The spent fuel pit cooling and purification system has a function to remove decay heat from spent fuel stored in the spent fuel pit and to remove solid and ion impurities in the spent fuel pit water, purifying it. Spent fuel pit pump, spent fuel pit cooler, and spent fuel pit demineralizer and filter are installed as well. Two spent fuel pit coolers operate to maintain the average temperature of the spent fuel pit water below 52 °C in the case that all fuel assemblies in the core are discharged and stored in the spent fuel pit in which spent fuel assemblies discharged from the previous cycles are already stored. Even operation with one spent fuel pit pump can maintain the average temperature of spent fuel pit water below 65 °C.

Exercises of Chapter 3

1. Give an overview of improvements for advanced standardization of LWRs from 1975 to 1985 in Japan.

2. Refer to the specifications (Tables 3.7 and 3.8) of high burnup 8 x 8 fuel (Step II). Determine size specifications of the fuel and water rods in the 10 x 10 fuel assembly to meet the conditions below.

• No change in channel box size

• The same fuel loading weight as Step II fuel

• Non-boiling region area inside the water rod is equal to or larger than Step II fuel.

Main specifications of high burnup 8 x 8 fuel (Step II): channel box inner width, 134 mm; number of fuel rods, 60; cladding outer diameter, 12.3 mm; cladding thickness, 0.86 mm; cladding inner diameter, 10.58 mm; pellet diam­eter, 10.4 mm; water rod inner diameter, 32 mm; inner cross-sectional area of channel box, about 179.3 cm2.

3. Calculate the following items for a BWR of 26.2 MWth/tU specific power.

(a) Cycle burnup at a cycle length of 12 months and load factor of 100 %.

(b) Core average burnup at the end of the equilibrium cycle for a discharge burnup of 45 GWd/t under the above fuel conditions.

(c) Achievable discharge burnup within the linear reactivity model for an extended cycle length to 15 months under the above fuel conditions.

4. Answer the following questions on reactivity control of a BWR in normal operation.

(a) Explain burnup reactivity control of the BWR.

(b) Set a typical void reactivity coefficient for the BWR and calculate the amount of reactivity control when the void fraction of the core is changed by5 % using core flow rate control.

(c) Explain the control rod position adjustment and core flow rate control during the cycle operation for the excess reactivity with cycle burnup as given in the figure below.

Explain challenges and measures in core design for high burnup based on the transition of design specifications of BWR fuel assembly.

Calculate the core average burnup at the end of the equilibrium cycle of the standard 3-loop PWR assuming a constant power with burnup under the follow­ing conditions.

Core thermal power: 2,652 MW Number of fuel assemblies in the core: 157 Initial uranium weight per assembly: 460 kg Number of fresh fuel assemblies: 60 Equilibrium cycle length: 413 days (full power)

7. Discuss design feasibility of a PWR core without soluble boron in the primary coolant for reactivity control.

8. Consider a PWR in hot full power normal operation with 700 ppm boron concentration in the primary coolant as shown in Fig. 3.38. The PWR was instantly put into hot shutdown. Calculate the boron concentration necessary for achieving criticality at the following times after shutdown, while maintaining the hot temperature.

(a) 8 h

(b) 20 h

(c) 90 h

9. Discuss measures against positive moderator temperature coefficients for the following two cases.

(a) In the process of reload core design, the moderator temperature coefficient for a fuel loading pattern was +2 pcm/°C at BOC and hot zero power. How many burnable poison rods are necessary to make the moderator temperature coef­ficient negative? Assume that the dependence of moderator temperature coef­ficient on boron concentration is the same as that in Fig. 3.35 and a burnable poison rod can reduce the critical boron concentration by about 0.5 ppm.

(b) For startup after refueling, the moderator temperature coefficient was measured as +2 pcm/°C at hot zero power. What operational limitations are available to make the moderator temperature coefficient negative at hot zero power? Refer to Figs. 3.45 and 3.42 for boron worth and control rod worth, respectively.