor is the most important safety system. In conjunction with the emergency core cooling system it provides protection against almost all faults. Section 3.1.5 has already outlined a number of trip signals and the trip values which might be used for a typical plant.

3.4.2.1 Scram Function

The prime function of the protective system is to ensure fast and reliable scram in response to a trip signal. To ensure that scram is obtained, the principle of redundancy is used, but to avoid spurious scrams, coincidence techniques are employed.

The logic of protective system action is as follows:

(a) A system acting on one signal from one monitor provides a minimum actuation but it does not provide safety against a failure in the single detec­tion or trip line.

(b) A system acting on one out of two trip lines provides redundancy against a single failure.

(c) A system acting on two out of three trip lines provides redundancy and coincidence and so protects against a spurious signal.

(d) A system acting on two out of four trip lines provides for one channel to fail or to be down for maintenance and still provides total safety.

Table 3.6 shows the scram channel redundancies and coincidences for a number of fast reactors. It can be seen that there is a divergence of opinion as to the correct way to instrument a reactor. Notice that EBR-II provides more trips in total although with less redundancy in some than the Fermi reactor.

TABLE 3.6

Reactor Safety System: Examples of Channel Redundancy and Coincidence

Techniques0

Trip EBR-II Dounreay RAPSODIE Fermi

Nuclear:

Period: source range

2

of

3

2

of

3

і

of

3

і

of 2

intermediate range

2

of

3

1

of

2

2

of

3

2

of 3

power range

2

of

3

Power level

2

of

3

2

of

3

2

of

3

2

of 3

Negative rate of change

of power

2

of 3

Thermal:

Flow: core inlet

1

of

2

1

of

l

blanket inlet

1

of

2

1

of

l

reactor outlet

1

of

l

2

of 3

core outlet

2

of

3

2

of

3

2

of 4

Temperature: upper plenum

2

of

4

core outlet

1

of

1

2

of

3

2

of

3

2

of 4

bulk sodium

2

of

4

Power-to-flow ratio

2

of

3

Upper plenum pressure

1

of

1

Bulk sodium level

1

of

1

Other:

Loss of power to pump

1

of

1

Gas blanket pressure

1

of

1

2

of

3

Seismograph

1

of

1

See Yevick and Amarosi {10).

Reactor scram in the fast system is accomplished by one of several methods: adding absorber material (Fermi), removing fuel material (DFR and EBR-II), and removing reflector material (CLEMENTINE).

The absorber is either boron carbide or tantalum. The former generates helium and requires replacement, while tantalum decreases the breeding by softening the spectrum, although it does increase the Doppler coefficient. The rod control drives are sometimes spring assisted either to increase the rate of fall throughout the fall or simply to give it an initial acceleration.

TABLE 3.7

Fermi Control-Rod Design Parameters’1

Parameter

Safety rods

Operating control rods

General

Reactor power (MWt)

200

200

Guide tube coolant flow (gal/min)

27

39

Rod coolant flow (gal/min)

11

29

Coolant temperature rise (°F)

90

110

Rod life (yr)

8.96

0.6b

Poison material

l0B contained (gm/rod)

535

88

B4C volume (cm3/rod)

554

158

l0B enrichment (at%)

57

32

10B burn-up (%)

7′

W

Gas release (liters/rod) (STP)

3.56

6.6“

Maximum B4C temperature (°F)

1000

1100

Poison containment tube

Design temperature (°F)

1200

1200

Maximum wall temperature (°F)

700

750

Thermal stress in tube (psi)

4000

8000

Internal pressure at end-of-life (psi)

660d

430і

Pressure stress at end-of-life (psi)

6800"

2400"

“ See Yevick and Amarosi (JO). b Based on 10% 10B burn-up.

" Limited by stress.

d Based on ASME Unfired Pressure Vessel Code where allowable fiber stress at 1200°F is 6800 psi.

image162

Table 3.7 shows the characteristics of the Fermi control rods and Fig. 3.4 shows the reactivity change as a control rod is inserted. No reactivity change is experienced for 0.35 sec. This includes a trip delay time and an initial rod insertion time for the end of the control rod to reach about a third of the way into the core. The peak reactivity change is felt by the time the end

reaches the bottom of the core. The time dependence of the reactivity insertion is the usual S-shaped curve which is taken into account in transient studies.

Table 3.8 shows the comparison of safety rod drive systems in Fermi, EBR-II, and DFR.

LA

-u

 

TABLE 3.8

Comparison of Fermi, EBR-II, and Dounreay Fast Reactor Control and Safety Rod Drive Systems0

Feature

 

Fermi

 

EBR-II

 

Dounreay

 

Подпись: Method of control Number and type of control rods Total reactivity Sk/k Shield plug design Coolant flow direction Drive location Method of connection of drive to rod Stroke Подпись:Подпись: 3 Safety Criteria

Peripheral fuel Central fuel backup

14 rods (12 peripheral control, 2 safety)

0.063-0.068

Double rotating

Up

On plug, in line with rods

Direct, relatively tight connection

14 in.

Peripheral fuel

Peripheral poison backup

12 rods (2 safety, 4 shutoff, 6 con­trol) 3 boron poison backup

More than 0.09

Double rotating

Down

Outside plug, offset actuator for rods

Located on carrier mating cone and pin

25 in.

Scram method

Safety rods dropped, drive follows fast to assure scram

Spring assisted

Actuation

Electromagnetic latch

Scram time total

About 0.9 sec

Type of drive shaft

Electric motor—driving ball nut and screw (external)

Position indicator

Digital readout gear driven

Speeds (in./min)

Safety: 1.6 out, 120 in. Shim: 0.4in/out Regulator: variable 1-10

Sealing

Metal О-rings and reciprocating metal bellows

See Yevick and Amarosi (70).

Подпись: 3.4 Safety Features

All control scram, pneumatic assisted

Safety rods only scram during start-up and refueling

Electromagnetic latch

About 0.32 sec

Electric motor—driving rack and pinion (external)

Selsyn system from pinion shaft

Fixed at 5 in/out

All rods scram. Control dropped with their drives, boron dropped with makeup piece only

Electromagnetic latch About 0.5 sec

Electric motor—gear to ball nut and screw (internal)

Special system from servo-arma­ture and search coil

Fixed at 0.18 out, 0.18 or 9 in Boron rods: 0.36

Aluminum gasket and reciprocating metal bellows

О-rings or other metal gaskets, no bellows. All seals static

сл

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