3a. Primary Circuit Flashcards
What are the four major components of the PWR primary circuit? How many of each?
RPV, Reactor Pressure Vessel
The Pressuriser - x1/r
SG, Steam generator - 2/3/4 (SZB IS 4)
RCP, Reactor coolant pumps - different designs; one per SG or two per SG for a 2 SG design
Fuel cycling happens over long periods (18months) what are the consequence of this and the mitigation strategies?
C: reactivity of the fuel decreases with age, this level needs to maintained over the course of operation.
M1: reduce boric acid conc in prim coolant
M2: use burnable poisons such as ‘gadolinium’ which has a high reactor cross-section but once neutrons are absorbed their isotopes have a v low capture cross-section
What are the materials of RPV? What are some of the significant features?
Main structure - ferritic steel (A533B & SA508)
Major features - forgings (joined using weldings)
Internal lining (cladding) - stainless steel (protects ferritic steel from boric acid corrosion)
What are the penetrations in the closure head used for and what are their main source of damage?
Use - passage of control rod drive mechanisms, thermocouples & instrumentation
Make up - Inconel tube attached to the inside of the vessel head by a partial penetration weld
Issues - Inconcel features are a source of Stress Corrosion Cracking (SCC)
What are the most significant threats to RPV structural integrity?
- Material degradation mechanisms
• Irradiation embrittlement - Potential severe fault condition
• Loss Of Coolant Accident (LOCA)
What is irradiation embrittlement?
Ferritic steels are especially susceptible to embrittlement when irradiated by neutrons and gamma rays.
At high temps this is initially ok and may even strengthen the material, however at low temps the fracture toughness is reduced radically.
With age, the transition temp reduces, which are temps in which the toughness is poor.
What is a LOCA and why is it bad?
Fault leads to significant thermal stresses on the RPV wall.
RPV are designed to withstand LOCA events, even with max size cracks are present and welds are irradiated.
What is the core baffle?
Set of perforated horizontal and vertical plates which form a cage around the core
Purpose - locate fuel elements and maintain core geometry
What is the core barrel and it’s main purpose?
Slides down into the reactor vessel and houses the fuel, the core barrel and all lower internals hang inside the reactor vessel from internal support ledge
Purpose - direct coolant flow path through reactor vessel
1. Coolant enters and hits outer surface of core barrel
2. Coolant forced downwards between reactor vessel wall and outer surface of the core barrel
3. Once at bottom, flow turned upwards inside the core barrel, passing over fuel assemblies
4. Hot water enters upper region and exits via outlet nozzles, which then goes to the steam generator to raise steam in secondary circuit
What are the typical neutron doses for RPV, Core barrel and core baffle? Why are they different?
(Lifetime doses)
Core baffle - 100dpa
Core barrel - 10dpa
RPV - 0.1dpa
The neutron shield, on the outside of the lower core barrel, protects the RPV. Which is important as the RPV is bearing the pressure load.
What are the primary fluid conditions?
Inlet / outlet temps; pressures / coolant flow
Inlet to RPV: 288C - 292C
Outlet from RPV: 323C - 324C
Pressure: 152 - 155 barg (MPa)
Coolant flow over core: 18.8 tonnes/s (4.7 tonnes/s per loop in 4 loop design)
Total volume of RCS: 400m3
What dictates primary coolant operation temperature in PWR?
Gen II/III work with liquid phase p.coolant, hence must be kept below 374C.
374C: critical point for H20 where there is no distinction between gas and liquid phases.
In practise temps and pressure are kept lower for material fatigue reasons etc (typically 324C)
This limit is called the Saturation Temp
What are the typical metrics of SG?
Pressures and temps, coolant temps; flow rate, materials
Design pressure & operating pressure: 17.2MPa & 15.5MPa
Max moisture at outlet: 0.25%
Reactor coolant flow: 4360kg/s
Reactor coolant inlet & outlet temp: 325.8C & 291.8C
Shell material: Mn-Mo steel
Channel head material: Carbon steel clad internally with stainless steel
Tube sheet material: Mo-Cr-No steel clad with Inconel on primary face
How does a SG work?
- Lower hemisphere split by a vertical plate that separates hotter (incoming) and cooler (outgoing) fluid
- Hotter fluid forced through tube plate, via RCP, and into U-shaped heat exchanger tubes
- U-tubes immersed in secondary coolant (liquid phase)
- Secondary coolant then boils and steam collects in the top part of the SG (after water droplets removed) which then exits the SG to the turbine.
Control systems ensure the U-tubes remain full immersed in secondary coolant at all times, matching the feed flow with the rate of steam egress.
What are the weak points of the SG?
Heat exchanger U-tubes £ their connections into the tubeplate.
Issues: SCC, vibration damage, sludge/corrosion products on the tube plates
